ANALYSIS OF FAILURE MODELS IN ADHESIVE BONDED LAP JOINTS
Guided by:K.S. SajikumarAsst. ProfessorMechanical EngineeringCET
Presented By:Roy Roshan ChandyM2 Machine Design, Roll No 1104CET
CONTENTS• Introduction• Adhesive bonding Vs Conventional mechanical bonding• Adhesive bonded joints• Configurations for adhesive bonds• Causes of failure of adhesive bonds• Failure mechanism of adhesive bonded joints• Formulation of interfacial crack with void• SERR calculation• Sample problem• Single notch specimen results• Double notch specimen results• Conclusions• References
INTRODUCTION• Composite materials are being used and studied
extensively today.• Applications:– military aircrafts, commercial airliners, automobiles,
robotics and repair of existing structures.• Composite parts are joined together using
adhesives or mechanical fasteners.• The transfer of load from the structure to the
substructures is through the joints• Strength and stress distribution of the joints need
to be studied.
ADHESIVE BONDING Vs CONVENTIONAL MECHANICAL BONDING
• Lower temperature manufacture of joints• Joints without blemish, distortion or protrusions • Net weight of the joint is minimised • Stresses are more uniformly distributed • Resulting structure is normally stiffer than for discretely
welded/fastened joints • Increased static strength, fatigue life , damping of
vibrations.• Complex geometries relatively easy to make • Reduced capital and labour costs• Process de-skilled or completely automated
ADHESIVE BONDING
• Adhesives can bond– most materials in common engineering use– especially useful where the substrates are different
materials.– Adhesively bonded lap joints show smooth load
transfer and have fewer points of stress concentration– Such joints also show weight and cost reduction
(aircraft industry)– It provides good acoustic, thermal, electric insulation
and liquid and gas tight sealing
ADHESIVE BONDED JOINTS
• Essential for highly-stressed applications• Bonded joints:– Are best loaded in compression– Give acceptable performance in shear
– Tension should be avoided• Especially peel: at least one component is
flexible• And cleavage: rigid components are involved.
ADHESIVE BONDED JOINTS
CONFIGURATIONS FOR ADHESIVE BONDS
Fig : (a) single lap (b) double lap (c) scarf (d) strap
APPLICABLE CONFIGURATIONS FOR ADHESIVE BONDED JOINTS
Compression good Shear OK
substrateadhesiveKEY:
AVOIDABLE CONFIGURATIONS FOR ADHESIVE BONDED JOINTS
• Peel (1 flexible) Cleavage (2 rigid)
x x
ADHESIVE BONDED JOINTS
X
ADHESIVE BONDED JOINTS
X
CAUSES OF FAILURE OF ADHESIVE BONDS
• Inadequate joint preparation• Failure of the substrate due to local stress
concentration• Bond failure due to moisture ingress or exposure to
ultraviolet light and high temperatures.• Progressive failure due to eccentric loading.• Failure of the adhesive due to inadequate mixing
or incorrect or prolonged storage. • Better to use a mechanical fixing as well as the
adhesive for critical applications.
FAILURE MECHANISM OF ADHESIVE BONDED JOINTS
• Interfacial Fractures: separation of adhesive from substrate at the interface between the two.– shear or normal stress or combination of both
exceeds the bond strength between adhesive and substrate.
• Cohesive Fractures: failure of adhesive at a point within the adhesive
Strain Energy Release Rate(SERR)
• A crack initiates when the value of SERR reaches Critical SERR value
• Critical Strain Energy Release Rate depends only on the material property of specimen considered.
• For adhesive bonded joints, Critical SERR depends on materials of substrate and adhesive.
• Interfacial failure is mixed mode failure and may include one or more of the failure modes.
FAILURE MODES
THEORIES CONSIDERED TO FIND SERR
• Virtual Crack ClosureTechnique (VCCT) and FEM were used to determine SERR
• Classical plate theory and VCCT were used to find SERR
• Classical Beam theory to calculate SERR.• Contour Integrals to find SERR• All the above theories considered the
adhesive bonded joints without flaws and voids
FORMULATION OF INTERFACIAL CRACK WITH VOID
• First Order Shear Deformation Theory (FSDT) is used• FSDT analyses stress and displacement • Void represents a region without presence of adhesive
between the two substrates.• Void could form when
– either adhesive at the location of void has evaporated or– Manufacturing defect that is not detected.
