Analysis of anisotropic in-plane corners

Singular behavior of the mechanical in-plane fields occurs at a laminate reinforcement patch corner due to the geometry and different material properties in the reinforced and non-reinforced domain, respectively. Adopting Lekhnitskii’s approach of the complex potential method, an asymptotic analysis of the mechanical fields is performed near laminate reinforcement patch corners. The mechanical in-plane fields at the two-dimensionally modeled interface corner can be determined in closed-form manner. Various configurations of interface corners are examined and their effect on the singular characteristics of the cross-sectional force field is studied. It is found that for a characterization of the singular behaviour of the in-plane forces each singular in-plane force term has to be considered and that the corresponding displacement modes are useful for understanding this behaviour.     

A cohesive zone model for predicting delamination suppression in z-pinned laminates

This paper presents a cohesive zone model based finite element analysis of delamination resistance of z-pin reinforced double cantilever beam (DCB). The main difference between this and existing cohesive zone models is that each z-pin bridging force is governed by a traction-separation law derived from a meso-mechanical model of the pin pullout process, which is independent of the fracture toughness of unreinforced laminate. Therefore, two different traction-separation laws are used: one representing the toughness of unreinforced laminate and the other the enhanced delamination toughness owing to the pin bridging action. This approach can account for the large scale bridging effect and avoid using concentrated pin forces, thus removing the mesh dependency and permitting more accurate analysis solution. Computations were performed using a simplified unit strip model. Predicted delamination growth and load vs. displacement relation are in excellent agreement with the prediction by a complete model, and both models are in good agreement with test measured load vs. displacement relation. For a pinned DCB specimen, the unit strip model can reduce the computing time by 85%.     

The effect of hybridization on the ballistic impact behavior of hybrid composite armors

In the present study, effect of hybridization on the hybrid composite armors under ballistic impact is investigated using hydrocode simulations. The hybrid composite armor is constructed using various combinations and stacking sequences of fiber reinforced composites having woven form of fibers specifically high specific-modulus/high specific-strength Kevlar fiber (KF), tough, high strain-to-failure fiber Glass fiber (GF) and high strength/high stiffness Carbon fiber (CF). Different combinations of composite armors studied are KF layer in GF laminate, GF layer in KF laminate, KF layer in CF laminate and CF layer in KF laminate at various positions of hybridized layers for a fixed thickness of the target. In this article the results obtained from the finite element model are validated for the case of KF layer in a GF laminate with experimental predictions reported in the literature in terms of energy absorption and residual velocity and good agreement is observed. Further, the effect of stacking sequence, projectile geometry and target thickness on the ballistic limit velocity, energy absorbed by the target and the residual velocity are presented for different combinations of hybrid composite armors. The simulations show that, at a fixed thickness of the hybrid composite armor, stacking sequence of hybridized layer shows significant effect on the ballistic performance. The results also indicate energy absorption and ballistic limit velocity are sensitive to projectile geometry. Specifically, it is found that arranging the KF layer at the rear side, GF layer in the exterior and CF layer on the front side offers good ballistic impact resistance. The hybrid composite armor consisting of a CF layer in KF laminate acquires maximum impact resistance and is the best choice for the design compared to that of other combinations studied.     

A finite element model of shape memory polymer composite beams for space applications

Structures made of shape memory polymer composite (SMPC), due to their ability to be formed into a desired compact loading shape and then transformed back to their original aperture by means of an applied stimulus, are an ideal solution to deployment problems of large and lightweight space structures. In the literature, there is a wide array of work on constitutive laws and qualitative analyses of SMP materials; dynamic equations and numerical solution methods for SMPC structures have rarely been addressed. In this work, a macroscopic model for the shape fixation and shape recovery processes of SMPC structures and a finite element formulation for relevant numerical solutions are developed. To demonstrate basic concepts, a cantilever SMPC beam is used in the presentation. In the development, a quasi‐static beam model that combines geometric nonlinearity in beam deflection with a temperature‐dependent constitutive law of SMP material is obtained, which is followed by derivation of the dynamic equations of the SMPC beam. Furthermore, several finite element models are devised for numerical solutions, which include both beam and shell elements. Finally, in numerical simulation, the quasi‐static SMPC beam model is used to show the physical behaviors of the SMPC beam in shape fixation and shape recovery. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of impact damages on the compressive strength of composite laminates

