Analytical and experimental investigation of fatigue crack propagation for polyethylene methacrylate

The fatigue crack growth of most polymers is sensitive to temperature. In this paper, tests on fatigue crack growth of polyethylene methacrylate were carried out and the fatigue crack growth rate was obtained at temperature range −50 to 90 °C and frequency 1 Hz. The fatigue crack propagation (FCP) properties of polyethylene methacrylate and metals were studied comparatively and a new modified formula for FCP rate was deduced to describe the polyethylene methacrylate FCP rates. The formula includes four parameters: the FCP threshold, Young's modulus, fracture toughness and stress ratio. The predicted curve based on this modified formula corresponds very well with the test data of polyethylene methacrylate at different temperatures. Therefore, the modified formula can be used to describe the FCP process.     

聚偏氟乙烯/聚氯乙烯相容性研究

采用溶解度参数法和混合焓变理论预测聚偏氟乙烯(PVDF)／聚氯乙烯(PVC)共混体系为部分相容体系,并用共溶剂法、黏度法、微分扫描量热法判断聚偏氟乙烯／聚氯乙烯的相容性．结果表明:两者属于部分相容体系,与理论预测相符,为后续制备聚偏氟乙烯／聚氯乙烯纤维膜奠定理论基础．     

An experimental investigation on fatigue crack growth of AL6XN stainless steel

The crack growth behavior of AL6XN stainless steel was experimentally investigated using round compact tension (CT) specimens. The influences of the R-ratio (the ratio of the minimum load over the maximum applied load in a cycle), the tensile and compressive overloads, and the loading sequence on crack growth were studied in detail. The results from the constant-amplitude experiments show a sensitivity of the crack growth rate to the R-ratio. The application of a tensile overload has a profound effect on crack growth, resulting in a significant retardation in the crack propagation rate. A compressive overload (underload) leads to a short-lived acceleration in crack growth. Results from the two-step high-low loading reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. A crack driving force parameter together with a modified Wheeler model is found to correlate the crack growth experiments well.     

纳米SiO2对聚偏氟乙烯超滤膜的影响研究

聚偏氟乙烯铸膜液中可以加入纳米无机粒子———SiO2,制成纳米无机-有机复合膜.复合膜较纯聚偏氟乙烯超滤膜的性能有一些改善,如果纳米SiO2量合适,膜的水通量增加而截留率不变.纳米SiO2可以大大增加铸膜液的黏度,使成膜更容易.同时纳米SiO2对成膜过程也有影响,从相图看,改变了双节点的位置,对非溶剂的容纳能力降低;从DSC测试曲线得知复合膜中PVDF的结晶度增加,所以说纳米SiO2对成膜产生了延时作用.     

品字形聚偏氟乙烯中空纤维膜的制备

采用溶液相转移法,通过特殊结构的纺丝喷头,制备了品字形PVDF中空纤维膜,讨论了PVDF固含量和芯液组成对品字形膜形态结构和性能的影响。结果表明,随PVDF固含量的增大,品字形膜支撑层空穴变小,膜丝外形融合程度减小,海绵体结构更致密,超滤水通量、透气系数和膜蒸馏通量相应减小,膜丝断裂强力增大;随芯液中溶剂DMAc含量的增加,品字形膜支撑层空穴变小,融合部分海绵体变致密,超滤水通量、透气系数减小,膜蒸馏通量略微提高,膜丝断裂强力增大;优化后的品字形膜的水通量较单芯膜有所降低,但断裂强力显著提高,接近单芯膜的3倍。     

Abaqus implementation of extended finite element method using a level set representation for three-dimensional fatigue crack growth and life predictions

