Evaluation of Residual Stress Using Instrumented Wedge Indentation : Stress Directionality and Magnitude

Abstract

Safety and reliability assessments are of vital importance in preventing accident because the failure of structures induces not only extensive damage but also loss of life, and requires huge effort and expense for recovery. In order to prevent unexpected accidents or failures, the stress states in structures must be considered. In particular, residual stress is defined as a stress state that exists in bulk materials or components without external load or other sources of stress. Residual stress arises in materials in almost every processing and manufacturing procedure. When residual stress is combined with external applied stress, some structure can fail at stresses beneath the yield strength of homogeneous and bulk material. In addition, residual stresses are detrimental to the performance and reliability of in-service structures. For these reasons, the quantitative assessment of residual stress is fundamental for the safe use and economical maintenance of industrial structures and facilities. Instrumented indentation testing (IIT) was developed to measure mechanical properties by analyzing the indentation load-depth curve. Over the last several decades, IIT has been extended beyond hardness and elastic modulus to methodologies evaluating for tensile properties, fracture toughness, fatigue characteristics, impact properties, interface adhesion and residual stress. IIT evaluates residual stress by looking at the difference in the indentation load-depth curve for the stress-free and stressed states. Previous research has evaluated the average surface residual stress using a Vickers indenter, and has also obtained information on the principal direction and stress ratio using a two-fold symmetric indenter, for example, Knoop indenter. As it can be necessary (as in testing curved pipes, or narrow welding regions) to evaluate nonequibiaxial residual stress within a small indent area, here we suggest a novel way to evaluate the directionality of the residual stress, p, using a wedge indenter characterized by two parameters, edge length and inclined angle. The present work describes a new wedge indentation model for evaluating surface nonequibiaxial residual stresses without change in indenter. We develop a wedge-indentation-mechanics model based on predetermined conversion factors determined by IITs for various uniaxial stressed states combined with finite element analysis (FEA) simulations. With this new model with pre-information on principal direction, two wedge indentation tests with respect to principal directions are required. On the other hand, without information on principal directions, four wedge indentations at intervals 45 degrees from some randomly chosen direction are needed. Principal directions and stress ratio are evaluated with two sets of load difference ratios at 90-degree intervals and a predetermined conversion factor ratio. The sum of the surface residual stress is obtained from the sum of load difference directionality with 90- degree intervals and the sum of conversion factors. To verify the suggested wedge indentation model, indentation tests were performed on 15 combinations of cruciform specimens, applied stress and various principal directions using stress-generating jigs. Additionally, the biaxial residual stress as evaluated using the new model are compared with values from the Vickers indentation model.