Cracking orientation and induced anisotropy of a ceramic matrix composite under off-axis loading

The effect of matrix microcracking on the stiffnesses of a carbon-fibre/SiC-matrix woven composite is studied by means of an ultrasonic method. It provides the whole set of the stiffness tensor coefficients which are inaccessible by classical strain measurements and which are required to identify anisotropic damage. The induced anisotropy depends on the loading direction. If a tensile solicitation in a fibre direction leads to stiffnesses decreases without any rotation of principal axes, a tensile solicitation of 45° from a fibre direction creates microcracks with a predominant orientation that does not coincide with the elastic symmetry axes, and induce a fully anisotropic elastic degradation.     

Impact of fibre orientation on tensile,bending and shear behaviors of a steel fibre reinforced concrete

Fibre orientation and density are known to have a significant influence on steel fibre reinforced concretes (SFRC) mechanical properties. In practice, parameters such as fresh state properties, restriction to concrete flowability and placing methods are likely to induce different fibre orientations in characterisation specimens and structural components. This difference in fibre orientation can impact the mechanical behavior of the structural component and therefore provide an unsafe design if not considered. This project consisted to produce a large SFRC slab, extract specimens with different fibre orientations, and submit specimens to tensile, bending and shear tests to evaluate the impact of fibre orientation and density on mechanical and post-peak strengths. Test results have shown that tensile and bending behaviors are mainly influenced by the fibre orientation, while the shear behavior is mainly impacted by fibre density. Test results were processed to allow comparison between tensile and bending tests. Linear correlations between tensile residual stresses and fibre orientation where found, linear or power type correlations according to bending residual stresses, as well as linear correlations between shear stresses and fibre density.     

Validation of the modified rule of mixtures using a combination of fibre orientation and fibre length measurements

The goal of this study was to investigate the fibre orientation distribution (FOD), and subsequent mechanical properties, of an injection moulded plate with two different number averaged fibre lengths, termed in this paper medium (1.35 mm) and long (2.40 mm). Fibre orientation measurements (FOD) were made using the 2D elliptical section method and an in-house developed image analyser. The samples were injected from a pin gate located at the centre and top of the plate. Expansion flow on the divergent flow front from this pin gate resulted in a core region with circumferential alignment, while through thickness shear resulted in the usual realignment of fibres in the flow direction either side of the core, termed the shell layers. Two interesting aspects were discovered from these measurements. First, and most importantly, the FOD was found to be independent of the two fibre lengths in this study, and so predominantly controlled by the mould shape and the interaction with the flow front. Second, the fibres in the core region were found to be much closer packed than those in the shell regions.The interaction between the flow front and the mould shape resulted in a range of FOD across the moulded plate, from equal in-plane orientation at the centre of the plate, to highly aligned at the plate edge. This gave a very useful set of samples from which to test out the well known modified rule of mixtures (MROM). Often the fibre orientation distribution cannot be measured directly, but indirectly using the modified rule of mixtures model in reverse. The samples from this moulding (at two different average fibre lengths) gave an excellent opportunity to validate this often used approach.Both the tensile modulus and strength (measured parallel to the injection direction) were found to show a strong correlation with the measured fibre orientation, with a significant increase in both measures between the centre and the edge of both plates. The increased length of the ‘long’ fibre plate was found to give only a small increase in tensile modulus but a much larger increase in tensile strength.The tensile modulus showed a linear dependence with the measured fourth order orientation tensor average, 〈cos⁴ θ〉, with respect to the injection direction of the plate, as predicted by the modified rule of mixtures. Excellent agreement was found between the measured modulus and the predictions from the modified rule of mixtures, based only on measured quantities (matrix modulus, fibre fraction and average fibre length) for both plates.     

