Residual Stress Intensity Factor Research Articles

The Influence of Residual Stress on Fatigue Crack Growth Rates in Stainless Steel Processed by Different Additive Manufacturing Methods

The properties and microstructure of Type 304L stainless steel produced by two additive manufacturing (AM) methods—directed energy deposition (DED) and powder bed fusion (PBF)—are evaluated and compared. Localized heating and steep temperature gradients of AM processes lead to significant residual stress and distinctive microstructures, which may be process-specific and influence mechanical behavior. Test data show that materials produced by DED and PDF have small differences in tensile strengths but clear differences in residual stress and microstructural features. Measured fatigue crack growth rates (FCGRs) for cracks propagating parallel to and perpendicular to the build directions differ between the two AM materials. To separate the influences of residual stress and microstructure, K-control test procedures with decreasing and constant stress intensity factor ranges are used to measure FCGRs in the near-threshold regime (crack growth rates ≤ 1 × 10−8 m/cycle). Residual stress is quantified by the residual stress intensity factor, Kres, measured by the online crack compliance method. Correcting the FCGR data for differences in Kres brings results for specimens of the two AM materials into agreement with each other and with results for wrought specimens, when the latter are corrected for crack closure. Differences in microstructure and tensile strength have an insignificant influence on FCGRs in these tests.

Re-Distribution of Welding Residual Stress in Fatigue Crack Propagation Considering Elastic–Plastic Behavior

The welding residual stress re-distribution behavior during fatigue crack propagation in butt-welded high-strength steel plates for ship construction is investigated based on experimental test results and numerical analyses. The specimens’ initial welding residual stresses are obtained from X-ray for middle tensile (MT) specimens cut from butt-welded high-strength steel plates. Then, fatigue crack propagation experiments on MT specimens are conducted, and a strain gauge is used to measure the residual stress re-distribution field around cracks. A practical fatigue crack propagation simulation procedure is developed with a dynamic update of in-situ welding residual stress, where the residual stress intensity factor Kres of the MT specimen is deduced. The stress ratio effect on Kres during fatigue crack propagation is analyzed and a good agreement between experimental and numerical results is achieved.

Fatigue crack growth prediction for shot-peened steel considering residual stress relaxation

The compressive residual stress field (CRSF) relaxes during the fatigue crack growth (FCG) process, and the pattern of this relaxation remains unclear. Residual stress (RS) relaxation leads to a change in residual stress intensity factor (RSIF), which significantly affects the fatigue crack growth rate (FCGR) and thus affects the assessment of fatigue crack growth rate. To this end, this work designed a series of experiments to explore the FCG behaviour in compressive residual stress fields (CRSFs) by considering RS relaxation. Firstly, shot peening(SP) treatments were conducted on the specimens. Then, residual stress fields (RSFs) at different crack propagation stages were measured to analyse the relationship between surface RS and inner RSF distribution. The derived weight function and the real-time RSF are combined to calculate the RSIF with/without considering RS relaxation. Finally, the effect of RS relaxation on the accuracy of FCGR assessment was investigated. The results show that the positive effect of CRS on the fatigue performance of the peened specimens is primarily embodied in the early stage of the FCG process. With the continuous relaxation of the CRSF, the difference in FCGR between the peened and unpeened specimens gradually decreases and tends to disappear. When the applied SIFs are large enough, the CRS would completely relax, and the inhibiting effect of CRSF on FCGR would fail. The proposed method in this work can effectively predict the FCGR of the peened specimen when considering the RS relaxation. This work could provide some meaningful insights for the assessment method of FCGR prediction in CRSF induced by surface strengthening treatment.

Evaluation of residual stress reproducibility and orientation dependent fatigue crack growth in powder bed fusion stainless steel

The complex thermal gradients of additive manufacturing (AM) result in residual stress and distinctive grain morphologies that influence mechanical performance and contribute to concern regarding the fatigue properties of AM parts. In this study, residual stress, microstructure, and fatigue crack growth rate (FCGR) results were compared in AM Type 304L stainless steel produced by laser powder bed fusion (PBF) on different systems using similar process parameters. Residual stress measured in the build direction was remarkably consistent in all PBF builds. Backscatter electron large area images revealed similar grain morphologies in the different builds, all of which exhibited elongated grains in the build direction and inhomogeneous grain size and shape. Fatigue crack growth investigated both parallel and perpendicular to the build direction revealed higher measured crack growth rates in the near-threshold regime in both orientations of the PBF material compared to wrought material. The difference in near-threshold fatigue crack growth rates is attributed primarily to the influence of processing-induced residual stress quantified by the residual stress intensity factor. These values revealed consistent FCGR effects in each orientation across specimens extracted from different builds and were then used to reveal a convergence of the corrected FCGR data of all PBF and wrought specimens.

