This paper presents the dynamic stress-strain properties of styrofoams under impact loading acting on a limited area. The specimens with the dimension of 100_??_mm×30tmm were bonded on the end of surface of the pendulum being impacted. The impact loading was applied to the center portion of the specimens by another impact pendulum with the diameter of 50φmm. The acceleration-time curves induced in the impacted pendulum were measured. The dynamic stress and strain were calculated by integrating these acceleration-time curves numerically. The effects of the strain rate and the number of impact loadings on the dynamic properties of styrofoams were examined in comparison with the results of the uniform impact loading on the whole_ surface area described in the previous report. The cushion factor-stress properties were also discussed. The results obtained are as follows: (1) Under localized impact loading, the strength and stiffness of styrofoams increase with increasing strain rate up to about 20/sec, beyond which they start to decrease. (2) A remarkable difference exists between the stress-strain properties under localized impact loading and those under uniform impact loading in the strain range beyond 20%. (3) The stiffness of styrofoams is remarkably reduced in the low strain range by the first application of localized impact loading, but the reduction becomes comparatively small in the higher strain range. The further reduction in stiffness is induced by the successive application of impact loadings. (4) The styrofoams seems to have the critical deformation rate at the strain rate of 35/sec under localized impact loading. In this strain rate, the dynamic stress-strain relationship becomes complicated because of the fracture in the vicinity of loading area. (5) The depth of the disintegration produced in styrofoams by localized impact loading can be indirectly estimated from the knee appeared on the cushion factor-stress curve.
Rotating bending fatigue tests were carried out on two kinds of solution-treated austenitic stainless steels (Type 321 and 347) at room temperature, 400, 500, 600 and 700°C with a frequency of 7500r.p.m. and their fatigue strength at 108 cycles was obtained by the staircase method. The specimens of Type 347 steel aged for 24hrs at 750°C were also tested at 600 and 700°C. The plastic deformation behavior under a constant stress amplitude was observed at room temperature, 500 and 700°C for Type 321 steel. The results were compared with those on other austenitic stainless steels (Type 304 and 316) which had been obtained previously by the authors. The results obtained are summarized as follows: (1) The following phenomena were observed commonly for the four kinds of solution-treated steels investigated. (i) Endurance limits are clearly found in the temperature range from room temperature to 600°C. At 700°C a clear endurance limit appears below 107 cycles, but the fatigue fracture starts to occur again after 107 cycles. (ii) The ratio of fatigue strength at 108 cycles to 0.2% proof stress is 1.2∼1.7 at 400∼600°C. (iii) The coaxing effect is distinctly observed in the temperature range from 400 to 600°C. At 700°C it is scarcely observed after 108 cycles. (2) The aged 347 steel showed no stepwise S-N curve at 700°C and its cycles to failure continued to increase with decreasing stress amplitude up to 108 cycles. (3) The plastic strain range for the stress amplitude near 0.2% proof stress decreased rapidly with increasing stress cycling at 500°C, and the material became to show substantially elastic behavior. (4) The above phenomena (1) commonly observed for the solution-treated steels are considered to be caused by the strengthening of the material due to aging during the fatigue test at 400∼700°C, and by the softening due to overaging after 106∼107 cycles at 700°C.
Recently, it has been noticed that the asphalt pavement flows remarkably as a result of heavy traffic. To eliminate or minimize the flow, it is necessary to increase the stability of asphalt mixture. The experiments described herein were undertaken to investigate the effect of asphalt penetration on the Marshall stability of asphaltic concrete. The experiments were performed under two different conditions; one at 60°C, and the other at various test temperatures between 25 to 60°C. The results obtained are summarized as follows: (1) At low temperatures such as 25°C, the optimum asphalt content (OAC) is independent of asphalt penetration, but the stability of asphalt mixture at OAC depends on the penetration; the smaller the penetration, the larger the Marshall stability. (2) At high temperatures such as 55 or 60°C, OAC changes with the penetration; the smaller the penetration, the smaller the OAC. But the asphalt penetration has little effect on the stability of asphalt mixture at OAC. Since the flow of asphalt pavement mostly takes place at high temperatures, it appears that the use of asphalt of low penetration is ineffective to decrease the flow.