A three-dimensional extended finite element method (X-FEM) coupled with a narrow band fast marching method (FMM) is developed and implemented in the Abaqus finite element package for curvilinear fatigue crack growth and life prediction analysis of metallic structures. Given the level set representation of arbitrary crack geometry, the narrow band FMM provides an efficient way to update the level set values of its evolving crack front. In order to capture the plasticity induced crack closure effect, an element partition and state recovery algorithm for dynamically allocated Gauss points is adopted for efficient integration of historical state variables in the near-tip plastic zone. An element-based penalty approach is also developed to model crack closure and friction. The proposed technique allows arbitrary insertion of initial cracks, independent of a base 3D model, and allows non-self-similar crack growth pattern without conforming to the existing mesh or local remeshing. Several validation examples are presented to demonstrate the extraction of accurate stress intensity factors for both static and growing cracks. Fatigue life prediction of a flawed helicopter lift frame under the ASTERIX spectrum load is presented to demonstrate the analysis procedure and capabilities of the method.     

Crack propagation of CCT foam specimen under low strain rate fatigue

The objective of this study is to investigate the effect of holes on the low strain rate fatigue properties of the nickel foam material and to understand the lifetime of this material which is subjected to the repeated loads. Failures of foam materials under single and repeated loads analogous to fatigue are essential to designers and users in military and aerospace structures. The material failure induced by repeated low strain rate loading becomes a critical issue because of significant loss of stiffness and compressive strength in the foam material. Testing methods to study low strain rate (that is, strain rate) fatigue are quite numerous; no single standard testing procedure is defined for studying the low strain rate fatigue property of a material. The increasing application of foam material in aerospace structures, owing to high specific stiffness and strength has attracted a great concern about the high sensitivity to low strain rate damage introduced during manufacture or in service, and the effects of such damage on structural degradation. To investigate this issue, this study sets up an experimental procedure to determine the low strain rate fatigue properties of nickel foam material. This study performs both experimental and numerical investigations to catch the low strain rate fatigue behavior of nickel foam with open-cell type. The experiments are conducted by rod up and down at the strain rate fatigue of loading. The crack length at the specific cycles are measured experimentally by taking pictures with a paper ruler attached on the surface of specimen and these values are apply to the computer simulations as crack seam model. The simulation result of stress intensity factors are compared with a well known theoretical calculation. Design life and probability of failure or survival at specified life can be calculated so that the fatigue life of nickel core material subjected to repeated low strain rate loading is predicted.     

Fracture mechanics in fatigue calculations as the assistance in structural design. Case study

A case study originated from a request to perform fatigue calculations on a partial penetration weld in a steel casting on a new heavy lift ship. The ship has a special lifting system for lifting and transporting topsides of offshore oil/gas platforms. There are many sizes of offshore platforms so the lift system is designed to move along the vessel on rails, which are part of the main deck of the ship hull. The loads passing into the rails during lift operations are large, and massive steel castings of complex shapes are used to distribute these loads into the hull. The castings are very thick so it is difficult to achieve full penetration welds, and therefore the initial design proposed partial penetration welds. In order to decide if the partial penetration welds were adequate, a fatigue assessment was carried out using a fracture mechanics approach based on BS7910. Different possibilities of the bevelling of castings edges in preparation for welding were considered in the stress analysis and in the crack growth estimations. In the areas of the ship hull which experienced high dynamic stress ranges none of the different possibilities showed acceptable fatigue life, and would demand re-design. The stress intensity factors obtained through the extensive finite element analysis were compared with the analytical solutions available in literature. Both results showed good correspondence.     

聚偏氟乙烯中空纤维膜的研制和应用

介绍了聚偏氟乙烯(PVDF)中空纤维膜的制备工艺，讨论了PVDF含量、添加剂、纺丝温度、干程及卷绕速度对膜性能的影响．并就PVDF中空纤维膜3种应用方法——连续膜过滤、膜生物反应器、双向流过滤的技术原理及应用作了介绍．     

Fracture and fatigue analysis of an agitator shaft with a circumferential notch

This paper is concerned with the fracture analysis of an agitator shaft of a large vessel and predicting its high cycle fatigue life. The agitator shaft has a circumferential notch around it and is subjected to remote bending and torque created by the mixing operation. The problem is comprised (i) the analyses of the bending force and torque acting on the agitator by using the analytical method, (ii) calculation of stress intensity factors under mode I and III loading conditions by using finite element method and, (iii) fatigue analysis of the agitator shaft failed in service.An agitator model is set up and data obtained from the agitator are processed to make more realistic approximations for bending forces, since they form a base for stress analysis, in which mode I stress intensity factors are evaluated. Mode I stress intensity factors obtained by finite element analysis are compared with the results provided by using the body force method.     

