Fatigue Strength Capacity Research Articles

DETERMINATION OF CRITICAL PLANES IN MULTIAXIAL FATIGUE WITH ESTIMATION OF SHEAR STRESS AMPLITUDE IN MECHANICAL COMPONENTS THROUGH DIFFERENT FAILURE CRITERIA

Fatigue strength analysis in mechanical components is fundamental in the development across various industrial and economic sectors worldwide. Different materials are used in the construction of structural and mechanical elements subject to intermittent and intense usage under various loading conditions. The evaluation of fatigue strength of materials is widespread in diverse computational research, which presents various characteristic parameters according to fatigue strength and material failure criteria. The presence of shear stresses in materials, mean stresses, their amplitudes, and critical planes of rupture seek to identify the assessment of the structural reliability of these elements. The Susmel and Lazzarin Method and the Findley Method, together with the evaluation of the shear stress in a loading history in materials, are capable of indicating, according to stipulated criteria and parameters, error indices, and affirming the accuracy of the fatigue strength capacity of materials. Thus, the determination of fatigue strength can indicate better design, construction, and maintenance parameters for studied elements.

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 strength of single-sided fillet welds in overlapping ultra-high-strength steel sheets

From the manufacturing viewpoint, overlapping thin sheets can provide a substantial geometrical improvement in welded hollow sections compared to butt-welded cross-sectional details. However, the plate eccentricity and non-penetrating fillet welds make the joints susceptible to fatigue failures under transversal cyclic loads. This work experimentally investigates the fatigue strength of overlap joints prepared with gas metal arc welding in the single-sided fillet weld configuration. Fatigue tests were carried out on the lap joints made of S960 ultra-high-strength steel (UHSS) grade under uniaxial constant amplitude axial loading employing both pulsating tension (applied stress ratio of R ≈ 0) and pulsating compression (R ≈ -∞). In addition, the lap joints were prepared with both straight welds (the welds transverse to the loading direction) and inclined welds (the welds with a 30° inclement angle from the transversal direction) to investigate the shear stress effects on the joints’ fatigue performance. Plasma butt-welded samples were tested as a reference join type. For the plasma butt-welded joints, the recommended detail category of FAT71 in the nominal stress system for weld root failures in single-sided butt welds was observed clearly conservative—a mean fatigue strength of Δσc,50% = 130 MPa with a fixed slope parameter of m = 3 was obtained. Compared to the butt-welded joints, a significant decrease in fatigue strength capacity was found in the lap joint specimens with misalignment factors of km > 3.0. The failure locations were also different in joints subjected to the tension and compression loads. The shear load did not majorly contribute to the changes in the fatigue strength capacity compared to the joints subjected to the transversal normal stress.

Influence of shear cut holes on the fatigue performance of hot-rolled 800 MPa automotive steel grades

Experimental fatigue tests were carried out for two t = 3 mm 800 MPa hot-rolled (HR) high-strength steel grades. These steels have good formability, and it is possible to use shear cutting to enable high productivity. The test campaign included unnotched base metal specimens to evaluate the fatigue performance of HR surface and notched specimens with shear-punched and electrical discharge-machined circular holes to evaluate the effect of cutting process and geometrical shape on the fatigue strength capacity. In this context, the properties of the shear-cut affected zone (SAZ) and resulting high tensile residual stresses, and their effects on the fatigue performance, were evaluated. The fatigue test results were first evaluated based on the nominal stress approach, followed by the applications of local approaches. The mean fatigue strength of the specimens with shear cut holes in the nominal stress system was 125 MPa (with a slope parameter of m = 3, stress range at the net cross section), which is at the comparable level to laser cut holes. Fractography analysis, conducted with scanning electron microscopy (SEM), showed local crack-like defects at the SAZ, and the characterization of local defect feature was utilized in the fatigue strength assessment using a multiparametric notch stress method applying effective stresses obtained by the critical distance approach.

Mode II Delamination under Static and Fatigue Loading of Adhesive Joints in Composite Materials Exposed to Saline Environment.

