Carbon composite materials of high strength and stiffness have been widely applied to the critical structural components of aerospace vehicles all over the world. In order to obtain the fundamental data for designing and processing, substructural models of aircraft rudder sections were experimentally fabricated as CFRP skin/aluminum honeycomb sandwitch structures and the static flexural tests were carried out on these models. The materials for carbon fiber/epoxy prepregs and adhesive systems, and the processes of angleply laminating, assembling by secondary bonding, and tooling techniques were investigated. In addition, the comparison between the tests on the CFRP model and aluminum one indicated the feasibility of the weight reduction over 25% by applying CFRP to the aircraft structure.
The rule of mixture for the coefficients of linear thermal expansion (CTE) of unidirectionally reinforced carbon fiber composites was investigated. The CTE of carbon fiber was obtained by the calculation using a model structure of the carbon fiber. Composites with quite different matrix properties were prepared and their CTE were measured near room temperature. The results were compared with the theoretical values based on various composite models. The applicability of Turner's equation to CTE in the longitudinal direction was confirmed experimentally. For the transverse CTE, a cylindrical composite model was proposed, which was applicable not only to matrices such as epoxy resins having lower moduli but also to matrices such as metals having higher moduli than those of carbon fiber in the transverse direction.
In the previous paper, it was shown that both the volume fraction of the reinforcement and the order of laminations had a marked influence on the relative stiffness of the CFRP-Al composite laminates which were reinforced over the entire span. In this paper, the bending behaviors of the partly reinforced structural model overlapped by shorter reinforcement is considered. The displacement and the strain distributions along the reference axes are calculated by the finite element method assuming anisotropic and isotropic characteristics for CFRP and Al layer, respectively. It is found that the length and the thickness of the reinforcement have a marked influence on bending behaviors of the partly reinforced composite laminates, and the results by FEM are considerably different from the results by the beam theory in the case of extremely short length of reinforcement. The results obtained in this paper provide the design data for deciding the length and the thickness of reinforcement in partly reinforced structures. Also the displacement function used in this paper can be applied effectively for the calculation of complicated displacement modes in the partly reinforced structural model in order to establish its bending behaviors.
Numerous investigations have been carried out on the mechanical behaviors of fiber reinforced plastics (FRP) with a single notch. But there are few studies on the effect of interference of many notches in FRP. This paper deals with the laminated composites containing multiple circular holes arranged as follows; (1) a single row of holes parallel to the load axis, (2) a single row of holes normal to the load axis, (3) a single row of holes with a certain angle to the load axis, (4) a diamond pattern of holes with one hole in its center, (5) a diamond pattern without a central hole. The relation between the breaking strength and the arrangement of holes in these materials subjected to tensile load is discussed, and the stress distribution in the vicinity of a hole is measured by photoelastic test and strain gauge test. The empirical equation involving the effect of interference of holes is proposed, which showes a good agreement with the experimental data.
Collapse of a FRP plate reinforced by roving cloth, with a hole, under uniaxial load appears to be caused by stress concentration and buckling in the case of the plate with smaller stiffness value in compression. This paper reports the above properties for the guidance in design and throws light on the interpretation of the mechanism of stress propagation in the fibrous reinforced composite material with a hole. To find the stress concentration factor and the critical buckling load, both experimental and theoretical approaches are taken. Experimentally the photoelastic coating method is used to analyze the stress distribution around the hole and the direct measurement of displacement along the centre axis on the surface of plate is adopted in the buckling test. On the numerical solutions, the finite element method is applied for the stress distribution and the buckling strength was treated as an eigenvalue problem. A good agreement is obtained between the results by the above theoretical and experimental methods. However, it is suggested that the analysis based on the assumption of homogeneity leads to a difficulty when applied to the mechanical interpretation about the behaviour of FRP. For the further precise treatment of the mechanics of FRP, both the effect of finite width and the non-linear behaviour of fibre bundle in orthogonal directions should be considered in future.
In case fiber reinforced plastics are used as construction members, they are often used as the form of combination of different FRP so called hybrid composites. It is necessary for designing the constructions to investigate the dynamical viscoelastic behavior of this material when it is subjected to forced vibration. Analysis of complex modulus of elasticity which expresses not only the rigidity of the material but also the degree of energy absorption as the vibration or sonic absorber is very advantageous problem for this material. For this purpose, the author established a theoretical method which relates the complex modulus of elasticity and the alignment of unit composites to the dynamical behavior of the hybrid composite. Resonance vibration tests by bending were carried out with test specimens, and the appropriateness of this method were verified. For the mathematical calculation, a new programing soft ware for electronic computer was developed. By this study we can control the dynamical viscoelastic properties of hybrid composite materials at the preparing process. Thus raw materials of composites are able to be used efficiently and the fundamental data for designing become to be obtainable.
