Analysis of crack propagation in an adhesive joint / M.D. Mohan Gift, J. Selvakumar, S. John Alexis (original) (raw)

FRACTURE STUDIES OF AN ADHESIVE JOINT INVOLVING COMPOSITION ALTERATION

Journal of Chemical and Pharmaceutical Sciences, 2015

Adhesive joints are widely used in industries because they have several advantages when compared to welded and riveted joints. One of the important factors is that they distribute the load and stresses uniformly over the entire bonded area providing good vibration resistance. Adhesive joints can readily bond dissimilar materials. The prediction of crack propagation validating the adhesive joint durability and toughness is a significant point that is addressed through various experimental methodologies based on the type of loading conditions. The analysis is hindered by the unpredictable adherend and adhesive behavior due to the loading conditions, the nature of crack propagation, and the geometry. The impact of hardener resin ratio alteration is a parameter that needs to be explored invalidating the joint toughness. The Double Cantilever Beam tests which are used for analyzing the fracture toughness for mode-1 loading in adhesive joints focus on adhesive thickness variation extensively. The alteration of composition and its role in influencing the crack propagation is explored from a limited perspective. An attempt is made in this work to analyze the adhesive composition variation and its impact on the joint toughness with the help of a DCB test involving three specimens incorporating variations in the hardener resin composition. The analytical and experimental results provided significant insights into the adhesive joint toughness validation.

Evaluation of structural adhesive joints fracture toughness without crack measurement

Materia-rio De Janeiro, 2021

The adhesive fracture resistance of structural adhesive joints in mode II, IIC, is accessed through the application of two models that prescind of the measurement of the crack length during the subjection of ENF specimens to three-point bending monotonic tests. The load vs transversal displacement results are used in two different approaches: a damage model and a cohesive zone model. The first one was used in test where the crack propagation was unstable and the second one was used for stable crack propagations. These initial results show the viability of the models and show that both: initial crack length and transversal load rate; have great influence in IIC evaluation. Keywords: structural adhesive joints, ENF specimens, fracture mechanics.

Fracture toughness of adhesively bonded joints

Engineering Fracture Mechanics, 1985

The fracture toughness of an epoxy-based film adhesive has been investigated using Mode I and combined Mode 1 + Mode II loadings. The opening Mode, Mode I, was realized by employing the Tapered Double Cantilever Beam (TDCB) specimen, while the mixed opening and shear modes, Mode I + Mode II, resulted from utilizing the Cracked Lap Shear (CLS) specimen. Fracture toughness was studied under conditions of constant elongation rate as well as sustained load. Furthermore, the environmental effects of both temperature (up to 70°C) and water were investigated in the sustained load case. Experimental results showed that the resistance to crack growth decreased in both the uniaxial and biaxial loading cases when sustained loads were applied, compared to the case of constant elongation rate. Higher temperatures (up to 70°C) in air did not cause any significant decrease in fracture toughness. However, the combination of water and temperature resulted in a significant decrease in the resistance to crack growth. Finally, mixed mode loading showed the most pronounced effect on the critical energy release rate parameter, GI,, as compared to a uniaxial, opening mode loading. The combined effect of biaxial load, temperature (up to 70°C) and water resulted in a drastic decrease in fracture toughness of the studied film adhesive in comparison to uniaxial loading ambient conditions. This result is of practical importance when fracture characteristics are entered in the design considerations for an adhesively bonded structure.

Determination of Decisive Parameters in the Crack Propagation Analysis of an Adhesive Joint

Indian Journal of Science and Technology, 2015

Validation of an adhesive joint strength is by and large done through crack propagation analysis influenced by the loading modalities leading to different testing methodologies. The DCB test method is extensively used under more-1 loading to study the fracture and delamination toughness of adhesive joints. The significance and suitability of using the test when the hardener-resin proportion of the adhesive is varied needs to be scrutinized. Three mild steel DCB specimens were analysed incorporating the proportion-variation. The analysis revealed the need of using the Cohesive Zone Model (CZM) to study the crack propagation in all the specimens as a pattern of inconsistent proportionality emerged between the resin proportion and the crack propagation. The graphs converged to a particular degree between the experimental and the analytical realms which further instigated the need of modeling of the entire specimen inclusive of the adhesive layer through Finite Element Analysis. The obtained results provided insights on stress distribution inside the adhesive layer when crack propagation takes place in the specimens. The proportion variation done in a systematic manner is seen as a key factor in improvisation techniques for analysis when efforts are undertaken to introduce modalities in crack propagation direction control.

