Experimental investigation of three-dimensional woven composites

This paper summarizes an extensive experimental study of composites reinforced with three-dimensional woven preforms subjected to tensile, compressive and in-plane shear loading. Three innovative three-dimensional woven architectures were examined that utilize large 12 K and 24 K IM7 carbon tows, including two ply to ply angle interlock architectures and one orthogonal architecture. Additionally, a two-dimensional quasi-isotropic woven material was evaluated for comparison. Loads were applied in both the warp and the weft directions for tensile and compressive loading. Digital image correlation was used to investigate full field strains leading up to quasi-static failure. Experimental results including ultimate strengths and moduli are analyzed alongside representative failure modes. The orthogonal woven material was found to have both greater strength and modulus in tension and compression, though a ply to ply woven architecture was found to outperform the remaining three-dimensional architectures. Recommendations are made for improving the manufacturing processes of certain three-dimensional woven architectures.     

A two-step optimization scheme for maximum stiffness design of laminated plates based on lamination parameters

The purpose of this paper is to propose an effective solution scheme of simultaneous optimization design of layup configuration and fiber distribution for maximum stiffness design of laminated plates. Firstly, a numerical analysis of the lamination parameters feasible region for a laminated plate consisting of various given number of ply groups (each ply group may have different thickness and all the fibers in one ply group are orientated in an identical direction) is carried out, and it is found that the feasible region based on only a few ply groups is very close to the overall one determined by infinite plies. Therefore, it is suggested that the feasible region of lamination parameters of a laminated plate could be approximately determined by the layup configuration of least ply groups. Secondly, a two-step simultaneous optimization scheme of layup configuration and fiber distribution for maximum stiffness design of laminated plates is proposed. Accordingly, by using ply thickness, fiber orientation angle and fiber volume fraction in a laminated plate of least ply groups as design variables, the optimal lamination parameters for maximum stiffness is obtained. Then, taking the optimal lamination parameters as the design objective, a detailed layup design optimization is implemented by considering some limitations on manufacturing, such as preset ply thickness, and specific fiber orientation angle and a limited maximum number of consecutive plies in the same fiber orientation. Numerical examples are also presented to validate the proposed two-step optimization scheme.     

考虑制造因素的变刚度层合板的抗屈曲铺层优化设计

与常规层合板相比,变刚度层合板的制造、有限元建模分析和铺层设计有其特殊之处。首先对设计时需考虑的制造因素进行了归纳,提出了变刚度层合板的铺层设计要求。然后给出了变刚度层合板的理想模型和考虑丝束宽度模型的建模方法。基于理想模型对ABAQUS的前处理模块进行二次开发,利用编制的参数化建模程序分析了不同铺放角的变刚度层合板的屈曲性能,并讨论了最小曲率半径对铺层的限制和变刚度设计提高屈曲载荷的机制。基于变刚度层合板的抗屈曲机制建立了一种铺层优化设计方法,使用遗传算法经两步优化得到最优铺层。对最优铺层建立考虑丝束宽度的模型以研究丝束宽度和铺层偏移对变刚度层合板抗屈曲铺层优化结果的影响。研究表明,在变刚度层合板的抗屈曲铺层优化中使用简化的理想模型通常来说是可行的。在考虑制造因素的情况下,优化后的变刚度层合板较常规层合板屈曲载荷有显著提高。     

Mechanical Properties of Stitched Multiaxial Fabric Reinforced Composites From Mannual Layup Process

This paper summarizes the consolidation behavior of E-glass stitched multiaxial non-crimp fabrics (NCFs) and the mechanical properties of the resulting composites from the manual layup process. NCFs offer many advantages over conventional woven and nonwoven fabrics in terms of processibility and properties. The reinforcement structures studied include six NCFs with different fiber orientation combination, number of sub-ply layers, and unit weight. The compressive and recovery behavior of the dry preforms is evaluated and related to their processibility in the hand lay-up process. Mechanical properties of specimens from the hand layup process are evaluated in tensile, compressive, and flexural tests. The test results and failure mechanisms are reported. The effect of consolidation quality on the mechanical properties is discussed.     