Fiber reinforced plastics (FRP) are often used for structural components where the strength under repeated load is required. Especially, carbon fiber reinforced plastics (CFRP) are known to have the excellent properties of high strength and elasticity. But the weakness of CFRP is that the fracture occurs abruptly and the failure strain is small when compared with other structural materials. So, the use of hybrid lamination is gradually attracting attention as a design philosophy for improving mechanical properties. This paper reports the results of bending fatigue tests on CFRP/GFRP hybrid composites plates. The constitutions of hybrid composites are two types of GFRP (roving cloth and mat construction) and one CFRP which is a unidirectional fiber-reinforced material. The flexural strength of GFRP plates is substantially improved by CFRP lamination. A beam theory approach which predicts these flexural properties with sufficient accuracy is developed. The fractural modes of the hybrid composites are different from those of the core materials of roving cloth and mat laminated plates. The hybrid composites with roving cloth GFRP show the zigzag fracture surface and the core mat GFRP presents the brittle failure acrossing the carbon fiber direction under the plain bending loads. The fatigue properties of CFRP/GFRP hybrid composites are improved in flexural strength and rigidity. The static flexural properties depend on the constitution of GFRP cores, but core material does not influence on the fatigue properties of hybrid composites.
The effect of specimen geometry on tensile impact fracture toughness, and the relation between toughness and breaking energy were investigated on glass fiber reinforced polycarbonate plates. The apparent strain fracture toughness of thick plates showed a constant value independent of the relative crack length and that of thin plates increased with increasing the relative crack length. However, the fracture toughness decreased with increasing the plate thickness at a given crack length. The fracture toughness of plates thicker than a given value reached a constant, but it was with much thinner plates than those satisfying the condition of plane strain fracture toughness defined by ASTM criteria. The toughness calculated by the fracture mechanics was in good agreement with that determined experimentally from the breaking energy.
The mechanical properties of GRP are more complicated than those of isotropic homogeneous materials because of its fibrous reinforcement. Especially, the shearing force at the interface between the layers of fiber bundle and resin must be taken into consideration when the ultimate strength is dealt with. This paper is intended to show that the shearing stress caused at the interface between the matrix and the reinforcement affects the ultimate strength of GRP beam under impact bending load, and furthermore that the variational stage of fracture caused by shearing stress or bending stress may exist. In order to clarify the phenomena described above, the measurement of the compressive force in the falling elastic bar and the flexural strain at the backface of the impact point, and the observation of the fracture aspect were carried out. Moreover, the stress wave propagation in the specimen was analyzed numerically for a short duration from the impact instant using the finite element method of Galerkin process. The factors causing fracture, especially about the shearing force at the interface, are discussed.
The Split Hopkinson Pressure Bar Method has been applied to the impact testing of Fiber Reinforced Plastics. In principle, the apparatus used here is similar to that used first by Kolsky in 1949. Under impact loading, metals show a typical plastic flow and their strain ranges observed are much wider than those of F.R.P. The maximum strain of F.R.P. is a few percents at most. Some problems associated with impact loading on F.R.P. have been discussed based on a simple theory. The effects of specimen dimensions and friction were also examined experimentally.
Low cycle tensile impact fatigue properties of glass fiber reinforced plastics were investigated using a drop weight impact tesing machine with primary emphasis on the progressive failure during fatigue. The materials tested were plain woven glass cloth reinforced polyester resin (cloth GRP) and chopped strand glass mat reinforced polyester resin (mat GRP). With a drop weight impact testing machine, a free fall from a fixed height can not produce a constant impact load if the rigidity of a specimen changes during fatigue. Therefore, the impulsive load during impact fatigue tests was monitored every time by using a wave memory, and when the decrease in maximum impulsive load was noticed a proper compensation for the drop height was made. The tensile impact fatigue strength was equal to or higher than the static strength up to 100 cycles of impact loading. The damage caused by the impact loading was not concentrated but extended over the whole specimen. The decrease in longitudinal extension modulus during fatigue showed a regular pattern on cloth GRP but not on mat GRP. The variation of the modulus for cloth GRP could be divided into three stages. The first stage is the region where the cycle ratio is less than about 0.1 and the modulus decreases rapidly. The third stage is the region where the cycle ratio is larger than about 0.9 and the modulus decreases rapidly again. The second stage is between these two stages and the modulus decrease gradually. It was found that the decrease in modulus in the first and second stages was mainly caused by the matrix but not by the fibers since the residual static strength scarecely changed until the cycle ratio exceeded 0.9. A gradual failure model with two components, namely STRONG part and WEAK part, was introduced to analyze such a decrease in modulus with the assumption of stochastically accumulated damage of WEAK part. The theoretical predictions showed a good agreement with the experimental results until the failure of the main load-carrying element could not be ignored anymore. Furthermore, it was found that the probabilities for the applied stress and the number of cycles during the damaging process of WEAK part were stochastically independent.