J.A.M. Ferreira

Evaluation of overload effects on fatigue crack growth and closure

Abstract Fatigue crack propagation tests with single tensile peak overloads have been performed in 6082-T6 aluminium alloy at several baseline Δ K levels and stress ratios of 0.05 and 0.25. The tests were carried out at constant Δ K conditions. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. The observed transient post-overload behaviour is discussed in terms of overload ratio, baseline Δ K level and stress ratio. The crack closure parameter U was obtained and compared with the crack growth transients. Experimental support is given for the hypothesis that plasticity-induced closure is the main cause of overload retardation for plane stress conditions. Predictions based on crack closure measurements show good correlation with the observed crack growth rates for all the post-overload transients when discontinuous closure is properly taken into account.

Analysis of fatigue and damage in glass-fibre-reinforced polypropylene composite materials

Abstract This paper concerns fatigue studies of polypropylene/glass-fibre thermoplastic composites produced from a bi-directional woven cloth of co-mingled E-glass fibres and polypropylene fibres with a fibre volume fraction V f of 0.338. The effect of lay-up design and load conditions on fatigue performance were investigated. The S-N curves, the rise in the temperature of the specimens, and the loss of stiffness during the tests, are discussed. Fatigue tests were performed in controlled displacement mode and in an imposed stress range. Similar results were obtained for both load conditions. The loss of stiffness was used as a damage parameter and related to the rise of temperature. The results show that the damage parameter E present a nearly linear relationship with the rise in temperature. A small deviation was probably caused by the stress release observed in the first period of fatigue life. ©

Effect of Interlayer Delamination on Mechanical Behavior of Carbon/Epoxy Laminates

This article presents the results of a current study on the influence of interlayer delaminations on the static and fatigue behavior of composite laminates. The composite was manufactured by a vacuum molding method using 12 balanced bi-directional carbon fiber layers and epoxy resin. Delaminations with different length were artificially introduced. The specimens with dog bone shape were cut from the original plates having 3 mm thickness and fiber weight fraction of 0.66. Static tests were performed in order to study the influence of delamination size on the laminate stiffness and strength. Complementary finite element analysis was carried out showing the influence of angular misalignments of fiber/matrix delaminations on the laminate stiffness. Fatigue tests were performed in load control for R = 0.05 and R = —1, with a loading frequency of 10 Hz, at room temperature. The artificial interlayer delaminations have a negligible influence on the fatigue strength for tensile cycle loadings, but produce significant decreases in strength for R = —1 fatigue loadings.

Effect of notch and test conditions on the fatigue of a glass-fibre-reinforced polypropylene composite

This paper is concerned with fatigue studies of polypropylene/glass-fibre thermoplastic composites produced from a bidirectional woven cloth of commingled E-glass and polypropylene fibres (the latter becoming the matrix after the application of heat and pressure). This composite was manufactured with a fibre volume fraction of 0.338. The effects of frequency, stress ratio, temperature and stress concentration on the fatigue performance were considered. The stress concentration was induced by circular holes in specimens loaded in tension. Stress/life fatigue curves, the rise in temperature of the specimens during the tests and the loss of stiffness were obtained and are discussed.

Static and fatigue behaviour of glass-fibre-reinforced polypropylene composites

Abstract This paper is concerned with fatigue of polypropylene/glass-fibre thermoplastic composites produced from a bi-directional woven cloth mixture of E glass fibres and polypropylene fibres. The latter becomes the matrix after the application of heat and pressure. This composite was manufactured with a fibre volume fraction Vf of 0.338. The effect of layer design on the static and fatigue performance was investigated. The S–N curves, the rise in the temperature of the specimens during the tests and the loss of stiffness, were obtained and discussed. The loss of stiffness was related to the rise of temperature and stress release observed in the material. The effect of load rate on the static properties was also studied and discussed accordingly.

