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Abstract ICCS

THE EFFECT OF PROTRUSION DENSITY ON COMPOSITE-METAL JOINTS WITH SURFI-SCULPT REINFORCEMENT

Wei Xiong 1*, 2*, Xichang Wang3, John P. Dear1*, Bamber R.K. Blackman 1*

1 Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom

2 First Aircraft Design Institute, Aviation Industry Corporation of China, Xi'an 710089, China

3 Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, China

Abstract

The effect of protrusion density on the static mechanical properties of composite-metal joints strengthened by surfi-sculpt protrusions has been experimentally studied with single lap joints. The CFRP composite adherends were constant thickness with a quasi-isotropic layup. The metallic adherends were Ti-6Al-4V alloy with a variable number of protrusions per unit area, manufactured by electron beam surfi-sculpt. Digital Image Correlation was used to measure the debonding on the overlap during the tests. Although the surfi-sculpt protrusions did not significantly affect the onset of debonding, they did resist the initial unstable failure mechanism and converted it into stable growth. The analysis indicated that the efficiency of the surface protrusions was different at the metal and composite ends of the overlap. This finding opens the possibility to vary the protrusion density across the overlap to meet specific damage tolerance criteria and optimise joint efficiency. Increasing the protrusion density significantly increased the ultimate failure load, joint extension and hence absorbed energy.

Keywords: Composite-metal joint, Surfi-sculpt, electron beam surfi-sculpt (EBS), density

1. Introduction

Joining dissimilar material such as carbon fibre reinforced plastic (CFRP) to metals effectively has been a significant challenge. The most common joining technologies are adhesive bonding and mechanically fastened joints. For bonded joints, adhesive bonding is usually sensitive to surface preparation and many such joints are limited in use to secondary loading applications due to their relatively low strengths. Moreover, inspection of the bond-line in service is also difficult. The subsequent failure of bonded joints is usually abrupt and catastrophic. For mechanically fastened joints, there is a need for holes to be formed in the composite and the intrinsic brittleness of CFRP makes them sensitive to these stress concentrations [1]. The use of mechanical fasteners therefore reduces structural integrity and also increases the weight, offsetting the potential weight saving gained from using the composite.

An innovative joining technology known as Surfi-Sculpt adopts the idea of z-pinning technology in a composite-composite joint by creating arrays of macro-scale surfi-sculpts on the surface of metal part. This allows a mechanical joint to form between the metal and the composite without the need for holes [2]. This advanced hybrid joint thus combines the advantages of a bonded and mechanically fastened joint. In this work, the electron beam surfi-sculpt (EBS) technique has been employed to create protrusions on the metal adherends using a process that uses multiple relative movements between an electron beam and the metal surface [3, 4]. Although some studies on the mechanical properties and damage tolerance of joints formed using this technology have been reported [5-14], little has been reported on the effects of protrusion density in hybrid joints.

The purpose of this study is to evaluate experimentally the effect of surfi-sculpt protrusion density on the mechanical properties of composite-metal joints strengthened by surfi-sculpt. Four joint types with different surfi-sculpt protrusion densities together with a reference joint have been tested and the initial damage, ultimate failure, debonding propagation and absorbed energy have been compared.

2. Single lap joint Manufacture

The single lap joints, as shown in Figure 1, has been adopted in the present work. Although single lap joints introduce normal stresses at the ends of the overlap which affects the apparent shear strength of the joint, they are easy to manufacture and hence are widely used [15]. Moreover, this manufacturing ease leads to joints with higher quality and greater reliability than using double lap joints to achieve a desired joint shape [10]. Single lap joints provide more freedom for composite layup design (stacking sequences) and are convenient for damage growth detection [12].

Figure 1 Single lap joint

The objective of this study is to investigate the effect of the surfi-sculpt protrusion density on the static mechanical properties of the advanced hybrid joints. The test matrix studied is shown in Table 1. Four types of joint with different surfi-sculpt protrusion densities were tested. Unstrengthened joints were also manufactured and tested as reference. For each type of joint, at least two specimens were tested.

Table 1 Test matrix

Joint type

CFRP nominal thickness (mm)

Composite layup

Surfi-sculpt array

Number of Protrusions

Joint No.

Reference

2.6

[45/0]3/45/[0/45]3

no surfi-sculpt

0

R

Protrusion

density effect

2.6

[45/0]3/45/[0/45]3

5, 6, 5, 6, 5, 6

33

DEN1

[45/0]3/45/[0/45]3

6, 7, 6, 7, 6, 7, 6, 7

52

DEN 2

[45/0]3/45/[0/45]3

7, 8, 7, 8, 7, 8, 7, 8

60

DEN 3

[45/0]3/45/[0/45]3

9,10,9,10,9,10,9,10,9,10

95

DEN 4

2.1. Materials

Ti-6Al-4V [4, 16] was used to manufacture the metal adherend. This alloy is frequently used in the manufacture of hybrid (metal-CFRP) structures in aerospace applications. The composite used to manufacture CFRP adherend was a hot-melt, epoxy prepreg SE 84LV [17]. The fibre of the prepreg is RC200T which is a woven fibre.

Electron beam surfi-sculpt (EBS) was adopted to manufacture surfi-sculpt protrusions on the bonding surface of the metal adherends [3, 4]. The electron beam impacts the metal surface through a special track. The external metal on the track is melted and this molten metal is displaced due to surface tension and vapour tension forming a projection [18]. After a portion of the molten metal solidifies, the EB scan is repeated one or more times. Desired protrusions can be formed with special tracks and using the optimized processing parameters of the EB. Figure 2 shows the surfi-sculpt protrusion shape obtained for the present work.

Figure 2 Surfi-sculpt protrusions

2.2. Joint manufacture

The integration of the metal and composite adherends was completed using vacuum bag processing [17]. Before the vacuum bag processing, the full thickness of the CFRP adherend was built up using the required number and sequence of prepregs (Table 1) and this was placed onto the metal adherends. During the vacuum bag processing, the protrusions were pressed into the uncured laminate with the pressure provided by the vacuum bag. The assembly was finally co-bonded on a hot plate, after which the composite was trimmed to size. Bespoke tooling was used to ensure thorough consolidation of the CFRP matrix and to minimise any misalignment between the laminate and the metal adherend. The lap joints manufactured had a width of 25.4 mm and a nominal overlal length of 30 mm.

