The recent development of analysis methodology for rapid crack propagation and arrest in gas pipelines

Several dynamic analysis issues relating to rapid crack propagation (RCP) and arrest in gas piplines were developed recently, and are presented in this paper. This is based on a fluid/structure/fracture interaction package, PFRAC. Some developments have been implemented into this finite element code to simulate the behavior of the fractured pipes. The criteria for crack initiation, propagation and arrest have been discussed. As the crack propagates along the pipeline, the gas pressure decompression ahead of the crack tip and an efficiency of a linear decay behind the crack have been used in the computation. For the calculation of crack driving force G, the numerical approaches using the nodal force release and energy balance methods are described. This paper also presents a novel analysis methodology that has been developed to investigate the suitability of crack arrestors. Several numerical results for the cracked steel pipes with arrestors are presented along with comparisons with pipes that do not have arrestors.     

Determination of material fracture toughness by a computational/experimental approach for rapid crack propagation in PE pipe

Based on an investigation of the Small Scale Steady State (S4) test, an integrated computational/ experimental approach has been developed in order to assess the fracture behaviour of polyethylene (PE) gas distribution pipe material during rapid crack propagation (RCP). This paper describes the use of the results obtained from the S4 test and program modified from PFRAC (Pipeline Fracture Analysis Code) to evaluate the fracture toughness of the material, G _d, which could not be directly obtained from the test, and to predict critical pressure, p _c, for RCP in a full scale PE pipe. The contact algorithms are developed to consider the opening pipe wall impact against a series of containment rings and the capabilities of PFRAC are also extended. Since G _d is evaluated, the investigations are made on it to the effect of temperature and wall thickness. In addition, procedures to evaluate the critical pressure for the S4 test pipe are also discussed.     

能量平衡结合有限元数值计算分析天然气管道裂纹稳定扩展问题

本文提出了在天然气管道裂纹稳定扩展问题中，应用能量平衡方法结合有限元数值计算结果来分析计算裂纹驱动力。能量数值计算采用了可模拟动裂纹在管道上扩展的有限元程序ＰＦＲＡＣ，它包括了对未开裂管道和裂纹扩展管道的能量计算。通过分析外力作功和内部能量在裂纹扩展时的变化率，应用能量平衡方法计算了裂纹驱动力，并与在ＰＦＲＡＣ程序中应用节点力释放方法计算的裂纹驱动力的结果进行了比较     

纤维增强PE材料增韧效果的研究

以聚乙烯(PE)材料为基体,应用玻璃纤维随机或定向分布,增加材料的强度、刚度和断裂韧性,是发展高压大口径复合材料天然气管道的需要。本文基于PFRAC程序的动态断裂分析能力[1],增加了各向异性材料的本构条件,发展了对纤维增强复合材料未开裂和开裂管道的计算功能。由力学性能的试验结果,提供了材料的本构关系,对未开裂和开裂的管道进行了计算分析。结果表明,PE管道经纤维增强之后,与纯PE材料的管道相比,其环向位移下降到53%(纤维随机分布)~5%(纤维沿管道轴向80度分布);裂纹驱动力相应下降到50%~17%,充分反映了纤维对PE材料的增强和增韧效果。     

断裂动力学有限元程序的开发及在天然气管道裂纹扩展问题上的应用

许多工程问题是由于流（气）体压力导致了结构开裂,裂纹迅速扩展或者止裂,这被认为是断裂动力学最前沿的研究领域。本文描述了应用于分析流体／结构／断裂耦合作用问题的计算程序ＰＦＲＡＣ（ＰｉｐｅｌｉｎｅＦＲａｃｔｕｒｅＡｎａｌｙｓｉｓＣｏｄｅ）,和它在天然气管道裂纹迅速扩展问题上的应用。基于断裂动力学的模式,提出了裂纹扩展和止裂的判据。对于裂纹驱动力的计算,给出了节点力释放技术和能量平衡方法的具体应用。根据实际管道开裂试验的数据,提出了便于非耦合作用计算的裂纹后面气体压力衰减模式。针对钢制和塑料制天然气工程管道,给出了部分计算和试验的结果。     

Effect of isothermal exposure on fatigue crack growth in boron-titanium composites

Fatigue crack growth testing has been applied to boron/Ti-6Al-4V composites in order to investigate simultaneously crack propagation mechanisms during the fatigue and overload portion of the experiments. It is concluded that linear elastic fracture mechanics (LEFM) for heterogeneous and anisotropic materials facilitates understanding of the rupture mechanisms and the assessment of failure work and toughness. The influence of isothermal exposure on crack propagation mechanisms has been pointed out. A short duration heat treatment at 850 C improves the composite toughness and reduces the fatigue crack growth rate although the fibrematrix (FM) interfacial bonding is increased. This effect has been related to a damage mechanism initiated in the interfacial reaction zone. In any case, the fatigue behaviour of the composite is controlled by the matrix and the capability of the fibres to function as crack arrestors. The impeding effect of the fibres is no longer effective when the thermal exposure duration is significant.     

