Yeh-Hung Lai

The cracked lap shear specimen revisited—a closed form solution

This paper revisits the cracked lap shear specimen and reports a closed form solution to determine three fracture parameters, the energy release rate, the fracture mode mixity, and the fracture efficiency parameter. The solution is based on a beam-column approach pioneered by Goland and Reissner. Because of the geometrically nonlinear nature of this specimen, it is found that the fracture parameters are functions of five independent nondimensional parameters. The closed form solution reported in this paper provides a simple and useful tool to design the cracked lap shear specimen and to understand the behavior of this test geometry. The results in this paper suggest that to design a specimen with an energy release rate less dependent on the crack length, the adherend thickness should be small compared to the specimen length, and the thicker or stiffer adherend should be used as the strap. For a specimen of unequal adherend thickness, using the thicker adherend as the strap would significantly reduce the likelihood of yielding in the adherend. Compared with the finite element analyses found in the literature, the closed form solution shows good agreement in energy release rates, but less satisfactory agreement in fracture mode mixities. Finally, the closed form solution is used to give a reasonable explanation of anomalous debond behavior in a series of fatigue experiment using the cracked lap shear specimen.

The Constrained Blister—A Nearly Constant Strain Energy Release Rate Test for Adhesives

Abstract A modification of the blister test permits nearly constant strain energy release rate testing of adhesive bonds. By constraining the deformation of the blister, a promising device for automated evaluation of critical strain energy release rates can be obtained. The procedure is especially amenable to viscoelastic and environmentally-assisted debonding processes. Preliminary experimental evaluation of the time-dependent adhesive fracture toughness of a tape product is included.

A study of the fracture efficiency parameter of blister tests for films and coatings

Analytical solutions for circular membranes with different boundary conditions are obtained based on the assumptions of large displacements, small rotations, and small strains. The solutions take into account prestresses of arbitrary magnitude. The maximum stresses and strain energy release rates for the standard and island blister tests are obtained. The fracture efficiency parameter, defined as the ratio between the strain energy release rate and the square of the maximum stress, is then introduced to study the geometric effect on the relationship between the failure along the interface and in the coating. The fracture efficiency parameter is also used to find the optimal specimen dimensions for a given blister test geometry. It is found that the prestresses have a significant effect on the parameter. Although the island blister test is capable of producing very high strain energy release rates at relatively low pressures by decreasing the size of the island, the fracture efficiency parameter does not i...

A Test Method for Accelerated Humidity Conditioning and Estimation of Adhesive Bond Durability

Abstract The ability to determine the durability of adhesive bonds remains an elusive task, especially when the service environment involves exposure to diluents such as water. Moisture continues to be of major concern for many adhesive bond systems for a number of reasons including: 1) many adhesives are hydrophilic, picking up significant amounts of moisture over time; 2) most adhesives and some adherends allow moisture permeation, eventually reaching the adhesive/adherend interface; 3) the high surface energies of metallic and certain other substrates result in moisture migrating to the adherend surfaces and displacing the adhesive from the substrates, and possibly oxidizing the adherend, etc., and 4) absorbed moisture induces swelling stresses which can reduce the bond strength. Recognition of this susceptibility to moisture has led to extensive studies aimed at evaluating the effects of moisture, developing an understanding of the responsible mechanisms, and predicting the performance of adhesive bon...

Analysis of the Notched Coating Adhesion Test

Abstract An analysis of the notched coating adhesion (NCA) test is presented. This simple adhesion test method is appropriate for measuring the interfacial fracture toughness of some classes of coatings and open-faced adhesive bonds. The NCA specimen consists of a single substrate coated with a thin layer of adhesive. The coating is notched to sever the coating and induce sharp interfacial debonds, and the specimen is then loaded in tension. The substrate strain at which coating debonding occurs is recorded and used to determine the critical strain energy release rate. Yielding of the substrate is permitted, and does not significantly affect the calculation of the strain energy release rate. Analytical and finite element analysis are used to quantify the available strain energy release rate for both steady state and laterally-constrained cases. The available strain energy release rate is shown to be quite insensitive to the initial debond length. The specimen geometry results in a mode mix which causes th...

An Elementary Plate Theory Prediction for Strain Energy Release Rate of the Constrained Blister Test

Abstract A technique based on elementary plate theory is proposed for estimating the strain energy release rate of the constrained blister specimen for the case of a relatively stiff blister adherend. The results of finite element analysis for an aluminum specimen confirm the applicability of the elementary plate theory approach for the constrained blister test. The paper also proposes an experimental scheme which could be automated to measure the necessary parameters to determine the strain energy release rate of the constrained blister specimen.

Using the fracture efficiency to compare adhesion tests

This paper reports the use of a novel concept, the fracture efficiency, to evaluate adhesion tests by deciding whether a particular test would be more or less likely to cause gross yielding or rupturing in the adherend/coating before the fracture condition is satisfied. Because of the technical importance and the difficulty in obtaining meaningful debond energies for coatings, much of the paper is devoted to the coating adhesion measurement problem. The fracture efficiency of the general coating delamination configuration is investigated and its theoretical limit is determined. The study suggests that it is unlikely to devise a new test with a significantly higher fracture efficiency than the existing tests. New experimental or analytical techniques considering the inelastic energy dissipation are needed for coating adhesion measurement. In addition to the fracture efficiency, the fracture mode mixity of the general coating delamination problem is also investigated. The conditions which may result in the contact of crack surfaces near the crack tip region are identified. The preferred configurations which would tend to induce interfacial delamination are also discussed. Finally, from the viewpoint of fracture efficiency, guidelines are given to aid in the selection of appropriate test geometries for coating adhesion measurement.

Numerical Analysis of the Constrained Blister Test

Abstract The constrained blister test is investigated through finite element analysis to determine the applicabilities and the limitations of the new technique. Numerical results confirm that the strain energy release rate asymptotically approaches a constant value. These results also show that the test technique and the approximate solution for strain energy release rate are applicable for some practical cases.

A comparison of energy release rates in different membrane blister and peel tests

Abstract The relationship between energy release rates and coating stresses in various coating adhesion fracture tests is investigated. In spite of the apparent difference in test geometries and loading conditions, an equivalent peel test can be found for each of the membrane peeling tests examined in this paper. The results suggest that these tests and other membrane peeling tests are special cases of the peel test if examined near the debond front. In addition to clarifying the relationship between peel tests and other membrane peeling tests, the limitations of all possible membrane peeling test geometries with coatings having a tensile prestress are investigated through the study of the fracture efficiency parameter for the most general coating peeling problem. The results suggest that developing high fracture efficiency tests for coating adhesion measurement seems unlikely.

Peel Stress Distributions between Adherends with Varying Curvature Mismatch

Abstract Previously-derived, closed-form solutions for the residual peel stresses developed between adherends with a slight but constant curvature mismatch are extended to the cases of two or more discrete curvatures and continuously varying curvatures. This beam on elastic foundation solution is now used to predict, for example, the peel stresses which develop when a molding strip is bonded to conform to a substrate with varying curvatures. Diagrams of typical stress distributions are given along with plots which allow the designer to estimate or minimize the resulting peel stresses for specific applications.