Fracture mechanics approach to predict delamination lifetime in Mode II under constant loads

The service lifetime of a laminated structure is one of the major concerns in the design of multilayered material systems. It is limited by the time required for acceptable delamination to propagate, under certain loading conditions, to attain a size perceived to be critical to the stiffness and/or the strength of the structure. This service lifetime could be predicted if the constitutive equation for the delamination rate is known. This paper describes an approach to determine the constitutive equation for delamination under Mode-II creep loading conditions. The approach is based on the principles of linear elastic fracture mechanics and uses an elevated temperature to accelerate the interlaminar fracture at constant loads. The experiments used double-cantilever beam test specimens fabricated as a model system of poly(methyl methacrylate) (PMMA) beams and epoxy adhesive. The effect of temperature on the experimental measurements has been considered. A form of Paris power law is suggested to forecast the service lifetime in terms of temperature, service load and the initial delamination size.     

An End-Loaded Shear Joint (ELSJ) Specimen to Measure the Critical Energy Release Rate in Mode II of Tough,Structural Adhesive Joints

This paper introduces a newly developed specimen type, which is used to measure the critical energy release rate of tough, structural adhesives loaded in shear. This End-Loaded Shear Joint (ELSJ) specimen is loaded until a shear crack propagates through the adhesive layer. When the crack propagation is stopped, by unloading the specimen, the critical energy release rate in mode II, G _IIc, can be obtained by correlating the energy dissipated during the test and the measured crack area on the fracture surface of the specimen. The paper presents the dimensions of the ELSJ specimen, the corresponding test setup and the evaluation method used to obtain G _IIc. An overview of the advantages and the limitations of the new specimen type shows the need for its development and improvement when compared to some state of the art experiments. The first results of ELSJ tests are shown and discussed, using the crash-optimized structural adhesive — Henkel Terokal 5077. The experimental results presented, focus on thin adhesive layers and quasi-static test velocities.     

Fatigue and failure behaviors of silver-filled electronically-conductive adhesive joints subjected to elevated humidity

Conductive adhesives are used in electronics packaging applications for hybrid, die-attach and display assemblies. There are a number of issues of concern in the design of joints bonded using electronically-conductive adhesives (ECAs). An important issue is the cyclic fatigue behavior of conductive adhesive joints under elevated humidity environments, in which failures may occur due to cyclic mechanical and/or thermal stresses. This paper addresses the effect of elevated humidity levels on the fatigue and failure behaviors of ECAs. For this purpose, joints were prepared using stainless-steel adherend specimens and a commercial ECA, and tested under monotonic and cyclic fatigue conditions, at two humidity levels, namely 20% and 90% relative humidity at 28°C. Furthermore, joint failure mechanisms were analyzed using optical techniques, and joint conductivity measurements. Load versus number of cycles (P–N) curves were generated using these specimens at three different load ratios (R), namely 0.1, 0.5 and 0.9, at a cyclic frequency of 150 Hz. The P–N curves were parallel and the failure modes were found to be predominantly interfacial, accompanied by a significant decrease in joint conductivity.     

A molecular mechanics approach to the adhesion of urea-formaldehyde resins to cellulose. Part 1. Crystalline Cellulose I

The energies of interaction of urea, methylol ureas, and urea-formaldehyde (UF) condensates, methylolated and non-methylolated, linear and branched, up to trimers, with the surfaces of an elementary model of the crystal of Cellulose I were obtained by molecular mechanics techniques. The results indicated, firstly, that methylolation enhances adhesion, especially at low molecular weights, while branching tends to decrease it; secondly, that adhesion of UF resins to the cellulose surface can be enhanced by shifting the resin preparation conditions to increase the proportion of species having higher specific adhesion. The theoretical results obtained are in agreement with published experimental evidence. While urea resins show stronger average affinity for cellulose than the average affinity of water, this trend is less marked than in phenol-formaldehyde (PF) resins. The results obtained also appear to infer that the lack of water resistance of UF resins is mainly due to the instability in water of the internal, covalent, aminoplastic bond rather than to UF adhesion to cellulosic substrates. Resin-substrate H-bonding was shown to be of lesser importance in UF than in PF resins.