A new structural health monitoring system for composite laminates

The increasing use of composite laminates in safety critical structures has prompted the development of a robust structural health monitoring system for laminates, which uses metastable ferrous alloy inserts embedded within the laminate during component construction to provide an indication of the peak tensile strain encountered by the laminate. The metastable ferrous alloy insert has an austenitic crystal structure at room temperature, but upon application of strain, this transforms to a thermodynamically stable martensite, resulting in a change in magnetic susceptibility, which can be correlated with the peak strain experienced by the material (strain memory effect). This paper presents the test results that show that it is possible to manufacture a smart laminate in this fashion, and that sufficient strain is experienced by the insert to provide a significant change in magnetic susceptibility, thereby warning of a high strain level in the laminate. Various insert geometries and laminate thicknesses are also tested for their effect on the susceptibility measurements.     

粘弹性基体复合材料层合板的固化残余应力和曲率变化

粘弹性基体复合材料层合板因铺层取向差异而产生的固化残余应力会随层合板在室温下放置时间的推移而变化, 这一过程可通过非对称层合板的曲率变化得到反映。本研究通过非对称板的曲率变化实验来观察板内残余应力的变化过程, 并根据细观复合材料力学理论和适宜的变形分析方法建立一种与残余应力最终稳定分布相对应的铺层修正折减模量矩阵, 用于进行层合板固化残余应力和曲率变化的渐近值预测, 并依此确定固化后残余应力的变化范围。     

Induction heating of continuous carbon-fibre-reinforced thermoplastics

This paper addresses the experimental investigation of induction heating of continuous carbon-fibre reinforced thermoplastics. The influence of the process parameters electromagnetic frequency, generator power, distance between induction coil and laminate, coil geometry and laminate lay-up on the heating rate and the heat distribution have been investigated in stationary experiments. It was found that all investigated parameters have significant influence on the heating behaviour and that a quadratic dependence is dominating. Heat is only generated when closed fibre loops exist, through which current can flow. The quality of the fibre junctions in a laminate, especially the contact length, was found to be of major importance. Thus, for example laminates with unidirectional fibre reinforcement, which do not contain fibre junctions, cannot be heated. Experimental evidence has shown that induction heating of carbon-fibre-reinforced thermoplastics is based on Joule losses.     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

Numerical and Experimental Study on the Effect of Lay-Up Type and Structural Elements on Thickness Uniformity of L-Shaped Laminates

Based on the two-dimensional resin flow and fiber compaction model developed by our group, we studied the cured laminate thickness uniformity of the L-shaped CF/BMI resin laminates and the effects of lay-up type and structural elements on it. Both the simulated and experimental data showed that the quasi-isotropic laminate thickness was more uniform than that of the [90°]_n laminates and the cured thickness of laminates molded by rigid convex tool was more uniform than that molded by the rigid concave tool. Lay-up type has a great influence on the cured laminate thickness uniformity. For the quasi-isotropic laminates, the structural elements, such as curvature radius, flat part length, and the number of plies, did not have much influence on the cured laminate thickness uniformity in the studied scope. For the [90°]_n laminates, the corner radius has a larger effect on the corner consolidation in comparison with the flat part length and the number of plies. According to the simulated results, resin pressure and consolidation time were largely affected by the lay-up type, due to the different permeability and compressibility. The rich resin defect was observed in the metallographic photos of the corner region of the [90°]_n laminates fabricated with the rigid concave tool, which demonstrated that the resin flow in the laminates played an important rule and validated the numerical prediction. Good agreement between the simulated results and experimental data demonstrated the reliability and universality of the numerical simulation method. These results are greatly helpful for the control of defects in angle-bended laminates and the optimization of cure cycle in autoclave process.     

Experimental study of damage propagation in Over-height Compact Tension tests

This paper describes an experimental investigation into progressive damage development in notched fibre-reinforced composites laminates. The Over-height Compact Tension (OCT) test captures the behaviour of laminates typical of large structures and permits the stable formation of a process zone ahead of the crack tip. This allows a study of the influence of sub-critical damage on progression of fibre failure. A range of lay-ups have been tested using dispersed and blocked plies in the thickness direction. The load vs. pin opening displacement (POD) curve is used to characterise the progressive failure of specimens. A number of interrupted tests were performed for each lay-up to capture the sub-critical damage process before the onset of fibre fracture. Results show that dispersed plies promote fibre failure and crack growth whilst blocked plies promote a larger amount of splitting and delamination which in turn causes a larger process zone and ultimately a tougher laminate.     

