An investigation on mechanical properties of anisotropic polymer solids is important not only for establishing the criteria for initiation of crazing and yielding, but also for solving practical problems. In this paper, as the first step of the strength study on anisotropic polymer solids, creep fracture of rolled polyvinyl chloride (PVC) plates has been investigated. The results showed that (1) the creep fracture is very sensitive to the orientation of specimen, or the angle between the specimen axis and the rolling direction, and (2) several kinds of distinctive marks which correspond to the fracture modes, i.e., delayed, ductile and brittle ones, are observable on the fracture surfaces. Their formation processes are also schematically illustrated.
Kneading of poly vinyl chloride (PVC) with maleic anhydride (MAN) using a heating roll mill was carried out in the presence of dicumyl peroxide (DCP) under the molten state. The addition reaction of MAN to PVC was found to occur. The amount of MAN attached to PVC increased with increasing peroxide or MAN. The best condition was the kneading of PVC with MAN at 140°C for 10min. The modified PVC with MAN was then reacted with several metal oxides, metal acetates, and other additives to obtain PVC ionomers. The physical properties of the PVC ionomers were also measured.
The behavior of glass fiber reinforced plastics (GRP) under bending load was investigated by using unsaturated polyester resin reinforced by roving cloths or chopped strand mat. The influence of glass content on their behavior was also investigated. The results obtained were as follows. (1) The GRP under bending load exhibited microcracks in their tension side at low stress level similarly as the behavior in their tensile tests. So, the strain in the tension side was slight larger than the strain in the compression side. (2) The elastic modulus in bending increases in proportion to the volumetric content of glass fiber β. But in M-GRP (reinforced by chopped strand mat), the effect of β on their elastic modulus was little. (3) For R-GRP specimens (reinforced by roving cloths) breaking usually occurred in the compression side, while for M-GRP it is in the tension side. (4) The bending strength increased in proportion to β to a limit value of β in the same way as the tensile strength, but the limit value for each specimen was slightly lower than that of the tensile strength.
The creed crack growth behavior of the SUS 304 stainless steel was investigated at 650°C using tension-type (CN and DEN) and bending-type (SEN and CT) specimens, in order to discriminate the most probable governing mechanical parameter of the phenomenon from several candidate mechanical parameters. The following mechanical parameters were examined in this study; stress intensity factor KI, elastic net section stress (σn)e, rigid-plastic net section stress (σn)p and modified J-integral J' (C* parameter). The effect of temperature on the creep crack growth behavior of center-notched (CN) specimens was also investigated. It was found that the creep crack growth rates could successfully be correlated with J', regardless of the types of specimens selected in this study, while the other three mechanical parameters, KI, (σn)e and (σn)p, gave poor correlations with the crack growth rates. This result was quite consistent with those in previous studies obtained by using several series of proportional specimens with different tension-type geometries (CN, DEN and round bar). It was concluded that J' is the most probable mechanical parameter which governs the creep crack growth behavior. Temperature increased the creep crack growth rates and the correlation curves of the crack growth rates with KI and (σn)e[≡(σn)p, in this case]moved upwards and approximately in parallel. However, the correlation curves with J' were almost unaffected by temperature. This may be a very convenient and useful property in various engineering applications.
Fatigue crack propagation rate and crack closure behavior under repeated two-step tests composed of loads above and below the threshold condition were studied on a medium carbon steel and an aluminum alloy. By means of electron fractography, striations corresponding to high and low level loadings were found on the fractured surface of both materials, which confirmed that crack propagation occurred even below the threshold level under varying loading conditions. And such behavior of crack propagation was observed even under the pulsating loading test as well as the reversed loading test. The acceleration of crack propagation was found to be contributed by both reasons of the acceleration under high level loading due to the increase in the effective stress intensity range and the above mentioned crack propagation below the threshold level. Furthermore, the effect of the cycle ratio of high level load to low level one on crack propagation rate was investigated.