Figure 3 Initial damage to CFRP

The insertion of the surfi-sculpt protrusions into the composite introduces resin rich zones and can cause some fibre damage, as shown in the scanning electron micrograph in Figure 3. To prevent the resin rich zones joining together the distance between two adjacent protrusions in both the horizontal and vertical directions was carefully controlled. Every other row of protrusions was offset by a distance equal to half the inter-protrusion distance along the row direction. Taking joint ‘DEN1’ for example, the final surfi-sculpt protrusion array design of joint DEN1 is shown in Figure 4, and contains three rows of 5 protrusions and three rows of 6 protrusions, giving 33 protrusions in total over the overlap area. As the bonding area and the protrusion distribution pattern of all strengthened joints were nominally the same, the surfi-sculpt protrusion number (i.e. number of protrusions on the overlap) is used to represent the corresponding surfi-sculpt protrusion density in this paper.

Figure 4 Surfi-sculpt protrusion array for joint design DEN1

3. Experimental

Single lap shear tests were conducted under displacement control on an Instron testing machine (model 3369) at ambient temperature. The cross head rate was 0.3 mm/min. This is a much lower rate than specified in ASTM D5868 [19] to allow for smaller changes in the compliance of the joint to be recorded. Prior to loading, an extensometer with a gauge length of 50 mm was attached to the specimen as shown in Figure 5. During the tests, the extension of the gauge length was recorded. DIC (Digital Image Correlation) [20] was also employed on the lateral side of the joint to identify the occurrence and the evolution of the debonding of the joint.

Figure 5 Single lap shear test setup

4. Results and discussion4.1. Load-displacement behaviour

Figure 6 Typical quasi-static load vs displacement curves for the joints

Figure 6 compares the typical load-displacement curve measured for the reference joint (control- no protrusions) with the curves for the strengthened joints manufactured with four different protrusion densities (DEN1 to DEN4).

The curve of the reference joint rises linearly up to 4 kN, at which point the stiffness of the joint experiences an abrupt change due to the initiation of debonding. The load at this point is referred to as the damage initiation load. The joint is then further damaged by unstable debonding after a further small increase in load (about 200 N). The resulting debonding propagates rapidly and the joint fails catastrophically because the joint geometry under the tensile load presents an increasing energy release rate with crack propagation and the interface is relatively brittle [21].

The curves for the strengthened joints also increase linearly up to the initial damage load point. It is noted that all joints, with same composite layup design, show the same slope (within the range of experimental error) indicating that the initial joint stiffness is not affected significantly by the different surfi-sculpts densities.

Joints DEN1-3 exhibited damage initiation at about 4.4 kN whereas joint DEN4 exhibited damage initiation at about 5 kN. Following damage initiation, all strengthened joints exhibit further deformation with a lower stiffness. This reduced stiffness value was broadly constant for all pin densities studied. Joints DEN1-3 failed after significant additional load increase (at approximately constant stiffness) at load values in the range of 9-11 kN. However, joint DEN4 exhibited an additional load-displacement behavior at about 13 kN. Thus, for joints DEN4, three different loading stages were observed. Firstly the initial linear stage up to damage initiation. Then the load increases further, almost linearly but with a lower slope, to about 13 kN. Then finally, with further displacement, the trace becomes nearly horizontal. Experimental observation shows that the surfi-sculpt protrusions near both the metal and composite ends of the overlap begin to be pulled out after this point. The load point of 13 kN is referred to here as the transition point. Because the protrusions pull out of the composite in an unstable manner and because there is very little further increase in strength following the transition point (only about 6%), it is suggested that the transition point is a more reliable point to define to characterize the joint failure. The data analysis of debonding propagation, joint extension and absorbed energy also support this suggestion, as is discussed further below.

4.2. Damage initiation load

Figure 7 Damage initiation load for the joints

Figure 7 shows the damage initiation load plotted against the number of protrusions present on the overlap area for the reference and strengthened joints. As the number of protrusions increases (i.e. as the density increases) the damage initiation load increases somewhat but appears to plateau for DEN4. According to the best fitted line through the data, when the protrusion number is less than 25, the damage initiation load remains approximately constant at 4 kN. When the protrusion number increases from 25 to 80, the damage initiation load increases almost linearly. Then the damage initiation load reaches a plateau value at about 5 kN at a protrusion number of 80, irrespective of further increases in protrusion number. The maximum increase in the damage initiation load was 23% for joint DEN4, relative to the reference joint.

4.3. Ultimate failure load

Figure 8 Ultimate failure load for the joints

Figure 8 compares the ultimate failure load for the reference and strengthened joints. All the strengthened joints were much stronger than the reference joints, and the ultimate failure load increased with increasing number of surfi-sculpt protrusions. DEN4 was the strongest joint (the ultimate failure load was 1. 34 kN) and achieved the maximum increase of 212% relative to the reference joint, R, (for which the ultimate failure load was 4.29 kN). The ultimate failure loads for joints DEN1, DEN2 and DEN3 were increased by 115%, 150% and 171% respectively, relative to the reference joint. However, the rate of increase in the ultimate failure load per additional protrusion decreased as the number of protrusions increased.

