Notch Stress Research Articles

A dual fracture mechanical approach for estimating notch stress intensity factor and T-stress using volumetric methods on API 5L pipe steel: Experimental study and numerical validation

This study focuses on the evaluation of the Notch Stress Intensity Factor (NSIF) as a key parameter for estimating the material's master failure curve in API 5L pipe steel. The research encompasses both experimental investigations and numerical validations to comprehensively assess the applicability of NSIF in predicting failure behavior. Through a series of controlled experiments, various notched specimens were subjected to different loading conditions, allowing the determination of NSIF values. Concurrently, a numerical framework was developed using finite element analysis to simulate the stress distribution near the notches. The experimental results were compared with the numerical simulations to validate the accuracy of the proposed approach. This research contributes to enhancing the understanding of how NSIF can serve as a reliable indicator of failure potential in industrial applications, particularly in the context of pipeline structures. The findings highlight the significance of NSIF-based predictions in ensuring the integrity and safety of materials under varying loading conditions.

Investigation of Effective Notch Stress at the Root of the Rib-to-Deck Weld in an OSD Box Girder Bridge Considering the Wheel Load Position

AbstractFatigue cracks have been reported in orthotropic steel deck bridges under severe traffic conditions in Japan, particularly root-deck and bead cracks, which initiate from the root of the rib-to-deck weld. The local stress directly influencing crack initiation was investigated using a finite element model based on a section of an actual bridge with a high incidence of cracks. The analysis focused on the weld root at the floor beam intersection and the span center of the bridge section. The model was subjected to loading in the transverse and longitudinal direction of the bridge combined with various wheel load configurations. The effect on the local stress properties was analyzed using the effective notch stress approach. A load position slightly off-center of the U-rib resulted in peak stress at the study locations. Its principal stress direction angle around the notch suggests a root-deck type of crack initiation. Testing several pavement stiffnesses revealed that using an SFRC pavement resulted in a 76%–84% reduction of peak effective notch stress compared to asphalt pavement. Furthermore, varying weld configurations demonstrated that lowering the weld penetration rate could alter the local stress position, influencing the crack initiation direction.

Evaluation of the fatigue life of AISI 347 specimens with small notches based on local strain considerations

The use of materials data for cyclic loading determined on unnotched specimens to predict the fatigue life of notched components has a long tradition, specifically when it comes to nuclear engineering. The basic principle is that the material behaviour of the unnotched specimens can be transferred to what the material is exposed to be in a notch root. This principle has been proven for large notches. However, what happens when the notch is relatively small? Can the fatigue life still be predicted on the grounds of what has been considered as the local strain or Neuber approach? A key reference in that regard is the Neuber equation or generally the load versus notch stress, notch strain – or any combination of those – relationship. In a larger study, unnotched and notched specimens from the metastable austenitic steel AISI 347 have been tested and characterized providing a database that may give some answers to the question raised above.The unnotched specimens considered here had a cross-section diameter of 10 mm while the notch radii of the notched components were only 0.5 and 0.35 mm, respectively. Notch radii of such dimensions will easily reach a fully plastic condition once the material has exceeded the yield limit, because the difference in load between yield start in the notch and full plastification is relatively small. This fully plastic state exceeds the validity of the Neuber equation and requires respective corrections. In this paper it is shown how such corrections can be obtained through a finite element analysis and how this applies to the cases mentioned above.

Fatigue assessment of structural components through the Effective Critical Plane factor

The integrity assessment of structural components under complex loading conditions relies on the evaluation of the fatigue damage typically arising from stress concentrations, such as geometric irregularities, notches, weld beads, grooves etc.. Various methodologies, including the Notch Stress Approach (NSA), the Theory of Critical Distances (TCD), the Strain Energy Density (SED), and the Critical Plane (CP) concept, have been pivotal in assessing fatigue strength for notched and welded components. Recent works combine some of the above mentioned methodologies, while other works propose to vary the embedded parameters accounting for the loading type or the fatigue lives, trying to improve the accuracy of the fatigue assessment process. This paper introduces a novel approach, the Effective Critical Plane (ECP), which is founded on the critical plane concept. The CP factor is, however, calculated starting from an averaged, over a small volume, stress–strain field. The size of the averaging volume is assumed to be a material parameter and is determined by a best fitting procedure over different experimental data sets. The novel approach is illustrated by means of the Fatemi-Socie and the Smith-Watson-Topper CP damage factors. Its potential application to other CP formulations is straightforward, as well. Literature experimental data for low carbon steel specimens possessing different notches and loading conditions are used to validate the method’s capability in accurately determining the fatigue life and to set the radius of the averaging volume for the given material and CP parameter. A spherical volume or circular area are used in case of fully 3D or 2D numerical models, respectively. Results are compared to those of some of already existing methods, namely SED, TCD and the Modified Wöhler Curve Method.

