Mean stress effects in strain–life fatigue

A mean stress equation can be incorporated into the strain–life curve in a manner that is consistent with the stress-based use of the same equation. Doing so for the Walker mean stress relationship gives excellent results for a number of strain–life data sets with non-zero mean stresses, including data on steels, one titanium alloy and aluminium alloys. This approach has a number of advantages: All data at all mean stresses can be combined into a single fitting procedure to determine the constants for the stress–life curve, which values also apply to the elastic strain term of the strain–life curve. The Walker parameter γ that also arises from this fitting is related to the sensitivity of the material to mean stress, giving this approach a versatility that is not possessed by other common mean stress methods. Where non-zero mean stress data are not available to obtain γ from fitting, an equation based on existing fitted values can be used to make estimates for steels. For precipitation-hardened aluminium alloys in the 2000 and 7000 series, an estimate of  γ= 0.5  may be applied, so that the method becomes similar to that of Smith, Watson and Topper. For other metals, a default estimate of  γ= 0.5  is suggested. For life estimates using the strain-based approach, it is recommended that the Walker mean stress method, incorporated into the strain–life curve, should be employed as an alternative to other methods, or perhaps to even replace them entirely.     

The influence of cyclic stress intensity threshold on fatigue life scatter

Understanding factors that contribute to scatter in fatigue lives of metallic structures (particularly airframes) subjected to identical spectrum is critical to maintaining safety and optimising designs. This paper first briefly discusses the sources of scatter, and then concentrates on the effect of variations in the “cyclic stress intensity threshold” (ΔK_thr) on fatigue crack growth. It shows that a version of the NASGRO equation can be used to account for the crack growth scatter seen in a number of classical fatigue experiments by accounting for variations in ΔK_thr. This is an important outcome for safety and is particularly useful when considering lead cracks for which ΔK_thr is small (approaching zero) as these cracks appear to commence growing soon after introduction into service.     

Rapid determination of fatigue life based on temperature evolution

This paper presents an approach to rapidly predict high cycle fatigue life based on the temperature evolution of a specimen under cyclic loading. The initial slope of the temperature evolution was proved as an indicator for rapid estimation of fatigue life theoretically. Meanwhile, a real-time temperature measurement system based upon high-precision semiconductor temperature sensors was developed to detect the temperature evolution based upon high-precision semiconductor temperature sensors. In order to verify the presented approach, constant-amplitude fatigue tests were carried out on A7N01 aluminium alloy and the welded joints, respectively. The predicted fatigue life based on proposed approach was in good agreement with experimental results.     

Study on the stress characteristic and fatigue life of the shredder pin

The hammer shredder is the key equipment for cyclic utilization of retired car bodies. The axes' pin is the main component of the shredder, but it is damaged easily. To improve the shredding efficiency and lengthen the changing life of the shredder pin, it is significant to study its stress characteristic and the fatigue life. In this article, the complex force conditions of the shredder pin at the normal working condition were studied briefly by mechanics analysis method. The stress characteristics and fatigue life of the pin were analyzed by the finite element transient analysis method. The maximum stress distribution law of the shredder pin was found and the fracture mechanism of the retired car bodies was revealed. Furthermore, the reliability of the simulation results was verified by the experiment. It was found that when the pin rotates around the main shaft counterclockwise, the maximum stress of the pin at 0° of the main shaft is higher than other positions. Additionally, along the axial direction, the maximum stress occurs at the cross section at a distance of 2/9 of the pin length from the pin head and at 0° of this section, which orientation is made when the pin is at 270° of the main shaft. Comparing four working modes, it was found that the working life of the shredder pin could be doubled by changing the sides of the pin and rotate it along its axes by 180°.     

Frequency-domain fatigue life estimation with mean stress correction

Two frequency-domain fatigue life calculation methods are presented which take into account the impact of the mean stress effect. The emphasis is set on the algorithm for fatigue life assessment of the method proposed by the authors. It is supplemented with a mean stress effect correction. Correction method is based on the direct transformation of the zero mean stress Power Spectral Density (PSD) due to mean stress. The method is verified on the basis of own results for the S355JR steel. The authors analyze five models for the designation of the probability density function used in the calculation process. The results are presented in the form of probability distributions after PSD transformation and the calculated fatigue life is being compared with the experimental life in fatigue comparison graphs. An analysis of the choice of a mean stress correction model is also widely discussed and a fatigue life estimation is also performed. The method proposed by the authors is being compared with the Kihl–Sarkani method for mean stress correction in frequency domain.     

