Querrißbildung in 0/90/0 CFK-Laminaten

Cross-ply Cracking in 0/90/0 CFRP Laminates Cross-ply cracking in carbon fibre reinforced plastics (CFRP) with thermoset as well as thermoplastic matrix systems was investigated on 0/90/0 laminates with varying 90°-ply thickness. Since in an angle ply laminate the 90°-ply fails first, the higher strength of the other plies cannot be taken advantage of. For this reason efforts have to be done to increase the transverse strength (strain) of fibre reinforced plastics. In the first place thus it is necessary to investigate the influence of the different parameters which contribute to the transverse strength. In this work the influence of matrix (fracture strain), fibre/matrix interface, voids and constraining effect of neighbouring plies is investigated. With the aid of two-parameter Weibull distributions of the 90°-ply fracture strain, which describe the phenomenon of multiple cracking in a specimen, it was found that the constraining effect due to neighbouring plies, improved fibre/matrix interface and matrix ductilty increase, whereas voids decrease the transverse fracture strain.     

Influence of fibre length and concentration on the properties of glass fibre-reinforced polypropylene: Part 3. Strength and strain at failure

In this report we present the results from the third part of a study on the influence of fibre length (0.1–50 mm) and concentration (3–60% w/w) on the properties of glass fibre-reinforced polypropylene laminates. These laminates were prepared in the laboratory using a wet deposition method and compared with samples prepared on a commercial melt impregnation GMT line. We found that laminate tensile strength increased linearly with fibre concentration up to 60% w/w. Laminate strength was also found to increase with increasing fibre length. At high values of fibre length (> 3–6 mm) the strength reached a plateau level which was directly dependent on fibre content. The matrix molecular weight appeared to have little direct influence on the level of laminate strength. However, the glass fibre sizing compatibility was found to have a strong effect on the tensile strength of both laboratory made wet deposited laminates and commercially prepared GMTs. The tensile strength of the GMT samples also showed a clear correlation with the measured fibre strength. A modified version of the Kelly-Tyson model gave calculated values of laminate strength which correlated well with the experimental data. We propose that the tensile strength of these laminates is governed by the properties of the fibres which have an orientation close to parallel with the loading direction.     

Compression moulding of glass and polypropylene composites for optimised macro- and micro- mechanical properties-1 commingled glass and polypropylene

The continuing desire in the automotive industry to reduce cost and weight while increasing safety requires innovative materials and processing routes. Glass-mat-reinforced thermoplastics have been used to produce semi-structural components but a higher and aligned glass fibre content is required in moulding materials for structural applications. Experimental design was used to investigate the non-isothermal processing of commingled fabrics which were woven from yarns of intimately mingled glass and polypropylene fibres. Processing models were generated by regression techniques to predict laminate properties over a range of processing conditions. Void contents were measured by image analysis techniques. Preheat temperature had the greatest effect on laminate flexural properties and porosity. A compaction time of 54 s was required to consolidate, cool and reduce the void content in laminates. A two-fold increase in stiffness was found compared with equivalent glass-mat-reinforced thermoplastic laminates. The intimate distribution of matrix and reinforcement reduced moulding pressures by a factor of 10.     

Determining a representative volume element capturing the morphology of fibre reinforced polymer composites

Production parameters have been found to influence the morphology of microstructure and damage behaviour in composite materials. To model the effects of this microstructural inhomogeneity, it is necessary, for practical purposes, to consider a limited, representative microstructure. A method to find a representative volume element (RVE) is investigated in which the Kolmogorov goodness-of-fit test is applied to a spatial distribution statistic, local population density, which is defined as a ratio between the fibre diameter and the area of that fibre’s enclosing Voronoi polygon. The method is applied to actual spatial distribution data recorded for a crossply composite laminate manufactured from 3M scotchply 1003 prepreg. For the composite laminate studied, a representative volume element size is determined through applying the Kolmogorov goodness-of-fit test at significance levels 0.05 and 0.10, resulting in sizes of approximately 300 × 300 μm.     

Compressive failure of finite size unidirectional composite laminates with a region of fibre waviness

The compressive behaviour of finite unidirectional composites with a region of misaligned reinforcement is investigated via finite element analyses. Models with and without fibre bending stiffness are compared, confirming that compressive strength is accurately predicted without modelling fibre bending stiffness for real composite components which typically have waviness defects of several millimetres wavelength. Various defect parameters are investigated. Results confirm the well-known sensitivity of compressive strength to misalignment angle, and also show that compressive strength falls rapidly with the proportion of laminate width covered by the wavy region. A simple empirical equation is proposed to model the effect of a single patch of waviness in finite specimens. Other parameters such as length and position of the wavy region are found to have a smaller effect on compressive strength. The modelling approach is finally adapted to model distributed waviness and thus determine the compressive strength of composites with realistic waviness defects.     