Energy-based prediction of failure in general symmetric laminates

This paper describes a new homogenisation technique for general symmetric laminates that enables progressive ply crack formation to be predicted in any number of plies having a variety of orientations. The approach involves (i) the analysis of non-uniformly spaced discrete ply cracks having a single orientation, (ii) a novel technique to homogenise the properties of the cracked ply so that discrete ply cracking can be analysed in plies having a different orientation, and (iii) the use of energy based methods to predict the progressive formation of ply cracks in any number of plies during loading. The analysis takes full account of the effects of thermally induced residual stresses. A key feature of the approach is the inclusion of a shear coupling term in the stress-strain relations for homogenised plies that ensures that the homogenised laminate has exactly the same effective properties as the laminate having a ply with discrete cracks in place of one of the homogenised plies. The model is applied to the prediction of the significant dependence of ply thickness and ply lay-up on laminate strength, and results for carbon fibre reinforced plastic laminates are compared favourably with published experimental results.     

Compression properties of z-pinned composite laminates

The effect of z-pinning on the in-plane compression properties and failure mechanisms of polymer laminates is experimentally studied in this paper. The reduction to the compression modulus, strength and fatigue performance of carbon/epoxy laminates with increasing volume content and diameter of pins is determined. The elastic modulus decreases at a quasi-linear rate with increasing pin content and pin diameter. Softening is caused by fiber waviness around the pins and reduced fiber volume content due to volumetric swelling of the laminate from the pins. A simple model is presented for calculating the compression modulus of pinned laminates that considers the softening effects of fiber waviness and fiber dilution. The compression strength and fatigue life also decrease with increasing volume content and diameter of the pins. The strength and fatigue properties are reduced by fiber kinking caused by fiber waviness around the pins and the reduced fiber content caused by swelling. The deterioration to the compression properties is also dependent on the fiber lay-up pattern of the laminate, with the magnitude of the loss in properties increasing with the percentage of 0° (load bearing) fibers in the laminate. The paper gives suggestions for minimizing the loss to the compression properties to laminates due to pinning.     

Repeated impact response of hand lay-up and vacuum infusion thick glass reinforced laminates

Vacuum infusion (VI) is being considered as a viable alternative to more traditional hand lay-up (HL). Main reason in favor of the more costly technique is the cleaner and friendlier work environment. Moreover, VI potentially offers another important benefit over HL in that prepreg levels of resin may be achieved, resulting in stronger and lighter laminates. The present paper compares the two manufacturing techniques on the basis of the response to repeated impact loading. The laminate is a thick non-symmetric glass-fiber-reinforced plastics intended for nautical application. Four impact velocities (1.5, 2.2, 3.1 and 3.8 m/s) were considered, and a minimum of four specimens for any given velocity were subjected to 40 repeated impacts or up to perforation. The impact response was evaluated in terms of damage progression by visual observation of the impacted specimens, evolution of the peak force and of the bending stiffness with the number of impacts and by calculating the Damage Index (DI), a damage variable recently proposed by the authors to monitor the penetration process in thick laminates. Results point out that for impact velocities for which no perforation occurs within test duration, the experimental data essentially overlap. On the contrary, for perforation tests, HL specimens survived more impacts before perforating absorbing more total energy than VI specimens. Plots of the DI variable against the number of impacts were observed to exhibit an initial linear portion, owing to a stable process of damage accumulation within the laminate, and to undergo an unstable growth a few impacts before perforation. When comparing the VI and HL specimens it was observed that, given an impact energy, the level of damage at first impact as well as the rate of stable damage accumulation is alike for the two sets of specimens. On the contrary, it is the number of impacts of the stable damage accumulation region which is lower for VI specimens.     

