Tensile static, fatigue and relaxation behaviour of closed cell electret PVDF foams

We present the experimental results related to the mechanical behaviour under tensile static, fatigue and compressive relaxation loading of closed cell PVDF electret foams under different loading ratio conditions. The specimens are statically loaded until 60% of their ultimate displacement, and subsequently subjected to cyclic loading under displacement control. The static tests show a stress-strain behaviour and failure mode similar to the one of other polymeric closed cell materials, such as polymetacrylimide-based foams. The fatigue tests show an evolution of the stiffness degradation over the levels of cycles characterised by three distinct phases. The behaviour of the energy dissipated versus the loading ratios and numbers of cycles applied is discussed in this paper, as well as the comparison between the compressive fatigue behavior recorded on similar classes of foams. The compressive relaxation behaviour shows a two-phase dependence over the loading time, with increasing modulus for higher loading ratios, and absence of an asymptotic modulus for long time exposure.     

带约束拉杆方形薄壁钢管/竹胶合板组合空芯柱轴心抗压性能

为解决方形薄壁钢管/竹胶合板组合空芯柱（SBCC）在压缩载荷下易开胶失效的问题,提出了一种新型的带横向约束拉杆的方形薄壁钢管/竹胶合板组合空芯柱（SBCCB）。首先,对9根SBCCB进行了轴心抗压试验,考察了SBCCB受压破坏形态,分析了SBCCB的截面尺寸、长细比和截面组合方式对其开胶载荷和极限承载载荷的影响,并将SBCCB的极限压应力与已有的SBCC数据进行了比较;然后,通过非线性回归分析,建立了SBCCB轴心抗压承载力计算公式。结果表明:SBCCB的轴心抗压失效破坏主要为柱端及柱身横向约束拉杆之间的竹胶合板开胶失效和竹胶合板材料破坏失效,其极限承载力不仅与截面尺寸和长细比相关,而且受截面组合方式影响。设置横向约束拉杆可有效减缓开胶失效,改变极限破坏模式,显著提高极限承载力;与SBCC的极限压应力相比,SBCCB的极限压应力平均提高26%。      

Numerical investigation of fatigue characteristics of concrete pavement

In concrete pavements, fatigue is one of the major causes of distress. Repeated loads result in the formation of cracks. The propagation of these cracks cause internal progressive damage within the structure, which ultimately leads to failure of the pavement due to fatigue. This paper presents a theoretical investigation of crack propagation within concrete pavement and its fatigue characteristics under cyclic loading. A numerical fatigue performance model has been developed for this purpose. The model is based on fictitious crack approach for the propagation of cracks and stress degradation approach for estimating the bridging stress under cyclic loading. Using the numerical model, a parametric study has been performed for a typical concrete pavement to evaluate its fatigue characteristics for different foundation strengths. The method can be used for prediction of crack propagation in concrete pavement under cyclic loading and gives an estimate of the incremental damage or the entire crack history of the pavement.     

Assessment of fatigue damage evolution in woven composite materials using infra-red techniques

Thermoelastic stress analysis (TSA) is used to study the growth of fatigue damage in single and two ply, 2 × 2 twill woven composite materials. Test specimens were subjected to a uniaxial tensile cyclic loading with maximum stresses of 10%, 15% and 20% of the ultimate failure stress. The development of fatigue damage locally within the weft yarns is monitored using high resolution TSA. The specimens were subsequently inspected using optical microscopy to evaluate the location and extent of cracks. Cracks were found in the weft fibres, running transverse to the loading direction. It is demonstrated that the lighter weight fabric is more resilient to damage progression. A signature pattern is identified in the TSA phase data that indicates the onset and presence of fatigue damage in the composite material.     

Fatigue failure in graphite fibre and glass fibre-polymer composites

Our studies have established that unidirectional graphite fibre composites show excellent fatigue resistance with only a 20 to 30% decrease in strength with cycling. Fatigue failures invariably occurred on the surfaces undergoing compression and were identified by scanning electron microscope studies as resulting from matrix failure adjacent to local fibre buckling failure zones. In contrast, glass fibre composites showed a much larger (70%) loss in strength under cyclic loading. At intermediate lives, failure occurred by the growth of matrix microcracks followed by delamination, while at long lives, the applied stress levels were below the microcrack initiation stress and behaviour was characterized by crack nucleation processes. These results have suggested a criterion for predicting high cycle fatigue strength which is based on the hypothesis that for failure to occur, the maximum applied effective cyclic strain in the composite must exceed a critical value which depends upon the fatigue response of the matrix material. The main assumption is that localized fatigue failures in the matrix are the predominant contributions to the ultimate fatigue failure of the composite.     