High temperature low cycle fatigue behaviour of UDIMET 720 Li superalloy

The low cycle fatigue behaviour of UDIMET 720 Li has been investigated at 700 °C in vacuum and air environments under strain control tests. Hold times (HTs) have been introduced at peak tensile strain in order to determine the effects of creep and fatigue interactions. The fatigue life of the alloy is practically independent of HT in both environments at high strain ranges while HT has beneficial effect at low strain ranges. The main factor that adversely affects the life of the alloy is oxidation, as the fatigue life is reduced in air compared to vacuum especially at high strain ranges. The increase in fatigue due to HT at lower strain ranges is attributed to stress relaxation. A life prediction model has shown that nucleation occurs after few cycles at high strain ranges whereas a large proportion of the fatigue life is consumed in the nucleation process at low strain ranges.     

Comparison of the simulation and experimental fatigue crack behaviors in the nanoseconds laser shocked aluminum alloy

This investigation was performed to compare the simulation and experimental results of the fatigue crack growth rates and behaviors of the 7050-T7451 aluminum alloy by nanoseconds laser shock processing (LSP). Forman–Newman–deKoning (FNK) model embedded in the Franc2D/L software was utilized to predict fatigue crack growth rate, which was conducted to weigh the stress intensity factor (SIF) changing on the surface cracks. LSP induced high compressive residual stresses that served to enhance fatigue properties by improving the resistance against fatigue crack initiation and propagation. The circulating times of crack growth obtained from the simulation and experimental values indicated a slower fatigue crack growth rates after LSP. The relationships between the elastic–plastic materials crack growth rates and the SIF changing after LSP are resolved.     

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.     

The effects of test temperature and humidity on the mixed-mode fatigue behavior of a toughened adhesive aluminum joint

The effects of test environment humidity and temperature on the fatigue threshold and crack growth behavior of P2-etched and commercial coil-coated (CC) aluminum adhesive joints were studied under mixed-mode loading using aluminum asymmetric double cantilever beam (ADCB) specimens. Under dry conditions, increasing the temperature to 80 °C had an insignificant effect on the fatigue threshold, but caused an increase in the crack growth rates. At 40 °C, the fatigue behavior was insensitive to moisture at higher crack growth rates, but became sensitive to moisture level in the test environment as crack growth rates slowed to the threshold. The effect of moisture and temperature were explained by the observed changes in the crack path, which in general moved progressively closer to the more highly-strained adherend as the applied strain energy release rate, and consequently the crack growth rate, decreased. Furthermore, the residual adhesive thickness on the more highly-strained adherend, t_r, increased with increasing temperature, and the crack path shifted to the hydrated aluminum oxide interface when the test environment was saturated with moisture. The degrading effect of a hot-wet environment was similar for both P2-etch and CC pretreatments. At higher crack growth rates, the joint fatigue performance was degraded solely due to the effect of the increased temperature, whereas at low crack growth rates, the fatigue performance was degraded predominantly because of elevated moisture.     

