Experimental Analysis of Static and Fatigue Strength Properties in Press-Fitted and Adhesively Bonded Steel–Aluminium Components

Press-fitted and adhesively bonded joints (Hybrid Joints) are increasingly used as an alternative way to traditional structural joining techniques. The main achievable benefits can be summarized in the possibility of maximizing the load transfer (torque or axial) and reducing both the weight and the stress field of the components, by taking advantage of the adhesive strength. Hybrid joints studies can be found in literature mainly on steel–steel components (Steel Hybrid Joints). The aim of this paper is to provide some relevant information on the static and fatigue strength properties in the case of steel–aluminium components (Mixed Hybrid Joints), from the experimental tests performed on a high strength, single-component adhesive which cures anaerobically. The use of the adhesive increases the press-fitted joint performances, with respect to its release force: the adhesive static shear strength is about 9 MPa, whereas the adhesive endurance limit is about 6 MPa, in presence of a stress ratio R = 0.1.     

Effect of Substrate Material on Fatigue Crack Propagation Rate of Adhesively Bonded DCB Joints

The effect of substrate material on the fatigue crack propagation rate was investigated using adhesively bonded DCB specimens with CFRP and aluminum substrates. The experimental results show that the increase in thickness of the adherend lowers the fatigue threshold, ΔG _th, and raises the crack growth parameter, n, irrespective of the substrate material, and that the crack growth parameter, n, for the aluminum joints is less than that for the CFRP joints. To elucidate the fatigue crack propagation behavior, fracture surface observation and finite element analysis have been conducted. Besides, Gurson's model is applied to the adhesive layer. SEM images show that numerous voids are formed in the fracture surface for the joints with aluminum substrate, but the growth of voids is suppressed for the joints with CFRP substrate. FEM results also show that the void area fraction for the joint with aluminum substrate is greater than that with CFRP substrate. Thus, the above experimental and numerical trends of voids correspond to the trends of the fatigue crack propagation behavior.     

Probabilistic modelling of fatigue crack initiation from pits and pit clusters in aluminium alloys

Abstract

A numerical model of crack initiation under high cycle fatigue loading from pits is investigated in this paper. A probability based pit growth model, which takes into account the influence of mechanical cyclic load and particle clusters present in alloys, is used for investigations. Critical pit sizes, calculated using linear elastic fracture mechanics principles, are used to determine the probability of crack initiation for different conditions. The results are critically compared to extract an insight on the parameters that control the pit growth behaviour and thereby the fatigue crack initiation.     

Fatigue properties of Al–Si casting alloy with cold sprayed Al/SiC coating

Abstract

The fatigue properties of Al–Si alloy cold sprayed Al and Al–SiC composite coatings have been studied. The specimens coated with composites reinforced with a large volume (25%) of fine SiC particles exhibited improved adhesion strength at the interface due to crater formation, and cyclic fatigue lives at room temperature more than three times those of uncoated specimens. In high temperature low cycle fatigue tests at 250°C, the pure Al coatings showed longer fatigue lives than the Al–SiC composite coatings, which is attributed to an increment in ductility at the surface retarding fatigue crack initiation.     

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.     

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.     

In situ study of martensitic transformation and nucleation and propagation of cracks in Cu-Ni-Al shape memory alloy

Abstract

The stress induced martensitic transformation and the relationship between it and the nucleation and propagation of cracks in the Cu-Ni-Al shape memory alloy were investigated through in situ tensile tests by SEM and TEM. The results indicated that the stress concentration ahead of the crack tip could induce formation of stacking faults and different types of martensites. Transmission electron microscope observations showed that the martensites could transform from one type to another type and even reversely to parent during loading. The microcracks nucleated along the martensite/parent interface and intersections between two martensites. When the crack propagated a certain distance, the stress concentration ahead of the crack tip was large enough to result in formation of slip bands, in this condition the microcrack nucleated along slip bands more easily.     

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.     

Dwell time effect on fatigue crack growth of RR1000 superalloy

Abstract

The requirement for improved understanding of the behaviour of turbine disc alloys at elevated temperatures has led to an increased interest in the contribution of time dependant mechanisms to high temperature fatigue crack growth. A study has been conducted on a new powder alloy to investigate the contribution of such mechanisms when the applied waveform is varied in terms of hold periods and the influence of limited thermal exposure is included. Variable waveform tests performed in air at 725°C have indicated that the addition of a hold time at maximum load in a fatigue cycle tends to increase the crack growth rate per cycle in the as heat treated material. Crack growth in thermally exposed material is retarded by up to a 10 s hold time and then accelerated as the hold time increases further. Rapid near crack tip stress relaxation induced by γ′ coarsening is proposed to have a beneficial effect on the severity of this type of damage which causes the crack growth rate reduction for short hold times.     

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.