Fatigue Resistance Curves Research Articles

On the slope of the fatigue resistance curve for laser stake‐welded T‐joints

ABSTRACTThe paper investigates the influence of highly localised stress distribution around the notch tips of the laser stake‐welded T‐joints to the slope of the fatigue resistance curve. The study considers experimental data of eight series involving joints under tension or bending loads. Various boundary conditions and plate thicknesses are considered. The stress distribution in the singularity‐dominated zone ahead of the notch tips is investigated by means of the finite element analysis. The aim is to relatively distinguish the stress distribution from one case to another. The growth rate of the elastic singular stress with respect to the distance from the tip is described by the dimensionless gradient. This gradient is equal to the slope of the linear stress‐distance function when presented in double‐logarithmic scale. The slope of the fatigue resistance curve varies approximately from 4 to 8. It is observed that the change of the slope can be closely associated with the gradient of the maximum principal stress evaluated in the plane that is orthogonal to the crack path. The orthogonal plane corresponds to the maximum principal stress direction. In contrast, there is a large scatter in the relation between the slope and the gradient evaluated in the commonly assumed crack plane. The study shows that the dimensionless gradient exhibits sensitivity towards plate thicknesses, local weld geometry and the loading condition.

Fatigue strength assessment of laser stake‐welded web‐core steel sandwich panels

ABSTRACTThis paper investigates the fatigue strength assessment of web‐core steel sandwich panels. The production of these structures is made possible by laser stake welding. The investigation in this study considered two series of panels, one being an empty steel structure and the other filled with in situ polyurethane foam in order to increase the panel stiffness. Both series were tested under cyclic bending loading condition (R = 0) until one of the panel joints failed completely. A 3D panel bending response was analysed using finite element method. The J‐integral values at the panel joints were obtained by means of plane strain finite element analysis and by using displacements from 3D panel response. The influence of the weld geometry on the J‐integral value was investigated. It was found that the J‐integral value is similar in the cases of the average and critical geometry. The contact between the joint plates is possible in some cases, but its influence proved to be insignificant for the fatigue strength assessment. The study further shows that by using the average geometry, the J‐integral approach was able to identify the critical panel joints and present the fatigue strength results from both panel series in a narrow scatterband. The fatigue strength at two million cycles obtained for the panels within this study was in agreement with the laser stake welds and other steel joint types from previous studies. However, the slope of the panels fatigue resistance curve was found to be shallower than in the case of joints.

Critical Characteristics of Fatigue Crack Growth and Resistance Curve of Carbon Steel SGV410

Critical Characteristics of Fatigue Crack Growth and Resistance Curve of Carbon Steel SGV410

ランダム負荷に対する疲労寿命予測の高精度化

This study focuses on the method of predicting the fatigue life of materials subjected to random loading. Since random stress caused by random loading is rigorously expressed in the frequency domain as stress power spectral density (PSD), fatigue life should be predicted using stress PSD. We propose two adjustment methods of improving the accuracy of fatigue life prediction using stress PSD in the frequency domain. The method proposed by Dirlik is widely used for predicting the fatigue life in the frequency domain; however, it overestimates fatigue damage caused by large stress amplitude when the slope of the fatigue resistance curve is large. To prevent this overestimation, we applied our two adjustment methods to fatigue life prediction for typical random stresses observed on mechanical products. As a result, the adjustment methods worked well in improving prediction accuracy. Lightweight and reliable products can be therefore designed by applying the proposed methods to the evaluation of fatigue life under random loading.

Fatigue assessment of laser stake-welded T-joints

This paper aimed to investigate the geometrical variations of laser stake-welded T-joints and their influence on fatigue assessment. Another aim was to determine which of the rref=1mm, rref=0.05mm, and J-integral assessment approaches are applicable for this joint type. Because of the specimen geometry and the loading condition in this study, J-integral approach was equivalent to mode I notch stress intensity approach. Accurate measurements of geometry were taken. Statistical analysis of the weld geometry showed that all the geometrical properties are normally distributed. The results of the fatigue assessment showed that all the approaches applied have a relatively small scatterband and are valid for this joint type. The fatigue strength at 2million cycles, estimated by all approaches, corresponded to the fatigue strength of other welded steel joints from the literature. However, in case of rref=0.05mm approach, fatigue strength at 2million cycles proposed in literature, appeared to be slightly conservative for laser stake-welded T-joints. All the approaches, applied in this study, had the slope of the fatigue resistance curves different from the usual m=3.