三维机织复合材料在拉压循环载荷下的疲劳性能

为研究三维机织复合材料在拉伸-压缩循环载荷下的疲劳性能,对材料进行了应力比R=-1的疲劳试验。在不同的载荷水平下,分别进行了纬向和经向两类拉压疲劳试验。试验获得了试样在疲劳载荷下的滞回曲线和全过程中剩余刚度比随寿命的变化曲线。结果表明,在拉伸-压缩循环载荷下,三维机织复合材料的疲劳损伤过程主要包含3个阶段,分别发生基体破坏、纱线横向裂纹扩展和纱线的最终断裂。基体的破碎和开胶、垂直于载荷方向排布的纱线撕裂和沿载荷方向排布的纱线断裂是试样内部的主要失效模式。试验还获得了纬向和经向拉压疲劳的拟合S-N曲线,可应用于工程中对该型材料进行疲劳寿命估算。该型材料的疲劳寿命在低应力区和高应力区均显示出较小的分散性,双对数坐标系下的拟合S-N曲线具有较好的线性度。     

CFRP筋的疲劳性能

进行了不同循环应力下CFRP筋的常温疲劳试验,选取CFRP筋70％的极限拉伸应力作为最大循环应力,在R=0．5（最小循环应力与最大循环应力的比）和R=0的应力率下,测量了CFRP筋的疲劳寿命曲线,研究了CFRP筋的疲劳性能．结果表明：CFRP筋的最大循环应力应控制在70％极限拉伸应力以下;在R=0．5应力率下,最大循环应力下降5％的极限拉伸应力时,CFRP筋的疲劳寿命增长10倍左右;最大循环应力分别为60％和50％极限拉伸应力时,R=0应力率下CFRP筋的疲劳寿命分别为R=0．5应力率下疲劳寿命的百分之一和十分之一．这说明CFRP筋具有很大的脆性,需要有足够的强度安全系数,才能发挥作用;根据试验拟合的疲劳寿命曲线,CFRP筋的疲劳性能远高于Q235光圆钢筋．     

Constant-life diagram modified for notch plasticity

A modified constant-life Haigh diagram has been formulated to account for plasticity occurring at stress concentrations under cyclic loading at sufficiently high stress ratios. The notch plasticity is assumed to occur within a range of elastic stress concentration factors, k_t, and cycles to failure, N_f, such that plastic straining occurs only during the first cycle of constant-amplitude cycling and straining remains elastic thereafter. This condition is expected to occur in high cycle fatigue at stress concentrations of moderate k_t loaded at high stress ratio. The validity of the model is assessed by means of fatigue data from Ti–6Al–4V notched specimens having a range of k_t. The model, purposely kept simple for ease of use as a design tool, is found to capture trends in the fatigue data not predicted using traditional straight-line models on a mean stress versus alternating stress constant-life plot.     

中碳贝氏体钢的室温单轴循环变形行为研究

为了了解中碳贝氏体钢支承辊的接触疲劳失效机制,对中碳贝氏体钢材料在室温单轴循环加载下的应变循环特性和棘轮行为进行了实验研究。讨论了材料的循环软/硬化特性及其对单轴棘轮行为的影响,同时揭示了该材料棘轮行为的平均应力、应力幅值及其加载历史的依赖性。实验研究表明:材料的循环软/硬化特性具有明显的应变幅值依赖性,进而导致材料在不同的应力水平下出现不同的棘轮行为。研究得到了一些有助于该类材料循环变形行为本构描述的结论。     

Fatigue behaviour of 3-d SiC/SiC Composites

A tension–tension fatigue damage analysis was performed using 3-d silicon carbide fibre reinforced (orthogonal) silicon carbide matrix (SiC/SiC) composites. Two groups of SiC/SiC specimens were tested. The first group consisted of samples without any oxidation protective top layer coating, whilst the latter one contained samples covered with a well fitting, chemical vapour deposited (CVD) SiC system. This coating is necessary for the material to sustain high temperatures. Both the coated and uncoated material had a fibre volume fraction of about 36% equally distributed in three rectangular directions. Load control fatigue tests were conducted at room temperature. The fatigue life was found to decrease by increasing the cyclic stress level. A power-law equation is proposed, which correlates the applied maximum stress during the fatigue test with the number of cycles to failure. In general, the presence of the coating layer decreases the static strength of the material. However, the nominal maximum cyclic stress for which the endurance fatigue limit appeared, remained unaffected by the presence of the oxidation protective SiC coating. Microstructural examination has also been performed on the fractured specimens and it reveals some of the failure mechanisms of the composite that appeared under quasi-static and dynamic loading.     

