Measurement of Highly Non-uniform Residual Stress Fields in Thin Plate Using a New Side Cut Destructive Method

A recently proposed Mechanical Strain Relaxation (MSR) technique for the measurement of residual stress in thin plates is presented. The measurement of residual stress involves making a single straight cut and collecting the relaxation displacement data from the side surface. In this work the method was applied to an Aluminium friction stir welded (FSW) specimen. The cut was introduced with a wire electrical discharge machining. The displacements were recorded with a 3D digital image correlation (DIC) method. The measured FSW residual stress profile agreed well with that measured by Energy Dispersive X-ray Diffraction (EDXRD). It was observed that the amount of plastic strain, caused by stress redistribution during the relaxation process, strongly depends on the direction of the propagation of the cut. In particular, if a cut is propagated along the thickness of a plate, then the effect of plastic flow on the measured residual stress is negligible. Another attractive feature of the method is that it is relatively insensitive to random experimental noise. Introduction Over the last two decades a wide range of mechanical strain relaxation (MSR) techniques have been developed and used to measure residual stresses in a wide variety of engineering components. Well known examples of MSR techniques are: hole drilling [1], slitting [2] and contour method [3]. Recently, new technique [4] has been proposed and it follows the general scheme of Mechanical Strain Relaxation (MSR) techniques, i.e. measuring the deformation due to removal of some stressed material in a body and then that measured relaxation is converted to stress. The main target of the proposed method is that the shape of in-plane residual stress field is highly non-uniform, including discontinuities. Other popular techniques, such as e.g. hole drilling, cannot be used in such cases because they assume stress uniformity within hole diameter [1] and many holes are required to measure residual stress field in relatively large measurement area, leading to complexity of the experiment [5]. During cutting residual stress is redistributed and this might cause plastic deformation where the magnitude of residual stress is close to yield [2]. However, it is shown that this plastic strain on relaxation can be mitigated by choosing the correct propagation of the cut [4]. For this is reason, the technique is attractive in practical application because the measurement can be conducted regardless of the propagation of the cut. It is not always possible for many techniques, for example the slitting method, the propagation of the cut is very important because non-uniform RS can only be measure along depth [2]. In this work, RS field in 3 mm thick FSW plates have been measured by the proposed method. The application is ideal for the technique because the specimen is very thin and the stress field is non-uniform. The techniques involves a single cut at the midsection of the longitudinal direction of the plate, propagated either from the front to the rear surface, called 'side cut', or from the top to bottom surface, called 'top cut' to observe the difference. 3D DIC were used to measure relaxation displacement on the half width of side surface. Then the residual stress field was reconstructed from the measured relaxation via analytical model. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 115-120 doi: http://dx.doi. org/10.21741/9781945291173-20 116 Analytical model The analytical model for the technique is based on the elastic solution for the problem of selfequilibrated loading at the one end of 2D semi-infinite strip of width 2c [6], see Fig. 1(a). This analytical model will be employed to reconstruct residual stress field in thin Friction stir welded (FSW) plate from the measured displacement fields on the side surface from 3D DIC, see Fig. 1(b). Other application of the elastic problem to the measurement of residual stress was investigated and described in [4]. Brief overview is given here. (a) (b) Fig. 1 : (a) The problem geometry for a 2D semi-infinite rectangular strip, of width 2c, with arbitrary self-equilibrated end load [4]. The boundary conditions are: ±c ∶ σ22 = σ12 = 0 and x1 = 0 ∶ ∫ σ22dx2 = c