Zur Ermittlung verformungsbedingter Makro- und Mikroeigenspannungen durch mechanische und röntgenographische Eigenspannungsanalysen

Analysis of deformation induced residual macro-and microstresses by mechanical and X-ray methods Residual stress distributions in plastically deformed tensile and bending specimens of perlitic steel were analysed using X-ray diffraction technique and incremental holedrilling method. After tensile loading compressive residual stresses are measured by X-ray analysis in the ferrite phase. Consequently X-ray analysis detects compressive microstresses. In the case of bending specimens residual macrostresses are superposed with residual microstresses after unloading. In no case identical residual stress values were measured by X-ray and hole drilling methods. Microstresses can be separated combining both measurement methods. Microstresses after tensile loading were found to be greater than in surface layers of respective bending samples subjected to the same amount of plastic strain.     

Monotonic and Cyclic Deformations of Case‐Hardened Steels Including Residual Stress Effects

Abstract: Monotonic and cyclic deformations of case‐hardened steel specimens under axial loading were investigated experimentally and analytically. A finite element (FE) model for the case‐hardened specimens was constructed to study multiaxial stresses due to different plastic flow behaviour between the case and the core, as well as to evaluate residual stress relaxation and redistribution subsequent to cyclic loading. The multiaxial stress is shown to increase the effective stress on the surface, and, therefore, unfavourable to yielding or fatigue crack nucleation. The residual stresses are shown to relax or redistribute, even in the elastic‐behaving region, when any part of a case‐hardened specimen or component undergoes plastic deformation. Multi‐layer models were used to analyse and predict monotonic and cyclic deformation behaviours of the case‐hardened specimen based on the core and case material properties, and the results are compared with the experimental as well as FE model results. The predicted monotonic stress–strain curves were close to the experimental curves, but the predicted cyclic stress–strain curves were higher than the experimental curves.     

Characterisation of residual stresses in heat treated,high strength aluminium alloy extrusions

Residual stresses were measured in rectilinear aluminium bars quenched using an aqueous polyoxyethylene glycol (PAG) solution or cold-water. Residual stresses were measured with neutron diffraction and a superposition-based method using mechanical strain release measurements. Three orthogonal stress components were measured along two transverse lines using neutron diffraction. The longitudinal residual stresses were mapped over a transverse cross-section using the contour technique. A primary slice removal technique mapped three orthogonal residual stresses over a transverse cross-section in the PAG extrusion. Residual stresses were found to vary from biaxial compressive in the part boundaries to triaxial tensile in the interiors. There was close correlation between the neutron diffraction and mechanical strain release techniques. PAG quenching demonstrated lower residual stresses.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

An experimental study on residual stress relaxation in low-cycle fatigue of Inconel 718Plus

Residual stress relaxation induced by the application of mechanical loads is determined by the nature of residual stress, the elasto-plastic material properties, and the type of applied load. Despite the importance of the first load cycle, analytical models available in the literature generally assumed residual stress relaxation as a continuous process. Residual stress induced by machining on Inconel 718Plus superalloy cylindrical specimens was measured before and after the application of load cycles under strain control. Low-cycle fatigue tests were carried out at room temperature for different strain amplitudes, and X-ray diffraction measurements were performed before and after 10 and 100 cycles. A comprehensive analytical model was derived to describe the relaxation process associated with the initial cycles and that associated with the continuous application of load cycle, which is based on the plastic strain energy per cycle W and requires the evaluation of parameters that are only dependent on the material and not on the strain amplitude.     

Manufacturing Residual Stress States in Heat Treatment Simulation of Bearing Rings

Residual stresses generated in the different steps of a manufacturing chain are attached great importance to the distortion due to heat treatment. Because the measurement of residual stress states is associated with large experimental work, the use of simulation would be qualified for the examination of the influence of manufacturing residual stresses on distortion. Due to this reason, a simple method for generating realistic residual stress states by the use of artificial thermal strain in a pre‐simulation step is presented. By changing the thermal strain distributions a wide range of residual stress distributions in a component can be generated. A typical residual stress distribution after machining was generated in a bearing ring. The calculated residual stress profile was in good agreement compared to XRD‐measurement. In a second simulation step the local development of stress relieving during heating was observed. Stress relieving can be attributed to local plastic deformation and rearrangement of stresses.     

