Evaluation of Deformation & Fracture Properties of Metallic Materials Using Instrumented Flat Punch Indentation

Abstract

Structural integrity is the ability of a structure or a component to withstand a designed service load, resisting structural failure due to fracture, deformation, or fatigue. To construct an item with structural integrity, an engineer must first consider the mechanical properties of a material, such as strength, hardness, or fracture toughness. In many cases, structural failures arise from a change in mechanical properties of the material due to degradation or embrittlement. In such cases, structural integrity assessment requires measurement of in-situ mechanical properties of in-service structural components. Fracture toughness, defined as the resistance to crack propagation, is one of the most important mechanical properties in fracture mechanical analysis of structural integrity. But standard fracture toughness testing is, like conventional mechanical testing, destructive and requires many specimens with specified geometry, so that measuring in-situ fracture toughness on in-service structural components is almost impossible. For this reason, nondestructive ways to measure in-situ mechanical properties as well as fracture toughness are highly desirable in order to improve the reliability of structural integrity assessment. Instrumented indentation testing, developed for nondestructive testing of in-field structures can be considered a solution to this problem. Many researchers have worked to estimate fracture toughness of metallic materials using instrumented indentation testing, trying to develop theoretical or experimental models. But these studies have some drawbacks arising from the many assumptions and empirical correlations made as an inevitable consequence of extrapolating from non-cracking to cracking resistance, that is, fracture toughness. In this study, new fracture toughness models are developed by theoretical and practical approaches based on fracture mechanics and contact mechanics to estimate the fracture toughness of metallic materials. First, in order to match the stress state beneath an indenter with that ahead of a crack tip, a flat punch indenter is selected instead of the spherical indenter generally used in indentation techniques. Using the flat punch indenter lets us derive a crack-like stress concentration at the edge of the indenter tip. Second, from this, the modeling is conceived as deriving virtual fracture toughness from flat punch indentation, not as in conventional methods correlating indentation deformation energy with fracture energy. Finally, the specimen size requirement in the fracture toughness testing standard, which has not been considered in previous indentation fracture toughness models but is very important for the validity of fracture toughness value, is made to correspond with the indenter size adjustment in indentation testing. Two distinct indentation fracture toughness models, a ductile fracture model and a brittle fracture model, have been established. In the ductile fracture model, the crack initiation point is determined by limit load and the indenter size is adjusted to a geometrical relationship between the acceptable crack extension and corresponding indentation depth. In the brittle fracture model, the crack initiation point is determined by the onset of nonlinear behavior in the indentation curve and the indenter size is adjusted to the standard thickness of a fracture toughness specimen. To verify these models, experimental results are compared with standard J test results and the results are considered to match well if they are within 20% error range. In addition to fracture toughness, tensile properties are also evaluated using flat punch indentation, and the yield strength and strain-hardening exponent can be evaluated by a simpler approach than the strength algorithm for spherical indentation. Further studies are recommended for improvement of indentation fracture toughness models and application to evaluation of the ductile-brittle transition temperature and in-field testing.