Residual Stress Intensity Research Articles

Is 2D numerical modelling of welding process able to capture the residual notch stress intensity factor values?

Because of its local nature, the influence of residual stresses on fatigue life of welded joints in the high cycle regime can be quantified by the residual notch stress intensity factor (R-NSIF). The standard methodology used for its assessment requires a very fine mesh and this is the reason why 2D numerical models were used first in literature. The calculation of R-NSIF values via 3D models is very recent. It was found possible thanks to rapid numerical techiques such as the peak stress method. This approach was proven to work well but R-NSIF values coming from 3D models were not consistent with those obtained through 2D models. This work is aimed at solving that apparent discrepancy. 2D and 3D welding process numerical simulations applied to a carbon steel were carried out taking into account effects of metallurgical transformations and by varying the main geometrical parameters of the plates. It was found that 2D models are able to capture the R-NSIF value that is observed in the stationary residual stress distribution of the corresponding 3D models.

Does metallurgy affect the residual notch stress intensity factor value induced by welding operations? A comprehensive study via a 3D numerical model

The residual notch stress intensity factor (R-NSIF) was found useful to quantify the influence of residual stresses on fatigue life of welded joints. Its distribution along the weld bead was recently assessed by using a 3D model. Those works focused only on the calibration of the model rather than an analysis of the R-NSIF values as a function of alloy metallurgy. This work wants to fill that gap. Both a carbon steel and an aluminum alloy were considered. Phase transformations are found to significantly influence the R-NSIF distribution with values that are more accurate compared to those obtained via previous models.

Residual stresses near cold-expanded hole at different stages of high-cycle fatigue by crack compliance data

Experimental method for a characterization of high-cycle fatigue evolution of residual stress near cold-expanded hole is developed and implemented. The technique is based on simultaneous measurements of deformation response to narrow notch, inserted in residual stress field, on opposite specimen’s faces by electronic speckle-pattern interferometry (ESPI). Two-side measurements of notch opening displacements are performed when a single notch, emanating from cold-expanded hole edge, is inserted. The transition from in-plane displacement component to residual stress intensity factor (SIF) values follows from the relationships of modified version of the crack compliance method. The approach provides a difference in residual stress values referred to mandrel entrance and exit surface. Notches are inserted at different stages of low-cycle fatigue without applying external load. The results obtained describe fine nuances of residual stress evolution, which cannot be considered as monotonic relaxation.

Residual Notch Stress Intensity Factors in Welded Joints Evaluated by 3D Numerical Simulations of Arc Welding Processes.

Approaches based on calculating Residual Notch Stress Intensity Factors (R-NSIFs) assume the weld toe to be a sharp V-notch that gives rise to a residual singular stress distribution close to the weld toe. Once R-NSIFs are determined, they might be included in local fatigue criteria for the structural strength assessment of welded joints based on NSIFs due to external cyclic loading. However, the numerical calculation of R-NSIFs through finite element (FE) simulations of the welding process requires extremely refined meshes to properly capture the residual stress singularity. In this context, the Peak Stress Method (PSM) has recently been adopted to estimate R-NSIFs due to residual stresses by means of coarse meshes of 2D 4-node plane or 3D 8-node brick elements. The aim of this work is to investigate the applicability of the PSM to estimate R-NSIFs in a butt-welded joint using coarse meshes of 3D 10-node tetra elements. The R-NSIF distribution at the weld toe line is estimated by applying the PSM to coarse meshes of 3D 10-node tetra elements, and the results are in agreement with those obtained using 3D 8-node brick elements. Accordingly, the PSM based on tetra elements further enhances the rapid estimation of R-NSIFs using coarse meshes and could be effective in analyzing complex 3D joint geometries.

The influence of structural imperfections on the LTB strength of I-beams

The structural imperfections, namely, the residual stresses, are generated during most manufacturing processes involving material deformation, heat treatment, machining or processing operations. The intensity and distribution of residual stresses are dependent on the techniques used in the production of steel sections (welding, rolling or cold formed techniques) and their dimensions. An appropriate evaluation of residual stress is of great importance for the structural performance of steel I-beams. Residual stresses can have a significant impact on the stability of structural members. In the case of I-section beam elements, such stresses can impact lateral torsional buckling (LTB) strength, particularly in the inelastic range. For the development of post buckling numerical analyses, the consideration of residual stress distribution models is fundamental for the correct determination of the LTB strength of I-beams. However, there are several models that attempt to represent the distribution and intensity of residual stresses. These models have a different influence on the LTB behavior of I-beams in numerical analysis. Therefore, this paper aims to investigate, through the development of post buckling numerical analyses with the ABAQUS software, the influence of the different residual stress distribution models on the LTB strength of I-beams. Seven different residual stress models were analyzed on simply supported I-beams subject to uniformly distributed loading applied in the shear center and in the upper flange. The results were compared with standard procedures. It was verified considerable divergences in LTB strength depending on the residual stress model applied. In addition, this paper presents a synthesis of the main residual stress distribution models. The analyses showed in this work shall help the choice of the residual stress distribution model on future numerical simulations.