Validation of on-line crack compliance data analysis methods for the residual stress intensity factor

Residual stress can significantly impact the fatigue crack growth rate (FCGR) observed in standard tests. In the present study, implementation of the on-line crack compliance (OLCC) method for determining the residual stress intensity factor, Kres, in FCGR tests is supported by validation data, details of novel data analysis methods, and strategies for data reduction. Slitting measurements on C(T) specimens of quenched (but not stress relieved) and aged aluminum alloy 7050-T74 provide reference profiles of Kres as a function of crack size. The OLCC method quantifies Kres profiles from stress intensity factor control and load control FCGR tests over a range of residual stress states. Novel OLCC data reduction techniques are developed, and data analysis attributes and limitations explored, contributing to an improved methodology for standardization. The results demonstrate that the on-line crack compliance method is a valid means of quantifying Kres during fatigue crack growth rate testing.

Effects of residual stress on orientation dependent fatigue crack growth rates in additively manufactured stainless steel

Localized heating and resulting temperature gradients during additive manufacturing (AM) create significant residual stress that influences mechanical behavior, such as fatigue performance. To quantify residual stress effects on fatigue crack growth in AM materials, crack growth rates parallel and perpendicular to the build direction in directed energy deposition (DED) Type 304L austenitic stainless steel were measured. The on-line crack compliance method was used to determine the residual stress intensity factor, Kres, while simultaneously collecting fatigue crack growth rate (FCGR) data. Constant applied alternating stress intensity factor (constant ΔKapp) tests revealed the primary influence on measured FCGR is the orientation dependent Kres. Critical analysis of the compliance data from decreasing ΔKapp tests was used to quantify Kres, which was then used to correct FCGR data in the near-threshold regime. Results demonstrated that the fatigue response of DED Type 304L is inherently similar to that of annealed wrought Type 304/304L.

The influence of welding heat input on Residual Notch Stress Intensity Factor

The influence of process parameters on the residual asymptotic stress distribution is investigated using a 3D numerical modeling of the welding process. Computational welding mechanics and the peak stress method are combined to calculate the residual Notch Stress Intensity Factors (R-NSIF) as a function of power input. Results are summarized in terms of heat input and discussed in view of a future guidelines proposal addressed to designers who are going to face a welding process optimization for fatigue life improvement.

The influence of residual stress on fatigue crack growth rates of additively manufactured Type 304L stainless steel

To examine the influence of residual stress on mechanical performance, specifically fatigue crack growth resistance, additively manufactured (AM) Type 304L stainless steel produced by directed energy deposition (DED) was evaluated and compared to conventional wrought Type 304/304L stainless steel. Increasing and decreasing alternating stress intensity factor (ΔK) tests were used to assess fatigue crack growth behavior over a range of crack growth rates in the near threshold regime (<10-8 m/cycle). Bulk residual stress and residual stress intensity factor (Kres) profiles of a fatigue specimen were measured using the incremental slitting method. Tensile residual stress at the edges of the DED materials led to positive values of Kres and faster fatigue crack growth rates in the DED material as compared to wrought material at the same applied ΔK. Correcting for the effects of Kres and crack closure in DED Type 304L and commercially available wrought Type 304/304L stainless steel shows that fatigue crack growth rates are similar at values of ΔK lower than 6 MPa·m0.5.

Effects of residual stress induced by laser shock peening on mixed-mode crack propagation behavior in 7075-T6 aluminum alloy panel

The effects of residual stress on mixed-mode crack propagation behavior of 7075-T6 aluminum alloy panel subject to multiple laser shock peening (LSP) impacts were investigated. Residual stress was obtained by an efficient LSP simulation termed continuous explicit-dynamic impact strategy. A numerical method, combining finite element (FE) method and residual stress intensity factor (SIF) analysis, was used to predict the mixed-mode (I/II) crack propagation in peened compact tension shear (CTS) sample. The effects of residual stress on the mixed-mode crack behaviors in different coverage areas were analyzed. It was found that the LSP-induced residual stress can change the crack path. Compressive residual stresses increase the effective crack length, and delay the crack propagation. The increased LSP coverage area and the decreased distance from the treated area to the crack tip are beneficial to the fatigue performance. The obvious crack turning point near the peened area is related to the significant change of mode II SIF components (KIIload and KIIres). The proposed numerical method provides a new idea for the damage tolerance analysis with respect to the applications of LSP technology in aircraft structures.