Synchrotron diffraction investigation of the distribution and influence of residual stresses in fatigue

This paper presents some results obtained from synchrotron diffraction investigations into two somewhat related areas of interest to the fatigue community. Firstly, the influence of fatigue cycling on the distribution and magnitude of residual strains and stresses and, secondly, the residual strains and stresses engendered around a growing fatigue crack. Its main premise is that modern tools such as automated synchrotron strain scanning offer the potential for more complete insight into the distribution of residual strains and stresses and their influence on fatigue performance. The first part of the work was accomplished using friction‐stir welded (FSW) and metal‐inert gas (MIG) welded specimens. The particular interest in these specimens was obtaining detailed knowledge regarding as‐welded variation in residual stresses between specimens, the location of peak values relative to local microstructure and stress concentrations, and of their modification during fatigue cycling. Such information may indicate a route forward to the selection of welding process parameters for optimised fatigue performance. The second part of the work considered an established fatigue crack in a compact tension (CT) specimen and examined the ability of synchrotron diffraction to characterize the stresses associated with the plastic enclave around a fatigue crack. This work is of interest in the context of better knowledge of crack‐tip shielding by plasticity‐induced closure and its incorporation into life prediction methodologies.     

Feature tests on welded components at higher temperatures—Material performance and residual stress evaluation

A reliable maintenance and service of energy generation plants would be impossible without the professional designing, manufacturing and monitoring of welded joints. The lifetime assessment factors of welded components as implemented in the design codes must be updated accounting for the modern materials and the advanced steam parameters used in the piping construction [Weld Strength for high temperature components design and operation (WELDON). European Project No. GRD2-2000-30363, Gampe U, Seliger P, Creep crack growth testing of P91 and P22 pipe bends. Int J Pressure Vessels Piping 2001;78:859-64, INTEGRITY of repair welds in high temperature plants operating under steady and cyclic loading conditions. European Project No. G5RD-CT-1999-00118].Within the EU 5th Framework RTD project ‘WELDON’ tests at high temperatures are performed on component-like feature test specimens like welded pipes and large tensiles in addition to laboratory specimens to study the geometry and size effects on the damage evaluation methodology. The circumferential welds of the pipes are subjected simultaneously to internal pressure and an axial load. The large tensile specimens manufactured from the welded pipes are subjected to uniaxial loading. These components are made from steel grade P22 and P91 and are equipped with gages for on-line monitoring of temperatures, deformations and strains.Residual stresses are measured on these components in as welded and after post-weld heat treatment (PWHT) conditions and are monitored during creep testing with interruptions and after failure. X-ray and hole drilling techniques are employed in these measurements. These data will be used to validate the FE modelling of residual stresses and damage assessment.The results obtained from long time creep tests, metallographic investigations of damage development in the different zones of the weldment and residual stress measurements are presented in this paper.     

Residual stress field determination in welds by means of X‐ray,synchrotron and neutron diffraction

To what extent the welding residual stresses influence fatigue is still unclear and matter of debate. An important reason for this lack of clarity is that the exact determination of residual stress fields in welds is complicated which leads to conservative assumptions about these stresses in the fatigue design codes. The advances in the diffraction analysis of materials offer the opportunity for the full‐field residual strain mapping in welds albeit at the cost of time and technical complexity. In this work residual stress field determination in welded S1100QL specimens by means of the x‐ray, synchrotron and neutron diffraction techniques was undertaken. The results revealed that the maximum values of surface residual stresses are not as frequently assumed, as high as the yield strength in small scale specimens. At the weld toe which could serve as a fatigue crack initiation site, even lower residual stresses than the weld centreline could be present. The in‐depth measurements revealed that the effective part of the residual stress field which could be decisive for the fatigue failure initiation is concentrated at the surface of the weld.     

Fatigue behavior of ferritic–pearlitic–bainitic steel in loading with positive mean stress

The effect of positive mean stress on the fatigue behavior of ferritic–pearlitic–bainitic steel has been studied. Specimens, produced from a massive forging, were cycled with two constant stress amplitudes and various positive mean stresses. Plastic strain amplitude and cyclic creep rate were measured during cyclic loading and the effect of the mean stress on saturated plastic strain amplitude and mean strain at half-life was established. Plastic strain amplitude is weakly dependent but creep strain increases with the mean stress exponentially. Fatigue life decreases with the mean stress for both stress amplitudes. The contributions of cyclic plastic strain and cyclic creep to the fatigue damage were evaluated and discussed in relation with the Manson-Coffin curve.     

