The residual stress caused by drilling in photoelastic plastics was investigated by using the photoelastic measurement. The distribution of residual stresses around the drilled hole on a circular plate is apparently axisymmetric when the hole is small as compared with the size of the plate. We can obtain the distribution of residual stresses on the circular plate with a small hole drilled at the center, by measuring the isochromatics which arises from the superposition of compression being applied along a diameter of the circular plate and the initial stresses. The specimens were circular discs, 7mm in thickness and 29mm in diameter, which were prepared by slicing the rods of polycarbonate. The discs were drilled at the center with straight shank twist drills under several different drilling conditions. The isoclinics of the discs drilled at the center were examined under a no loading state, so as to ascertain that the distribution of residual stresses was approximately axisymmetric. Higher residual stresses were caused by smaller diameter of drill and faster rotational drill speed. The maximum value of residual stresses was about one-tenth of the yield stress of the material. The residual stresses were relieved by annealing the discs at 130°C for five hours.
The effects of Mn and Cr on tensile properties of Al-Mg alloys were investigated by the tensile test performed in the temperature range from room temperature to 400°C. The compositional ranges of the test alloys were Mg=0-5%. Mn=0-1.0% and Cr=0-0.5%, respectively. Specimens were preannealed at 450°C for 1hr to bring them up close to the steady state at high temperatures, and then heated up to the test temperature and held for 30min. The results obtained are as follows: (1) The strength of alloy at relatively low temperatures depends largely on Mg content and increases with increasing its content. On the other hand, Mn and Cr have little effects, increasing slightly the strength but decreasing the elongation with the increase of their contents. (2) It would be satisfactory to consider that the strength of alloy at relatively low temperatures is primarily caused by the effect of Mg dissolved in Al, which improves the solid solution hardenability and work hardenability. (3) With rising temperature, the contributions of Mn and Cr on the strength gradually increase but that of Mg decrease, on the contrary. The strength of alloy, therefore, turns out to be dependent on Mn or Cr content rather than on Mg. (4) The behavior at relatively high temperatures is caused by disappearance of the effect induced by the dissolved Mg on account of the expansion of crystal lattice and of the high diffusion rate of Mg in Al. It is also caused by the contribution of Mn and Cr on the maintenance of strength owing to the small diffusion rates of these elements and the derivative effect of delayed recrystallization.
The observation has been made on the surface cracks initiated on extremely pure aluminum fatigued by tension and compression. Plate specimens, 0.7mm thick were completely annealed and then electropolished. The resulting grain sizes were from 0.2 to 1mm diameters. The tests were made with the strain amplitudes of 0.05% and 0.25%. Their surfaces were observed with an optical microscope and a scanning electron microscope. The slip band patterns observed could be classified in three types: x1, long and straight one, x2, wavy one, and x3, meshlike one. Specimen I subjected to the smaller strain amplitude of 1.7×106 cycles revealed the many intergranular cracks when the slip bands were removed by a slight electropolishing. By examining the relation between the silp bands and the cracks, the following facts could be clarified: The slip bands of pattern x1 which lie in direction of from 40° of 55°to the stress axis were in high angles with the cracks along the grain boundaries. On the specimen II fatigued at the larger strain amplitude, the intergranular cracks could also be observed after 2×104 cycles. The cracks propagated along the grain boundaries until about a half of fracture life (1.4×105 cycles). The relation between the cracks and the slip bands was the same as that which was observed on the specimen I. The following conclusion may be driven: The glides are impeded at the grain boundary and many dislocations pile up there. The grain boundary should be destroyed. Those microcracks join with each other and grow up into an observable crack.
In the experiment, length of crack, which had started from a notch hole, was measured by a microscope, under the basically tested conditions of Δε=1.0%, ν=5cpm. Additional test conditions are some combinations of Δε=0.3, 0.5, 1.5%, ν=0.1, 1.0, 10, 30, 60cpm. The temperature level is between R. T. and 600°C. The results obtained are summarized as follows: (1) During a strain controlled low cycle fatigue test at 5cpm, the rate of crack propagation is the largest at the blue-brittleness temperature range (200-300°C) and the smallest at the high temperature range (500-600°C). Temperature dependence of the rate of crack propagation seems to be opposite of that of tensile ductility. The period of crack initiation becomes shorter as a temperature increases. (2) At higher temperatures (≥400°C) and low frequencies, numerous micro cracks are initiated in the vicinity of a main crack tip. The main crack propagates rather rapidly by coalescing these micro cracks. Temperature dependence of the rate of crack propagation is effectively influenced by the change of test frequency. (3) The increase in the rate of crack propagation due to coalescing process is estimated qualitatively with the assumption that the micro cracks prohibit the decrease of ductility of a material in the vicinity of a main crack tip.
