Thermal and mechanical fatigue analysis of CFRP laminates

[0°/90°]_s and [±45°]_s CFRP laminated plates were analysed using a finite element formulation for their fatigue behaviour. A fatigue criterion which is based on the laminate interlaminar stresses and the basic lamina fatigue parameters was used. Thermal effects were included in the formulation. In particular, initial thermal stresses resulting from the curing of the laminate were also included in the analysis. The results showed that both laminates had predicted S-N behaviour similar to that from experiments of past investigators. Also, the fatigue behaviour for the [±45°]_s laminate between room temperature and the curing temperature were found to be the same. However, in the case of the [0°/90°]_s laminate the fatigue strength at high temperatures was found to be lower than that at low temperatures.     

A damage identification technique for CF/EP composite laminates using distributed piezoelectric transducers

In this study, a damage identification approach was developed for carbon fibre/epoxy composite laminates with localized internal delamination. Propagation of the Lamb wave in laminates and its interaction with the delamination were examined. The fundamental symmetric Lamb wave mode, S₀, and the lowest order shear wave mode, S₀^′, were chosen to predict damage location. A real-time active diagnosis system was therefore established. This technique uses distributed piezoelectric transducers to generate and monitor the ultrasonic Lamb wave with narrowband frequency. The two-way switches were employed to minimize the number of transducers. A signal-processing scheme based on the time–frequency spectrographic analysis was utilised to extract useful diagnostic information. Also, an optimal identification method was applied on damage searching procedure to reduce errors and obtain the diagnostic results promptly. Experiments were conducted on [0/−45/45/90]_s CF/EP laminates to verify this diagnosis system. The results obtained show that satisfactory detection accuracy could be achieved.     

The effects of geometric parameters on the failure strength for pin-loaded multi-directional fiber-glass reinforced epoxy laminate

A numerical and experimental study was carried out to determine the failure of mechanically fastened fiber-reinforced laminated composite joints. E/glass–epoxy composites were manufactured to fabricate the specimens. Mechanical properties and strengths of the composite were obtained experimentally. Tests have been carried out on single pinned joints in [0/90/0]_s and [90/0/90]_s laminated composites. A parametric study considering geometries was performed to identify the failure characteristics of the pin-loaded laminated composite. Data obtained from pin-loaded laminate tests were compared with the ones calculated from a finite element model (PDNLPIN computer code). Damage accumulations in the laminates were evaluated by using Hashin's failure criteria combined with the proposed property degradation model. Based on the results, ply orientation and geometries of composites could be crucial for pinned laminated composite joints.     

Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, G_Ic of unstitched and stitched glass/polyester composites is investigated in this paper. The G_Ic of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/−30, 45/−45, 60/−60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/−30, 45/−45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch². The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on G_Ic is studied. Double cantilever beam (DCB) tests are carried out and the G_Ic is determined using modified beam theory. The G_Ic of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The G_Ic values of unstitched 45/−45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on G_Ic is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the G_Ic and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle.     

正交铺层PIP-SiC_(f)/SiC复合材料的水淬失效行为EI北大核心CSCD

铺层方式是影响预浸料铺贴工艺制备SiC_(f)/SiC复合材料抗热震性能的关键因素之一。以先驱体浸渍裂解(PIP)工艺制备的SiC_(f)/SiC复合材料为研究对象,基于内聚力模型,对不同正交铺层方式的方形试样开展1200℃高温水淬实验及有限元仿真研究。结果表明:水淬过程中试样表面温度急剧下降,芯部与表面存在较大温差,顶角与芯部温差最高可达1077℃,导致复合材料的主要失效模式为层间开裂和基体开裂,且试样层间开裂位置与铺层方式有关。由模拟结果可知,由于正交铺层方式的不同,试样的层间开裂位移也存在显著差异:[0/90]与[0/90/0]两种铺层方式层间主裂纹开裂位移约为0.61 mm,而[0/0/90/90]铺层主开裂位移仅为0.04 mm。此外,随着水淬时间的增加,层间主裂纹开裂位移逐渐增大,而次裂纹在开裂后逐渐发生闭合现象。     