The effects of low temperature fatigue on the RRR and strength of pure aluminum

Low temperature fatigue effects on residual resistivity ratio (RRR = rho_{273 K}/rho_{4.2K}) and strength of 300 and 1000 RRR aluminum are reported. The objective of this investigation is to select the best initial purity for the stabilizer aluminum used in energy storage magnets. Monolythic centimeter diameter specimens were fatigued at 4.2 K to strains (ε) reaching 0.3 percent. The resistivity ratio rapidly decreases during the first 100 cycles and approaches saturation (RRRf) after about 1000 cycles for all strains tested. The RRRfvalues are different for different initial resistivity ratio (RRRi) values, but all tend to come together at 0.3% strain independent of RRRi. The maximum specimen stress (sigma_{max}) is reached after about 1000 cycles also, and approaches a common value (sigma_{max} = εE/2, where ε is the strain range and E the elastic modulus) independent of RRRi. Thus high purity aluminum becomes "fully hard" at equilibrium and behaves elastically. The impact of fatigue damage on conductor design and choice of stabilizer purity is considered.     

超深矿井提升机卷筒动态应力与疲劳寿命研究

超深矿井提升机卷筒在钢丝绳多层缠绕时所受钢丝绳的缠绕力错综复杂,在长期作业过程中卷筒结构可能会萌生裂纹进而发生疲劳破坏,所以有必要对卷筒结构在多层缠绕实际工作条件下的疲劳寿命展开研究。基于板壳理论,将超深矿井提升机卷筒视为均匀受压的旋转对称壳体,通过对卷筒结构负荷构成进行分析,建立了钢丝绳对卷筒作用力的数学模型,并根据提升系统动力学原理建立了钢丝绳满载上提和空载下放的动力学模型,确定了卷筒结构的动态载荷。通过对卷筒结构作业过程强度分析得出卷筒结构在整个工作循环中的应力-时间历程曲线。结合不同存活率的S-N曲线和累积损伤理论,分析了卷筒结构在不同负载下的疲劳寿命。研究结果表明：提升机满载上提和空载下放过程中卷筒结构的最大等效应力均出现在等速阶段,且其最大等效应力出现在卷筒内壁支轮与支环的中部;在给定的存活率下卷筒结构的疲劳寿命与工作负载密切相关。研究结果为多层缠绕复杂工况下卷筒的力学分析与科学设计提供了理论支撑。     

Effects of mean stress and crack closure on fatigue life of spot welds

The effects of mean stress and crack closure on fatigue life of spot welds were investigated. A review showed that several of the previously proposed mean stress corrections give similar results. Fatigue tests on shear and peel loaded specimens were carried out, and the results agreed with the corrections reviewed. The present study shows that crack closure explains the mean stress effects observed. The crack opening force for spot welds was obtained, both experimentally from F–N curves with different load ratios and analytically from the available mean stress corrections. This was verified with detailed finite element simulations. Finally, the experiments and simulations indicate that the use of linear damage accumulation in fatigue life prediction of spot welds can be non‐conservative.     

The relative effects of residual stresses and weld toe geometry on fatigue life of weldments

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.     

Investigation of low temperature effects on fatigue behavior of PVDF

Fatigue behavior of polyvinylidiene fluoride (PVDF) pipes is investigated under low temperatures to characterize the temperature effects. The analysis included experimental evaluation of fatigue life for test samples taken directly from the manufactured pipes used for service as opposed to compression molded compact tension samples used in previous works. In this test, short sections from an extruded pipe are used to better represent the material service conditions. A compact test chamber was designed to control the test temperature. The samples were loaded into the test rig and allowed to cool for 30 min ensuring a constant and even temperature distribution. Cooling was done in a sealed test chamber using carbon dioxide gas. Two test temperatures of −20 °C and −10 °C were chosen since they represent typical temperature during which failure occurs during actual pipe service in cold environments. Fractured surfaces were inspected and fatigue data were analyzed using a standard procedure for calculation of fatigue life with a semi-elliptical surface crack assumption was performed; from which parameters of the Paris law for fatigue fracture were obtained. Comparing the results with previous works it is found that they capture the trend of the PVDF material behavior for high temperature.     

