Notch Stress Research Articles

Stress magnification effect of initial deformation on the notch stress field and fatigue strength of thin plate welded joints

Compared with thick plate welded joint, the welding joint of thin plate will produce initial deformation due to its low bending rigidity. The existence of initial deformation will cause the welded structure to produce secondary bending effect, which will produce greater stress magnification effect at the weld toe and seriously affect the fatigue strength of thin plate welded joints. Therefore, based on the correction formula of thick plate, considering the influence of initial deformation and geometric nonlinearity of thin plate, this paper deduces the stress magnification factor formula at the weld toe of T-shaped and cruciform specimens. The accuracy of the revised formula is further verified by comparing the notch stress calculated by the modified formula with the FE results. Finally, the modified formula is applied to the notch stress and fatigue evaluation of typical thin plate welded joints respectively. The results show that the proposed notch stress calculation formula can fully consider the stress amplification effect of thin plate structure, and can be used to quickly evaluate the notch stress field and fatigue strength of thin plate welded joints.

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.

IKAROS is required for the measured response of NOTCH target genes upon external NOTCH signaling.

The tumor suppressor IKAROS binds and represses multiple NOTCH target genes. For their induction upon NOTCH signaling, IKAROS is removed and replaced by NOTCH Intracellular Domain (NICD)-associated proteins. However, IKAROS remains associated to other NOTCH activated genes upon signaling and induction. Whether IKAROS could participate to the induction of this second group of NOTCH activated genes is unknown. We analyzed the combined effect of IKAROS abrogation and NOTCH signaling on the expression of NOTCH activated genes in erythroid cells. In IKAROS-deleted cells, we observed that many of these genes were either overexpressed or no longer responsive to NOTCH signaling. IKAROS is then required for the organization of bivalent chromatin and poised transcription of NOTCH activated genes belonging to either of the aforementioned groups. Furthermore, we show that IKAROS-dependent poised organization of the NOTCH target Cdkn1a is also required for its adequate induction upon genotoxic insults. These results highlight the critical role played by IKAROS in establishing bivalent chromatin and transcriptional poised state at target genes for their activation by NOTCH or other stress signals.

Fracture assessments of blunt V-notches using the coherent gradient sensing (CGS) method

In this paper, the coherent gradient sensing (CGS) method is extended to study the local stress fields and fracture behavior of a blunt V-notch tip. First, the optical governing equations of the CGS method are derived based on Filippi’s stress equation for blunt V-notches and the concept of the notch stress intensity factor (NSIF). Moreover, the CGS fringe patterns in the vicinity of the blunt V-notch tip with different opening angles and notch radii are theoretically synthesized. Next, the fracture property of a blunt V-notch is experimentally studied using a series of three-point-bending polymethyl methacrylate (PMMA) specimens with different geometrical configurations. Finally, the generalized NSIFs up to the initiation of a fracture are evaluated using the proposed CGS method. The CGS fringe patterns in the experimental results exhibit a reasonable agreement with the theoretical fitted fringes, and the NSIFs and notch fracture toughness at the blunt V-notch tip are consistent with those obtained in previous theoretical studies and finite element simulations.

Local Stress–Strain Estimation for Tenon Joint Structure under Multiaxial Cyclic Loading at Non-isothermal High Temperature

In this paper, a method based on the improved effective stress concentration factor is proposed to estimate the local stress–strain for tenon joint structure under multiaxial cyclic loading at non-isothermal high temperature. In order to consider the effect of changing temperature on material properties, the impact caused by temperature on the stress–strain response needs to be introduced to the parameters of material constitutive model. So that, each material parameter in viscoplastic constitutive model is expressed as a function of temperature. In addition, the computing method of the effective stress concentration factor is improved so as to solve the problem of the overestimation for local stress–strain of tenon joint structure in the Neuber rule. Moreover, the proposed notch correction method is coupled with the Chaboche unified viscoplastic constitutive model to estimate the local stress–strain of tenon joint structure at non-isothermal high-temperature condition. To verify the reliability of the proposed method, the calculation results are compared with the nonlinear finite element analysis results under various loading paths at non-isothermal high-temperature condition. The results showed that the proposed method can accurately estimate the notch stress and strain under multiaxial thermo-mechanical cyclic loading.