Mechanical Properties of Natural-Fibre-Mat- Reinforced Thermoplastics based on Flax Fibres and Polypropylene

Thermoplastic composites based on flax fibres and a polypropylene (PP) matrix were manufactured using (i) a film-stacking method based on random fibre mats and (ii) a paper making process based on chopped fibres. The influence of fibre length and fibre content on stiffness, strength and impact strength of these so-called natural-fibre-mat-reinforced thermoplastics (NMTs) is reported and compared with data for glass-mat-reinforced thermoplastics (GMTs), including the influence of the use of maleic-anhydride grafted PP for improved interfacial adhesion. In addition some preliminary data on the influence of fibre diameter on composite stiffness and strength is reported. The data is compared with the existing micro-mechanical models for strength and stiffness. A good agreement was found between theory and experiment in case of stiffness whereas in the case of strength the experimental values fall well below the theoretical predictions. Results indicated that NMTs are of interest for low-cost engineering applications and can compete with commercial GMTs when a high stiffness per unit weight is desirable. Results also indicated that future research towards significant improvements in tensile and impact strength of these types of composites should focus on the optimisation of fibre strength rather than interfacial bond strength.     

Mechanical Properties of Natural Fibre Mat Reinforced Thermoplastic

The use of natural fibres instead of man made fibres, as reinforcements in thermoplastics, gives interesting alternatives for production of low cost and ecologically friendly composites. In this work different commercially available semi-finished natural fibre mat reinforced thermoplastics (NMT) composites have been studied. Mechanical properties and microstructure of different NMT composites were investigated and compared to conventional GMT (glass fibre mat reinforced thermoplastic) composites and pure polypropylene (PP). The study included also NMT composites manufacturing processing parameters as processing temperatures and pressure during compression moulding. The results showed that NMT composites have a high stiffness compared to pure polymer and the NMT with a high fibre content (50% by weight) showed even better stiffness than the GMT. The GMT composites had superior strength and impact properties compared to the NMT which might be due to the relatively low strength of the natural fibres but also to poor adhesion to the PP matrix. The NMT materials showed a large dependence on direction and are therefore believed to have more fibres oriented in one direction. The stronger direction (0°) of the NMT was in some cases as much as 45% better than the 90° direction.     

Development of kenaf fibre reinforced polymer laminate for shear strengthening of reinforced concrete beam

Fabrication of green fibre composite laminate for strengthening of reinforced concrete structure is one of the current interests in the field of construction industry. The aim of this research was to develop kenaf fibre reinforced polymer (KFRP) laminate for shear strengthening of reinforced concrete beam. Comprehensive design and theoretical models were also proposed for KFRP laminate shear strengthened beam. In the experimental programme, KFRP laminate had been fabricated with various fibre content to obtain optimal mix ratio. Physical and mechanical properties of KFRP laminates were experimentally investigated. Three reinforced concrete beam specimens were prepared for structural investigations. Results showed that KFRP laminate with maximum fibre content had the highest tensile strength and the laminate was found to be elastic isotropic in nature. The KFRP laminate strengthened beam had 100 % higher shear crack load and 33 % ultimate failure load as compared to un-strengthened control beam. It reduced the numbers and width of cracks and had shown strain compatibility behavior with shear reinforcement. The failure load, ductility, crack patterns and strain characteristics of KFRP laminate strengthened beam were found to be closely comparable with CFRP laminate strengthened beam. The experimental results satisfactorily verified the proposed design and theoretical models.     