Tensile fracture characterization of adhesive joints by standard and optical techniques

Engineering Fracture Mechanics, 2015

The use of adhesive joints has increased in recent decades due to its competitive features compared with traditional methods. This work aims to estimate the tensile critical strain energy release rate (G IC ) of adhesive joints by the Double-Cantilever Beam (DCB) test. The J-integral is used since it enables obtaining the tensile Cohesive Zone Model (CZM) law. An optical measuring method was developed for assessing the crack tip opening (d n ) and adherends rotation (h o ). The proposed CZM laws were best approximated by a triangular shape for the brittle adhesive and a trapezoidal shape for the two ductile adhesives.

Fracture mechanics characterisation of medium-size adhesive joint specimens

Medium-size specimens (∼ 2 m in length), consisting of two glass-fibre beams bonded together by an adhesive layer were tested in four point bending to determine their load carrying capacity. Specimens having different thickness were tested. Except for one specimen, the cracking occurred as cracking along the adhesive layer; initially cracking occurred along the adhesive/laminate interface, but after some crack extension the cracking took place inside the laminate (for one specimen the later part of the cracking occurred unstably along the adhesive/laminate interface). Crack bridging by fibres was observed. The measured applied moment at steady-state crack growth was compared with predictions based on independent mixed mode fracture resistance measurements made on small laboratory specimens. The predicted and measured strength values of the medium-size specimens were found to be in good agreement with each other. Thus, the scaling from small specimens to medium-size specimens was successfully achieved.

Multi-material adhesive joints with thick bond-lines: Crack onset and crack deflection

Composite Structures, 2021

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines (≈10 mm) under global mode I loading. The role of adherend-adhesive modulus-mismatch and pre-crack length are scrutinized. The parameters controlling the crack path directional stability are also discussed. Single-material (i.e. steel-steel and GFRP-GFRP) and bi-material (i.e. steel-GFRP) double-cantilever beam joints bonded with a structural epoxy adhesive are tested. The joints are modelled analytically, considering a beam on elastic-plastic foundation, to include characteristic length scales of the problem (e.g. adhesive thickness, plastic zone) and numerically using Finite Element Model. An empirical relation, in terms of geometrical and material properties of the joints, that defines the transition between non-cohesive and cohesive fracture onset is found. Above a specific pre-crack length the stress singularity at pre-crack tip rules over the stress singularity near bi-material corners, resulting in mid-adhesive thickness cohesive fracture onset. However, the cracking direction rapidly deflects out from the adhesive layer centre-line. Positive T-stress along the crack tip is found to be one of the factors for the unstable crack path.

Fatigue crack growth analysis of different adhesive systems: Effects of mode mixity and load level

Fatigue & Fracture of Engineering Materials & Structures, 2019

The aim of this paper is to investigate the fatigue fracture behavior of three different adhesive systems (epoxy-based, acrylic, and a rubber-like adhesive). To achieve this, double cantilever beam specimens were manufactured with different adhesives and tested under several mode mixities and different load levels. Fatigue crack growth rate was evaluated through a Paris law equation. For postprocessing, the compliance-based beam method was used. Results showed that the variation of the threshold energy with load level is more pronounced for the epoxy-based adhesive. The crack propagation life is higher for the acrylic adhesive. Although, for pure mode I conditions, the normalized threshold of the rubber-like adhesive is lower, for pure mode II, it was higher than the epoxy-based adhesive. Due to the normalization by the static fracture energy, the slope of the Paris law was approximately constant for all the adhesive systems.