چكيده به لاتين
In composite materials, microscopic residual stresses are induced by mismatch of mechanical and physical properties of fiber and matrix. These stresses may affect the sample shape and performance. These stresses also increase the probability of crack initiation. That is why residual stress measurement is vital for composites and polymeric parts. In this research, a new non-destructive and non-contact method called zone heating method is introduced for residual stress measurement in composites and polymeric parts. In zone heating method, a small spot (2 to 4 mm diameter) of sample is heated above its glass transition temperature. A laser beam was applied for this task. The temperature of heated spot is controlled using an infrared thermometer. After this step, a dramatic stiffness reduction occurs in the spot. This stiffness reduction results in a stress relief. Strains due to this relief are measured using a simplified digital image correlation (DIC) method. Finally these strains are related to residual stresses by means of a close form solution and a numerical simulation.
In this research at the first step, a known load was applied to a polymeric sample. Moreover, the zone heating and the DIC methods were used to measure the stress induced in the sample. Then measured strains were used to compute the residual stress by both analytical and numerical techniques. In analytical method, thermal properties of polymer are neglected, so it has less accuracy (16.5% in this case). Numerical method involves thermal properties of polymer so; it is amore accurate method (2% in this case).
In the next step, the possibility of using this technique on polymeric composites is discussed. Numerical simulation of applying this method to composites proves that if the temperature is controlled carefully and the camera has enough resolution, then this technique is capable of measurement of residual stresses. As a sample in this section, a diagram was obtained which relates the microscopic residual stress to measured strain from the zone heating technique.
