Engineering assessment of cracked structures subjected to dynamic loads using fracture mechanics assessment

This paper presents practical guidance on the assessment of cracked structures subjected to dynamic loading. General reviews of fracture behaviour of structures subjected to dynamic loading are presented. A series of finite element (FE) analyses have been carried out to study the effects of dynamic loading on both fracture toughness specimens under rapid loads and cracked connections in steel framed structures under earthquake loads. FE results of submodel analyses of cracked connections are compared with the results from a simplified method. The simplified method can reduce the analysis time enormously and allows design engineers to assess the possibility of connection fractures, or determine approximate values of toughness and defect size requirements for given peak stress and strain level.     

Failure resistance of drilling rig casing pipes with an axial crack

Working conditions of casing pipes in drilling rigs can significantly influence the initiation and development of damage in the material, and therefore also the safe service of the entire system. In this work, an integrity assessment of a pipe with initial defect (machined surface crack) is presented. The position of this defect is on the external surface; unlike transport pipes, where internal surface is often endangered due to the contact with the fluid, casing pipes are also often exposed to damages at the external surface. A pipe segment exposed to internal pressure is examined experimentally and numerically, using the finite element method. Experimental setup included tracking of crack mouth opening displacement (CMOD) values, as well as J integral. Criteria for pipe failure are determined on the finite element (FE) models of the pipe; fracture initiation and plastic collapse are considered as failure mechanisms. Several 3D models with different crack sizes are evaluated. 2D plane strain models are also examined, to determine the applicability limits of this simplified approach. Integrity assessment criteria for the analysed geometries are discussed. Assessment of fracture resistance of the pipeline material is also considered in this work. Besides the standard SENB specimens, Ring specimens cut from the pipe are tested, and the results are compared. Both specimen geometries are modelled using local approach to fracture, by application of the micromechanical Complete Gurson model (CGM), developed by Z.L. Zhang. It is shown that the Ring specimens have similar fracture conditions under bending load as SENB specimens. Since they are much simpler to fabricate from the pipe than standard specimens, it is concluded that they can be used for assessment of fracture of the pipes with axial cracks.     

The stress field in a [0°/90°] laminated composite plate containing a finite line crack

This paper deals with the construction of an asymptotic solution for the stress field in a laminated composite plate, with [0/90] stacking sequence. The plate contains through the thickness a line crack of length 2c and its perimeter boundaries are sufficiently far away from the crack so that no edge effects are present. The stress field is derived explicitly, and includes a correction factor to account for the laminate effects in the third dimension. The stress _zz is a maximum at an angle of 3° (see Figure 3). The stress field may now be used to bridge the gap between macro and micro mechanics and to derive a series of fracture criteria, at the micro and macro level, which ultimately will provide us with a better understanding of the formation of the damaged zone ahead of the crack tip. For example, the stress field is used to derive one such approximate fracture criterion for mode I loading and for a self similar type of fracture, similar to that of Griffith. This criterion shows how the periodic length of the material lay-up microstructure effects the fracturing characteristics of the material system. Comparison with some experimental observations for two different material systems shows a fairly good agreement which substantiates the predicted influence.     

Influence of cracked concrete models on the nonlinear analysis of High Strength Steel Fibre Reinforced Concrete corbels

In the present study, new material constitutive relationships for High Strength Steel Fibre Reinforced Concrete (HSSFRC) have been formulated by means of a regression analysis of many experimental data (available in literature) by using SPSS (Statistical Package for the Social Sciences) program. These proposed material constitutive relationships have been employed for formulating the material finite element models to study the behaviour of HSSFRC corbels.     

On the structural integrity assessment of elastic-plastic redundant cracked structures

