A new damage model for composite laminates

Aircraft composite structures must have high stiffness and strength with low weight, which can guarantee the increase of the pay-load for airplanes without losing airworthiness. However, the mechanical behavior of composite laminates is very complex due the inherent anisotropy and heterogeneity. Many researchers have developed different failure progressive analyses and damage models in order to predict the complex failure mechanisms. This work presents a damage model and progressive failure analysis that requires simple experimental tests and that achieves good accuracy. Firstly, the paper explains damage initiation and propagation criteria and a procedure to identify the material parameters. In the second stage, the model was implemented as a UMAT (User Material Subroutine), which is linked to finite element software, ABAQUS™, in order to predict the composite structures behavior. Afterwards, some case studies, mainly off-axis coupons under tensile or compression loads, with different types of stacking sequence were analyzed using the proposed material model. Finally, the computational results were compared to the experimental results, verifying the capability of the damage model in order to predict the composite structure behavior.     

复合材料高周弯曲疲劳试验与寿命预测

利用一阶弯曲共振现象,开展了复合材料悬臂梁高周弯曲疲劳试验。为了取代传统的金属疲劳理论,根据复合材料疲劳损伤渐进扩展的特点,发展了新的数值方法应用于复合材料的疲劳分析。研究局部疲劳损伤模型和周期跳跃技术,开发了复合材料悬臂梁高周弯曲疲劳的半解析法Matlab疲劳损伤分析程序;另一方面,通过开发UMAT子程序,实现了疲劳损伤模型和周期跳跃技术在商业有限元软件ABAQUS中的应用。分别使用半解析法和有限元法分析复合材料悬臂梁高周弯曲疲劳的损伤累积破坏过程,预测了其高周弯曲疲劳寿命,数值预测结果与试验结果较好吻合。     

K-cor夹层结构的平压性能

采用欧拉杆屈曲模型,以端部约束系数 μ 定量表征夹层结构Z-pin端部结合程度, μ 取值越小,Z-pin端部和面板结合程度越高,压缩载荷下更不易失稳。以不同参数K/X-cor结构的压缩试验为背景,分别研究了Z-pin直径和折弯长度对压缩试验性能的影响。通过显微观察并利用"混合法则"比较了Z-pin直径对K/X-cor结构的增强效率。从减重方面考虑,探讨了折弯长度对K-cor结构比强度和比刚度的影响,并得到最优折弯长度半解析表达式。结果表明: K-cor结构中Z-pin增强效率高且对结构损伤小,远优于X-cor结构;其比强度和比刚度随折弯长度的增加呈先增加后降低的趋势,不同参数的K-cor结构折弯长度存在最佳值。     

Prediction of thin‐rimmed gear crack propagation from a factorial design approach

Gear failure involving bending fatigue can have catastrophic consequences depending on the propagation path direction. Therefore, anticipating and preventing eventual critical fracture are crucial at the design stage. However, none of the methods available can give rapid and quantitative evaluation of gear fatigue crack evolution. Aiming to provide fast predictions of crack propagation paths, this paper proposes a factorial design approach for gear bending fatigue simulation. Six parameters related to gear geometry and initial crack configuration are considered in this study. Factorial design experiments are numerically conducted with an efficient 2D boundary element model assuming linear elasticity. Then, bending fatigue damage is modelled using polynomial functions. The resulting prediction model can instantly establish the crack trajectory in thin‐rimmed gear for any cycle numbers. Application of the approach is illustrated by several case studies, while its precision and reliability are demonstrated through an exhaustive validation procedure.     

面外载荷下含孔复合材料点阵夹层结构弯曲强度数值计算方法

通过实验手段和有限元方法针对含孔碳纤维增强树脂基复合材料(Carbon Fiber Reinforced Polymer,CFRP)点阵夹层结构在面外载荷作用下的失效模式及其影响因素进行了研究。首先通过实验获得了含孔CFRP点阵夹层结构的失效模式,其次建立了其有限元渐进损伤失效分析模型,基于该模型对开孔形状、开孔率及开孔位置对结构弯曲强度的影响进行了探讨。结果表明,当面板较厚时,含孔CFRP点阵夹层结构的主要失效模式为节点脱粘和面板皱曲;有限元计算结果和实验结果吻合较好,极限承载力的误差约为9.1%;当开孔率φ1.3%时,CFRP点阵夹层结构的弯曲承载能力与开孔形状基本无关;当开孔率1.3%≤φ8.5%时,含圆形孔夹层结构的弯曲承载能力较大;当开孔率φ8.5%时,含方孔夹层结构的弯曲承载能力较大;当开孔位于点阵夹层结构的几何中心或边缘时,对弯曲承载能力影响较大。     

