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A new method for evaluating the interfacial properties of fibrous composites based on a fragmentation technique has been proposed by use of the gradual multi-fiber composite, in which the inter-fiber spacing is gradually changed. The results showed that as the inter-fiber distance increased, the aspect ratio of broken fibers decreased while the fibre/matrix interfacial shear strength increased. When the reciprocal of the inter-fiber distance was taken for the above relationships, both the aspect ratio and interfacial shear strength were found to show a saturated value. This means that the gradual multi-fiber composite indicates an upper bound in aspect ratio and a lower bound in interfacial shear strength, while the single-fiber composite shows a lower bound in aspect ratio and an upper bound in interfacial shear strength. It was concluded that this fragmentation test could be a new method for composite evaluation, since reducing the difference between these two bounds is effective for composite strengthening. In addition an elasto-plastic finite-element analysis was carried out to relate the above results with the fiber stress distribution around fiber breaks. It is proved that the bound obtained in the gradual multi-fiber composite test is closely concerned with stress concentrations caused by a group of multi-fiber breaks.

A creep-rupture simulation of continuous fiber-reinforced composites was proposed, based on a shear-lag model. This simulation model is composed of tension-carrying elastic fibers and shear-carrying viscoelastic matrix including the effect of stress history. Simultaneous integro-differential equations obtained from their force equilibrium were semi-discretized with respect to space coordinate and solved analytically with respect to time. The result showed that the unrecovered stress region and stress concentration region around a broked fiber were increased with time, similarly to a conventional anlaytical model. On the other hand, since the above equations are changed with each fiber breakage, it is uncertain which initial condition of the equations should be used in the simulation. To solve this problem, the break-influence superposition (BIS) method was applied to the simulation, in which independent solutions with different fiber breakage points are superposed with weight coefficients. It is proved that the BIS method can reasonably predict a complicated viscoelastic behaviour caused by initially and later broken fibers. Finally, the simulation was carried out for the composites reinforced by the fibers without time-dependent breakage to predict a possibility of creep-rupture.

A Monte-Carlo simulation technique based on a finite-element method has been developed in order to clarify the effect of interfacial shear strength on the tensile strength and reliability of fibrous composites. In the simulation a boron/epoxy monolayer composite was modelled, and five hundred simulations were carried out for various interfacial shear strengths. The interfacial shear strength value which raised the average strength of the composite corresponded approximately to the value which reduced the coefficient of variation. This implies the existence of an optimum value of interfacial shear strength which can increase the strength and reliability. The simulated strength and reliability were closely related to the degree and type of damage around a fiber break. That is to say, large-scale debonding caused by a weak interfacial bond and matrix cracking caused by a strong bond reduced the number of fiber breaks accumulated up to the maximum stress, and decreased the strength and reliability. On the other hand, small-scale debonding promoted comparatively the cumulative effect of fiber breaks and played a key role in increasing the composite strength and reliability.

Environmental compatibility of polymer composites has become an important characteristic as the need to reduce environmental hazards is increasing worldwide. Many incidents taking place around the world are enough to bring us around to this point of view. A catastrophic earthquake and tsunami devastated the Pacific coast of north-eastern Japan on 11 March 2011. The earthquake, which was the most powerful earthquake ever measured in Japan, was of magnitude 9.0 on the Richter scale. About 19 000 were dead and missing. Three prefectures in the Tohoku (north-eastern) region of Japan, Miyagi, Iwate, and Fukushima, were most severely damaged. Reconstruction is yet to take place in many of the affected cities and towns. The area around the Fukushima Daiichi Nuclear Power Plant was evacuated owing to radioactive contamination. It is said that complete restoration will take more than 30 years, because the influence of the Chernobyl nuclear power plant disaster, which happened more than 25 years ago, continues to be felt. In Fukushima prefecture, many residents are still forced to lead lives as long-term refugees, and the residents in certain areas outside the refuge zone continue to live under threat of radiation that is much higher than is normal. The damage caused by radioactivity has also been considerable: it has already affected the soil of schoolyards, tapwater, grass, agricultural products, marine products, and so on, in large areas within the Fukushima prefecture. It is not clear how much of this damage is due to sea pollution and how long its effects will last in the future. Against such a background, a planned conversion to renewable natural power sources as recommended by the energy policy, depending on nuclear power generation, attracts attention. For instance, it has been decided to abolish nuclear power generation systems in Germany; they propose to convert from 16% of total energy generation from the natural power sources at present to 35% by 2020 and to 80% by 2050 [1]. In the report ‘‘The Green New Deal’’ published in 2009 [2], promotion of use and development of alternative and renewable energy, improvement in energy efficiency, greenhouse gas reduction, and so on, have also been proposed. Today

