B.R.K. Blackman

The use of a cohesive zone model to study the fracture of fibre composites and adhesively-bonded joints

Analytical solutions for beam specimens used in fracture-mechanics testing of composites and adhesively-bonded joints typically use a beam on an elastic foundation model which assumes that a non-infinite, linear-elastic stiffness exists for the beam on the elastic foundation in the region ahead of the crack tip. Such an approach therefore assumes an elastic-stiffness model but without the need to assume a critical, limiting value of the stress,σmax, for the crack tip region. Hence, they yield asingle fracture parameter, namely the fracture energy,Gc. However, the corresponding value ofσmax that results can, of course, be calculated from knowledge of the value ofGc. On the other hand, fracture models and criteria have been developed which are based on the approach thattwo parameters exist to describe the fracture process: namelyGcandσmax. Hereσmax is assumed to be a critical,limiting maximum value of the stress in the damage zone ahead of the crack and is often assumed to have some physical significance. A general representation of the two-parameter failure criteria approach is that of the cohesive zone model (CZM). In the present paper, the two-parameter CZM approach has been coupled mainly with finite-element analysis (FEA) methods. The main aims of the present work are to explore whether the value ofσmax has a unique value for a given problem and whether any physical significance can be ascribed to this parameter. In some instances, both FEA and analytical methods are used to provide a useful crosscheck of the two different approaches and the two different analysis methods.

Standard Test Methods for Delamination Resistance of Composite Materials: Current Status

This paper presents a survey of the current status of test methods for the measurement of delamination resistance of composite materials, with particular emphasis on the work performed in this area by ESIS, the European Structural Integrity Society. First, existing mode I fracture test standards are described. We then present work currently underway, both to extend the range of application of these mode I tests and to standardise mode II, mixed mode (I/II) and mode III tests. Finally, we discuss tests to characterise fatigue crack propagation.

Measuring the mode I adhesive fracture energy, GIC, of structural adhesive joints: the results of an international round-robin

The results of an inter-laboratory round-robin test programme designed to validate a new protocol for determining the mode I adhesive fracture energy, GIC, of structural adhesive joints are presented. The analysis schemes employed by the protocol are described and critically compared in the light of these results. The importance of a number of validity checks on the data analyses are discussed and the accuracy and precision of the test method is determined according to existing International standards. The values of GIC deduced were shown to be independent of the test geometry of the joint (i.e. DCB versus TDCB) but dependent upon the substrate material used. Additional studies have shown that the substrate dependence was due to the cured adhesive in the different joints possessing different values of glass transition temperature.

The calculation of adhesive fracture energies in mode I: revisiting the tapered double cantilever beam (TDCB) test

Abstract Analytical corrections have been derived for a beam theory analysis for the adhesively bonded tapered double cantilever beam test specimen to account for the effects of beam root rotation and for the real, as opposed to idealised, profile of the beam as required experimentally. A number of adhesive–substrate combinations were tested according to a new test protocol and the new analysis method for data reduction is compared critically with the existing simple beam theory and experimental compliance approaches. Correcting the beam theory for root rotation effects is shown to be more important than correcting only for the effects of shear deformation of the substrates. Results from a finite element analysis, using a cohesive zone model, also showed close agreement with the proposed new corrected beam theory analysis method.

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The failure of fibre composites and adhesively bonded fibre composites under high rates of test, up to rates of about 15 m s−1 were studied in detail. The present paper. Part I of the series, considers the experimental aspects of the mode I fracture of the fibre composite materials and joints. Part II will analyse the dynamic effects which are invariably associated with high-rate tests, and will show how these effects influence the observed behaviour of the test specimens. Part III will report the results from mode II and mixed-mode I/II tests on the fibre composite materials.

