Interface Fracture Behaviour of Industrial Adhesives: A Novel Evaluation Approach for Adhesive...

Symposium on Innovations in

Adhesives and their Applications Munich, Germany, February 14-15, 2017

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INTERFACE FRACTURE BEHAVIOUR OF INDUSTRIAL ADHESIVES: A NOVEL EVALUATION APPROACH FOR ADHESIVE SELECTION

M.H. Brandtner-Hafnera*

a BRANDTNER-HAFNER Fracture Analytics, Raiffeisenstraße 11/4/5, 7072 Mörbisch am See, Austria * Contact person: [email protected]

www.fractureanalytics.com

ABSTRACT For adhesives, the prevailing situation shows that material parameters obtained from popular standardised tensile strength and peel tests cannot be used to make fracture mechanical statements. Unfortunately, they merely giving the maximum bonding strength of the adhesively bonded composite. Specifically, since they are pure measures assuming continuum mechanics, they are not appropriate for choosing the right adhesive under fracture mechanical aspects. Furthermore, popular fracture mechanical tests such as DCB or ENF do not take account the complex and non-linear nature of industrial adhesives, which leads to misleading results. To overcome such limitations, an innovative fracture testing method was newly applied enabling stable crack propagation behaviour after maximum load has been exceeded. This in turn leads to an understanding of interface fracture mechanisms and therefore leads to fruitful conclusions of how to choose the best adhesive/adherend system. KEYWORDS: Interface Fracture Behaviour, Adhesively Bonded Composites, Fracture Energy, Adhesive Selection 1. INTRODUCTION

1.1. Review A major drawback of standardised classic mechanical testing systems [1,2] is that they do not provide any information about cracking- and damaging behaviour of the interface region of the adhesive composite system under investigation. Furthermore, no statements can be made if crack propagation is brittle or tough. This influences stability of failure, which means that for example a running crack could lead to a catastrophically failure of the whole structure. This engineering approach of using just maximum value parameters such as maximum force or maximum tensile strength is not suitable for selecting the optimal adhesive/adherend system. Additionally, no statement can be made if a structure will fail brittle or tough by relying just on tensile strength alone.

The theoretical fracture mechanics concept of this study is based on the nonlinear fracture energy criterion of Hillerborg et al. [3], which is known as the fictitious crack model (FCM) or cohesive zone model (CZM). The basic evaluation parameter characterising the fracture mechanical behaviour used into this non-linear fracture criterion is the specific fracture energy GF, which can be regarded as a fracture toughness and fracture resistance parameter, respectively. It can be determined by a method introduced by [4]. It ensures a stable testing environment which enables physical statements about formation of cracks as well as information on post-cracking behaviour beyond the maximal load. Further, it is not necessary to know the position and shape of the process zone a priori. The cracking process thus can be characterised by strain-softening diagrams and specific fracture energy [4].

Please cite this paper as follows: M.H. Brandtner-Hafner, Interface Fracture Behaviour of Industrial Adhesives: A Novel Evaluation Approach for Adhesive Selection, In: Proceedings of the IN-ADHESIVES Symposium on Innovations in Adhesives and their Applications, pp. 221-229, Munich, Germany, 2017.



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INTERFACE FRACTURE BEHAVIOUR OF INDUSTRIAL ADHESIVES: A NOVEL EVALUATION APPROACH FOR ADHESIVE SELECTION Dr. Martin BRANDTNER-HAFNER Fracture Mechanics Professional

For this study, this was done for all three modes of crack separation according to Irwin [5] shown in figure 1 below:

Figure 1: Crack separation modes [5]. As an explanation to that, mode I means crack opening (left picture), mode II in-plane shear (middle) and mode III out-of-the-plane shear or tearing (right picture). Reviewing technical literature, there are two basic concepts used mainly for characterising industrial adhesives and bonded composites. The first is stress based such as the K-concept (stress intensify factor, SIF) [6,7] whereas the second principle is related to energy such as the strain energy release rate Gc [8,9]. The first one is only valid for homogenous, isotropic materials with strict linear-elastic material behaviour obeying Hooke`s law [4]. This might hold only for very brittle materials such as glass or ceramics. However, when it comes to quasi-brittle (i.e. concrete) or even very tough structures (i.e. rubber), they do not show linear-elastic but elastic-plastic behaviour [4]. For that, linear-elastic fracture mechanics (LEFM) is not suitable for describing non-linear cracking processes of tough adhesive composites [4]. Therefore, in this study, the non-linear fracture mechanical approach of Hillerborg`s cohesive zone model (CZM) [3] is the theoretical framework on which the cracking behaviour is described. The major benefit over stress intensify factor K (SIF) and strain energy release rate Gc is the independence of crack geometry as well as the ability to deal with non-linearity effects appearing in a large damage zone compared to specimen size. Since the above mentioned cases are just dealing with linear-elastic components,