The methods currently used for the assessment of the structural integrity of cracked structures do not consider, for the evaluation of the stresses acting on the flawed sections, the singularity introduced by the presence of the cracks. The effect of this presence is more pronounced in structures constituted of ductile materials, that behave in the elastic-plastic regime, because they are able to sustain a significant crack propagation before fracture.The formulation provided by the so-called “simplified procedures for the evaluation of cracked structures” allows the designer to consider the existence of cracks in the integrity assessments. The work herein presented, which is the follow-up of a previous one that defines the methodology to be applied taking into account the crack presence [Gorrochategui I. Comportamiento en Rotura de Tuberías de Alta Tenacidad con Sustentación Hiperestática en Régimen Elastoplástico, Tesis Doctoral, Escuela Técnica Superior de Ingenieros de Caminos, Canales y Puertos, Universidad de Cantabria, Diciembre 1993, Gorrochategui I, Gutiérrez-Solana F. A method for the assessment of hypersyatic cracked structures in the elastic-plastic regime, Engng Fract Mech 1998;61:519-35, Gorrochategui I, Gutiérrez-Solana F, González JJ, Varona JM. An extension of the application of elastic-plastic assessment to cracked pipework systems. Nucl Engng Des 1998;182:141-7], illustrates the way to perform this type of evaluation through the resolution of a realistic example. After a review of the currently existing “simplified procedures”, the Engineering Treatment Model (ETM) [Schwalbe K-H, Zerbst U, Kim Y-J, Brocks W, Cornec A, Heerens J, et al. The ETM Method for Assessing the Significance of Crack-Like Defects in Engineering Structures, Comprising the Versions ETM 97/1 and ETM 97/2. GKSS- Forschungszentrum Geesthacht GmbH, 1998] has been selected for this application and its selection is justified.The results of these calculations show the benefits obtained in the estimation of the maximum load bearing capacity of cracked structures if the existence of the flaws is considered.     

用等离子体促进渣─金属液间的反应

分析了等离子体的冶金属性，简要报导了用等离子体强化钒在渣铁间的还原，及在等离子体条件下用V2O5代替钒铁对钢液进行还原合金化的研究结果，提出直接在钢包内进行还原合金化的可能性及前景．     

乙烯废碱液的苛化法再生工艺研究

采用苛化法对乙烯裂解废碱液进行处理，Na2CO3的苛化转化率在80％以上(平衡反应液的苛化率在90％以上)；废碱液的反复再生和利用对碱液的吸收性能无显著影响，再生碱液可满足吸收裂解气中CO2等酸性气的要求。     

单盘含裂纹转子系统的非线性响应分析与实验研究

建立含裂纹Jeffcott转子的动力学运动方程。针对裂纹深度对转子系统动态响应的影响及转子盘摆振响应的特点进行仿真计算和试验研究。数值仿真表明,对于较浅的裂纹,系统响应为周期性运动,在某些转速下会出现倍周期运动,同时还出现各种倍频分量。此时的摆振运动常包含激振频率的倍频成分。当裂纹较深时,盘的摆振运动与横向振动都会出现各种非协调响应。试验结果表明,当轴上存在裂纹时,盘的横向振动响应和摆振响应中都会出现高次谐波分量,这些分量虽然从频率成分上讲是相同的,但各分量的大小有明显差异。这些结论对于转子裂纹故障的监测与诊断具有新的意义。     

有挤压油膜阻尼器支承的多故障转子系统的非线性响应特性研究

研究了有挤压油膜阻尼器、转子与静子碰摩和转轴上的横向裂纹等多种非线性因素作用下的转子系统故障的非线性特性。发现这类转子系统具有丰富的非线性特性 ,三条通向混沌的道路都存在 ,即阵发性通向混沌 ,倍周期分叉通向混沌和拟周期通向混沌的道路。系统响应的最大振动幅度包含了大量的跳跃现象 ,这些跳跃所在的参数点与周期解分叉的参数点相重合。(以前已经发现了这种跳跃现象 ,但没有发现与周期解分叉有关系 )保持不平衡参数在一个小区间内能够使系统抑制多种故障 ,并使系统能够保持稳定的周期一解。较大的挤压油膜阻尼器参数对系统的振动有明显的抑制作用。在裂纹角和不平衡间的夹角符合β=π时 ,系统具有最小的振动幅度     

膜片联轴器对转子系统动态特性的影响研究分析

用有限元分析膜片联轴器联接转子系统的振动特性与发生裂纹、碰摩故障时的动态特性与响应。利用各单跨转子临界转速与转子系统临界转速的比较分析,研究膜片联轴器对转子系统弯曲振动、弯扭耦合振动的影响,以及转子发生裂纹、碰摩故障时膜片联轴器的影响,并提出膜片联轴器在机器故障诊断与维护中的有利作用。