New unloading compliance correlation for throughwall circumferentially cracked pipe to measure crack growth during fracture tests

Crack growth is generally measured during fracture experiment of specimen or component. The unloading compliance technique is commonly used for this purpose because of its simplicity. It infers the crack length from unloading compliance of cracked component. The pre‐requisite of this technique is the availability of an equation that correlates crack length with unloading compliance. While such correlations are available for compact tension and three‐point bend specimens, it is not available for big components such as pipe or pipe bend. Development of such a correlation for throughwall circumferentially cracked (TCC) straight pipe under bending, therefore, forms the objective of the present study. However, the challenge to develop such correlation for TCC pipe is that the equation should contain not only crack length as a function but also the current deformation or load level as a parameter. This is attributed to the fact that the circular cross section of the pipe ovalizes during deformation leading to change of bending stiffness of the cracked body. New compliance correlations have been proposed for TCC pipe under bending load considering these complexities. Elastic‐perfectly plastic material behaviour has been assumed to characterize the material stress–strain response. However, it has been shown that error due to this approximation with respect to the actual stress–strain behaviour is negligible if one chooses flow stress equal to average of yield and ultimate strength. The proposed correlations are expressed in terms of normalized parameters to make them independent of specific values of geometric dimensions such as radius, thickness and span length of four‐point bending loading system. Effectiveness of this normalization has also been verified by carrying out a sensitivity study.     

Mesoscopic modeling of crack arrestor delamination in Al–Li: primary crack shielding and {T}-stress effect

This work explores the effects of grain orientation and T-stress on crack-front shielding and delamination cracking in modern aluminum–lithium alloys. In the crack-arrestor configuration of interest here, a primary crack extends through the grain thickness while triggering delaminations over susceptible grain boundaries that lie normal to the plane of the primary crack. A three-dimensional, small-scale-yielding framework that employs a gradient-enhanced, crystal plasticity material model reveals key features of the strain/stress fields in a simulation of pancake-shaped grains with alternating orientation near a primary crack front. The alternating grain configurations exhibit a soft/stiff behavior and alternating out-of-plane, L–T shear stress—effects observed in recently-published experiments completed by the authors and others on various Al–Li alloys. Both texture effects contribute to highly localized driving forces for delamination cracking while concurrently shielding the primary crack. Moreover, texture does not act to shield the arrestor delamination planes, thereby favoring arrestor delamination development over primary crack growth. A compressive T-stress further enhances shielding of the primary crack—a result which aids in understanding marked differences in observed fracture behavior of tested M(T) and C(T) specimens of Al–Li alloys.     

Transferability of specimen J–R curve to straight pipe with circumferential surface flaw

Specimen J–R curve is extensively used for structural integrity of large components. It is well known that J–R curve heavily depends on constraint level ahead of crack tip in remaining ligament. In earlier work, it was demonstrated that J–R curve from Three Point Bending (TPB) specimen is transferable to straight pipe with circumferential through wall crack. In this paper, the transferability of J–R curve is investigated from TPB specimen to pipe with circumferential surface crack. A 16 in. diameter pipe with circumferential surface crack and TPB specimen machined from same piping material (SA333Gr6 Steel) are tested. Consequently, 3D finite element analysis (FEA) has been performed on surface cracked pipe and TPB specimen. Crack‐initiation load is also predicted for surface cracked pipe by FEA and compared with experimental result. J–R curve is calculated for the pipe using experimental data, that is, load, load line displacement and crack growth. J–R curve of pipe is compared with TPB specimen and it is found that the pipe is predicting much higher J–R curve than TPB. This difference of J–R curve is investigated by evaluating stress triaxiality in remaining ligament for both cases. Stress triaxiality is quantified using triaxiality factor (h) ahead of crack tip for pipe and TPB specimen. It is found that the TPB specimen has considerably higher constraint level than pipe with surface crack, which is well supported by trend of J–R curves for specimen and pipe. A study has also been carried out to investigate the effect of internal pressure on the stress triaxiality. It is found that there is negligible difference in stress triaxiality because of internal pressure. The stress triaxiality is re‐established as a qualitative parameter to assess the transferability of J–R curve from specimen to component.     

