预成形体渗透率预测及剪切变形的影响

准确预测预成型体渗透率对复合材料液态成型工艺过程仿真有重要意义,铺覆过程中织物发生的剪切变形对局部渗透率有很大影响。本工作考虑纱线的可渗透性,对织物单胞内的树脂流动建立了统一的流动控制方程,同时建立了逼真的正交单胞几何模型,基于Adams-Bashforth差分格式和Chorin投影法构造了数值求解树脂流动控制方程的高分辨率TVD格式,利用达西定律预测了单胞的渗透率,算例表明该算法预测值与实验值有较好的吻合,验证了算法的准确性。在正交单胞渗透率求解的基础上,采用贴体坐标法完成了单胞剪切变形后流动控制方程从物理域到计算域的转换,进而实现了剪切单胞渗透率的数值预测,考察了单胞主渗透率比与剪切角的关系,通过与文献中数据对比证明了该剪切单胞渗透率预测算法的合理性。     

预定型平纹织物的剪切模型

预定型织物是一种用于纺织复合材料液态成型的新型材料, 可以提高复合材料构件的形状精度和尺寸精度。由于织物中存在定型剂, 使织物材料的性能发生改变。基于像框剪切试验, 建立预定型平纹织物剪切变形的理论模型。与干态织物相比, 重点分析了预定型织物中纱线的弯曲刚度和纱线摩擦系数的变化对剪切性能的影响; 同时模型中考虑了剪切过程中纱线轴向力的变化对剪切性能的影响。另外, 利用立式显微镜观察了在纱线挤压阶段纱线宽度变化的规律, 考虑了定型剂和织物结构对纱线宽度变化的影响。根据平衡方程得到预定型平纹织物的剪切模型, 通过与试验结果比较, 该模型可以较好地预测预定型平纹织物的剪切变形性能。      

基于曲面信息的平面织物铺覆改进渔网算法

通过分析复合材料平面织物预成形体在铺覆过程中的变形模式,提出铺覆仿真计算的织物变形基本假设。分析已存在的几种渔网铺覆算法,提出一种基于曲面信息的改进渔网算法（SIB渔网法）,该算法利用相邻节点处的曲面切向量及曲率信息来确定当前节点位置,避免了高强度的迭代计算,提高了仿真精度。研究采用非均匀有理B样条（NURBS）技术表达的复杂曲面求交算法。通过不同算法对球面、马鞍面仿真的结果对比,验证了SIB渔网法对复杂曲面铺覆计算的精确性及稳定性。     

基于自由变形技术的二维机织物细观模型

为了避免理想化织物模型横截面恒定、纱线间相互渗透的问题,生成具有真实感的二维机织物三维细观模型,提出一种基于自由变形技术的几何变形方法。首先通过理想化的纱线中心线轨迹、横截面建立织物初始几何模型,然后应用自由变形技术对纱线进行变形。在变形过程中,所有纱线横截面在空间位置和参数的约束下进行自由变形,所有横截面变形后的控制网格组成纱线的控制网格,以驱动纱线的整体变形,最终生成具有真实感的织物细观模型。变形过程中纱线间的接触应用基于射线的碰撞检测技术处理。该方法可以扩展并应用于其他织物结构,且可以输出到其他软件中进行模拟计算。      

压边力对非平衡平纹机织物预制体成型作用规律

为研究压边力对非平衡玻璃纤维平纹机织物预制体铺覆成型的作用规律,针对非平衡平纹机织物在成型过程中的大变形特征,基于非正交材料本构,建立织物面内材料变形本构模型;同时考虑面外弯曲刚度,结合壳单元,建立织物的膜(面内变形)-壳(面外变形)双层模型;利用商业有限元软件ABAQUS,结合实验方法,研究织物在不同压边力条件下的成...     

数控铣削加工精度预测模型的建立

依据铣削加工的特点,将影响加工精度的误差从坐标分解的角度分为三大类:刀架坐标系、主轴坐标系和工艺基准坐标系。在充分考虑机床几何(运动)误差、工艺系统受力变形、工艺系统热变形等对加工精度的影响的情况下,通过坐标变换将所有误差集中纳入工艺基准坐标系,采用齐次坐标变换法建立了加工精度的预测模型。所建的加工精度模型具有开发性和实用性,已在实际数控铣削加工中得到验证。     

