Fatigue crack growth of filament wound GRP pipes with a surface crack under cyclic internal pressure

In this study, fatigue damage behavior of (±75₃) filament wound composite pipes with a surface crack under alternating internal pressure was investigated. The specimens were tested at room temperature and exposed to open ended fatigue tests in which the pipe can be deformed freely in the axial direction. The tests were carried out in accordance with the ASTM D-2992 standard. The alternating internal pressure was generated by conventional hydraulic oil. The low cycle tests were performed with 0.42 Hz frequency and R = 0.05 stress ratio. Glass reinforced polymer pipes (GRP) are made of E-glass/epoxy and have (±75₃) configuration. Surface cracks were machined in the axial direction of the pipes which have depth-to-thickness ratios a/t = 0.25–0.38–0.50 and depth to length ratio of a/c = 0.2. Tests were performed at three different loads of 50%, 40%, and 30% of ultimate hoop stress strength of unnotched pipes. The failure behavior of GRP pipes during the test was observed and fatigue test results were presented by means of (S–N) curves and delamination damage zone area-cycle (A–N) curves.     

Fatigue resistance of welded tees under pulsed internal pressure

The authors developed a method of evaluating the fatigue endurance of — welded tees under the effect of internal pressure. It is assumed that fracture takes place as a result of development of an angular quarter- circle crack along the weld line in the plane of the axes of the connected pipes. A semiempirical equation for determining the concentration factor of circumferential membrane stresses is proposed. The results of tests of welded tees made of 20 steel showed satisfactory agreement with the calculated values.Translated from Problemy Prochnosti, No. 3, pp. 85–89, March, 1993.     

Investigation and analysis of depressurization of vessels operating under cyclic internal liquid pressure

The limiting state of local depressurization is investigated in experiments on large-scale cylindrical simulators of pressure vessels with surface cracks. The local depressurization characteristic is formulated by using the concept of the coefficient of elastoplastic strain intensity.Translated from Problemy Prochnosti, No. 9, pp. 17–21, Semptember 1990.     

循环荷载作用下钢桥面铺装疲劳损伤失效行为研究

针对钢桥面铺装工程中普遍采用的改性沥青(Stone Matrix Asphalt,SMA)、浇筑式沥青(Guss asphalt,GA)、环氧沥青(Epoxy asphalt,EP)混合料双层铺装结构,进行了循环车载作用下钢桥面与沥青混凝土铺装疲劳损伤特性理论分析与试验研究。基于疲劳损伤度,研究了钢桥面铺装疲劳损伤失效行为和疲劳开裂过程中损伤场、应力和应变场动态演变机制,推导出疲劳失效时的损伤场、应力和应变场计算表达式,并给出钢桥面铺装疲劳寿命理论公式。以三座钢箱梁桥桥面铺装(润扬长江大桥2005,南京长江三桥2005,苏通大桥2008)为例,对不同铺装结构组合方案下的复合梁进行疲劳试验分析和使用寿命理论预测。实例研究结果表明,钢桥面铺装疲劳损伤失效行为预估模型合理可行;相较于改性沥青、浇筑式沥青,环氧沥青混合料具有较强高的强度低变形能力,更适合于大跨径钢桥面铺装抗疲劳的设计要求;由环氧沥青混合料组合而成的“双层环氧沥青混凝土”和“浇注式沥青混凝土(下层)+环氧沥青混凝土(上层)”的抗疲劳性能优于其它沥青混合料铺装结构组合方案,同等厚度组合情况下疲劳使用寿命可延长1倍~2倍以上;“双层环氧沥青混凝土”已应用于润扬长江大桥、南京长江三桥和苏通长江大桥钢桥面工程,并已成功运行10年以上,其跟踪观测结果良好。     

External surface cracks in shells under cyclic internal pressure

The stress analysis and fatigue crack growth behaviour of a part‐through‐cracked double‐curvature thin‐walled shell is examined. An external surface crack is assumed to lie in one of the principal curvature planes of the shell, and to present a semi‐elliptical shape. The stress intensity factors (SIFs) along the crack front for different elementary opening stresses acting on the crack faces are determined through a three‐dimensional finite element analysis. Then approximate values of SIF in the case of a cracked pressure vessel are computed by employing the above results together with the superposition principle and the power series expansion of the actual opening stress. Finally, a numerical simulation procedure is carried out to predict the crack growth under cyclic internal pressure. Some results are compared with those of other authors.     