Figure 9 Ultimate failure load increase rate for the joints

To compare the efficiency of additional protrusions for the various joints, a parameter called the ultimate failure load increase rate (UFLIR) has been used. The UFLIR is defined as:

UFL : Ultimate failure load

: The number of surfi-sculpt protrusions of the joint DEN(i);

DEN(0 ) refers to the Reference joint

Figure 9 shows the comparison of UFLIR values for DEN1 to DEN4. It is clear that the values of UFLIR and hence the efficiency of additional protrusions decreases with increasing protrusion number. DEN1, with the minimum number of protrusions achieved the largest UFLIR. On average, per additional protrusion, the ultimate failure load was increased by 150 N compared with reference joint. DEN4, with the maximum number of protrusions, achieved the smallest value of UFLIR. On average, per additional protrusion, the ultimate failure load increased by 96 N compared with the reference joint. Moreover, it should be noticed that higher density will cause more serious damage into composite adherend and short distance between protrusion make the linkage of resin rich zone more possible, thus decreasing the joint’s mechanical properties significantly. Besides, during manufacture, it is found that the protrusion density higher than DEN4 makes it extremely difficult for protrusions to insert composite adherend and make protrusions lose their function completely. For this material system and manufacture method, the optimum protrusion density is near that of DEN4.

4.4. Damage mechanisms and damage evolution4.4.1. Damage mechanisms

The initially overlapped surfaces of the failed joints were inspected after each test. Visual inspection of the bonding areas on the metal and composite adherends showed evidence of the damage mechanisms that had occurred in the strengthened joints. In each case all the protrusions had failed and the plane of the fracture surfaces through the residual protrusions was not parallel to the bonding surface, as is shown in Figure 10 (a). For a protrusion, its initial height is defined as h. Following fracture, the residual protrusion shows a different height at its left and right sides, as is shown in Figure 10 (b). Here the height on the right side is termed the damage initiation height, . It is determined by the external force acting on a protrusion. The height at the left side is termed the rear fracture height . It results mainly from the internal microstructure of the re-solidified protrusion. Because was found not to be repeatable, the height of the residual protrusion (residual height) has been defined as the height at the damage initiation point on protrusions,, as shown in Figure 10 (b).

hrear

(a) Force diagram of a protrusion (b) The residual protrusion

Figure 10 Protrusion force diagram and residual height definition

For joints DEN1-4, the distribution pattern of the residual heights were similar (shown in Figure 11): the residual protrusions near the metal adherend end are the highest, then the residual height decreases from the metal adherend end to composite adherend end. The lowest residual protrusions heights were near composite adherend end, at about 2/3 length of the bond line from the metal adherend end. Then the residual height increased from the lowest position to the composite adherend end, but the height of the residual protrusions near the composite adherend end is lower than those near the metal adherend end. Figure 11 also shows that the residual height on the joint DEN4 (with the maximum surfi-sculpt protrusion number) is higher than that on the joint DEN1 (with the minimum surfi-sculpt protrusion number).

(a) Joint DEN1

(b) Joint DEN4

Figure 11 The joint damage and protrusion residual height distribution

The variation in the residual protrusion heights indicate that they experience different load conditions across the joint. This results in different failures being attained. It is believed all the differences are the result of the joint deformation in thickness direction.

Figure 12 shows a typical elastic deformation of a single lap joint under a tensile load. The joint experiences bending deflection normal to the external load in addition to a tensile deformation parallel to the external load, caused by the unsymmetrical geometry of the single lap joint. To compare the joint deformation in the normal (Z direction) and the protrusion residual height, the coordination is defined as shown in Figure 13: The origin is located at the bonding interface at the metal end of the overlap. The joint bending deflection are in the Z direction.

Figure 12 Typical single lap joint deformation under tensile load

Figure 13 Joint coordination

The residual height value is always positive but the joint deflection is negative near metal end, so the absolute value of the normalized joint bending deflection, shown as the dashed line in Figure 14, is compared with the residual heights along the bond line for the various joints. These deflection values are normalized by the deflection value at X=0, such that the normalized deflection =1 when X=0. The joint deflection shows a bilinear trend. The point (X=21) shows no deflection and is defined as the neutral point. From the neutral point to the metal end (X=0), the deflection increases linearly and reaches the largest value at the metal end. The same trend is evident from the neutral point to the composite end (X=29), but the deflection at the composite end is smaller than at the metal end. It is evident that the residual height of the protrusions in the joints is proportional to the protrusion density. The residual height of protrusions in the joints (given by the data points in Figure 14) show bilinear trends along the bond line as was noted for the joint deflections. The minimum residual height occurs in the vicinity of the neutral point, according to the bilinear lines fitted through the data for joints DEN1-4. The maximum residual height occurs near the metal end and the residual height near the composite end is smaller than that near the metal end. It is believed that the crack opening displacement at a large deflection point is larger than that at a smaller deflection point, so the force (applied to the metal protrusion by the composite adherend) is applied further from the metal protrusion root, thus the critical damage initiation location is higher, which leads to larger residual height .

Figure 14 The comparison of joint deflection with residual height

Table 2 Damage mechanisms in hybrid joints

Joint

Damage Mechanisms

R

Unstable bond line debonding

DEN1

Composite compression failure; protrusion breakage (maximum 0.15 mm)

DEN2


DEN3


DEN4


The damage mechanisms observed are summarized in Table 2. The reference joints failed by rapid, unstable debonding and two adherends separate catastrophically, immediately following damage initiation. Joint DEN1 only sees the breakage of protrusions. Joints DEN2-4 exhibit similar damage mechanisms with the residual height increasing with increasing protrusion density.

4.4.2. Damage evolution

The DIC measurements taken from a camera facing the X-Z plane allowed the crack growth from both ends of the overlap to be determined during the tests. From the recorded images, the strains were analyzed. Figure 15 shows a typical distribution for a test. The length of the crack formed by the debonding of the interface from the composite end of the overlap is defined as and the length of the similarly formed crack from the metal end of the overlap is defined as . The total crack length is given by the sum of .

Figure 15 DIC data from overlap region showing debonding

Joints DEN1-4 exhibited debonding from the composite end first, prior to attaining the damage initiation load. Then the joints started to debond from the metal end at a load equivalent to the damage initiation load and the debonding continued to propagate from both ends of the overlap until ultimate failure. The maximum total crack length for joints DEN1-3 reached about half of the overlap length prior to final failure. However, for joint DEN4, the total crack length grew across the entire overlap prior to failure.