Development of Web-Based Fatigue Analysis System for Machine Components and Structures

A fatigue analysis program to calculate fatigue lives of mechanical components and structures from FE(Finite Element) results is developed. The useful characteristic of this program is operated under Web environment. So, any designer who design fatigue strength of components and structures can use without other program installation. For the assessment of multi-axial fatigue damage, signed equivalent stress method and critical plane approach have been employed. Each method is compared and the results of Signed von Mises stress method has similar to the results of Smith-Waston-Topper's parameter using critical plane approach. The results were compared with those from commercial program FE-Fatigue6.0 and it was observed that fatigue life and cumulative damage distribution calculated applying same fatigue resistance curve. The results of calculated fatigue life using Web based program agree well with those from FE-Fatigue6.0.

Fatigue resistance of continuous glass fiber/polypropylene composites: Temperature dependence

AbstractThe effect of testing temperature on the fatigue resistance of continuous glass fiber/polypropylene (CGF/PP) composites was studied. Fatigue resistance curves (or S‐N curves) were obtained at −40°C, 23°C and 50°C. Both on an absolute stress basis and on a normalized stress basis (with respect to the yield stress at the temperature considered), the S‐N curves showed that CGF/PP composites had excellent fatigue performance at 23°C and that their performance was actually improved at −40°C (below Tg of the PP matrix). The S‐N curves at 50°C showed that, although the composite flexural strength was reduced because of PP matrix softening, their fatigue performance remained relatively high, as it is controlled by the CGF reinforcement. Comparison with a CGF/thermoset isophthalic polyester composite of identical fiber architecture and similar flexural strength at 23°C indicated that the properties of the thermoplastic PP matrix provided improved fatigue resistance, both on an absolute and a normalized basis, especially below the glass transition temperature. It was concluded that the fact that the fatigue performance of the CGF/polyester composite is only weakly temperature‐dependent, while that of the CGF/PP composite is strongly temperature‐dependent, does not necessarily mean that it shows superior performance. Polym. Compos. 25:622–629, 2004. © 2004 Society of Plastics Engineers.

Fatigue resistance of continuous glass fiber/polypropylene composites: consolidation dependence

The effect of consolidation on the fatigue resistance of continuous glass fiber/polypropylene (CGF/PP) composites was studied. Five sets of conditions, chosen to approach plant-like processing conditions, were used to obtain samples with varying consolidation quality. These conditions resulted in different void content, degree of crystallinity, size of crystalline entities (spherulites) and level of fiber dispersion. Although they led to different short-term (flexural, tensile or short-beam shear) mechanical results, a single linear correlation could be established between the interlaminar shear strength (ISS) and the void content for all consolidation conditions except one. The fatigue resistance of the CGF/PP composites showed that these consolidation conditions also resulted in different fatigue damage evolution and fatigue resistance curves (or S–N curves). From the S–N curves, two useful correlations could be established: the first is a linear correlation between the Basquin's fatigue strength and the short-term flexural strength; the second is a linear correlation between the Basquin's fatigue sensitivity and ISS. The results emphasize the predominance of the fiber–matrix interface in determining the general mechanical properties of continuous fiber composites and suggest that short-term flexural and short-beam shear tests can be useful empirical indicators of the long-term performance of CGF/PP composites.

Influence of load amplitude and uniaxial tensile properties on fatigue crack growth

Fatigue resistance curves are developed for a circular bar specimen containing an initial penny-shaped crack. They appear as straight lines in the strain energy density factor, S, versus crack length, a, plot as the stress amplitude (and/or mean stress) and uniaxial tensile properties are varied. Subcritical crack growth is seen to increase with decreasing load amplitude and increasing fracture toughness. Results for cases other than those analyzed may be obtained by extrapolation of the data that involves rotating the S- a lines. The loadtime history influence on fatigue crack growth is accounted for through the hysteresis strain energy density function. Only a single parameter is needed that can be determined either from a standard fatigue crack growth test or uniaxial fatigue test of a smooth specimen. This parameter remains constant for a fixed material as load amplitude and specimen size are changed. Reinterpretation of the obtained results is made by application of the commonly recognized two-parameter crack growth relation involving the stress intensity factor range Δ K. These parameters are shown to vary appreciably with changes in load amplitude and hence cannot be regarded as material constants.