they are not very representative in terms of describing real fracture mechanisms. As a fact, they represent just a simplification of damage reality occurring and therefore are not appropriate for damage analysis of adhesively bonded composites [4]. 1.2. Interface fracture mechanics of adhesively bonded composites Theoretical framework based on K-factors (SIF) used to describe interface fracture behaviour of adhesively bonded composites are subject to major shortcomings. One of this is the prevailing of complex K-factors with the phenomenon of highly oscillating and penetrating crack banks [10,11]. Besides, K-values are subject to very high mean variations, have very unphysical units [MPa·m-1/2] and therefore questionable meaning of explaining real fracture processes. For adhesively bonded composites, the critical point regarding crack behaviour is within the interface region, which is the contact layer or zone between the adhesive/adherent composite. There, damage mechanisms can occur in terms of “failure modes” standardised by [12] depicted in figure 2:

Figure 2: Damage mechanisms of adhesively bonded

composites for mode I [12]. According to the ASTM D5573 standard, locations of fracture can be categorized as follows [13]: (1) Adhesive layer (cohesive failure):

in this case, the adhesive is weak. (i.e. A and B),



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(2) Interface (interfacial failure): in this case, the adhesive bonding is weak. (i.e. C)

(3) Mixed failure: (1) + (2). (i.e. D and E) (4) Adherends or its surface layer (adherend failure). (i.e. F) Basically, fracture behaviour of adhesive composites is non-linear [4]. The same holds for bonded sandwich panels containing two interfaces. Additionally, the main reason of how a crack finally turns out is the extent of toughness of the compound. This can be characterised experimentally leading to a fracture mechanical fingerprint of the whole structure gaining insights of its prevailing material law. Finally, for the interesting reader a comprehensive overview about interface fracture mechanics can be found at [11]. Another important aspect of understanding the mechanism of adhesive cracking is related to the shape and extend of the so called “fracture process zone”, which is a sub-area within the “plastic zone”. For those, the tougher the material, the greater its extent. Such areas are consuming fracture energy used to propagate the crack and therefore are an essential indicator of the damage behaviour of an adhesively bonded composite [4]. Figure 3 shows different stages of process zones of a bi-material composite of different toughness and stiffness. The upper part (white) is brittle, whereas the lower part (grey) is tough [14]. For picture 3a, linear-elastic fracture mechanics can be applied as the whole compound is very brittle and the process zone is very small compared to the specimen size. In figure 3b, the plastic zone (PL) is far more extended than before as this composite is less brittle. This means, elastic-plastic mechanics must be applied. Turning to picture 3c, the fracture process zone is very large compared to the specimen and to picture 3a. There, non-linear and quasi-brittle material behaviour can be observed. This is valid for example for concrete, wood, several types of plastics and adhesives. It should be noted that it is the interface region where the crack starts to propagate as there the fracture resistance is much lower than in the

adherends (white and grey) and a starter notch has been created.

Figure 3: Overview of fracture process zones of different adherend materials [14]

In order to understand the influence of fracture energy of adhesives, it is important to use stiff and rigid adherends. In order to ensure this, for this study bonded parts were made of pearwood, which ensures a high stiffness. A further important issue noteworthy to address for investigating interface fracture behaviour of adhesives is the crack propagation tendency illustrated in figure 4 below [15]. There, A stands for unstable, B for semi-stable and C for stable crack propagation. The test method introduced by [4] enables condition C and therefore clear statements on post-cracking behaviour of the tested adhesive.

Figure 4: Types of crack propagation behaviour [15].



