失效模式约束下层级褶皱结构的多目标优化

基于欧拉梁和弹性薄板理论,该文分析了二级层级褶皱结构的板模型在剪切或压缩载荷下对应的六种失效模式及对应的名义应力。通过比较名义应力值,控制发生的失效模式类型,并结合多目标优化算法,同时进行最小重量和最小挠度的性能优化。该文建立了两种优化模型：一是控制首次发生的失效模式类型;二是根据损失大小将失效模式分级,并按照等级序列发生。在此基础上,讨论了两种优化模型的Pareto前沿图中典型设计点的性能指标,最后采用有限元分析验证失效模式类型。结果表明：两种优化模型得到的结构设计,分别与控制发生的类型一致,综合性能得到极大提升;第二种模型考虑了应力值的差异,防止结构突然发生整体失效,更加安全。     

An improved formal failure analysis approach for safety-critical system based on MBSA

It is important to analyze the failure in safety-critical system because a disaster may occur once any type of failure mode and/or failure effect is neglected or misjudged. In order to conduct the failure analysis more effectively and efficiently, the concept of formal modeling is introduced. This paper improved the model-based safety analysis (MBSA) working process to optimize the formal failure analysis approach of safety-critical system.As the core works of MBSA process, the formal modeling and model extension aim to build an integrated system model which can be used for analyzing the failure behaviors in the system by model checking. However, in order to automatically check if there are any potential failures in the structured system model and whether the model satisfies the specified system properties and requirements using model checker, model transformation is normally needed, which can introduced potential errors during the transformation. Moreover, different model checkers generally require the system models to be expressed in a particular input language, which increases the difficulty of modeling as well. In order to avoid these problems and improve the efficiency of failure analysis work, this paper focused on how to build an unified model of safety-critical system quickly and accurately using symbolic language SMV, and conduct automatic verification using the corresponding open-source model checker NuSMV soon afterwards. After the model checking, the formal verification results such as counter-examples generated by model checking need to be transformed into traditional failure analysis artifacts, such as FMEA and/or FTA, to guide the iterative improvement of system development conveniently. Therefore, to solve the transformation from formal verification conclusions to traditional failure analysis results is another key point of this paper. Finally, a case study about airborne equipment is provided to validate the proposed method.     

Identifying Counterexamples Without Variability in Software Product Line Model Checking

Product detection based on state abstraction technologies in the software product line (SPL) is more complex when compared to a single system. This variability constitutes a new complexity, and the counterexample may be valid for some products but spurious for others. In this paper, we found that spurious products are primarily due to the failure states, which correspond to the spurious counterexamples. The violated products correspond to the real counterexamples. Hence, identifying counterexamples is a critical problem in detecting violated products. In our approach, we obtain the violated products through the genuine counterexamples, which have no failure state, to avoid the tedious computation of identifying spurious products dealt with by the existing algorithm. This can be executed in parallel to improve the efficiency further. Experimental results show that our approach performs well, varying with the growth of the system scale. By analyzing counterexamples in the abstract model, we observed that spurious products occur in the failure state. The approach helps in identifying whether a counterexample is spurious or genuine. The approach also helps to check whether a failure state exists in the counterexample. The performance evaluation shows that the proposed approach helps significantly in improving the efficiency of abstraction-based SPL model checking.     

Prediction model for the load-carrying capacity of nailed timber joints subjected to plug shear

Timber joints can experience ductile failures as modeled by Johansen in the European Yield Model adapted in Eurocode 5, or they can fail in a brittle manner. In nailed or screwed joints where the fastener does not protrude through the timber, plug shear failure can occur where an entire plug defined by the perimeter of the joint is torn away from the timber. The brittle plug shear failure, which can occur in joints loaded in tension parallel to the grain, results in a lower joint resistance than the ductile failure modes. The aim of this study is to evaluate existing prediction formulas for plug shear failure in timber connections, compare them to test results and observations, and propose a new prediction formula. Test results from four different experimental studies are presented. Using hypothesis testing, a prediction formula for plug shear failure is proposed based on 92 experiments. The resistance of the tensile failure mode of plug shear failure is best modelled by the tensile resistance of the end face of the plug. The resistance of the shear failure mode of plug shear failure is best modelled by the shear resistance of the bottom area of the plug taking into account the volume effect on shear strength. The model currently in Annex A of Eurocode 5 is overestimating the plug shear resistance compared to the test results analysed in this research. To avoid plug shear failure, short and wide joints are preferred, minimising the number of fasteners in line with the load and grain direction.     

