Residual Stress Intensity 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.

Oxygen inward diffusion and complex reorganization of the reaction layer in ion-exchanged glasses evidenced by Auger electron spectroscopy

Alkali silicate glasses subjected to low-temperature ion exchange in molten salt are being probed by Auger electron spectroscopy (AES). Spot analyses and elemental line scans across fresh fracture surfaces of broken glass specimens reveal oxygen enrichment of reaction layer where Na+-for-K+ ion exchange occurred and exceptional similarity in shapes of concentration depth profiles of potassium and oxygen. The probe provides the first direct evidence that smaller-for-larger ion replacement is accompanied by inward oxygen transport into reaction zone where potassium cations rearrange former sodium sites in accordance with their steric demands. Such chemical and structural rearrangements entail defect formation and glass network depolymerization and fragmentation, which define stress buildup and relaxation, residual stress intensity and profile in the surface layer of chemically strengthened glasses.

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.

Investigation of the Intensity of Residual Stresses in a Growing Body Obtained by the Electric Arc Method

Investigation of the Intensity of Residual Stresses in a Growing Body Obtained by the Electric Arc Method

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.

Theoretical Basis for the Photoelastic Residual Stress Evaluation in Misaligned Cubic Crystals

Photoelasticity is a fast and powerful technique for internal stress detection and quality control in crystals; to fully exploit its possibilities, an appropriate theoretical analysis must be developed for different crystallographic structure and observation planes. For a cubic crystal specimen whose geometry is non-coherent with its crystallographic directions (i.e., observation planes and crystallographic directions are not parallel), we write a set of equations that allow an estimate of the refraction indices as a function of the residual stress. This is obtained upon the assumption that the residual stress may be represented by a plane stress parallel to the observation face. For cubic crystals, we obtain an explicit estimate of the residual stress intensity; this can be achieved provided we know the piezo-optic tensor component, the orientation of two non-parallel specimen faces with respect to the crystallographic axes, and that we can measure the principal directions of the refractive indices on the observation face.

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.

Optimization of modes of pendular surface plastic deformation for intensification of the stress-strain state of carbon steel parts

The results of modeling on the influence of the main parameters and modes of the pendulum SPD process on the stress-strain state (SSS) of the surface layer are presented. It has been established that the obtained dependences of the SSS indicators on the technological parameters of the pendulum RPM process are not linear, for the solution of which polynomial equations are used as input data in the Statictiska 10.1 software package. Using the software package Statictiska 10.1, the optimal modes of the pendulum RPM process were determined. It has been established that the optimal modes of the pendulum SPD, which allow simultaneously obtaining the maximum intensity values of temporary (560–580 MPa) and residual compressive stresses (340–350 MPa) of the surface layer of hardened parts, are quite close. To optimize the value of work hardening (h), a significant increase in the value of the radial interference and a decrease in the value of the longitudinal feed are required, which lead to the approach of the intensity of temporary stresses to the u l timate strength of the material (600 MPa), and the intensity of residual stresses to the yield strength of the material (360 MPa). Such regimes lead to a decrease, and perhaps even to a complete removal of the residual stresses of the hardened layer, as well as to a decrease in the degree of work hardening. Thus, the rational maximum value of the hardened layer with pendulum RPM is 2.45 mm.

Effect of parameters of reversible surface plastic deformation on temperature in contact zone of tool with workpiece

The article presents the results of experimental studies to determine the influence of the main technological parameters of reverse surface plastic deformation (SPD) on the temperature in the contact zone of the working tool with the workpiece. To implement the proposed method of finishing and hardening processing, a device has been developed that provides a reverse circular movement of the working tool. Using a FLIR SC7000 series thermal imager, the temperature in the contact zone of the working tool with the workpiece was determined. Experimental studies have shown that the value of the temperature on the surface of the workpiece with reverse SPD varies from 42 to 123 °C and depends on the main technological parameters of the process. It has been established that the temperature exceeding 123 °C at the contact leads to a decrease in the maximum intensity of residual stresses in the surface layer of the part. Based on the results of experimental studies and using the PyCharm computer program with the Python programming language, optimal hardening modes were determined that provide the minimum surface temperature of the part, which makes it possible to maintain the quality of the surface layer after hardening.