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2. METHODS 2.1. Mechanical testing Basically, pull-out tests or tensile strength tests were performed on unnotched specimens [1,2] according to figure 5. This can be described mathematically by equation (1):

proj

TT A

F (1)

With T as tensile strength, FT as tensile force and Aproj as projected area perpendicular to the stress direction. This value can be regarded as “adhesive bonding strength”. Since this is a normal stress, only notch tensile strength from mode I is comparable. Hence, the author introduced notation RV as a global term describing composite or adhesive bonding strength for all three modes of cracking. Specimens for testing tensile strength where made up of unnotched bonded pear wood blocks of 50x50 mm area and thickness of 30 mm each as pictured in figure 5:

Figure 5: Specimen for pull-out test [4]. 2.2. Fracture mechanical testing In order to compare stresses gathered from mechanical tensile strength tests [1,2], notch tensile strength for mode I was measured and compared. However, this characteristic value alone does not account for cracking effects. Thus, another parameter describing non-linear cracking behaviour in terms of fracture resistance is the specific fracture energy [4].

According to Hillerborg [16], GF represents the fracture energy per unit area of the fracture surface. This is the projected area on a plane perpendicular to the stress direction, also called “ligament area”. It is located within the damage zone of the specimen. This may not be confused with the area enclosed by the --curve representing an energy per unit volume absorbed in the whole specimen [16]. GF in turn is only released within the fracture process zone (also: plastic or damage zone) describing cohesive energy and may not be confused with the linear-elastic strain energy release rate Gc [17], which is a completely different parameter derived from beam theory solution [18]. It is based upon linear-elastic behaviour and depends on the exactly determined crack length, which has to be monitored. However, this cannot be valid for adhesively bonded composites, where highly elastic-plastic material properties prevail leading to a distinct damage zone which cannot be ignored. This damage zone is formed by secondary mechanisms such as fibre bridging, microcracking, crazing and others, which are also called “shielding mechanisms” [19]. Figure 6 gives an overview of such mechanism.

Figure 6: Crack shielding mechanism [19].



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Types of shielding mechanisms: a) Interface deboning, b) Microcracking, c) Yielding, d) Wake region, e) Fibre bridging and fibre pull-out, f) Grain bridging and interlocking, g) Viscoelastic bridging (crazing). Such mechanisms are responsible for delaying crack propagation, which is an important criteria for adhesive selection. Especially g (crazing) is the most responsible shielding mechanism for adhesives, which is very desirable regarding healing behaviour. In the best case, this could lead to a crack arrest and therefore for the structure there is no need to repair. Basically, nowadays DCB-tests are predominantly used in order to gain values of Gc [20] for mode I and ENF-tests [21] for mode II, respectively. However, for adhesive composites, their adherends are elastically bended during testing. Unfortunately, the elastic energy is stored thus exceeding the resistance against crack propagation which in turn may lead to unstable cracking. As a consequence, post-cracking-behaviour cannot be characterised. A critical review of the DCB- and ENF-testing techniques can be found in [22,23], indicating that more suitable alternatives have to be applied [4].

Also, it must be considered that G is different from that in G (fracture energy). This is often neglected in literature and leads to errors and confusion in fracture energy terminology and its applications [4]. As a conclusion, the separation law of interface can be described by measuring the specific fracture energy GF, which can be regarded as resistance against crack growth [4]. This can be used for all three modes of cracking [5].

3. EXPERIMENTS Specimens for fracture mechanical testing where made of single edge notched blocks of bonded pearwood (ligament area: 55x50 mm). This was done for six different types of industrial adhesives illustrated in table 1:

Table 1: Overview of adhesive candidates [4]

Adhesive Candidate

Chemical Basis

A Acrylate 1K

B MS-Polymer 1K

C Polyurethane 2K

D Acrylonitrile 1K

E Polyurethane 1K

F Polyurethane 1K

All candidates had different chemical and mechanical properties in order to represent a broad range of common industrial adhesive systems. Specimens were glued and stored for seven days. After that, they were notched by a saw of 3 mm thickness. Tests were performed at room temperature (20°C) and 43% humidity. Details can be found in [4]. 4. RESULTS

Investigations on interface fracture behaviour of adhesively bonded composites with innovative newly adopted test methods introduced by [4] lead to interesting insights of understanding the relationship of strength, stiffness and toughness of adhesive composites. Correlating these three engineering parameters, valuable statements in terms of finding the benchmark of choosing the best adhesive/adherend combination can be made. This can be viewed in figure 7, where six different industrial adhesive candidates for mode I loading named A, B, C, D, E and F are illustrated. There, spherical colourful demonstration helps to identify their fracture stability tendency accompanied by strength-stiffness relationship.