This paper deals with long fiber-reinforced composite strands using continuous ramie yarns, in which polypropylene including MA-PP (maleic anhydride-grafted polypropylene) was used as a matrix material. The composite strands were fabricated by a new method called multi-pin-assisted resin impregnation (M-PaRI) process, for which the equipment was newly applied after conventional extrusion process. The composite strands were then pelletized and injection molded. Tensile strength and Young's modulus of the resultant short ramie/PP reinforced composites were investigated. Results show that this new process improved the mechanical properties of injection-molded specimens.

A theory is developed to obtain the Weibull scale and shape parameters for in situ fibre strength utilizing the data obtained from a single-fibre-composite (SFC) test. It is well known that during the SFC test, the fibre fractures several times along its length at successive weak points that are randomly located. The SFC technique, although most commonly used for measuring the fibre/matrix interfacial shear strength, is an excellent way to determine the fibre flaw spacings and the in situ fibre failure stress at every fracture location. In the present technique, the SFC specimen is partitioned into a relatively large number of small sections of equal length such that each section will have either one or no fibre fracture point. Because all the sections may not include the fibre fracture points because of their random nature, the test data are regarded as “censored data”. In other words, the theory is constructed for the estimation of Weibull parameters that takes into account the censored nature of the data. The Weibull parameters predicted using the present theory are in the same range as those obtained from the single fibre tension tests. For several reasons the values obtained from the SFC test tend to be slightly higher than those obtained from the simple tension tests. However, the in situ fibre strength values obtained using SFC technique may be more realistic and thus may be more useful in modelling composite strength.

The purpose of this study is to evaluate the statistical properties of creep-rupture lifetime in boron fibers by using a time-dependent Weibull distribution, from the viewpoint of materials reliability engineering. Axial creep-rupture tests on boron fibers were carried out under several constant loads and constant loading rates in air at 300°C, and the relevant Weibull parameters were estimated from the lifetime and strength data obtained in the above tests. The results showed that the estimated lifetime distribution curves were not in good agreement with the experimental data at other applied loads. The SEM observation for the fracture surfaces indicated that the kind of defect causing the long-term rupture was completely different from that of the short-term rupture. Therefore, a mixed time-dependent Weibull distribution based on a mixed distribution model was proposed for evaluating the scatter of the creep-rupture lifetime data. It was proved that, finally, the distribution proposed here was suitably applicable for predicting the creep-rupture lifetime distribution of boron fibers.

BN coating on sintered SiC fibers (Tyranno SA) by continuous CVD process was developed for the interphase of SiC/SiC composites. BN coating by the continuous CVD process was conducted at 1580°C and pressure of around 1 Torr. The reactant gases were NH 3 and BCl 3 and diluted by Ar. And yarn speed was 4.5 mm/s. Conventional experiments showed that the BN coating thickness across the fiber tow varied significantly. Hence, the CVD process was modified by not only adjustments of the amount of reactant gases, but also loosing the yarn bundle by reduction of the yarn tension and setting a new furnace for removing the sizing agent of the fiber tow prior to the CVD furnace. The BN coating thickness by the modified process was much more uniform than that by the conventional process. The coated BN was ordered structure, nearly stoichiometric and pure with little O and C. The strength of the BN coated SA fiber retained 90% of the strength of as-received SA fiber.

Tensile tests of a 10 vol% boron-aluminum composite were carried out at room temperature, 150°C and 300°C, in order to investigate the validity of the finite-element simulation proposed in the previous report which can evaluate the tensile strength of fiber-reinforced metals. The simulated average strengths were in good agreement with the experimental results, though the scatter of strength was somewhat underestimated. The fiber stress distributions around a broken fiber were analyzed by FEM, by taking into account the matrix constitutive equation at each temperature. From the results of analysis and the stochastic model based on a Markov chain theory, the relation between the fiber stress distributions and the statistical properties of composite strength obtained in the experiment and the simulation was discussed. It was pointed out that, consequently, the statistical properties of room temperature strength would be maintained to some degree at elevated temperatures, unless the fibers were degraded severely. Additionally, the effect of thermal stresses occurring in the composite on the tensile strength was investigated.

