Novel method for measuring surface residual stress using flat-ended cylindrical indentation

Instrumented indentation is a promising technique for estimating surface residual stresses and mechanical properties in engineering components.The relative difference between the indentation loads for unstressed and stressed specimens was selected as the key parameter for measuring surface residual stresses in flat-ended cylindrical indentations.Based on the equivalent material method and finite element simulations,a dimensionless mapping model with six constants was established between the relative load difference,constitutive model parameters,and normalized residual stress.A novel method for measuring the surface residual stress and constitutive model parameters of metallic material through flat-ended cylindrical indentations was proposed using this model and a mechanical properties determination method.Numerical simulations were conducted using numerous elastoplastic materials with different residual stresses to verify the proposed model;good agreements were observed between the predicted residual stresses and those previously applied in finite element analysis.Flat-ended cylindrical indentation tests were performed on four metallic materials using cruciform specimens subjected to various equibiaxial stresses.The results exhibited good conformance between the stress–strain curves obtained using the proposed method and those from traditional tensile tests,and the absolute differences between the predicted residual stresses and applied stresses were within 40 MPa in most cases.

Micromechanical characterization of microwave dielectric ceramic BaO-Sm_(2)O_(3)-5TiO_(2),by indentation and scratch methods

Mechanical characterization of dielectric ceramics,which have drawn extensive attention in wireless communication,remains challenging.The micromechanical properties with the microstructures of dielectric ceramic BaO-Sm_(2)O_(3)-5TiO_(2)(BST)were assessed by nanoindentation,microhardness,and microscratch tests under different indenters,along with the X-ray diffraction(XRD),scanning electron microscopy(SEM),and Raman spectroscopy.Accurate determination of elastic modulus(Err)(i.e.,260 GPa)and indentation hardness(Hrr)(i.e.,16.2 GPa)of brittle BST ceramic by the instrumented indentation technique requires low loads with little indentation-induced damage.The elastic modulus and indentation hardness were analyzed by different methodologies such as energy-based approach,displacement-based approach,and elastic recovery of Knoop imprint.Consistent values(about 3.1 MPa·m^(1/2))of fracture toughness(Kc)of BST ceramic were obtained by different methods such as the Vickers indenter-induced cracking method,energy-based nanoindentation approaches,and linear elastic fracture mechanics(LEFM)-based scratch approach with a spherical indenter,demonstrating successful applications of indentation and scratch methods in characterizing fracture properties of brittle solids.The deterioration of elastic modulus or indentation hardness with the increase in indentation load(F)is caused by indentation-induced damage and can be used to determine the fracture toughness of material by energy-based nanoindentation approaches,and the critical void volume fraction(f^(*))is 0.27(or 0.18)if elastic modulus(or indentation hardness)of the brittle BST ceramic is used.The fracture work at the critical load corresponding to the initial decrease in elastic modulus or indentation hardness can also be used to assess the fracture toughness of brittle solids,opening new venues of the application of nanoindentation test as a means to characterize the fracture toughness of brittle ceramics.