Shock response of 3D woven composites: An experimental investigation

A modified shock tube was used to determine the effect of shock wave loading on 3D woven composite panels. The shock wave, which produces a short duration steeply rising pressure pulse when impacting the panel, was used to load the panels. The out of plane deformation response was measured using a full field Digital Image Correlation (DIC) technique. The results allow for measurements of full field displacements and strains in the samples. Three distinct textile composite architectures, corresponding to different amounts of Z-fiber (fiber tows that bind the different textile layers together) were investigated. Two separate shock intensities were used. Matrix micro-cracking was observed to be the mechanism by which failure is initiated, and this micro-cracking was found to occur closest to the center of the panel where the outer-surface straining is highest. Fiber tow failure was absent in the shock strengths studied in the present work. The results suggest that the 6% Z-fiber architecture provided the largest panel stiffness and the least amount of damage. This result suggests that this may be the optimal architecture and density for orthogonally woven Z-fiber reinforced composites, however due to the complex nature of the problem the same architecture with a different tow (and fiber) volume fraction may yield different results.     

Shock loading of 3D woven composites: A validated finite element investigation

In a companion paper in this issue (Pankow et al., 2010 [1]), experimental results for the deformation response of 3D glass fiber textile composites subjected to shock wave pulse pressure loading were presented. In this paper, finite element models are developed to simulate the deformation response and damage development observed in the experiments. Two levels of models are presented, with the first focused on a layered, homogenous orthotropic model that examined the bulk macroscopic deformation response. In the second model, each layer is represented to capture the interactions between fiber tows and matrix, taking into account the discrete non-homogeneous material distribution in each layer. Both models incorporated a user defined subroutine within the commercial software ABAQUS to capture matrix micro-cracking, which is responsible for damage development and growth. Results from the computational models correlated well with experimental results and observed locations of matrix micro-cracking.     

Effect of stitching and weave architecture on the high strain rate compression response of affordable woven carbon/epoxy composites

In this study, experimental investigations on stitched and unstitched woven carbon/epoxy laminates under high strain rate compression loading are discussed. Stitched/unstitched laminates are fabricated with aerospace grade plain and satin weave fabrics with room temperature curing SC-15 epoxy resin using affordable vacuum assisted resin infusion molding process. The samples are subjected to high strain rate loading using modified compression split Hopkinson’s pressure bar at three different strain rates ranging from 320 to 1149 s⁻¹. Results are discussed in terms of unstitched/stitched configuration, fabric type and loading directions. Dynamic compression properties are compared with those of static loading. Failure mechanisms are characterized through optical and scanning microscopy.     

三维正交机织复合材料的动态压缩性能

本文作者利用分离式Hopkinson压杆装置对玻璃纤维三维正交机织复合材料进行了高应变率下面外、面内方向的压缩试验,并在万能试验机下进行了相应的准静态压缩。获得3 个高应变率及准静态下的应力-应变曲线,观察了试样的破坏形貌。结果表明：玻纤三维正交机织复合材料是应变率敏感材料,最大应力、压缩模量随着应变率的增大而增大。三维正交结构使复合材料体现出各向异性：面外的最大应力、失效应变比面内大;面内的压缩模量大于面外,且压缩模量对于应变率的变化比面外方向敏感;经纬向相比,纬向的最大应力大于经向。     

3D机织复合材料多向接头有限元分析

为了探究3D机织复合材料桁架接头的机械性能,采用有限元软件ANSYS对3D机织复合材料多向接头所在桁架总体进行有限元模拟,模拟中根据纤维走向对多向接头不同轴向圆管建立相应坐标系,并赋予材料属性,使用MPC多点约束法施加载荷.求解分析后结果表明:模拟结果与实测结果中最大应变的位置与数值基本吻合,确定了模拟的有效性;将最大应力与破坏应力对比发现接头在当前载荷下可能发生轻微破坏,破坏位置应位于副管顶部;通过模拟判断了实测中发生轻微响声的原因;将4种角联锁结构的多向接头模拟结果对比发现,带有衬经结构的复合材料为多向接头最佳材料.此次模拟补充了实测中无法得到的数据,为接头的优化设计和实际使用提供一定的帮助.     

