In order to clarify the behavior of pitting damage, the rolling contact fatigue tests were performed under the pure rolling and the sliding-rolling contact conditions on the annealed S45C roller specimens, and especially the surface and the cross section of roller specimens during the rolling contact fatigue tests were observed with microscopes. The summarized results are as follows: (1) The pits develop along the forward flow formed in the subsurface layer of roller under both the pure rolling and the sliding-rolling contact conditions, and the shapes of pits are on the whole irregular in the pure rolled specimens but are fan-shaped in the sliding-rolled specimens. No subsurface origin cracks are observed in both contact conditions. (2) The damages, such as the scratching or adhesive wears, the micro-pits caused by the repetitions of the normal and tangential stresses and so on, appear on the roller surface before the formation of pits. They are observed mainly in ferrite grains. And the dislocation microstructures of the roller surface exhibit distinct substructures at the formation of pitting cracks.
The flow characteristics of foam polystyrene which contains 1.5wt% butane were investigated and compared with those of polystyrene used for general purpose. The apparent viscosity of foam polystyrene was lower than that of general polystyrene. This may be due to that the Brownian motion of polymer molecules becomes more active by the existence of minute bubbles and reduces the intermolecular force or friction (shearing stress). The non-Newtonian flow index of foam polystyrene was smaller than that of general polystyrene. This may be due to that the random Brownian motion of molecules becomes more active and thus makes harder the imperfect orientation of molecules arising from the increase in shear rate. The activation energy of flow of foam polystyrene was slightly smaller than that of general polystyrene, which may be due to that the molecules of foam polystyrene melt become more movable by the existence of minute bubbles. Besides, the influences of pressure and temperature upon foaming pattern and foaming ratio were investigated, and it was found that the greatest foaming ratio can be obtained at the optimum temperature (215°C) for extruding general polystyrene, and, at constant temperature, a good skin of extrudate can be obtained only when the extruding pressure exceeds a certain limit value.
The creep strain rate of metals under varying load and temperature depends not only on the instantaneous load and temperature but also on the history of load and temperature conditions, because the microstructure of the metal changes according to the history of these conditions, and affects the degree of resistant stress, or so-called internal stress, which lowers the effect of applied stress upon the movement of mobile dislocations. As the first step in deriving a general deformation law, the authors examined the change of internal stress and microstructure during transient creep by the stress dip testing and the X-ray diffraction technique. Results obtained are summarized as follows: (1) The internal stress increases during transient creep but remains constant during steady state creep. As the applied stress becomes larger, the magnitude of the internal stress becomes larger. The ratio of internal stress during steady state creep σiss to applied stress σ, that is σiss/σ, does not remain constant but decreases with increasing applied stress. (2) The creep strain rate is related to the effective stress (σe=σ-σi) independently of applied stress and strain. (3) The dislocation density measured by the X-ray diffraction technique is related to the internal stress as σi=7.8+1.50×10-4√ρ (4) Vickers hardness HV is related to the internal stress as σi=0.214HV-12.4
For the purpose of investigating the intrinsic nature of the impact strength of screws and establishing the ways of designing and utilizing the bolts under impact tensile load, the screw and bolt specimens with or without nuts were subjected to impact tensile load from the nut side, and the behaviours of high-speed impact tensile fracture of screws were examined. The bolt materials used were made of several kinds of carbon steel, stainless steel and IN steel, and the impact velocity ranged from static to 60m/sec. The results are summarized as follows: (1) To examine the high-speed impact tensile characteristics of screws, it is appropriate as the test condition to employ a specimen fitted with a nut. (2) The impact tensile strength of the screws made of materials having large elongation characteristics, such as S10C-D, S15C-D, S20C-D, 13 Cr steel and IN steel, was greater than that of the round bars at the root. The strength is generally comparable, up to the impact velocity of about 10m/sec, to the impact tensile strength of an equivalent round bar with a diameter of de=(pitch diameter of thread+root diameter)/2. However, beyond the impact velocity of 20m/sec, the latter value exceeds the former and the notch effect appears conspicuously. (3) In the case of a material having small elongation characteristics, such as S55C-D (δ3.7: 25%), the static tensile strength corresponds to that of the abovementioned equivalent round bar. Although a considerable dispersion was observed for the impact tensile strength, its mean value seems to correspond to the impact tensile strength of the equivalent round bar. In the case of a high carbon steel (C: 1.1%), the static tensile strength similarly corresponds to that of the equivalent round bar, but in the impact test all the specimens inclusive of those having a root diameter smaller than the diameter of the equivalent round bar showed a break in the screw portion prior to a break in the smooth portion. Therefore, the abovementioned concept of impact tensile strength of the equivalent round bar is not applicable to this case. Accordingly, a material having large elongation characteristics under high-speed deformation is required for impact-proof screws.