Fatigue life prediction in AlMgSi1 lap joint weldments

The fatigue behaviour of an AlMgSi1 alloy lap welded joint and the improvement in fatigue strength due to post-weld heat treatment were investigated. Two series of fatigue tests were performed: one in as-welded specimens and another in heat-treated specimens. The prediction of crack initiation life under a plane strain condition was made based on the local strain approach using Morrows modified equation. The local inelastic stresses and strains at the weld toe were calculated for the heat-treated specimens using three methods: Neuber, Glinka and finite element analysis. The accuracy of the different methods were evaluated and discussed.

Fatigue and fretting fatigue of ion-nitrided 34CrNiMo6 steel

Abstract This work is concerned with the surface treatment (ion nitriding) of fretting fatigue and fatigue resistance of 34CrNiMo6. Tests are made on a servo-hydraulic machine under tension for both treated and non-treated specimens. The test parameters involve the applied displacements δ ±80–±170 μm; fretting pressure σ n =1000–1400 MPa; fatigue stress amplitude σ a =380–680 MPa and stress ratio R =−1. The ion nitriding process improves both fatigue and fretting fatigue lives. Subsurface crack initiation from internal discontinuities was found for ion-nitrided specimens.

Fracture assessment of PMMA/Si kitchen sinks made from acrylic casting dispersion

Abstract Acrylic casting dispersion is used to fabricate kitchen sinks made of PMMA/Si which stands for poly-methyl methacrylate mixed with a fine dispersion of silica. The composite is suited for sanitary wares that are subjected to severe temperatures, giving rise to high thermal stresses. This leads to failure by cracking in the region near the drain hole. Such conditions are investigated by determining the mechanical and fracture properties for two different chemical compositions of PMMA. They are referred to by trade marks from ICI as Asterite and Amatis. Data are obtained for temperatures from 0 to 95 °. Extensometry data are also obtained to the thermal stresses around the drain hole subjected to alternating flow of cold and hot water. Critical crack lengths are determined from the critical stress intensity factor and maximum local tensile stresses based on measured data. They are estimated to be 1.5 mm and 4.0 mm, respectively, for the Asterite and Amatis composites. These results confirm with practice where the cracking of the former was observed while the latter did not crack.

A comparative study of the fatigue behaviour of GRP hand lay-up and pultruded phenolic composites

This paper presents a study of the fatigue behaviour of E-glass-type fibre-reinforced phenolic composites manufactured by the pultrusion and the hand lay-up processes, with the main objective of assessing the influence of the manufacturing process on the fatigue life. For the hand lay-up process fatigue data are shown for composites with fibres aligned in the 0° longitudinal tensile loading directions, 90/0/90 orientations and 0/90/0 fibres with tensile loading applied also in the longitudinal direction. The results were obtained for two different values of frequency, stress ratio and temperature. In the pultruded specimens fatigue life data are presented at room temperature and only one value of stress ratio and frequency. The results were obtained for five sets of manufacturing parameters including a woven-type composite. With the exception of the woven pultruded specimens, which gave low fatigue strength values, the remaining types of pultruded composite have shown adequate fatigue strength values but lower than the results obtained for the longitudinally loaded 0° hand lay-up specimens. The hand-lay-up composite of the unidirectional fibre orientation (0°) have shown higher values of fatigue strength at the elevated temperature of 200°C in comparison with the room temperature values. The variations obtained in fatigue strength amongst the different types of composites are assessed through the analysis of data and also by the failure mechanisms detected by SEM observations.

Influence of weld and plate geometry on the fatigue strength of cruciform joints

Abstract Results of fatigue life prediction in cruciform joints are reported. Using Fracture Mechanics methods, fatigue life was computed as a function of initial flaw size, plate length and thickness, weld geometry and loading mode. The Bueckner weight function method was used for computing the stress intensity factor using a 2-D finite element program. The results have shown that plate length has little or negligible influence on fatigue strength provided that the plate length to thickness ratio is below 20 to 50 depending on the plate thickness. The attachment thickness to main plate thickness ratio also has little or no effect on fatigue strength. The plate thickness itself and the loading type (tension and bending), however, do affect the fatigue. Their effects are quantified and introduced into the SN-curve for design.