Ply cracking and stiffness degradation in cross-ply laminates under biaxial extension, bending and thermal loading

Transverse ply cracking often leads to the loss of stiffness and reduction in thermal expansion coefficients. This paper presents the thermoelastic degradation of general cross-ply laminates, containing transverse ply cracks, subjected to biaxial extension, bending and thermal loading. The stress and displacement fields are calculated by using the state space equation method [Zhang D, Ye JQ, Sheng HY. Free-edge and ply cracking effect in cross-ply laminated composites under uniform extension and thermal loading. Compos Struct [in press].]. By this approach, a laminated plate may be composed of an arbitrary number of orthotropic layers, each of which may have different material properties and thickness. The method takes into account all independent material constants and guarantees continuous fields of all interlaminar stresses across interfaces between material layers. After introducing the concept of the effective thermoelastic properties of a laminate, the degradations of axial elastic moduli, Poisson’s ratios, thermal expansion coefficients and flexural moduli are predicted and compared with numerical results from other methods or available test results. It is found that the theory provides good predictions of the stiffness degradation in both symmetric and antisymmetric cross-ply laminates. The predictions of stiffness reduction in nonsymmetric cross-ply laminates can be used as benchmark test for other methods.     

Modelling of chip separation in machining unidirectional FRP composites by stiffness degradation concept

Progressive failure of unidirectional glass fiber-reinforced polymer composites (FRP) was studied using finite element analysis in orthogonal machining. Chip formation process and damage modes such as matrix cracking, fiber–matrix debonding and fiber breaking were modelled by degrading the material properties. Damage analysis was carried out using Hashin, Maximum stress and Hoffman failure criteria. After damage was detected, selective stiffness degradation was applied to the workpiece material. The objective of this study is to better understand the chip formation process and to analyse the cutting-induced damage from initiation stage until complete chip formation. The effect of the fiber orientation on cutting forces and sub-surface damage was investigated with different failure criteria. The results were addressed in terms of cutting forces evolution and damage progression in the composite structure during machining. It was demonstrated that the use of the stiffness degradation concept with the appropriate failure criterion responds potentially in a predictable fashion to changes in chip formation process for machining of FRPs.     

Matrix fatigue cracking in fiber composites

A model is developed for fatigue growth of matrix cracks in metals reinforced with aligned continuous elastic fibers. The mechanics of elastic cracks bridged by frictionally constrained fibers is used to develop the model, which provides estimates of the tip value of the stress intensity factor amplitude, ΔK_TIP. It is found that when the applied load amplitude is held fixed during fatigue crack growth, ΔK_TIP, and thus the rate of growth approach an asymptotic value independent of crack length. The residual strength after fatigue crack growth is also discussed. In some cases, the residual strength is unaffected by prior fatigue growth. But, in another regime, the matrix crack length allows fibers to begin breaking before the matrix crack grows. The strength is then inversely proportional to the square root of fatigue crack length.     

Characterization of stiffness degradation caused by fatigue damage in textile composites using circumferential plate acoustic waves

The aim of this paper is to show the possibility of fatigue damage characterization in GFRP-tube-like components by using circumferential plate waves. For that purpose, fatigue tests with different loading directions have been conducted and the stiffness degradation has been monitored. After a preset number of loading cycles, non-destructive ultrasonic tests using circumferential plate waves were performed. The correlation between damage induced amplitude changes of the plate waves and stiffness degradation in non-crimped fabric composites is discussed. The results indicate that the technique is applicable to fatigue damage assessment in complex-shaped components of composite materials and is relevant to a wide field of applications.     

Restrained shrinkage cracking in fibre-reinforced cementitious composites

The influence of seven types of fibres on restrained shrinkage cracking in Portland cement pastes and mortars is investigated at various fibre volume fractions. Linear specimens with one-dimensional restraint were subjected to a drying environment soon after casting. Fibres belonging to two major categories-macro (large) and micro (fine)-were investigated. These two categories of fibres were found to result in distinctly different cracking patterns due, in part, to their different reinforcing mechanisms. An attempt is made to relate the observed cracking patterns with the perceptible micromechanical processes. A ‘fibre efficiency factor’ is proposed which appears to be an appropriate basis for grading the fibres.     

Some factors controlling transverse cracking in cross-plied composites

Journal of Materials Science -     

Fatigue life prediction of nanoparticle/fibrous polymeric composites based on the micromechanical and normalized stiffness degradation approaches

In this paper, a new model based on the micromechanical and normalized stiffness degradation approaches is established. It has been assumed that during the fatigue condition, only material properties of composites (fiber and matrix) were degraded and nanofillers remain intact under different states of stress. A normalized stiffness degradation model was proposed for laminated fibrous composites reinforced with nanoparticles to derive a novel model to predict the stiffness reduction. The developed model is capable of predicting the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nanofillers. The new fatigue model is verified by applying it to different experimental data provided by different researchers. The obtained results by the new fatigue model are in very good agreement with the experimental data of nano-silica glass/epoxy composites under constant cyclic stress amplitude fatigue and also for silica/epoxy nanocomposites in various states of stress with negligible error.     

