Bonded Joint Specimens Research Articles

Shear induced toughening in bonded joints: experiments and analysis

Bonded joint specimens were fabricated from composite adherends and either an epoxy or a urethane adhesive. In mixed-mode fracture experiments, the epoxy bonded specimens generally failed by subinterfacial fracture in the composite, while specimens bonded with urethane failed very close to the adhesive/substrate interface. For the epoxy bonded specimens, fracture toughness did not change significantly with mode-mix, but for urethane bonded joints, fracture toughness increased with increasing shear load. Finite element analysis, which modeled specimens bonded with the two adhesives, showed similar trends. The different toughening behaviors for the two bonded joints can be attributed to dissipation of energy through inelastic deformation, which was insignificant in the epoxy-bonded joints but substantial when the urethane was used as the bonding agent.

Plastic yielding of a film adhesive under multiaxial stresses

This article presents a study of the plastic yielding behaviour of a rubber-toughened film adhesive (FM73) subjected to triaxial stresses. Experiments were carried out using a bonded joint specimen similar to the Iosipescu specimens originally designed for use with fibre composite materials. The experimental results reveal that the plastic yielding of the FM73 adhesive exhibits a complex dependence on the hydrostatic stress, especially in the compression regime. It is found that the conventional yield criteria widely employed to model adhesives, such as the modified Tresca criterion, the modified von Mises criterion, and the linear Drucker–Prager criterion, are unable to characterise the yield locus. To overcome this difficulty, the modified Drucker–Prager/cap plasticity model that is commonly associated with geological materials is adopted in the present study, which provides a good correlation with the experimental data.

Mixed mode failure criteria for a glass/epoxy composite and an adhesively bonded composite/composite joint

Mixed mode initiation failure criteria for the delamination of a glass/epoxy composite and its composite/composite bonded joint are compared. Tests were conducted under mode I (DCB), mode II (ENF) and mixed mode I/II (MMB) loading. Mode partitioning was performed using beam theory. The initiation failure point was defined both by the non-linearity point on the load/displacement curve and by the first acoustic signal. The main results concern test stability and fracture energies: crack propagation was unstable in adhesively bonded joint specimens under mode I loading but stable under mode II loads. This is in contrast to the expected behaviour and to that seen in composite interlaminar fracture. Fracture energies are much higher for the joints than for the composite specimens. The fracture envelopes obtained can now be used in the prediction of the failure loads for structural joints.

Fatigue failure criterion of adhesively bonded CFRP/metal joints under multiaxial stress conditions

Abstract In order to investigate the fatigue failure criterion of adhesively bonded CFRP/metal joints under multiaxial stress conditions, a series of fatigue tests was carried out on three kinds of adhesively bonded joint specimens; butt joint, scarf joint and thick-adherend lap-shear joint, where these joints have considerably uniform stress distributions in the adhesive layer. In addition, stress distributions of these joints were analysed by the finite element method. The results show that the fatigue limits of respective joints are governed by the maximum principal stress except when negative hydrostatic pressure acts on the adhesive.

Stress Analysis of the Cracked-Lap-Shear Specimen: An ASTM Round-Robin

Abstract An ASTM round-robin was conducted to evaluate the state of the art in stress analysis of adhesively bonded joint specimens. Specifically, the participants were asked to calculate the strain-energy-release rate for two different geometry cracked-lap-shear (CLS) specimens at four different debond lengths. The various analytical techniques consisted of two- and three-dimensional finite element analysis, beam theory, plate theory, and a combination of beam theory and finite element analysis. The results were examined in terms of the total strain-energy-release rate and the Mode I to Mode II ratio as a function of debond length for each specimen geometry. These results basically clustered into two groups: geometric linear or geometric nonlinear analysis. The geometric nonlinear analysis is required to properly analyze the CLS specimens. The three-dimensional finite element analysis gave indications of edge closure plus some Mode III loading. Each participant described their analytical technique and results. Nine laboratories participated.

Effect of moisture on the static and viscoelastic shear properties of epoxy adhesives

The effects of moisture on the structural properties of two commercially available adhesives, FM73M and FM300M, are investigated. The experimental study consists of static and viscoelastic shear measurements made from bonded joint specimens soon after they were cured and after lengthy exposure in 63% and 95% relative humidity environments. Static shear modulus and shear creep compliance data for each adhesive at each moisture level throughout a wide range of temperatures are illustrated. Also shown are the effects of temperature and moisture on the ultimate shear strength behaviour, and the temperature and moisture viscoelastic shift functions. It is concluded that the effect of moisture as an external plasticizer on the shear properties of these adhesives is equivalent to raising the environmental temperature.

Developing Failure Criteria for Adhesive Joints Under Complex Loading

This paper describes an investigation of the failure behavior of bonded joints under a wide range of in-plane loading. Combinations of tension, shear, and bending loads were applied to bonded joint specimens using a unique computer-controlled loading system. Failure criteria were developed for initiation of crack growth or the onset of nonlinear behavior based on a computation of energy dissipated by the failure process. Failure surfaces were constructed from these data for the range of loadings studied. A strain energy release rate formulation was developed for bonded materials which fail under shear, tension, and bending loading by interfacial crack growth. This formulation was used to analyze specimens which failed by debonding along the interface. Results of these studies were also compared with failure criteria obtained using a larger scale specimen on the basis of strain energy release rate.