十字叠层板横向剪切刚度的计算方法

本文研究了十字叠层板横向剪切刚度系数的计算方法.指出前人采用的横向剪应力为抛物线分布的假设不够准确,使C_ii的计算值产生误差.本文首先推导了横向剪应力沿厚度分布的计算公式,然后用幂级数拟合了近似权函数,用权函数法和能量法对两种铺层结构的碳/环氧十字叠层板的C_ii进行了计算.对比了两种方法的结果,并与文献[1]的方法作了对比.结果表明,对于对称层板本文权函数法与文献[1]的结果基本一致;而对于非对称叠层板,二者差别明显.能量法结果与权函数法对比,前者更精确.     

Novel test method for accurate characterization of intralaminar fracture toughness in CFRP laminates

A novel initial crack insertion method, “intralaminar film insertion method”, was proposed to investigate the fracture toughness of unidirectional carbon fiber reinforced plastic (CFRP) laminates when the crack propagates inside the ply and not in the interlayer resin-rich area. Here, a release film was inserted inside a single lamina during the resin impregnation process of prepreg manufacturing. Mode I intralaminar fracture toughness tests were carried out for conventional CFRP laminates and interlayer toughened CFRP laminates. For comparison, two conventional methods were used to introduce initial cracks. One is the “interlaminar film method”, where a release film is inserted between two prepreg plies during the lay-up process. The other is the “machined slit method”, where a slit notch is machined in parallel to the layer of CFRP laminates. It was demonstrated that the proposed “intralaminar film method” can correctly evaluate the intralaminar fracture toughness of both conventional CFRP laminate and interlayer toughened CFRP laminate from the initial value to the propagation value. For this range, it was also found that the intralaminar fracture toughness of interlayer toughened CFRP laminate was the same as that of conventional CFRP laminate. Thus, the intralaminar fracture toughness was not influenced by interlayer toughening.     

Schädigungsentwicklung bei Ermüdung verschiedener CFK-Laminate

Fatigue Damage Development of Various CFRP-Laminates The behaviour of a woven carbon-fibre reinforced laminate in a balanced eight-shaft satin weave style was compared to non-woven laminates with an equivalent cross-ply lay-up (50% of the fibres in the 0° and 50% in the 90° direction) Two types of non-woven laminates were investigated consisting of continuous fibres and aligned discontinuous fibres, both produced from carbon fibre prepregs. The static strength and the fatigue behaviour was measured. Stiffness reduction was monitored during fatigue loading as a damage analogue to which the mechanisms of damage could be associated. Similarities and differences in the fatigue behaviour and damage development of the three laminates will be discussed.     

Tool–part interaction in composites processing. Part I: experimental investigation and analytical model

The ability to process composite structures with a high degree of dimensional control remains a barrier to further implementation of composite materials in commercial applications. Of the numerous types of process induced deformations that occur, the warpage of flat laminates due to tool–part interaction remains a poorly understood phenomenon. This paper presents an experimental study of the effect of process conditions and part aspect ratio on tool–part interaction induced warpage. For a given lay-up and material, part aspect ratio was found to have a much greater influence than autoclave pressure on warpage, while the tool surface condition was not observed to have any significant effect. The results of the study are embodied in an empirical relation, which can be a useful guide to predict laminate warpage over a range of industrially relevant conditions. In addition, a simple analytical model is proposed which agrees well with the experimentally determined relationships. A complementary numerical model is presented in a companion paper.     

Design of multistable composites for application in adaptive structures

During the manufacturing process and operating conditions of multilayered fibre-reinforced composites with variable fibre orientations, residual stresses build up due to the directional expansion of the unidirectionally reinforced single layers. Dependent on the laminate lay-up, these inhomogeneous residual stresses, which are primarily caused by thermal effects, moisture absorption and chemical shrinkage, can lead to large multistable out-of-plane deformations in the case of unsymmetric laminates. Instead of avoiding these laminate's curvatures, they can be advantageously used for technical applications such as novel adaptive structures. In order to adjust the laminate deformations to technical requirements, a dimensioning tool based on a modified stability analysis in combination with a novel optimisation procedure has been developed and experimentally verified. Based on the theoretical investigations, an adaptive prototype of a multistable composite with integrated smart alloys has been designed and manufactured.     