Damage driven crack initiation and propagation in ductile metals using XFEM

Originally Continuum Damage Mechanics and Fracture Mechanics evolved separately. However, when it comes to ductile fracture, an unified approach is quite beneficial for an accurate modelling of this phenomenon. Ductile materials may undergo moderate to large plastic deformations and internal degradation phenomena which are well described by continuum theories. Nevertheless in the final stages of failure, a discontinuous methodology is essential to represent surface decohesion and macro-crack propagation. In this work, XFEM is combined with the Lemaitre ductile damage model in a way that crack initiation and propagation are governed by the evolution of damage. The model was built under a finite strain assumption and a non-local integral formulation is applied to avoid pathological mesh dependence. The efficiency of the proposed methodology is evaluated through various numerical examples.     

An improved crack tracking algorithm with self-correction ability of the crack path and its application in a continuum damage model

In this paper, a new procedure for predicting the crack propagation and fracture behavior in quasi-brittle materials is presented based on continuum damage mechanics. Several crack tracking algorithms are widely used for failure analysis, among which the local tracking algorithm is very simple and easy to implement with low computational cost. However, in the prediction of crack growth with the traditional local tracking algorithm, it can be often seen that the sharp changes in the crack path such as U-turns can occur. An improved local tracking algorithm with self-correction ability of the crack path is proposed to overcome these difficulties of the traditional crack tracking algorithms. A continuum damage model, in addition to the present crack tracking algorithm, can greatly enhance the performance of failure prediction in quasi-brittle materials. The present approach has the advantages that it can well predict the crack propagation path and can avoid mesh size and mesh bias dependence without the embedment of discontinuities or nodal enrichment. The present model is incorporated into ANSYS by the ANSYS Parameter Design Language to simulate the initiation and propagation of the discrete crack in engineering applications. The numerical results by this model are compared with the ones by the extended finite element method and experiments, and good agreements are achieved.     

含分层损伤复合材料加筋层合板的动承载能力

采用有限元方法研究了含穿透分层损伤复合材料加筋层合板的动力响应和承载能力。根据复合材料层合板一阶剪切理论, 推导了复合材料层合板单元的刚度阵和质量阵列式;同时采用Adams 应变能法与Rayleigh阻尼模型相结合的方法, 构造了相应的阻尼阵列式;为了防止在低阶模态中分层处出现的上、下子板不合理的嵌入现象, 建立了含分层损伤复合材料加筋层合板动力分析中分层分析模型和虚拟界面联接模型。并采用Tsai提出的刚度退化准则和动力响应分析的精细积分法, 对在动荷载作用下含分层损伤复合材料加筋层合板结构进行了破坏和承载能力分析。通过典型算例分析, 分别讨论了外载频率、分层深度、筋的位置以及破坏过程中刚度退化对含损伤复合材料加筋层合板动力响应特征和承载能力的影响, 得到了一些具有理论和工程价值的结论。     

Fatigue and fracture analysis of a seven‐wire stainless steel strand under axial and bending loads

Fatigue failure of cables and strands is a common and complex problem. Failure is typically caused by different combinations of time‐variable bending and axial forces. In addition to these loads, contact stresses between wires may play an important role in the fatigue failure of cables. The present work aims to provide deep insight into the fatigue failure of a seven‐wire stainless steel strand subjected to a combination of variable axial and bending loads. To avoid side effects in the analysis, fatigue failure of the strand close to the clamps is prevented. Several tests were performed with a new device specifically designed to avoid failure near the clamps. Thus, failure is always produced at the middle length of the specimen. Test simulations were performed by employing the finite element method. The numerical results were validated via comparisons with experimental data. Finally, life prediction curves were obtained.     