The fatigue behaviour of Kevlar-29 fibres

Single Kevlar-29 fibres have been subjected to a variety of tensile cyclic and steady loading conditions. The dispersion of tensile strengths of the samples tested was found to be inherent to the fibre due to the distribution of defects in it and not due to variations of diameter between samples. Cyclic loading was found to produce both longer and shorter lifetimes than those recorded under steady loads equal to the maximum cyclic load. Longer lifetimes indicated failure due to creep mechanisms whereas shorter lifetimes, seen with greater load amplitudes, suggest a fatigue mechanism. No difference was seen in the fracture morphologies of Kevlar-29 fibres broken under simple tensile, fatigue and creep conditions because of the complex splitting which occurs in all cases.     

Biodegradable behaviour and fatigue life of ZEK100 magnesium alloy in simulated physiological environment

The fatigue life of ZEK100 magnesium alloy in the phosphate buffered solution for various immersion intervals was investigated by experiments and theoretical predictions. The biodegradable behaviours of ZEK100 magnesium alloy were also studied. Microstructure observation showed that the corrosion behaviours were characterized by pitting corrosion. The corrosion rate decreased a lot in the initial 7 d and then almost stayed unchanged. After 28 d immersion, the elastic modulus almost kept stable, while the yield strength and the ultimate strength decreased a lot, which indicated that corrosion had important effects on the tensile mechanical properties. It showed that the fatigue life of the samples under the same stress conditions decreased with increasing immersion time under the asymmetric stress‐controlled cyclic loading. Considering the effect of corrosion on the material failure, a modified fatigue life model was proposed for magnesium alloy under corrosion.     

Experimental investigation on bending fatigue failure of corrugated paperboard

A series of 3‐point bending fatigue tests were conducted to investigate the bending fatigue behaviour of flute type B and C corrugated paperboard samples under cyclic loading. The S‐N curve was obtained. The fatigue failure of corrugated paperboard may be described by both Basquin‐type and exponential‐type S‐N curves; however, the exponential‐type S‐N curve is more appropriate. The stiffness is gradually degraded with almost same energy dissipation in most stress cycles, but it decreases abruptly with the enlarged energy dissipation when the testing cycle is very close to the ultimate cycles of fatigue failure. The corrugated board deforms constantly under the action of cyclic loading, and no visible crack appears. The fatigue failure modes and mechanisms are same for the corrugated boards with B‐flute and C‐flute; however, the S‐N curve of corrugated board is closely related to the flute structure. The results obtained in this paper may be applied to the dynamic design and accelerated vibration test of stacked corrugated boxes.     

Accumulation of Damage in A336 GR5 Structural Steel Subject to Complex Stress Loading

Abstract: Changes in the mechanical characteristics of the structural steel because of fatigue damage accumulation are investigated in this article. Cyclic loading was performed in a complex stress state. Tubular specimens were loaded by axial force and torque, with loading performed along proportional and non‐proportional paths in the strain space. Material characteristics in the form of the stress–strain curve and yield surface were determined in the ‘as received’ state and after cyclic loading. It was found that accumulation of the fatigue damage because of cyclic loading in a complex stress state along proportional and non‐proportional paths in the strain space is manifested by the increase in inelastic response. The rate of damage was found to be higher for non‐proportional (circular) loading paths than that for proportional loading with the same strain amplitude.     

Zur thermischen Ermüdung der Magnesium‐Basislegierung AZ91

Thermal fatigue of magnesium‐base alloy AZ91 Thermal fatigue tests of the magnesium‐base alloy AZ91 were carried out under total strain control and out‐of‐phase‐loading conditions in a temperature range between ‐50°C and +190°C. Specimens produced by a vacuum die casting process were loaded under constant total strain and uniaxial homogeneous stress. To simulate the influence of different mean stresses, experiments were started at different temperature levels, e.g. the lower, mean or upper temperature of the thermal cycle. The thermal fatigue behavior is described by the resulting stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. Depending on the maximum temperature and the number of loading cycles, cyclic softening as well as cyclic hardening behavior is observed. Due to the complex interaction of deformation, recovery and recrystallization processes and as a consequence of the individual temperature and deformation history, thermal fatigue processes of the material investigated cannot be assessed using results of isothermal experiments alone. The upper temperatures or the resp. temperature amplitudes determine the total fatigue lifetime.     