Investigation of residual stresses in a repair‐welded rail head considering solid‐state phase transformation

Repair welding for recovery from local damage of a rail head surface is known to cause high residual stress and can accelerate fatigue in the rail. This study examines repair‐welded rails by applying experimental and numerical approaches. In the former approach, two newly manufactured rail specimens and four repair‐welded rail specimens with two different weld depths were prepared, and their residual stresses were measured with a sectioning method. In the latter approach, a finite element repair welding simulation model was developed that adopted a prescribed temperature method with a moving block as an input heat source, and the thermal strain caused by the volume change due to solid‐state phase transformation was considered. Overall, the residual stresses correlated well between the experimental and numerical approaches. The measured high compressive residual stress of ?290 MPa seems to be beneficial to prevent a crack initiation in the rail surface.     

Confirmation of Principal Residual Stress Directions in Rectilinear Components by Neutron Diffraction

Abstract: Large magnitude residual stresses in precipitation hardened aluminium alloys are a consequence of inhomogeneous plastic deformation arising from thermal strains. Thermal gradients develop because of the metallurgical requirement to quench the material rapidly from the solution treatment temperature. Characterisation of the resulting residual stresses can be usefully performed by neutron diffraction. For rectilinear shapes, it is usually assumed that the orthogonal directions of the sample are coincident with the principal stress directions. To test this assumption, residual strains were measured in a rectilinear block of the aluminium alloy 7449 using the SALSA neutron strain analyser. The strains at a single point in the forging were measured as a function of orientation; that is the forging was rotated around the three orthogonal axes of a coordinate system with its origin at the measurement point. Analysis of the neutron diffraction data allowed the determination of the full three‐dimensional strain and stress tensors which confirmed that the orthogonal directions of the forging did coincide with the principal stress directions.     

Combined experimental–numerical study on residual stresses induced by a single impact as elementary process of mechanical peening

Peening processes can be used as a fatigue enhancement treatment for metallic structures by locally introducing compressive residual stresses. A combined experimental–numerical study on a single-impact process with a drop tower on the aluminium alloy AA5754, representing the elementary process of mechanical peening, has been performed to investigate different impact parameters on the residual stress profile. Residual stresses have been measured using high-energy X-Ray diffraction. A three-dimensional finite element model is used to predict the residual stresses numerically. The elastic strain components from the numerical results are used to calculate residual stresses by assuming either a plane stress or a plane strain state for different specimen thickness to assess the validity of respective assumption. The validity of the numerical simulation is evaluated based on comparisons of the elastic strain profiles and the percentage loss in kinetic energy of the steel ball due to the impact for four different energies, showing overall a good agreement in the experimental–numerical comparisons.     

Residual stress measurements of computer aided design/computer aided manufacturing (CAD/CAM) machined dental ceramics

Residual stress analysis is becoming more important in terms of understanding the strength and fatigue behaviour of ceramic materials. The residual stresses after computer aided design/computer aided manufacturing (CAD/CAM) machining according to dental practice were analysed for two different kinds of dental ceramics, a feldspathic porcelain and a glass-ceramic. A mechanical strain gauge element was used to measure the deformation of dental test inlays during material removal by etching the surface of the sample. From these data the residual stress depth profile could be calculated for crystalline as well as amorphous materials. The strain gauge results were compared to X-ray diffraction data. The depth profile of the residual stress for both ceramics showed compressive stress at the surface of the machined ceramics, changing towards tensile stress at a depth of 10 to 15 μm from the surface. Ceramics with pronounced plastic deformation behaviour in CAD/CAM machining revealed higher residual stresses as well as a more distinct stress anisotropy in terms of grinding direction. This revised version was published online in November 2006 with corrections to the Cover Date.     

Eigenspannungen als Messgröße zur Qualitätssicherung

Residual stress as measured quantity in quality assurance The x‐ray method is used in the Volkswagen Automotive Group at different world‐wide locations as a standard procedure to determine residual stress. As an important measured quantity in the quality assurance residual stresses are specified in drawings of different components (e.g. gears, coil springs). Measurements are carried out during quality assurance, production, damage analyse, technical development department and design control of components from suppliers. An optimum efficiency can be achieved, if it exists a unique correlation between residual stress values and loading capacity of components, and if the processes to introduce residual stress can be optimized using residual stress measurements during the production.     

Effect of mean stress on residual stress relaxation in steel specimens

Abstract

Residual stress relaxation was investigated by subjecting specimens with various mean stress levels to strain controlled cyclic loading. The material studied was mild steel in three different conditions. The mean stress levels ranged from 100 to 200 MPa, and two strain amplitudes, 0·05 and 0·06%, were studied in detail. The residual stresses in the specimens were measured before and after mean stress relaxation experiments. It was found that experimental factors such as temperature variations and crack growth have a significant influence on the results. Based on the experimental results, it is proposed that the mean stress relaxation exponent should be divided into two parts: mean stress dependent and mean stress independent. The first includes the contribution of quasi-static relaxation, i.e. mean stress dependent plastic deformation. The second part includes the contribution of cycle dependent mean stress relaxation, which does not depend on the mean stress.     