Effect of thickness on the compressive performance of ballistically impacted carbon fibre reinforced plastic (CFRP) laminates

The response of structural elements under impact conditions is a particularly important consideration in the design of components made from composite materials. The understanding of this response includes both the impact behaviour and the influence of some design parameters and material properties. Thus, the dependence of the residual compressive strength of ballistically impacted carbon fibre reinforced plastic (CFRP) laminates on their thickness has been examined. A previously verified model developed by the authors, has been applied resulting in rather interesting findings about the effect of the thickness on the sensitivity of a laminate to impact. The model takes into account the number of plies, the impact energy and the stacking sequence. Experimental results derived from the literature have been used for the verification of the model and a close agreement between theoretical predictions and experimental results was found. Also, it can be concluded that the present work helps to optimize laminate impact behaviour by varying the laminate thickness. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

CFRP感应加热线圈中心区域温度场

感应加热技术是实现碳纤维增强树脂复合材料(CFRP)低能耗高效固化成型的有效方法,提高CFRP感应加热温度场均匀性是保证成型质量的关键,而线圈中心区域温度场均匀性是保证材料整体温度均匀性的关键。根据电磁加热原理建立了CFRP有限元多场耦合的分析模型,通过对模拟计算和实验过程的温度场升温及分布情况的对比分析,证明了本仿真可以准确模拟CFRP感应加热温度场分布。根据图像的熵值理论将温度场均匀性通过熵值大小进行表示,实现了CFRP感应加热温度场均匀性的量化分析,并通过有限元模型计算研究了线圈直径及线圈与材料间距对线圈中心区域温度场均匀性的影响,得到了中心区域温度场均匀性与线圈直径及材料间距之间的关系曲线,为组合式线圈均匀加热CFRP提供了线圈直径及材料间距大小选择的理论依据。      

Strength and failure modes in resistance welded thermoplastic composite joints: Effect of fibre–matrix adhesion and fibre orientation

The strength and failure modes of resistance welded thermoplastic composites were investigated. Special attention was paid to the effect of basic characteristics of the adherends such as fibre–matrix adhesion and fibre orientation. 8HS woven GF/PEI composites were resistance welded. Intralaminar failure was found to be the major failure mechanism for the well welded joints, consisting of either fibre–matrix debonding or laminate tearing. An improved fibre–matrix adhesion was found to result in significantly higher lap shear strength. Besides, the main apparent orientation of the fibres on the welding surfaces was found to have an effect on the strength of the joints.     

A parametric study of material damping in fibre-reinforced plastics

In this paper, the important parameters causing energy dissipation in unidirectional glass fibre-reinforced epoxy laminae are investigated. Three parameters are considered: matrix and fibre damping and the effect of the interface. An energy balance approach is used to find closed-form relationships between material properties and design parameters. This approach shows that the fibre or matrix contribution to damping lies in the partition of elastic strain energy, which is highly dependent on the elastic properties of the fibres and matrix. A three-phase model is developed which clearly shows that a poor quality interface, with low elastic stiffnesses, has a significant effect on the energy damping capacity of the unidirectional laminate. However, in the case of a normal quality interface, more sensitive and accurate damping measurement methods are needed to identify the contribution of interface damping to the global laminate damping behaviour.     

Failure modes of fibre reinforced composites: The effects of strain rate and fibre content

The many aspects of high speed response of fibre reinforced composite materials have received the attention of a large number of investigators. Nevertheless, the understanding of the mechanisms governing failure under high speed loadings remain largely unknown. The effect of rate and fibre content on failure mechanisms was investigated by viewing fractured surfaces of tensile specimens using a scanning electron microscope (SEM). Tensile tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. A second laminate (with random continuous glass reinforcement) was tested in tension at varying fibre volume fractions in order to ascertain the relationship between fibre content and failure mechanisms. The results suggest a brittle tensile failure in fibres of the woven laminate. In addition, the matrix was observed to play a greater role in the failure process as speed was increased, resulting in increased matrix damage and bunch fibre pull-out. The results also indicated that increasing the fibre volume fraction increased the likelihood of a matrix dominated failure mode.     

Characterization of thermally oxidized carbon fibre surfaces by ESCA,wetting techniques and scanning probe microscopy,and the interaction with polyethylmethacrylate. Development of a biocompatible composite material

The development of biocompatible weft knitted carbon fibre-reinforced thermoplastics needs optimization of each composite component: fibre, matrix and interface. The material investigated was a composite of polyethylmethacrylate reinforced with a knitted and sized T300 carbon fibre. After chemical removal of the fibre sizing, the fibres were thermally oxidized at temperatures between 400 and 600°C. Angle-resolved photoelectron spectroscopy (ESCA) and Wilhelmy surface energy measurements have been used to describe the modification of the surface chemistry by thermal oxidation. The surface morphology, visualized by scanning probe microscopy and scanning electron microscopy, indicates an increased surface roughness. The interaction between fibre and matrix was investigated by observing the microscopic wetting behaviour of the thermoplastic at sinter temperature by the solid-body wetting technique. It is found that the strength and failure mechanisms of the knitted fibre-reinforced composite are determined by the interface properties.     