Elastic–plastic fatigue crack growth analysis under variable amplitude loading spectra

Most fatigue loaded components or structures experience a variety of stress histories under typical operating loading conditions. In the case of constant amplitude loading the fatigue crack growth depends only on the component geometry, applied loading and material properties. In the case of variable amplitude loading the fatigue crack growth depends also on the preceding cyclic loading history. Various load sequences may induce different load-interaction effects which can cause either acceleration or deceleration of fatigue crack growth. The recently modified two-parameter fatigue crack growth model based on the local stress–strain material behaviour at the crack tip [1,2] was used to account for the variable amplitude loading effects. The experimental verification of the proposed model was performed using 7075-T6 aluminum alloy, Ti-17 titanium alloy, and 350WT steel. The good agreement between theoretical and experimental data shows the ability of the model to predict the fatigue life under different types of variable amplitude loading spectra.     

The effects of texture in titanium alloys for engineering components under fatigue

The mechanical response of textured Ti 6/4 plate material is assessed through an evaluation of monotonic properties under tension and torsion loading and fatigue testing of plain section and notched specimen geometries. Significant variations in modulus, yield strength, ultimate tensile strength and ductility are demonstrated for testpieces taken from the plate materials parallel to either the transverse or longitudinal rolling direction. Cyclic performance is also shown to be sensitive to orientation with different cyclic stress–strain curves defined in the two orientations. The relationship between the principal stress axis and the dominant basal plane texture is shown to control fatigue crack initiation lives and the ultimate mode of fracture. Whilst loading parallel to the transverse direction offers the strongest monotonic and cyclic stress–strain response, fatigue tests performed on specimens orientated parallel to the longitudinal rolling direction provide the optimum cyclic life. These effects are discussed with reference to the inherent, anisotropic mechanical response of α+β titanium alloys, which results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.     

Interlaminar shear fatigue of pultruded graphite fibre-polyester composites

The effect of cyclic loading on the interlaminar shear strength of pultruded graphite fibre-polyester was determined. Two fibre volume fractions, 0.5 and 0.33, were studied. The results indicate that the deterioration in the interlaminar shear strength with cycling is significantly greater than in flexural fatigue. The higher volume fraction material showed a greater drop in the interlaminar shear strength than the lower volume fraction material. Unlike the monotonic strengths, the effect of the fibre volume fraction on interlaminar shear fatigue strength at high cycles is small, indicating that there is little advantage in increasing the fibre volume fraction to improve the interlaminar shear strength in high cycle fatigue environments. A critical stress was determined above which interlaminar shear fatigue failure did not occur within 10⁷ cycles for the materials tested.     

Fatigue behavior of laminated composites with a circular hole under in-plane uniaxial random loading

A modified fiber failure fatigue model is presented to evaluate the behavior of laminated composites with a central circular hole under in-plane uniaxial random/block loading. The analytical model presented is based on minimum strength model and fiber failure criterion under static loading available in the literature. The analysis starts with the determination of location of a characteristic curve around the hole and the stress state along the characteristic curve under in-plane uniaxial fatigue loading. Number of cycles to failure and location of failure are determined under given fatigue loading condition. Based on ply-by-ply analysis, ultimate fatigue failure and the corresponding number of cycles are determined. Degradation of material strength as a function of applied number of cycles is considered in the model presented. Random loading case is analyzed based on rainflow counting technique. Analytical predictions are compared with the experimental results for uniaxial block loading.     

STATIC AND CYCLIC FRETTING FATIGUE BEHAVIOUR OF SILICON NITRIDE

A cyclic fretting fatigue test machine was constructed. The piezoelectric bimorphs were used as actuators for cyclic loading and fretting motion at the resonance frequency of the system. Fretting fatigue tests under static and cyclic loading conditions were carried out using HIP-sintered silicon nitride. From the experimental results, it was found that fretting fatigue strengths under the two test conditions were identical and hence the effect of cyclic loading on fretting fatigue strength of silicon nitride was almost negligible. A fretting crack initiated in a very early stage of the fatigue life at the position of the maximum frictional stress in the contact area. Fretting fatigue life prediction based on fracture mechanics was also carried out. The predicted lives were in good agreement with the experimental results.