For the reference joint, there is only debonding from one end of the overlap (the composite end) prior to the ultimate failure of the joint. The initial debonding is about 2 mm and then the debonding crack propagates quickly to 3 mm with a 200N load increase and then the joint fails with total debonding.

Debonding at composite end

DIL


DIL

(a) (b)

Figure 16 Nonlinear behaviour caused by debonding from composite end for joints

The reason that the debonding started from the composite end in all joints is that this end experienced the largest normal (mode I opening) stress (Although the deflection on the metal end is higher, an elastic finite element analysis model of the joints shows that the opening stress is higher at composite end). For joints DEN1-4 the debonding did not lead to early catastrophic failure but resulted in the smooth and continuum nonlinear load-displacement behaviour prior the damage initiation point, as shown in Figure 16. The debonding propagation was delayed by the nearest row of protrusions. The joint deflection at the composite end was so small that the joint stiffness was only slightly affected by the debonding at the composite end.

Debonding from the metal end of the overlap occurred after the damage initiation load and was also resisted by the nearest row of protrusions which the crack encountered. However, the joint deflection at the metal end is larger than that on the composite end and the joint stiffness is therefore affected more significantly by the debonding. The debonding from the metal end thus leads to significant changes in the load-displacement curve (due to reduction in stiffness) and the load at this point corresponds to the damage initiation load, as shown in Figure 16.

Figure 17 shows development of the total debonding length for joints DEN1-4 versus the applied load. Joints DEN1-3 all show bilinear behaviour, which comprises of two stages. The first stage is a short and rapid debonding propagation stage, during which the debonding length rises quickly to about 4 mm. The second stage is characterized by the slow propagation of debonding with a significantly reduced slope with increasing load up to the ultimate failure load. However, joint DEN4 shows an additional third stage: stage two leads finally to a transition point rather than ultimate failure. Following this transition point, the propagation of the debonding became rapid and unstable. This instability following the transition suggests that the transition point should be considered as the ultimate failure point for this joint.

Bondline length

Figure 17 Debonding length of strengthened joints vs load

Figure 17 also indicates that there is no clear correlation between the initial debonding (i.e. intersection of each line on the load axis) and the number of protrusions present. It is believed that these initial values are influenced significantly by manufacturing details such as the existence of a resin fillet at the ends of the overlap. The knee between stages one and two is observed to always occur after about 4 mm of debond growth. Beyond the knee (i.e. in stage two) the surfi-sculpt protrusions are effectively resisting the rapid debond growth, resulting in a much more gradual crack growth than was observed in stage one. In stage two, the slope of the line fitted to the total crack length versus load data, defined as the debonding length growth rate, decreases with increasing protrusion density.

Figure 18 Normalized debonding length growth rate

Figure 18 shows the normalized (with respect to DEN1) crack growth rate for the joints DEN1-4. Compared with DEN1, joints DEN2 and DEN3 achieve only a 5% and 12% decrease in crack growth rate respectively. However, DEN4 achieves a 47% decrease in crack growth rate. Thus, it is clear that although the protrusions do not affect the load at which debonding initiates in the joint, their existence significantly decreases the subsequent crack growth rate.

(a) Debonding length from composite end

(b) Debonding length from metal end

Figure 19 Debonding growth from each end of the overlap vs load

It is of interest to partition the total crack growth into the two components: acomp and am. Figure 19 shows these partitioned values. Both acomp and am have similar propagation behaviour and are similar to the total crack growth behaviour shown in Figure 17. The debonding from the composite end and the metal end in joints DEN1-3 show bilinear behavior. The debonding in joint DEN4 again shows three stages: the bilinear stages (the first two stages) and the last unstable debonding growth stage after the transition point. It is evident that during stage one, there is about twice as much crack growth from the composite end (4 mm) than from the metal end (2 mm). Both debonding lengths are 1mm less than the distance from the corresponding adherend end to the nearest protrusion respectively, which indicates that the initial unstable debonding on both ends is resisted by the nearest protrusions during the first stage.

Figure 20 Debonding length from each end of bond line

Figure 20 shows the average debonding length from each end of the overlap at the end of the second stage (i.e. at the ultimate failure point) versus the protrusion number for the joints. When the protrusion number increased from DEN1 to DEN2, both debonding lengths increased significantly and reached a similar length of about 11 mm. This corresponded to an increase in the ultimate failure load for the DEN2 joint of 34% compared to DEN1. When the protrusion number was increased further to DEN3 and then DEN4, the crack lengths from both ends of the overlap decrease but remain similar.

Although increased protrusion density can increase the resistance of the joint to debonding, at the same time it increases the ultimate load and this increased load leads to a longer debonding length. From DEN1 to DEN2, the increased resistance is not able to resist the debonding resulting from this increased ultimate load, leading to longer debonding lengths from both ends of the overlap. However, when the protrusion density is higher than DEN2, the protrusion density is high enough to resist the debonding caused by the increased ultimate failure load.

Figure 21 Normalized debonding length growth rate vs protrusion density

Figure 21 shows the debonding propagation rate during the second stage (stable debonding propagation stage) for joints DEN1-4. The slope (mm/N) obtained from the fit to the data in the second stage of Figure 19 (a) and (b) is normalized by the debonding rate of DEN1 at the metal end. The results indicate that the debonding from the metal end of the overlap grows faster than the debonding from the composite end for all joints. The debonding growth rate (mm/N) from the composite end was about 58% of that from metal end. It is also clear that the reduction in the rate of debonding growth is greater at the metal end than that at the composite end. In joint design, Figure 21 can allow for the tailoring of the protrusion density to achieve an allowable debonding growth rate by using different protrusion densities at the two ends of the overlap (higher at the metal end than the composite end). This finding may have significant potential to inform future joint design with sculpted metallic adherends.