Abstract

By simplifying the impact damages as a single delamination near the surface with an elliptical boundary, the approximate solution of total strain energy release rates can be derived as a function of delamination major axis, minor axis, external compressive strain, Possion's ratio of parent medium, extensional and bending stiffnesses of sublaminate. A linear relation of residual strength versus strain energy release rate can be constructed by correlating the approximate solution with test data of compressive residual strain (strength) after impact (CSAI), indicating that the dimension between the delamination major and minor axis should be dependent. In addition, the delamination aspect ratio is found to be not only a function of the specimen geometry and the extensional stiffness, but also a function of laminate thickness. The approximate solution provides a method for predicting the post impacted strength of composite laminate for only either thick laminate or thin laminate with low impact energy.     

The low velocity impact response of an aluminium honeycomb sandwich structure

The low velocity impact response of two aluminium honeycomb sandwich structures has been investigated by conducting drop-weight impact tests using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the glass fibre reinforced/epoxy skins and aluminium core was investigated through a series of flexure, shear and indentation tests. Here, it was found that the flexural modulus of the composite skins and the shear modulus of the aluminium honeycomb core did not exhibit any strain-rate sensitivity over the conditions investigated here. In addition, it was found that the indentation characteristics of this lightweight sandwich structure can be analysed using a Meyer indentation law, the parameters of which did not exhibit any sensitivity to crosshead displacement rate.

The impact response of the aluminium honeycomb sandwich structures was modelled using a simple energy-balance model which accounts for energy absorption in bending, shear and contact effects. Agreement between the energy-balance model and the experimental data was found to be good, particularly at low energies where damage was localised to the core material immediate to the point of impact. The energy balance was also used to identify energy partitioning during the impact event. Here, it was shown that the partition of the incident energy depends strongly on the geometry of the impacting projectile.     

SMAs复合材料层板大变形分析计算

在复合材料层板结构中铺设形状记忆合金丝,利用形状记忆合金弹性模量随相变状态的不同变化,受限回复时可以产生很大的回复力的特点,除改善和增强复合材料层板本身性能外,还能使复合材料层板产生弯曲,扭转等形状变化。如何正确有效地计算其力学特性,是复合材料层板形状控制以及设计和应用形状记忆合金复合材料智能结构的基础。本文在复合材料层板的有限元分析中考虑形状记忆合金丝的热力学特性以及大变形回复的影响,并通过实验进行分析和对比,得到一些有意义的结论。      

高导热石墨烯-碳纤维混杂增强热致形状记忆复合材料研究进展及发展趋势

热致形状记忆复合材料(SMPC)是一种能够对外界温度刺激做出响应的智能材料,与传统热致SMPC相比,高导热石墨烯(GR)-碳纤维(CF)混杂增强热致SMPC具有形状记忆性能优良、比强度高和导热性强等一系列优异性能,近年来受到人们广泛的关注并开展了相关研究。本文从形状记忆材料相关历史起源与应用入手,聚焦GR-CF混杂增强热致SMPC研究前沿问题,分别对该复合材料浸渗规律、成型工艺、形状记忆性能强化规律和弯曲失效规律四个方面的国内外研究现状进行了文献综述,并结合现有研究情况对其中出现的难题进行了探讨,最后指出了该热致SMPC未来有待深入研究的方向。      

On the experimental investigation of crash energy absorption in laminate splaying collapse mode of FRP tubular components

In this experimental work the crash energy absorption of fibre reinforced plastic (FRP) tubular components that collapse in laminate splaying mode is investigated by means of a new testing method, the “curling test”. This test method was used trying rectangular carbon, aramid and glass FRP strips—in which the reinforcing fibres were in the form of reinforcing woven fabric (carbon and aramid FRP specimens) and multi-axial fibre reinforcements (glass FRP specimens). Apart from the analysis of the system of bending and friction forces acting on the specimens during the curling tests in comparison with the forces acting in the case the laminate splaying collapse mode and the observations related to the deformation and crushing induced on the FRP specimens by this force combination, the analysis of the test results focused on the influence of the most important geometric and laminate material properties—such as thickness, flexural rigidity, number of reinforcing fibre layers, laminate stacking sequence and constituent material mechanical properties—on the specific energy absorption and the peak load.     

