A Benchmark Study on Casting Residual Stress
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INCREASED demand due to regulatory conformance, such as Tier IV norms, is pushing iron casting designs toward optimization that will result in weight reduction, manufacturability advantages, and reliability improvements.[1] Castings need to be designed with closer tolerances by incorporating hitherto unknowns, such as residual stresses arising due to thermal gradients and phase and microstructural changes during solidification phenomenon. Residual stresses affect both the dimensional stability and the durability of a component.[2] Traditionally, the residual stresses were accounted for in the design through increased safety factors or conservative assumptions on the stress magnitude that reflected worst-case scenarios. Both of these approaches result in less than optimum designs. In critical designs, the residual stresses are often measured and incorporated in the durability assessment. The most common residual stress techniques measure stress/strain through relaxation of stresses by cutting or drilling or by measuring the strain in the interplanar spacing through diffraction and are briefly described as follows. Hole drilling:[3] This technique measures the strain relief after drilling a hole, which is surrounded by a strain gauge rosette. The technique is limited to the depth of the drill bit diameter and is not accurate for stresses above 50 pct of the yield strength of the E.M. JOHNSON, Staff Engineer, is with the Moline Technology Innovation Center–John Deere, Moline, IL 61265. T.R. WATKINS, Senior Research Staff Member, and J.E. SCHMIDLIN, Technical Staff Member, are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6040. Contact e-mail: [email protected] S.A. DUTLER, Client Development Manager, is with MAGMA Foundry Technologies, Inc., Schaumburg, IL 60173. Manuscript submitted February 15, 2011. Article published online October 8, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
material. Hole drilling is also not feasible in notches, which is typically where high residual stresses develop. Diffraction techniques:[4,5] These techniques use X-rays or neutrons to measure the interplanar spacing change/ difference of the stressed part relative to a stress-free reference. Laboratory X-ray sources are limited to surface measurements, and special etching techniques are needed to obtain the stresses through the thickness of the part. Both neutron and high-energy synchrotron X-ray diffraction have the ability to penetrate through the thickness and measure the internal stresses, but are not accessible for everyday design needs. Experimental measurement of residual stress in a casting through neutron or X-ray diffraction, sectioning or hole drilling, and magnetic, electric, or photoelastic measurements is a very difficult and time-consuming exercise. If validated, the use of computational methods in design, such as structural finite element (FE) analysis, provides an attractive alternative to experimental measurements. These methods made it possible to calculate the stress a component will see in use and have been used effe
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