Modeling strategies of 3D woven composites: A review

Due to advancements made in 3D weaving process, 3D woven composites have evolved as an attractive structural material for multi-directional load bearing and impact applications, due to their unique transverse properties such as stiffness, strength, fracture toughness and damage resistance. Substantial progress has been made in recent years for the development of new modeling techniques in design and analysis to understand the unique mechanical behavior of 3D woven composites. This paper systematically reviews the modeling techniques along with their capabilities and limitations for characterization of the micro-geometry, mechanical/thermo-mechanical behavior and impact behavior of 3D woven composites. Advantages, disadvantages and applications of 3D woven composites have also been delineated. In addition, this reference list provides a good database for future research on 3D woven composites.     

Modeling the Response of 3D Textile Composites under Compressive Loads to Predict Compressive Strength

The compression response of 3D woven textile composites (3DWC) that consist of glass fiber tows and a polymer matrix material is studied using a combination of experiments and finite element based analyses. A previous study reported by the authors consisted of an experimental investigation of 3DWC under high strain rate loading, Pankow, Salvi, Waas, Yen, and Ghiorse (2011). Those experimental results were explained by using the finite element method to analyze the high rate deformation response of representative volume elements (RVEs) of the 3DWC, Pankow, Waas, Yen, and Ghiorse (2012). In this paper, the same modeling strategy is used to examine the quasi-static, compressive deformation response of 3DWC. The effect of using different numbers of the textile repeat unit architecture in the RVE, on the predicted compression strength, is examined. The transitions in failure modes that are seen in experiments are seen to be captured by the model that is presented here.     

三维机织复合材料在拉压循环载荷下的疲劳性能

为研究三维机织复合材料在拉伸-压缩循环载荷下的疲劳性能,对材料进行了应力比R=-1的疲劳试验。在不同的载荷水平下,分别进行了纬向和经向两类拉压疲劳试验。试验获得了试样在疲劳载荷下的滞回曲线和全过程中剩余刚度比随寿命的变化曲线。结果表明,在拉伸-压缩循环载荷下,三维机织复合材料的疲劳损伤过程主要包含3个阶段,分别发生基体破坏、纱线横向裂纹扩展和纱线的最终断裂。基体的破碎和开胶、垂直于载荷方向排布的纱线撕裂和沿载荷方向排布的纱线断裂是试样内部的主要失效模式。试验还获得了纬向和经向拉压疲劳的拟合S-N曲线,可应用于工程中对该型材料进行疲劳寿命估算。该型材料的疲劳寿命在低应力区和高应力区均显示出较小的分散性,双对数坐标系下的拟合S-N曲线具有较好的线性度。     

Z-fiber influence on high speed penetration of 3D orthogonal woven fiber composites

In the current work we present a computational investigation of high speed penetration response of 3D orthogonal woven fiber composites (3D OWC) utilizing sub-unit cell, meso-level partitioned damage mechanics with the specific aim of understanding the role of Z-fibers in the mechanical response. In our model, two primary sources of nonlinearities have been addressed – one resulting from the strain rate dependence and large deformation of the composite constituents and the other from evolving failure. We reduce a number of arbitrary parameters typically present in high speed models by taking advantage of specific geometrical properties of 3D OWC which prevent extensive delamination. This property allows us to partition the structure into resin impregnated fibers assumed to be wholly responsible for the progressive damage behavior and bulk resin which is identified as the source of visco-plasticity and strain rate dependence. The fibers are modeled as anisotropic linear elastic with strain rate dependent progressive damage evolution. The resin is modeled using an advanced high strain rate large deformation Mulliken–Boyce polymer model (Mulliken and Boyce 2006) together with a terminal thermo-mechanical failure criterion. The projectile is assumed to be cylindrical, isothermal, rigid and impacting at right angles to the plate. The shape of the damaged area and the extent of penetrative damage compares favorably with experiments. We find that Z-fibers aid in improving penetration and impact resistance by both energy absorption and structural engagement. However, we also find that they are susceptible to localized de-bonding especially around the winding crowns. In addition, we found crucial differences in mechanical response in wave propagation brought about by the interplay of fiber architecture and damage with respect to simplified membrane models.Finally, the Z-fibers were found to influence the shape and nature of the damaged area in the fibers compared to layered composites where the matrix damage is spread more evenly while the fiber damage is restricted towards the fiber axes directions.     