Generalized SCF Formula of Out-Of-Plane Gusset Welded Joints and Assessment of Fatigue Life Extension by Additional Weld.

The existing S-N curves by effective notch stress to assess the fatigue life of gusset welded joints can result in reduced accuracy due to the oversimplification of bead geometries. The present work proposes the parametric formulae of stress concentration factor (SCF) for as-welded gusset joints based on the spline model, by which the effective notch stress can be accurately calculated for fatigue resistance assessment. The spline model is also modified to make it applicable to the additional weld. The fatigue resistance of as-welded and additional-welded specimens is assessed considering the geometric effects and weld profiles. The results show that the error of SCFs by the proposed formulae is proven to be smaller than 5%. The additional weld can increase the fatigue life by as great as 9.4 times, mainly because the increasing weld toe radius and weld leg length lead to the smaller SCF. The proposed series of S-N curves, considering different SCFs, can be used to assess the welded joints with various geometric parameters and weld profiles.

Fatigue analysis of wire arc additive manufactured (3D printed) components with unmilled surface

First investigations have been carried out on the characterisation of the as-welded surface roughness of unmilled wire arc additive manufactured (WAAM) components, followed by an experimental determination of the fatigue strength. Subsequently, randomly based finite-element (FE) simulations are carried out to expand the findings. The statistical characteristics of the surface provide the input values for the Monte-Carlo (MC) simulation. Here, the fatigue strengths of single-wire and multiple-wire welded components have been compared applying the effective notch stress concept performing over 2200 simulations. Finally, a fatigue detail classification for surfaces of WAAM components is given following the principles of EN 1993-1-9 [1].

On notch fracture mechanics

Different stress distributions for an elastic behavior are presented as analytical expressions for an ideal crack, a sharp notch and a blunt notch. The elastic plastic distribution at a blunt notch tip is analyzed. The concept of the notch stress intensity factor is deduced from the definition of the effective stress and the effective distance. The impacts of the notch radius and constraint on the critical notch stress intensity factor are presented. The paper ends with the presentation of the crack driving forceJρfor a notch in the elastic case and the impact of the notch radius on the notch fracture toughnessJρ,c. The notch fracture toughnessJρ,cis a measure of the fracture resistance which increases linearly with the notch radius due to the plastic work in the notch plastic zone. If this notch plastic zone does not invade totally the ligament, the notch fracture toughnessJρ,cis constant. This occurs when the notch radius is less than a critical one and there is no need to use the cracked specimen to measure a lower bound of the fracture resistance.

Local stress analysis of blunt V-notches using the digital gradient sensing method.

In this study, the digital gradient sensing (DGS) method is extended to determine the local stress fields at the blunt V-notch tip. Completely analytical expressions for angular deflections of light beams in the vicinity of the blunt V-notch tip are deduced based on Filippi's stress equation. Under plane stress conditions, the patterns of angular deflection related to two orthogonal stress gradients are theoretically plotted, and the effects of notch angle and notch radius are synthetically investigated, respectively. New procedures for the evaluation of local stress components and generalized notch stress intensity factor (NSIF) are presented using the experimental patterns of angular deflection contours. The NSIF values at the blunt V-notch tip in polymer materials with different geometric parameters under three-point-bend loading conditions are extracted from the DGS measurements. A good agreement is observed between the experimental data and the finite element simulations, which can verify the effectiveness and accuracy of the proposed DGS method.

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.