复合材料开孔层板压缩渐进损伤试验

为研究碳纤维增强树脂基复合材料开孔层板在压缩加载过程中的损伤起始、演化方式和损伤特点,采用微距拍摄、逐级加载超声C扫描、X光扫描和扫描电子显微镜观测4种观测手段对国产CCF300/5228A[45/0/-45/90]4s、[452/02/-452/902]2s、[454/04/-454/904]s3种铺层方式的开孔层板进行了压缩试验研究。对压缩载荷作用下开孔层板的损伤起始和损伤演化进行了观察和对比。对试验中观测到的纤维微屈曲、纤维挤出、孔边开裂和分层扩展等现象之间的关系进行了分析和说明。试验结果表明:压缩载荷下45°和90°铺层相邻位置为层板易分层位置,含45°和90°铺层相邻位置的开孔层板渐进损伤过程较为明显:开孔层板在压缩载荷下较早出现损伤,损伤的起始和演化缓解了孔边应力集中,促使压缩应变能在孔边逐步释放,推迟开孔层板压缩破坏的发生,提高层板压缩承载能力。研究结果可为材料结构损伤容限设计提供依据。     

Failure of delaminated carbon/epoxy composite beams under cyclic compression

Experimental study of the response of laminated [(45/90/ — 45/0)₆]_s carbon/ epoxy virgin and pre-cracked composite beams to compressive cyclic loading is reported. The development of failure, in the form of delaminations, recorded continuously on video tape during the tests, revealed formation of delaminations close to the external faces of the beam. Some of the delaminations developed instantly and some propagated along the beam in jumps. Total failure occurred when the stress in the central ‘undamaged’ section of the beam reached a critical value. Calculation of the buckling load of unsymmetric laminated beams supports the experimental finding that a pre-crack close to the center of the specimen has no effect on the fatigue behavior of the beam. Prediction of time to first delamination crack, based on previous results, agree with the present experimental observation.     

聚酰亚胺树脂基MT300/KH420复合材料高温力学性能(Ⅱ)——弯曲性能

从宏、微观的角度研究了碳纤维增强聚酰亚胺树脂基MT300/KH420复合材料的高温力学性能,重点揭示了MT300/KH420复合材料[0°]14和[±45°/0°/90°/+45°/0°2]s层合板在常温~500℃的弯曲性能变化规律。研究表明:MT300/KH420复合材料高温力学性能优异,[0°]14层合板在420℃的弯曲强度保持在51%以上,弯曲模量在500℃以内变化很小。[0°]14层合板在常温下断口粗糙,且贯穿厚度,表现为脆性破坏;随温度升高,树脂流动性增强,呈现出黏弹效应,破坏逐渐集中在加载点处,在500℃,部分树脂热解,纤维束脱离基体并氧化。[±45°/0°/90°/+45°/0°2]s层合板高温弯曲性能较为稳定,主要破坏为上、下表面沿45°方向开裂,并伴有层间分离,在500℃出现严重分层破坏;相比于受基体控制的层合板弯曲性能,温度对受纤维控制的层合板弯曲性能影响较小。     

Numerical investigations of free edge effects in integrally stiffened layered composite panels

A linear finite element analysis is conducted to examine the free edge stresses and the displacement behavior of an integrally stiffened layered composite panel loaded under uniform inplane tension. Symmetric (+Φ, −Φ, 0, −Φ, +Φ) graphite-epoxy laminates with various fiber orientations in the off-axis plies are considered. The quadratic stress criterion, the Tsai-Wu criterion and the Mises equivalent stresses are used to determine a risk parameter for onset of delamination, first ply failure and matrix cracking in the neat resin. The results of the analysis show that the interlaminar stresses at the +Φ/−Φ and −Φ/0 interfaces increase rapidly in the skin-stringer transition. This behavior is observed at the free edge as well as at some distance from it. The magnitude of the interlaminar stresses in the skin-stringer transition is strongly influenced by the fiber orientations of the off-axis plies. In addition, the overall displacements depend on the magnitude of the off-axis ply angle. It is found that for Φ < 30° the deformations of the stiffener section are dominated by bending, whereas for 45° < Φ < 75° the deformations are dominated by torsion. The failure analysis shows that ply and matrix failure tend to occur prior to delamination for the considered configurations.     

Bearing strength of carbon epoxy laminates under static and dynamic loading

An experimental study has been carried out to investigate both the static and dynamic bearing strengths of a pinned-joint carbon epoxy composite plate with [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking configurations. The static and dynamic experiments have been carried out according to the ASTM D953 standards and ASTM STP 749, respectively [ASTM D 953-D, Standard Test method for Bearing Strength of Plastics, ASTM Designation. 342; Joining of Composite Materials, ASTM STP 749, American Society for Testing and Materials (1981) 131]. The ratio of the edge distance to the pin diameter (E/D), and that of the width to the pin diameter (W/D) of the specimens were varied to obtain the static bearing strength and the S–N fatigue curve. The experiments show that the static bearing strengths reach their upper limit when E/D and W/D ratios are equal to or greater than 4 for both [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking sequences. The fatigue strength, on the other hand, reduces by up to 65% as E/D and W/D ratios increase for both stacking configurations.     