Tri-dimensional numerical modelling of plasticity induced fatigue crack closure

Fatigue crack closure has been studied numerically with 2D finite element models as alternative to experimental methods. In a few cases fatigue crack closure behaviour has been analyzed with tri-dimensional models. In this work, a compact C(T) aluminium specimen has been modelled tri-dimensionally. The minimum element size is smaller than common accepted recommendation. This parameter limits the mesh size along the thickness. The number of divisions of the thickness is a special issue studied in this work. Different thicknesses, range of loads and stress relations have been calculated. The results show the crack behaviour through the thickness. The plastic zone is visualized and quantified. An abrupt transition is observed in a thin external slice of the specimen; only captured with a fine meshing of the thickness. Crack closure/opening along the thickness have been computed. Results show that closure is only relevant in this small external area and its size is not affected by the thickness. Results match experimental observation and have been correlated with experimental tests.     

The criterion for the unstable failure of cracked stainless steel piping subject to a combination of applied loadings

For extreme accident conditions, the applied loadings on a cracked piping system are complex and can be a combination of loads and displacements applied to various parts of the system. In investigating the problem of crack instability for such conditions, this paper analyses the model where a straight pipe, containing a circumferential through-wall crack at its mid-length position, is subject to bending deformation as a result of rotations applied at its built-in ends through rotational springs and a transverse load applied at an intermediate position along the pipe, again through an appropriate spring system. It is thereby possible to examine the effect of a wide range of loading combinations on the crack instability criterion, as derived using the tearing modulus methodology. One important conclusion is the underscoring of the view that the loading characteristics at the pipe-ends have a very important effect on crack instability.     

用于测量陶瓷断裂韧性的Vickers和Knoop压痕裂纹技术

采用 Vickers 压痕裂纹测量法或 Knoop 压痕～强度法等压痕技术进行了陶瓷的断裂韧性测量。反应烧结 Si_3N_4,热压 Si_3N_4和 SiC 等结构陶瓷用于研究显微结构非均质性和热残余应力对断裂韧性值的影响。人工水晶用于研究晶体学取向和断裂韧性之间的关系。研究发现,Knoop 压痕～强度法比 Vickers 压痕裂纹测量法更适合于非等轴晶系的晶体断裂韧性测量。其原因是 Knoop 压痕的主裂纹和缺口试样的裂纹都沿着同样的晶面扩展,而 Vickers 径向裂纹对却须沿着相互正交的晶面扩展,仅其中的一个晶面与缺口试样的裂纹扩展面一致。在 Vickers 压痕裂纹技术的基础上,用以计算 K_(1c)值不含弹性模量 E 项的 Evans 式(21),似乎能显示出显微结构对 K_(1c)值的影响;含有弹性模量 E 项的 Evans 式(22),似乎能反映出残余热应力对 K_(?)值的影响。所有上述的陶瓷材料,除了反应烧结 Si_3N_4,其压痕和裂纹关系 a~2～C~(3/2)的线性回归相关系数都相当,甚至>0.99。这表明了 Vickers 压痕技术在测定陶瓷的断裂韧性方面具有一定的实用性。     

Evaluation of an unusual superheated steam pipe failure

An examination has been made of a steam generator outlet pipe that failed locally after some 100,000 h of operation. Failure took place by cracking along preferentially formed lines (or planes) of graphite nodules. Grain boundary decohesion and void formation ahead of the crack tips were very local in nature, voids being formed very close to the propagating cracks only. The graphitization is concluded to have occurred locally because of inhomogeneity in carbon content.     

Fractographic study of high-density polyethylene pipe

High-density polyethylene (HDPE) pipe is now being used as an alternative to medium-density polyethylene (MDPE) for gas, water, sewage and waste-water distribution systems. Laboratory tests appear to show that HDPE is more able to suppress rapid crack propagation (RCP), whilst remaining sufficient resistance under the operational circumstances that lead to the type of slow crack growth observed in service failures. There have been many fractographic studies on MDPE pipe materials, actual pipe and fittings, but little on HDPE. A fractographic study of the type of HDPE pipe in current production has been undertaken. For these tests, whole pipe sections were subjected to either static or dynamic internal (water) pressurization fatigue loading. Failure mechanisms are discussed based on the fracture morphologies resulting from these tests. A further argument for good resistance of HDPE pipe to RCP is suggested. © 1998 Chapman & Hall     

Fatigue behavior of surface cracked filament wound pipes with high tangential strength in corrosive environment

The aim of this study is to examine the corrosion fatigue behavior of filament wound composite pipes with a surface crack under alternating internal pressure. The filament wound pipes are composed of multi-layered E-glass/epoxy composites with a [±75°]₃ lay-up. The surface notches were formed on the outer surface of the pipe along the pipe axis. Dilute (0.6 M) HCl acid was applied to the surface crack region by a corrosion cell mounted on the outer surface of the pipe. The results of an experimental investigation into the corrosion fatigue tests are conducted to observe the oil leakage failure and the crack propagation of the composite pipe subjected internal pressure loading with an open ended condition in which the pipe can be deformed freely in the axial direction. The internal pressure was generated by conventional hydraulic oil for fatigue loading. The fatigue tests are performed at 0.42 Hz frequency and a stress ratio of R = 0.05 in accordance with ASTM D-2992 standard. The oil leakage from the crack tip was observed after the crack propagation reached to the critical stress intensity level. The fatigue crack propagation behavior with the environment exposure was strongly dependent on the crack parameters such as crack-depth ratio and crack-aspect ratio. The micro structure of the fracture surface with the effect of environment and the fatigue loading were also observed.     