Modelling and experimental validation of material deformation at different zones of welded structural-steel under multiaxial loading

The fatigue life assessment of welds represents an engineering challenge regarding material inhomogeneity, welding residual stresses, notch stress effects and multiaxial loading conditions. In a weldment due to the non-uniform heating process the material in the weld metal and adjacent regions may exhibit heterogeneous mechanical properties. This makes the local stress-strain evaluations even more complicated under fatigue loading conditions. Therefore it is essential to characterize the variation of mechanical and particularly cyclic properties of the material in the vicinity of the weld notches. To improve the predictions of fatigue life estimation, numerical modelling techniques are complemented by experimental studies.The present work investigates the effects of weld induced microstructure variations in the multiaxial cyclic properties of the ferritic-pearlitic steels. A numerical model to describe the cyclic plastic deformation behavior of constructional steel S355J2H is established. Two material states present in the vicinity of a weld, the base material and the heat-affected zone (HAZ) are considered. Special attention was given to the non-proportional hardening of the material. Experimental results for both materials are used to calibrate the isotropic and kinematic hardening constants in the Chaboche plasticity model. The calibrated model is applied to multiaxial loading conditions. To account for non-proportional hardening effects, a constitutive motivated parameter by Tanaka is implemented. Therefore, first, the proportional results are reproduced and then the non-proportionality parameter is incorporated into the isotropic hardening law. The results of the calibration procedure for both proportional and non-proportional multiaxial load paths are discussed regarding its applicability in fatigue analysis and limitations of the model.

Fatigue assessment of laser beam and friction stir welded joints made of aluminium

Available fatigue recommendations are typically developed for arc-welded joints. Some of them may need to be adjusted to fit fatigue life of joints manufactured with non-conventional welding techniques since they show different mechanical properties. Laser beam and friction stir welded butt and overlap joints made from aluminium are addressed in this paper focused on a comparison between different fatigue assessment methods. For an evaluation, S-N data from published papers was collected. Additionally, fatigue tests on laser beam and friction stir welded overlap joints were carried out. For a fatigue assessment, the nominal stress, notch stress and effective stress approach were applied. The comparison of the endurable nominal stresses showed a comparatively high fatigue strength of the tested friction stir welded overlap joints in comparison to literature data. The notches at the interface of the overlap joints were observed to have an asymmetric geometry compared to the common symmetric one, frequently found in literature, leading to a lower stress concentration. A comparison between endurable nominal stresses and the classes FAT 28 for butt welded and FAT 12 for overlap welded joints resulted in a conservative design for the majority of specimens. An evaluation based on notch stresses lead to partly non-conservative results that could be explained by the mild notches present at the laser-beam welded butt joints. For the effective stress approach, an evaluation of the micro-structural length was performed and a FAT-value is proposed.

Evaluation of generalized stress intensity factors at blunt V-notch tip in polymer materials using transmitted caustics

The paper deals with the notch stress intensity factors (NSIFs) at a blunt V-notch tip in polymer materials using transmitted caustics. First, the governing equations of transmitted caustics which relate to the experimental measurements and the linear elastic stress fields at blunt V-shaped notches are established. Second, a series of caustic curves at the blunt V-notch tip under mode I loading are simulated. Finally, fracture experiments of polymer materials with a blunt V-shaped notch under three-point-bend loading are performed. The NSIFs obtained from caustic measurements show a good agreement with numerical results.

Effective-notch-stress-based fatigue evaluation of rib-deck welds integrating the full-range S—N curve concept

Compared with the nominal and hot-spot stress, the effective notch stress could present more accurate stress state by involving the notch stress induced by weld bead. Inspired by the concept of full-range SN curve, a new notch stress-based fatigue evaluation framework is proposed in this study. The proposed fatigue evaluation framework reasonably considers the material types, welding residual stress, mean stress effect and notch stress concentration. Fatigue testing data of 60 rib-deck welded specimens are extracted to validate the feasibility of the proposed framework. Field monitoring in a real cable-stayed bridge was also carried out as a case study to present the application of the proposed framework. Results show that the proposed fatigue evaluation framework is feasible and reliable for the evaluation of fatigue testing results of rib-deck welded specimens. Instead of the Goodman method and SWT method, Walker equation is recommended to take into consideration the mean stress effect. In engineering application, the proposed fatigue evaluation framework provides more conservative prediction results compared with code recommended FAT225 SN curve, which is applicable to important components where fatigue cracking is not allowed.