Noise in a point-contact dc SQUID

Measurements of five variations of a toroidal point-contact dc superconducting quantum interference device (SQUID) are presented. The energy resolution and other parameters of these SQUIDs are examined and compared with the predictions of the Resistively Shunted Junction model. For these SQUIDs, the measured minimum energy resolution was approximately 2×10^–30 J/Hz. Excess noise in the point contacts was found to limit the energy resolution of the SQUIDs. A comparison between the typical junction parameters and noise obtained for our niobium-niobium point contacts and those of others is given. TheI–V characteristics of the junctions showed the effects of Joule heating. The white voltage noise spectral density was found to have an approximately parabolic dependence on the average voltage for bias currents larger than the critical current. While this parabolic dependence is consistent with heating effects in the junctions, the amplitude of the noise cannot be explained in terms of a heating model. The low-frequency noise of the point contacts has also been investigated.     

Fatigue crack growth and delamination behaviours of advanced Al‐Li alloy laminate under single tensile overload

In this paper, fatigue crack growth and delamination behaviours of a new fibre metal laminate (FML) named as Al‐Li alloy laminate were tested under different single tensile overloads and compared with those of glass laminate aluminium reinforced epoxy. The results indicate that the crack growth rate of Al‐Li alloy laminate after overload applied can quickly get back to its original level when the crack grows outside of the overload plastic zone. The overload has no influence on the delamination shape and size of Al‐Li alloy laminate. These results are obviously different from those found in the present study for GLARE, in which the crack growth rate cannot recover after overload, even though the crack is far beyond the overload plastic zone. A kink nearby the location of overload applied was found in the obtained delamination shape. This study provides some new results for better understanding the damage tolerance mechanism of FMLs.     

Influence of the Physical Structure of Flax Fibres on the Mechanical Properties of Flax Fibre Reinforced Polypropylene Composites

This study investigates the influence of the physical structure of flax fibres on the mechanical properties of polypropylene (PP) composites. Due to their composite-like structure, flax fibres have relatively weak lateral bonds which are in particular present in flax fibres that are often used in natural fibre mat reinforced thermoplastics (NMT). These weak bonds can be partly removed by combing the fibres. In order to study the influence of the physical structure of flax fibres on NMT tensile and flexural properties, uncombed and combed flax fibre reinforced PP composites were manufactured via a wet laid process. The influence of improved fibre-matrix adhesion was studied using maleic-anhydride grafted PP. Results indicated that the flax physical structure has a significant effect on flax-PP composite properties and that the flax fibre reinforced PP properties are similar to values predicted with existing micromechanical models. The tensile modulus of flax-PP composites can fairly compete with commercial glass mat reinforced thermoplastic (GMT) modulus, the strength, however, both tensile and flexural, can not. In order to rise the strength of flax fibre reinforced PP composites to the level of GMT strength, the flax fibres have to be further isolated to elementary flax fibres.     

Thermal NDI of resistance welded composite structures

This paper presents a thermography-based approach for the non-destructive inspection (NDI) of thermoplastic composite welds. Termed HELTHY (Heat Emitting Layer for THermographY), the technique makes use of the heating element that is used in the welding process as the source of heat for thermographic inspection. Two techniques were utilised in observing surface signal variations: direct temperature measurement and lockin. An infrared camera was used to acquire the surface temperatures and lockin post-processing to obtain phase images of the weld. Both glass and carbon fibre reinforced thermoplastics of various thicknesses were investigated to determine material influences on the technique. Defects of different size were simulated using inserts placed at the weld line before consolidation. The resulting images were then compared to those from ultrasonic inspection. It was found that HELTHY shows great potential for application in-service NDI of resistance welded structures.     

Optimization of CFRP laminate dynamic properties using combinations of short/continuous fibres and stiff/flexible resin matrices

High volume fraction carbon fibre-reinforced plastics (CFRPs) have a stiffness-to-weight ratio much greater than some conventional metallic materials but the material damping is low. Damping may be increased by the use of short fibres or a matrix material with high dissipation. Experimental and theoretical studies are described which show that a lamina can be made which has high damping; the effects of lamina properties on those of the laminate have been investigated. The effects of fibre length, matrix type and fibre orientation have been assessed with the objective of optimizing the dynamic properties of laminated CFRP plates using combinations of short/continuous fibres and conventional/highly dissipative resin matrices.