Fatigue and monotonic loading crack nucleation and propagation in bimodal grain size aluminum alloy

Abstract

Bimodal and nanocrystalline (NC), or ultra-fine-grain (UFG) aluminum alloys are being investigated as stronger replacements for conventional polycrystalline aluminum alloys. Higher strengths are achieved by reducing the grain size of a metal; however, as the grain size is reduced the ductility diminishes. One solution that limits this decrease in ductility is the addition of a percentage of microcrystalline or coarse grains (CGs) into a nanocrystalline alloy, creating a bimodal microstructure which offers a better balance of strength and ductility. Two- and three-dimensional microstructural finite element (FE) simulations of monotonic and fatigue failures in Al 5083 having bimodal grain structures are conducted. To reduce the computational time and facilitate the modeling of microstructural features, a global–local model is developed. Macroscopic linear-elastic and nonlinear plastic properties for each of the bimodal compositions are first used to simulate the tensile and fatigue tests in a global FE model. Subsequently, a local model that represents a single element at the center of the global model is built with distinct CGs distributed throughout an UFG matrix. 10% of the elements in this model are defined as CGs, after which NC and polycrystalline properties are assigned to the UFG and CG regions, respectively. Available fatigue test data are utilized to generate a low cycle fatigue damage model for bimodal grains size Al 5083 and obtain the damage model constants for varied levels of coarse grains. This fatigue damage model is then used in conjunction with a finite element continuum damage modeling approach, namely, successive initiation, to predict the damage and crack initiation sites and propagation paths in bimodal alloys. The successive initiation method is used to continually accumulate damage in elements and initiate and propagate the crack through grains that reach the failure criteria defined for monotonic and cyclic loading. It is observed from the monotonic FE model that using ultimate stress as the failure criteria, cracks initiate on the boundaries between CGs and UFGs, and propagate through the UFG matrix around the CG until they become large enough to extend all the way through the UFG region. In the cyclic FE models, the crack is observed to initiate in a CG and propagate along the CG and the surrounding UFG matrix until it is large enough to cause failure.