Numerical approaches and experimental verification of the conical indentation techniques for residual stress evaluation
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Conical indentation methods to determine residual stress are proposed by examining the finite element solutions based on the incremental plasticity theory. We first note that hardness depends on the magnitude and sign of residual stress and material properties and can change by up to 20% over a specific range of elastic tensile and compressive residual stress, although some prior indentation studies reported that hardness is hardly affected by residual stress. By analyzing the characteristics of conical indentation, we then select some normalized indentation parameters, which are free from the effect of indenter tip rounding. Adopting dimensional analysis, we present practical conical indentation methods for the evaluation of elastic/plastic equi- and nonequi-biaxial residual stresses. The validity of developed approaches is confirmed by applying them to the experimental evaluation of four-point bending stress.
I. INTRODUCTION
Residual stresses are formed by diverse processes. The residual stresses in materials affect the behavior of materials, including fatigue, fracture, corrosion, abrasion, and friction. For this reason, various experimental measuring techniques have been developed, e.g., neutron and x-ray tilt techniques, strain/curvature measurements, beam bending, hole drilling, layer removal, and chemical etching.1,2 Each of these methods, however, has a shortcoming with respect to accuracy, sensitivity, resolution, cost, specimen preparation, material type, and geometry of structure. An indentation test is another method that can evaluate residual stresses. It is nondestructive and easy to use; moreover, it can be applied to small specimens and parts in present structural use. To evaluate residual stresses using an indentation test, it is necessary to secure the indentation data at the non-residual state. In the first stage, attention was focused on the variation of hardness with the direction and magnitude of residual stresses. Tsui et al.3 and Bolshakov et al.4 investigated the effects of residual stresses on hardness, contact area, and elastic modulus using experimental work and finite element analyses (FEA). They showed that residual stress was not related to material hardness but closely to the pileup of material. On the assumption that material hardness is independent of triaxial stress, Suresh and Giannakopoulos (SG)1 suggested a novel methodola)
Address all correspondence to this author. e-mail: [email protected] Present address: Korea Atomic Energy Research Institute, Yuseong-gu, Daejeon 305-353, Republic of Korea. DOI: 10.1557/JMR.2010.0275
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J. Mater. Res., Vol. 25, No. 11, Nov 2010 Downloaded: 04 Apr 2015
ogy to determine surface equi-biaxial residual stress with sharp indentation, invoking the invariance of contact pressure (or hardness). For a given residual stress sR, they assumed the following relation, Pomax ¼ Pmax þ sR fc A ;
ð1Þ
where Pmax and Pomax are the maximum loads at the same indentation depth hmax with and without residual stress, resp
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