Craze growth from a crack tip is an interesting problem in connection with delayed crack growth. When subjected to some stress in a certain environmental condition, high polymeric solids may craze at a crack tip. The craze zone initiated in front of the crack tip may develop with elapse of time. Unfortunately, not so many studies about this problem have not been carried out, although it is important in relation to the environmental delayed crack growth. The present experiment was carried out to investigate the relation of craze zone length in front of a crack tip to testing time. The results were compared with the theory proposed by Marshall et al. It was found that their theory was not satisfactory enough to explain the craze growth over the whole range of testing time. A new model taking a creep effect into account was proposed. The theoretical predictions derived from the model were compared with the experimental results. Fairly good agreement was obtained.
For the purpose of explaining the mechanism of so-called“fatigue hardening”, the static tension tests and the fatigue tests under pulsating tension with a constant stress amplitude were performed on a extremely low cabon steel and the relations among its yield strength, grain size, strain hardening exponent and strain ageing capacity were studied. The effects of decarburizing and denitriding on the mechanical properties were also investigated. From the fatigue experiments, the static tension properties during fatigue stressing and the variation of plastic strain amplitude under constant stress amplitude were clarified. Several significant results were obtained as follows: The values of the constants in the Petch relation on yield stress increase with strain ageing capacity. The strain hardening exponent does not depend on decarburizing and denitriding but its value increases with grain size. It is found that denitriding is much more effective than decarburizing to eliminate strain ageing. Three stages are recognized in the fatigue process of unnotched material. The hardening in STAGE I results mainly from strain hardening, while strain ageing seems to be the dominant factor of fatigue hardening in STAGE II.
This paper presents the results of constant load low-cycle rotating bending fatigue tests that were carried out on nodular cast iron and grey cast iron with the speed of stress repetition at 10 and 100rpm. The deflection values of specimens (vertical and horizontal deflections) were monitored and recorded during the tests. All tests were conducted at room temperature. The results obtained are summarized as follows: (1) The low-cycle fatigue process of the nodular cast iron and the grey cast iron can be expressed by the horizontal and vertical deflections. (2) Under the low-cycle rotating bending tests, the relations between N and σa/σB for both the nodular cast iron and the grey cast iron can be given by almost the same straight line on the logarithmic scale coordinates. (N: number of cycles to failure, σa: stress amplitude, σB: tensile strength.)
The effects of ausforming on the ductile-brittle transition temperature of SKD 61-steel (H 11-steel) were examined by measuring the hardness and determining the transition temperature from the intersection of the straight line parts of the plot of the logarithmic Vickers hardness vs. the reciprocal of absolute temperature. The results are summarized as follows: (1) In the measured range from 60°C to -196°C, the ausformed steel has two transition temperatures T* and T** in the hardness test. (2) The ductile-brittle transition temperature of the ausformed steel when plotted as a function of rolling ratio changes a little (within 20°C), having a slight maximum. Among several factors influencing the ductile-brittle transition temperature of ausformed steels, the grain size and the dislocation density are considered to be very important. As a rule, these two factors in the ausforming give opposing influences on the ductile-brittle transition temperature, so that the ductile-brittle transition temperature of this ausformed steel remains almost unchanged as compared with that of a conventional heat-treated steel. Therefore, the ausforming is considered to be a strengthening treatment without having embrittlement rather than a treatment to improve toughness.
The solid-state reaction of ZrO2 and TiO2 from 1380°to 1530°C and its kinetics were investigated. The reaction rate was determined from X-ray diffraction intensities of the strongest peaks of the reactants and product ZrTiO4. The marker experiment indicates that diffusion of Ti4+ or O2- through ZrTiO4 controls the reaction rate. The activation energy for the formation of ZrTiO4 was about 45∼57kcal/mole. The effects of the minor component added and atmosphere on the reaction rate of ZrTiO4 formation were also discussed.
Optical absorption spectra of Ni2+ and Cr3+ have been determined over the range 350∼2500nm in binary silicate, borate, phosphate and germanate glasses. The values of the crystal field strength Dq and the Racah parameters B and C for the glasses were calculated from the observed band positions. The assignments for the bands ν1∼ν5 and νT1 of Ni2+ and Cr3+ in these glasses were made and the calculated band positions of ν1∼ν5 were determined as shown in Table I. The increase of Dq in the composition range of approximately 25∼45mol. per cent PbO for the lead-borate glass is interpreted based on the evidences that the four-co-ordinated borons are present and the type of lead-oxygen association becomes more covalent bonding.