Stem to prevent periprosthetic fracture after notching in total knee arthroplasty.

Improper osteotomy during total knee arthroplasty (TKA) can lead to anterior femoral notching, which increases the risk of periprosthetic fractures due to stress concentration. One potential solution is the addition of an intramedullary stem to the femoral component. However, the optimal stem length remains unclear. In this study, we aimed to determine the optimal stem length using finite element models. Finite element models of femurs were developed with unstemmed prostheses and prostheses with stem lengths of 50, 75, and 100 mm. Under squat loading conditions, the von Mises stress at the notch and stress distribution on four transversal sections of the femur were analyzed. Additionally, micromotion of the prosthesis-bone interface was evaluated to assess initial stability. The unstemmed prosthesis exhibited a von Mises stress of 191.8 MPa at the notch, which decreased to 43.1, 8.8, and 23.5 MPa for stem lengths of 50, 75, and 100 mm, respectively. The stress reduction on four selected femoral transversal sections compared with the unstemmed prosthesis was 40.0%, 84.4%, and 67.1% for stem lengths of 50, 75, and 100 mm, respectively. Micromotion analysis showed a maximum of 118.8 μm for the unstemmed prosthesis, which decreased significantly with the application of stems, particularly at the anterior flange. Intramedullary stems effectively reduced stress concentration at the femoral notch. The 50-mm stem length provided the optimal combination of reduced notch stress, minimized stress-shielding effect, and decreased micromotion at the anterior flange.

Mode-{I, III} multiaxial fatigue of welded joints in steel maritime structures: Total stress based resistance incorporating strength and mechanism contributions

Arc-welded joints in steel maritime structures are typically identified as weakest links in terms of fatigue limit state performance. Multiaxiality can be involved, consisting of predominant mode-I and non-negligible mode-III components. Aiming to answer the question if a cracked geometry based fatigue strength parameter would outperform an intact geometry based one like the effective notch stress, the total stress is adopted. A von Mises type of criterion is defined at the critical fracture plane and includes mode specific and material characteristic strength and mechanism contributions. A lifetime dependent shear strength coefficient is introduced to cover the resistance curves intercepts and slopes, whereas the total stress parameter contains the mean stress contribution as well as the (mixed) mode dependent notch and crack tip elastoplasticity coefficients, reflecting an interaction mechanism. Cycle counting includes a cycle-by-cycle non-proportionality measure and damage accumulation is based on a linear model. Evaluating mid-cycle fatigue resistance data, the total stress and effective notch stress performance turns out to be similar. However, the total stress related elastoplasticity coefficients are an explicit and sensitive measure to incorporate the actual physics of the fatigue damage process, whereas the material characteristic lengths for the effective notch stress seem to be more implicit and less sensitive ones.

Experimental determination of mode I notch stress intensity factors using valid strain gage locations

Accurate experimental determination of mode I notch stress intensity factors (NSIFs) of various sharp V-notched configurations using strain gages plays an important role in notch fracture mechanics. However, practically feasible, economical, and efficient strain gage techniques are hardly available for the sharp V-notched problems. Considering problems associated with the sharp V-notched bodies, the authors recently proposed a strong theoretical basis for an efficient single strain gage technique with valid gage locations for the accurate measurement of mode I NSIFs of sharp V-notched configurations. The main thrust of the present investigation is the experimental substantiation of the above strain gage technique using different V-notched configurations. It has been shown that these valid locations are governed by two parameters rmin and rmax of a sharp V-notched configuration. The results of the present investigation clearly demonstrate that very accurate NSIFs can be obtained from a single strain gage located at the suggested valid locations, which need not necessarily be in the vicinity of a notch tip. Further, the results also show that highly erroneous NSIFs can result if the gages are located at invalid radial locations. The results explicitly indicate that adopting the valid gage locations is beneficial rather than pasting the gages based on guesswork or other speculations.