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Figure 7: Strength-stiffness distribution for mode I loading [4].

As a conclusion, it can be stated that not strength and stiffness (described by the Young’s modulus) is the ultimate engineering design parameter, but the resistance against crack propagation (described by specific fracture energy GF). Contrarily to the major opinion of common engineering practice, from the author’s point of view it is not very constructive just to rely on two classical mechanic parameters such as tensile strength and Young’s modulus alone. Not only they lack on dealing with non-linearity effects involved with cracking, they do not take into account fracture toughness, fracture sensitivity and fracture stability. Following this thought, table 2 illustrates the candidates shown in figure 7 above in terms of fracture stability tendency, whereas the size of sphere represents the fracture resistance and the colour the fracture stability tendency according to [15].

Table 2: Fracture stability of composites [4]

Adhesive Candidate

Fracture stability tendency

E, F Uncritical (green)

B Semi-critical (yellow)

A, D, C Critical (red)

Summarising that, it is suggested to prefer adhesive systems such as E and F in order to obtain high fracture resistance and thus achieve a low risk of growing cracks become propagating. It is very important for the whole component structure to prevent instable collapse

if the tensile strength has been exceeded and cracks were already initiated. Turning to figure 8a, strength distributions for mode I,II and III loading can be found in terms of coloured bar charts. There, RV stands for “composite strength” accounting for both normal and shear stress, respectively. Likewise, is also accomplished for specific fracture energy GF describing the crack propagation resistance for all three separation modes (also shown in figure 8b).

Figure 8: GF & Rv distribution for mode I/II/III [4].

5. DISCUSSION According to findings described above, it can be concluded that for adhesively bonded composites it is recommended to focus on bonding systems based upon 1-K Polyurethane. This is because fracture toughness and resistance are far higher than for Acrylate or Acrylonitrile based adhesives. The key finding of this study therefore is to focus on bonding systems leading to a maximum fracture resistance in terms of high GF values. Comparing both strength as well as fracture property results for mode I/II/III loading from

a

b



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figure 8, it is noticeable that both properties may often oppose each other. This means, that high tensile strength does not automatically lead to high fracture resistance and vice versa. This remarkable effect is throwing new light into preconceived opinions namely “the higher the bonding strength the better the adhesive”. As this is not true in terms of cracking behaviour, prevailing dogmas in engineering should be forced to be corrected by generating a “fracture mechanical conscience” in the design and material selection process. 6. SUMMARY Summarising the findings, following conclusions can be drawn by the author: 1) Conventional material parameters, such as

tensile strength, Young’s modulus, etc. are not sufficient for characterising material behaviour of industrial adhesives.

2) Pull-out tests [1,2] are not appropriate to describe cracking behaviour of industrial adhesives and bonded composites.

3) Fracture mechanics indeed may be applied, however, there are still too many rival streams and theories not able to explain distinct questions satisfactorily. Also, precaution is required in using the terminology and syntax of “fracture energy”, as in literature there is still much confusion about the correct meaning and definition [4].

4) If fracture mechanics is used, still improperly approaches are applied, for example linear elastic fracture mechanics (LEFM) for adhesives and bonded composites [6,7,8,9].

5) As a matter of fact, still a multitude of research and testing is done based on setups with questionable results, such as DCB [20] or ENF [21] methods critically reviewed in [22,23].

Finally, it is time to review standardised and still dominating fracture mechanical testing methods. To address inconsistencies, this study was the first applying an innovative stable test

setting introduced by [4] on adhesively bonded composites. As a final statement, more future emphasis should be laid on establishing a global fracture mechanical standard for industrial adhesives and bonded composites in order to obtain clear results so that the cracking behaviour could be addressed more realistically.

ADDITIONAL INFORMATION

This paper is based upon the doctoral thesis of the author Martin Brandtner-Hafner [4]. As this thesis is currently restricted due to patent applications, no further detailed information on the test setup can be given. For the interested reader, further information can be requested directly by the author by e-mail at [email protected] or via web at www.fractureanalytics.com.