A numerical technique based on the Monte-Carlo method in a shear-lag model is developed to simulate the tensile strength and fracture process for a unidirectional carbon-fiber-reinforced-plastic (CFRP) composite. The technique improves on the conventional approach by using an rmin method that can determine the stresses working on the fiber and matrix elements as the damage progresses. The rmin method is based on tracking the incremental ratio of the strength of an element to its stress. The present model includes the effect of the sliding frictional forces around fiber breaks caused by debonding between the fiber and matrix. Statistical properties of the tensile strength were obtained through simulation runs involving 100 samples for each value of the frictional force parameter. Also studied was the size effect in composite strength with increasing numbers of fibers, N, where it was found numerically that mean strength varies linearly as 1 [l n ( N )] 1 2 and coefficient of variation varies linearly as 1 l n ( N ) , as suggested from a simple theory.

A winding machine with heat treatment was newly developed to strengthen ramie yarns. During the treatment process, ramie yarn in normal or wet state was wound on a winding machine and passed continu- ously through a heater at 100 and 150 � C, respectively, with different winding speeds and tensions. Higher tensile strength and stiffness of ramie yarn was achieved after heat treatment on wet yarn and winding speed had a sig- nificant influence on the tensile properties of yarns. How- ever, a little decline in tensile strength was found for ramie yarns after heat treatment in normal state. This implies that the water-swollened structure of ramie yarn during the heat treatment is crucial in strengthening yarns. In the case of heat treatment on wet yarns, the effect of winding tension on the tensile properties of yarns was studied. It was found that the tensile strengths and Young's moduli of ramie yarns first increased and then reached equilibrium as the winding tension was increased. The crystallinity calculated from X-ray diffraction dia- grams showed a slight decrease in heat-treated ramie yarns whereas the crystalline orientation factors had no appreciable change. It was considered that the improved effect was related to the more oriented molecular chains in amorphous region and optimized yarn structure. V C 2010

A tensile test was carried out on the two kinds of SiC fiber-reinforced Al alloy composite wires, SiC/pure-Al and SiC/Al-5.7 % Ni, which were quenched or annealed. The effect of the matrix hardness and microstructural change on the tensile strength and its scatter has been investigated. The results showed that the SiC/Al-5.7 % Ni composite wires maintained a higher strength than the SiC/pure-Al composite wires after the wires were quenched from a temperature up to 450°C, and it had less strength than SiC/pure-Al after being heated to a temperature over 550°C. The scatter of the strength of SiC/Al-5.7 % Ni wire increased as quenching temperature increased, but the strength of SiC/pure-Al wire had no change in scatter. The results above were explained by the matrix microstructure, and by the tensile strength of the extracted fibers.

A Monte-Carlo simulation method for evaluating the tensile strength of the unidirectional fiber reinforced metal matrix composites is proposed on the basis of the elastic-plastic finite element method. In this simulation model, the two one-dimensional line elements representing the reinforcing fibers are incorporated into two sides along the y-axis of a 4-nodes isoparametric element (plane stress), which represents the metal matrix. Furthermore, for estimating reasonably each element stress increment necessary to the fiber-break, the γmin method is applied to the simulation procedure. This simulation is performed for boron/alumium composite monolayer. The simulated result shows the larger stress concentration around the broken fiber than that of the previously proposed simulation by means of the shear-lag model. Therefore, the former simulation lowers the mean values of tensile strength and increases its scatter, as compared with the later. Such a statistical tendency is in good agreement with the results obtained by comparing the previous experiment with the shear-lag simulation method.

A Monte-Carlo simulation technique, using a shear-lag model, is proposed to elucidate the tensile fracture process and the tensile strength of unidirectional fiber reinforced metal matrix composite materials, on the condition that the fiber behaves elastically and the matrix metal is approximated as an elastic linear hardening plastic material in accordance with the kinematic hardening model. In the simulation procedure, a conventional technique is modified by γmin method based on the minimum value of the incremental ratios which can be calculated from the element strengths and the stresses working on each fiber element and each matrix element. The simulated results show that the average value and the coefficient of variation of tensile strength are improved by an increase in the Weibull's shape parameter of the fiber, although the average value does not reach the value estimated from a rule of mixture. Then, in order to investigate the validity of the present technique, the simulated strength data are compared with the theoretical cumulative distribution curve given by a recursion analysis technique. Additionally, it is shown that the structural uncertainty, e.g. random fiber spacing, lowers the composite average strength and increases its scatter.