混凝土梁柱子结构连续倒塌动力效应的试验研究

近年来,结构的抗连续倒塌问题在国内外引起了广泛关注。RC框架作为实际工程中最常见的结构形式被广泛研究。然而,已有RC框架梁柱子结构的连续倒塌性能相关研究多基于确定性分析且主要针对静力拆除构件工况。该文基于OpenSees分别建立了典型RC框架梁柱子结构的静力和动力连续倒塌分析有限元模型,通过试验对比验证了模型的准确性。在此基础上,考虑结构截面几何属性、材料特性等不确定性因素,基于拉丁超立方抽样生成不同模型并分析了结构不确定性对RC框架梁柱子结构静力和动力抗连续倒塌性能的影响。参数不确定性分析结果表明：悬链线机制对RC框架梁柱子结构的抗连续倒塌性能十分重要;而以梁端转角达到0.20 rad作为RC框架梁柱子结构动力失效指标在一定程度上偏于保守。参数敏感性分析结果表明：纵筋屈服强度、极限强度为影响RC框架梁柱子结构抗连续倒塌能力的主要因素。     

RC框架梁柱子结构抗连续倒塌性能不确定性分析

近年来,结构的抗连续倒塌问题在国内外引起了广泛关注.RC框架作为实际工程中最常见的结构形式被广泛研究.然而,已有RC框架梁柱子结构的连续倒塌性能相关研究多基于确定性分析且主要针对静力拆除构件工况.该文基于OpenSees分别建立了典型RC框架梁柱子结构的静力和动力连续倒塌分析有限元模型,通过试验对比验证了模型的准确性....     

Ductile fracture simulation of 304 stainless steel pipes with two circumferential surface cracks

In this paper, ductile fracture behaviours of 304 stainless steel pipes with two circumferential surface cracks under pure bending are simulated using finite element damage analyses. Simulations are based on the stress‐modified fracture strain model with the concept that the critical accumulated damage for progressive cracking is assumed to be dependent on an element size. The proposed method can predict not only maximum loads but also complex ductile fracture patterns observed in experiments.     

复合材料三维损伤模型在大开口结构强度预测中的应用

为了预测复合材料大开口结构的强度及损伤扩展情况,开展了3种不同复合材料三维损伤退化模型(瞬间退化模型、渐进损伤退化模型和连续损伤退化模型)在复合材料大开口结构强度预测中的应用研究,完成了含口框加强件的复合材料大开口结构面内纯剪切试验,并采用3种损伤退化模型对试验件及相关文献算例进行了数值模拟。数值模拟结果与试验结果的对比表明:3种损伤退化模型均可以较为精确地预测复合材料大开口结构的极限强度;与其他模型相比,连续损伤退化模型的预测精度最高、通用性最好且网格依赖度最低。研究结果为复合材料大开口结构力学性能的研究提供了理论指导。     

Damage Model and Progressive Failure Analyses for Filament Wound Composite Laminates

Recent improvements in manufacturing processes and materials properties associated with excellent mechanical characteristics and low weight have made composite materials very attractive for application on civil aircraft structures. However, even new designs are still very conservative, because the composite failure phenomenon is very complex. Several failure criteria and theories have been developed to describe the damage process and how it evolves, but the solution of the problem is still open. Moreover, modern filament winding techniques have been used to produce a wide variety of structural shapes not only cylindrical parts, but also “flat” laminates. Therefore, this work presents the development of a damage model and its application to simulate the progressive failure of flat composite laminates made using a filament winding process. The damage model was implemented as a UMAT (User Material Subroutine), in ABAQUS^TM Finite Element (FE) framework. Progressive failure analyses were carried out using FE simulation in order to simulate the failure of flat filament wound composite structures under different loading conditions. In addition, experimental tests were performed in order to identify parameters related to the material model, as well as to evaluate both the potential and the limitations of the model. The difference between numerical and the average experimental results in a four point bending set-up is only 1.6 % at maximum load amplitude. Another important issue is that the model parameters are not so complicated to be identified. This characteristic makes this model very attractive to be applied in an industrial environment.     