Accelerated fatigue fracture of polyethylene pipe material: crack layer analysis

A fatigue accelerated pipe test method has been developed which is able to quickly differentiate pipes according to their crack propagation resistance. The differences in brittle crack propagation resistance between two pipe materials was quantified and related to the specific enthalpy of damage and the dissipation coefficient via the Crack Layer Theory. These values show that the ethylene/hexene copolymer was more resistant to brittle cracking than was the ethylene/butene copolymer, because in the latter a larger percentage of the irreversible work per cycle was expended on the fracture process. The ethylene/butene copolymer also had a smaller intrinsic toughness than did the ethylene/hexene copolymer, as manifested by its lower specific enthalpy of damage. These results agree with the kinetics of crack propagation, as well as the known field performance of these two materials.     

Crack front curvature: Influence and effects on the crack tip fields in bi-dimensional specimens

Crack front curvature is evidence in most experimental crack advance test. When classical linear elastic fracture mechanic theory deals with bi-dimensional crack configurations, it ignores the three-dimensional effects of crack propagation. Issues as the influence of the specimen thickness and the crack front curvature are not considered. Previous numerical studies have shed light on out-of-plane plastic zone development or stress state. Nevertheless, these numerical studies are based on the assumption that the crack front is ideally straight; despite it is well known that the crack front has some kind of curvature. In the present work, a CT aluminium specimen has been modelled three-dimensionally and several calculations have been made considering a huge combination of different single load levels, specimen thicknesses and crack front curvatures. Due to the abrupt transition from plane strain to plane stress, an ultrafine mesh along the thickness has been applied. The analysis of the evolution of the plastic zone and the stress state along the thickness provides information about the combined influence of these parameters on fracture mechanics.     

Effect of stitch density and stitch thread thickness on low-velocity impact damage of stitched composites

Effect of stitch density and stitch thread thickness on low-velocity impact damage of stitched composites is investigated experimentally. Physical examination on damage surfaces shows that stitches perform as crack initiators, as well as crack arrestors. Longer matrix cracks are observed in densely-stitched composites, in contrast to isolated matrix cracks found in moderately-stitched composites. Ultrasonic C-scan evidently compares the delamination areas and concludes that specimens with higher stitch density and thread thickness are more capable of impeding delamination growth by effectively bridging delamination cracks. Load–time curves reveal that the onset of delamination is not influenced by stitch density and thread thickness. Energy consumption for the impact event is evaluated and discussed with the conclusion that, although absorbed energy is independent of stitch density and thread thickness, the proportion of energy consumption for damage mechanisms like delamination, matrix cracks and stitch debonding are different for laminated composites stitched with different stitch parameters.     

A methodology for fatigue crack growth and residual strength prediction with applications to aircraft fuselages

The FRANC3D/STAGS software system has been developed to model curvilinear crack growth in aircraft fuselages. Simulations of fatigue crack growth have been reported previously (Potyondy et al. 1995). This paper presents two enhancements to this system. One is the generalization of the representation of cracks that allows the system to represent realistic damaged structures more accurately. With this capability, parameters that may affect the trajectory of a fatigue crack are studied. Results are compared with measurements from a full-scale test. The second enhancement is to model stable tearing for residual strength prediction. A stable tearing simulation along a crack path that captures the material nonlinearities inherent at the crack tip is performed. The CTOA (Crack Tip Opening Angle) is used as a crack growth criterion to characterize the fracture process under conditions of general yielding. Residual strength of cracked structures is predicted.     

Stress Corrosion Cracking and Oil Leakage in a Control Oil Piping System

This article describes the results of an investigation concerning with the failure of a pipe which was carrying oil from the control oil unit to the steam turbine control valves servomotor. The failure was in the form of a crack, propagating horizontally along the pipe. The crack initiated on the outside of the pipe. The cause of the failure was investigated by conducting visual examination, detailed macro and microstructural examinations and determining the composition of material from the failed pipe. The composition of the pipe material was analyzed by atomic absorption spectrometer. The failure of the oil pipe was attributed to stress corrosion cracking. The pipe material is A312 TP 304L. Recommendation to minimize such failures includes coating the pipe to prevent contact with chloride from the surrounding marine environment.