J.M. Costa

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.

Effect of water and fiber length on the mechanical properties of polypropylene matrix composites

This paper presents the results of a current study on polypropylene matrix composites processed by injection, with two different glass fiber lengths and five different volume fractions. Physical and mechanical properties were obtained, namely flexural strength, stiffness modulus and fracture toughness. The mechanical properties of the composites increased significantly with the increase of the fibers volume fraction in agreement with the Counto model. The effect of water immersion time was also analysed. Immersion in water promotes a marked decrease in mechanical properties in the early seven-ten days, and afterwards tends to stabilize. Water causes a decrease of the relative strength which increases with fiber volume fraction and reaches about 29 % and 32 % for 20 % of 4.5 mm fiber length and for 25 % of 12 mm fiber length respectively, after 28 days immersion in water. Fracture toughness increases with fiber volume fraction and is always higher for 12 mm fiber length composites than for 4.5 mm fiber length composites.

Fatigue and impact response of gel-coated glass mats/polyester composites

Composites based on polyester matrices reinforced with fiber glass mats are widely used in the automotive industry, incorporating in many cases a gel coat surface layer. This paper presents the results of a current study in polyester/short fiber glass mat composites processed by hand lay-up moulding. Different lay-up architectures were tested under impact and fatigue bending loadings. Furthermore, the influence of the gel coat surface layer on fatigue and impact loading performances was also analysed. The composites tested with gel coat layer under compression loading exhibit significant higher bending stiffness, static and fatigue strength than when the gel coat layer was under tensile loading. The impact response showed a slight variation of the peak load and on the elastic recuperation and a significant increase on maximum displacement and absorbed energy with the increase of the incident impact energy. Both energy recovery, in percentage of the incident impact energy, and residual bending strength showed a marked and almost linear decreasing with the increase of the impact energy for all loading conditions, while the absorbed energy increased with the incident impact energy, reaching about 80% of impact energy for the highest impact energies