Glulam beams reinforced with FRP externally-bonded: theoretical and experimental evaluation

The glued- laminated lumber (glulam) technique is an efficient process for the rational use of wood. Fiber-reinforced polymer (FRPs) associated with glulam beams provide significant improvements in strength and stiffness and alter the failure mode of these structural elements. In this context, this paper presents guidance for glulam beam production, an experimental analysis of glulam beams made of Pinus caribea var. hondurensis species without and with externally-bonded FRP and theoretical models to evaluate reinforced glulam beams (bending strength and stiffness). Concerning the bending strength of the beams, this paper aims only to analyze the limit state of ultimate strength in compression and tension. A specific disposal was used in order to avoid lateral buckling, once the tested beams have a higher ratio height-to-width. The results indicate the need of production control so as to guarantee a higher efficiency of the glulam beams. The FRP introduced in the tensile section of glulam beams resulted in improvements on their bending strength and stiffness due to the reinforcement thickness increase. During the beams testing, two failure stages were observed. The first was a tensile failure on the sheet positioned under the reinforcement layer, while the second occurred as a result of a preliminary compression yielding on the upper side of the lumber, followed by both a shear failure on the fiber-lumber interface and a tensile failure in wood. The model shows a good correlation between the experimental and estimated results.     

Structural design of a composite bicycle fork

Despite the wide literature on the mechanical behaviour of carbon/epoxy composites, it is rare to find practical methodological approaches in finite element design of structural components made by laminate layup. Through the case study of a special bicycle fork needed in a Student Team prototype, this paper proposes a simplified methodology as starting point for educational and manufacturing purposes. In order to compare two manufacturing solutions in terms of stiffness, strength and failure mode, a numerical model was implemented. Since the project requirements imposed to avoid standard destructive testing, the model validation was based on a posteriori linear stiffness comparison with the manufactured component. The slight discrepancies between experimental and numerical results were discussed in order to check their origin and to assess the reliability of the model. The overall methodology, even if complain with only a part of the safety standard requirements, shows to be reliable enough and can be the basis for further extension and refinement.     

Predicting mode-II delamination suppression in z-pinned laminates

A finite element model for predicting delamination resistance of z-pin reinforced laminates under the mode-II load condition is presented. End notched flexure specimen is simulated using a cohesive zone model. The main difference of this approach to previously published cohesive zone models is that the individual bridging force exerted by z-pin is governed by a specific traction-separation law derived from a unit-cell model of single pin failure process, which is independent of the fracture toughness of the unreinforced laminate. Therefore, two separate traction-separation laws are employed; one represents unreinforced laminate properties and the other for the enhanced delamination toughness owing to the pin bridging action. This approach can account for the so-called large scale bridging effect and avoid using concentrated pin forces in numerical models, thus removing the mesh-size dependency and permitting more accurate and reliable computational solutions.     

矩形截面框架柱斜向受剪承载力计算模型

为避免矩形截面框架柱在斜向水平地震作用下发生脆性破坏,通过试验研究14根框架柱在斜向水平荷载作用下的受力性能和破坏机理。以桁架-拱模型为基础建立计算矩形截面框架柱斜向受剪承载力的空间桁架-拱模型,该模型主要考虑了框架柱截面面积、混凝土和箍筋材料强度、配箍率、轴压力以及混凝土强度折减系数对其斜向受剪承载力的影响。最后,利用该文以及其他试验共计30根矩形截面框架柱的斜向受剪承载力试验结果对此空间桁架-拱模型的可行性和适用性进行了验证,试验结果与计算值的对比表明利用该模型可以安全计算矩形截面框架柱斜向受剪承载力。     

