J.K. Spelt

Mixed-mode fracture characterization of adhesive joints

A novel load jig is presented which allows mixed-mode fracture testing of adhesive joints and composite laminates over the entire range from mode I to mode II, by using a single equal adherend double-cantilever-beam specimen. Experiments performed with the load jig showed that GIIC was approximately three times higher than GIC for the tested adhesive system consisting of FPL-etched 7075-T6 aluminium adherends bonded with Cybond 4523GB (American Cyanamid) epoxy adhesive. Experimental data showed that GC was independent of crack length and that there was no dependence of GIC on adherend thickness. Comparison of GIIC values obtained by using the load jig to test conventional end notch flexure (ENF) specimens indicated that there are relatively small friction effects between crack faces in mode II testing of ENF specimens. The experimental data were also used to evaluate three different analytical techniques for the mode partitioning of unequal adherend specimens.

Fracture load predictions for adhesive joints

Abstract An engineering approach to fracture load predictions for adhesive joints is presented. The approach is based on the premise that the in-situ strength of the bondline can be characterized by the fracture envelope (critical energy release rate as a function of the mode of loading), for a specific adhesive system. By using the J integral for large deformations together with large-deformation beam theory, a simple closed-form expression is obtained for the energy release rate per unit area extension when a crack propagates in the bondline of a generalized adhesive joint (adhesive sandwich). This technique, together with a published method for mode partitioning, enables fracture load prediction by comparing the calculated fracture parameters with the critical ones from the fracture envelope. The approach is shown to predict fracture loads accurately for a variety of joints including the cracked lap shear (CLS), the single lap shear (SLS) and the double strap (DS) joint.

Measurement of adhesive joint fracture properties as a function of environmental degradation

Abstract The degradation of the fracture strength of two epoxy adhesives was measured using a new approach to accelerated aging. ‘Open-faced’ specimens were prepared by applying adhesive to aluminum plates and, after curing, exposing them to a range of temperatures and humidities. At various times, the adhesive layer (in either the ‘wet’ or ‘dry’ states) was bonded to a second aluminum adherend to form a double-cantilever-beam fracture specimen. The critical strain energy release rate was then measured at several mode ratios by ensuring that the crack followed a path in the ‘primary’ adhesive. This approach yielded fracture data which unambiguously corresponded to a particular, uniform state of degradation. Moreover, the rate of water absorption was greatly accelerated. The ‘wet’ state fracture data illustrated the combined effects of water plasticization (reversible upon drying) and degradation, while the ‘dry’ state data showed only the irreversible effects of degradation.

The effect of geometry on the fracture of adhesive joints

Abstract In this paper, a recently suggested method for fracture load prediction of adhesive joints is demonstrated to compare well with experimental data for aluminium joints bonded with a rubber-toughened structural epoxy (Permabond ESP 310). The method is also used to investigate the effect of varying geometric parameters such as adherend lengths and thicknesses on the strength of adhesive joints such as the single lap shear, cracked lap shear and double strap joints.

Observations of fatigue crack initiation and propagation in an epoxy adhesive

Abstract Fatigue crack initiation and propagation were investigated in structural adhesive joints consisting of 7075T6 aluminium adherends bonded with a mineral filled structural epoxy (Cybond 4523GB, American Cyanamid). Three types of joints were tested to achieve mode I (double-cantilever beam specimen, DCB), mixed mode I–II (cracked lap shear specimen, CLS), and mode II (end notch flexure specimen, ENF). All tests were conducted under ambient conditions with load ratio of 0.1 at a frequency of 30 Hz. Fatigue loading significantly reduced the strain energy release rate (G) required to initiate a crack compared with static and quasi-static loading. For the load ranges tested, fatigue precracks doubled the time to cause a resumption of crack growth under mode I loading. Negligible differences in crack initiation times (time to generate a crack from a fillet or resume extension of an existing crack) were observed for mixed-mode I–II and mode II specimens with cracks starting from fast mode I precracks, intact fillets and fatigue precracks. For the adhesive system tested, the relative influence of the mode ratio depended on whether the rate of crack propagation was plotted versus Gmax or %Gc (percentage of the quasi-static critical energy release rate at the particular mode ratio). When expressed as a function of %Gc, debonding rates were greatest under mixed-mode conditions at a given %Gc, and were indistinguishable under mode I and mode II loading. However, when expressed as a function of Gmax, the propagation rates at a given Gmax were the same under mixed-mode and mode I loading, and smaller under mode II loading. This means that the allowable loads for joints in fatigue will depend on the mode ratio; for mixed-mode joints it will be a smaller fraction of the quasi-static allowable load than for mode I or mode II joints. Threshold energy release rates (Gmax) under mode I and mixed mode I–II loading were essentially the same, and were obtained equally from extrapolated crack propagation rates or crack initiation times. For this adhesive system, it is recommended that adhesive joint design be based on threshold values for zero crack growth, because crack propagation rates show too much scatter to be relied upon for the prediction of in-service subcritical crack growth, particularly under mode I and mode II loading.

