Crack growth and residual stress in Al-Li metal matrix composites under far-field cyclic compression

Abstract

The growth of cracks under far-field cyclic compressive loading in aluminium-lithium (Al-Li) alloys reinforced with SiC particulates is investigated in notched compact tension specimens (CT). When cracks were initiated from the root of the notch, progressive deceleration occurred with the initial crack growth being largest. After crack arrest, analysis indicated that the initial residual stress diminished as the crack became non-propagating and at arrest the crack faces appeared to be open. When the crack closure loads were determined, it was shown that not all the stress amplitude produced crack growth and opening. This effect of crack closure was enhanced for small stress fields when the effective stress intensity dropped to the fatigue threshold of the alloy. For large residual stress fields the effective stress intensity range was well above the threshold and the initial crack growth rates were largest in the alloy containing the reinforcement particles. A residual strain model was used to determine the residual stress introduced in the root of the notch from the first compressive preload. It is shown that the fatigue crack growth was confined to a region of tensile stress within the residual stress field and the initial crack propagation rates were enhanced by the presence of the reinforcement. A dependence of the stress magnitude on growth rates was also established — the greater the residual stress at the root of the notch the larger the growth rates. The reinforcement had an additional “amplification” effect in terms of tensile distance from the notch. The effective stress intensity range, ΔK, was investigated using compliance measurements and a model is introduced which explains the underlying features and mechanism of accelerated growth in both alloys, taking into account the reinforcement phase, plastic zone-size dependence and the residual stress field of the MMC.