Failure analysis of drill pipe: A review

Although the drill pipe is a simple tool in the drilling industry, it is the most used and important one. The drillstring failure has two general forms: wash-out and twist-off. These failures begin with mechanical, thermal and corrosion fatigue cracks. On the basis of previously conducted researches, the failure of drillstring is not unknown. However, the occurrence of drillstring failure still is too much. The main reason for this can be due to complex loading, combined stresses and different types of vibrations and moreover, the corrosive and erosive behavior of drilling mud that makes the total condition of the well so difficult to analyze. In this paper, the attention is focused on the analysis of drill pipe failure and the fatigue crack is represented for the root cause of drill pipe failure. The aims of most of published papers are to examine how and why a specific drill pipe is failed and just the main reason of failure for that drill pipe is discussed in detail. In this paper all metallurgical and mechanical aspects of failure that can occur for a drill pipe, are considered and discussed. And finally, this comprehensive review leads to a conclusion that categorizes the primary sources of drill pipe failure into seven major groups and makes some recommendations to avoid them.     

Failure analysis of anchors in shear under simulated seismic loads

This paper presents a study of the failure analysis of anchors in shear in simulated seismic loads. Ductile failure is critical for anchor design in seismic applications to avoid brittle catastrophe while existing design codes and guidelines that usually accept steel failure as ductile failure are insufficient, and may overestimate shear capacities and ductility. In this study ductility was evaluated based on both effective confinement due to anchor reinforcement and ductile steel. Those anchors with the proposed reinforcement showed a significantly high strength and exhibited great ductility due to effective confinement. Three types of anchor steel were also evaluated. The influences of failure modes, shear capacities and ductility of anchors were taken in account by defining exposed length. Test results indicated both effective confinement and specified ductile steel could ensure anchor have good seismic performance.     

An equivalent model for mechanical analysis of single crystal cooled blade with film cooling holes using crystal plasticity

Modern cooled turbine blades made of single crystal superalloy contain lots of film cooling holes. It is difficult to simulate the mechanical behavior of the blade as so many holes. Developing precise equivalent model is an effective method to solve this problem. In this work, an equivalent model based on crystallographic theory has been presented to deal with film cooling holes. A unit cell model with film cooling hole and an equivalent model without cooling hole are established to determine the equivalent parameters first. And then the flat specimen made of single crystal with 14 holes is designed to verify the feasibility of the equivalent method. The tensile test and finite element simulation are carried out to study the load-displacement curve of specimen. The experimental results and fracture surface of specimens show that the equivalent method based on local failure is reasonable. The method is a useful way to analyze the mechanical behavior of cooled blade with film cooling holes and to avoid the fracture of the cooled blade.     

Calibration and validation of a large scale abrasive wear model by coupling DEM-FEM: Local failure prediction from abrasive wear of tipper bodies during unloading of granular material

Handling of granular materials like rocks, pebbles and sand can expose equipment to abrasive wear that can result in local failure. In some cases this can have far reaching economic significance such as the costs of replacement, the costs from machine downtime and lost production. Models for predicting wear can be found from lab scale tests, but are difficult to apply in large scale applications. An important property is the flow behaviour of granular material during its transportation in a granular material handling system. In order to effectively predict abrasive wear in large scale applications, models for solid structure, granular material flow and wear behaviour have to be coupled. In this work; the finite element method is used to model the structure of the tipper body and the discrete element method is used to model the granular material. To couple the structure response to granular flow behaviour a contact model is used. A calibration of the wear constant in Archard's wear law is obtained from measurement data of rotating drum tests, using the representative material combination used in a tipper unloading case. This wear model is then used in a full scale tipper body simulation to predict the absolute wear and validated against field measurement. A good agreement between numerical calculation and field measurement regarding the spatial position and size of wear areas were found. This combination of numerical methods gives new possibilities to understand the wear process and is one step towards more physically correct models for large scale predictions between tipper bodies and granular material. Numerical tools can give future opportunities to optimise material selections and geometry with the intension to increase functionality, life of large scale wear applications and avoid local failure.     