Fatigue strength of woven fabric composites with drilled and moulded-in holes

The fatigue strength of glass-fiber woven-roving composites with a circular hole has been examined. Circular holes of two types, drilled and moulded-in, were considered. Experiments on the fatigue life and damage processes in (0, 90)_s and (45, −45)_s woven-roving composites with hole diameters of 5, 10, 14 and 19 mm were conducted. Experimental results show that laminates with moulded-in circular holes exhibit higher fatigue strength than those with drilled holes as a consequence of the altered mode of failure. Photographs of fatigue failure processes and specimen elongation with life are presented. The failure phenomena of laminates with drilled and moulded-in holes are compared and discussed.     

Vibration control of an active laminated beam

The dynamic response of a carbon fibre reinforced plastic [0/ + 45/ − 45/0]_s active laminated beam covered by piezoelectric layers is considered. A simple collocated displacement control strategy is implemented by letting the active layers work as actuators and sensors and by constructing an analogue control circuit the characteristics of which are given in some detail. The open- and closed-loop steady-state response of the cantilever beam are then experimentally tested in the range of 40–2000 Hz and the effectiveness of the control mechanism is successfully demonstrated.     

Growth of delamination cracks due to bending in a [905/05/905] laminate

A study of the growth of delamination due to bending in a [90₅/0₅/90₅] graphite/epoxy laminate is presented. A plane strain finite element analysis is used to determine the strain energy release rate during delamination in a three-point-bend specimen. A closed form expression for energy release rate is obtained from classical lamination theory and Timoshenko beam theory and is compared with the finite element analysis result. These results combined with the experimentally determined critical load, P_crit, are used to calculate the critical strain energy release rate, G_C. The critical energy release rate is also determined experimentally by the compliance method.     

Stress and strength analysis of pin joints in laminated anisotropic plates

Mechanical joints in composites can be tailored to achieve improved performance and better life by appropriately selecting the laminate parameters. In order to gain the best advantage of this possibility of tailoring the laminate, it is necessary to understand the influence of laminate parameters on the behaviour of joints in composites. Most of the earlier studies in this direction were based on simplified assumptions regarding load transfer at the pin-plate interface and such studies were only carried out on orthotropic and quasi-isotropic laminates. In the present study, a more rigorous analysis is carried out to study pin joints in laminates with anisotropic properties. Two types of laminates with (0/ + ₄/90)_s and (0/ ± ₂/90)_s layups made out of graphite epoxy T300/5208 material system are considered. The analysis mainly concentrates on clearance fit in which the pin is of smaller diameter compared to the hole. The main aspect of the analysis of pin joints is the changing contact between the pin and the plate with increasing load levels. The analysis is carried out by an iterative finite element technique and a computationally efficient routine is developed for this purpose. Numerical studies indicate that the location and magnitude of the peak stresses along the hole boundary are functions of fibre angle and the overall anisotropic properties. It is also shown that the conventional assumption of cosine distribution for the contact pressure between pin and the plate in the analysis lead to underestimation of bearing failure load and overestimation of shear and tensile failure loads in typical (0/90₅)_s cross-ply laminates.     

Effect of damage levels and curing stresses on delamination growth behaviour emanating from circular holes in laminated FRP composites

This paper deals with the study of the effect of drilling induced delamination damage levels and residual thermal stresses (developed during manufacturing process of cooling the laminate form curing temperature to room temperature) on delamination growth behaviour emanating form circular holes in graphite/epoxy laminated FRP composites. Two sets of full three dimensional finite element analyses (one with the residual thermal stresses developed while curing the laminate and the other without residual thermal stresses i.e. with mechanical loading only) have been performed to calculate the displacements and interlaminar stresses along the delaminated interfaces responsible for the delamination onset and propagation. Modified crack closure integral (MCCI) techniques based on the concepts of linear elastic fracture mechanics (LEFM) have been used to calculate the distribution of individual modes of strain energy release rates (SERR) to investigate the interlaminar delamination initiation and propagation characteristics. Asymmetric variations of SERR obtained along the delamination front are caused by the overlapping stress fields due to the coupling effect of thermal and mechanical loadings. It is found that parameters such as ply orientation, drilling induced damage levels and material heterogeneity at the delaminated interface dictate the interlaminar fracture behaviour of laminated FRP composites.     