纤维增强复合材料自动化成型中织物变形研究进展

纤维增强树脂基复合材料具有高比强度和高比刚度等优异特性，被广泛应用于汽车领域以实现汽车轻量化。然而，纤维织物的多尺度特性使其在复杂形状的车身零部件成型中变形机制十分复杂，极易产生成型缺陷，削弱成型后的复合材料力学性能。表征织物变形特性，揭示织物变形机制，是准确预测织物复杂形状成型，指导织物成型工艺参数合理设计的重要基础。因此，本文针对织物在成型过程中的压缩、剪切、弯曲和滑移4种关键变形特性及在复杂几何形状中成型特性的研究进展进行综述，详细介绍了织物上述变形和成型特性的研究方法及研究热点。本文将为织物的变形机制研究、复杂形状成型精准预测和工艺参数合理设计提供指导，推动复合材料在汽车领域中的大规模应用。     

复合材料机翼整体成型技术研究

对给定外型与尺寸的整体成型复合材料机翼进行了设计、 制备及力学性能实验研究。采用有限元分析软件, 对空心复合材料机翼进行静力学分析, 得到了承载效率与机翼几何尺寸的关系, 并确定了最优结构尺寸与复合材料纤维铺层厚度。采用石蜡芯模辅助气囊法成型技术, 制备了整体成型复合材料机翼, 并进行了三点弯曲实验测定, 分析了其破坏机制。三点弯曲实验研究发现, 整体成型复合材料机翼的破坏模式为上蒙皮的局部屈曲失效, 屈曲后仍有一定的承载能力。      

万能工具显微镜纵向示值误差的影响因素及解决方法

一、概述 万能工具显微镜是一种光机电配合的几何量精密测量仪器,由仪器的纵横向导轨组成平面的x、y坐标系,由对应的标尺提供坐标位置尺寸,测量范围x轴为200mm、y轴为100mm。     

一种新型球面等积六边形网格系统生成算法

分析了构建球面等积网格系统的基本原理和常用剖分方法的特点,提出了在二十面体展开图上对顶点和面统一编码的研究思路,将两个相邻三角面合并为一个四边形并建立坐标系描述孔径为4的新型递归剖分网格,最后借助施奈德等积多面体投影将平面网格映射到球面,得到了将孔径为4的等积六边形剖分产生的球面网格系统ISEA4H-3.实验结果表明,用这种算法生成的ISEA4H-3网格的几何属性优于现有球面六边形网格,更适用于海量空间信息管理,具有较强的应用价值.     

动物骨折治疗用金属骨板局部增量成形新工艺

目的目前动物骨折常用的锁定骨板内固定技术(Point-contact Reconstruction Compress Locking, PRCL)需要采用多个工具配合手动完成骨板成形,针对该过程中精度不可控、效率低等问题,提出一种弯扭复合成形模具,发展一种局部增量成形金属骨板的方法。方法 PRCL骨板固定治疗中,为了贴合受伤骨骼,治疗前骨板需经过面内弯曲、弯曲以及扭转3类变形。通过调整弯扭复合成形模具的空间位置及模具不同组成部分的相对位置,实现不同区域内不同变形量的面内弯曲、弯曲或扭转。应用数值方法分析验证弯扭复合成形模具及成形方法的适用性,基于DEFORM软件建立工业纯钛TA2骨板局部增量成形过程有限元模型,分析具有两个成形区的TA2骨板局部增量成形特征。结果塑性变形仅发生在复合模具附近,对已变形区无影响,会引起未成形区的刚性位移;骨板长度方向受力小于其宽度和厚度方向受力,面内弯曲需要较大的成形载荷。结论所发展的模具和方法可实现预期的骨板成形,也适用于其他PRCL金属骨板的成形。     

Investigation into the changes in bending stiffness of a textile reinforced composite due to in-plane fabric shear: Part 2 – Numerical analysis

Numerical investigations were pursued in an effort to understand the relationship between changes in the stiffness of plain-weave fabric-reinforced plates and the degree of in-plane shear within the fabric reinforcement. These numerical studies were motivated by an experimental study where the measurable geometric changes discerned among plates with different levels of in-plane shear were (1) the reorientation of the fibers within the plane of the plate, (2) an increase in thickness with increasing in-plane shear, and (3) the change in width of the fiber tows as function of in-plane shear. Finite element models were used to investigate the individual contributions of these geometric changes on the bending stiffness of the plates. For the material system considered in this study, the reorientation of the fibers and the change in plate thickness as a function of the state of shear were concluded to be the dominant factors affecting the bending stiffness of the plates. The change in cross-section orientation about the tow axis was determined to be insignificant.     