A fatigue process consists of crack initiation and crack propagation. So the authors have tried to estimate the fatigue life to failure, Nf, as the sum of the crack initiation life Ni and the crack propagation life Np. The authors also assumed that the effect of the mean stress on the fatigue damage is different in each of these two processes. Then, the actual service load patterns have been analysed by applying the range-pair-mean count method as follows. Each stress range pair σa with mean stress level σm, that is S(σa, σm), is translated into a stress amplitude σt by taking σti=σa+mσm, where m=0.4σw/σs, in the crack initiation, and σtp=σa+kσm for σa>|σm|, k=0.6∼1.0, or σtp=(1+k)(σa+hσm)/(1+h) for σm>σa>0, where h=0∼0.5, in the crack propagation process. Then the fatigue damage FN' which is produced in the N'th stress cycle, is expressed as FN'=eAσt+D·e-γ(N'-1) where A, D and γ are constants. These constants, which are given as Ai, Di and γi for the crack initiation and Ap, Dp and γp for the crack propagation, can be determined from the fatigue test results. The fatigue tests on annealed low carbon steels under plane bending condition were conducted. The surfaces of the test pieces were observed occasionally with an optical microscope and by the replica method, and the micro-cracks were detected. The fatigue tests under varying mean stresses in stepwise were also conducted. Those test results were analysed by means of the method mentioned above, and the estimated values had a good agreement with the test results. It was also clarified that the fatigue damage is affected by the stress waveforms and their repeating speeds.
Fatigue crack propagation in epoxy resins containing 0 wt% to 45wt% plasticizer was examined. In this experiment, double-cantilever type specimens were used. The stress intensity factor was adopted to express the stress near the crack tip. When specimen was cyclically loaded, not only the range of stress intensity factor ΔK but also the maximum stress intensity factor Kmax or the mean stress intensity factor Kmean were taken into consideration. Under the assumption of dl/dN=A(Kmax)α1(ΔK)α2 or dl/dN=B(Kmean)β1(ΔK)β2(dl/dN means the fatigue crack propagation rate) as the fatigue crack propagation law, dl/dN was measured for several combinations of ΔK and Kmax or Kmean and the values of constants A, α1, α2, B, β1 and β2 were evaluated from the values of log(ΔK) and log(Kmax) or log(Kmean) by means of least squares analysis. As the results, it was found that as the percentage of plasticizer increases, A and B increase by two orders, α1 and β1 decrease by 40%, and α2 and β2 decrease slightly. For the specimen without any plasticizer, both α1 and α2 are equal to about 5, and for the specimen with 40 wt% of plasticizer, both β1 and β2 are equal to about 4. It is interesting that the exponent of ΔK remains constant in spite of the decrease in exponent of Kmax or Kmean. The mechanical properties vary widely from the brittle fracture type to the ductile fracture type as the percentage of plasticizer increases, and it seems that the above interesting results about the fatigue crack propagation behavior in epoxy resins containing plasticizer are principally attributed to the difference in their mechanical properties.
