Fatigue crack growth resistance and fracture toughness of selectively reinforced aluminium alloys

• Main Author: Milan, Marcelo Tadeu
• Published: University of Birmingham 2002
• Subjects: 620; Material degradation & corrosion & fracture mechanics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.251924

The main aim of this work was to investigate the fatigue crack growth resistance and fracture toughness of selectively reinforced Al alloys. In such bimaterials, the crack growth resistance is affected by the failure mechanism, the direction of crack approach to the interface and by the conflict between the elastic-plastic mismatch and residual stresses. When the crack approaches the interface from the composite side, in the A12124 based bimaterials, the fatigue crack growth rate is reduced below "composite only" values by the compressive residual stress, although the elastic-plastic mismatch was expected to cause the opposite effect. In the A16061 based bimaterials, although some crack deceleration is also observed, fatigue crack growth rates are above the "composite only" values presumably because these bimaterials have lower compressive residual stress and higher plastic mismatch near the interface. After crossing the interface, the crack driving force is affected by closure mechanisms developed on the composite side of the crack wake. Conversely, when the crack grows from the Al alloy side, for both A12124 and A16061 based bimaterials, the crack growth rate is mainly reduced by the elastic-plastic mismatch. After crossing the interface, the crack driving force is well described by the thermal residual stresses, unless a crack tip deflection reduces the Mode I near tip stresses. In a fracture toughness test, when the pre-crack tip is in the composite side of the A12124 based bimaterials, KQ(5%) values are increased above "composite only" values presumably due to the compressive residual stresses and despite the amplification of the crack driving force from the elastic-plastic mismatch. In the A16061 based bimaterials, due to the higher plastic mismatch and lower compressive residual stresses, KQ(5%) values are below "composite only" values. Additionally, for all bimaterials, KQ(5%) values increase if the pre-crack tip is closer to the interface. When the crack propagates, it extends to the interface, bifurcates and arrests. The load then had to be increased to promote the onset of plastic collapse. The crack tip blunting and deflection mechanism increases the toughness attained at the onset of plastic collapse of the bimaterials above "Al alloy only" values. Conversely, if the pre-crack tip is in the Al alloy side, the final failure is deduced to occur when damage accumulated on the composite side links to the pre-crack tip. When the pre-crack tip is at 2.0mm from the interface, for the A12124 based bimaterials, KQ(5%) values are in general lower than the "A12124 only" value due to the tensile residual stresses. For the A16061 based bimaterials, KQ(5%) values are as high as the "A16061 only" value presumably due to the higher plastic mismatch and lower tensile residual stress of such bimaterials. Additionally, for all bimaterials, KQ(5%) values increase if the precrack tip is at 0.5mm from the interface. If the pre-crack tip is at 2.0mm from the interface, Kerit and Scrit values of the bimaterial are higher than the "Al alloy only" value and this is deduced to be due to the increase in the elastic-plastic mismatch shielding and by delayed critical particle damage within the composite side. At 0.5mm from the interface, Keritt and Scrit values are reduced and this is deduced to be because both the near tip tensile residual stress is higher and critical particle damage occurs earlier on the composite side; moreover, the unreinforced Al alloy layer is thinner and the damage on the composite side is deduced to link more easily to the pre-crack tip. For a constant particle size, there is an optimum particle volume fraction in which both Kerit and Scrit values are maximised with respect to a specific pre-crack tip position.