Micromechanical Coupled Study of Crack Growth Initiation Criterion in Pressure Vessel Steel

We present results of the combined design-theoretical investigation of the mechanism of crack growth at the onset of ductile fracture of NPP reactor pressure vessels. Micromechanical approach to the prediction of ductile fracture has been applied, according to which the volume fraction of voids in the deformed material is determined by the finite-element method. On the basis of CT-specimen tests and known damage parameters, determined for smooth spherical specimens, we propose a micromechanical criterion of crack growth initiation for ductile fracture.     

Micromechanical Analysis of Interphase Damage for Fiber Reinforced Composite Laminates

In the present study, the initiation and evolution of the interphase damage and their influences on the global stress-strain relation of composite laminates are predicted by finite element analysis on a micromechanical unit cell model. A thin layer of interphase elements is introduced and its stress-strain relation is derived based on a cohesive law which describes both normal and tangential separations at the interface between the fiber and matrix. In addition, a viscous term is added to the cohesive law to overcome the convergence difficulty induced by the so-called snap-back instability in the numerical analysis. The matrix behavior is described by a recently developed nonlinear viscoelastic constitutive model. As application examples, glass fiber/epoxy unidirectional laminates under off-axis loadings are analyzed. One-quarter of the unit cell is used in the analysis accounting for the geometrical symmetry of the model, and the corresponding periodic boundary conditions for combined global shear and normal loading are derived. Results show that the initiation and evolution of the interphase damage can be well simulated and the predicted global stress-strain responses are in good agreement with the experimental results.     

近距离爆炸荷载作用下H型钢柱毁伤数值分析

鉴于现代武器弹药的高命中精度,H型钢柱作为多数钢结构厂房的重要承重构件,研究其在近距离爆炸荷载作用下的毁伤具有重要意义.根据相似法则,对某典型钢结构厂房H型钢柱作缩尺变换,利用LS-DYNA有限元软件,采用流固耦合方法,对不同比例距离下9组装药近距离爆炸冲击H型钢柱相似模型作数值模拟.通过对比H性钢柱遭受到爆炸荷载作用...     

Analysis of Fatigue Damage under Block Loading in a Low Carbon Steel

Abstract: The fatigue damage accumulation behaviour of the P355NL1 steel is characterised using block loading fatigue tests. First, the constant amplitude low‐cycle fatigue behaviour of the P355NL1 steel is evaluated through strain‐controlled fatigue tests of smooth specimens. Both fatigue and cyclic elastoplastic behaviours are analysed. Then, block loading is applied to identify the key features of the fatigue damage accumulation phenomena for the P355NL1 steel. The block loading is composed of two distinct low‐cycle constant amplitude strain‐controlled blocks. The first block is applied for a predefined number of loading cycles, being followed by a second block which is applied until failure. The block loading illustrates that fatigue damage evolves nonlinearly with the number of load cycles as a function of the strain amplitude. These observations suggest a nonlinear damage accumulation rule with load sequence effects. The linear Palmgren–Miner's rule used extensively in design is not verified for the P355NL1 steel. Finally, using the generated experimental data, the cyclic elastoplastic behaviour of the P355NL1 steel is modelled using a continuum plasticity model with nonlinear kinematic hardening, available in the commercial finite element code ansys ®.     

Nb-Ti微合金高强钢动态再结晶动力学及临界条件

在Gleeble-3500 热模拟试验机上对Nb-Ti微合金高强钢进行了热模拟压缩试验,研究了其在变形温度为900~1 100 ℃、应变速率为0. 01~5 s-1 、最大变形量为70%条件下的动态再结晶行为。对流变曲线的分析及微观组织观察结果表明,低温高应变速率下流变曲线未显现出典型动态再结晶特征,但此条件下已发生动态再结晶。使用双曲正弦形Arrhenius关系计算的Nb-Ti微合金钢变形激活能为404 kJ/mol。利用加工硬化原理和Cingara-McQueen模型确定了动态再结晶初始临界应力和应变,分析了由Cingara-McQueen模型计算临界应力值偏高的原因,建立了临界应力、应变和Z参数之间的定量关系,得到了动态再结晶临界应力和应变方程:σ_c=0.335Z0.144,εc=0.005 9Z0.079。通过对θ-ε曲线进行分析,建立了最大软化速率处应变(εm)和变形条件的关系。在此基础上使用Avrami型动态再结晶动力学模型计算了不同变形条件下的再结晶体积分数,结果表明此模型可准确预测Nb-Ti微合金高强钢动力学。     

