Influence of intergranular corrosion on the low-cycle fatigue of AMg5 alloy

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 24, No. 2, pp. 96–97, March–April, 1988.     

Specific features of the fatigue curves under fretting corrosion conditions

The specific features of fatigue curves obtained for specimens undergoing fretting corrosion are discussed, and an accelerated method of constructing fatigue curves of this kind is proposed. The method is based on the known criteria ( and N_k) of the fatigue strength and on the results of fatigue tests on steels OKhNZM, 10, 65G and other materials.     

Substantiation of the predicted fatigue curve for structural elements of aluminum alloy

The paper provides a more specific definition of the method for calculating fatigue life of aluminum alloy structural elements by a local stress-strain state in pulsating regular loading. The calculated results are compared to the test data for specimens with a free and filled holes and for lugs. Recommendations on the method application for calculating fatigue life by nominal stresses are given.     

Ti6Al4V合金在血清溶液中的扭动微动腐蚀行为研究

在新型恒温扭动微动腐蚀实验装置上,通过改变角位移幅值,在恒温37℃的25%血清溶液中对Ti6Al4V合金的扭动微动腐蚀行为进行了研究。实验结果表明,角位移幅值对扭动微动的运行区域及腐蚀行为有重要的影响,摩擦扭矩-角位移幅值(T-θ)曲线分别呈直线型、椭圆型和平行四边形型,Ti6Al4V合金的扭动微动运行区分别呈现部分滑移区、混合区及滑移区等3个微动运行区。当角位移幅值较小时,扭动磨损发生在接触边缘,损伤轻微;扭动微动对腐蚀几乎不产生影响,腐蚀电流和腐蚀电位随时间的曲线波动不大;随着角位移幅值的增大,接触表面产生强烈的塑性变形,损伤严重。腐蚀电流和腐蚀电位随时间的变化曲线表明,扭动微动对腐蚀的影响较大,呈磨损加速腐蚀的特征。     

Influence of the parameters of nitriding on the subsurface hardening of VT6 titanium alloy

We study the influence of the parameters of nitriding (temperature, time of holding, and pressure of the active gas) on the formation of nitride coatings based on VT16 titanium alloy. We give recommendations concerning the possibility of combination of the prescribed thermal treatment with the thermochemical (nitriding) treatment of the alloy aimed at guaranteeing the required level of subsurface hardening. It is shown that the decrease in the partial pressure of nitrogen to 1–10 Pa increases the depth of the hardened zone and ensures the required level of subsurface hardening. The procedure of heating in a vacuum (1 mPa) performed prior to the action of nitrogen improves the surface quality of the alloy. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 49–54, November–December, 2007.     

Fracture mechanics based fretting fatigue life predictions in Ti-6Al-4V

A fracture mechanics based crack propagation analysis is developed to work directly with the output of a contact mechanics stress analysis for fretting fatigue. A series of remote load fatigue tests were conducted on specimens that had previously been subjected to fretting fatigue loading conditions. The growth of these prior fretting induced cracks were monitored and compared to results from the crack propagation analysis. A combined fatigue crack formation and propagation analysis was then applied to other fretting fatigue experiments with good success. The creation of fretting fatigue stress-life curves is also demonstrated.     

Influence of contact pressure on fretting fatigue behaviour of AA 2014 alloy with dissimilar mating material

The effect of contact pressure on the fretting fatigue behaviour of 2014 Al alloy which has been solution heat treated and age hardened (T6 heat treatment) with dissimilar mating materials, was investigated. The fretting fatigue configuration involved bridge‐type contact pads on a flat fatigue specimen. Specimens were made of 2014 Al alloy and bridge‐type pads were made AISI 4140 steel. All the fatigue tests were conducted at a rotational speed of 5000 rpm with a rotating bending fatigue machine (R=?1), using S–N curves to evaluate the fatigue and fretting fatigue properties. The fretting fatigue strength of the material subject to a T6 heat treatment condition at 1 × 10⁷ cycles was dramatically reduced, as compared to that without fretting and with as‐cast. The fretting fatigue life exhibited a variable behaviour with an increase in the contact pressure. A scanning electron microscope was employed to observe the fretting scars and fracture surfaces of the specimens. This analysis showed that cracks originated at the contact surface and crack orientations were approximately ±56 ° from perpendicular to the loading direction.     

Effect of loading cycle asymmetry,stress concentration,and fretting corrosion on the fatigue resistance of alloy D16T

Experimental data are presented for the fatigue strength of smooth specimens, specimens with a stress concentrator, and specimens under fretting corrosion conditions for alloy D16AT at three levels of loading cycle asymmetry coefficient R = –1, 0, and –0.5. It is established that fatigue resistance for alloy D16AT under fretting corrosion conditions is governed by the amplitude of operating stresses but it is practically independent of changes in loading cycle asymmetry coefficient. The possibility is demonstrated of calculating the fatigue limit for specimens with a stress concentrator with different loading cycle asymmetry from known values of the threshold stress intensity factor, the theoretical stress concentration factor, and the length of an nondeveloping crack at fatigue limit. Limiting stress amplitude diagrams for smooth specimens under fretting corrosion conditions have a different nature for the relationship between amplitude and average stresses of the loading cycle and therefore they cannot be described by a general analytical expression.Translated from Problemy Prochnosti, No. 5, pp. 47–51 May, 1990.     

A procedure for estimating the total life in fretting fatigue

ABSTRACT This paper proposes a procedure for estimating the total fatigue life in fretting fatigue. It separately analyses the fatigue crack initiation and propagation lives. The correlation between crack initiation and propagation is made considering a non‐arbitrary crack initiation length provided by the model. The number of cycles to initiate a crack is obtained from the stress distribution beneath the contact zone and a multiaxial fatigue crack initiation criterion. The propagation of the crack is considered using different fatigue crack propagation laws, including some modifications in order to take the short crack growth into account. The results obtained by this method are compared with the fatigue lives obtained in various fretting fatigue tests under spherical contact with 7075‐T6 aluminium alloy.     

Influence of fretting on the fatigue strength at the vise clamp-specimen interface

Fretting fatigue is one of the most important phenomena for inducing a significant reduction of fatigue strength and consequently, leading to unexpected failure accidents of the engineering structures even at very low stresses. In the present study, both plain and fretting fatigue tests with zero mean stress were carried out on two different types of steel, low-carbon steel and martensitic stainless steel, by means of a reversed bending fatigue testing machine. The drop in the fatigue strengths through fretting at vise clamp-specimen interface were significant for both tested steels. The fretting processes produced a reduction in fatigue strength of about 27% for low-carbon steel and 16% for martensitic stainless steel.     

Characterization of fretting fatigue in self-piercing riveted aluminium alloy sheets

A study was conducted to characterize fretting fatigue in self‐piercing riveted single‐lap joints of aluminium alloy 5754 sheets. The experimental results showed that fretting occurred at three different positions in the joint. It was established that fretting led to surface work‐hardening and crack initiation as well as early stage crack propagation. Crack initiated at the surface of the riveted sheets as a result of high stress concentration and propagated oblique to the mating surface under the effect of fretting fatigue. The depth of damage due to fretting depended on the applied load and the cycle time. Microhardness measurements allowed the estimation of the depth of damage due to fretting. These results were observed to correlate well with the length of crack propagation.