采用数字图像相关方法的莫来石纤维增强气凝胶复合材料力学试验

基于V型缺口试样双轨剪切法设计了面内剪切试验方案,开展了莫来石纤维增强气凝胶复合材料的室温面内剪切和弯曲性能试验,采用数字图像相关方法对试样表面的位移场和应变场进行测量,并分析了力学行为和破坏模式。结果表明:设计的试验方案可以在测试区域获得均匀的剪切应变场,适用于莫来石纤维增强气凝胶复合材料的面内剪切性能测试。试验获得的面内剪切模量和强度分别为248 MPa和0.95 MPa,弯曲模量和强度分别为294 MPa和2.08 MPa。面内剪切载荷下,试样的裂纹萌生于缺口尖端附近,并沿两缺口连线方向扩展。根据弯曲正应变场的分布特点,发现试样中性层与几何对称面不重合,验证了该材料拉压模量不同的性质。采用数字图像相关方法获得的中性层位置和理论计算值比较接近,相对误差在10%左右。     

In-plane permeability of sheared fabrics

Shear is the main mode of deformation in the draping of fabrics over complex mould geometries in composites manufacturing. Hence, the measurement and prediction of the in-plane permeability of sheared fabrics is a crucial task for the design of resin transfer moulding and other composites processing techniques of complex shaped articles. A mathematical model has been developed and applied to predict the in-plane permeability in the two principal directions and the angle of the flow ellipse for sheared assemblies of bi-directional woven fabrics that are in-plane isotropic to flow when unsheared. Modelling was accompanied by in-plane permeability measurements for unsheared and sheared woven fabric assemblies, and a comparison of this experimental permeability data with the proposed model proved encouraging. A study into the change of the areal density of different woven fabrics with shear angle has also been included.     

Twist springback characteristics of dual-phase steel sheet after non-axisymmetric deep drawing

This paper aims to investigate the twist springback characteristics of advanced high strength steel sheet subjected to deep drawing. A C-rail benchmark, which leads to a particularly pronounced twist springback characteristics, was developed. For an accurate numerical modeling of the process, a non-quadratic anisotropic yield criterion integrated with combined isotropic and kinematic hardening model was used to describe the strain-stress behavior including anisotropy and Bauschinger effects. The corresponding mechanical experiments, namely uniaxial tension and forward-reverse simple shear tests were performed to determine the material parameters. The digital image correlation technique was applied for component tests as well as the deformation and stress-strain analysis. The experimental validation of the elastic-plastic finite element model was assessed by comparing maximum in-plane strain, thickness reduction distribution and twist springback of the drawn rail. To explore the source of twist springback, the deformation associated with in-plane stress and bending moment was analyzed. The results indicate that the bending moment before springback caused by non-symmetric stress states play an important role in twist springback and control. Certain regions of the die radius were varied in a numerical analysis to control the bending moment for the minimization of twist springback as well as the preliminary results of the relationship between the ratio of variable die radius and twist springback.     

A size-dependent third-order shear deformable plate model incorporating strain gradient effects for mechanical analysis of functionally graded circular/annular microplates

In this paper, we develop a novel size-dependent plate model for the axisymmetric bending, buckling and free vibration analysis of functionally graded circular/annular microplates based on the strain gradient elasticity theory. The displacement field is chosen by using a refined third-order shear deformation theory which assumes that the in-plane and transverse displacements are partitioned into bending and shear components and satisfies the zero traction boundary conditions on the top and bottom surfaces of the microplate. Besides, the present model contains three material length scale parameters to capture the size effect. The material properties of the microplate are assumed to vary in the thickness direction and estimated through the classical rule of mixture. By using Hamilton's principle, the equations of motion and boundary conditions are obtained. Afterward, the differential quadrature method is adopted to discretise the governing differential equations along with various types of edge supports and therefore the deflection, critical buckling load and natural frequency can be determined. Convergence and comparison studies are carried out to establish the reliability and accuracy of the numerical results. Finally, a parametric study is conducted to investigate the influences of material length scale parameters, gradient index, thickness-to-outer radius ratio, outer-to-inner radius ratio and boundary conditions on the mechanical characteristics of the microplate.     

Tensile extension properties and deformation mechanisms of multiaxial non-crimp fabrics

The deformation resistance and mechanisms of biaxial and triaxial non-crimp fabrics under bias extension loading is experimentally investigated. The bias deformation resistance of multiaxial non-crimp fabrics is found to be substantially lower than conventional single layer woven fabric due to the unique deformation mechanisms of rotation, sliding and compaction of the fibre tows. It is shown that the amount, line-tension and location of the warp-knitted stitches used to bind non-crimp fabrics have a major influence on the bias deformation resistance. It is found that lowering the amount and line-tension of the stitches facilities the tow sliding mechanism that reduces the deformation resistance of non-crimp fabrics. Fabric deformation is also affected by the location of the stitches, with stitches that pass between, rather than through, the tows reducing the resistance to bias deformation, again due to increasing ease of the tow sliding process. The warp knitting conditions needed to maximise the deformation limits of non-crimp fabrics are determined.     