Using synchroton radiation-based micro-computer tomography (SR μ-CT) for the measurement of fibre orientations in cellulose fibre-reinforced polylactide (PLA) composites

The present work deals with the measurement of fibre orientation angles in composites. A study by Bax and Müssig [1] investigated the mechanical properties (tensile and impact characteristics) of injection-moulded flax and Cordenka-reinforced polylactide (PLA) composites with fibre mass fractions between 10 and 30 %. Raising the fibre content from 10 to 30 % resulted in an increase in tensile characteristics, but it was noted that a reinforcement with 10 % flax fibres led to poorer tensile strength as compared to the neat PLA matrix. This behaviour was not expected and needs clarification. Therefore, test specimens with a fibre content of 10 and 30 mass % were examined for their fibre orientations and void content. For the investigations, microcomputer tomography images were created by monochromatic synchrotron radiation. Fibre orientation angles of these micrographs were determined with an adapted measuring mask of the Fibreshape software. It could be shown that the fibre orientation in the composite is dependent on the fibre mass fraction and the type of fibre. No voids were found in all the investigated composites. The average fibre orientation angle of 10 % flax/PLA showed a larger deviation from the longitudinal axis of the test specimen than the other samples, and is made primarily responsible for the lower tensile strength of this composite.     

Impact and post-impact properties of a carbon fibre non-crimp fabric and a twill weave composite

The impact and post-impact static and fatigue tensile properties of a carbon fibre/epoxy NCF composite were determined and compared to those of a carbon fibre/epoxy woven fabric composite, for two impact energies (3.5 and 7 J). The projected damage area after impact was larger for the NCF composite than that for the woven fabric composite for both impact energies. Impacted samples were subjected to static tensile tests and tensile–tensile fatigue tests. It was found that even a relatively low energy impact has already a significant negative influence on the residual properties in both static and fatigue tests, in the fibre direction as well as in the matrix dominated direction. In the matrix dominated directions the post-impact behaviour of the two materials is very similar. In the fibre direction, however, the properties of the non-crimp fabric composite are degraded more by an impact than those of the woven fabric composite.     

Strength and fracture properties of industrially prepared steel fibre reinforced concrete

The paper reports on a study of steel fibre reinforced concrete (SFRC) which was prepared using normal industrial mixing, compaction and curing conditions. Both strength (compressive and tensile) and fracture (toughness measurements) characteristics have been investigated with test specimens prepared from 5 m long SFRC piles. The piles contained only steel fibre reinforcement and were manufactured in exactly the same way as ordinary piles.Slight differences in the tensile strengths (determined via torsion tests) were observed due to the existence of preferential fibre orientation. Flexural tests on notched beams (to evaluate fracture characteristics) produced a much more stable, reproducible, test than that observed for un-notched beams. Hence, it is concluded that the notched beam is a better geometry in terms of test stability and reliability. The results showed that tests specimens taken from industrially prepared SFRC displayed similar characteristics compared to that observed with test specimens prepared under laboratory conditions, with regards to the strength, fracture characteristics and, in particular, the variation observed.     

Fatigue and post-fatigue behaviour of carbon/epoxy non-crimp fabric composites

This paper focuses on the static, fatigue and post-fatigue tensile properties of a biaxial carbon/epoxy non-crimp fabric composite. In a series of quasi-static tensile tests, the stress–strain level where damage initiates was determined. This stress level was then used as the maximum stress level in tensile–tensile fatigue tests in the fibre direction. It was found that in fibre direction, this load level can be considered safe for fatigue up to very high cycle numbers. The damage evolution during the tests was monitored at certain cycle times with X-ray radiography. The post-fatigue residual static tensile properties were determined after different numbers of cycles. A series of tensile–tensile fatigue tests at various higher stress levels allowed for the fatigue life curves to be constructed in each of the four testing directions. This revealed that the damage initiation load level is well below the practical fatigue limit of the material.     

Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase

The knowledge of the behaviour of flax fibres is of crucial importance for their use as a reinforcement for composites materials. Flax fibres were tested under tensile loading and in repeated loading–unloading experiments. We have shown that fibre stiffness increases with the strain.This phenomenon is attributed to the orientation of the fibrils with the axis of the fibre when a strain occurs. By using micro-mechanical equations, the Young's modulus of a flax fibre is estimated by taking into account the composition of the fibre and the evolution of the orientation of the fibrils during a tensile test. A good agreement is found between experimental and calculated results. The origin of the large spread observed in the mechanical characteristics is analysed here.     

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.     