Numerical analysis of the influence of maximum residual thermal stresses on the intensity factor between the matrix and particle interfaces in metal matrix composite

A critical problem in the application of metal matrix composites is the presence of high residual thermal stresses induced during the development process. These thermally induced stresses are generally detrimental to the service life of this type of composite. This article discusses the influence of maximum residual stresses on the intensity factor. The results interpreted in terms of damage, allowed us to identify the risk zones; characterized by a significant level of maximum residual stresses (S11Max, S22Max, S33Max), namely the particle/Matrix interface. The results also show that the loading conditions and the inter-distance between matrix and particle with two interfacial cracks have an important effect on max residual stresses and stress intensity factors.

Quantitative Fracture Mechanics Approach to Fatigue Life Improvement by Shot Peening Application

Stress intensity factor has been a favorite tool in fracture mechanics due to its simplicity. Here, it is used to study the influence of the shot peening application in fatigue life improvement. Using a CT-specimen, the study of shot peening effect on fatigue life was performed. Here, the compressive residual stress is converted into residual stress intensity factor, Kres.. Together with the applied stress intensity factor, Kapl., the superposition principle can be used. The result of the investigation clarifies quantitatively what has been widely believed that one of the easiest ways to improve fatigue life is by perfecting the surface via shot peening. Compressive stresses are beneficial not only in postponing fatigue crack initiation but also in preventing the crack propagation, which lead to increasing fatigue strength.

Quantitative analysis of overload effect at crack tip on crack growth rate of 7005 aluminum alloy

A new fatigue crack growth rate model is proposed considering overload effect, which could quantitatively describe the crack growth rate considering overload from variable-amplitude load. Residual stress intensity factor Kq caused by overload was put forward by defining stress ratio Rc and obtained by XFEM. Three typical overload situations are studied and reappearance of the threshold stage was elaborated by compact tension tests. The effective scope of the overload is the width of the plastic zone, within the region the stress ratio rises quickly and then become stable. Threshold stage reappeared only when overloads by the higher preload effect were conserved.

RESIDUAL STRESSES IN A THERMOELASTIC CYLINDER RESULTING FROM LAYER-BY-LAYER SURFACING

The article investigates the residual stresses arising in a thermoelastic cylinder as a result of layer-by-layer deposition of material on its lateral surface. Residual stresses are defined as the limiting values of internal stresses developing during the technological process. Internal stresses are caused by incompatible deformations that accumulate in the body as a result of joining parts with different temperatures. For the analysis of internal stresses, an analytical solution of the axisymmetric quasi-static problem of thermoelasticity for a layer-by-layer growing cylinder is constructed. It is shown that the distribution of residual stresses dependson the scenario of the surfacing process. In this case, the supply of additional heat to the growing body can significantly reduce the unevenness of the temperature fields and reduce the intensity of residual stresses. The most effective is uneven heating, which can be realized, for example, by the action of an alternating current with a tunable excitation frequency. This is illustrated by the calculations performed using the constructedanalytical solution.

Mixed mode interface fracture toughness of a multi-directional composite – UD/woven pair

Twenty-seven mixed mode Brazilian disk specimens were tested to obtain the fracture toughness of a delamination between two fiber reinforced composite plies in a multi-directional laminate, manufactured by means of a wet-layup. Using finite element analyses and conservative integrals, mechanical and residual curing stress intensity factors were found and used to calculate the interface energy release rate and phase angles. Based on these results, a three-dimensional failure criterion is proposed to obtain a failure surface. This criterion may be used to predict failure of a structure containing this material and interface.

A new way of identifying, predicting and regulating residual stress after chip-forming machining

Residual stresses are the result of distortion of the equilibrium forces on the surface of the material, which leads to its deformation and distribution of unevenness of the machined surfaces. The paper presents a new approach for the prediction of residual stress distribution after milling. This method for solving residual stress and predictions is highly dependent on material properties and therefore provides substantial improvement and modelling of the transformation states of the material that is necessary to reliably predict its behaviour under the conditions of the functional load. It has been found that roughness and roughness control determine the intensity of residual stress. Samples made of hardened steel X37CrMoV5-1 were used as the experimental material. A Proto iXRD diffractometer was used for residual stress measurement. The validity of the proposed method is confirmed by the good agreements between the theoretical, experimental results and results obtained from a literature survey.