This study investigates the fatigue delamination behavior of adhesive joints in epoxy carbon composite materials under Mode II fracture loading. The joints were characterized using the End-Notched Flexure (ENF) test, comprising adhesive joints formed by bonding two unidirectional carbon fiber epoxy matrix laminates with epoxy adhesive. These joints were subjected to different exposure periods (1, 2, 4, and 12 weeks) in a saline environment. Prior to dynamic fatigue testing, critical Mode II energy release rate values were determined through quasi-static tests, serving as a reference for subsequent fatigue characterization. This study aimed to comprehend how exposure duration to a saline environment affected the initial stage of fatigue delamination growth and employed a probabilistic model based on the Weibull distribution to analyze the experimental data. The results, gathered over a two-year experimental program, revealed varying behaviors in adhesive joint resistance to delamination based on exposure duration. A noteworthy reduction in fatigue strength capacity was observed, with fracture energies for infinite fatigue life reaching approximately 20% of their static loading capacity. This study sheds light on the deterioration of adhesive joints when exposed to a saline environment.

Overload and variable amplitude load effects on the fatigue strength of welded joints

Different load spectra and individual load peaks might substantially relax high residual stresses as well as induce compressive residual stresses in welded components and, consequently, affect the fatigue performance of these joints. Consideration of peak loads and resulting relaxation of residual stresses in fatigue analyses can substantially enhance the accuracy of life prediction. The aim of the current study is to experimentally investigate the fatigue strength of welded joints subjected to different levels of overloads and variable amplitude (VA) loads and to develop local fatigue assessment method to account for the relaxation of residual stresses via a mean stress correction using the 4R method. The 4R method applies a local stress ratio for the mean stress correction considering material strength, residual stresses, applied stress ratio of external loading and local weld geometry in elastic–plastic material behaviour. Fatigue tests were carried out on fillet-welded longitudinal gusset joints made of S700 high-strength steels under applied stress ratio R = 0–0.1. A mild strength steel (S355) and ultra-high-strength steel (S1100) were selected as reference steel grades for the fatigue testing to study the material strength effects. Numerical analyses were conducted to evaluate the fatigue notch factors using the effective notch stress concept with the reference radius of rref = 1.0 mm and theory of critical distance (TCD) using the point method. The experimental results indicated that a substantial improvement in the fatigue strength capacity can be claimed in the joints subjected to tensile overloads, particularly in the studied S700 and S1100 steels. The higher-level overload (0.8fy), corresponding to the nominal cross-sectional area, improved the mean fatigue strength of the welded joints manufactured of high-strength S700 steel by approximately 60%, while the lower overload (0.6fy) improved the mean fatigue strength by 20%. In addition, a use of equivalent nominal stresses for the joints subjected to VA loads resulted in conservative assessments when employing S–N curves obtained for the CA loading. The 4R method, via the local mean stress correction for individual cycles, provided higher accuracy for the fatigue assessments.

On the interaction of axial and bending loads in the weld root fatigue strength assessment of load-carrying cruciform joints

Current design standards and recommendations incorporate the nominal stress system for assessing the fatigue strength capacity of load-carrying cruciform (LCX) joints with the fillet welds and failing from the weld root. Thus far, bending-loaded joints have not been addressed in these standards. The aim of the present study is to investigate the fatigue performance of LCX joints subjected to cyclic axial and bending loads. Firstly, fatigue test data sets of such joints subjected to axial loading and bending loads in the adjoined plate component are extracted from the literature, and statistical analyses are carried out to evaluate the fatigue strength capacity using the nominal weld stresses (NWSs). Secondly, experimental fatigue tests are carried out on LCX joints made of ultra-high-strength steel (UHSS) grade using constant amplitude loading and subjected to combined axial and bending load to study the load interaction effects on the fatigue strength capacity. The results showed that the FAT36 detail category for the weld root failures is applicable for bending-loaded joints when applying NWSs calculated on the basis of effective throat thickness of weld and assuming linear-elastic stress distribution over the joint section. The effective notch stress analyses showed unconservative results for the tested joints, when applying FAT225 design curve with the reference radius of rref = 1.0 mm.

