Nondestructive Evaluation of Metal Strength, Toughness, and Ductility Through Frictional Sliding

Abstract

Traditional assessment of mechanical properties requires the removal of a standardized specimen for destructive laboratory testing. A nondestructive in-situ method is a cost-effective and efficient solution for applications where sample cutouts are not feasible. This work describes developments in contact mechanics that use frictional sliding to evaluate the material strength and toughness of steel pressure vessels and pipelines. Hardness, Strength, and Ductility (HSD) testing is a portable implementation of frictional sliding that provides a tensile stress-strain curve for assessment of the yield, ultimate tensile strength (UTS), and strain hardening exponent for power-law hardening metals. HSD testing incorporates four styluses of different geometry that generate grooves on the surface of a material as they travel. The measured geometry of these grooves along with the normal reaction forces on the stylus are correlated to representative tensile stress-strain values through finite element analysis (FEA) simulations. These principles have been extended to account for nonlinear strength behavior through the thickness of seam-welded steel pipes by using a combination of the HSD surface measurement, microstructure grain size, and chemistry. Frictional sliding tests are also used to assess material variation across a welded seam to identify different welding processes and the effectiveness of post-weld-heat-treatments (PWHT). A second implementation of frictional sliding is Nondestructive Toughness Testing (NDTT), which provides an NDE solution for measuring fracture toughness by using a wedge-shaped stylus with an internal stretch passage to generate a Mode I tensile loading condition on the surface of a sample. The test produces a raised fractured surface whose height provides an indication of the materials ability to stretch near a propagating crack and is correlated to the crack-tip-opening-displacement (CTOD) measured from traditional laboratory toughness testing. Experiments on pipeline steel indicate that NDTT can provide an index of fracture toughness to benchmark materials tested under similar conditions. Implementation of these new instruments to gather data for integrity management programs, fitness for service assessments, and quality control of new manufacturing will help to reduce risk and uncertainty in structural applications.