Fatigue crack growth in thick-walled cylinders under pulsating internal pressure

The stress intensity factor is derived for both single and multiple longitudinal, elliptical cracks in the wall of a pressurized thick cylinder of given geometry. For this purpose, it is found necessary to combine known solutions to the stress intensity factor for a straight longitudinal crack with the effects of a curved crack front and multiple cracking. The analysis is appraised from a number of fatigue tests reported for % Ni-Cr-Mo cylinders with diameter ratios of between 2 and 3 under repeated and fluctuating pressure cycles. When cylinders with poorly finished bores are assumed to be initially flawed, it is found that their fatigue lives under high ranges of pressure may be predicted reliably for the single crack propagation failures observed. This analysis employs published WOL or SEN fatigue crack growth data for the alloy. The enhancement in fatigue life that results from an improved surface finish has enabled that proportion of life expended during the initiation phase to be determined. It is further shown that the observed effect of mean stress and surface finish on the fatigue limit may be quantified with a change to the threshold of stress intensity for crack growth. A number of tests were conducted with two-step changes to the amplitude of the pressure cycle. In this instance, nonlinear, stress dependent, cumulative damage rules are shown to offer no advantage over Miner's rule in the prediction of fatigue life.     

Fatigue and fracture characteristics of aluminum alloy cylinders under internal pressure

This paper presents the results of tensile, plane-strain fracture-toughness, and fatigue evaluations of die forged and premium cast hydraulic cylinders of six high strength aluminum alloys. Comparisons are made of fatigue strengths at stress levels related to design values, and of relations between values of K_Ic and the conditions under which unstable crack growth was observed in the fatigue tests.     

Fatigue of bonded steel interfaces under cyclic shear loading and static normal stress

Hybrid joints that combine mechanical fastening and bonding provide potential joining alternatives for high strength steel structures. An experimental and analytical research program has been initiated to assess the static and cyclic Mode II shear strength of epoxy bonded steel interfaces subjected to static normal (Mode I) pre-stress. During cyclic loading, shear stress vs. interface displacement could be divided into elastic and inelastic regions with a power law equation describing the inelastic curve. Hysteresis loop shape varied with normal stress, shear stress and accumulated fatigue cycles. A shear stress amplitude threshold equal to about 50% of the fracture shear stress was observed.     

Dynamic analysis of multi-layered filament-wound composite pipes subjected to cyclic internal pressure and cyclic temperature

Based on the three-dimensional anisotropic elasticity, the stress analysis of multi-layered filament-wound composite pipes subjected to cyclic internal pressure and temperature loading is conducted in this article. The time-dependent stress, strain and deformation distributions are numerically obtained by the use of the finite difference technique. The pressure and temperature are considered to be symmetrical about the axis of the cylinder and independent of the axial coordinate. Each layer of the pipes is made of a homogeneous, anisotropic and linearly elastic material and it is assumed that the material properties do not change with increasing the temperature. The shear extension coupling is also considered because of lay-up angles. Numerical results obtained from the present model are compared with other published results and good agreement has been achieved.     

Acoustic emission analysis of composite pressure vessels under constant and cyclic pressure

The use of acoustic emission (AE) for the detection of damage in carbon fibre composite pressure vessels was evaluated for constant and cyclic internal gas pressure loading conditions. AE was capable of monitoring the initiation and accumulation of damage events in a composite pressure vessel (CPVs), although it was not possible to reliably distinguish carbon fibre breakage from other microscopic damage events (e.g. matrix cracks, fibre/matrix interfacial cracks). AE tests performed on the carbon fibre laminate used as the skin of pressure vessels revealed that the development of damage is highly variable under constant pressure, with large differences in the rupture life and acoustic emission events at final failure. Numerical analysis of the skin laminate under constant tensile stress revealed that the high variability in the stress rupture life is due mainly to the stochastic behaviour of the carbon fibre rupture process.     

Fatigue crack propagation under cyclic thermal stress

Structures used at elevated temperature subject to severe cyclic thermal stress. Therefore, accurate prediction procedures for thermal fatigue crack growth should be applied to rationalise component flaw assessment. Fatigue crack propagation tests under thermal stress were carried out using an modified type 316 stainless steel (316FR), which is a candidate material for the fast reactor in Japan. Thermal stress of the tests was generated by cyclically changed temperature distribution through thickness in a plate by induction heating and air-cooling. Numerical analysis was also carried out to examine the applicability of the J integral under cyclic thermal stress. The J integral under elasto-plastic condition under thermal stress is close to the elastically calculated J integral. Prediction by J integral tends to be conservative for deeper cracks, and modification of the J integral value using crack opening ratio gives good agreement with the experimental crack growth.     