Distributed damage in composites,theory and verification

A model that accounts for the effect of structural changes, resulting from the application of loads, in composite materials is developed. Such changes are incorporated in the theory through a tensor form of a damage variable. The model extends the theory of elasticity and it accounts for certain important properties observed in composite materials. The parameters involved in the theory are identified and determined from available experimental data. Back prediction of test results, for two different composite materials, verifies the theory.     

Correlation between resin material variables and transverse cracking in composites

In this paper, the correlation between the resin material variables and the transverse cracking in composites is established. A theoretical model based on the fracture mechanics principle is built to describe thein situ failure process of transverse cracking. The central concept of the model is that the fracture is controlled by the plastic zone developed at the crack tip. Based on an approximate crack tip stress distribution, a quantitative representation is found to relate the laminate transverse cracking fracture toughness,G _c(comp), to certain resin properties: fracture toughness,G _c(resin), yield stress, _y, Young's modulus,E, and residual stress build-up, _R.G _c(comp) values of several fibre-glass/epoxy laminate systems were measured using the double torsion technique. The experimental results are found to be interpreted reasonably well by the theory. As a result, a clear picture of transverse cracking emerges. It seems that _y ²/E plays a more dominant role thanG _c(resin) in controllingG _c(comp). The residual stress _R can weaken the laminate significantly when its level is high. It is also shown that the failure model discussed here can be readily applied to laminate delamination failure as well as adhesive bond fracture.     

Simulating cracking and failure in multilayer composites: Multiple failure

Summary A model has been devised for the failure in a layered composite with long-time static loading. Each layer is considered as reinforced in one direction by continuous fibers with any cross section. Damage accumulation is accompanied by changes in the elastic and strength characteristics for each layer and for the composite as a whole.The model has been used in a scheme for forecasting the working life of a layered composite on long-time static loading on the assumption that the damage accumulates uniformly in each layer.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 3, pp. 44–50, May–June, 1991.     

Stochastic dynamics with fatigue-induced stiffness degradation

This article deals with stochastic dynamics coupled with simultaneous evolution of degradation of the system properties. We provide a general formulations and interpretations of various response-degradation problems associated with randomly vibrating systems and then an efficient analysis of vibratory systems with fatigue-induced stiffness degradation is presented. First, an approach to the random vibration problem with empirical characterization of stiffness degradation is sketched out and then the efficient sequential method for a coupled analysis of the response-degradation is presented in detail including numerical illustration of the results.     

Matrix cracking initiated by fibre breaks in model composites

Fracture of resin in a composite material can be initiated by a tensile break in a fibre. This process has been investigated for a simple model composite, consisting of two inextensible rods placed along the axis of a cylindrical elastic block and touching in the centre. The rods represent a broken fibre. Energy release rates,G, were calculated by finite element methods for a circular crack growing outwards from the point where the rod ends separated as they were pulled apart. Results are compared with experimental observations on cracking of a silicone rubber cylinder containing two steel rods. It was found that a crack grew outwards under increasing load until its radius reached a certain size, approximately half-way to the surface of the resin cylinder. At this point,G reached a minimum value and then increased. Simultaneously, the crack accelerated and the sample broke. Forces required to propagate the crack were successfully predicted by linear elastic fracture mechanics at all stages of crack growth and for a wide range of fibre and sample radii. In particular, good agreement was obtained with the maximum force that the model system could support, i.e. the breaking load. When the sample was surrounded by a rigid tube, representing neighbouring fibres surrounding the broken one, growth of a crack required an increasing load at all stages. The sample finally fractured when the broken fibre pulled out with resin still attached to it. Application of these results to unidirectional fibre-reinforced materials is discussed.     

Interfacial degradation in fibre-reinforced cement-based composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Interfacial degradation in fibre-reinforced cement-based composites

Abstracts are not published in this journal     

Multiple cracking in aligned polypropylene fibre reinforced cement composites

Multiple cracking has been shown to occur in hardened cement paste reinforced with aligned polypropylene fibres of elastic modulus considerably lower than that of the cement paste. The effect of fibre volume fraction on the distribution of matrix cracks has been studied and good agreement found with existing theory. Factors which enable fibre/matrix contact to be maintained during the multiple cracking process, despite the unfavourable Poisson's ratio contraction of polypropylene have been discussed. These include the lateral displacement of one surface of a crack relative to the other and also the lateral displacement of matrix material next to the crack surface relative to the fibre array. This latter mechanism is shown to apply to other aligned composites and calculations based on a simple theory predict that multiple cracking should occur in all composites provided that a critical size is exceeded.