纤维增强形状记忆聚合物复合材料及其航天应用

形状记忆聚合物(SMP)是一种能够保持临时形状,并在外界刺激下自发回复到其初始形状的智能材料,具有高形状固定率、高形状回复率、转变温度可调、变形能力强、质量轻等优点,但其应用受到响应方式单一和承载能力差的限制,通过向聚合物中添加功能颗粒或增强纤维制成形状记忆聚合物复合材料(SMPC),可有效解决这一问题。首先介绍了SMP形状记忆效应的原理,然后阐述了纤维增强型SMPC有限变形过程中纤维的微屈曲行为。最后对可变形结构在航天领域的应用进行了论述。      

Invariant Based Transversely-Isotropic Material and Failure Model for Fiber-Reinforced Polymers

In this article, a constitutive formulation of a transversely-isotropic material and failure model for fiber-reinforced polymers is presented comprising pre-failure material nonlinearities, a novel invariant based quadratic failure criterion (IQC) as well as post failure material softening. The failure surface of the IQ criterion is assumed to take the influence of triaxiality on fracture into account. Further, a distinction between fiber failure and inter-fiber failure is conducted. Material softening is governed by a fracture energy formulation and the introduction of an internal length. The constitutive model is implemented into a programming user interface of the commercial finite element program Abaqus. As results, different laminate lay-ups are modelled and exposed to different stress states in an FE analysis. The obtained failure surfaces and stress strain curves for each laminate lay-up are compared to experimental data. As further applications of the material model presented, a curved composite beam, showing delamination, and a 0 /90 /0 -rod, showing the characteristic damage state in the 90 layer, are simulated and compared to tests.     

Glare层板疲劳性能研究综述

Glare层板是由铝合金和玻璃纤维增强复合材料组成的一种混杂结构材料,是一种新型的航空结构材料。Glare层板具有优异的抗裂纹扩展能力,广泛地应用于飞机较为关键的疲劳结构件上。目前,国外许多研究机构对Glare层板疲劳性能开展了大量的试验研究,综合分析了国外Glare层板的研究文献,对4所研究机构(荷兰Delft理工大学、美国MIT大学、美国California大学和日本Shiga Prefecture大学)开展的Glare层板疲劳性能试验研究进行了分析与总结,简述了不同类型Glare层板抗疲劳裂纹扩展的能力,并提出了今后进一步的研究方向及工作重点。     

Strength prediction in CFRP woven laminate bolted double-lap joints under quasi-static loading using XFEM

The current paper is concerned with modelling damage and fracture in woven fabric composite double-lap bolted joints that fail by net-tension. A 3-D finite element model is used, which incorporates bolt clamp-up, to model a range of CFRP bolted joints, which were also tested experimentally. The effects of laminate lay-up, joint geometry, hole size and bolt clamp-up torque were considered. An Extended Finite Element (XFEM) approach is used to simulate damage growth, with traction–separation parameters that are based on previously reported, independent experimental measurements for the strength and toughness of the woven fabric materials under investigation. Good agreement between the predicted and measured bearing stress at failure was obtained.     

Predicting mode-II delamination suppression in z-pinned laminates

A finite element model for predicting delamination resistance of z-pin reinforced laminates under the mode-II load condition is presented. End notched flexure specimen is simulated using a cohesive zone model. The main difference of this approach to previously published cohesive zone models is that the individual bridging force exerted by z-pin is governed by a specific traction-separation law derived from a unit-cell model of single pin failure process, which is independent of the fracture toughness of the unreinforced laminate. Therefore, two separate traction-separation laws are employed; one represents unreinforced laminate properties and the other for the enhanced delamination toughness owing to the pin bridging action. This approach can account for the so-called large scale bridging effect and avoid using concentrated pin forces in numerical models, thus removing the mesh-size dependency and permitting more accurate and reliable computational solutions.     

The impact resistance of polypropylene-based fibre–metal laminates

The high velocity impact response of a range of polypropylene-based fibre–metal laminate (FML) structures has been investigated. Initial tests were conducted on simple FML sandwich structures based on 2024-O and 2024-T3 aluminium alloy skins and a polypropylene fibre reinforced polypropylene (PP/PP) composite core. Here, it was shown that laminates based on the stronger 2024-T3 alloy offered a superior perforation resistance to those based on the 2024-O system. Tests were also conducted on multi-layered materials in which the composite plies were dispersed between more than two aluminium sheets. For a given target thickness, the multi-layered laminates offered a superior perforation resistance to the sandwich laminates. The perforation resistances of the various laminates investigated here were compared by determining the specific perforation energy (s.p.e.) of each system. Here, the sandwich FMLs based on the low density PP/PP core out-performed the multi-layer systems, offering s.p.e.’s roughly double that exhibited by a similar Kevlar-based laminate.A closer examination of the panels highlighted a number of failure mechanisms such as ductile tearing, delamination and fibre failure in the composite plies as well as permanent plastic deformation, thinning and shear fracture in the metal layers. Finally, the perforation threshold of all of the FML structures was predicted using the Reid–Wen perforation model. Here, it was found that the predictions offered by this simple model were in good agreement with the experimental data.     