Nearly Mode I Fracture Toughness and Fatigue Delamination Propagation in a Multidirectional Laminate Fabricated by a Wet-Layup

Five double cantilever beam specimens were tested quasi-statically to obtain a GIR resistance curve. In addition, nine double cantilever beam specimens were tested in fatigue to obtain a Paris-type relation to describe the delamination propagation rate da/dN where a is delamination length and N is the cycle number. Displacement ratios of Rd = 0.10 and 0.48 were used for five and four specimens, respectively. The specimens were fabricated by means of a wet-layup process from carbon fiber reinforced polymer plies. The interface containing the delamination was between a unidirectional fabric and a woven ply. The fracture toughness and fatigue delamination propagation protocols are outlined. The mechanical and thermal residual stress intensity factors were obtained by means of finite element analyses and the conservative M-integral along the delamination front. They were superposed to determine the total stress intensity factors. It was found that the total mode I stress intensity factor dominates the other two stress intensity factors. Thus, nearly mode I deformation was achieved. Interpolation expressions for the mechanical and thermal residual stress intensity factors were determined using three and two-dimensional fittings, respectively. Results are presented with an expression for GIR determined. Moreover, the fatigue data is described including threshold values and master-curves. These results shed light on the behavior of delamination propagation in multidirectional laminate composites.

Optimal Design of Co/In/Cu Sputtering Target Assembly Using Finite Element Method and Taguchi Method

Target assembly is a key consumable material for producing thin film used in the electronic packaging and devices. The residual stresses induced during the process of soldering are detrimental to the performance of target assembly. In this work, the intensity and distribution of the soldering residual stress of Co/In/Cu target assembly subjected to a 20 W/(m2 K) cooling condition corresponding to the actual air cooling process were studied, based on finite element simulation and Taguchi method, to optimize the sputtering target assembly. Effects of different control factors, including solder material, thickness of solder layer, target and backing plate, on the soldering residual stress of target assembly are investigated. The maximum residual stress is calculated as 9.28 MPa in the target located at 0.16 mm from target–solder layer interface and at a distance of 0.78 mm from symmetry axis. The optimal design in target assembly has the combination of indium solder material, cobalt target at 12 mm thick, solder layer at 0.8 mm thick, copper backing plate at 15 mm thick. Moreover, solder material is the most important factor among control factors in the target assembly.

Recovery of fatigue life using laser peening on 2024‐T351 aluminium sheet containing scratch damage: The role of residual stress

AbstractThe aim of the current work was to study the effect of laser shock peening (LSP) when applied to 2‐mm thick 2024‐T351 aluminium samples containing scratch‐like defects in the form of V‐shaped scribes 50 to 150 μm deep. The scribes decreased fatigue life to 5% of that of the pristine material. The effect of laser peening on fatigue life was dependent on the specifics of the peen treatment, ranging from further reductions in life to restoration of the fatigue life to 61% of pristine material. Fatigue life was markedly sensitive to near‐surface tensile residual stress, even if a compressive residual stress field was present at greater depth. Fatigue life after peening was also dependent on sample distortion generated during the peening process. Sample distortion modified local stresses generated by externally applied loads, producing additional life changes. Models based on residual stress intensity and crack closure concepts were successfully applied to predict fatigue life recovery.

Calculation of 3D residual notch stress intensity factors by means of the peak stress method

In the recent literature new approaches were developed to include the residual stress effects in fatigue strength assessments of welded joints. Among these, the local approach based on the calculation of the Residual Notch Stress Intensity Factors (R-NSIFs) seems very promising. Unfortunately, its main drawback is linked to the difficulty to assess such parameters by means of numerical simulation of the welding process, because of the high mesh density required to capture the residual stress field singularity. In this scenario, new numerical methodologies are needed which use coarse meshes, such as the Peak Stress Method (PSM). In this work, an investigation is described aimed at analyzing the PSM applicability to estimate the R-NSIF values in a 3D butt-welded joint. It was shown that the R-NSIF values obtained ‘via’ the PSM by using 3D models are quite higher (about 45%) as compared to estimations obtained by means of a 2D model, under generalized plain strain condition. This was attributed to the limitation of 2D models to correctly assess the residual stress field induced by welding processes.

