Crack retardation mechanism due to overload in base material and laser welds of Al alloys

Abstract

In order to determine retardation mechanisms due to overload and to predict the subsequent evolution of crack growth rate, investigations are conducted on crack retardation due to single tensile overload in laser weld and base material of AA6056-T6 Al alloys sheets. The effect of such overloads with different load ratios on the fatigue crack propagation behavior of the homogenous base metal and welded C(T) 100 specimens was compared in terms of crack growth rate and fracture surface features using experimental and FE analysis methods. The retardation due to overload is described in term of affected regions ahead of the crack tip. The size and shape of the crack tip plastic zone and the damage profile induced during the application of the overload in base material are predicted by FE analysis in conjunction with a porous metal plasticity model. The results show that the mechanisms of retardation in undermatched welds are substantially different from that of homogenous base material. The more significant crack retardation due to overload has been observed in the laser weld of AA6056-T6. Based on SEM observations of fracture surfaces and damage profiles predicted by the proposed FE model, the shape of the crack front formed during the application of overload can be correlated. During the overload, the crack-front extends to a new shape which can be predicted by the ductile damage model; the higher the load the more curved the resulting crack-front. These outcomes are used to determine the dominated retardation mechanisms and the significance of retardation observed in each region ahead of crack tip and finally to define the minimum crack growth rate occurred after overload.

Fatigue Crack Growth Life Prediction of Screw Blade in Screw Sand Washing Machine based on Austen Growth Law

“…This algorithm includes recalculating the crack geometry's stress intensity factor for every cycle. In addition, crack retardation is taken into consideration in the calculation of crack growth [14,15].…”

Effect of temperature on the fracture behaviour of heat‐treated Al–Cu–Li alloy laser welds under low‐cycle fatigue loading

“…Daneshpour et al 32 have shown the effect of single and multiple tensile loads on the fatigue crack propagation in the 2139 aluminium alloy plates and their laser welds. Later, the same authors have investigated crack retardation mechanisms during the fatigue tests of the AA6056‐T6 alloy and its welds 33 . Liu et al 34,35 have focused on computer simulations of the fatigue crack growth behaviour in the laser welded butt joints and T‐joints of the 6156 aluminium alloy, as well as the model verification by the experimental results.…”

Experimental investigation and numerical prediction of fatigue crack growth of 2024-T4 aluminum alloy

“…However, some other materials exhibit an insignificant R-ratio effect on the crack growth, which is attributed to the fast mean stress relaxation of this kind of material under asymmetric cyclic loading [9][10][11]. When a single tensile overload was introduced in the fatigue crack growth experiments under constant amplitude loading, the delayed retardation or even full arrest of the subsequent crack growth was observed for a positive R-ratio [12][13][14][15][16][17]. The retardation degree of subsequent crack growth is determined by overloading ratio, which is defined as the overload magnitude over the maximum value of constant amplitude loading.…”

“…The retardation degree of subsequent crack growth is determined by overloading ratio, which is defined as the overload magnitude over the maximum value of constant amplitude loading. Right after the application of the tensile overload, many materials [7,9,[15][16][17] display a short-lived acceleration of crack growth, which was attributed by Bichler [17] to an additional damage in front of the crack tip or the formation of overload plastic zone. For the high-low sequence loading, a retardation phenomenon similar to the overloading case was found when the two successive loading steps have the same R-ratio; however, if the maximum load is identical, the prior higher loading step has an insignificant influence on the subsequent crack growth rate [2,3,9].…”