Fatigue crack initiation assessment of welded joints accounting for residual stress

The impact of residual stresses on the fatigue crack initiation life of welded joints is evaluated by the finite element method. The residual stresses of nonload‐carrying cruciform joints, induced by welding and ultrasonic impact treatment, are modelled by initial stresses, using the linear superposition principle. An alternative approach of using modified stress‐strain curves in the highly stressed zone is also proposed to account for the residual stress effect on the local stress‐strain history. An evaluation of the fatigue crack initiation life of welded joints based on the local strain approach is carried out. The predicted results show the effect of residual stresses and agree well with published experimental results of as‐welded and ultrasonic impact treated specimens, demonstrating the applicability of both approaches. The proposed approaches may provide effective tools to evaluate the residual stress effect on the fatigue crack initiation life of welded joints.     

Eigenspannungen in der Oberfläche gas- und badnitrierter Stahlproben

Residual stresses in the surface of gas and bath nitrided steel specimens . Residual stresses in the steels Ck 45, 34 Cr 4, 34 CrAl 6, and Ck 15 were measured by X-ray diffraction. Cylindrical specimens of the heat-treated steels were nitrided in ammonia-gas and in a Tenifer®-salt- bath. The course of residual stresses of Ith order was determined by removing thin layers. With the carbon steel Ck 15 ist shown the effect of tempering on residual stresses in bath-nitrided and quenched specimens.     

Fatigue life prediction for butt‐welded joints considering weld‐induced residual stresses and initial damage,relaxation of residual stress,and elasto‐plastic fatigue damage

Fatigue damage of butt‐welded joints is investigated by a damage mechanics method. First, the weld‐induced residual stresses are determined by using a sequentially coupled thermo‐mechanical finite element analysis. The plastic damage of material is then calculated with the use of Lemaitre's plastic damage model. Second, during the subsequent fatigue damage analysis, the residual stresses are superimposed on the fatigue loading, and the weld‐induced plastic damage is considered as the initial damage via an elasto‐plastic fatigue damage model. Finally, the fatigue damage evolution, the relaxation of residual stress, and the fatigue lives of the joints are evaluated using a numerical implementation. The predicted results agree well with the experimental data.     

Fatigue crack initiation life estimation in a steel welded joint by the use of a two-scale damage model

This work deals with the fatigue behaviour of S355NL steel welded joints classically used in naval structures. The approach suggested here, in order to estimate the fatigue crack initiation life, can be split into two stages. First, stabilized stress–strain cycles are obtained in all points of the welded joint by a finite element analysis, taking constant or variable amplitude loadings into account. This calculation takes account of: base metal elastic–plastic behaviour, variable yield stress based on hardness measurements in various zones of the weld, local geometry at the weld toe and residual stresses if any. Second, if a fast elastic shakedown occurs, a two-scale damage model based on Lemaitre et al. 's work is used as a post-processor in order to estimate the fatigue crack initiation life. Material parameters for this model were identified from two Wöhler curves established for base metal. As a validation, four-point bending fatigue tests were carried out on welded specimens supplied by 'DCNS company'. Two load ratios were considered: 0.1 and 0.3. Residual stress measurements by X-ray diffraction completed this analysis. Comparisons between experimental and calculated fatigue lives are promising for the considered loadings. An exploitation of this method is planned for another welding process.     

Behaviour of stresses in circumferential butt welds of steel pipe under superimposed axial tension loading

A computational procedure is presented for analyzing behaviour of stresses in circumferential butt welds of carbon steel pipe subject to superimposed mechanical loading. Three-dimensional uncoupled thermo-mechanical finite element (FE) analysis method is developed in order to predict the weld residual stress states in circumferentially butt-welded steel pipe. The FE method is verified through the experimental work. Then, three-dimensional elastic–plastic FE analysis is carried out to investigate the behaviour of stresses in steel pipe circumferential welds undergoing superimposed axial tension loading using the weld residual stresses and plastic strains obtained from the thermo-mechanical FE method. The simulated results show that spatial variations of the weld residual stresses are present along the circumference and a rapid change of the residual stresses exists at the weld start/stop position, therefore three-dimensional FE analysis is essential to accurately simulate the circumferential welding of steel pipe. Moreover, when axial tension loading is applied to the circumferentially welded steel pipe, bending moment is generated at the weld area caused by the circumferential shrinkage of the weld during welding, thus affecting the axial and hoop stress evolutions in the course of mechanical loading.