Fiber reinforced plastics (FRP) are being used for many purposes as structural materials. The special features of FRP are their lightweight, possibility of one-body plastic moulding operations and resistance of corrosion among others. But the one-body moulding operation is limited to a small size and is not suitable for making a large-size liquid tank, container or fishing boat. Therefore, in order to answer increasing demands for large-size products, the bonding structure must be incorporated with FRP. This paper concerns with the low cycle fatigue strength of the FRP adhesive bonding structure, which are used for single lap joints. All of the joint specimens used in this investigation were glass roving cloth FRP materials bonded with epoxy-polyamides adhesive. The determinations of fatigue strength and fracture process of adhesive joints yielded the relations between static strength and fatigue strength as well as the stress-strain curves for FRP adhesive joints. Furthermore, the endurance of adhesive bonded joints were investigated on the specimens exposed to the accelerating exposed testing machine (weathering tester) for the time periods of 0, 500 and 1000 hours. The static and fatigue strengths of the adhesive bonded joints and the life of the bonded strength were examined. The breaking surface of the bonded joints showed the interfacial fracture, which occured between the fiber and resin matrix. The exposed specimens showed higher bond strength than the unexposed standard ones. Some considerations on the design of the FRP adhesive bonded structures are also presented.
We previously reported the correlation of physical properties of plastics (ultimate strength and relative elongation at break) with changes of their internal structures during the ultraviolet ray deterioration from kinetic point of view. Hereunder presented is the report of kinetic studies about the oxidation of water soluble plastic, PVA (polyvinyl alcohol), under ultraviolet rays in the atmosphere of nitrogen and oxygen. The differences in the mechanisms of deterioration under the different conditions of surrounding gas were investigated by the infrared absorption method. At the same time, the crystallinity and stereospecific (syndiotactic diad) were determined from the ratio of absorbances of crystallization (1141cm-1) and stereospecific bands (916cm-1, 849cm-1). The results of these experiments are as follows: (1) The rate of increase or decrease in the absorption band for each functional group is a first order reaction. (2) The remarkable increase in the absorptions of carbonyl groups, such as esteric type (1738cm-1), aldehydic type (1720cm-1), ketonic type (1709cm-1) and acidic type (1700cm-1), are recognized and their quantities are in the order of ketonic, acidic, aldehydic and esteric in nitrogen atmosphere and of aldehydic, ketonic, acidic and esteric in oxygen. (3) The mechanism of the photo-oxidation is closely related with the mechanisms of formation of these carbonyl groups. Especially, it seems that the deterioration is remarkably accelerated by the chain breaking caused by the formation of aldehydic and acidic carbonyl groups and by the crosslinking caused by the formation of esteric carbonyl group. (4) The values of crystallinity and syndiotactic diad, calculated from the ratio of absorbances (D1141/D1420, D916/D849), tend to increase. (5) The formation of ozonides is recognized and the ozone-initiated reaction is expected to take place from the consideration of the reaction process or the mechanism of deterioration. (6) The coloration is attributed to the chromophores (>C=O) and the bathochromic effect of auxochromes (-OH), and the degree of coloration is larger in nitrogen atmosphere than in oxygen.
We previously reported the kinetic studies of the accelerated aging of plastic films (polystyrene, polyvinyl chloride and polypropylene) in ultraviolet rays. But in case of polypropylene no remarkable change of internal structure (measured by the infrared spectrophotometer) was observed because of the inhibitory effects of several additives. Hereunder presented is the report of kinetic studies of the accelerated aging of polypropylene samples with or without various inhibitors such as ultraviolet absorbers and antioxidants in ultraviolet rays. Particularly, the inhibitory effects of the samples with various inhibitors were investigated from kinetic point of view. The results from these experiments are as follows: (1) The mechanism of deterioration of pure polypropylene is autoxidation which is induced by the hydroperoxide. The breakup of polymer chain frequently accompanies with the formation of carbonyl groups (ketonic and aldehydic types) and consequently the deterioration is accelerated. (2) Normal-propyl gallate (antioxidant) acts as an effective inhibitor for the deterioration of polypropylene. (3) Phenyl salicylate, 4-hydroxy benzophenone (ultraviolet absorbers) and β-naphthol (antioxidant) show little inhibitory effects. (4) In case of the sample with 4-hydroxy benzophenone the accelerative effect is observed rather than the inhibitory effect. It is presumed that this additive acts as an accelerator but on this matter more detailed experiments are required.