REFERENCES

[1] ASTM D897-08(2016), Standard Test Method for Tensile Properties of Adhesive Bonds, ASTM International, West Conshohocken, PA, 2016. [2] DIN EN 14293:2006-10, Adhesives - Adhesives for bonding parquet to subfloor - Test methods and minimum requirements, Beuth, Berlin, 2016. [3] A. Hillerborg, M. Modéer, P.-E. Petersson, Analysis of crack formation and crack growth in concrete by means of fracture mechanics and finite elements, Cement and Concrete Research 6(6), 773-781, 1976. [4] M.H. Brandtner-Hafner, Interface fracture mechanics and healing of wood-adhesive-composites at mode I, II, III loading, Doctoral Thesis, Vienna University of Technology, 2016. [5] G.R. Irwin, Handbuch der Physik, Vol. VI, p. 49, Springer, Berlin, 1958.



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[6] S. Ammann, P. Niemz, Mixed-mode fracture toughness of bond lines of PRF and PUR adhesives in European beech wood, Holzforschung 69(4), 415-420, 2015. [7] P.S. Watson, S. Clauss, S. Ammann, P. Niemz, Fracture Properties of Adhesive Joints under Mechanical Stresses, Wood Research 58(1), 43-56, 2013. [8] B. Blackman, A. Kinloch, Fracture tests on structural adhesive joints, European Structural Integrity Society 28, 225-267, 2001. [9] S. Azari, M. Eskandarian, M. Papini, J.A. Schroeder, J.K. Spelt, Fracture load predictions and measurements for highly toughened epoxy adhesive joints, Engineering Fracture Mechanics 76(13), 2039-2055, 2009. [10] D. Gross, T. Seelig, Bruchmechanik: Mit einer Einführung in die Mikromechanik, 4. Auflage, Springer, Berlin, 2007. [11] L. Banks-Sills, Interface fracture mechanics: theory and experiment, International Journal of Fracture 191(1-2), 131-146, 2015. [12] ASTM D5573-99(2012), Standard Practice for Classifying Failure Modes in Fiber-Reinforced-Plastic (FRP) Joints, ASTM International, West Conshohocken, PA, 2012. [13] M. Afendi, T. Teramoto, H.B. Bakri, Strength prediction of epoxy adhesively bonded scarf joints of dissimilar adherends, International journal of adhesion and adhesives 31(6), 402-411, 2011. [14] T. Cordes, Mechanische Eigenschaften von Grenzschichten zwischen zementgebundenen Materialien, Doctoral Thesis, University of Innsbruck, 2013. [15] J. Nakayama, Direct measurement of fracture energies of brittle heterogeneous materials, Journal of the American Ceramic Society 48(11), 583-587, 1965. [16] A. Hillerborg, The theoretical basis of a method to determine the fracture energy GF of

concrete, Materials and structures 18(4), 291-296, 1985. [17] G.R. Irwin, J.A. Kies, Critical energy release rate analysis of fracture strength, Welding Journal Research Supplement, 33, 193–8, 1954 [18] S. Timoshenko, History of strength of materials, McGraw-Hill, New York, 1953. [19] M. Sakai, R.C. Bradt, Fracture toughness testing of brittle materials, International Materials Reviews, 38(2), 53-78, 1993. [20] ASTM D5528-13, Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites, ASTM International, West Conshohocken, PA, 2013. [21] ASTM D 7905-14, Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites, ASTM International, West Conshohocken, PA, 2014. [22] J.J.L. Morais, M.F.S.F. de Moura, F.A.M. Pereira, J. Xavier, N. Dourado, M.I.R. Dias, J.M.T. Azevedo, The double cantilever beam test applied to mode I fracture characterization of cortical bone tissue, Journal of the Mechanical Behavior of Biomedical Materials 3(6), 446-453, 2010. [23] D.O. Adams, Fracture mechanics testing of composites, CompositesWorld 1(2016), Article, Online, http://www.compositesworld.com/articles/fracture-mechanics-testing-of-composites

Interface Fracture Behaviour of Industrial Adhesives: A Novel Evaluation Approach for Adhesive...

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