The effect of eccentricity on dynamic crack propagation behaviour of rubber-modified PMMA models

Rubber-modified materials show a complex fracture behaviour. In a previous paper the influence of a rigid PMMA circular inclusion with a rubber ring surrounding it embedded in a PMMA matrix, on the dynamic fracture mode was investigated. In the present experimental study, the same model system was used to investigate the effect of eccentricity of the initial crack path from the specimen centreline on which the complex inclusion centre is located, on the dynamic crack propagation mode. This effect was studied by using high-speed photography and dynamic caustics. It was shown that the eccentricity of the initial crack path largely determines both the crack propagation delay observed as the crack approaches the complex inclusion and the bifurcation of the crack. It was also found that the maximum crack propagation velocity, the length of the crack path along the interface, as well as the angle of incidence with which the propagating crack meets the interface, increase as the eccentricity increases.     

Effects of pre-cracking methods on fracture behaviour of an Araldite-F epoxy and its rubber-modified systems

The fracture behaviour of an Araldite-F epoxy and its rubber-modified systems was evaluated using compact tension specimens pre-cracked by three methods, namely, razor blade pressing, razor blade tapping and fatigue pre-cracking. The results show that the razor blade tapping method produces a lowest critical stress intensity factor, K _lc, while the razor blade pressing produces an abnormally high K _lc, being about five times higher than the former for the pure epoxy. Transmission polarized optical microscopy reveals that the crack tip produced by razor blade pressing in the pure epoxy specimen was completely surrounded by a plastic deformation zone with compressive residual stress, but the crack tip produced by razor blade tapping was free of residual stress and plastic deformation. It was found that the sensitivity of the fracture toughness value to the pre-cracking methods decreases after the pure epoxy was modified by 10% core-shell rubber or 10% liquid rubber.     

Effect of notch severity on fatigue fracture in a rubber-modified glassy polymer

The research concerned the effect of notch severity on the fatigue behaviour of series of rubber-modified glassy polymers, consisting of a styrene-acrylonitrile copolymer with different amounts of an olefin rubber. Tests were conducted under displacement control and two different loading conditions. Both stages of fatigue lifetimes, that is, fracture initiation and crack propagation, were examined. It was observed that the initial notch severity determines the duration of the crack-initiation stage, while crack propagation does not depend on it. The crack velocity appears to be controlled by the maximum applied stress intensity factor, and the correlation does not depend on the rubber content. The results obtained have been interpreted by considering three different zones in the specimen during the fracture process: a far-field viscoelastic continuum, a process zone and a failure zone.     

Three-dimensional elastoplastic finite element modelling of deformation and fracture behaviour of rubber-modified polycarbonates at different triaxiality

A periodic face-centred cuboidal cell model is provided to account for inter-particle interaction, and a particle-crack tip interaction model is developed to study the interaction between a blunting model I crack tip and the closest array of initially spherical rubber particles in an effective medium. Three-dimensional elastoplastic finite element analysis has been preformed to study the deformation and fracture behaviour of rubber-modified polycarbonates. The effective elastoplastic constitutive relation is derived by the method of homogenisation and local stress and strain distributions are obtained to explore the role of rubber cavitation in the toughening process at different stress triaxiality. 3D elastoplastic finite element results are compatible with experimental observations, that is, rubber particles can act as stress concentrators to initiate crazing or shear yielding in the matrix but they behave differently from voids at high triaxiality. Rubber cavitation plays an important role in the toughening process under high tensile triaxial stresses.     

Reliability of PMMA bone cement fixation: fracture and fatigue crack-growth behaviour

Fracture mechanics tests were performed to characterize the fracture toughness and fatigue crack-growth behaviour of polymethylmethacrylate (PMMA) bone cement, commonly used in joint replacement surgery. Compact tension specimens of various thicknesses were prepared and tested in both air and Ringers solution. Contrary to previous reports citing toughness as a single valued parameter, the PMMA was found to exhibit resistance-curve behaviour with a plateau toughness of 0.6 MPa m1/2 in air, and 2.0 MPa m1/2 in Ringers solution. The increased toughness in Ringers solution is thought to arise from the plasticizing effect of the environment. Under cyclic loads, the material displayed true mechanical fatigue failure in both environments; fatigue crack-growth rates, da/dN, were measured over the range 10-10 to 10-6 m/cycle and found to display a power-law dependence on the stress intensity range, K. The cement was found to be more resistant to fatigue-crack propagation in Ringers solution than in air. Wear debris was observed on the fatigue fracture surfaces, particularly those produced in air. These findings and the validity of using a linear-elastic fracture mechanics approach for viscoelastic materials are discussed in the context of providing more reliable and fracture-resistant cemented joints.     

