Materials Science Forum Vols. 768-769

Abstract: A conical slit cell for depth-resolved diffraction of high-energy X-rays was used for residual stress analysis at the high-energy materials science synchrotron beamline HEMS at PETRA III. With a conical slit width of 20 µm and beam cross-sections of 50 µm, a spatial resolution in beam direction of 0.8 mm was achieved. The setup was used for residual stress analysis in a drawn steel wire with 8.3 mm diameter. The residual stress results were in very good agreement with results of a FE simulation.

Abstract: The incremental hole-drilling method is a well-known mechanical measurement procedure for the analysis of residual stresses. The newly developed PRISM® technology by Stresstech Group measures stress relaxation optically using electronic speckle pattern interferometry (ESPI). In case of autofrettaged components, the large amount of compressive residual stresses and the radius of the pressurized bores can be challenging for the measurement system. This research discusses the applicability of the measurement principle for autofrettaged cylinders made of steel AISI 4140. The residual stresses are measured after AF and after subsequent boring and reaming. The experimental residual stress depth profiles are compared to numerically acquired results from a finite element analysis (FEA) with the software code ABAQUS. Sample preparation will be considered as the parts have to be sectioned in half in order to access the measurement position. Following this, the influence of the boring and reaming operation on the final residual stress distribution as well as the accuracy of the presented measurement setup will be discussed. Finally, the usability of the FEA method in early design stages is discussed in order to predict the final residual stress distribution after AF and a following post-machining operation.

Abstract: The purpose of this paper is to demonstrate a) application of the contour method in measuring residual stresses in large complex geometries such as a welded pipe, b) the power of combining the contour measurement approach with other measurement methods in order to map multiple components of stress tensor in structures, c) the capability of the contour method for characterising residual stresses where there are steep gradients of stress, for example in an electron beam welded plate, and d) mapping multiple components of residual stress in thick structures using multiple cuts.

Abstract: Quantitative residual stress depth profile measurements are common in metallic parts but not in glass or plastic. This paper describes some experimental stress depth profile measurements using hole-drilling with electronic speckle pattern interferometry (ESPI) in two types of glasses and two thermoplastics. Stress depth profiles in laminated and toughened glass specimens show the expected low stresses in the former and significant compressive stresses near the surface in the latter for the as-is condition. The stress curves shift towards tensile stresses during slight bending deformation, as expected. The bending devices initially used for experiments with Bayblend® and Makrolon® were deforming the specimens too severely to allow proper, static, measurements. Significant stress reductions due to creep were measured for extended bending times. Subsequent measurements for less severe bending show similar stress curve characteristics. Residual stresses in the as-is condition apparently account for some of the differences to the predicted bending stress profile.

Abstract: The paper presents a method for measuring residual stresses in normal thermo-bimetal Fe-Ni-Mn/Invar strips with a thickness of 0.76 mm. For this purpose, a setup was designed which permits to remove layers from a strip substrate by electrochemical etching. Residual stresses in the directions that are longitudinal and transversal to rolling are determined by the curvature method based on the layer growing/removing techniques. As a reference, residual stresses were also determined by hole-drilling technique. Tensile and compressive residual stresses arose both in active and in passive layers and were considerably higher when determined by the hole-drilling technique.

Abstract: Residual Stress measurement has gained interests among researchers for many years. Slitting method is one of the destructive techniques that relies on the introduction of an increasing cut to a part containing residual stresses. Similar to all other mechanical strain relief techniques, slitting suffers from the level of plasticity occurs within the sample while cutting. In the present research, slitting method was simulated using finite element analysis. The correct performance of the slitting method procedure was explored using known residual stress fields. Then, simulations of the quenching process of beam samples were performed for three different temperature levels; 400°C, 600°C and 850°C. The experimental procedures of the slitting method on the quenched samples at these temperatures were then carried out. The influence of three stress levels on the ability of the slitting method was discussed. Interesting results were observed.

Abstract: In the present study an Artificial Neural Network (ANN) approach is proposed for residual stresses estimation in engineering components using indentation technique. First of all, load-penetration curves of indentation tests for tensile and compressive residual stresses are studied using Finite Element Method (FEM) for materials with different yield stresses and work-hardening exponents. Then, experimental tests are carried out on samples made of 316L steel without residual stresses. In the next step, multi-layer feed forward ANNs are created and trained based on 80% of obtained numerical data using Back-Error Propagation (BEP) algorithm. Then the trained ANNs are tested against the remaining data. The obtained results show that the predicted residual stresses are in good agreement with the actual data.

Abstract: The incremental hole-drilling method is the method of choice to determine residual stress depth distributions with limited costs and minor destruction of the investigated component. With a spatial resolution of commonly two millimeters in diameter and one millimeter in depth especially the effects of frequently used surface treatments like e.g. shot peening or deep rolling can be reliably detected if the in depth residual stress gradients are relatively smooth. Nevertheless up to now the quantitative accuracy of the method is poor for residual stress analyses close to the materials surface up to depths of approximately 0.2 mm and in the case of steep in-depth residual stress gradients or oscillating residual stress depth distributions. In this paper, residual stress depth distributions of a broad range introduced by mechanical surface-treatments in flat specimens were analyzed with the hole-drilling method and compared with the results measured by X-ray diffraction as the reference. It comes out, that arbitrary residual stress depth distributions can be successfully determined with a modified differential evaluation formalism. For this purpose, often neglected well known weak points of the hole-drilling method were considered and improved, e.g. hole geometry, numerical calibration and data conditioning. Especially, the proposed strategy of data conditioning results in an almost user-independent evaluation formalism.

Abstract: A hybrid experimental-numerical methodology is applied to evaluate the unwanted stresses induced by hole-drilling in two 5000 and 7000 series aluminium alloys. The influence of the cutting speed of ultra-high speed drills powered by turbine systems by compressed air, which are commonly used in the hole-drilling equipments for residual stress measurements, is analyzed. The comparison of the effect of different drilling conditions on the drilled material using a quantitative approach is now possible. The applied methodology can play an important role on the improvement and optimization of the hole-drilling technique for residual stress measurements in particular and the drilling process in general.

Abstract: Thick film systems with coating thicknesses between 50 and 1000 µm are often fabricated by thermal spray processes. During the deposition and due to the substrate pre-treatment residual stresses, which influence the coating properties, develop. Due to the substrate preconditioning thermal spray coatings exhibit a large interfacial roughness. This study investigates the application of the incremental hole-drilling method on thermal spray coatings. The focus is on the influence of the interfacial roughness on the residual stress evaluation. A systematic FE-study was carried out in order to minimize the final error for the residual stress evaluation. The simulation results are transferred to experimental hole-drilling results of a thermally sprayed model thick film system. Finally, the hole-drilling results are compared to the residual stress depth profile that was determined by X-ray diffraction in combination with successive electrochemical layer removal. The results clearly show that the effect of the interfacial roughness can be neglected for residual stress calculation if the mean coating thickness is properly considered for calculation of the calibration function / parameters.