高应力条件下爆破振动对岩石破坏研究

文章介绍了爆破振动的检测方法及主要过程,利用爆破振动时频分析得出某质点振动参数,利用动态应力理论将爆破振动对岩石作用转化为动态应力.以大理岩为例分析高应力条件振动波动态应力对岩石的反复压缩卸载过程,并指出在高应力条件下,爆破振动对岩石的破坏作用更加明显.     

汽车炸弹钢箱梁内部爆炸局部破坏效应分析

采用ALE多物质流固耦合算法,对汽车炸弹 (TNT当量200kg~1500kg) 在双层桥梁下层桥面典型位置爆炸的局部破坏效应进行了数值模拟,研究了内爆炸冲击作用下钢箱梁的响应过程、破坏模式、破坏参数及其主要影响因素。结果表明：破坏模式及破坏参数与爆炸位置和TNT当量密切相关,爆炸位置对桥梁主要受力体系的受损程度影响较为明显,加劲肋对其垂直方向的破口具有约束作用,箱体对冲击波的约束效应使破坏作用加剧。合理设置加劲肋、加强重要构件和设置防爆层等措施有利于提高桥梁结构抗爆能力。     

Micromechanical Modeling of Impact Damage Mechanisms in Unidirectional Composite Laminates

Composite laminates are susceptible to the transverse impact loads resulting in significant damage such as matrix cracking, fiber breakage and delamination. In this paper, a micromechanical model is developed to predict the impact damage of composite laminates based on microstructure and various failure models of laminates. The fiber and matrix are represented by the isotropic and elastic-plastic solid, and their impact failure behaviors are modeled based on shear damage model. The delaminaton failure is modeling by the interface element controlled by cohesive damage model. Impact damage mechanisms of laminate are analyzed by using the micromechanical model proposed. In addition, the effects of impact energy and laminated type on impact damage behavior of laminates are investigated. Due to the damage of the surrounding matrix near the impact point caused by the fiber deformation, the surface damage area of laminate is larger than the area of ??impact projectile. The shape of the damage area is roughly rectangle or elliptical with the major axis extending parallel to the fiber direction in the surface layer of laminate. The alternating laminated type with two fiber directions is more propitious to improve the impact resistance of laminates.     

起爆方式对变壁厚药型罩形成毁伤元影响的数值仿真

为获得起爆方式对变壁厚药型罩形成毁伤元的影响规律,采用有限元分析软件AUTODYN-2D对变壁厚药型罩装药结构在中心点起爆、环起爆、点环起爆和面起爆4种起爆方式下形成毁伤元的过程进行数值仿真研究。结果表明:采用不同的起爆方式,装药会产生不同的爆轰波形,变壁厚药型罩会分别形成向前压拢型EFP、向后翻转型EFP、准球形EFP和倒锥形EFP。4种不同形态的毁伤元最终速度相差不大,但达到飞行稳定的时间不同,采用中心点起爆形成的向前压拢型EFP所需时间最短。     

Micromechanical Modelling of Time-Dependent Stress-Corrosion Behaviour of High-Strength Steel

This paper provides micromechanical bases to explain the time-dependent stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue precrack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different time-dependent behaviour is observed in both steels and even in the same steel in distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly oriented in the case of the hot rolled bar and markedly oriented in the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.     

Micromechanical Characteristics of the Surface Layer of 45 Steel After Electric-Spark Treatment

Materials Science - We analyze the principal micromechanical characteristics of the surface layers of 45 steel with electricspark coatings, namely, their microhardness, microplasticity (creep),...     