Internal stress effects on fatigue crack initiation at notches

In this paper the effects of the internal stresses on crack initiation at notches is investigated. Internal stresses are induced by applying two pre-loading cycles at a given load ratio, R = S_MIN/S_MAX, to the ‘virgin’ notched specimen. After pre-loading, blocks of 10⁶ cycles with a constant nominal stress range, ΔS, slightly larger than the nominal threshold stress range, ΔS_th, are applied with increasing minimum stress, S_MIN, in each subsequent block until a crack initiates. The lowest S_MIN above which a crack initiates is recorded as S_IN. The process is repeated for a wide range of positive and negative load ratios in order to obtain S_IN/S_MAX vs. load ratio curve. This curve for positive load ratios surprisingly resembles the K_PR/K_MAX plot reported in the literature for a cracked specimen. The results are explained in terms of compressive internal stress and the associated local clamping action at the notch cyclic plastic zone. Both the analytical and finite element calculations show a fairly good agreement with the experimental results.     

Mean stress and ratcheting corrections in fatigue life prediction of metals

Ratcheting occurs easily because of the presence of mean stress during the stress‐control fatigue of engineering components. For ductility exhaustion dominated fatigue failure, a new fatigue life prediction model is developed by introducing the mean ratcheting strain rate to incorporate the effects of ratcheting and mean stress on fatigue life. The prediction accuracy of the proposed model was compared with that of the generalised damage parameter, Xia–Kujawski–Ellyin, Walker and Goswami models. Specifically, the model predictions and tested lives were compared using nine sets of experimental data from the literature. In the statistical analysis of these five models, the proposed model provides the highest accuracy and robust life predictions with the lowest model prediction errors.     

The effects of creep and fatigue stress ratio on the long-term behaviour of angle-ply CFRP

The long-term behaviour of ±45°-angle-ply laminates of carbon/epoxy was studied. Due to the absence of 0°-layers angle-ply laminates are subject to cyclic creep. The creep strain evolution was investigated by experimental and analytical means. To predict the total strain depending on the applied stress level and load time an empirical creep law based on power law functions was adopted. Good agreement between experiment and prediction was found. The interaction between creep and fatigue was used to estimate a lower bound of the endurance limit based on creep predictions exclusively.     

Modelling the mean stress effect on fatigue life of fibre reinforced plastics

The mean stress influence on fatigue life of carbon and glass fibre reinforced plastics is investigated in detail. A new phenomenological approach is presented to model the mean stress effect in various material systems and fibre dominated stacking sequences. The model is calibrated to fatigue data via a developed fitting-routine that is based on least squares method. The calibration input data is one Woehler curve at R = 0.1 and the ultimate static strengths in tension and compression loading. The characterization effort is reduced by this significantly. Finally the method is verified successfully by fatigue data of several material systems.     

Residual stress effects in fatigue crack growth

The paper describes the influence of residual stresses on the propagation of fatigue cracks in low-alloy steels used in the construction of North Sea Oil platforms. The results show faster crack growth in thicker plates and the development of secondary fatigue cracks in the residual stress field. Characteristics of fatigue crack growth are discussed.     

Multiaxial stress effects on fatigue behavior of filled natural rubber

This work explores multiaxial stress effects on fatigue crack nucleation and growth in filled natural rubber based on experiments using short thin-walled cylindrical specimens subjected to axial and twist displacements. Cyclic stress–strain response exhibits significant initial softening relative to the monotonic response, followed by a more gradual additional softening. Irreversible breakage of various types of bonds is believed to cause the initial softening, while the presence of fillers and their influence on network chain breakage is believed to cause the additional softening. Crack nucleation planes and crack growth paths were monitored and cracks were observed to form on specific plane(s). Correlations of crack nucleation lives and growth rates with respect to the peak maximum principal strain were obtained and are discussed.