Fatigue life estimation of notched specimens of modified 9Cr-1Mo steel under uniaxial cyclic loading

Accuracy in the estimation of low cycle fatigue life of modified 9Cr-1Mo steel notched specimen by different analytical methods such as linear rule, Neuber’s rule, strain energy density method and numerical method such as finite element analysis have been studied in this investigation. The fatigue tests on notched specimens having notch radius of 1.25 mm, 2.5 mm and 5.0 mm were carried out at 823 K with net stress amplitudes of 250 MPa, 300 MPa and 350 MPa. The fatigue tests on smooth specimens were carried out with strain amplitudes ranging from ±0.3% to ±0.8% with a strain rate of 3 × 10^?3 s^?1 at 823 K to evaluate the fatigue life of notched specimen through strain-life approach. In order to predict the cyclic stress response of the material, Chaboche non-linear hardening model was employed considering two back stress components. Predicted hysteresis loops for smooth specimen were well in agreement with experimental results. Estimated fatigue lives of notched specimens by analytical methods and finite element analysis were within a factor ±16 and ±2.5 of the experimental lives respectively.     

Fatigue properties of rock salt subjected to interval cyclic pressure

Due to the influence of gas pressure operational modes, surrounding rock of salt carven underground gas storage always suffers combined stress composed of cyclic pressure and intervals of no stress (or small stress). We conducted comparisons between conventional fatigue tests and (six groups of) interval fatigue tests, which combine spaced stress cycles and normal stress cycles. Experimental measurements demonstrate that the combined cyclic stress has a strong impact on the fatigue activity of rock salt. In interval fatigue tests, the residual strain of a spaced stress cycle is notably larger than that of a normal stress cycle. As the conventional tests can be actually considered as a kind of fatigue tests with transitory intervals, the accumulative rate of residual deformation increases with the duration of the interval in all test groups. The testing results show the fatigue lives of samples from interval fatigue tests dramatically reduce in a certain range; when intervals extend beyond the value 120 s, fatigue lives perform with a slight rise. Based on the S–N curve and the S–T curve, an experiential model fitting the relationship between fatigue life and interval was established.     

Fatigue of welded steel joints under wideband loadings

Fatigue tests were conducted on high-strength welded steel cruciform-shaped specimens subjected to random loadings to investigate the effects of loading intensity, nonnormality and frequency bandwidth on the rate of fatigue damage accumulation. The test result are compared with predictions made using the Rayleigh approximation and rainflow analysis in terms of cycles and times to failure. Results indicate that nonnormality can significantly increase the rate of fatigue damage accumulation and result in nonconservative fatigue life estimates if it is effect is not accounted for properly. Likewise, frequency content was also found to influence the rate of fatigue damage accumulation, but to a lesser extent than nonnormality.     

Distributed brillouin scattering sensor for discrimination of wall-thinning defects in steel pipe under internal pressure

A distributed Brillouin scattering sensor has been employed to identify several inner wall cutouts in an end-capped steel pipe by measuring the axial and hoop strain distributions along the outer surface of the pipe. The locations of structural indentations that constitute 50-60% of the inner pipe wall are found and distinguished by use of their corresponding strain-pressure data. These results are quantified in terms of the fiber orientation, defect size and depth, and behavior relative to those of unperturbed pipe sections.     

高速列车荷载作用下铸钢焊接节点的疲劳分析

钢结构特别是焊接钢结构对动荷载特别敏感。武广客运专线武汉火车站采用了新型的“桥建合一”的结构型式,在长期往复动荷载作用下,上部大跨度钢结构的疲劳寿命令人关注。论文建立了铸钢节点包括焊缝的精细有限元模型,基于热点应力法和Miner线性累积损伤理论,分析了武汉站上部大跨度钢结构10管相贯焊接铸钢节点的疲劳寿命。分析结果表明：列车振动荷载引起的动应力对铸钢节点的疲劳寿命影响不大,满足结构使用寿命100年的要求。分析方法可为国内今后类似结构的疲劳寿命分析提供参考。     

Fatigue crack initiation and propagation under cyclic contact loading

A computational model for contact fatigue damage analysis of gear teeth flanks is presented in this paper. The model considers the conditions required for the surface fatigue crack initiation and then allows for proper simulation of the fatigue crack propagation that leads to the appearance of small pits on the contact surface. The fatigue process leading to pitting is divided into crack initiation and a crack propagation period.The model for prediction of identification of critical material areas and the number of loading cycles, required for the initial fatigue crack to appear, is based on Coffin-Manson relations between deformations and loading cycles, and comprises characteristic material fatigue parameters. The computational approach is based on continuum mechanics, where a homogenous and elastic material model is assumed and results of cyclic loading conditions are obtained using the finite element method analysis.The short crack theory together with the finite element method is then used for simulation of the fatigue crack growth. The virtual crack extension (VCE) method, implemented in the finite element method, is used for simulating the fatigue crack growth from the initial crack up to the formation of the surface pit. The relationship between the stress intensity factor K and crack length a, which is needed for determination of the required number of loading cycles N_p for a crack propagation from the initial to the critical length, is shown.