Experimental investigation of vibratory finishing of aluminum

Abstract The normal contact forces in a vibratory finishing machine were measured and compared with the resulting changes in surface roughness and hardness of two aluminum alloys, AA1100-O and AA6061-T6. The principal variables were the media size, degree of lubrication and the duration of the vibratory finishing. The changes in hardness and roughness were found to depend mostly on the lubrication condition, the media roughness, and the size of the media, since these influenced the interaction between the media and the workpiece, and hence the extent of plastic surface deformation per impact. The impact force parameters such as the average force, maximum force, and impulse, however, did not vary appreciably amongst the three media for dry and water-wet conditions. Thus, the differences observed in hardness and roughness were due to smaller scale differences in the impact contact conditions. On average, a sensing disk with a diameter approximately equal to that of the media was in contact with media for approximately 30% of the total finishing time. This was consistent with videotaped observations showing that the media was loosely-packed as it flowed past the workpiece, with relatively large gaps in the packing near the workpiece surface.

Failure load prediction of structural adhesive joints: Part 1: Analytical method

Abstract This paper describes an analytical method for calculating the strain energy release rate of cracked adhesive joints. The calculations proceed from a knowledge of the reactions in the adherends at the end of the joint overlap. For joints with equal adherends, a simple method exists for determining the Mode I and Mode II components of the energy release rate. The equations make it relatively easy to apply fracture mechanics failure criteria to arbitrarily loaded adhesive joints. In a subsequent paper, it is shown that by treating uncracked joints as having a crack, with the crack tip coinciding with the location of the spew fillet, the load required to propagate a crack in a cracked joint serves as a reliable conservative estimate of the load required to propagate a crack in an uncracked joint. The present method is suitable, therefore, for failure load predictions of structural adhesive joints in design applications.

Analysis of the Peel Test: Prediction of Adherend Plastic Dissipation and Extraction of Fracture Energy in Metal-to-Metal Adhesive Joints

The peel test has been widely used for the mechanical measurement of the adhesion phenomenon. However the proportion of the energy input dissipated plastically within the adherend is a major concern in analyzing peel test data. This paper presents an analytical approach to predict the adherend plastic dissipation in the peel test for metal-to-metal adhesive joints, thereby allowing the fracture energy to be extracted from the test data using an energy balance approach. Expressions are developed for the deflection of an elastic-plastic beam on an elastic foundation, which is then combined with known solutions for the deformation of an elastic-plastic strip under large displacement. The model takes into account both the adhesive and adherend compliance effects on the plastic dissipation. Numerical predictions of the model are presented to gain insight into the effects of adherend properties and peel angle on plastic dissipation in the peel test. It is demonstrated that experimental results with various adherend properties and peel angles are consistent with the predictions of the model. An important conclusion is that for typical structural adhesives, the effects of plastic dissipation may be kept small by using a relatively low yield strength alloy with a thickness much smaller than the critical thickness at which the plastic dissipation effect is a maximum. The extraction of the fracture energy from the test data is also discussed with regard to the mixed-mode nature of the peel test.

Contact forces and mechanisms in a vibratory finisher

The normal and tangential contact forces in a vibratory finishing machine were measured using a newly developed force sensor. A video system was used to record the motion of the finishing media as it collided with the test surface. Media contact occurred in three different modes. The ratio of the normal and tangential forces was compared with the measured friction coefficient under dry and water-wet conditions. This confirmed that media sliding occurred under water-wet conditions.

Simulation of interference effects in particle streams following impact with a flat surface: Part I. Theory and analysis

Abstract The interference between an incident stream of spheres and those rebounding from a flat surface is described using a computer model. The model was capable of examining the effect of the following parameters on the severity and frequency of inter-particle collisions: stream angle of incidence, nozzle divergence angle, incident particle velocity and flux, particle size, particle–particle and particle–surface impact parameters, and stand-off distance. The collision dynamics for systems consisting in excess of 10 4 particles were obtained. Frictionless inter-particle collisions were assumed, but friction for particles impacting the surface was considered. For all collisions, a coefficient of restitution model of impact behaviour was assumed. Dimensionless parameters used to aid in the presentation of the results were identified, and a companion paper [Simulation of interference effects in particle streams following impact with a flat surface. Part II. Parametric study and implications for erosion testing and blast cleaning, submitted for publication] presents a parametric study of their role in erosion testing and blast cleaning processes.