Fatigue Limit Of Welded Joints Research Articles

A two-scale thermodynamic entropy model for rapid high-cycle fatigue property evaluation of welded joints

This investigation attempts to propose a two-scale thermodynamic entropy model that can quantify the micro-scale fatigue behaviors from the macroscopic level. Both non-damaging anelastic and damaging inelastic microstructural motions that cause entropy generation at different stress amplitude blocks of specimens are considered. On this basis, a thermodynamically based correlation between the macro-stress amplitude and the microstructural motion is constructed following the framework of thermodynamics. Butt joints made of low carbon steel are used as the experimental materials in this work. The fatigue limit of welded joints is estimated in a short time using the developed approach in combination with the correlation coefficient optimization method. Finally, the fracture fatigue entropy (FFE) model, corresponding to the two-scale theory, is developed to achieve fast prediction of fatigue life by considering the threshold value associated with the irreversible inelastic entropy generation in high-cycle fatigue. The results show that the predicted data are in satisfactory agreement with the experimental data.

Extension of the peak stress method to estimate the fatigue limit of welded joints by means of the cyclic R-curve method

A new simplified and effective method has been formalised to estimate the Constant Amplitude Fatigue Limit (CAFL) of stress-relieved steel welded joints subjected to uniaxial push–pull loading and failing from the weld toe. Starting from the sharp V-notch assumption of the NSIF approach and the cyclic R-curve of the material in the heat affected zone, the proposed method identifies the CAFL as threshold level of the local stress field at the V-notched weld toe in the uncracked configuration. Such threshold stress field assures the crack arrest at the V-notched weld toe, according to the cyclic R-curve analysis. The method has been validated against experimental results and proved effective for a straightforward assessment of the CAFL of welded joints, as the stable crack propagation analysis of classical fracture mechanics approaches can be avoided.

Analytical model for assessing fatigue limit of welded joints of ferritic-pearlitic steels

Introduction. Microdefects and zones with stress concentration in welded joints cause fatigue macrocracks. Such damage is potentially dangerous, especially if the fatigue life of the structure is almost exhausted. In this case, the crack size is close to the critical value, and it is crucial to determine its length. The paper considers the development of an engineering analytical model for assessing the critical crack length and endurance limit of welded joints with the formed grain in the structure of ferrite-pearlitic steels after welding. Materials and Methods. The theory and methods of fracture mechanics at the mesoscale are used. A simple analytical dependence is obtained, which provides determining the critical dimensions of a macrocrack for ferrite-pearlite steels without using the Griffiths formula. . The calculation results of the critical crack lengths of various steels depending on their yield strength are presented. An analytical dependence of the endurance limit calculation for the most dangerous symmetric loading cycle, according to the standard set of mechanical characteristics and the average grain diameter of ferrite-pearlite steel, is presented. Results. Structural deformation analysis of the crack propagation process has been performed. On its basis, an engineering technique for assessing the endurance limit is developed. A mathematical model that enables to calculate the endurance limit and the critical crack length in the components of welded assemblies of large-sized facilities, considering periodic loads of a symmetrical cycle, is developed. Using this model, it is possible to estimate the degree of metal sensitivity to the original characteristics (yield stress, Poisson's ratio, grain diameter, relative constriction, Young's modulus, power-law hardening coefficient, etc.). Discussion and Conclusion. Under stresses corresponding to the steel endurance limit, the critical crack opening rates of the tip and edges approach each other. Energetically, this moment approximately corresponds to the transition of the crack to an unstable state. The accumulation of one-sided plastic deformations causes the limiting state of plasticity of the region adjacent to the crack tip and its avalanche-like or sharply accelerated motion. This critical area is interrelated with the grain diameter of the material, the characteristic of critical plasticity and the critical opening at the crack tip at the fatigue limit. The proposed analytical dependences can be used to assess the residual life and the fatigue limit of welded structures, the influence of various factors on the fatigue limit of welded joints of ferrite-pearlitic steels used in mechanical engineering, shipbuilding, pipeline transport, etc