大气压等离子体射流改性船体钢表面亲水性研究

在室温下,采用大气压等离子体射流对船体钢进行表面改性,通过水接触角测量、扫描电镜、X射线光电子能谱等分析测试方法研究了等离子体射流处理前后船体钢表面润湿性、表面形貌及化学特性的变化。研究结果表明,船体钢经大气压空气等离子体射流处理后在其表面引入了大量含氧基团,处理2s表面的水接触角就可以降到30°以下;处理后材料表面的亲水性受处理时间及放电电流的影响且在空气中放置时会出现老化效应,处理时间越长,老化效应越弱。     

基于声发射信号参数分析的耐火材料损伤聚类研究

耐火材料声发射信号可分为单一突发型和叠加突发型两种,通过分析其波形特征,将上升时间、持续时间、幅度、能量、计数和质心频率作为区分这两种信号的特征参数,利用K均值聚类方法,实现耐火材料不同损伤类型的声发射信号分类。利用该方法对MgO-C耐火材料受压损伤信号进行分析,研究结果表明:单一突发型信号主要频率成分单一集中,信号主要呈现50～60kHz和150～160kHz两种频率段,叠加突发型信号主要频率成分为上两种频率成分的叠加;将信号分为3类,第1类和第2类信号反映了具有计数少、持续时间短和能量小特点的单一突发型信号,第2类信号的损伤强度大于第1类,第3类信号反映了具有计数多、持续时间长和能量大特点的叠加突发型信号。     

工况载荷下传递路径分析方法

阐述传递路径分析(TPA)基本原理,通过对比几种主要载荷识别方法优劣,提出综合利用试验和仿真手段进行载荷识别方法。运用该方法进行车内噪声分析,并通过对比试验结果证明方法可行性。在此基础上进行工况载荷下整车TPA分析,根据分析结果对车辆进行优化,取得显著效果。     

Optimization of the Parameters of a Porous Coating under Conditions of Pool Boiling of Propane

Results of an investigation of the influence of the parameters of a sintered porous copper coating on heat transfer under conditions of propane boiling are reported. The experiments were carried out within the ranges of specific heat fluxes of q = 10²–6.4·10⁴ W/m² and of saturation vapor pressures of p _s = 0.48–1.08 MPa (T _s = 0–30°C). Optimum parameters of the porous coating were determined within the investigated saturation pressure range. A dimensionless equation is suggested for determination of the heattransfer rate in propane boiling on sintered porous copper coatings.     

Mathematical Modeling of Damage Accumulation under the Conditions of Creep and Corrosion Cracking of Structural Materials

Materials Science - We develop a mathematical model of damage accumulation in structural materials under the conditions of creep and corrosion cracking. The model is tested to determine the service...     

含脱粘损伤的复合材料加筋板压缩破坏渐进损伤数值分析

本文提出了基于连续损伤力学的复合材料层合加筋壁板渐进损伤分析模型。该模型采用界面单元模拟筋条和壁板之间的连接界面,连接界面和复合材料层板分别采用Quads准则和Hashin准则作为失效判据,基于内嵌连续损伤状态变量的材料刚度退化方案,采用非线性有限元方法,研究了复合材料加筋壁板在压缩载荷下的破坏过程,分析了结构相应失效模式的细观失效机理。数值分析结果与实验数据吻合较好,证明该方法的合理性和有效性,并详细探讨了界面单元关键参数和层板铺设角度对加筋壁板结构力学响应的影响规律,得到了一些富有价值的结论。     

Mechanism of Work-hardening for Austenitic Manganese Steel under Non-severe Impact Loading Conditions

The effect of C,Mn and heat-treatment onwork-hardening of austenitic Mn steel and thework-hardening mechanism have been investigatedunder non-severe impact loading condition.Theresults show that the ability of work-hardening in-creases with the increase of C and aging tempera-ture but decreases with Mn.The work-hardeningwith high austenitic stability results mainly fromdislocations,and that with low austenitic stabilityresults mainly from combined effects of strain-in-duced martensite and high density of dislocationsunder non-severe impact loading conditions.Thewear resistance of medium manganese steel (Mn7)is 1.64-2.46 times that of Hadfield steel (Mnl3).     

The Formation of Carbide-Free Bainite in High-Carbon High-Silicon Steel under Isothermal Conditions

It is shown that a carbide-free bainite structure can be formed in high-carbon steel of the Fe–Si–Mn–Cr–V system using a traditional furnace facility. The structural aspects of bainitic transformation developing under isothermal conditions at 300°C have been studied by the methods of X-ray diffraction and transmission electron microscopy. Orientation relationships between crystalline lattices of γ and α phases have been established. A superequilibrium carbon concentration in the bainite α phase has been determined.