Evaluation of Impact Property through Analysis of Dynamic Deformation during Indentation Impact Test of Metallic Materials

Abstract

The safety and reliability assessment of structural components is important because seemingly minor flaws can cause extensive damage to or total failure of the system. Damage initiated by small breakages in huge systems such as heavy industries, power plants or nuclear facilities may occasion large economic costs and also hazard to human life. In particular, extreme service conditions for structural components can reduce performance and lessen lifetime. Thus, understanding the performance of structural components under such conditions is of major importance in assessing total system service. However, measuring the properties of in-use materials is not easy because conventional mechanical tests are difficult to apply in the field. In = nuclear power plants, the primary mechanical fracture performance for safety assessment is generally defined as Charpy V-notch impact testing (CVN testing). However, CVN testing of in-field structural components is severely limited because the CVN testing process includes fracture of the specimen. Thus, instrumented indentation testing (IIT) can be a reasonable method for measuring mechanical properties of in-field facilities because it requires neither specific sample dimensions nor large samples. Also, it offers the advantage that measuring localized properties can make it easier to find weak points by property mapping. Because of these advantages, many attempts have been made to assess the fracture performance of structural components through indentation testing. However, correlating the generally used fracture performance in CVN tests with indentation properties is difficult because indentation results are defined from static deformation behavior while the CVN test is based on dynamic deformation behavior. Additional strain rate theories must be considered to overcome these limitations. Here a solution is suggested: a technique for indentation testing under dynamic conditions called indentation impact testing. The present study focuses on absorbed energy because CVN test results are given in energy terms. Determination of the absorbed indentation impact energy corresponding to CVN energy is proposed based on similarities in geometry, stress distribution, and crack history in CVN testing and indentation impact testing. And the corresponding indentation impact energy is interpreted in terms of stages of indentation crack formation (flaw initiation, crack enlargement, crack propagation) using strain rate criteria that are estimated from the change in plastic zone expansion rate and definition of the indentation strain rate. The values of indentation impact energy were estimated for more than 14 different metallic materials and compared with the values from standard Charpy V-notch impact tests. The results showed good agreement between the two methods (within ±20% error range) and displayed very similar tendency. These results showed that indentation impact testing is effective in inspection of in-service structural components to monitor the CVN energy variation in materials due to degradation. Also, this test can be suggested an alternative method to Charpy V-notch impact testing when the components are restricted in various ways, e.g., small-size specimen or limited region available for testing.