Damage modes in 3D glass fiber epoxy woven composites under high rate of impact loading

A qualitative analysis of experimental results from small caliber ballistic impact and dynamic indentation on a 3D glass fiber reinforced composite are presented. Microscopic analysis of the damaged specimens revealed that the current 3D weaving scheme creates inherently two weak planes which act as potential sites for delamination in the above experiments. It is concluded that while the z-yarns may be effective in limiting the delamination damage at low loads and at low rates of impact, at high loads and high loading rates delamination continues to be the dominant failure mode in 3D woven composites. It is shown that dynamic indentation can be used to capture the progression of damage during impact of 3D woven composites.     

Dynamic fracture toughness of a high strength armor steel

This paper summarizes the results of a research being carried out to determine fracture behavior both in static and dynamic conditions of high strength armor steel Armox500T. In this research, notched specimens were cut to be tested in three-point bending test. Specimens were pre-cracked by flexural fatigue. Thereafter, some specimens were tested in bending up to rupture to determine the static fracture toughness K_IC. To obtain fracture toughness in dynamic conditions, a split Hopkinson bar modified to perform three-point bending tests was used. In this device, displacements and velocities of the specimen were measured, as well as the rupture time by means of fracture detection sensors, glued to the specimens. After that, a numerical simulation of the test was performed by using LS DYNA hydrocode, obtaining stresses and strain histories around the crack tip. From these results, the stress intensity factor history was derived. By using the rupture time, measured by the sensors, the value of the fracture toughness computed was unrealistic. Therefore, the use of a numerical procedure to obtain the rupture time was decided, by comparing experimental results of velocities at the transmission bar with numerical results obtained with several rupture times. With this procedure, the computation of dynamic fracture toughness was possible. The method shows that the measurement of the dynamic fracture toughness is possible without the needs of using crack sensors or strain gauges. It can be observed that fracture toughness of this steel under static and dynamic conditions is quite similar.     

三维机织复合材料结构表征与实体造型

利用经纱脉络描述法解决了任意三维机织复合材料的结构表征问题;提出了单胞分解法与亚胞向量概念,实现了三维结构的二维化、数字化,根据数字表征结果和经典截面假设构造了初始断裂模型;使用一种迭代算法对初始断裂模型进行优化,使纱线束逐步逼近真实形态,进而实现三维实体造型,在此基础上进行网格剖分。研究结果表明,迭代算法能够使纱线形态趋于自然,使造型结果接近实际。数值分析结果表明,迭代算法中的经纬纱退让系数介于0.6～0.7之间时,迭代收敛速度最快。     

基于细观结构的2.5维机织复合材料强度预测模型

采用经纱矩形截面及纬纱六边形截面假设,将经纱的屈曲轨迹简化为折线形式,建立了2.5维机织复合材料单胞几何模型。以单胞为研究对象,引入改进的三维Hashin失效准则和Mises准则作为组分材料的失效判据,采用不同的刚度退化方式来表征不同的失效模式,建立了基于逐渐损伤理论的强度预测模型。利用有限元分析（FEA）技术,开发了相应的参数化2.5维机织复合材料逐渐损伤分析程序,预测了浅交弯联结构不同机织参数2.5维机织复合材料的拉伸强度,并模拟了经向拉伸和纬向拉伸的损伤扩展过程。与静拉伸试验结果相比,拉伸强度的预测误差在10%以内;模拟的失效模式与试验结果吻合较好。     

Modeling compressive response of 3D woven textile composites accounting for microscale geometric uncertainties

The compressive response of 3D woven textile composites (3DWTC), that consist of glass fiber tows and an epoxy matrix material, is studied using a finite element (FE)-based mechanics model. A parametric Representative Unit Cell (RUC) model is developed in a fully three-dimensional setting with geometry and textile architecture for modeling the textile microstructure. The RUC model accounts for the nonlinear behavior of the fiber tows and matrix. The computational model is utilized to predict the compressive strength of 3DWTC and its dependence on various geometrical and material parameters. The finite element model is coupled with a probabilistic analysis tool to provide probabilistic estimates for 3DWTC compressive strength. The reported results are found to be in good agreement with experimental data.     