Concept of “chemical notch” to study stress-corrosion

Abstract In this article, a new partially immersed slow strain rate (SSR) and corrosion-fatigue (CF) test methods are described and discussed. Partial immersion of a specimen in 3.5 % NaCl solution creates an interface between the solution and air. We call this a “chemical notch” stress concentration factor. The outcome of this demonstrates that the SSR test condition on a smooth specimen will fail at the interface if the machined notch stress concentration factor is less than the “chemical notch” stress concentration factor. This “chemical notch” concept can be applied to correlate corrosion-fatigue S–N data for a specific material/environment system.

Fatigue experiment and assessment of Q420qFNH weathering steel in the unequal-thickness butt-welded joint

The use of unequal-thickness butt-welded joints is widespread in bridge engineering, material savings mean a decrease in cost, which can lead to lower overall costs and significant benefits Because of their unique structures and propensity for stress concentration and weld defects, it is crucial to investigate their fatigue performance. In this study, 15 sets of unequal-thickness butt-welded joints based on Q420qFNH were subjected to fatigue tests. ABAQUS modelling and a stress-extraction process utilising distance iterations were employed in this study. The key findings demonstrated that the fatigue performance of these weathering steel butt joints was superior, and the stress variation at these joints exhibited greater complexity and resulted in a more pronounced multi-crack fusion effect. Additionally, an attempt was made to evaluate the fatigue behaviour of other welded joints and unequal-thickness butt-welded joints using the same approach as that presented herein. The fatigue evaluation method based on nominal stress yielded a significant scatter of 1:Ts = 4.21. Iterations were conducted based on the scattering at various critical distances. The results indicate that based on the theory of critical distance, the scatter of 1:Ts = 2.304 is lower than the scatter of 1:Ts = 2.502 associated with the notch stress method. These findings suggest that employing this method for fatigue assessment yields favourable outcomes across diverse welded joints.

Fatigue life evaluation of laser welded lap joints of dissimilar aluminum alloys

Abstract Constant amplitude fatigue tests were conducted on 6061/7075 dissimilar aluminum alloy laser welded lap specimens, as well as weld line cross-section hardness measurements. The fatigue test results show that the specimens exhibit multiple fracture modes that exit near the weld seam. The microhardness data on weld line cross-section from 7075 side to 6061 side display a sharp change and the softening phenomenon is serious. The hardness variation in heat affected zone of laser welding is very shallow, and its hardness is close to that of the base material. It was found that there are slag inclusions and pores in the weld seam when observing the fatigue fracture surface using SEM, and a small amount of secondary cracks were generated. However, stress concentration plays a dominant role in causing specimen fracture under fatigue loading, rather than welding defects. Defective specimens are found to have higher fatigue strength. The fatigue life prediction results obtained by the notch stress method and the hot spot stress method are both conservative and fall within two factor lines. The hot spot stress method has relatively higher accuracy for life prediction. The accuracy of both methods in predicting life is influenced by the location of the fracture.

Fatigue enhancement of welded thin-walled tubular joints made of lean duplex steel

In the present work, the fatigue strength and enhancement techniques of welded thin-walled (wall thickness of t = 2 mm) square hollow section (SHS) joints made of lean duplex stainless steel (grade EN 1.4162) are experimentally and numerically studied. Experimental fatigue tests were carried out on gas metal arc-welded tubular X-joints subjected to fully reversed (applied stress ratio of R = −1) constant amplitude in-plane bending moment. Geometrical improvement techniques were investigated by applying fully penetrating fillet welds and using structural reinforcements. The applications of high-frequency mechanical impact (HFMI) treatment were examined in the context of further enhancing the fatigue strength of the geometrically improved joints. In addition to the experimental work carried out in this study, the fatigue test data of welded thin-walled SHS and rectangular hollow section (RHS) joints were extracted from literature. Structural stress concentration factors were numerically and analytically obtained to validate the fatigue assessment model for welded thin-walled tubular joints using the structural hot-spot stress method. In addition, the effective notch stress (ENS) concept with the reference radius of rref = 0.05 mm was implemented. Compared to the joints in the as-welded state, fatigue strength improvements of 1.53–2.7 were found in the geometrically improved joints. On the other hand, the HFMI treatment did not lead to an additional increase in the fatigue strength capacity of the studied AW joints. The CIDECT guidelines for the structural hot-spot stress method were found conservative for thin-walled joints (t < 3 mm) and could be recommended for fatigue design.