Intensification of stress state at deformation site under local action of a deformation tool

This paper discusses the development of a new procedure for finishing and hardening treatment by changing the kinematics of a deformation tool having a double-radius shape of the profile of the working surface, along with the determination of the stress-strain state in the site of elastic-plastic deformation and residual stresses in the hardened zone of the surface layer. The SOLIDWORKS 2018 software for 3D design and the ANSYS Work-bench 19.1 soft-ware using the finite element method were used to build a mathematical model of local loading. The temporary and residual stresses, the strain state in the loading zone, the depth of the plastic layer and the maximum value of the relative plastic deformation under various loading procedures of the working tool were determined. It was established that, at the reversible rotation of a double-radius roller, the values of temporary stresses are over 15% higher compared to those during static hardening, while the residual stresses are 5.7% higher. With the reversible rotation of the double-radius roll-er, the value of the maximum strain intensity becomes 2.11 times higher than that during static hardening. The intensity of the maximum residual stresses during the reverse rotation of the double-radius roller occurs at a depth 3 times greater than the indentation of the double-radius roller, rather than on the surface of the sample. The results of computer model-ling and numerical calculations indicate that the procedure of reversible rotation of a toroidal double-radius roller has the greatest influence on the intensity of the stress state in the deformation site, while the procedure of static hardening by a single-radius roller has the least impact. The obtained results suggest that the proposed technological process of surface plastic deformation based on the reversible rotation of the working tool will allow the radial tension to be reduced while maintaining the high quality of the surface layer of machine parts.

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.

Thermal stresses in growing thermoviscoelastic cylinder and their evolution in the course of selective laser melting processing

AbstractThe evolution in time of stresses and strains in a solid during its manufacturing through selective laser melting (SLM) process is studied. The technological technique, which allows to reduce the intensity of internal and residual stresses is proposed and theoretically grounded. The basic concept of the technique is that the additional in‐homogeneous inductive heating in the vicinity of the growing boundary is applied in the processing. The term “growing boundary” refers to the part of the boundary, on which the additional melted material is added. The evolutionary problem for a growing thermoelastic body is considered in approach of thermal stresses (for slow additive processes) and in the approach of full coupled thermoelasticity (for fast processing). The time depend distribution of internal stresses and their residual counterparts has been evaluated in analytical form in the case of model cylindrical problem. The analysis of the influence of frequency and amplitude of electromagnetic induction as well as viscoelastic properties of heated material being added has been carried out. The results are analyzed numerically for Titanium and Copper materials.

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.

Determination of the stress-deformed state of cylindrical parts at circular oscillation of a sectorial working tool

It is described the effect of the frequency of circular oscillations of a sectorial working tool on the stressstrain state of cylindrical parts. As a result of computer modeling, it was revealed that under the same loading conditions, the action of a working tool with different frequencies of circular oscillations creates different values of temporary and residual stresses. In the case of movement in one direction of the tool and the workpiece, the minimum value of stresses is formed, and with their oncoming movement, the value of the maximum stresses increases by 15...40 %. The resulting value of the residual stresses generally increases. With an increase in the frequency of circular oscillation of the working tool, the intensity of residual stresses increases by 1.2...2 times. The intensity of deformation changes in direct proportion to the value of the frequency of the circular oscillation and the depth of plastic deformation reaches 2.1...2.8 mm.

Residual stress distribution in thick double-V butt welds with varying groove configuration, restraints and mechanical tensioning

This paper aims to investigate the effect of different weld groove configurations on the residual stresses dispersed in 40 mm thick multipass high strength low alloy (HSLA) steel butt-welds. Two welded joints having double-V type groove geometry were prepared, one of which had an equal double-V groove, and the other had an unequal double-V groove appearance. The locked-in residual stresses present in the weld zone and heat-affected zone (HAZ) were evaluated using both computational and experimental approaches. Furthermore, the effect of restrained condition on the trend of residual stresses of the equal double-V groove weld was analyzed using computational modeling technique with varying weld pass sequence. Lastly, a feasibility study was conducted on the implementation of mechanical tensioning technique to mitigate the weld-induced residual stresses in thick multipass butt-welded joints. The findings show that using an unequal-V groove joint can effectively reduce residual stress levels in the bottom half of the weld. A restrained welding condition with a single directional weld pass sequence can adversely impact the intensity of residual stresses across the weldment. The transverse residual stress levels can be reduced in the weld zone and its vicinity by an amount equal to the corresponding applied mechanical lateral tensioning load.