Preventing Stress Corrosion Cracking of Spent Nuclear Fuel Dry Storage Canisters

Multi-purpose canisters (MPCs) are being employed for dry canister storage of spent fuel at nuclear plant sites as a temporary approach until an interim or permanent dry storage site(s) is available. For storage in coastal or lakeside regions and even humid environments, the corrosive nature of the moist air with entrained chlorides can make the welded regions of the canisters susceptible to pitting and chloride induced stress corrosion cracking (CISCC). In this report we evaluate CISCC lifetimes of welded 316L stainless steel canister plates showing in excess of 19 times increase of laser peened panel sections vs. those left as-welded using ASTM G36 (2013) accelerated corrosive testing. Specifically cracks never developed or propagated into the laser peening area. We also provide measurements of residual stress in test plates and related calculations of stress intensity and depth expected in the full canister geometry. We discuss the relevance of stress depth to pitting depth and crack growth rates. For this project welded 316L stainless steel panels were configured to MPC geometry and laser peened in the same manner as deployed for actual canisters. Our two-dimensional stress mapping shows that high energy laser peening provides deep (>5 mm) plastic compression. Literature information is not clear as to specific dependence of crack initiation and growth rates on variables including temperature, specifics of chlorine exposure, pit nucleation and residual stress intensity and depth. In our CISCC testing, although extensive cracking developed quickly (less than 18 hours) in the unpeened area, no cracking developed at all in the laser peened sections after 340 hour exposure. We further show that cracking that initiated and grew in unpeened regions arrested upon propagating to the laser peened boundary. We performed detailed finite element stress analysis (FEA) to evaluate the difference in expected retention of stress generated by the peening of test panels and the more constrained geometry of actual storage canisters. Our analysis shows that the 4 mm depth of stress measured in unconstrained test panels correlates to actual stress depth in the confined canister geometry of 6 mm, that is approximately 2 mm deeper thus satisfying a key stress depth requirement. The accelerated CISCC tests indicate that MPC design life can be dramatically improved by laser peening thereby making the MPCs a viable technical and economic solution to prevent CISCC. Following qualification testing and system configuration, a high energy laser peening process was deployed to peen welds of the dry fuel canisters for the San Onofre Nuclear Power Plant. The laser peening thereby helps ensure that the storage canisters will remain free from chlorine induced stress corrosion cracking.

The Welding of Dissimilar Joints with Low Temperature Martensitic Transformation Filler Material

Abstract The welding of dissimilar joints is very common in systems used in oil exploration and production in deep waters. This commonly involves welding low carbon steel pipes with valves, pumps or other pipes, all of them made of alloy steel. Inconel has been used in such welding as filler material; however, this welding is preceded by a process of buttering in the low alloy steel side and a stress relief heat treatment. An alternative technique presented in this paper is the use of a welding filler metal that, during the cooling process, undergoes a martensitic transformation at a relatively low temperature, so that the resulting deformation processing offsets the contraction that occurs as a result of the cooling process. This study evaluates the application of Maraging 250 weld in a dissimilar joint of the oil industry. Tests were performed on joints of ASTM steels A516 grade 70 and AISI 8630M welded with Maraging 250. The residual stress profiles, hardness, microstructure, tensile strength and impact energy absorbed were evaluated. The welded joint with Maraging 250 showed a high intensity of compressive residual stress. The microstructure of different welding regions was analysed and compared.

A conservative approach for mode I‐II fatigue analysis under residual stresses: The RSIF proportionality conjecture

Residual stresses are a crucial aspect of fatigue life predictions of welded structures. However, their implementation in fatigue crack growth simulations can be tedious. In this paper, a novel approach is proposed that considers the effects of residual stresses in a worst‐case scenario by applying the maximum tangential stress criterion and maximising the crack growth rate. Results demonstrate that computationally demanding analysis of residual stresses can be avoided by assuming the residual stress intensity factors are proportional to the stress intensity factors induced by the externally applied cyclic load. This approach is referred to as the residual stress intensity factors proportionality conjecture, which is particularly useful for inspection planning of complex structures subject to mixed‐mode crack propagation situations, where the computational recovery of current residual stress states by consideration of their temporal evolvement is cumbersome.

Investigation of fatigue crack models by micro-scale measurement of crack tip deformation

The paper describes displacement measurements taken in the vicinity of a fatigue crack tip using digital image correlation. In-situ loading is employed in a scanning electron microscope to get measurements very close to the tip. The results are compared to the usual elastic model of crack tip deformation and to the Christopher, James and Patterson model, which is critically discussed. It is shown that the use of an elastic model with a residual stress intensity caused by crack tip shielding gives a good representation of the experimental results.

An application of stochastic modeling to pitting of Spent Nuclear Fuel canisters

Chloride induced stress corrosion cracking (CISCC) is one of the main factors affecting the integrity of used nuclear fuel in dry storage canisters, especially at coastal sites. CISCC has complex interactions associated with environment, stress and materials properties. This paper is focused on the development of probability distribution functions for maximum pit depth based on experimental data created at Lawrence Livermore National Laboratory (LLNL) that have not been fully analyzed before. The LLNL report outlined the key variables of pit initiation, termination and growth rate that may lead to CISCC, with the final aim of prediction of canister life and optimization of the time interval inspection of the canisters. The key parameters characterizing the probability distributions of pits at each stage depend on environment, material susceptibility conditions, and residual stress intensity. A basic stochastic approach to estimate probability distributions based on the median and maximum pit depths observed in experimental data succeeds in reproducing the experimental results, and can be used to estimate the distribution of maximum depth at future times.