New Method for Fatigue Characterization via Cyclical Instrumented Indentation Testing

New materials and structures with complex microstructural distributions are being rapidly developed using advanced manufacturing approaches, such as Additive Manufacturing (AM). For example, it is possible to fabricate metals using AM processes, such as Direct Metal Laser Sintering (DMLS), with spatial variations at sub-millimeter scales in textured grain structures, residual porosity, and precipitate morphologies at sub-millimeter scales. This poses new challenges for characterizing the corresponding variation in material behavior, particularly fatigue, for which conventional techniques were not designed. Therefore, new techniques are needed to quickly and accurately quantify these spatial variations. To this end, a method was developed for locally characterizing the fatigue behavior of materials, particularly metals and metal-matrix composites, by extending indentation methods to fatigue characterization using cyclic instrumented testing to measure the localized stiffness, which degrades due to the accumulation of fatigue damage under the indenter. A high frequency electro mechanical load frame equipped with an indenter was used to characterize three different metals: 4340 steel, 9310 steel, and brass. The S-N curve was then generated by applying a linear transformation to convert the measured stiffness to an equivalent alternating stress. The results correlated well with those obtained using conventional fatigue characterization techniques, including capturing the endurance limits. Finite element analysis with fatigue damage modeling was also used to quantify the evolution of stresses near the indenter due to the fatigue damage, while optical microscopy provided validation of the observed accumulation of plastic deformation associated with cyclic indentation loading.