The effects of the biaxial stretching of leather on fibre orientation and tensile modulus

Leather has been subjected to different degrees of equal biaxial strain (up to 20%) during drying and its tensile modulus has been measured when dry. The collagen fibre orientation distribution in the dried leather has been assessed using wide angle X-ray diffraction. It was found that drying under biaxial strain caused the tensile modulus to increase markedly (by up to 400% at 20% biaxial strains) but with a dependence on the angle of test axis in relation to the principal axes of biaxial strain. The fibre orientation distribution in planes parallel to the surface was affected less by biaxial strain than in planes perpendicular to the surface and it is concluded that the latter type of fibre reorientation is the main factor responsible for the observed increases in tensile modulus.     

Creating three-dimensional (3D) fiber networks with out-of-plane auxetic behavior over large deformations

Fiber networks with out-of-plane auxetic behavior have been sporadically investigated. One of the major challenges is to design such materials with giant negative Poisson’s ratio over large deformations. Here in, we report a systematic investigation to create three-dimensional (3D) fiber networks in the form of needlepunched nonwoven materials with out-of-plane auxetic behavior over large deformations via theoretical modeling and extensive set of experiments. The experimental matrix has encapsulated the key parameters of the needlepunching nonwoven process. Under uniaxial tensile loading, the anisotropy coupled with local fiber densification in networks has yielded large negative Poisson’s ratio (up to ?5.7) specifically in the preferential direction. The in-plane and out-of-plane Poisson’s ratios of fiber networks have been predicted and, subsequently, compared with the experimental results. Fiber orientation was found to be a core parameter that modulated the in-plane Poisson’s ratio of fiber networks. A parametric analysis has revealed the interplay between the anisotropy of the fiber network and the out-of-plane Poisson’s ratio based upon constant volume consideration.     

轧制和退火过程中等规聚丙烯的力学性能

借助拉伸试验对轧制和退火后的等规聚丙烯进行了轧向及横向上力学性能的研究。结果发现,结晶度和大分子链的取向是影响材料力学性能的主要原因,力学性能的变化主要发生在轧制过程中。轧制后,大分子链沿轧向排列,出现各向异性:轧向上拥有高的拉伸强度和低的延伸率。结晶度的变化主要是影响材料的弹性模量和横向上的屈服强度。退火后,大分子链的取向发生较小的变化。非晶部分的再晶化与无序化共同影响材料的性能,在伸拉强度没有减少的基础上增大了材料轧向上的延伸率。     

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 7. Interface strength and fibre strain in injection moulded long fibre PP at high fibre content

The mechanical performance of injection moulded long glass fibre reinforced polypropylene with a glass fibre content in the range 0–73% by weight has been investigated. The composite modulus exhibited a linear dependence on fibre content over the full range of the study. Composite strength and impact resistance exhibited a maximum in performance in the 40–50% by weight reinforcement content range. The residual fibre length, average fibre orientation, interfacial shear strength, and fibre strain at composite failure in the samples have been characterised. These parameters were also found to be fibre concentration dependent. The interfacial shear strength was found to be influenced by both physical and chemical contributions. Theoretical calculations of the composite strength using the measured micromechanical parameters enabled the observed maximum in tensile strength to be well modelled.     

Analysis of knitted fabric reinforced composites: Part II. Stiffness and strength

The influence of the knit structure on the stiffness and strength in tensile and in share loading of glass warp knitted fabric epoxy composites is studied. The average strength depends on the fibre content and on the linear density of the yarn. The anisotropy in tensile and shear properties is related to the orientation tensor components a₁₁₁₁ and a₁₁₂₂, respectively. By making use of these relationships, a knit structure can be evaluated with regard to the mechanical properties of its composite with only two measurements: (1) measurement of the achievable fibre content; and (2) measurement of the fibre orientations.     

Structure and anisotropy of stiffness in glass fibre-reinforced thermoplastics

Techniques which are readily available, and which may be considered suitable for the qualitative or quantitative assessment of fibre orientation distribution in short glass fibre-reinforced thermoplastics are reviewed. The results of using several of these techniques in structural studies on injection mouldings of glass fibre-reinforced grades of polypropylene and polyamide 66 are presented. Uniaxial tensile creep tests were carried out on specimens cut from the mouldings and the anisotropy of stiffness of each moulding is compared with that predicted from the structural studies. Certain of the structural techniques are considered to be unreliable or of restricted applicability and it is concluded that the technique of contact micro-radiography is the most versatile; being capable of yielding reliable qualitative or quantitative information on fibre orientation distribution. Detailed structural studies on edge-gated injection moulded discs, using the technique of contact micro-radiography, show that the fibre orientation distribution varies dramatically through the thickness of the mouldings, even in cases where uniaxial tensile creep tests suggest isotropy of stiffness in the plane of the moulding. Care must therefore be taken when seeking to relate flexural data to tensile data and strength data to stiffness data.     

