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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