非对称/非均衡复合材料层压板弯曲与扭转耦合变形试验方法

对几种铺层的非对称/非均衡层压板在纯弯载荷下的耦合变形进行了模拟分析, 并进行了板宽、 板厚等参数的影响分析, 在此基础上设计了一套四点弯曲柔性支持装置。采用该装置对两种铺层的非对称/非均衡层压板进行了柔性四点弯曲试验, 并与常规四点弯曲试验进行了对比。柔性四点弯曲装置对非对称/非均衡层压板施加弯曲载荷的同时没有限制试件的扭转变形, 而常规四点弯曲几乎完全限制了非对称/非均衡层压板的扭转变形。柔性四点弯曲加载装置基本上达到了设计要求, 可用于非对称/非均衡层压板弯-扭耦合系数的测试。      

Macroscale assessment of low-velocity impact on hybrid composite laminates

Composites are usually brittle materials and have low impact properties. Structural dimensions, stacking sequence, ply materials, ply thicknesses and ply angles are standard variables that influence composite‘s performance against impact loads. Stacking sequence in hybrid laminates affects the failure and impact resistance. Failure mechanisms at the low-velocity impact of a rigid object in hybrid laminates are complex, and the subsurface damage in a composite laminate cannot be detected directly. However, various simulation platforms make it easy to see the impact damage between the plies of laminate. This paper numerically investigated the effect of stack sequence and hybridization of two fiber types against low-velocity impact. The current study adopted four-layer composite laminates of carbon and glass fiber layers with a stacking plan [C/C/C/C], [C/G/C/G] and [G/C/G/C], having lay-up angles as [0°/45°/−45°/90°]. Keeping the impactor mass and the incident velocity constant, the laminates were subjected to low-velocity impact. The damage contours for a failure mode were recorded and compared at the ply level. The numerical study resulted in impact imitations showing comparisons of the damage contours using Hashin failure criteria. Hybrid laminates display better performance in absorbing impact energies; however, hybrid laminates experienced more subsurface damage due to more impact energy absorption.     

Forming induced wrinkling of composite laminates: A numerical study on wrinkling mechanisms

When manufacturing composite aircraft components consisting of uni-directional prepreg laminates, Hot Drape Forming (HDF) is sometimes used. One issue with HDF is that, in contrast to hand lay-up where normally only one ply is laid up at a time, multiple plies are formed together. This limits the in-plane deformability of the stack, thus increasing the risk of out-of-plane wrinkling during forming. In this paper mechanisms responsible for creating different types of wrinkles are explained. It is shown through simulations how the wrinkles are created as a result of interaction between two layers with specific fibre directions or due to compression of the entire stack. The simulations are compared to experimental results with good agreement.     

复合材料等厚层板热压成型中树脂流动过程数值模拟

树脂基复合材料热压成型过程中树脂流动在很大程度上决定着层板纤维含量、 孔隙含量以及层板尺寸 , 根据有效应力原理与达西渗流定律建立了描述复合材料等厚层板热压成型过程树脂流动与纤维密实的数学模型 , 采用有限单元方法实现了热压成型中纤维密实均匀状况的预报。分析了温度边界条件、 铺层方式对树脂流动过程的影响。结果表明: 温度边界条件对计算结果影响比较大 ; 铺层方式对层板厚度以及纤维体积分数分布规律影响非常大 ; 边界条件以及材料参数的准确性直接影响计算结果的可靠性。以 T700S/环氧 5228单向层板为例进行了实验验证 , 结果表明计算与实验结果的一致性非常好。      

On the use of Levy's method for symmetrically laminated composite plates

For rectangular plates with two opposite edges simply supported and no bending twisting coupling, Levy's method can yield exact solutions for static deflections, free vibrations and buckling loads. However, three distinct solutions are obtained, depending on the relative magnitude of the plate rigidities, which in turn are functions of material properties, laminate thickness and lay-up. In this communication a stiffness invariant formulation is used to identify those laminates which fall under the three cases defined.     

Modelling complex progressive failure in notched composite laminates with varying sizes and stacking sequences

The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.