层合板低速冲击损伤的数值模拟

为了分析复合材料层合板在冲击载荷作用下,层合板的损伤演化发展情况,结合采用单元失效技术和铺层刚度退化技术建立层合板低速冲击的三维有限元分析模型。在该模型中,当有限元模型中的单元发生某种冲击失效形式时,定义该单元发生部分失效,并将其刚度适当的退化。计算发现冲击背面容易产生基体开裂,并由此导致分层发生,而且靠近冲击背面的界面所产生的分层面积要较靠近冲击正面的界面的分层面积要大;随着冲击能量的增大,分层面积也增大;当冲击能量很高时,铺层内会出现纤维断裂,同时在层合板的边界处也容易出现损伤,计算结果和试验结果吻合。     

基于应变损伤模型的复合材料层合板低速冲击数值模拟

提出了一种基于应变的复合材料损伤模型,考虑了复合材料冲击过程中出现的面内纤维断裂与压缩,基体开裂与挤裂。在使用Abaqus软件进行数值模拟计算时,自编的用户子程序VUMAT和Cohe-sive模型分别实现了复合材料面板的损伤和层间分层。通过对层合板在不同能量下的低速冲击的有限元模拟发现,模拟得到的分层损伤形状和面积、冲头最大挠度、接触力和凹坑深度都与试验结果吻合较好。     

材料重复冲击损伤模型与数值计算方法研究

基于连续介质损伤力学,将有效应力概念和应变等效假设应用于Johnson-Cook本构模型中,建立了重复冲击载荷作用下材料力学性能退化的损伤模型,以及考虑材料性能退化对冲击应力应变响应影响的数值计算方法。首先,推导了基于损伤耦合J-C本构模型的应力应变数值计算关系式,并在ABAQUS中进行二次开发,采用两种方法实现了考虑损伤影响的应力应变数值计算。进一步,建立了试件在重复冲击下损伤累积的计算方法,并采用所建方法对含缺口三点弯试件的重复冲击损伤进行了数值计算,研究了缺口根部应力应变及损伤度,并对试件整体损伤规律进行了分析。进一步开展了缺口三点弯试件的重复冲击试验,通过计算结果与试验结果的对比验证了建立的数值计算方法的可行性和适用性。与无损伤耦合模型计算结果相比,损伤耦合模型更能合理反映出缺口根部材料的力学性能退化过程以及损伤累积与冲击响应之间的相互影响。     

冲锤处流体阻力对高能射流式液动锤性能影响

应用计算流体动力学(computational fluid dynamics,简称CFD)动态分析技术,对SC-86H型高能射流式液动锤试验样机流场特性进行了研究,计算得出了相关性能参数,并通过实验装置对液动锤不同输入流量下的冲击频率进行了测试。将液动锤冲击频率的模拟计算结果与实测结果进行了对比,结果表明:若不考虑冲锤处流体阻力的影响,液动锤冲击频率计算值与实测值相比明显偏大,最小相对误差达18.9%;而将冲锤处流体阻力的作用考虑在内,冲击频率的计算值与实测值比较接近,最大相对误差为8.0%,大幅提高了数值计算结果与实测值之间的吻合程度。这说明冲锤高速运动产生的轴向流体阻力不容忽略,设法减小冲锤处流体阻力的大小,有望成为提高高能射流式液动锤冲击功和能量利用率的重要途径。     

冲击速度和温度对层合板冲后压剩余强度的影响

为研究冲击速度和温度对层合板冲后压的影响,建立考虑温度影响的渐进损伤模型预测材料的损伤模式,并分析层合板在低速冲击下的损伤机理.首先将模型预测的冲后压位移应力曲线与实验进行对照,进一步分析不同冲击速度对压痕深度、分层区域面积和剩余强度的影响,最后在2.3 m/s冲击速度下分析了不同温度对剩余强度和分层区域面积的影响.     