4.5. Joint extension and energy absorption

Figure 22 Joint extension at failure for all joints

Figure 22 shows the comparison of extension versus protrusion density for all joints tested. All the strengthened joints can withstand higher extension than reference joints. It should be noticed that although joint DEN4 shows a higher extension value at the maximum load point, it has a much longer error bar, indicating that the extension value is not repeatable. In contrast, the extension value at the transition point is more repeatable. The joint extension at failure values increase with the protrusion number. Joint DEN4, with highest protrusion density, shows that the average extension was increased by 423% compared with reference joint. Joint DEN1, with the lowest protrusion density, achieves the least extension increase of 223% compared with reference joint. The extension of joints DEN2-3 were increased by 287%. The figure also shows that the rate of increase of extension decreases with increasing protrusion number.

Figure 23 Energy absorption

The energy absorbed by the joint is defined as the area under the load-displacement curve. The increase in both the ultimate failure load and extension of the joints results in a remarkable increase in energy absorption. Figure 23 shows the values of the energy absorbed by the various joints. It also shows that although joint DEN4 shows a much more absorbed energy at the maximum load point, it has a much larger uncertainty, indicating that the absorbed energy is not repeatable at the maximum point. However, the absorbed energy is more repeatable when measured at the transition point. This confirms the greater reliability of defining final failure as the transition point for joints exhibiting a three stage failure process. The absorbed energy increases linearly with the surfi-sculpt protrusion number, indicating that most energy absorption is due to the failure of surfi-sculpt protrusions. Joint DEN4, with the most surfi-sculpt protrusions, shows that the average absorbed energy was increased by 1616% compared with reference joint. Joint DEN1, with the minimum surfi-sculpt protrusions, achieved the least energy absorption increase of 672% compared with reference joint. The absorbed energy of joints DEN2 and DEN3 were increased by 957% and 1012% respectively.

5. Conclusions

The effect of the surfi-sculpt protrusion density on the static mechanical properties of advanced hybrid joints has been experimentally studied. Four joint types with different protrusion densities were tested in tension and the corresponding results were compared with that of reference joints without protrusions present. The maximum load capacity and best damage tolerance was achieved with the highest surfi-sculpt density, i.e. the joints of type DEN4.

Digital image correlation analysis was used to measure the growth of the debonding along the joint overlap during the tests. It was shown that although the protrusions did not affect the joint stiffness and the onset of debonding significantly, the protrusions did resist the initial unstable failure mechanism and converted it into stable growth. With increasing protrusion density, the debonding propagation rate decreased and the failure mode was shown to change from debonding to intra-laminar failure of the composite and fracture of the metallic protrusions in a mixed shear and bending mode. The analysis also showed the efficiency of protrusions was different at the two ends of the overlap, i.e. at the metal and composite ends respectively. This opens the possibility that the protrusion density could be designed to vary across the overlap to meet a damage tolerance criterion and also to optimise protrusion efficiency. Finally, increasing the protrusion density was shown to significantly increase the ultimate failure load, joint extension and hence absorbed energy. Simulations are being conducted to further understand these observations.

Acknowledgements

The strong support from the Aviation Industry Corporation of China (AVIC) the First Aircraft Institute (FAI) and Beijing Aeronautical Manufacturing Technology Research Institute (BAMTRI) for this funded research is much appreciated. The research was performed at the AVIC Centre for Structural Design and Manufacture at Imperial College London.

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Figure

Figure 1 Single lap joint

Figure 2 Surfi-sculpt protrusions

Figure 3 Initial damage to CFRP

Figure 4 Surfi-sculpt protrusion array for joint design DEN1

Figure 5 Single lap shear test setup

Figure 6 Typical quasi-static load vs displacement curves for the joints

Figure 7 Damage initiation load for the joints

Figure 8 Ultimate failure load for the joints

Figure 9 Ultimate failure load increase rate for the joints

(a) Force diagram of a protrusion (b) The residual protrusion

Figure 10 Protrusion force diagram and residual height definition

(a) Joint DEN1

(b) Joint DEN4

Figure 11 The joint damage and protrusion residual height distribution

Figure 12 Typical single lap joint deformation under tensile load

Figure 13 Joint coordination

Figure 14 The comparison of joint deflection with residual height

Figure 15 DIC data from overlap region showing debonding


DIL


DIL

(a) (b)

Figure 16 Nonlinear behaviour caused by debonding from composite end for joints

Bondline length

Figure 17 Debonding length of strengthened joints vs load

Figure 18 Normalized debonding length growth rate

(a) Debonding length from composite end

(b) Debonding length from metal end

Figure 19 Debonding growth from each end of the overlap vs load

Figure 20 Debonding length from each end of bond line

Figure 21 Normalized debonding length growth rate vs protrusion density

Figure 22 Joint extension at failure for all joints

Figure 23 Energy absorption

Tables

Table 1 Test matrix

Joint type

CFRP nominal thickness (mm)

Composite layup

Surfi-sculpt array

Number of Protrusions

Joint No.