Fatigue strength assessment of ultra-high-strength steel fillet weld joints using 4R method

A fatigue analysis on experimentally tested transverse fillet-welded non-load-carrying T- and X-joints made of S960 and S1100 ultra-high-strength steel was carried out in the present study. The test data consisted of welded joints in the as-welded, high frequency mechanical impact-treated, and tungsten inert gas-dressed conditions, that were fatigue tested using uniaxial constant amplitude loading with an applied stress ratio of R = 0.1–0.5. The weld geometry and residual stress measurements were carried out, and for each joint, the fatigue strength was assessed using a multiparametric notch stress approach, entitled the 4R method, which incorporates the consideration of four parameters, i.e. notch stress range Δσk(r), applied stress ratio R, material ultimate strength Rm, and residual stress σres in the fatigue assessment. In the 4R method, the local cyclic elastic-plastic behavior at the notch root is obtained, and the Smith-Watson-Topper parameter is applied to conduct a mean stress correction to commeasure all results into a single S-N curve. The results showed that the applied stress ratio had a distinct influence on the fatigue strength capacity for both joints in the as-welded and post-weld treated conditions when using the conventional stress-based approaches, i.e. nominal stress, structural hot-spot stress and effective notch stress concepts. Nevertheless, the 4R method resulted in a good agreement between the experimental test results and the fatigue strength assessments, regardless of the load and joint conditions.

Fatigue strength capacity of load-carrying fillet welds on ultra-high-strength steel plates subjected to out-of-plane bending

Weld root fatigue strength capacity is an important design criterion in load-carrying (LC) fillet welded joints subjected to cyclic loads. This paper elaborates on the weld root fatigue strength capacity of fillet welded LC joints made of ultra-high-strength steel (UHSS) and subjected to out-of-plane bending. Experimental fatigue tests are carried out using constant amplitude loading with an applied stress ratio of R = 0.1 with both pure axial, i.e. DOB = 0 (degree of bending, bending stress divided by total stress) and bending, i.e. DOB = 1.0, load conditions. The applicability of different approaches – nominal weld stress, effective notch stress concepts, and 2D linear elastic fracture mechanics (LEFM) – for the fatigue strength assessment of weld root capacity is evaluated. Furthermore, a parametric LEFM analysis is used to evaluate the effect of weld penetration on the root fatigue strength capacity in axial and bending loading. The results indicate that in the case of bending, nominal weld stress can be calculated using the linear stress distribution over the joint section and FAT36 as a reference curve. In the bending loading, for the joints failing from the weld toe, a mean fatigue strength of up to 185 MPa in the nominal stress system was achieved, indicating that the reference curve FAT63 is overly conservative. The ENS concept with FAT225 seemed to be slightly unconservative for assessing the root fatigue strength capacity. LEFM analyses revealed that in the case of increasing weld penetration and bending loading, weld root fatigue strength capacity seemed to correlate with the nominal weld stress calculated using effective weld throat thickness, while in axial loading, weld stress should be calculated using external throat thickness summed with penetration length.

Analisys of Welding Crack on The Under Frame of Wagon for Cement Bags Transportation Using Euro Code, Measurement and Finite Element

The Underframe of Wagon steel structure have both complex geometry and loading conditions producing complex Underframe structure behaviour which is hard to estimate and analyse using the traditional fatigue calculation methods. The impacts of the loading direction, loading type and detail geometrical shapes, i.e. Underframe components which are working in a group, need to be considered carefully in the stress analysis. Using the assumption of elastic behavior for all the underframe structural systems, in which sources of stress increased that have significant effects on the fatigue strength capacity are several included, can over/lower yield stress values to be analyzed in fatigue design. The application of the advanced stress assessment methods in the welding area using the Euro Code calculation studied in this paper produces more accurate stress results that can explain welding crack during operation in The car body of Wagon Steel. Based on Euro Code stress assesment, FEM analysis and measurement can be known that crack on the welding joint of wagon bottom part due to lack finishing of the welding joint. The IIW recommendations can be use in order to repair welding joint finishing to avoid crack fatigue.