The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites

In Liquid Composite Molding (LCM) processes, a fibrous reinforcement preform is placed or draped over a mold surface, the mold is closed and a resin is either injected under pressure or infused under vacuum to cover all the spaces in between the fibers of the preform to create a composite part. LCM is used in a variety of manufacturing applications, from the aerospace to the medical industries. In this manufacturing process, the properties of the fibrous reinforcement inside the closed mold is of great concern. Preform structure, volume fraction, and permeability all influence the processing characteristics and final part integrity. When preform fabrics are draped over a mold surface, the geometry and characteristics of both the bulk fabric and fiber tow bundles change as the fabric shears to conform to the mold curvature. Numerical simulations can predict resin flow in dual scale fabrics in which one can separately track the filling of the fiber tows in addition to flow of resin within the bulk fabric. The effect of the deformation of the bulk fabric due to draping over the tool surface has been previously addressed by accounting for the change in fiber volume fraction and permeability during the filling of a mold. In this work, we investigate the effect of shearing of the fiber tows in addition to bulk deformation during the dual scale filling. We model the influence of change in fiber tow characteristics due to draping and deformation on mold filling and compare it with the results when the fiber tow deformation effect is ignored. Model experiments are designed and conducted with a dual scale fabric to characterize the change in permeability of fiber tow with deformation angle. Simulations which account for dual scale shear demonstrate that the tow saturation rate is affected, requiring longer fill times, or higher pressures to completely saturate fiber tows in areas of a mold with high local shear. This should prove useful in design of components for applications in which it is imperative to ensure that there are no unfilled fiber tows in the final fabricated component.     

Stretching and bending deformations due to normal and shear tractions of doubly curved shells using third-order shear and normal deformable theory

ABSTRACT

We analyze static infinitesimal deformations of doubly curved shells using a third-order shear and normal deformable theory (TSNDT) and delineate effects of the curvilinear length/thickness ratio, a/h, radius of curvature/curvilinear length, R/a, and the ratio of the two principal radii on through-the-thickness stresses, strain energies of the in-plane and the transverse shear and normal deformations, and strain energies of stretching and bending deformations for loads that include uniform normal tractions on a major surface and equal and opposite tangential tractions on the two major surfaces. In the TSNDT the three displacement components at a point are represented as complete polynomials of degree three in the thickness coordinate. Advantages of the TSNDT include not needing a shear correction factor, allowing stresses for monolithic shells to be computed from the constitutive relation and the shell theory displacements, and considering general tractions on bounding surfaces. For laminated shells we use an equivalent single layer TSNDT and find the in-plane stresses from the constitutive relations and the transverse stresses with a one-step stress recovery scheme. The in-house developed finite element software is first verified by comparing displacements and stresses in the shell computed from it with those from either analytical or numerical solutions of the corresponding 3D problems. The strain energy of a spherical shell is found to approach that of a plate when R/a exceeds 10. For a thick clamped shell of aspect ratio 5 subjected to uniform normal traction on the outer surface, the in-plane and the transverse deformations contribute equally to the total strain energy for R/a greater than 5. However, for a cantilever shell of aspect ratio 5 subjected to equal and opposite uniform tangential tractions on the two major surfaces, the strain energy of in-plane deformations equals 95–98% of the total strain energy. Numerical results presented herein for several problems provide insights into different deformation modes, help designers decide when to consider effects of transverse deformations, and use the TSNDT for optimizing doubly curved shells.     

考虑剪切变形时薄壁箱梁的挠曲分析

为分析剪切效应对薄壁箱梁受力特性的影响,利用微板的面内剪切及平衡微分方程,分别推导出不考虑和考虑薄壁箱梁各板面内剪切效应的弯曲位移函数。选取剪切效应引起的附加挠度作为广义位移,通过定义的剪切广义力矩及剪切翘曲位移函数,将剪切变形状态从全梁挠曲变形状态中分离出来,作为独立的变形状态进行分析。为满足全截面翘曲应力的自平衡条件,引入两个截面特性参数对广义剪切翘曲位移函数进行了修正。数值算例表明,按该文推导的薄壁箱梁剪切弯曲位移函数计算的两跨连续梁跨中截面应力与实测值及有限元值吻合良好。挠度计算表明:剪切效应使得该箱梁在集中和均布荷载作用下跨中挠度分别增大27%和24%。