Numerical Investigation of Dynamic Response and Failure Mechanisms for Composite Lattice Sandwich Structure under Different Slamming Loads

This paper mainly investigates the slamming dynamic response and progressive damage evolution of the composite lattice sandwich structure under different slamming velocities and deadrise angles. Based on the Coupled Eulerian–Lagrangian (CEL) method, an integrated numerical model of sandwich structures is developed to simulate the slamming process, in which the progressive damage evolution of composite material is considered with VUMAT subroutine. The reliability and accuracy of the corresponding numerical models are verified through the comparison between numerical and experimental results. Then, the typical slamming behavior of composite lattice sandwich structure is analyzed in detail, including hydrodynamic force, jet flow/water pressure distribution, progressive damage evolution and absorption energy. Subsequently, the influences of slamming velocity and deadrise angle on the hydrodynamic response and damage modes of the sandwich structures are investigated based on the developed numerical models. The results demonstrate that the slamming velocity and deadrise angle have significant influences on the hydroelastic behavior and damage modes of composite lattice sandwich structures. In the process of slamming, matrix damage and delamination damage are more prone to appear in the chine region, while the fiber damage more likely occurs under the higher slamming velocity and lower deadrise angle cases.

     

张力拉矫机的二自由度行星齿轮减速箱故障诊断研究

张力拉矫机是生产带钢非常重要的关键性设备,某钢厂冷轧酸洗线的张力拉矫机主减速箱是二自由度差动轮系结构,行星包中的太阳轮、行星轮和大齿圈、行星架都同时运转,其运行轨迹复杂。为研究1#锥齿减速箱和主减速箱振动大的原因,对其进行振动信号测试和频谱分析,分析结论得出,引起主减速箱和1#锥齿减速箱振动大的主要原因是主减速箱中行星包中齿轮啮合不良,并且推测出行星轮有严重损伤。实际检测结果表明,基于传统的频谱分析能够识别出行星齿轮箱的故障。     

重载齿轮弯曲疲劳寿命测试方法研究现状

重载齿轮是大型机械装置(推土机、挖掘机、装甲车等)传动系统的核心部件,它的主要功能是按照规定的转速比传递运动和转矩。随着科学技术的发展和军事装备的更新换代,重载齿轮的研究除了在材料性能、齿形设计、承载能力等方面取得了新成就外,另一个突出的进步就是在齿轮性能测试技术方面获得了很多成果,使得一些过去难以定量研究的问题(如齿轮的疲劳强度、齿轮传动品质等)都有了比较实用的测量手段。而在重载齿轮疲劳性能研究中,相对于接触疲劳产生的齿面点蚀、胶合、磨损等微小破坏而引起齿轮传动效率降低,啮合不到位等现象,弯曲疲劳则会直接导致齿根产生裂纹甚至形成断齿现象,造成重大事故。因此,准确测试重载齿轮的弯曲疲劳寿命,分析弯曲疲劳性能,进而优化齿轮设计,提升齿轮性能,对监测因弯曲疲劳失效所引起设备故障以及避免服役过程中发生重大事故具有重要意义。重载齿轮弯曲疲劳寿命受多方面因素的影响,其中包括材料性能、加工尺寸、制备工艺以及测试手段等,因此对其弯曲疲劳寿命的定量测试一直是各国研究人员关注的热点话题。关于重载齿轮弯曲疲劳寿命的研究可以归纳为以下三方面:弯曲疲劳原理探究方面已发展到声发射信号检测、光学图像分形理论计算、计算机有限元数学模拟等多方面的实际应用;性能检测实验已有单齿/双齿脉冲加载、动态啮合式加载等多种试验方法;数据处理方面已发展出升降法、成组法、雨流法以及多种S-N曲线拟合的数据处理手段。这些分析方法以及测试手段的应用可以大大节省实验成本、提高分析效率、减少试验误差,进而提高重载齿轮弯曲疲劳寿命检测的准确性。本文从重载齿轮弯曲疲劳寿命的测试原理、试验方法以及测试数据处理三方面出发,根据国内外研究现状,对重载齿轮的弯曲疲劳性能进行机理性与实验性的探究,为测试重载齿轮的弯曲疲劳寿命提供有效的理论依据、具体的测试方法以及准确的数据处理手段。     

Material influence on mechanical properties of gears with alternative microstructures

Case hardening represents the most important heat treatment process to increase the load carrying capacity of gear components. Beside predominantly martensitic surface layers with low proportion of retained austenite, there were investigated material structures with differing properties in preceding research work.

In a previous publication, the results of three variants made of the material 20MnCr5, which is typically used for gears, were presented. The reference heat treatment was a conventional carburizing with subsequent case hardening. The second variant was also gas carburized, but with a high proportion of retained austenite. The last presented variant had a bainitic structure in the surface layer. The second and the third variant showed a similar tooth root bending strength compared to the reference. The numbers of the pitting resistance were significantly higher than for the reference variant.