Inter-ply stitching optimisation of highly drapeable multi-ply preforms

An efficient finite element model has been developed in Abaqus/Explicit to solve highly non-linear fabric forming problems, using a non-orthogonal constitutive relation and membrane elements to model bi-axial fabrics. 1D cable-spring elements have been defined to model localised inter-ply stitch-bonds, introduced to facilitate automated handling of multi-ply preforms. Forming simulation results indicate that stitch placement cannot be optimised intuitively to avoid forming defects. A genetic algorithm has been developed to optimise the stitch pattern, minimising shear deformation in multi-ply stitched preforms. The quality of the shear angle distribution has been assessed using a maximum value criterion (MAXVC) and a Weibull distribution quantile criterion (WBLQC). Both criteria are suitable for local stitch optimisation, producing acceptable solutions towards the global optimum. The convergence rate is higher for MAXVC, while WBLQC is more effective for finding a solution closer to the global optimum. The derived solutions show that optimised patterns of through-thickness stitches can improve the formability of multi-ply preforms compared with an unstitched reference case, as strain re-distribution homogenises the shear angles in each ply.     

Study of layup influences on the nonlinear behavior of composites by evaluation of ply stiffness reduction

The influence of the stacking sequence on the nonlinear response of composite laminates is investigated. It is shown that a layup dependency solely emerges from damage evolution mechanisms, whereas damage initiation and viscoelastic and viscoplastic strain accumulation are not affected by the layup. This is a result of a proposed procedure that enables the evaluation of the stiffness reduction on lamina level. The residual ply stiffness components can be determined at large deformations and for various laminates under in-plane loading conditions. A finite element study is utilized to characterize the properties of a ply containing discrete cracks. The relationship between transverse and shear stiffness reduction is derived from the FE results. This allows the combined determination of the residual lamina moduli from an axial laminate stiffness. The analysis approach is validated by angle-ply specimens with different layups.     

Fractographic observations on delamination growth and the subsequent migration through the laminate

This paper describes fractographic observations from the detailed examination of delamination fracture surfaces and offers an interpretation of the key growth mechanisms. Firstly, the relationship between toughness, delamination failure criteria and fracture morphology is presented and the influence of cusp formation and deformation on toughness is discussed. Observations regarding delaminations migrating through the lamina at multidirectional ply interfaces are then discussed. It is demonstrated how this migration process can be avoided in fracture toughness coupons and consequently the toughness of multidirectional ply interfaces can be characterised. The influence of migration on delamination growth from embedded defects in laminates under compression is presented, and these results are extended to demonstrate how migration influences damage growth in structures. The paper concludes by making recommendations for realistic modelling of migration, and suggests how it can be exploited in damage tolerant structural design.     

真空导入模塑工艺中织物预成型体的可压缩性

采用真空加载方法研究了循环加载、织物形态、纤维种类、织物层数、铺层方式和混杂方式等参数对真空导入模塑工艺(VIMP)中纤维织物预成型体压缩行为的影响。结果表明: 预成型体纤维体积分数随着压缩循环加载次数的增加而逐渐增大, 但增幅呈现逐渐减小的趋势; 在相同的压缩载荷下, 预成型体的纤维体积分数随着织物层数的增加而增大, 但增幅很小, 对于VIMP制备复合材料构件基本可以忽略; 纤维预成型体在压缩载荷下的响应方式与织物形态、纤维种类、铺层方式和混杂方式等因素密切相关, 单向铺层比正交铺层更容易压缩而获得较高的纤维体积分数, 夹芯混杂比层间混杂方式更容易压缩。      

A non-local criterion for modelling unbalanced woven ply laminates with stress concentrations

A non-local ply scale criterion [Hochard C, Lahellec N, Bordreuil C. A ply scale non-local fibre rupture criterion for CFRP woven ply laminated structures. Compos Struct 2007;80:321–26] was previously developed for predicting the failure of balanced woven ply structures with stress concentrations. This non-local criterion was based on the mean values determined over a Fracture Characteristic Volume (FCV) corresponding to a cylinder with a circular area and the same thickness as the ply. This non-local approach along with a ply scale continuum damage behavioural model was implemented in the ABAQUS Finite Element Code. The behavioural model was developed from a classical Continuum Damage Mechanics (CDM) model [Ladevèze P. A damage computational method for composite structures. Comput Struct 1992;44:79–87]. In the present study, this approach was extended to the case of unbalanced woven ply. The FCV approach and the CDM behavioural model are presented and comparisons are made between the experimental data and the modelling predictions obtained on plates with open holes, notches and saw cuts.     