三维正交机织复合材料低周弯曲疲劳力学性能有限元模拟

为了深入理解三维正交机织复合材料（3DOWC）疲劳性能,改进材料抗疲劳设计,结合三维正交机织复合材料试样经纱方向准静态三点弯曲及60%应力水平下的三点弯曲疲劳实验与ABAQUS有限元软件,构建了全尺寸三维实体模型,研究了三维正交机织复合材料在低周循环载荷下的弯曲疲劳性能,经分析得到循环加载下模型应力分布情况和疲劳损伤形态。结果表明：经纱为材料最重要的承载部件,中间加载区域为材料应力集中区,损伤主要位于应力集中区的Z纱通道处的经纱上,随着循环增加,逐渐在中心加载区域的上部和下部形成三角形损伤区域,该研究在复合材料设计与优化中具有指导意义。     

Mesh generation and geometrical modelling of 3D woven composites with variable tow cross-sections

The modelling of 3D woven composites represents a key part in both material and component design. Current modelling techniques can struggle to mesh correctly accurate unit cell models and apply the complex but necessary periodic boundary conditions required, often with sacrifices made in idealisation of the weave architecture. An automated voxel meshing technique suitable for modelling failure in 3D woven composite unit cells has been developed, which is generic in nature and allows incorporation of architectural deformities within weaves, including tow rotations and misalignments. The model requires node points with only five independent variables to define the unit cell geometry. Application of a smoothing algorithm to the tow/matrix interface voxels produces a suitable surface for modelling tow/matrix debonding, leading to a more accurate representation of the stress field.     

Effect of cooling rate on the high strain rate properties of boron steel

In this work, the effect of cooling rate on the high strain rate behavior of hardened boron steel was investigated. A furnace was used to austenize boron sheet metal blanks which were then quenched in various media. The four measured cooling rates during the solid state transformation were: 25 (compressed air quench), 45 (compressed air quench), 250 (oil quench) and 2200 °C/s (water quench). Micro-hardness measurements and optical microscopy verified the expected as-quenched microstructure for the various cooling rates. Miniature dog-bone specimens were machined from the quenched blanks and tested in tension at a quasi-static rate, 0.003 s⁻¹ (Instron) and a high rate, 960 s⁻¹ (split Hopkinson tensile bar). The resulting stress vs. strain curves showed that the UTS increased from 1270 MPa to 1430 MPa as strain rate increased for the specimens cooled at 25 °C/s, while the UTS increased from 1615 MPa to 1635 MPa for the specimens cooled at 2200 °C/s. The high rate tests showed increased ductility for the 25, 45 and 250 °C/s specimens, while the specimens cooled at 2200 °C/s showed a slight decrease. The Hollomon hardening curve was fit to the true stress vs. true strain curves and showed that the mechanical response of the high rate tests exhibited a greater rate of hardening prior to fracture than the quasi-static tests. The hardening rate also increased for the specimens quenched at higher cooling rates. Optical micrographs of the fractured specimens showed that the failure mechanism transformed from a ductile-shear mode at the lower cooling rates to a shear mode at the high cooling rates.     

Interlaminar shear properties of polymer matrix composites: Strain rate effect

Studies are carried out on interlaminar shear behavior of typical polymer matrix composites under high strain rate shear loading. Torsional split Hopkinson bar (TSHB) apparatus is used for the studies in the shear strain rate range of 496–1000/s. Experimental details, specimen configuration and development, data acquisition and processing are presented. Interlaminar shear strength and shear modulus are presented as a function of shear strain rate. The results are presented for typical plain weave carbon/epoxy and plain weave E-glass/epoxy composites. For comparison, studies are presented at quasi-static loading. It is observed that the interlaminar shear strength at high strain rate is enhanced compared with that at quasi-static loading. Further, it is observed that the interlaminar shear strength increases with increasing shear strain rate within the range of shear strain rate considered.     

Determination of physicomechanical properties of soft soils from medium to high strain rates

A method is presented for obtaining materials parameters, for quartz sand, over a wide range of strain rates. This involves the use of a modified Kolsky bar and plate impact experiments. Data were analysed based on the determination of longitudinal and lateral stresses in the Kolsky method. These values were combined with the longitudinal stress data from shock experiments to provide a stress–compression curve. From this curve, other parameters can be determined.     

3D机织复合材料卫星桁架接头的抗弯刚度研究

为了研制高性能复合材料卫星桁架接头,采用三维有限元分析技术,分析了三维整体织物增强的碳纤维卫星桁架接头三通制件的等效抗弯刚度和内部应力分布,并与制件的三点弯曲实测结果进行了对比.结果表明:三通制件的等效抗弯刚度可以用有限元软件快速、方便、准确地计算出来,并可提供制件任意部位的应力分布状态,为优化制件结构设计提供理论依据.弯曲破坏实验的破坏载荷未能真实地反映接头的最大抗弯破坏承载能力,3个方管的根部应力尚未达到材料的破坏极限.