Prediction of the Fatigue Life of Cast Steel Containing Shrinkage Porosity

Abstract

A simulation methodology for predicting the fatigue life of cast steel components with shrinkage porosity is developed and validated through comparison with previously performed measurements. A X-ray tomography technique is used to reconstruct the porosity distribution in 25 test specimens with average porosities ranging from 8 to 21 pct. The porosity field is imported into finite element analysis (FEA) software to determine the complex stress field resulting from the porosity. In the stress simulation, the elastic mechanical properties are made a function of the local porosity volume fraction. A multiaxial strain-life simulation is then performed to determine the fatigue life. An adaptive subgrid model is developed to reduce the dependence of the fatigue life predictions on the numerical mesh chosen and to account for the effects of porosity that is too small to be resolved in the simulations. The subgrid model employs a spatially variable fatigue notch factor that is dependent on the local pore radius relative to the finite element node spacing. A probabilistic pore size distribution model is used to estimate the radius of the largest pore as a function of the local pore volume fraction. It is found that, with the adaptive subgrid model and the addition of a uniform background microporosity field with a maximum pore radius of 100 lm, the measured and predicted fatigue lives for nearly all 25 test specimens fall within one decade. Because the fatigue lives of the specimens vary by more than four orders of magnitude for the same nominal stress amplitude and for similar average porosity fractions, the results demonstrate the importance of taking into account in the simulations the distribution of the porosity in the specimens.

Effect of Porosity on Deformation, Damage, and Fracture of Cast Steel

“…Equation [2] was determined from comparing, and achieving excellent agreement between, measured and predicted strains for cylindrical test specimens having a wide range of porosity. [2] Equation [3] was determined through simulations of computer generated representative elemental volumes of porous structures. [20] The effect of porosity on stiffness [2] was studied using locally mapped porosity at FEA nodes determined from radiography.…”

“…In previous studies, the effects of large amounts of porosity on stiffness and fatigue life were investigated. [2,3] Porosity less than a few percent does not result in a measurable loss of stiffness, or large stress concentrations, or stress redistribution, but it greatly affects fatigue resistance. [4,5] Also, the presence of low-level porosity will reduce the ductility of metals since microvoids pre-exist before any stress is applied and the nucleation stage is bypassed.…”

Study of the Effect of Shrinkage Porosity on Strength Low Carbon Cast Steel

“…Hardin and Beckermann in [7,8] proposed the methodology to predict the fatigue life of the components made of the cast steel with the shrinkage porosity. They checked data from X-ray tomography measurements of porosity in the castings.…”

Empirical formulation of stress concentration factor around an arbitrary-sized spherical dual-cavity system and its application to aluminum die castings

“…It is believed that a region of high stress concentration is created around certain gas pores, which accelerates the fatigue crack initiation process and reduces the fatigue lives of such components. Some researchers have used X-ray computed tomography (CT) to visualize and track the fatigue crack initiating gas pores [3] and to quantify the stress concentrations around those pores by image-based finite element methods [4,5]. While it is possible to identify which gas pore has caused fatigue crack initiation by comparing X-ray CT images from before and after the fatigue tests, it is not possible to predict the vulnerability of such gas pores without conducting fatigue tests.…”