典型螺栓连接CFRP薄壁C型柱轴压失效行为:试验及数值模拟

为了研究典型螺栓连接碳纤维增强树脂复合材料(CFRP)薄壁C型柱的轴压失效模式及吸能特性,进行了5组不同铺层方式C型柱的准静态轴压试验,即[0/90]_4s、[±45]_4s、[±45/90₂/0₄]_s、[±45/90/0₂/90/0₂]_s、[90/±45/0]_2s,获得其失效形貌及载荷-位移曲线。采用Lavadèze单层壳单元模型、Puck-Yamada失效准则、层间胶粘单元及螺栓模型,建立C型柱层合壳模型进行轴压仿真,并与试验失效形貌、载荷-位移曲线及吸能特性评估指标进行对比分析。结果表明:0°、±45°、90°纤维可以显著影响C型柱轴压失效模式及吸能特性。在轴压载荷下,±45°纤维铺设C型柱发生局部屈曲失效模式,吸能特性差。±45°纤维铺设在外部,0°和90°纤维交替铺设在内部的C型柱,其轴压失效过程平稳,吸能特性好。与C型柱轴压试验结果相比,层合壳模型获得的整体变形和局部失效形貌吻合较好,载荷-位移曲线变化趋势和吸能特性评价指标基本一致。研究结果对CFRP薄壁C型柱吸能设计具有一定的指导意义。      

Parametric optimisation of composite shell structures for an aircraft Krueger flap

This paper details the conceptual design optimisation of the configuration and composite lay-ups used to replace the conventional honeycomb stiffened structure of a Krueger flap. The multiple composite laminates selected for redesigning the lay-ups within an initial symmetrical quasi-isotropic ply configuration of [0/45/−45/90]_s, had to demonstrate full orthotropic characteristics. In order to construct a numerical process to optimise the required multi-layered composite shells, a commercial finite element code, Ansys, was used to develop a parametric analysis file. This analysis subroutine was then integrated into an Ansys Parametric Design Language code embedding the objective of the optimisation process––mass minimisation––as well as all the constraints and the allowable domains of the parameters. The paper, in its conclusion, presents a comparison between the original product and the optimal design, and reviews the advantages of the future implementation of this design.     

Prediction for delamination initiation around holes in symmetric laminates

The purpose of this paper is to present an analytical technique for predicting the delamination initiation load and delamination location for balanced symmetric laminates containing a hole. Basically, this approach includes two phases. First, the stress distribution around the hole region is calculated by the authors' recently developed efficient method. Second, the delamination initiation load and delamination location are predicted by using an average stress failure hypothesis together with the Hashin-Rotem delamination criterion. A computer program was developed to calculate the delamination initiation load and location for notched laminates with different ply orientations, different material properties and different stacking sequences. Numerical results for (θ/ – θ/0°)_s and (0°/90°)_s composite laminates generated by the current approach are compared with existing solutions. The results show that the present method is very efficient and useful for the purpose of engineering design.     

开孔碳纤维层合板层间应力分析

针对具有典型铺设角 的开孔碳纤维层合板, 采用三维有限元数值模拟方法, 分析了在单向拉伸载荷作用下孔边附近的层间应力, 讨论了界面层参数对层间应力的影响, 详细给出了典型铺设角之间层间应力的分布规律和最大层间应力产生的位置。结果表明: 对于相同铺设角的界面层, 沿厚度方向的位置影响层间应力的大小, 但不影响分布趋势; 而铺层顺序(如 或 )对层间应力的大小和分布趋势影响则较小。最大层间正应力产生于 的界面层, 位于与拉伸方向成90°的位置, 是外加拉伸应力的51%; 最大层间剪应力产生于 的界面层, 最大层间环向剪应力位于与拉伸方向成74°的位置, 是外加拉伸应力的64%; 而最大层间径向剪应力位于与拉伸方向成66°的位置, 是外加拉伸应力的25%。      

Matrix cracking behavior of K3B/IM7 composite laminates subject to static and fatigue loading

The matrix cracking behavior of a new high-performance thermoplastic composite material, K3B/IM7, was systematically investigated. Laminates in various grouped thickness and ply stacking sequences, [0₂/90₂/0₂], [0₂/90₄/0₂], and a quasi-isotropic laminate [+45/0/−45/90]_s were tested under static and tension–tension fatigue loading. Depending on the stacking sequence of the laminates and the type of loading, various matrix cracking behavior were found. Under static loading, the matrix cracks were mainly close to the specimen edges. A few cracks were found to penetrate the specimen width, even when the load was large enough to break the specimen. However, under fatigue cyclic load, the edge initiated cracks propagated fully across the specimen width. Combined with the fatigue Paris Rule and considering the ply thickness and stacking sequence, the energy release rate method was applied to predict the relations between the loading strain amplitude and fatigue cycles for matrix cracking failure.