The phenomena of pull-out in composite materials at a high rate of loading were investigated experimentally and numerically. The finite element method taking account of existence of a virtual shear deformation layer as an interface between a fiber and the matrix, or as a matrix layer, was applied. The experimental results were not explained by the classical theory of one dimensional stress wave propagation where the rigid deformation was supposed to be on the shear layer. On the other hand, the experimental results show a good agreement with the numerical one obtained by F. E. M. using the hypothesis of the interaction between the matrix and the fiber in the shear deformation layer. The numerical results show that the inter-laminar shear stress between the fiber and the matrix is time-dependent. It also depends on the applied load-time history, namely, the pulse rise time. Under step pulse loading, the inter-laminar shear stress oscillates, finally decaying to the value under static loading. However, in any case, the maximum shear stress occurs at the loaded end.
To investigate the time-and temperature dependences of the mechanical behaviors in the transversal direction of fibers in CFRP (carbon fiber unidirectionally-reinforced plastics), three point bending experiments were conducted at various temperatures for CFRP and epoxy resin which was used as the matrix of CFRP. The results obtained are summarized as follows: (1) Both the CFRP and the matrix have characteristic creep compliances dependent upon time and temperature; that is, they show so-called viscoelastic behaviors. (2) The master curve of their creep compliances can be constructed, using their thermo-rheologically simple properties. As for the time and temperature shift factors, they are entirely identical, from the quantitative point of view. (3) The relation between the creep compliances of the matrix and the CFRP is uniquely determined by several factors such as the volume fraction of fibers, but independent of time and temperature. The relation is in good agreement with that given by the approximate averaging method which is one of the mixture laws.
Elastic moduli of glass fibers drawn under various conditions were measured by a dynamic methods. (1) The Young's modulus and the resonant frequency of glass fibers increased with increasing of spinning speed, and with decreasing of spinning temperature. (2) The apparent activation energy for thermal relaxation was 14∼17kcal/mol. (3) The effect of hydrofluoric acid etching on elastic moduli of glass fibers was studied. On etching the Young's modulus increased rapidly until the surface layer had been removed down to the depth of about 10∼15% of fiber radius. With further etching, however, the Young's modulus increased only slightly. These results were used to discuss the difference in glass structure between the surface layer and the interior of glass fiber.
The earth grounds are often subjected to dynamic or repeated loadings. Although the constitutive relationship for clay under static shearing load has been established, the behaviour of clay under the repeated loading is not clarified yet. In this study, undrained repeated loading tests were performed on saturated cohesive soil in order to know the dynamic properties of soil. The triaxial apparatus used has a servo-mechanism and can controll the stress condition automatically and precisely, so that the mean principal stress is kept constant during shearing. The excess pore water pressure generated only by dilatancy can be measured directly in this specially designed apparatus. This paper describes the mechanical behaviours of normally and over-consolidated saturated cohesive soils which stay inside the state boundary surface at the time of repeated loading. A special attension was given to explore the dilatancy and pore pressure characteristics during the mean stress constant tests.
The effect of prestrain on the transition behaviour of fracture in a lamellar pearlite steel containing free ferrites has been investigated by means of low temperature tensile tests. The results obtained are summarized as follows: (1) The types of initiation of micro-cracks can be divided into the following classes according to the temperature range. Below TC, almost all of micro-cracks occur at the boundary of pearlite colonies (type A) and micro-cracks in the free ferrite (type B) are observed barely. Between TC and TB, only those of type A are observed. Both type A and type C, which represents micro-cracks grown into the pearlite colony from a boundary of pearlite colony and a free ferrite, are observed near TB in the range between TB and TD·In the vicinity of TD, transgranular micro-cracks in the pearlite colony (type D) are observed in addition to those of types A and C. (2) The non-ductile transition temperature rises with increasing prestrain. (3) When prestrained, the peak of the fracture stress-temperature curve near the ductile transition temperature becomes very weak. The fracture stress of the prestrained specimens is lower than that of the non-strained.
In order to discuss the effect of interaction between corrosion fatigue and stress corrosion cracking on the fracture strength of metals, zero to tension low-cycle fatigue tests with a hold time were conducted under a constant tensile stress on notched sheet specimens of aluminium alloy in salt water (3.0% NaCl). The results obtained are summarized as follows: (1) From the low-cycle corrosion fatigue tests with trapezoidal stress wave, the number of cycles to rupture was found to decrease with increasing stress hold time and was approximately proportional to the cyclic frequency fa. (2) The tests of alternate loading of stress cycling and holding a constant stress showed that, when the proportion of the number of stress cycles to a period of stress wave form was large, the number of cycles to rupture remarkably decreased as compared with that of corrosion fatigue. (3) The crack propagation rate, da/dN, was enhanced by the crack growth during the hold time. The crack growth was remarkably affected by the number of stress cycling in one period of stress wave form and was larger than that of stress corrosion cracking. (4) The effects of the interaction between stress cycling and hold time on crack propagation rate and fracture life were found to change complicatedly with the ratio of the stress cycle time to the hold time in one period of stress wave form.
In the present paper, the change in strength caused by the difference in the form of cast iron was examined by using round, square and triangular bars with the same sectional area. It was found that the strength increased in order of round, square and triangular form. On the assumptions that (1) the cooling rate in casting depends on the ratio of the surface area to the volume of the castings and (2) the mechanical strength is closely associated with the cooling rate, the following expression was obtained for the tensile strength. σB=K(1/V/S)n where, S is the surface of the cast iron, V is the volume and K and n are experimental constants. Furthermore, microstructural observations were made to understand the above phenomena. In the case of the bars with small sectional area, the graphite structure of E type, appearad in all the bars. The change in mechanical strength caused by the difference in form can mainly be explained from the size, form and distribution pattern of the graphite appearing in cast iron.