Fatigue life evaluation of welded joints under multiaxial loading for different stress concepts using an extended Gough-Pollard criterion

Fatigue life evaluation of welded joints under multiaxial loading usually refers to stresses normal to the weld and shear stresses. Stresses parallel to the weld are not considered in most experiments or the well-known Gough-Pollard criterion. Hence, the Gough-Pollard criterion has recently been extended to include all stress components at the weld surface. In this paper, both the original and, for the first time, the extended Gough-Pollard criterion are applied to different welded specimens under multiaxial loading that includes stresses parallel to the weld. As shown, the original criterion is insufficient to evaluate such stress states. This is because the calculated fatigue life becomes less conservative as the stresses parallel to the weld become more significant. The extended criterion, on the other hand, shows greatly improved accuracy while significantly reducing the likelihood of non-conservative results. In conclusion, the extended Gough-Pollard criterion can describe fatigue life under multiaxial loading better than the original version and provides reliable and conservative results for welded joints. The main findings are valid for the nominal, the hot spot, and the notch stress concept.

Sequence effects on the life estimation of welded tubular structures made of S355J2H under uniaxial fatigue loading

The use of hollow sections to form lightweight structures is widespread in common steel processing industries such as crane, commercial vehicle, steel bridge and agricultural machinery construction. The hollow sections are mainly designed as truss or frame structures, in some cases using high-strength and higher-strength steels in order to achieve optimum utilization of the component and material. A new collection of fatigue life data covering sequence effects and the accuracy of the linear damage accumulation is presented. Effects of the shape of the applied load spectra and sequence effects of different amplitudes have been investigated. This document covers tubes of 4 to 8 mm thickness made by low-carbon or mild steel S355J2H. In general, it was found that the spectrum shape and the loading sequence have an influence on the service life. Depending on the shape of the spectrum, random tests tended to lead to shorter service lives than tests with block-loading sequences. An influence of overloads was also found for the tests with interspersed overloads. Typical maximum linear damage sums taken from recommendations and codes of 0.2 or sometimes 0.5 are exceeded for all spectra investigated and in some of the cases even significantly above 1.0. Transferability of the recommendations to component-type structures like tubular joints needs revision to lift its lightweight potential. Using stress concentration factors (SCF) from finite element analysis, typical strength values for the structural and effective notch stress concepts are checked. All joints investigated show a significantly higher strength compared to the IIW recommendations using the structural stress approach or compared to the DVS 0905 with the effective notch stress approach.

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Fatigue behaviour of root crack in stiffener-to-deck plate weld at crossbeam of orthotropic bridge decks

Steel Orthotropic Bridge Decks (OBDs) are widely used in long-span and movable bridges. Fatigue resistance analysis plays an important role in the design or assessment of OBDs. One possible fatigue failure is the crack initiating from the weld root of stiffener-to-deck plate connections at crossbeams. A full-scale experimental investigation in this study using a 20 mm thick deck plate with a dimension of 9.4 m × 5.1 m, including three crossbeams, represents the modern designed OBDs. The experiments show an arrest of crack propagation with a final crack depth of approximately 75% of the deck plate thickness. On the contrary, through thickness cracks develop in deck plates of 10 or 12 mm. Hot spot stress based fatigue detail categories (DC) using various failure criteria derived from the tests. Analysis with the effective notch stress shows that the DC has low sensitivity to the amount of weld penetration. The results of analyses with the eXtended Finite Element Method (XFEM), employed to analyse the fatigue crack propagation path and crack arrest, are in line with the experimental study.

Investigations on the influence of angular and linear misalignment on the fatigue strength of HFMI-treated structural steel butt joints

The effect of High-Frequency Mechanical Impact (HFMI) treatment on the fatigue resistance of structural steel S355 butt welds with macro geometric imperfections outside of the usual fatigue design limits was investigated. Thin-walled specimens (t = 8 mm) were manufactured with different degrees of angular and linear misalignment and fatigue tested in the as welded and HFMI treated condition. The fatigue test results show that HFMI treatment can compensate the fatigue strength-reducing effects of macro geometric imperfections (ΔσC,HFMI = 127 N/mm2, m = 5), but also that linear and angular misalignment does not reduce the fatigue strength of butt welds as severely as rules and guidelines suggest (ΔσC,AW = 107 N/mm2, m = 3). Underestimating the fatigue strength determined by testing by more than 40%, numerical analysis and strain gauge measurements show that the application of the effective notch stress approach and the structural hot-spot stress approach delivers conservative assessment results, which might not always be representative for the actual fatigue strength benefit that can be gained by HFMI treatment.