A study of frequency and temperature effects on fatigue crack growth resistance of CPVC

Excellent corrosion resistance of chlorinated polyvinyl chloride (CPVC) makes it an attractive material for piping systems carrying corrosive materials. The relatively high glass transition temperature of CPVC has increased its use in hot water distribution. Establishing a relationship that describes the effect of test frequency on fatigue crack propagation (FCP) rate of polymers is an interesting challenge. FCP rates can decrease increase or remain constant with increasing test frequency. Moreover, FCP sensitivity to frequency of some polymers is known to be dependent on test temperature. In this study, fatigue crack propagation in a commercial grade chlorinated vinyl chloride (CPVC) over the frequency and temperature ranges of 0.1-10 Hz and −10 °C to 70 °C, respectively, was investigated. FCP tests were conducted on single edge notch (SEN) specimens prepared from 100-mm injection molded CPVC pipefittings. The crack growth rate (da/dN) was correlated with the stress intensity range ΔK. The FCP rate was found to be insensitive to frequency at sub room temperatures. The fatigue crack propagation resistance of CPVC was enhanced with increasing cyclic frequency at 50 and 70 °C. Frequency effect on FCP rate was found to be higher in the low frequency range.Macro-fractographic analysis of fracture surface showed that stepwise crack propagation existed at 0.1 and 1 Hz for all temperatures of interest.     

Influence of temperature on impact fracture behavior of an alloy steel

In this paper, the influence of temperature on impact toughness and fracture behavior of alloy steel (AISI Classification 8320) is presented and discussed. Impact toughness decreased with a decrease in test temperature. The extrinsic influence of temperature on impact toughness–fracture resistance relationships is rationalized in light of the conjoint and mutually interactive influences of intrinsic microstructural features, local stress states and macroscopic fracture behavior.     

Fatigue behavior and modeling of short fiber reinforced polymer composites including anisotropy and temperature effects

Effects of anisotropy and temperature on cyclic deformation and fatigue behavior of two short glass fiber reinforced polymer composites were investigated. Fatigue tests were conducted under fully-reversed (R = −1) and positive stress ratios (R = 0.1 and 0.3) with specimens of different thicknesses, different fiber orientations, and at temperatures of −40 °C, 23 °C, and 125 °C. In samples with 90° fiber orientation angle, considerable effect of thickness on fatigue strength was observed. Effect of mold flow direction was significant at all temperatures and stress ratios and the Tsai–Hill criterion was used to predict off-axis fatigue strengths. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain–life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios.     

异形聚偏氟乙烯中空纤维膜的研制

利用特殊结构的纺丝喷头,通过溶液相转移法,研制具有异形结构如:一字形多芯、品字形多芯结构的聚偏氟乙烯(PVDF)中空纤维膜.系统研究了纺丝入水距离、异形膜形状对膜形态结构与性能的影响.结果表明:随纺丝入水距离的增大,异形膜的超滤水通量、透气系数、膜蒸馏通量、抗压密系数、断裂强力及破裂压力减小;异形膜中品字形三芯中空纤维膜的断裂强力较单芯中空纤维膜有很大的提高,而且破裂压力和抗压密系数在异形膜中最大,纺丝成形稳定性最佳.     

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

Fatigue crack growth rates have been experimentally determined for the superalloy GH2036 (in Chinese series) at an elevated temperature of 550 °C under pure low cycle fatigue (LCF) and combined high and low cycle fatigue (CCF) loading conditions by establishing a CCF test rig and using corner-notched specimens. These studies reveal decelerated crack growth rates under CCF loading compared to pure LCF loading, and crack propagation accelerates as the dwell time prolongs. Then the mechanism of fatigue crack growth at different loadings has been discussed by using scanning electron microscope (SEM) analyses of the fracture surface.     

Effect of material plasticity on fatigue crack propagation under complex stress state

The maximum energy release rate criterion, i.e., G _max criterion, is commonly used for crack propagation analysis. However, this fracture criterion is based on the elastic macroscopic strength of materials. In the present investigation, a modification has been made to G _max criterion to implement the consideration of the plastic strain energy. This criterion is extended to study the fatigue crack growth characteristics of mixed mode cracks in steel pipes. To predict crack propagation due to fatigue loads, a new elasto-plastic energy model is presented. This new model includes the effects of material properties like strain hardening exponent n, yield strength σ_y and fracture toughness and stress intensity factor ranges. The results obtained are compared with those obtained using the commonly employed crack growth law and the experimental data.