A new mechanism for the formation of ply wrinkles due to shear between plies

The formation of out-of-plane ply deformation causes significant reductions in the mechanical properties of composites. In-plane fibre misalignments also cause reductions in the compressive strength, yet the origins of these defects are misunderstood. This paper presents a new mechanism for the formation of wrinkles, which is based upon the shear forces generated as a result of mismatches in the coefficient of thermal expansion of composite and tool, as well as the process of ply slippage that occurs during consolidation into radii. Using a U-shaped tool, defects in composite spars have been characterised using light microscopy, showing that the tool geometry and prepreg bridging leads to “instability sites,” which lead to wrinkles up to 750 μm in height as well as in-plane misalignment of 0° plies of up to 50°. Increasing the frictional shear stress through omission of release film prevents the formation of wrinkles, supporting the mechanism presented in this paper.     

Production Principles and Technological Development of Novel Woven Spacer Preforms and Integrated Stiffener Structures

3D textile preforms offer a high potential to increase mechanical properties of composites and/or decrease manufacturing costs. Within the scope of this study, production principles were developed for complex spacer preforms and integrated stiffeners. These principles were applied through technological further development of the well-known face-to-face and terry weaving techniques. Various woven preforms were produced with Glass fibre/Polypropylene (GF/PP) Commingled yarns, however, the technology is suitable for any type of reinforcement yarns. U-shaped woven spacer preform was consolidated into a sandwich composite component for lightweight applications.     

Manufacturing and ultimate mechanical performance of carbon fibre-reinforced epoxy composite suspension push-rods for a Formula 1 racing car

The contemporary Formula 1 racing car makes extensive use of advanced composite materials in its construction. The design, manufacture and ultimate performance under compression of composite suspension push-rods, that typically could be used in a Grand Prix racing car, are described in this present paper. An aerofoil cross-section has been used based on different lay-ups of carbon/epoxy composite. One push-rod was manufactured using a uniform layup of unidirectional and woven cross-ply prepreg, whilst a further three push-rods were manufactured with a tapered layup of unidirectional and woven cross-ply prepreg. Failure mechanisms including fibre microbuckling, fibre kinking and fibre fracture were observed, whilst comparisons have been made between the experimentally observed failure strains and those that were predicted using simple buckling theory. The ultimate compressive strength of the structural component was significantly less than that of the carbon/epoxy composite.     

Experimental and analytical assessment of axial crushing of circular hybrid tubes under quasi-static load

In the present article, axial crushing behavior of circular aluminum/glass–epoxy hybrid tubes is studied experimentally and analytically. 48 quasi-static axial crushing experiments are carried out on bare metal and hybrid tubes to evaluate the effect of different parameters such as metal and composite wall thicknesses and stacking sequence of composite layers on the crashworthiness characteristics. The specimens are made in two types of layups including angle ply pattern [±θ]_s and multi angle ply pattern (different ply angles). The experimental results reveal that stacking sequence has a considerable effect on crashworthiness characteristics, for example for layup [90/0/0/90], the absorbed energy is more than three times of aluminum tube with the same aluminum wall thickness. Also the aforementioned layup has better energy absorption compared to [90/90/90/90] which has been previously proposed as the best layup.     

自动铺带铺层贴合形成机制

探讨了自动铺带工艺过程中相邻铺层界面间的贴合形成机制,将铺层界面间贴合过程建模为树脂补片在平行板之间挤压流动从而浸润界面的过程。指出在树脂流体黏度一定的情况下,树脂对界面浸润效果越好,则界面贴合能力越强。利用流体力学理论推导出树脂流体对界面浸润效果的计算公式,并分析了铺放压力、铺放速度等因素对浸润效果以及贴合能力的影响,提出以剥离力作为量化铺层间贴合能力的指标。在自行研制的自动铺带工艺参数实验平台上进行了一系列实验,实验结果表明,铺层间贴合能力随铺放速度增大而变差,随铺放压力、模具温度、压辊半径增大而变强,这一规律验证了所提出的树脂流体对界面浸润效果计算公式的正确性。     

Characterisation and modelling of the cured shapes of arbitrary layup bistable composite laminates

Asymmetric laminates can have a bistable response to loading, making them of particular interest for applications requiring a large deflection with relatively small and removable energy input. After nearly 30 years of research effort the effects of ply orientation and laminate geometry on the room-temperature shapes of such laminates are well understood and the temperature dependent deformations of laminates of arbitrary layup can be quantitatively predicted. With attention switching to the design of mechanisms incorporating bistable laminates for actuation there is an increasing importance placed on the accuracy and the sensitivity of modelling techniques to predict out-of-plane deflections. This paper presents the results of a novel experimental investigation to map the surface profiles of a series of arbitrary layup laminates. It is concluded that while existing modelling techniques are successful in accurately predicting room-temperature shapes, the sensitivity of solutions to imperfections is significant.     

