The paper focuses on the analysis of the eccentricity effect in the measurement of the hole-drilling residual stress. Relaxed strains were evaluated by computational simulation of the hole-drilling experiment using the finite element method. Errors induced by eccentricity were estimated for elastic and elastic-plastic states in area around the drilled hole due to the stress concentration. The invariance of the stress change with depth was assumed. The correction of eccentricity and plasticity effects in evaluation of residual stresses was realized within the EVAL 7 software (SINT Technology). The analysis shows that in elastic state the eccentricity and angular position of the drilled hole have a significant effect on relative residual stress errors. Correction according to the HDM method is very effective in this case. If the relative error of 5 % is allowed, which is in engineering practice acceptable, eccentricity of ±0.05 mm could be accepted without correction. When the combination of eccentricity and plasticity occurs, the correction of plasticity is more important in method 13-EXT-UN.
One of the most popular and widely used technique for measuring residual stresses is the hole-drilling method. By this method, it is possible to evaluate only biaxial residual stresses located in plane parallel to the surface, but some processes produce a triaxial stress state. For this reason, the evaluation of triaxial stress state by the method used for biaxial state was assessed in this paper. A hole-drilling experiment was simulated by the finite element method for two different stress states. The first stress state considered constant residual stresses in all directions. The second one considered constant residual stresses in a plane parallel to the surface and the residual stress in a direction perpendicular to the surface was equal to zero on the surface and increased with depth. Both states were simulated for various ratios of stress in a direction perpendicular to the stresses in plane. The obtained results show that residual stress in a perpendicular direction affects the evaluation of residual stresses in plane. If the residual stress in the perpendicular direction is high compared to stresses in plane, the error produced by the evaluation of triaxial stress state by the method for biaxial stress state can also be high.
1 Introduction
Background
The hole-drilling method is a well-known and widely used technique for the determination of residual stresses, but is limited to materials with linear elastic behaviour. This can be a problem when high residual stresses are measured, since a local yielding can occur due to stress concentration around the drilled hole.
Objective
If the residual stress exceeds about 80% of the material yield stress, the error caused by the plasticity effect becomes significant. In order to correctly evaluate high uniform residual stresses, a universal procedure for a correction of the plasticity effect is introduced.
Methods
The procedure uses a neural network and is capable of correcting any combination of uniform residual stresses with magnitudes up to the material yield stress. It also covers a wide range of material parameters, hole diameters, and strain gauge rosettes and it is independent of the orientation of the strain gauge rosette.
Results
The correction procedure was tested by more than a million randomly generated stress states that covered the entire range of input parameters and performs remarkably well, since the error of the corrected residual stresses is negligible even for the states with residual stress magnitudes equal to the material yield stress.
Conclusions
The proposed correction extends the application range of the hole-drilling method to high uniform residual stresses and therefore can be very useful for practical measurements.
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