F. Ellyin

A unified periodical boundary conditions for representative volume elements of composites and applications

Abstract An explicit unified form of boundary conditions for a periodic representative volume element (RVE) is presented which satisfies the periodicity conditions, and is suitable for any combination of multiaxial loads. Starting from a simple 2-D example, we demonstrate that the “homogeneous boundary conditions” are not only over-constrained but they may also violate the boundary traction periodicity conditions. Subsequently, the proposed method is applied to: (a) the simultaneous prediction of nine elastic constants of a unidirectional laminate by applying multiaxial loads to a cubic unit cell model; (b) the prediction of in-plane elastic moduli for [± θ ] n angle-ply laminates. To facilitate the analysis, a meso/micro rhombohedral RVE model has been developed for the [± θ ] n angle-ply laminates. The results obtained are in good agreement with the available theoretical and experimental results.

Fatigue damage, crack growth and life prediction

Some general concepts concerning fatigue. Cyclic stress-strain response. Phenomenological approach to fatigue life prediction under uniaxial loading. Fatigue failure under multiaxial states of stress. Multiaxial experimental facilities. Constitutive laws for transient and stable behaviour of inelastic solids. Fatigue crack growth. Fatigue of notched members. Growth and behaviour of small cracks. Probabilistic fatigue crack growth. References. Index.

Effect of mean stress and ratcheting strain on fatigue life of steel

Detailed results of stress-controlled fatigue tests with tensile mean stress on ASTM A-516 Gr. 70 steel are presented. By using different preloading procedures, the ratcheting effect on the fatigue life of materials was separated from the mean stress effect. A previously proposed energy-based life prediction criterion is extended to include the effects of both mean stress and ratcheting. A comparison between the theoretical prediction with the present test results is found to be quite satisfactory.

Effect of stress ratio on the fatigue of unidirectional glass fibre/epoxy composite laminae

Abstract Fatigue behaviour of unidirectional glass fibre/epoxy composite laminae under tension-tension and tension-compression loading is investigated. Stress/life experimental data were obtained for fibre orientation angles, θ, of 0, 19, 45, 71 and 90°. These tests were performed under stress ratios, R ( = σ min σ max ) , of 0.5, 0 and −1. A fatigue failure criterion is proposed based on the input strain energy. This criterion takes into account both the fibre orientation angle and the value of the stress ratio. A non-dimensional form of this criterion collapses all data points, obtained from different combinations of fibre orientation angles and stress ratios, onto a single curve.

Cyclic response and inelastic strain energy in low cycle fatigue

Abstract The cyclic response of ASTM A-516 Gr 70 carbon low alloy steel subjected to fully-reversed constant strain- or stress-controlled cycles has been determined. The cyclic stress/strain relationship of the material was obtained through a least squares fit technique. Stable hysteresis loops at half life for various strain ranges are presented. The material does not exhibit Masing-type behaviour. The total inelastic strain energy is calculated by a new method and is in good agreement with the measured values. Comparison is also made with other proposed relationships. The total strain energy dissipated at failure may be expressed as W f = KN f α

A fatigue failure criterion for fiber reinforced composite laminae

Abstract A fatigue failure criterion for unidirectionally fiber reinforced laminae, under oscillatory states of combined plane stress, has been developed. The salient feature of this approach is based on the premise that the damage caused in the material due to cyclic loading, is related to the mechanical energy input. The predictions of the proposed criterion are compared with the experimental data, and are shown to be in good agreement.

A study of fatigue crack closure by elastic-plastic finite element analysis for constant-amplitude loading

A comprehensive elastic-plastic constitutive model is employed in a finite element analysis of fatigue crack closure. An improved node release scheme is used to simulate crack growth during cyclic loading, which eliminates the associated numerical difficulties. New definitions of crack opening and closing stresses are presented in this paper. Special attention is paid to a discussion of some basic concepts of fatigue crack growth and crack closure behaviour. Residual tensile deformation and residual compressive stress are found to be two major factors in determining the crack opening stress. A comparison of crack tip node release at the maximum or minimum load of each cycle is made and the disadvantage of releasing crack tip node at the minimum load are pointed out.

Fatigue Failure Model for Fibre-Reinforced Materials under General Loading Conditions:

A new model is presented which predicts the fatigue failure of fibrereinforced materials (FRMs) under multiaxial stresses, varying minimum to maximum cyclic stress ratios and different fibre orientations with respect to the loading directions. The concept of multidirectional elementary blocks is introduced. The model predicts the fatigue failure of these and unidirectional elementary blocks, based on the establishment of a reference stress versus life line and the determination of two simple functions. This methodolgy reduces considerably the number of parameters to be determined experimentally. Correlation with published fatigue failure results is presented. It is shown that the model succeeds in accurately predicting fatigue failure of different unidirectional and bidirectional FRMs subjected to uniaxial and biaxial stresses and different minimum to maximum cyclic stress ratios.

In-Phase and Out-of-Phase Multiaxial Fatigue

In this investigation, thin-walled circular cylindrical specimens fabricated from a low alloy pressure vessel steel (ASTM A-516 Gr. 70) were subjected to various multiaxial loading conditions. The tests were conducted under strain-controlled condition, and loading was provided through an axial actuator and internal and external pressure across the specimen wall

Influence of the filament winding tension on physical and mechanical properties of reinforced composites

Abstract In this experimental investigation the influence of the applied tow tension during filament winding on the physical and mechanical properties of glass-fibre reinforced polymeric composite tubulars, was studied. Pressure retaining tubular products used in the transportation/storage of fluids are generally subjected to a variety of loading conditions during their service life; thus tubular specimens were tested under different biaxial loading ratios. The stress/strain response was recorded and functional and structural failure envelopes were developed. These envelopes indicate the leakage and final failure characteristics of the components, respectively. The mechanical properties were analysed in conjunction with the measured physical properties: ‘fibre volume fraction’ and ‘effective wall thickness’. Experimental findings demonstrate that the component strength depends on the degree of fibre tensioning. Under fibre-dominated loading conditions, higher winding tension leads to an improved resistance against failure of tubular components, whereas under matrix-dominated loading failure is delayed by reduced fibre tensioning.