In situ ESEM study of the deformation of elementary flax fibres

The deformation behaviour of single elementary flax fibres was investigated in an ESEM, using a modified loop test. Plastic deformation starts on the compressive side of the loop, whereas fibre failure occurs on the tensile side of the loop. The primary and the secondary cell wall show a different deformation behaviour. The primary cell wall breaks in a brittle manner, whereas in the secondary cell wall, due to its fibrillar nature, a coarse crack grows, bridged by fibrils. The secondary cell wall was found to split relatively easily along the length direction, indicating that the lateral strength of the fibre is lower than its tensile strength, which also accounts for the lower compressive strength of the fibre compared to its tensile strength.     

Elastic moduli,thermal expansion and microstructure of copper-matrix composite reinforced by continuous graphite fibres

Copper-matrix composites reinforced by continuous graphite fibres (C_g) were processed by hot-pressing layers of metallic prepregs, each fibre within the yarns having previously been coated with copper by electroplating. The electrodeposition and consolidation conditions were optimized to minimize the residual porosity, which could be considered as negligible. One-dimensional (1D) and two-dimensional composites were obtained by this technique. In addition to the good metallurgical quality of the matrix, examination of the fibre/matrix interphase by Auger electron spectroscopy confirmed the excellent chemical compatibility between copper and graphite. As a consequence, the ultimate tensile strength of fibres extracted from the matrix remained nearly unchanged. The thermal expansion coefficients of 1 D C_g/Cu composite materials were determined between 100 and 300°C, along the two orthogonal directions. Values ranging from 8 to 9 × 10⁻⁶ °C⁻¹ in the composite plane and from 16 to 18 × 10⁻⁶ °C⁻¹ in the orthogonal direction were obtained. These results, which are related to the strong anisotropy of the ex-pitch graphite fibre, are correlated to the theoretical values found with the rule of mixtures. The poor Young's modulus and the tensile strength values are correlated to the microstructure of the fibre/matrix interphase.     

Dry sliding wear of an Al2O3 continuous fibre reinforced Al-Cu alloy against steel counterface

The tribological properties of Al₂O₃ continuous fibre reinforced Al-4.43 wt %Cu alloy composites with a fibres' volume fraction of about 0.55 were measured for five types of fibre orientations under a dry sliding contact with a bearing steel. Fibres were in a plain perpendicular to wear surface and parallel to sliding direction, and had the angles 0°, 45°, 90°, or 135° with respect to the direction of motion of the counterface; or were anti-parallel the sliding direction. The results show obvious dependence of wear characteristics on fibres orientation: for the 45°, 90°, and 135° orientations, the larger the fibres' angle, the lower the volume loss; while the 0° orientation resulted in a higher steady-state wear rate than those of the 45°, 90°, and 135°, orientations, except that the anti-parallel orientation caused the highest volume loss at all sliding distances. The wear mechanism was inferred as a oxidation-microgrooving process through the analyses of worn surface and subsurface with the aid of optical microscope and scanning electron microscope. Also it was found that the fibres' broken and subsurface deformation had played an important role in causing wear anisotropy.     

Non-hookean stress-strain response and changes in crystallite orientation of carbon fibres

The non-hookean stress-strain response of carbon fibres was investigated in relation to changes in crystallite orientation with tensile stress. Various one-dimensional array models and a mosaic model were examined. Amongst these models, only the mosaic model in which the stress of the crystallites can be transmitted in both the transverse and the axial directions showed any quantitative agreement with the measured increases in the tensile modulus and the crystallite orientation with tensile stress. This suggests that deformation of the crystallites is constrained with increasing tensile stress. It was also found that the ratio of the tensile stress of the fibre to that of the crystallites is close to the crystallite volume fraction rather than the ratio of the fibre density to the crystallite density.