The rotating bending fatigue tests were perfomed on the specimens of nodular cast iron with unnotch, semicircular notch and 60°V-notch at room temperature, 150°C, 300°C, 400°C, 550°C, and 800°C with the frequency of 3600rpm. Their values of fatigue strength were obtained at 107 cycles. The results are as follows: (1) The fatigue strength of the unnotched specimen decreases with the rising test temperatures. At about 400°C, however, the fatigue strength takes the slightly high value due to the blue brittleness. Beyond the range of 500°C to 550°C, the fatigue strength decreases rapidly. (2) The fatigue strength of the notched specimens decreases with the rising test temperatures. At about 300°C to 400°C, however, the fatigue strength shows a near constant value. Beyond 400°C, the fatigue strength decreases rapidly. (3) The value of fatigue notch factor gradually increases with the elevating temperatures above room temperature and then increases rapidly beyond about 400°C to 500°C.
In this report, the fatigue properties of an ausformed 12-Cr stainless steel have been clarified by means of S-N curves, crack growth rates and microscopic observations of propagating cracks and fractured surfaces. The results obtained are as follows: (1) The endurance limits of a plane specimen are improved in proportion to the reduction ratio of ausforming and the fatigue lives at a definite stress level are lengthened markedly by ausforming. At the same time, the crack growth rate slows down in the ausformed specimen. (2) The modes of crack growth, precipitation of carbides and fractographs differ clearly between conventional and ausforming processes and these differences seem to cause the above results. (3) The crack growth rates are represented by the equations which contain a stress intensity factor and a parameter of mean stress in each process. By these equations, we can recognize that the effect of mean stress on crack growth rate becomes larger for the ausformed specimen. (4) The fractographs show that the fracture surface of the conventional heat treated specimens exhibit either relatively rough striations or intergranular fracture patterns, while the fracture surfaces of the ausformed specimens are almost covered by the mixed patterns of rather irregular striations and flat areas which look like rub marks.
It is well-known that the corrosion rate of iron and steel is affected by the Fe3+ concentration in acidified solutions. Experimental works on oxidation from Fe2+ to Fe3+ by the dissolved oxygen in various electrolytes have been carried out. Generally, the process is first order with respect to Fe2+, and it depends much on the pH. The reaction rate increased with decreasing pH in HCl, while the rate decreased with decreasing pH in other solutions such as phosphate, sulfate and citrate buffer solutions. The reaction rate was less than 10-3min-1 in solutions of pH<4, or the increment of Fe3+ was only 10-5M/min in solutions containing Fe2+ of 10-2M. The limiting current density for the cathodic process of those Fe3+ to Fe2+ would be 1.6×10-5ma/cm2, which is very small in comparison with the corrosion rate of Fe in aerated solutions. Therefore, it is concluded that the effect of oxidation of Fe2+ by the dissolved oxygen is negligible on corrosion of iron and steel.
The mechanical properties of unsaturated-polyester resin mortar is adversely affected by water when it hardens. This is a serious problem when it is applied as cast-in-place mortar, because in a field work there are few places where water can completely be eliminated. The demand for the cast-in-place resin mortar, however, is becoming greater. Therefore, the decrease in strength of resin mortar caused by water must be solved. Two kinds of effect of water have been proposed. (1) When resin mortar is placed or joined in water, it occludes water. This occluded water decreases the mechanical strength of resin mortar by abstracting the complete adhesion between resin and aggregate. (2) Aggregate for resin mortar is required to be completely dry. Water content of aggregate is usually kept less than 0.3%. In this paper, aggregate was treated with silane coupling agent in order to minimize the influence of water. Three kinds of silane coupling agents, γ-methacryloxypropyl trimethoxysilane, vinyltriethoxysilane and vinyltris (β-methoxyethoxy) silane were used. The results are as follows: (a) When the resin mortar made of untreated aggregate had been molded in water, it showed 13% lower tensile strength than that which was hardened in air, while the resin mortar made of treated aggregate preserved the strength as much as 95%. (b) As for the tensile strength of the joint, the resin mortar made of untreated aggregate showed 40% lower strength than that which was joined in air, and more than 88% of the strength could be preserved with the resin mortar made of treated aggregate. (c) The effect of water content in aggregate on the tensile strength of the resin mortar was examined. The resin mortar made of untreated aggregate showed a steep strength drop even at small water content. The resin mortar made of treated aggregate showed only a mild strength drop even up to 10% water content.