S.L. Ogin

Matrix cracking and stiffness reduction during the fatigue of a (0/90)s GFRP laminate

Abstract Stiffness reduction due to matrix cracking in a (0/90) s glass fibre reinforced plastic (GFRP) laminate has been studied under both quasi-static and fatigue loading. The stiffness reduction is shown to be directly proportional to the density of cracks which accumulate in the transverse ply. A model for the transverse ply crack growth during fatigue gives good agreement with the experimentally determined stiffness reduction curves.

The use of the PeakForceTM quantitative nanomechanical mapping AFM-based method for high-resolution Young's modulus measurement of polymers

PeakForceTM quantitative nanomechanical mapping (QNMTM) is a new atomic force microscopy technique for measuring Youngs modulus of materials with high spatial resolution and surface sensitivity by probing at the nanoscale. In this work, modulus results from PeakForce™ QNM™ using three different probes are presented for a number of different polymers with a range of Youngs moduli that were measured independently by instrumented (nano) indentation testing (IIT). The results from the diamond and silicon AFM probes were consistent and in reasonable agreement with IIT values for the majority of samples. It is concluded that the technique is complementary to IIT; calibration requirements and potential improvements to the technique are discussed.

On matrix crack growth in quasi-isotropic laminates—I. Experimental investigation

Abstract Matrix crack growth behaviour under quasi-static and mechanical fatigue loadings has been studied in quasiisotropic ( 0/90 −45/ + 45 ) s GRP laminates. Detailed experimental observations were made on the initiation and growth of individual cracks in ± 45 ° and 90 ° plies. Degradation of elastic properties of the laminates due to matrix cracking was observed and recorded. The quasistatic results were compared with baseline data from a crossply ( 0 90 ) s laminate. Distinctly different crack growth behaviour was observed in quasi-isotropic laminates under quasi-static and fatigue loadings, suggesting a strong influence of load history. A higher density of ± 45 ° cracks and hence greater stiffness reduction appear to be the characteristics of fatigue damage in quasi-isotropic laminates.

Damage accumulation in woven-fabric CFRP laminates under tensile loading : Part 1. Observations of damage accumulation

Abstract Damage development in composite laminates reinforced with two-, four- and six-layer carbon-fibre-reinforced polyimide eight-harness satin-weave fabric has been observed as a function of applied strain. Residual properties (Young’s modulus, Poisson’s ratio and residual strain), together with damage accumulation, have been recorded as a function of applied strain. Distinct differences which emerge in the behavior of the residual properties of the laminates with increasing applied strain are related to the different types of damage which accumulate in the laminates.

Application of the Paris Equation to the Fatigue Growth of Transverse Ply Cracks

A model based on a stress intensity factor for a growing transverse ply crack is outlined. The model is applied to experimental observations of crack growth in a trans parent 0/90/0 glass fibre/epoxy laminate under fatigue loading. The crack growth rate is found to be independent of crack length but to depend on the spacing between cracks. Under static loading and fatigue loading at high maximum stress, cracks grow by fast frac ture. Slow crack growth is observed at lower maximum fatigue stresses and in the later stages of fatigue tests at higher stresses when the crack spacing is small. Crack growth rates can be described using a Paris relation.

Characterisation and modelling of the notched tensile fracture of woven quasi-isotropic GFRP laminates

Damage growth and fracture at circular holes in quasi-isotropic laminates, assembled from four layers of woven glass fabric, loaded in tension has been studied. A comprehensive notch-edge damage analysis was performed by a combination of recording tests on video, plan-view photography and a de-ply technique, enabling a layer-by-layer analysis. It was shown that the notch-edge damage initiation and propagation comprised matrix cracking, fracture of the 0° tows, delamination and longitudinal splitting. The tows in the fabric layers oriented at 45° within the damage zone remained intact up to the maximum load in a tensile test. The well-known semi-empirical point-stress and average-stress criteria resulted in accurate notched strength predictions with the average-stress criterion being better than the point-stress criterion. A recently developed critical damage growth model, requiring as input parameters the unnotched strength and fracture toughness of the laminate which were measured from independent experiments, yielded accurate notched strength predictions (to better than 10%), while being very easy to implement. Predicted damage zone lengths from the critical damage growth model generally agreed well with experimental observations.

On transverse matrix cracking in cross-ply laminates loaded in simple bending

Abstract A one-dimensional analysis of a cross-ply laminate, containing cracked transverse plies, loaded in flexure is presented. Simple bending theory is used in conjunction with a shear-lag analysis, to calculate the degraded longitudinal modulus of a cracked transverse ply, enabling the flexural modulus of the laminate to be determined. The solution is shown to agree well with a more sophisticated stress transfer model in the literature. The analysis is then extended to calculate the applied bending moment at transverse crack onset under flexural loading using a fracture mechanics approach. The results suggest that the in situ transverse ply stress at which matrix cracking commences for the beam loaded in flexure is very close to the stress level at which the same ply would crack if the laminate were loaded in tension.

On matrix crack growth in quasi-isotropic laminates - II. Finite element analysis

Abstract Multiple matrix crack growth behaviour in a quasiisotropic ( 0/90 −45/ + 45 ) s laminate has been simulated by using generalised plane-strain finite element models. Representative models have been developed to describe the sequential matrix cracking events observed from the experiments, as reported in Part I of this paper. These models provide quantitative predictions of stiffness reduction due to matrix cracking which are in good agreement with the experimental results. These results show that 90 ° and + 45 ° cracking are the major damage events governing the degradation of the laminate elastic properties with the contribution from the −45 ° cracking being less important. The models provide information concerning stress distributions in the vicinity of existing cracks, which enable the transfer lengths for cracks in the 90 ° ply to be estimated and may facilitate the study of failure mechanisms at the local level.

A STRESS INTENSITY FACTOR APPROACH TO THE FATIGUE GROWTH OF TRANSVERSE PLY CRACKS

Abstract A model has been developed for the stiffness reduction due to transverse ply crack growth during the fatigue of a (0/90)s glass-fibre reinforced plastic laminate. A stress intensity factor is derived for a transverse ply crack and related to the stiffness reduction rate by the Paris law. The model gives good agreement with experimental data and can be used to maintain a constant stiffness reduction rate by incremental load-adding. The effect of frequency is considered.

Intra-laminar fracture in angle-ply laminates

The development of matrix cracking in (0/θ/0) GFRP laminates, where θ = 45°, 54°, 75° and 90°, has been investigated. Crack initiation and propagation in the θ° ply were studied separately by using unnotched and notched samples. Details of average crack initiation/propagation strains and associated ply stresses, calculated from laminate theory, as functions of off-axis ply angle are presented and discussed for all laminates along with investigations of notch depth effects and crack growth patterns. It is suggested that to a first approximation crack propagation is governed by the transverse stress while crack initiation has a more complex and as yet unknown dependence on stress state.