Investigation of the time elapsed effect on residual stress measurement in a composite plate by DIC method

In composite plates, there are generally amounts of residual stresses that can have adverse effects in final parts. So, it is very important to know about the residual stress's type, location and amounts in composite plates. The measurement methods of the residual stress are commonly indirect and the displacement and strain is calculated at first. Two-dimensional digital image correlation (2D DIC) is a practical and effective tool for quantitative in-plane deformation measurement of a planar object surface and now widely accepted and commonly used in the field of experimental mechanics. In the hole drilling method, the residual stress is measured immediately just after drilling and the other researchers have paid less attention to the amount and pattern of changes in released stress around the hole after drilling at different times. It can be stated that the novelty of this research may identify the optimal measurement time after the drilling and the holes with different diameters is considered and the displacement and strain fields are measured at different times. So in this research, the residual stress measurement of a composite plate in the time elapsed is discussed. Also, the effects of hole's diameter and elapsed time after drilling in amount of released residual stresses are investigated by 2D DIC method. The pointwise local least-squares fitting technique (PLS) is used for strain estimation from displacement fields. The residual stress is calculated by the estimated strain, determination of calibration constants in orthotropic materials and considering the properties of composite plate that has already been calculated. The finite element method (FEM) is used for determination of calibration constants and the correction factor in FEM is obtained by comparing of the FEM and experimental tensile test results.