Application of the crack-bridging model for fracture resistance (R-curve behaviour) to PMMA

The rising fracture toughness behaviour of PMMA was characterized using a crack-bridging model originally developed for coarse-grained alumina that predicts a mechanism for crack resistance from the bridging of unbroken grains behind the crack tip. Based on the published experimental observation of PMMA, the craze zone behind the crack tip was thought to be analogous to the effective grain-bridging zone in the model in which the fibrils in the craze zone were related to the restraining interfacial ligaments. Self-consistent results in terms of the model were obtained which indicates that the crack-bridging model can be used to account for the fibril-toughening mechanism in PMMA.     

Effect of strain rate on the fracture behaviour of collagen

The strain-rate dependence of collagen fibre, a viscoelastic material, was studied both in the native and dry conditions. The strain rate effect was observed in the stress-strain, plastic set behaviour of both dry and wet collagen fibres. Fractured ends of the broken fibres, observed using scanning electron microscopy, showed that the fracture behaviour was different at high and low strain rates. The results are compared with those for elastoidin.     

A fracture mechanics analysis of the effect of rubber content on rubber-modified polystyrene

Consideration is given to achieving test conditons in which linear elastic fracture mechanics may be employed to measure toughness in these rather ductile materials. This is achieved by testing wide specimens (100 mm) and these are employed to measureK _c over a wide range of temperatures for materials with several rubber levels. It is shown that bothK _c and the yield stress are determined by the extent of crazing and that, at the low rates used here, the full crazing capacity of the polystyrene is utilized by the addition of about 1% of rubber.     

Temperature effects on quasi-static fracture of PMMA

The effect of temperature on the quasi-static fracture behaviour of PMMA is examined. It is found that the critical stress intensity factor, K ₁₀, at crack growth initiation decreases with temperature up to a certain critical temperature, T _c. Above T _c, K ₁₀ increases rapidly. The speeds with which slow crack growth could be maintained without transition to brittle fracture were found to be significantly higher at high temperatures.     

Temperature effects in the fracture of PMMA

Experiments are described in which the fracture toughness,K _c, of PMMA has been determined in the temperature range –190 to + 80° C and over the crack speed range of 10^–2 to 10² mm sec^–1. Single edge notch tension was used for instability measurements but the other data were obtained using the double torsion method. In the range –80 to + 80°C the variations inK _c may be described in terms of modulus changes and a constant crack opening displacement criterion. Crack instabilities are correlated with isothermal-adiabatic transitions at the crack tip. Below –80° C there is an inverted rate dependence associated with thermal effects during post-instability crack propagation.     

Effect of fibre misalignment on the fracture behaviour of fibre-reinforced composites

Deviations from ideal parallel packing in a unidirectional fibre-reinforced composite affect its resistance to splitting. In order to relate, quantitatively, the failure processes to such misorientation, it is necessary to characterize the departure from ideality and to measure the resistance to failure. Experimental observations are presented relating to: (i) a tendency for the carbon fibres in a tow to group into bundles that deviate somewhat from being parallel with each other, and (ii) the fracture toughness for the splitting of several imperfectly aligned composites. Statistical representations are offered for quantifying or modelling the degree of misalignment.     

Fracture behaviour of unmodified and rubber-modified epoxies under hydrostatic pressure

The fracture toughness and uniaxial tensile yield strengths of unmodified and CTBN-rubber-modified epoxies were measured under hydrostatic pressure. The purpose of these experiments was to learn how suppressing cavitation in rubber particles affects the deformation mechanisms and the fracture toughness of rubber-modified epoxy. It was found that the cavitation of CTBN-rubber could be suppressed at a relatively low pressure (between 30 and 38 M Pa). With cavitation suppressed, the rubber particles are unable to induce massive shearyielding in the epoxy matrix, and the fracture toughness of the rubber-modified epoxy is no higher than that of the unmodified epoxy in the pressure range studied. Unmodified epoxy shows a brittle-to-ductile transition in fracture toughness test. The reason for this transition is the postponement of the cracking process by applied pressure.Work performed while on a sabbatical leave at the University of Michigan.