The Analysis of Micro-Scale Deformation and Fracture of Carbonized Elastomer-Based Composites by In Situ SEM.

Carbonized elastomer-based composites (CECs) possess a number of attractive features in terms of thermomechanical and electromechanical performance, durability in aggressive media and facile net-shape formability, but their relatively low ductility and strength limit their suitability for structural engineering applications. Prospective applications such as structural elements of micro-electro-mechanical systems MEMS can be envisaged since smaller principal dimensions reduce the susceptibility of components to residual stress accumulation during carbonization and to brittle fracture in general. We report the results of in situ in-SEM study of microdeformation and fracture behavior of CECs based on nitrile butadiene rubber (NBR) elastomeric matrices filled with carbon and silicon carbide. Nanostructured carbon composite materials were manufactured via compounding of elastomeric substance with carbon and SiC fillers using mixing rolling mill, vulcanization, and low-temperature carbonization. Double-edge notched tensile (DENT) specimens of vulcanized and carbonized elastomeric composites were subjected to in situ tensile testing in the chamber of the scanning electron microscope (SEM) Tescan Vega 3 using a Deben microtest 1 kN tensile stage. The series of acquired SEM images were analyzed by means of digital image correlation (DIC) using Ncorr open-source software to map the spatial distribution of strain. These maps were correlated with finite element modeling (FEM) simulations to refine the values of elastic moduli. Moreover, the elastic moduli were derived from unloading curve nanoindentation hardness measurements carried out using a NanoScan-4D tester and interpreted using the Oliver–Pharr method. Carbonization causes a significant increase of elastic moduli from 0.86 ± 0.07 GPa to 14.12 ± 1.20 GPa for the composite with graphite and carbon black fillers. Nanoindentation measurements yield somewhat lower values, namely, 0.25 ± 0.02 GPa and 9.83 ± 1.10 GPa before and after carbonization, respectively. The analysis of fractography images suggests that crack initiation, growth and propagation may occur both at the notch stress concentrator or relatively far from the notch. Possible causes of such response are discussed, namely, (1) residual stresses introduced by processing; (2) shape and size of fillers; and (3) the emanation and accumulation of gases in composites during carbonization.

A novel high-frequency fatigue testing methodology for small thin-walled structures in the HCF/VHCF regime

A novel high-frequency testing methodology for small thin-walled component-like structures is presented that enables fatigue tests up to 1e9 cycles within 7–8 days. A shaker excites the designed thin-walled (min.0.1 mm) specimen near its resonant frequency and a non-contact relative displacement measurement allows to set a defined notch stress. Calculated and experimentally determined resonant frequencies deviate max.by −1.9%. Furthermore, vibration and temperature measurements prove the stability of the technique, no self heating of the specimen is detected. Validation tests with an X5CrNiCuNb16-4 steel show a typical S-N behaviour for this material, whereby cracks always appear in the intended notch.

Effects of grinding treatment on surface properties and deformation microstructure in alloy 304L