The calculation of adhesive fracture energies from double-cantilever beam test specimens

The purpose of this letter is to demonstrate that, when using the DCB test specimen to study joints that consist of bonded polymeric fibre-composite substrates, careful attention needs to be paid to the method used to analyse the experimental data

The impact wedge-peel performance of structural adhesives

The impact wedge-peel (IWP) test is an International Standard (ISO 11343) method that is employed to measure the resistance to cleavage fracture of structural adhesives at a relatively high test-rate of 2 to 3 m/s. In the present work this test has been employed to evaluate the performance of a range of structural adhesives when used to bond either steel or aluminium-alloy substrates. Firstly, a novel test arrangement for performing these tests, using a high-speed servohydraulic machine, is described. Tests were performed at 10−4 and 2 m/s and at test temperatures of −40 and 23°C. High-speed photography was also used to investigate the failure of the IWP test specimens. Both stable and unstable types of crack growth were recorded, with the crack propagating cohesively through the adhesive layer in all cases. The methods of data analysis recommended by the International Standard are critically reviewed, and some shortcomings are highlighted. Secondly, the results from the IWP test are then directly correlated to the measured values of the adhesive fracture energies, Gc, of the adhesives, which were determined using a fracture-mechanics approach. Finally, it is demonstrated that, from knowledge of the value of Gc of the adhesive, coupled with a finite-element analysis of the IWP test geometry, the failure behaviour of the IWP specimen may be successfully modelled and predicted.

Mode II delamination

Publisher Summary This chapter elaborates the different aspects of the mode II delamination. Introducing shear loads into composite materials has presented major problems to the composite industry. Tests to determine shear moduli and strengths abound and all are open to criticism. Shear loading of cracked specimens involves many of the difficulties associated with these shear tests, but adds others, such as friction between sliding crack faces, instabilities in some of the specimen geometries, and non-linear behavior. When a crack propagates by the relative sliding of two crack faces, the friction between these faces can have a significant effect on the measured fracture toughness. It is advisable to perform both methods of data analysis—the beam theory and compliance calibration—because differences between the two can give an indication of the presence of fracture mechanisms that may invalidate some of the test data. Large differences between propagation values for the two methods may reflect multiple crack propagation. The recommendation in the ESIS protocol is to include either a thin film of PTFE or a pencil lead between the two crack faces to minimize friction.

The effects of pre-bond moisture on the fracture behaviour of adhesively-bonded composite joints

The results of an investigation into the effects of pre-bond moisture absorbed by fibre-composite substrates prior to bonding with various structural epoxy adhesives are presented. Substrates were bonded in the as-received condition (where substrates had been exposed to atmospheric moisture for periods of greater than three months) and were also bonded in the fully-dried condition (after drying under vacuum at 105°C for 28 days). Additionally, substrates were conditioned by water submersion for various durations prior to bonding. Double cantilever beam tests were performed on the resulting joints to determine the adhesive fracture energy, G IC. The effect of pre-bond moisture on the glass transition temperature of the adhesive was also determined. One adhesive was shown to exhibit an extreme sensitivity to pre-bond moisture. A severe reduction in fracture energy accompanied a change in the fracture morphology and Tg. Other adhesives were shown to be relatively insensitive to the levels of pre-bond moisture introduced.

Fracture tests on structural adhesive joints

Publisher Summary This chapter focuses on fracture tests on structural adhesive joints. The protocol is based upon a linear-elastic fracture-mechanics (LEFM) approach and is designed to be used to determine the value of the adhesive fracture energy, of structural adhesives under Mode I loading. The specimens in the protocol are bonded joints based upon double-cantilever beam (DCB) and the tapered double-cantilever beam (TDCB) specimens. The DCB specimen of the protocol is well-suited for testing joints consisting of an adhesive which is bonding relatively thin sheets of fiber-composite materials, but may also be used when metallic substrates which possess a relatively high yield stress are being employed. The adhesively-bonded steel joints were tested in both the DCB and TDCB test geometry. It is observed that the glass transition temperature of the particular rubber-toughened adhesive is very much dependent upon the exact heating cycle used to cure the adhesive, and the amount of water present in the CFRP substrates prior to forming the joint.