Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

Tensile and fatigue behaviour of sinter hardened Fe–1·5Mo–2Cu–0·6C steels

Abstract

In the present work the tensile and axial fatigue behaviour of sintered hardened Fe–1·5Mo–2Cu–0·5C at three density levels (6·8, 7·0 and 7·2 g cm^–3) have been studied. The materials were tested under the as sintered condition, and after tempering at 180 and at 240°C. The results show that steels under the as sintered condition posses a high hardness but a brittle tensile deformation and fracture behaviour. Tempering at 180 and 250°C induces the disappearance of brittleness and tensile fracture is thus ductile although very localised at the necks. Fatigue strength is determined by the resistance of the materials to the internal damage evolution due to the nucleation of small cracks at the pores edges, and their coalescence into a long crack. Tempering induces an increase in the fatigue resistance. The greatest fatigue strength at 2 × 10⁶ cycles is displayed by the steel with a density of 7·2 g cm^–3 and tempered at 180°C.     

基于ABAQUS和FATIGUE的风电转盘轴承疲劳寿命计算

为更加方便计算转盘轴承的疲劳寿命,提出一种基于ABAQUS和FATIGUE的转盘轴承疲劳寿命计算方法。首先在ABAQUS中完成转盘轴承的接触应力计算,建立转盘轴承的有限元模型;接着将计算结果导入FATIGUE中计算其疲劳寿命。结果表明该方法切实有效。     

Predicting optimum hollowness of normally loaded cylindrical rollers using finite element analysis

Abstract

Fatigue life investigations have been made for hollow rollers in pure normal loading. Different hollowness percentages between 20 and 80% have been tested to find the optimum percentage hollowness that gives the longest fatigue life. Two main models were built for this purpose: model 1 with two identically sized rollers and model 2 with two non-identically sized rollers. In each model, two cases have been studied: when both rollers are hollow and when one roller is hollow while the other one is solid. The Ioannides–Harris (IH) theory was used to calculate the relative fatigue life of the hollow rollers with respect to solid rollers under the same loading. Investigations have been made for five different materials: CVD 52100, carburised steel, VIMVAR M50, M50NiL and induction hardened steel. The finite element package ABAQUS has been used to study the stress and deformations in the loaded rollers. In general, the optimum hollowness percentage with the longest fatigue life ranges between 60 and 70% based on the kind of the material, whether the rollers are of the same or different size and whether one or both rollers are hollow. Using the IH theory for fatigue life calculation resulted in having infinite fatigue life for those rollers made of induction hardened steel that relatively has high fatigue limit value. Rollers in the optimum range are flexible enough to get the best redistribution of stress in the contact zone. For models of a hollow cylindrical roller in contact with a solid roller, the optimum hollowness is around 70%. When both cylindrical rollers are hollow, the optimum hollowness decreases between 60 and 65%. At the optimum hollowness, small differences in the fatigue life have been found between models of one hollow roller and models of two hollow rollers, even though having both hollow rollers means less weight, thus saving more material and more stability for the system.     

On mechanism of fatigue fracture in interstitial free steel sheet

Abstract

The mechanism of fatigue fracture in an interstitial free steel sheet has been studied. The process can be divided into four regimes:

(i) the fatigue crack initiates on the specimen surface, from the mesocracks along the grain boundaries in stage I

(ii) propagates mostly in an opening mode through grain boundaries in stage II

(iii) propagates through microscopic striations and transverse intergranular cracking briefly in stage III

(iv) the crack path changes from flat to slant along with through thickness necking and it propagates to failure through discrete crack jumps in stage IV. The crack jumps are associated with crack progression marks (CPMs), the spacing of which increases exponentially from few micrometres to few hundred micrometres with crack length.     

Static and fatigue performance of weld bonded dual phase steel sheets

Abstract

Lap joints of dual phase steel sheets of 1·0 mm were prepared by adhesive bonding, spot welding and weld bonding processes using a one component epoxy base structural adhesive. Mechanical properties of the joints were evaluated by tensile shear and fatigue tests. The size of the weld nugget for both spot weld and weld bond was measured for different welding parameters (current, time) and compared. For identical welding parameters, weld bonded nuggets exhibit higher nugget diameter. Tensile shear strength of weld bonded joints is 40 and 58% higher than spot welded joints and 15 and 39% higher than adhesive bonded joints and for DP590 and DP780 steels respectively. Considering 10⁶ cycles, the endurance limit of weld bonded joint is much higher than spot welded joint but smaller than adhesive bonded joints. Overall the performance of weld bonded joints is superior to those of resistance spot welding.     

Integrated fracture mechanics approach to analyse fatigue behaviour of welded joints

Abstract

Current fracture mechanics methods for fatigue assessment, including those that consider thresholds for crack propagation, are based on long crack behaviour. The present work is concerned with an attempt to predict the fatigue strength of welded joints using a fracture mechanics approach that takes into account the fatigue behaviour of short cracks. The methodology estimates the fatigue crack propagation rate as a function of the difference between the applied driving force and the material threshold for crack propagation, which is a function of crack length. The fatigue strength of butt welded specimens stressed transversely was analysed. Experimental results from the literature were used for comparison. Estimations are obtained by using only the fatigue limit and the fatigue propagation threshold for long cracks, and the applied stress distribution along the crack path obtained from simple finite element models. The influence of plate thickness, initial crack length, and reinforcement angle on fatigue strength of butt welded joints was analysed. Results show good agreement with experimental trends.     

Design criteria for rolling contact fatigue resistance in back-up rolls

Abstract

This research centres on surface initiated damage on back-up rolls whereby rolling contact fatigue cracks can propagate into the rolls potentially reaching the internal stress fields and leading to catastrophic failure and has sought to establish design criteria for avoiding such failures. The project objectives have been achieved by examining field evidence, determining the loading and tribological conditions at the work roll/back-up roll interface and investigating both theoretically and experimentally the mechanisms involved in rolling contact fatigue in this case. The presented rolling contact, fatigue and fracture mechanics model includes criteria for crack branching either upwards (i.e.relative safety) or downwards (i.e.potentially catastrophic) and the link between these two cases is related, quantitatively, to the properties of the roll material. After linking mechanics to microstructure, the influence of work roll test disc surface roughness on both the surface wear of and the interaction between wear and rolling contact fatigue at the surface of back-up roll test discs has been quantified using the results obtained from experimental simulations carried out on a rolling–sliding testing machine. Finally practical quantitative recommendations are made for the mechanical and microstructural design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both work rolls and back-up rolls presented to the mill.     

Fatigue damage evaluation of adhesively bonded butt joints with a rubber-modified epoxy adhesive

A damage evolution of adhesively bonded butt joints with a rubber-modified adhesive has been investigated under cyclic loading. An isotropic continuum damage model coupled with a kinetic law of damage evolution was applied to the butt joint. To solve the kinetic law, analytic and numerical methods were tried: the former solution was derived with some simplifications and the latter one was derived rigorously without simplications. On comparing the analytic solutions with the numerical ones, it was confirmed that differences in the two solutions were small. Furthermore, the estimated S-N curves based on the analytic equation agreed well with experimental data.