The formation and propagation of fatigue cracks were studied under repeated impact and rotary bending, and the following results were obtained. (1) In the case of repeated impact, the number of blows required for the formation of fatigue crack and the impact energy per blow are related by a hyperbolic law. In the case of rotary bending, the number of stress repetitions required for the crack formation and the stress intensity are also related by a hyperboliciaw. (2) The relation between the total number of blows required for the break-down of the specimen and the impact energy per blow of repeated impact is given by a hyperbolic law. The relation between the total number of repetitions and the stress intensity of rotary bending is also given by a hypetbolic law. (3) The relation. between the number of blows required for the stage from the crack formation to the break-down of the specimen and the impact energy per blow is approximately given by an exponential law. A similar relation exists also in the case of rotary bedding. (4) In the case of repeated impact, the ratio of the number of blows required for the formation of fatigue crack to the total number of blows required for the break-down of the specimen decreases by an amount varying from about 35 to 20%, as the impact energy per blow diminishes within a certain ranse. On the contrary, in the case of rotary bending, the ratio of the number of stress repetitions required for the formation of fatigue crack to the total number of repetitions increases by an amount varying from about 50 to 90%, as the stress intensity diminishes. Thus the behaviour of materials under the external force differs markedly according to the nature of stressing. (5) The mean initial velocity of propagation of crack becomes less as the impact energy diminishes, showing a hyperbolic law.

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Very few reports have, hitherto, been given of the investigation on high speed steels containing more than 20% Co. In the first and second reports, we have made clear on the effect of each of various alloying elements on high speed steel; In. the present investigation the similar studies, the changes of hardness due to the different heat treatment, and effect of each alloying elements on durability, have been carried on of special high speed steels containing 20 and 25% Co. The results obtained from the present investigation are as follows: (1) High speed steel containing a high percentage of Co are greatly affected by the heat treatment. (2) In such high speed steels containing a high. percentage of Co, higher durability is obtained when content of C is less than 0.4%. (3) The mot appropriate Cr contents in both cases of 10% and 15% Co is 3_??_4%. (4) W contents at 15_??_17% in both cases give the highest durability, and more conted of W results rather less durability. (5) The contents of V up to 1% rapidly increase the durability, and the further increase of V makes its influence comparatively weaker, and it reaches its maximum point at about 1.8%. (6) The Mo content up to 1.5% does not give much influence upon durability, but the further increase of Mo percentage brings less durability.

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It is known that fatigue limit of steel is lowererd by the hard chromium plating, but the details of this phenomenon are not fully tested and the cause is not explained. Using the rotating beam fatigue machine, we tested the fatigue limit of the test pieces of the following four types, each of which are Ni-Cr steel test pieces electrically plated with hard chromium of different thickness. Four types are as follows: (1) plated, (2) annealed, (3) with the plated surface finished with emery paper and (4) with the surface finished after annealing. of the above four cases the relations between the fatigue limit and the thickness of plating are representediin Fig. 3. The photographs of electron diffraction pattern of the hard chromium plating before and after (broken and not broken) the fatigue test are shown in Fig. 5, and are found no change in its pattern. From this fact we concluded that the plated chromium layer itself has no relation to the lowering effect. It is expected that the graininess of the plated surface will act as notch to the test piece and lower the limit. This expectation is verified by the following two facts: (i). seeing the curve (1) of Fig. 3, we find the following: the fact that the thinner the plating is, the lower the limit becoines, corresponds to the fact that the thinner the plating is, the sharper the graininess becomes, and (ii) comparing the curve (1) with (3), (2) with (4) of Fig. 3, we see that the finished smooth one has the higher limit than the plated grainy one Also by the same reason the annealed crack of the plating lowers the limit, which is clearly seen by comparing the curve (1) with (2), (3) with (4) of Fig. 3. From the fact that the deplated test pieces have the same fatigue limit as the paper finished ones and that at the static test the deplated one shows hydrogen brittleness, we conclude that the hydrogen absorbed into the steel during plating will lower the limit at a few percent.The internal stress between plating and steel is considered to act as the mean stress to the fatigue test and may lower the limit. But the part attributed to this effect will be small, if exist, because the fatigue limit of the annealed one, which has no internal stress, is lower than that of non-annealed one. Thus we attribute the lowering effect of fatigue limit of steel by the hard chromium plating to the graininess of the plated surface, the annealed crack, and the absorbed hydrogen.

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The corrosion resistance of mild steel, 13% Cr steel, 18% Cr steel and 18-S Cr-Ni steel against nitric and sulphuric acids mixed has been examined. The corrosion tests are carried out in 98% nitric and 98% sulphuric acids mixed and in 98% nitric and 30% fuming sulphuric acids mixed at 30° and 50°, by measuring the weight loss for 10 hours, and the corrosion rate is expressed as g/m²/hr. The results obtained are shown in Fig. 1_??_12.

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Effects of repeated bending stresses and aging upon damping capacities and electric resistances of SDR and SDH have been investigated. At first the damping capacity of SDR is independent of the amplitude, but after the repeated stress of about ±10 kg/mm² was applied for several million times, the damping capacity comes to increase with the amplitude and of this it recovers after 1 or 2 days.
When SDR and SDH are quenched from 500° and aged at room temperature, the electric resistance, the natural frequency and the damping capacity decrease after several hundreds hours, a phenomenon much like that of “Rückbildung”, and then increase again. This phenomenon is observed also when the repeated bending stresses are applied several million times on the specimen of SDH.

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The strong wrought aluminium alloys, SDH, SDR, ESDW and ESDT, and clad sheets, SDCH, SDCR and ESDC have been fatiguetested by the method of 3% salt water spray, with Schenck's vibratory endurance testing machine. The S-N diagram thus obtained indicates the scattering of results for the test particulary in the corroded condition. No difinite conclusion was obtained from this test, but the results are as follows: (1) The fatigue strength and orrosionfatigue resistance of ESD has the highest value in these specimens. (2) The ratio of fatigue strength of the simple sheet to the clad sheet are in the range of 47% to 28% in air, the decreasing percentage of fatigue strength in the salt water spray to that in air changes from 88% to 39% in the sheet, and from 71%, to 51% in the clad sheet. (3) From the microscopic study, many hair cracks are seen to be produced in the surface layer of a clad material, and this phenomenon indicates the decrease of its 6, tigue resistance.

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