Post-test impression creep evaluation methods and findings for improved code of practice

The impression creep test is a small sample testing technique that has been used mostly for qualitative assessments intended for risk ranking of in-service components. Electric Power Research Institute (EPRI) has conducted testing to determine the limitations of the test method. This involved extensive analytical and physical posttest assessments that have led to recommendations to improve the current code of practice. Analytical assessments for determination of minimum strain rates suggest that having a standard procedure would be very beneficial to the testing community. Physical assessments that involve indentation depth measurements, hardness mapping and grain structure evaluations have provided methods for determining the affected area of deformation under the indentation surface. These methods may help in validating and revising current FEA models. Additional FEA modeling may also help answer further questions regarding applicable indentation range and appropriate testing conditions (stresses and temperatures). EPRI will continue to conduct impression creep testing to further evaluate the limitations of the test method and address questions from the work discussed in this paper.