Extraction of residual stresses by instrumented indentation based on force-contact area (F-S) measurement

In the past decades, residual stresses have attracted wide attention due to their significant influences on material’s strength, fatigue life, and dimensional stability. Various residual stress measurement methods have been developed such as X-ray diffraction, neutron diffraction, crack compliance, and hole drilling. These methods may suffer from different disadvantages including radiation, high cost, destructive, unportable, etc. In this work, an in situ residual stress measurement method was proposed based on instrumented indentation using a piezoelectric bimorph cantilever. A Vickers’ indenter was fabricated onto the free end of the cantilever for pressing into the sample and a strain gauge was bonded on the cantilever to monitor the indentation load. During testing, the contact area was extracted by tracking the cantilever’s contact resonance frequency based on the electromechanical impendence method. Different from traditional indentation-based methods that use a single hardness value to compute the residual stress, here the indentation force-contact area (F-S) curves with and without residual stresses were measured to derive the residual stress based on an empirical model. Experiments were then conducted on a specially designed CrMnCu specimen with different applied stresses. Results show that the measured residual stress values agreed well with the applied stresses monitored by a strain gauge. The proposed residual stress measurement method holds great promise for in situ residual stress estimation due to its portable apparatus, simple operation procedure and insensitiveness to testing environment.