Reference

2.6

[45/0]3/45/[0/45]3

no surfi-sculpt

0

R

Protrusion

density effect

2.6

[45/0]3/45/[0/45]3

5, 6, 5, 6, 5, 6

33

DEN1

[45/0]3/45/[0/45]3

6, 7, 6, 7, 6, 7, 6, 7

52

DEN 2

[45/0]3/45/[0/45]3

7, 8, 7, 8, 7, 8, 7, 8

60

DEN 3

[45/0]3/45/[0/45]3

9,10,9,10,9,10,9,10,9,10

95

DEN 4

Table 2 Damage mechanisms in hybrid joints

Joint

Damage Mechanisms

R

Unstable bond line debonding

DEN1


DEN2


DEN3


DEN4


6x6-1(1)3469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.9013700.355731225296442691.18577075098814232.01581027667984182.49011857707509864.7430830039525693DEN13958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.94530999999924.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464377.11462450592885447.82608695652173928.18181818181818179.249011857707509810.19762845849802311.02766798418972313.16205533596837914.5849802371541497x8-1(1)2509.17922565.17383000000022959.439940000000200.588235294117647083.1764705882352939DEN22959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.8564499999993.176470588235293944.05882352941176454.58823529411764675.88235294117647017.05882352941176458.35294117647058710.11764705882352910.58823529411764912.58823529411764913.76470588235294215.52941176470588216.1176470588235298x8-1(1)3295.48754999999983314.77417000000013586.832033608.814940000000200.472440944881889760.826771653543307062.8346456692913384DEN33608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.274412.83464566929133843.77952755905511814.01499999999999974.72440944881889726.25984251968503898.385826771653544410.39370078740157411.33858267716535414.0551181102362214.76377952755905614.881889763779528DEN45993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.4404300000015.08366533864541835.768.2800000000000011911.0412.2410x10-1(3)13230.44043000000113688.7968814066.19140999999914140.5927699999991414212.2414.87999999999999917.5219.7999999999999973010x10-1(1)4145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.960939999999900.61.562.0434.43999999999999955.08366533864541836x6-1-303469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.945309999999200.355731225296442691.18577075098814232.01581027667984182.49011857707509864.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464377.11462450592885447.82608695652173928.18181818181818179.249011857707509810.19762845849802311.02766798418972313.16205533596837914.5849802371541497x8-1-302509.17922565.17383000000022959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999900.588235294117647083.176470588235293944.05882352941176454.58823529411764675.88235294117647017.05882352941176458.35294117647058710.11764705882352910.58823529411764912.58823529411764913.76470588235294215.52941176470588216.1176470588235298x8-1-303295.48754999999983314.77417000000013586.832033608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.2744100.472440944881889760.826771653543307062.83464566929133843.77952755905511814.01499999999999974.72440944881889726.25984251968503898.385826771653544410.39370078740157411.33858267716535414.0551181102362214.76377952755905614.88188976377952810x10-1-304145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.44043000000113688.7968814066.19140999999914140.5927699999991414200.61.562.0434.43999999999999955.08366533864541835.768.2800000000000011911.0412.2414.87999999999999917.5219.79999999999999730

Load (N)

Total debonding length (mm)

3352609510.955050237969328420.882601797990481110.52564780539397149

Surfi-sculpt protrusion number

Normalized debonding rate

DEN13958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.94530999999924.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464375.92885375494071195.92885375494071196.04743083003952546.40316205533596877.11462450592885357.2332015810276688.18181818181818178.89328063241106656x6-1(1)3469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.9013700.355731225296442691.18577075098814232.01581027667984182.49011857707509864.74308300395256937x8-1(1)2509.17922565.17383000000022959.439940000000200.588235294117647083.1764705882352939DEN23502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999944.05882352941176454.1176470588235294.1176470588235294.58823529411764674.82352941176470564.94117647058823554.94117647058823556.94117647058823557.29411764705882347.76470588235294118.2352941176470588x8-1(1)3295.48754999999983314.77417000000013586.832033608.814940000000200.472440944881889760.826771653543307062.8346456692913384DEN33608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.274412.83464566929133843.77952755905511814.01499999999999974.0157480314960634.13385826771653524.37007874015748054.84251968503937045.66929133858267696.96850393700787417.08661417322834637.086614173228346310x10-1(1)4145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.960939999999900.61.562.04333.3505976095617527DEN45993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.4404300000013.35059760956175273.64.24.44000000000000045.4610x10-1(3)13230.44043000000113688.7968814066.19140999999914140.5927699999991414266.848.279999999999999410.199999999999999156x6-1-103469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.945309999999200.355731225296442691.18577075098814232.01581027667984182.49011857707509864.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464375.92885375494071195.92885375494071196.04743083003952546.40316205533596877.11462450592885357.2332015810276688.18181818181818178.89328063241106657x8-12509.17922565.17383000000022959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999900.588235294117647083.176470588235293944.05882352941176454.1176470588235294.1176470588235294.58823529411764674.82352941176470564.94117647058823554.94117647058823556.94117647058823557.29411764705882347.76470588235294118.2352941176470588x8-13295.48754999999983314.77417000000013586.832033608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.2744100.472440944881889760.826771653543307062.83464566929133843.77952755905511814.01499999999999974.0157480314960634.13385826771653524.37007874015748054.84251968503937045.66929133858267696.96850393700787417.08661417322834637.086614173228346310x10-14145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.44043000000113688.7968814066.19140999999914140.5927699999991414200.61.562.04333.35059760956175273.64.24.44000000000000045.466.848.279999999999999410.19999999999999915

Load (N)

Debonding length acomp (mm)

6x6-1(1)5736.444345779.125979999999801.1857707509881423DEN16043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.94530999999921.89723320158102762.13438735177865622.84584980237154153.08300395256916993.79446640316205524.98023715415019735.6916996047430837x8-1(1)3743.65601000000023794.406254237.354980000000100.470588235294117641.7647058823529411DEN24237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.8564499999991.76470588235294112.47058823529411783.52941176470588225.17647058823529445.64705882352941215.64705882352941216.47058823529411787.76470588235294117.8823529411764718x8-1(1)5331.156255347.69141000000045510.903320000000300.708661417322834612.1259842519685042DEN35510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.274412.12598425196850424.0157480314960635.55118110236220465.66929133858267697.08661417322834637.67716535433070837.795275590551181510x10-1(1)5683.49608999999965734.230959999999901.44DEN45993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.4404300000011.73306772908366542.164.084.55999999999999965.646.2410x10-1(3)13230.44043000000113688.7968814066.19140999999914140.592769999999141426.248.03999999999999919.249.6156x6-1-203469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.945309999999201.18577075098814231.89723320158102762.13438735177865622.84584980237154153.08300395256916993.79446640316205524.98023715415019735.6916996047430837x8-1-202509.17922565.17383000000022959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999900.470588235294117641.76470588235294112.47058823529411783.52941176470588225.17647058823529445.64705882352941215.64705882352941216.47058823529411787.76470588235294117.8823529411764718x8-1-203295.48754999999983314.77417000000013586.832033608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.2744100.708661417322834612.12598425196850424.0157480314960635.55118110236220465.66929133858267697.08661417322834637.67716535433070837.795275590551181510x10-1-204145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.44043000000113688.7968814066.19140999999914140.5927699999991414201.441.73306772908366542.164.084.55999999999999965.646.248.03999999999999919.249.615