The measurements of fatigue crack propagation rate in 7075-T6 aluminum alloy and mild steel SS41 sheet specimens have been made to determine the effect of stress ratio on the propagation rate. The effect was clearly recognized in 7075-T6, but not in SS41. This observation was well explained in terms of the accumulated plastic strain rate, which is defined as the proportion of the accumulated plastic strain in the monotonic plastic zone to the nominal fracture plastic strain. The plastic strain in the monotonic plastic zone for Mode I of crack opening was calculated by analogy to the plastic strain distribution in Mode III given by Rice. In 7075-T6, the accumulated plastic strain rate clearly increased in proportion to the stress ratio, but in SS41, the rate hardly increased even if the stress ratio increased. It was concluded that the effect of stress ratio on propagation rate depends on the degree of increasing rate of the accumulated plastic strain rate corresponding to the increase in stress ratio, which in turn depends on the degree of the nominal fracture plastic strain. This conclusion has been substantiated through a survey of the correlation between the effect of stress ratio and the nominal fracture plastic strain on various materials reported heretofore. It was established that the effect of stress ratio is clearly recognized in the materials having nominal fracture plastic strains of less than 50% (i.e., the reduction of area is less than 33%), but none is recognized in the materials having nominal fracture plastic strains of more than 80% (i.e., the reduction of area is more than 44%). By considering the relation between the accumulated plastic strain rate and the material's constant c in fatigue crack propagation rate law (dl/dN=c(ΔK)m), it is shown that the effect of stress ratio can be estimated from the accumulated plastic strain rate quantitatively.
In an experimental study of acousto-elastic effect, some anomalous phenomena in ultrasonic velocity, that could not be interpreted by the lattice anharmonicity, have been observed. In this paper, we have observed in detail the behaviors of an ultrasonic velocity propagating in a copper specimen (99.98% of purity) perpendicularly to the stress axis, as the uniaxial tensile and compressive stresses exceeding the range of plastic deformation were applied or released stepwisely. The results obtained are summarized as follows: (1) The decrease of the traveling time (in a traveling path of 10mm) after one minute of stress application under a constant stress level (ΔTm) seems to be related with the pinning of dislocation lines similar to the Köster effect. (2) The difference between the traveling time at To and that after stress release (ΔTor) increased with the plastic strain (εply). The dislocation is in un-stable state when the plastic strain appears, so that its mobility becomes larger. Even in the elastic range, ΔTor was observed. (3) The change in traveling time (ΔTa) during stress application increased parabolically with the stress in the elastic range, similar to (ΔTr) during stress release. When some plastic strain pre-existed, they decreased at first depending on the amount of the plastic strain, reached a minimum, and then increased with the stress.
The variation of mechanical properties due to the mass effect in cast iron was examined on the specimens cut from several parts of a gray cast iron flywheel by subjecting them the repetition of heating and cooling, and of annealing. The conditions of heat treatment were as follows: (1) from 800°C to 600°C, (2) from 650°C to 350°C, and from 350°C to 150°C, and (3) from 800°C, 650°C, or 350°C to the room temperature, respectively. In the case (1), the change in mechanical properties of each part decreased rapidly in the early stage of heat repetition, and every part yielded almost the same strength after heating cycles of more than 50 times. On the other hand, their microscopic structures showed that the area percentage of graphite and ferrite increased with increasing number of heat repetition, and a good correspondence existed between the changes in the structure and in mechanical properties. In the case (2), the same tendency with the case (1) could be recognized between the changes in mechanical property and microscopic structure, but since these changes were small, a larger number of heat cycles was necessary for the homogenization of mechanical properties of every part. And, in the heat cycles at low temperatures (350°C_??_150°C), the changes in mechanical properties and microscopic structures were not detected even after 100 cycles. In the case (3), that is, in the repetition of annealing, the changes in mechanical properties and microscopic structures revealed almost the same tendency with those in the case of the heat treatment mentioned above.