Residual Stress Prediction during TIG and MIG Welding Process of Steel - A Review

In this study, a forensic review of residual stress predictions and its impact on welded joints especially the steel pipes during the Tungsten inert gas welding and metal inert gas welding processes were reviewed. It was established that in the case of MIG welding, the effect of residual stress can be reduced after treatment. In the case of TIG welding process, increase in current will lead to deformation and residual stresses. Estimating the residual stress is a challenging task. It is common to predict residual stress using a notch stress intensity factor, a numerical simulation approach coupled with a thermal-mechanical finite element model. Thus, adequate design in welding parameters will help in the optimization of the process to achieve a safe and reliable welded joint during TIG and MIG welding process.

Evaluation of the fatigue resistance of butt-welded joints in towers of wind turbines — a comparison of experimental studies with small scale and component tests as well as numerical based approaches with local concepts

Wind turbines must endure a high number of load cycles during their service lifetime. Therefore, the fatigue strength verification plays an important role. Butt-welded joints are one of the most common structural details in the tower structure of wind turbines. In general, the nominal stress method is used for their fatigue verification. The Eurocode 3 part 1–9 is the current design standard for this field of application and defines the fatigue resistance by pre-defined FAT-classes. This paper presents recent results of fatigue tests on small-scaled specimens and large components with transverse butt welds to discuss the validity of the FAT-class. In addition, results from numerical simulations for the verification with the effective notch stress and the crack propagation approach are used for comparison. For this purpose, macroscopic measurements of the weld seam geometry and a 3D scan were used for a realistic consideration of the notch effect in the simulation. Based on the consistency between the numerical results and the fatigue tests, the influence of the seam geometry on the fatigue resistance was worked out. Furthermore, a prediction of the fatigue strength of butt-welded joints with plate thicknesses up to 80 mm was carried out.

Determination of notch stress intensity factors for V-notched specimens under mode I loading using the 3D-Digital image correlation and strain energy approach

In this paper, the fracture behaviour of Single Edge Notched Tension Specimen (SENT) is investigated by using the three-dimensional digital image correlation (3D-DIC) method and the mode I notch stress intensity factors (NSIFs) for the SENT specimen is derived. Initially, the concept of NSIFs and different experimental methods are reviewed. This is followed by a review of the theory of the DIC method. Then, the step-by-step process for setting up a 3D-DIC full-field experimental system for the SENT specimens tested is described. In the experiments, the SENT specimens were tested with pinned and clamped boundary conditions (BCs) and different notch initial opening angles. Furthermore, the analytical stress fields around the sharp V-notches are presented. Combining with the strain energy approach (SEA), the experimental strain fields are analysed to calculate the mode I NSIFs. Moreover, the presented numerical mode I NSIF values for the SENT model are validated by the available results in the literature. Finally, the experimental results obtained from the 3D-DIC method are compared with the numerical results from finite element analysis (FEA) to verify the feasibility and accuracy of the 3D-DIC method. The effect of different BCs of pinned and clamped ends is discussed.

Effect of angular distortion and axial misalignment on the fatigue strength of welded and ground mild steel cruciform joints

AbstractThe focus of this work is to investigate the fatigue strength of a cruciform joint one-sided through-welded by a HV seam with a backing layer made of a 10-mm-thick S355 mild steel. The aim is firstly to analyse the effect of the deformation state incorporating angular distortion and axial misalignment on the fatigue strength of the tested specimens and secondly, the analysis of the benefit by grinding as post-treatment method on the resulting fatigue performance. The deformation impact is numerically investigated utilizing geometry measurements of the specimens as well as strain gauge measurements for validation. Thereby, the near notch stress under clamping condition without any load and under an additional static nominal load is analysed. Based on these investigations, the applied stress range for each tested specimen is converted to a “deformation-free” condition by means of the IIW-recommended km factor. The statistical analyses of the data points with and without this modification reveal a significant impact by the deformation state on both the evaluated fatigue strength and the scatter band. Moreover, this affects the evaluated benefit by grinding as post-treatment, whereas an increase of more than 40% in the fatigue strength at 2 million cycles is observed using the modified data points.