Load (N)

Debonding length am (mm)

From composite end335260958.928571428571428810.7661290322580649.44881889763779456From metal end335260956.190476190476190711.3709677419354848.85826771653543336.24


Debonding length (mm)

From composite end335260950.581337737407101480.513625103220478940.4351775392237820.30305532617671344From metal end3352609510.86374896779521060.758051197357555660.51775392237819973



6x6-1(1)3469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.9013700.355731225296442691.18577075098814232.01581027667984182.49011857707509864.7430830039525693DEN13958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.94530999999924.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464377.11462450592885447.82608695652173928.18181818181818179.249011857707509810.19762845849802311.02766798418972313.16205533596837914.5849802371541497x8-1(1)2509.17922565.17383000000022959.439940000000200.588235294117647083.1764705882352939DEN22959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.8564499999993.176470588235293944.05882352941176454.58823529411764675.88235294117647017.05882352941176458.35294117647058710.11764705882352910.58823529411764912.58823529411764913.76470588235294215.52941176470588216.1176470588235298x8-1(1)3295.48754999999983314.77417000000013586.832033608.814940000000200.472440944881889760.826771653543307062.8346456692913384DEN33608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.274412.83464566929133843.77952755905511814.01499999999999974.72440944881889726.25984251968503898.385826771653544410.39370078740157411.33858267716535414.0551181102362214.76377952755905614.881889763779528DEN45993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.4404300000015.08366533864541835.768.2800000000000011911.0412.2410x10-1(3)13230.44043000000113688.7968814066.19140999999914140.5927699999991414212.2414.87999999999999917.5219.7999999999999973010x10-1(1)4145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.960939999999900.61.562.0434.43999999999999955.08366533864541836x6-1-303469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.945309999999200.355731225296442691.18577075098814232.01581027667984182.49011857707509864.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464377.11462450592885447.82608695652173928.18181818181818179.249011857707509810.19762845849802311.02766798418972313.16205533596837914.5849802371541497x8-1-302509.17922565.17383000000022959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999900.588235294117647083.176470588235293944.05882352941176454.58823529411764675.88235294117647017.05882352941176458.35294117647058710.11764705882352910.58823529411764912.58823529411764913.76470588235294215.52941176470588216.1176470588235298x8-1-303295.48754999999983314.77417000000013586.832033608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.2744100.472440944881889760.826771653543307062.83464566929133843.77952755905511814.01499999999999974.72440944881889726.25984251968503898.385826771653544410.39370078740157411.33858267716535414.0551181102362214.76377952755905614.88188976377952810x10-1-304145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.44043000000113688.7968814066.19140999999914140.5927699999991414200.61.562.0434.43999999999999955.08366533864541835.768.2800000000000011911.0412.2414.87999999999999917.5219.79999999999999730

Load (N)

Total debonding length (mm)

3352609510.955050237969328420.882601797990481110.52564780539397149



DEN13958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.94530999999924.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464375.92885375494071195.92885375494071196.04743083003952546.40316205533596877.11462450592885357.2332015810276688.18181818181818178.89328063241106656x6-1(1)3469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.9013700.355731225296442691.18577075098814232.01581027667984182.49011857707509864.74308300395256937x8-1(1)2509.17922565.17383000000022959.439940000000200.588235294117647083.1764705882352939DEN23502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999944.05882352941176454.1176470588235294.1176470588235294.58823529411764674.82352941176470564.94117647058823554.94117647058823556.94117647058823557.29411764705882347.76470588235294118.2352941176470588x8-1(1)3295.48754999999983314.77417000000013586.832033608.814940000000200.472440944881889760.826771653543307062.8346456692913384DEN33608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.274412.83464566929133843.77952755905511814.01499999999999974.0157480314960634.13385826771653524.37007874015748054.84251968503937045.66929133858267696.96850393700787417.08661417322834637.086614173228346310x10-1(1)4145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.960939999999900.61.562.04333.3505976095617527DEN45993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.4404300000013.35059760956175273.64.24.44000000000000045.4610x10-1(3)13230.44043000000113688.7968814066.19140999999914140.5927699999991414266.848.279999999999999410.199999999999999156x6-1-103469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.945309999999200.355731225296442691.18577075098814232.01581027667984182.49011857707509864.74308300395256935.33596837944664065.45454545454545415.6916996047430835.86896634327464375.92885375494071195.92885375494071196.04743083003952546.40316205533596877.11462450592885357.2332015810276688.18181818181818178.89328063241106657x8-12509.17922565.17383000000022959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999900.588235294117647083.176470588235293944.05882352941176454.1176470588235294.1176470588235294.58823529411764674.82352941176470564.94117647058823554.94117647058823556.94117647058823557.29411764705882347.76470588235294118.2352941176470588x8-13295.48754999999983314.77417000000013586.832033608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.2744100.472440944881889760.826771653543307062.83464566929133843.77952755905511814.01499999999999974.0157480314960634.13385826771653524.37007874015748054.84251968503937045.66929133858267696.96850393700787417.08661417322834637.086614173228346310x10-14145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.44043000000113688.7968814066.19140999999914140.5927699999991414200.61.562.04333.35059760956175273.64.24.44000000000000045.466.848.279999999999999410.19999999999999915

Load (N)

Debonding length acomp (mm)