This paper presents the results of further investigations on the influence of different microstructures on the gear load carrying capacity. For this purpose, gears made of 18CrNiMo7‑6 were tested with regard to their load carrying capacity. 18CrNiMo7‑6 is a case hardening steel like 20MnCr5, too, which is often applied in big gear components. The tooth root bending strength was approximately constant, whereas the pitting resistance decreased compared to the corresponding variants of the material 20MnCr5. In comparison to the carburized 20MnCr5 reference variant, only the 18CrNiMo7‑6 variant with a large proportion of retained austenite showed a higher pitting resistance.

The tooth root bending strength investigations took place in the cycle regime of limited life as well in the high cycle fatigue regime. The pitting resistance was only determined in the cycle regime of limited life.

     

Influence of different load models on gear crack path shapes and fatigue lives

A computational model for determination of the service life of gears with regard to bending fatigue at gear tooth root is presented. In conventional fatigue models of the gear tooth root, it is usual to approximate actual gear load with a pulsating force acting at the highest point of the single tooth contact. However, in actual gear operation, the magnitude as well as the position of the force changes as the gear rotates. A study to determine the effect of moving gear tooth load on the gear service life is performed. The fatigue process leading to tooth breakage is divided into crack‐initiation and crack‐propagation period. The critical plane damage model has been used to determine the number of stress cycles required for the fatigue crack initiation. The finite‐element method and linear elastic fracture mechanics theories are then used for the further simulation of the fatigue crack growth.     

缝合泡沫夹芯复合材料低速冲击的多尺度数值方法

为了发展缝合泡沫夹芯复合材料低速冲击损伤的多尺度分析方法, 建立了缝合泡沫简化力学模型, 将缝合泡沫等效为缝线树脂柱增强的正交各向异性芯材, 其材料参数由各组分性能及所占体积分数根据均一化理论计算得出; 同时, 建立冲击试验有限元模型, 通过界面元模拟面板与芯材之间的层间分层。采用GENOA渐进损伤分析模块对缝合结构冲击动态响应过程进行数值模拟, 并将计算结果与试验记录进行对比分析。结果表明: 缝合可以减小面板破坏面积, 抑制面板与泡沫分层的扩展; 但缝纫会对结构造成初始损伤, 较高的缝合密度使芯材刚度增加, 不利于泡沫结构的缓冲吸能。数值模拟结果与试验记录吻合良好, 验证了多尺度分析方法的正确性。     

PC框架结构基于易损性的“强柱弱梁”设计方法研究

研究和震害均表明,按规范设计的钢筋混凝土框架结构很难实现\     

2.5D机织复合材料压缩性能实验与数值模拟

为了研究2.5D机织复合材料的压缩损伤和失效机制,验证双尺度渐进损伤有限元数值模拟方法的有效性,对这类复合材料分别沿经纱方向和纬纱方向进行了准静态压缩实验,获得了其相应的应力-应变曲线,并测定了材料的初始弹性模量和极限强度。在此基础上,利用双尺度渐进损伤有限元数值方法模拟分析了材料的压缩应力-应变响应和损伤演化行为,取得了与实验吻合较好的模拟结果。结果表明:2.5D机织复合材料在纬向压缩下的主要失效模式是纬纱的轴向压溃与断裂,可获得相对较高的压缩强度;但在经向压缩下,经纱因弯曲会承受附加弯矩作用,从而对周围基体造成挤压,故在经纱轴向断裂之前容易出现经纱之间基体的压溃和纱线之间的分层开裂,使强度降低,不利于发挥纤维的承载优势。     

Shape of the power spectral density matrix components: Influence on fatigue damage

The damage estimation for a structure under random loading is a challenge in fatigue assessment, especially when the loading is multiaxial. The comparison of the effect of different spectra on fatigue damage is essential when the structure can be subjected to different types of loadings. Therefore, in the present paper, the expected fatigue damage produced on metallic structures by combined bending and torsion stationary proportional and nonproportional loading is evaluated varying the shape of spectra of the normal and shear stress tensor components.     

Analogies between progressive collapse of structures and fracture of materials

The analogy between structural progressive collapse and Fracture Mechanics is consistent either for phenomenological, technological and theoretical aspects. In this paper a general energy criterion suitable for fracture in heterogeneous materials is applied to study the progressive collapse of simple structures with cohesive post peak behavior: elementary frames and fiber bundles. The analyses put into evidence some interesting scale effects induced by ductility and dynamics. In particular, a power law describing the decrease of the reduced dynamic critical load with the structural scale and a second order ductile-brittle transition, have been found. These results can be usefully applied in robustness oriented structural design. Moreover, the study of the influence of the extent of the starting damage in structures with different sizes suggests that, the elementary cells of complex framed structures can play a role similar to the microstructure of materials. In conclusion, a new approach to the problem of collapse into complex structures by means of the tools of Fracture Mechanics is proposed.