用接触－冲击方法研究复合材料层板线载荷冲击问题

用接触－冲击分析研究了复合材料层板线载荷冲击响应及损伤机理。分析中，考虑层板的材料性质、铺展顺序和长、宽、厚度等参数的影响。数值计算结果与实验结果完全符合。     

用接触－冲击方法研究复合材料层板线载荷冲击问题

用接触－冲击分析研究了复合材料层板线载荷冲击响应及损伤机理。分析中，考虑层板的材料性质、铺展顺序和长、宽、厚度等参数的影响。数值计算结果与实验结果完全符合。     

考虑行走弹性影响下的车辆换挡过程分析

为研究行走弹性对工程车辆行走换挡过程的影响，建立了考虑驱动轮轮胎变形引起的车辆行走弹性影响的整车行走动力学模型，该模型适用于换挡过程分析。计算与分析了典型装载机动力换挡过程，将考虑或不考虑行走弹性时计算得到的整车加速度和瞬时冲击度的变化过程分别进行了对比，利用实车试验对单一换挡过程进行了验证；并将考虑行走弹性时升挡过程中变速箱输入输出转速变化过程、离合器滑摩功率特性的计算结果与不考虑行走弹性时换挡过程的计算结果进行了对比。研究结果表明：与不考虑行走弹性的情况相比，行走弹性变形使得换挡过程中整车加速度的变化更加平缓；换入挡离合器有两次滑摩过程，冲击度更小，换挡时间更长；离合器接合时的冲击度与其接入时间有关；滑摩功更小。     

齿轮磨损和接触疲劳竞争失效可靠性模型

为了计算齿轮可靠度,针对磨损失效和疲劳失效2种常见的失效方式,分别建立了失效概率模型,在此基础上计算出可靠度,并考虑了服从泊松过程的冲击对磨损和接触应力的作用。磨损采用累积磨损的形式,建立了磨损-磨损阈值干涉模型。接触疲劳失效考虑随机强度退化,建立了动态强度-应力干涉模型。以某矿用挖掘机提升减速器中的齿轮为研究对象进行可靠性分析,对模型和方法进行验证,得到了考虑冲击作用的2种失效模式竞争的齿轮可靠度计算结果,分析了不同冲击参数下可靠度值。结果表明,模型对于估计寿命、材料选取等工程问题有一定的参考价值。     

具有缓冲的气压驱动水平冲击台参数计算

建立了气压驱动水平冲击台两自由度阻尼减振的数学模型;对不考虑缓冲和考虑缓冲的水平冲击台基本参数进行了计算比较,给出了具体工程实例;分析了减振系统对冲击峰值加速度、脉冲宽度的影响。     

Ultimate strength of composite laminates with free-edge delamination

The present paper deals with the free edge effect of composite laminates by using a generalized quasi-three dimensional analysis and experimental verification of an analysis performed for laminates with Teflon inserted on interfaces to simulate initial free-edge delamination. We performed tensile tests for laminates [30₂/?30₂/90]_s carbon-epoxy laminates with free-edge delamination under uniaxial tension. The experiment reveals that extensional stiffness of the laminate decreases by the initiation of the delamination, and that strength of the laminate without delamination is smaller than that of the laminates with delamination. Generalized quasi-three dimensional finite element technique, which employs energy release rate and maximum stress criteria, is developed to estimate behavior of the laminate after initial delamination. The numerical result by use of this technique predicts the ultimate strength of the laminates with sufficient accuracy according as the comparison with an experimental stress-strain curve. In the experiment conducted both for the laminate with initial delamination and the laminate without initial delamination, an unexpected results were obtained that is the ultimate load of the laminate without initial delamination was lower than that of the laminate with initial delamination. We presented clear explanation on the phenomenon occurred and developed the method to predict the nonlinear behavior of the laminate with or without initial delamination.     

层间增韧复合材料在静压痕力下的损伤阻抗和渐进损伤分析

在复合材料层合板层间植入韧性层是提高复合材料韧性和抗冲击能力的有效方法。为了研究层间增韧对层合板损伤阻抗的改善作用，文中通过准静态压痕试验研究层间增韧复合材料在准静态压痕力下的损伤和破坏行为，利用超声C扫描测量分层损伤面积。试验结果表明，层间增韧复合材料具有较高的分层起始载荷和分层起始能量，损伤阻抗显著提高。在相同的载荷水平下，具有较小的分层损伤面积。文中还采用有限元方法对层间增韧复合材料在静压痕力下的分层和铺层失效进行数值分析，有限元计算结果与试验结果吻合较好。     

Energy release rate of postbuckled laminates with a through-width delamination

The strain energy release rate is calculated for buckled one-dimensional delamination (through-width delamination) in composite laminates subjected to in-plane compression. A crack closure method based on plate finite elements is used in this analysis. For some laminates containing a one-dimensional delamination in cylindrical bending, closed form solutions are available. The present finite element solutions show excellent agreement with the analytical solutions. The strain energy release rate for various types of laminates is also calculated using the present finite element method. The results show that the strain energy release rate strongly depends on the type of laminate.     

