An experimental technique to evaluate free-edge residual stresses in layered material systems

An experimental technique that can accurately determine residual stresses on the free edge of a layered material has been developed. The method utilizes moiré interferometry and material removal principles to obtain the strain along the free edge through the thickness of the material. The technique is referred to as the Material Removal for Free-Edge Evaluation (MRFEE). The equations that define the MRFEE method are derived from the constitutive relationship. Using sample sectioning and layer removal, the boundary conditions along the free edge of the layered material system are modified resulting in a stress-free point. The stresses that were originally at this point are released, resulting in displacements detectable by moiré interferometry. The measured strains are used in the MRFEE equations to determine the original stress state in the material. The demonstration of the method is performed on a sample of 2024-T3 aluminum with a layer of AF-191 U epoxy film adhesive. Analytical predictions are made using the B-Spline Analysis Method (BSAM). At the stress free point an anomaly is present that masks the experimental displacement of the material. The cause of the anomaly is the moiré diffraction grating, Which has comparable material properties to that of the AF-191-U, resulting in material stiffening and shear lag of the displacements. Through the use of analytical predictions and extrapolations of experimental data, the issue of the anomaly is resolved and the MRFEE method is successfully demonstrated. The development and demonstration of the MRFEE method is a significant contribution to the field of residual stress assessment.