Fatigue analysis of 7075-T6 aluminum alloy under high compressive residual stresses by considering the non-propagating crack region

This article is a numerical investigation to determine the fatigue life of cold expanded fastener holes in 7075-T6 aluminum alloy. To estimate the numerical fatigue life, a combination of initiation and propagation lives was used. Three multiaxial fatigue criteria were employed to estimate the initiation life, while the AFGROW software was employed to predict the propagation life. The current research encountered the difficulty of high compressive residual stress at the edge of the cold expanded hole which hinders the crack growth. This overload zone near the hole edge is called the non-propagating crack region. Three models were considered to simulate the effect of the non-propagating crack region. Each model tried to provide the ability of the crack growth in this area by modifying factors such as residual stress and stress intensity factors. The S–N diagrams show longer lives for the cold expanded specimens. Moreover, to ensure the accuracy of the numerical results, they were compared with the experimental data in which the numerical results were in good agreement with the experimental data.

Residual Notch Stress Intensity Factor assessment via 3D welding numerical simulation: influence of power input

In fusion welded joints fatigue crack originates from the weld toe or weld route due to geometrical discontinuities inducing high stress concentration. This holds true even in presence of residual stress which was found to follow the same asymptotic distribution theorized first by Williams and then, in the elastic-plastic filed, by Hutchinson, Rice and Rosengren. Solid state phase transformations play a fundamental rule in determining the sign of the singular residual stress field. This obviously will influence the crack initiation life in the high cycle fatigue regime where plastic effects at the weld toe can be neglected. In this scenario, the influence of process parameters on the residual asymptotic stress distribution is not yet investigated using a 3D numerical modeling of the welding process. This work is aimed at filling this gap. Computational welding mechanics and the peak stress method are combined to calculate the residual Notch Stress Intensity Factors (R-NSIF) as a function of power input. Results are summarized in terms of heat input and discussed in view of a future guidelines proposal addressed to designers who are going to face a welding process optimization for fatigue life improvement.

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.

Influence of stress ratio on residual stress evolution near cold-expanded hole due to low-cycle fatigue by crack compliance data

Modified version of the crack compliance method is used for determination of stress intensity factor (SIF) related to narrow notches emanating from cold-expanded holes. These notches are inserted at different stages of low-cycle fatigue under constant external load. It is shown how residual SIF values, generated by residual stress field influence, can be separated from total experimental SIF values. Residual SIF values, obtained at different stage of low-cycle fatigue with the same stress range Δσ = 350 MPa but different stress R = –0.4 and R = –1.0, provide quantitative description of residual stress evolution near cold-expanded hole. It shown that maximal residual stress relaxation of order 20 per cent occurs at 95 lifetime per cent for both loading programs.

Application of stress intensity factor superposition in residual stress fields considering crack closure

The correlation between stress intensity factor (SIF) range and fatigue crack growth is a powerful tool for fail-safe design approaches applied to lightweight structures. The key role is precise calculation of the SIFs of fatigue load cycles. Advanced material processing can shape residual stresses and makes SIF calculation a challenging task. While the consideration of tensile residual stresses is successfully tackled by SIF superposition, the treatment of compressive residual stresses needs still clarification. This work demonstrates the application of the SIF superposition principle in regions containing high compressive residual stresses leading to crack closure effects. Crack closure depends on the combined load of residual and applied stresses and is interpreted as a change of crack geometry in this work. Thus the relation between the source, i.e. the applied or residual stress, and its consequence, i.e. the corresponding SIFs, depends on the interaction of the sources, i.e. the combined load. Due to this interaction, residual stress-induced changes of the fatigue behaviour cannot be linked to the residual or applied SIF only. This work proposes two alternative definitions of applied and residual SIF, allowing a clear correlation between either residual or applied SIF to fatigue behaviour changes.

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.