Highly aligned flax/polypropylene nonwoven preforms for thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from unidirectional yarns and woven fabrics are known to perform significantly better than composites made from random nonwoven mats, but unidirectional yarns and fabrics are much more expensive to manufacture than random nonwoven mats. Here, we report on highly aligned natural fibre nonwoven mats that can be used as a replacement for unidirectional woven fabrics. A drawing operation is added to the conventional nonwoven process to improve fibre alignment in the nonwoven preforms and the final composites. The modified nonwoven manufacturing process is much simpler and cheaper than the unidirectional woven fabric process because of the elimination of expensive spinning and weaving operations. The composites fabricated from the highly aligned nonwoven mats showed similar mechanical strength as the composites made from unidirectional woven fabrics.     

First-ply failure prediction of an unsymmetrical laminated ellipsoidal woven GFRP composite shell with incorporated surface-bounded sensors and internally pressurized

First-ply failure of an unsymmetrical laminated ellipsoidal woven Glass Fiber Reinforced Polymer (GFRP) composite shell internally pressurized was investigated analytically using the linear interpolation technique. The shell's boundary was fixed at its end. Tsai-Wu failure criterion was used as the composite failure design factor. The analytical results, including critical internal pressure and strains in global directions, were validated with the experimental results for some arbitrarily selected points on the shell surface along meridian axis. Manufacturing of laminated ellipsoidal composite shells was performed by using the Vacuum Infusion Process (VIP), a novel method commonly adopted for the fabrication of laminated composite shells. Surface-bounded sensors were installed on the shells' surface to measure the strain values after the internal pressure was applied. According to the analytical investigation findings, the failure factor was critical at the innermost ply. In addition, for each ply, the shell's edge was observed to be the region with the highest failure factor. The experimental findings confirmed that the failure occurred in the regions close to the shell's edge, as predicated by the analytical approach. The results from both approaches were in a close agreement. Subsequently, the effect of various parameters including thickness, aspect ratio, and stacking sequence on the first-ply failure of laminated ellipsoidal woven GFRP composite shell were investigated and the critical mechanical factors to avoid failure were determined.     

3D modeling of textile composite preforms

The purpose of this paper is to present three-dimensional models of fabric reinforcements for composite components by using computer aided geometric design (CAGD) techniques. A novel approach to structural representation of fabric preforms is described in both parametric and graphic forms. The philosophy behind the development of the computer generated model of a composite fabric reinforcement is discussed. The model described here is a general one, capable of producing a 3D representation of any 2D and 3D fabrics. The computer program also provides a very useful visualisation of the structure. Various structures of fabrics used as reinforcements for composite materials, such as woven, braided, knit, and multi-axial 3D weave, are demonstrated in virtual realistic forms.     

A generalized damage model for woven ply laminates under static and fatigue loading conditions

A generalized non-linear cumulative damage model for woven ply laminates subjected to static and fatigue loading is developed in this paper. The damage, consisting of small cracks running parallel to the fibers, leads to a loss of stiffness in the warp, weft and shear directions. The model presented here describes the evolution of the damage up to failure of the first ply. By replacing the woven ply by two stacked unidirectional plies corresponding to the warp and weft thicknesses, this general model is extended to cover a broad range of plies, from quasi-unidirectional to balanced woven plies. A continuum damage approach (CDM) is then used to define the behaviour of the two virtual unidirectional plies under static and fatigue loading conditions. The model is applied here to an unbalanced woven ply with glass reinforcement and the results of the simulations are compared with experimental data.     

厚壳玻璃钢艇体选材、铺层设计及工艺

本文涉及玻璃钢猎扫雷艇艇体材料的优选、铺层设计和工艺等内容。试验确定了以１６２９间苯聚酯树脂为基体、以９００ｇ／ｍ２无碱无捻粗纱短切毡（Ｍ）和８１０ｇ／ｍ２无碱无捻粗纱方格布（Ｒ）为增强体以及（Ｍ＋Ｒ）为基本铺层设计单元的方案。试验表明，按上述材料和铺层制作的层板，性能达到Ｌｅｒｉｃｉ和ＭＨＣ－５１艇壳层板的指标要求。