Method for calculating the fatigue limit of welded joints of different thicknesses after high-frequency mechanical forging

Method for calculating the fatigue limit of welded joints of different thicknesses after high-frequency mechanical forging

Prediction of fatigue limit improvement in needle peened welded joints containing crack-like defects

PurposeThe purpose of this paper is to estimate the acceptable defect size amax after needle peening (NP) and predict the fatigue limit improvement through the use of NP for an austenitic stainless steel welded joint containing an artificial semi-circular slit on a weld toe.Design/methodology/approachResidual stress and hardness distribution were measured. Microstructures around the weld toe were observed to clarify the cause for the change in hardness after NP. Finite element method analysis was used to analyze the change in the stress concentration following NP. Fracture mechanics was used to evaluate amax after NP. The fatigue limits before and after NP were predicted by determining amax for several levels of stress amplitude.FindingsThe tensile residual stress induced at the surface of the weld toe prior to NP changed to a compressive residual stress after NP. The residual stress near the surface layer after NP exceeded the yield stress prior to NP due to the increase in yield stress as a result of work hardening as well as the generation of a deformation-induced martensitic structure. The stress concentration was reduced due to the shape improvement caused by NP. The estimation value of amax after NP and the prediction results of fatigue limits were in good agreement with the fatigue test results.Practical implicationsThe proposed method is useful in improving the reliability of welded joints used in large steel structures, transportation equipments and industrial machines.Originality/valueFrom an engineering perspective, it is essential to estimate amax and the fatigue limit of welded joints with crack-like defects. However, it is unclear as to whether it is possible to predict amax and the effects of NP on the fatigue limit for stainless steel welded joints.

The effect of seismic loading on the fatigue strength of welded joints

Earthquakes sometimes damage steel structures. Structures which are not seriously damaged are still used after earthquakes but their fatigue strength may have been reduced by the large cyclic loadings imposed by the earthquakes. In order to clarify the effect of seismic loading on the fatigue strength of welded joints, high cycle fatigue and variable amplitude fatigue tests after a number of large initial strain cycles were performed. The large strain cycles formed a short crack at the toe of the weld in a low cycle fatigue that triggered a high cycle fatigue strength reduction. The high cycle fatigue limit of welded joints after initial strain cycles is governed by the threshold stress intensity factor of the short crack. The formation of short cracks also enhanced the damage accumulation for subsequent variable amplitude loading. It is important to keep all of the stress variations after earthquake below the fatigue limit of the cracked welded joints to avoid fatigue damage accumulation after an earthquake.

熱延鋼板アーク溶接継手の疲労限度に及ぼす金属学的性質の影響

The effects of material properties of steel sheet, such as mechanical properties, microstructure and chemical composition, on the fatigue limit of gas shielded metal arc welded (GMAW) joints were investigated.Axial tension fatigue tests were conducted to lap-fillet welded joints of seven different hot-rolled sheet steels.Followings are main results. (1) The fatigue limit of welded joints varies to large extent, depending on the steels tested. (2) The fatigue limit depends mainly on toe radius of weld bead, which determines the stress concentration factor of welds. And the fatigue limit of welded joint is slightly affected by the fatigue limit of base metal. (3) There is a close correlation between chemical compositions of base metal and the fatigue limit of welded joint. The increase of Si content improves the fatigue limit remarkably, because it enlarges the toe radius of weld.

Probabilistic calculation of the fatigue limit of welded joints in stationary loading

Probabilistic calculation of the fatigue limit of welded joints in stationary loading

Effect of low temperature on the fatigue limit of welded joints

It was f i r s t o b s e r v e d in [1] tha t a change in t h e shape of the f a t i gue c u r v e o c c u r s at low t e m p e r a t u r e t he i n c r e m e n t of t he fa t igue l i m i t at h igh s t r e s s e s i s c o n s i d e r a b l y l a r g e r than at s t a n d a r d t e m p e r a t u r e (F ig . 1). The need fo r a p h y s i c a l exp l ana t i on of t h i s phenomenon was a l s o no ted in [1].