• Assumption: both adhesive and substrate are– Linear elastic– Isotropic– Homogenous And– Specimen undergoes plane strain deformation
• hU, hL thickness of upper and lower substrates in mm • ƞ adhesive thickness in mm • P applied tensile load per unit width in N/m on the right end
of specimen• Left end is fixed• Adhesive is mostly deformed in shear. • Assume that
• We assume that during use an interfacial crack of length Lc has developed right after the first notch
• there is a void where no adhesive exists between the two substrates
• A simplifying assumption is that prior to the application of the load P, the two substrates and the adhesive are stress free.
• hU, hL, ƞ are small as compared to their lengths and widths
ASTM D3165 SPECIMEN
ASTM D3165 geometry including an interfacial crack and a void
Discretizaion of the specimen into subregions.
Equations for a substrate
Equations for adhesive
FREE BODY DIAGRAM
VCCT
ASTM D3165 specimen with an initial interfacial crack of length a, virtual crack extension of length b.
SERR CALCULATION
W also equals the energy released during the virtual extension of the crack through distance b.
• Thus for unit width of the specimen in the y-direction, the SERR is given
• The mode-mixity parameter, β, is defined
• For pure mode I failure, β= 0• For pure mode II failure β=π/2
SERR CALCULATION
SAMPLE PROBLEM
• Substrates are made of 2024-T3 aluminum (Eal=73 GPa, ϑAl=0.33) bonded together with a 0.1 mm thick epoxy adhesive (Eadh=1.64 GPa, ϑadh=0.35).
• Lo=50.8 mm • The notch-size Ln=1.6 mm • The lengths L1 and L8 of substrates outside the overlap equal to
25.4mm• The length, Lv, of the void is varied• hU=hL=1.6 mm.• Two configurations considered:
– the double-notch and – the single-notch( no right notch)
SINGLE NOTCH SPECIMEN RESULTS
Comparison of the adhesive shear stress distributions
inset labeled overlap configuration lists values of Li/Lo, Lv/Lo, (Lo-Li-Lv)/Lo.
Comparison of the adhesive normal stress distributions
inset labeled overlap configuration lists values of Li/Lo, Lv/Lo, (Lo-Li-Lv)/Lo.
Comparison of the resultant axial force
Comparison of the resultant shear force
Comparison of the resultant bending moment
Comparison of FSDT and FE-VCCT
Effect of substrate thickness on the total SERR
Comparison of the mode-mixity parameter
COMPARISON OF RESULTS
DOUBLE NOTCH SPECIMEN RESULTS
Comparison of the adhesive shear stress distributions
the inset labeled overlap configuration lists valuesof Li/Lo, Lv/Lo, (Lo-Li-Lv)/Lo.
Comparison of the adhesive normal stress distributions
the inset labeled overlap configuration lists values of Li/Lo, Lv/Lo, (Lo-Li-Lv)/Lo
Comparison of FSDT and FE-VCCT
Effect of substrate thickness on the total SERR
Comparison of the mode-mixity parameter
CONCLUSIONS
• SERR and the mode-mixity parameter β found from results of the FSDT differ by less than 4% from the corresponding values computed from the solution of the problem by the finite element method
• The presence of a void and where it is located have minimal effects on values of the SERR and β.
REFERENCES• Alireza Chadegani,Romesh C.Batra , Analysis of adhesive-bonded single-lap
joint with an interfacial crack and a void, International Journal of Adhesion & Adhesives,2011;31:455-465
• Da Silva LFM, das Neves PJC, Adams RD, Spelt JK. Analytical models of adhesively bonded joints—part I:literature survey. International Journal of Adhesion & Adhesives, 2009;29:319 –30
• Da Silva LFM, das Neves PJC, Adams RD, Spelt JK. Analytical models of adhesively bonded joints—part II: Comparative study. International Journal of Adhesion & Adhesives, 2009;29:331 –41
• Madhusudhana, K. S. and Narasimhan, R. Experimental and numerical investigations of mixed mode crack growth resistance of a ductile adhesive joint. Eng. Fract.Mech., 2002, 69(7), 865–883.
• M D Banea and L F M da Silva, Adhesively bonded joints in composite materials: An overview, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials Design and Applications 2009 223: 1
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