碳纤维束铆钉锚固下CFRP布加固梁的力学性能

粘贴碳纤维布(CFRP)加固梁时,通常需对其进行锚固,避免发生因CFRP布过早剥离而无法充分利用其抗拉强度。但当梁施工空间较小,梁侧面形状不规则时,CFRP布锚固较困难,采用新型碳纤维束铆钉锚固的方式能够很好地解决这一弊端。本文通过对四组不同锚固方式下的CFRP布加固梁进行四点受弯承载力试验,对比分析加载过程中各梁的力学性能和破坏规律,研究表明碳纤维束铆钉锚固下CFRP布加固梁强度和刚度都有显著增强。进一步对碳纤维束铆钉锚固方法优化,得到最佳碳纤维束铆钉锚固深度约为80mm,最佳锚固间距约为200mm。     

A study of the low-cycle fatigue failure of a galvanised steel lighting column

This paper presents the results of a low-cycle fatigue test on a galvanised steel lighting column. The aim of the test was to simulate the behaviour of the column undergoing large amplitude resonant vibration caused by wind. A metallurgical study of the failure revealed the significant role of the galvanised coating in the failure process. Results from a detailed 3D finite element model are also used to explain the failure mechanism.The swage joint in the column was confirmed as a failure location by both experiment and finite element analysis. This in itself is not surprising and the position of the fatigue failure is consistent with those observed in the field. Of more importance is the fact that the experiment shows that galvanising can lead to premature failure of such columns. This is a highly significant conclusion as it implies that improving the weld detail in an effort to improve fatigue life may be ineffective for lighting columns coated in this manner.Given the detrimental effect of galvanising on fatigue performance and the fact that the most severe corrosion will be on the outside of columns, then the fatigue life of such structures may benefit if the inner surface was not galvanised in high stress regions. An alternative improvement would be the use of a galvanising coating with higher toughness and less susceptibility to cracking and damage.Attention is drawn to the need for a better understanding of the fatigue performance of galvanised steel columns resulting from large amplitude wind induced resonant vibration. The approach adopted so far for lighting column resonant vibration, has been to try and avoid it. While this is a laudable objective, clearly this has not always been possible, as designs push the limits permitted by Codes of Practice.     

Behaviour of voids in a shear field

When voids are present in a ductile material subject to a shear dominated stress state under low stress triaxiality the voids collapse to micro-cracks, which subsequently rotate and elongate in the shear field. In the present plane strain analyses for cylindrical voids a surface load normal to a plane connecting the ends of the micro-crack is used as an approximate representation of contact stresses during frictionless sliding. In a previous study of the same problem the author applied hydrostatic pressure inside the nearly closed micro-crack to approximate contact conditions. The transverse surface loads used in the present analyses avoid the tendency to unrealistically elongate the voids. It is found that even though the model applied here gives significantly later occurrence of a maximum overall shear stress than that found by using hydrostatic pressure, the present model does predict a maximum in all the cases analyzed and thus illustrates the micro-mechanism leading to failure of the material by localization of plastic flow.     

A Robustness Approach to Reliability

Reliability of products is here regarded with respect to failure avoidance rather than probability of failure. To avoid failures, we emphasize variation and suggest some powerful tools for handling failures due to variation. Thus, instead of technical calculation of probabilities from data that usually are too weak for correct results, we emphasize the statistical thinking that puts the designers focus on the critical product functions. Making the design insensitive to unavoidable variation is called robust design and is handled by (i) identification and classification of variation, (ii) design of experiments to find robust solutions, and (iii) statistically based estimations of proper safety margins. Extensions of the classical failure mode and effect analysis (FMEA) are presented. The first extension consists of identifying failure modes caused by variation in the traditional bottom–up FMEA analysis. The second variation mode and effect analysis (VMEA) is a top–down analysis, taking the product characteristics as a starting point and analyzing how sensitive these characteristics are to variation. In cases when there is sufficient detailed information of potential failure causes, the VMEA can be applied in its most advanced mode, the probabilistic VMEA. Variation is then measured as statistical standard deviations, and sensitivities are measured as partial derivatives. This method gives the opportunity to dimension tolerances and safety margins to avoid failures caused by both unavoidable variation and lack of knowledge regarding failure processes. Copyright © 2012 John Wiley & Sons, Ltd.