6x6-1(1)5736.444345779.125979999999801.1857707509881423DEN16043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.94530999999921.89723320158102762.13438735177865622.84584980237154153.08300395256916993.79446640316205524.98023715415019735.6916996047430837x8-1(1)3743.65601000000023794.406254237.354980000000100.470588235294117641.7647058823529411DEN24237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.8564499999991.76470588235294112.47058823529411783.52941176470588225.17647058823529445.64705882352941215.64705882352941216.47058823529411787.76470588235294117.8823529411764718x8-1(1)5331.156255347.69141000000045510.903320000000300.708661417322834612.1259842519685042DEN35510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.274412.12598425196850424.0157480314960635.55118110236220465.66929133858267697.08661417322834637.67716535433070837.795275590551181510x10-1(1)5683.49608999999965734.230959999999901.44DEN45993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.4404300000011.73306772908366542.164.084.55999999999999965.646.2410x10-1(3)13230.44043000000113688.7968814066.19140999999914140.592769999999141426.248.03999999999999919.249.6156x6-1-203469.11063499.58154000000013529.91699000000023559.50293000000013917.32227000000013958.901374507.27880999999985021.72753999999995548.64794999999965736.444345779.12597999999986043.18505999999986573.37694999999997102.91503999999997641.17235999999968187.53075999999968721.39160000000089205.945309999999201.18577075098814231.89723320158102762.13438735177865622.84584980237154153.08300395256916993.79446640316205524.98023715415019735.6916996047430837x8-1-202509.17922565.17383000000022959.43994000000023502.838623743.65601000000023794.406254237.35498000000014808.31444999999996294.60595999999996858.13965000000017416.343758265.81151999999939013.97655999999929536.198239999999710095.85644999999900.470588235294117641.76470588235294112.47058823529411783.52941176470588225.17647058823529445.64705882352941215.64705882352941216.47058823529411787.76470588235294117.8823529411764718x8-1-203295.48754999999983314.77417000000013586.832033608.81494000000024689.87207000000035331.156255347.69141000000045510.90332000000036388.93896000000047810.57031000000018869.74022999999949993.785159999999510851.3232411707.2744100.708661417322834612.12598425196850424.0157480314960635.55118110236220465.66929133858267697.08661417322834637.67716535433070837.795275590551181510x10-1-204145.36376999999994206.63231999999974766.28369000000025038.52001999999995683.49608999999965734.23095999999995993.96093999999996743.03075999999967989.20604999999989227.757809999999211449.6562513230.44043000000113688.7968814066.19140999999914140.5927699999991414201.441.73306772908366542.164.084.55999999999999965.646.248.03999999999999919.249.615

Load (N)

Debonding length am (mm)

From composite end335260958.928571428571428810.7661290322580649.44881889763779456From metal end335260956.190476190476190711.3709677419354848.85826771653543336.24


Debonding length (mm)

From composite end335260950.581337737407101480.513625103220478940.4351775392237820.30305532617671344From metal end3352609510.86374896779521060.758051197357555660.51775392237819973



1

30

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0

2000

4000

6000

8000

10000

12000

14000

Damage Initiation

Transition Point

Load (N)

Displacement (mm)

R

DEN1

DEN2

DEN3

DEN4

0

20

40

60

80

100

0

1000

2000

3000

4000

5000

DEN4 (22.9%)

DEN3 (16%)

DEN2 (10%)

DEN1(4%)

R

Initiation Damage load (N)


0

20

40

60

80

100

0

2000

4000

6000

8000

10000

12000

14000

Ultimate failure load (N)


R

DEN1(114%)

DEN2 (150%)

DEN3 (171%)

DEN4 (212.03%)

0

10

20

30

40

50

60

70

80

90

100

0

20

40

60

80

100

120

140

160

Ultimate failure load increase rate(N)


DEN2 (123N)

DEN1(150N)

DEN3 (122N)

DEN4 (96 N)

0

5

10

15

20

25

0.0

0.2

0.4

0.6

0.8

1.0

Bonding line deflection

DEN1

DEN2

DEN3

DEN4

X (mm)

Normalized joint Deflection (absolute value)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Residual' Height (mm)

0.0

0.1

0.2

0.3

0

5000

10000

Nonlinear behavior

Load (N)

Displacement (mm)

R

DEN1

DEN2

DEN3

DEN4

DEN1

DEN2

DEN3

DEN4

0

20

40

60

80

100

0.0

0.1

0.2

0.3

0.4

DEN4

(Maximum load point)

(714.25%)

Extension (mm)

Surfi-sulpt density

R

DEN1

(223%)

DEN2

(287%)

DEN3

(288%)

DEN4

(Transition point)

(423%)

0

20

40

60

80

100

0

1000

2000

3000

4000

DEN4


(3339%)

Absorbed engery (N.mm)

Surfi-sulpt density

R

DEN1

(673%)

DEN2

(957%)

DEN3

(1012%)

DEN4

(Transition point)

(1616%)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0

2000

4000

6000

8000

10000

12000

14000

Damage Initiation

Transition Point

Load (N)

Displacement (mm)

R

DEN1

DEN2

DEN3

DEN4

0

20

40

60

80

100

0

1000

2000

3000

4000

5000

DEN4 (22.9%)

DEN3 (16%)

DEN2 (10%)

DEN1(4%)

R

Initiation Damage load (N)


0

20

40

60

80

100

0

2000

4000

6000

8000

10000

12000

14000

Ultimate failure load (N)


R

DEN1(114%)

DEN2 (150%)

DEN3 (171%)

DEN4 (212.03%)

0

10

20

30

40

50

60

70

80

90

100

0

20

40

60

80

100

120

140

160

Ultimate failure load increase rate(N)


DEN2 (123N)

DEN1(150N)

DEN3 (122N)

DEN4 (96 N)

0

5

10

15

20

25

0.0

0.2

0.4

0.6

0.8

1.0

Bonding line deflection

DEN1

DEN2

DEN3

DEN4

X (mm)

Normalized joint Deflection (absolute value)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Residual' Height (mm)

0

20

40

60

80

100

0.0

0.1

0.2

0.3

0.4

DEN4


(714.25%)

Extension (mm)

Surfi-sulpt density

R

DEN1

(223%)

DEN2

(287%)

DEN3

(288%)

DEN4

(Transition point)

(423%)

0

20

40

60

80

100

0

1000

2000

3000

4000

DEN4


(3339%)

Absorbed engery (N.mm)

Surfi-sulpt density

R

DEN1

(673%)

DEN2

(957%)

DEN3

(1012%)

DEN4

(Transition point)

(1616%)

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