Investigation into laminate design of open carbon–fibre/epoxy sections by quasi–static and dynamic crushing

The effect laminate design on the crush performance of carbon–fibre/epoxy “DLR” crush elements has been experimentally investigated. A quasi-isotropic lay-up was found to result in the highest Specific Energy Absorption (SEA) for Four-Harness (4HS) reinforced laminates; however a hybrid of unidirectional weave and 4HS fabric produced the highest SEA of 114 kJ/kg. Interleaving with thin thermoplastic films increased the steady state crushing force, however the increase in laminate density associated with the addition of the film caused no improvement and in some cases a reduction in SEA depending on material and lay-up. Dynamic crush testing of selected laminate designs resulted in a reduction in SEA of between 6% and 15% compared to the quasi–static case.     

雷击冲击力作用下复合材料层合板动力学响应及损伤特性

采用基于连续介质损伤理论（CDM）的复合材料三维渐进损伤分析模型,以ABAQUS有限元分析软件为平台,结合VUMAT子程序,对雷击冲击力作用下的复合材料层合板进行了三维动力学分析,研究了雷击冲击力作用下层合板的动力学响应及损伤特性。结果表明,在雷击冲击力作用下,层合板做降幅振荡运动,冲击力做功与层合板内能和动能相互转换,同时伴随着黏性耗散能,冲击力做功大小可用雷电流库伦量与作用积分的函数表示;层合板损伤由外力做功大小决定,对于同种材料,基体、纤维及分层损伤分别存在不同的损伤能量临界值,当冲击力做功大于该值,层合板会产生对应的损伤;在相同边界支持条件下,冲击力总功最大值决定了不同损伤类型损伤状态变量的大小,与波形参数和峰值电流无关。     

Experimental testing and phenomenological modelling of the fragmentation process of braided carbon/epoxy composite tubes under axial and oblique impact

A new simulation technique is presented for the phenomenological modelling of stable fragmentation in fibre reinforced composite structures under dynamic compressive loading. An explicit crash code is used for implementation of a hybrid modelling technique, in which two distinct material models act simultaneously. The first model is implemented in a multi-layered shell element and uses a unidirectional composites fracture criterion to predict potential ply fracture mechanisms on a macroscopic scale. This model is, however, unable to represent the complex localised fracture mechanisms that occur on a meso (sub-ply) scale under compression fragmentation loading. Therefore, a second constitutive model is added to capture the energy absorbing process within the fragmentation zone, utilising an Energy Absorbing Contact (EAC) formulation between the composite structure and the impacting body. The essential benefits of the procedure are that it requires minimal input data that can be obtained from simple fragmentation tests, and that the procedure is computationally efficient enabling application to large scale industrial structures. The EAC theory is discussed, together with the required material model parameters. A series of dynamic axial and oblique impact tests and simulations of cylindrical continuous carbon fibre reinforced composite tubes have been performed to validate the approach. Furthermore, the application to more complex load cases including combinations of fragmentation and global structural fracture have also shown a good correlation with test results.     

纤维缠绕复合材料壳体刚度衰减模型数值模拟

应用微分几何理论,推导出纤维缠绕复合材料壳体的非测地线缠绕轨迹、包角方程及绕丝头运动方程,得到缠绕过程的动态仿真模拟数据。将封头处变化的缠绕角、厚度等实际工艺参数直接用于壳体结构的理论分析。采用叠层的增量本构关系,模拟层合板壳结构的损伤过程,建立了损伤后刚度衰减模型及刚度退化准则,并通过实验确定了刚度衰减系数。应用此模型对纤维缠绕复合材料压力容器进行了数值分析。结果表明:纤维缠绕复合材料压力容器封头处损伤会导致其弯曲刚度降低,这是影响轴向变形的重要因素。