Different types of sandpapers were used to grind surface of 304L alloy, and the effects of grinding treatment on surface properties and deformation microstructure were studied. Three dimensional profilometer, Vickers hardness tester, X-ray diffraction (XRD), scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) were used to characterize the microstructure and mechanical properties. The results showed that with the increase of grinding particle size the surface roughness and hardness increased, while the absolute value of residual compressive stress decreased, however the grinding particle size had no effect on the average tension strength because the groove did not lead to the effect of notch stress concentration. After grinded by coarse sandpaper (No. 60), the surface metal deformed in the way of martensite shear from γ-austenite to α′-martensite, which conformed to K-S model, and surface nano-crystal layers with a thickness of about 2.3 μm, 1.5 μm and 0.6 μm were found at the groove peak, slope and valley, respectively. By comparison, after grinded by fine sandpaper (No. 2000), the nano-crystallization did not occur, and a high-density dislocation layer with a thickness of more than 1 μm was observed at the groove peak, slope and valley. • Hardness increase and compressive stress decrease with the increase of roughness. • Roughness had no effect on tension strength and notch stress concentration. • Surface metal deforms in the way of γto nano-α′ after grinded by No. 60 sandpaper. • Thickness of nano-α′ layer at groove peak, slope and valley decreases gradually. • Surface metal deforms as dislocation slip after grinded by No. 2000 sandpaper.

An overview and comparative assessment of approaches to multi-axial fatigue of welded components in codes and standards

Three types of commonly used local stress approaches recognized by Codes and Standards for fatigue of welded joints are reviewed and evaluated with respect to a comprehensive collection of multi-axial fatigue test data available in literature. These local stress approaches are surface-extrapolated hot spot stress, effective notch stress and traction structural stress. The hot spot stress and notch stress approaches are based on those stipulated in Eurocode 3 (2005) and IIW Recommendations (2016). Traction structural stress is based mainly on ASME VIII Div 2 Code (2007) and its latest development. The results show traction structural stress method offers a significantly improved data correlation over hot spot stress and notch stress approaches.

Fatigue assessment of laser beam welds between AlSi10Mg AM-structures and conventionally manufactured aluminum by local approaches

The fatigue assessment of dissimilar aluminum materials is made by notch stresses for laser beam welded joints between additively manufactured AlSi10Mg and conventionally manufactured EN AW-6082 T6. The assessment is extended from the joint types of single side welded lap joints and peel T-joints both with an I-shape seam weld to lap joints with fillet welds on both sides. The results of notch stresses based on the von Mises stress hypothesis for lap joints with fillet welds exceed FAT 142 and even FAT 160 recommended for weld toe failure, assuming a slope of the design S-N curve of k = 5 in case of thin and flexible structures. Notch stresses of both joint types with I-shape seam welds are in good agreement. Recommended notch stress fatigue classes of FAT 160 or higher cannot be applied in a safe fatigue assessment for these joint types, since some lower stress values (for peel T-joints) are found. The scatter of an overall notch stress S-N curve for LBW joints with I-shape and fillet welds is increased.From the structural stress assessment of a LBW aluminum component, a sufficiently low difference in the fatigue life between the FE-based estimation and the experiment is obtained. However, this result is an outcome of a specific modeling and meshing approach. Consideration of multiaxial load cases or a general validation is not part of this study.

Studying Thermal Shock Resistance of Ceramic Materials Based on Their Structural Sensitivity to a Stress Concentrator

A method for determining the thermal shock resistance of ceramic materials is proposed based on their structural sensitivity to a stress concentrator controlling fracture during thermal loading. For this purpose, a prismatic sample with a specially created notch (stress concentrator) was subjected to thermal cycling realized by heating it to 850°C and cooling with a water jet (20°C). The coolant was supplied to the notch apex of the heated sample (local thermal shock (LTS) technique). By using the LTS technique, the thermal shock resistance of the following 3 types of ceramic materials was studied: α-Al2O3(type 1), α-Al2O3– β-SiC whiskers (20 vol.%) (type 2) and t-ZrO2 partially stabilized with Y2O3 (3.5 mol.%) (type 3). Type 2 ceramics has the highest thermal shock resistance, type 3 ceramics—the lowest, and type 1 ceramics occupies an intermediate position among the studied materials, which is associated with the specifics of formation and distribution of thermal defects in the structure of these ceramic materials as a result of using the LTS technique. The advantage of the latter consists in an explicit reproducibility of the results of determining thermal shock resistance (RT) within the studied batch of samples (measurement accuracy 3 – 5%).