The compressive behavior of CFRP laminates with delamination of different shapes using numerical and experimental approaches

Composite laminates often produce delamination due to a series of factors during the manufacture and service process. In order to research the effect of containing oblique elliptical and circular initial delamination damage on the compressive strength of composite laminates, numerical and experimental methods are used in this paper. Finite element models (FEMs) and the progressive damage subroutine USDFLD are developed to predict the damage initiation and extension behavior of the intralaminar of the laminate. Interlaminar damage is predicted based on cohesive zone models (CZM). At the same time, four compression tests containing different initial delamination damage are performed. The results show that experimental measurements of compression tests concur with the numerical predictions and validate the FEMs. The strength and stiffness of the specimens gradually decrease with the increase of the initial delamination area. It reveals that, for the compression process, the delamination damage extends from the edge of the initial damage to the surroundings. The bearing capacity of the composite laminates is mainly determined by the material properties of the fibers and matrix, rather than the initial delamination.

     

Effect of various surface preparation techniques on the delamination properties of vacuum infused Carbon fiber reinforced aluminum laminates (CARALL): Experimentation and numerical simulation

Carbon fiber reinforced aluminum laminates (CARALL) are one of the aluminum based Fiber metal laminates (FMLs) which, due to their high strength to weight ratio and good impact resistance are greatly replacing aluminum alloys in aircraft structures. In this research work, interlaminate shear strength of Vacuum assisted resin transfer molding (VARTM) manufactured CARALL has been investigated. Numerical simulation model incorporated with real time material data has been developed to predict the delamination behavior of CARALL laminates. Standard CARALL specimens with different surface morphologies were prepared by electric discharge machining, mechanical, chemical and electrochemical surface treatments. T-peel tests were carried out according to standard ASTM D1876-08 to find out inter laminate shear strength. FMLs made out of mechanically, chemically and electrochemically cleaned metal sheets depicted high interlaminate shear strength. SEM micrographs of failed surfaces verify the high adhesive strength of epoxy. Developed numerical simulation model accurately predicts the delamination behavior of CARALL as observed during experimentation.     

Hole drilling with abrasive fluidjets

Small-hole drilling in several materials using abrasive fluidjets—abrasive-waterjets and abrasive cryogenic jets with liquefied nitrogen as the working fluid—were investigated through laboratory tests, numerical simulation, and phenomenological analysis. Drilling is accomplished by an abrasive slurry for abrasive-waterjets and primarily by a stream of dry abrasives for abrasive cryogenic jets as the liquefied nitrogen changes phase after exiting the mixing tube. Scaling factors were successfully derived through analysis of profiles digitized from hole images to collapse the data. For abrasive-waterjets, water is nearly incompressible; a high stagnation pressure inside the blind hole was developed causing delamination in laminates and cracking in the thermal barrier coating. The return abrasive slurry also possesses considerable residue erosion power that could cause damage to the hole entrance due to secondary wear. Neither delamination in laminates nor cracking in the thermal barrier coating is however observed when abrasive cryogenic jets were used.     

基于短纤维增韧的复合材料制孔毛刺和分层抑制研究

由于复合材料的各向异性、树脂导热性差和层间韧性低,在二次机械加工特别是钻削制孔过程中,复合材料易产生毛刺、分层等缺陷和损伤,将短纤维层间增韧方法用于钻削损伤的抑制研究。制备低密度芳纶短纤维薄膜,采用低压接触成型工艺制备了含短纤维增韧与未增韧的复合材料层合板,进而在加工试验台上进行钻削试验。通过对试件加工孔的红外无损检测和显微观测,研究转速和短纤维增韧对复合材料制孔损伤的影响,结果表明提高转速和短纤维界面增韧可改善制孔质量。基于短纤维与基体间相互作用,揭示其增韧机理是由于短纤维在层间形成的丰富桥联抑制了分层扩展,同时短纤维与层间树脂复杂的破坏机制而产生额外的能量耗散,并讨论短纤维参数对增韧效果的影响。该方法为复合材料高质量加工提供借鉴意义。     

Evaluation of material properties and internal damage within CFRP and CFRP/Al sandwich panels due to impact loading

In the present paper, damage development within Carbon fiber reinforced plastic (CFRP) laminates and CFRP/Aluminum (Al) honeycomb core sandwich panels by impact loading was evaluated, and change in material properties due to the damage development was investigated. Falling weight impact tests, 3-point bending tests and cross-sectional observation were carried out. As results, it is found that falling rate of bending elastic modulus due to internal damage in the laminate only is lower than that in the upper face-sheet of the sandwich panel, and that difference in the falling rate between them becomes maximum at the impact energy of 5.1 J. As a result of investigating the relationship between reduction in bending elastic modulus and internal damage development, the reduction is caused by delamination within CFRP laminates mainly. Since total length of cracks in CFRP/Al honeycomb core sandwich panels is smaller than that in the laminates only, the sandwich panels have high impact tolerance because of absorption of impact energy by damaging Al honeycomb core.