Literatures show that very few tests have been made on the fatigue strength of prestressed concrete beams. This paper reports the results of fatigue and static tests on two kinds of posttentioned prestressed concrete beams. The Losenhausen fatigue testing machine was used for these tests.
The distinctive characteristics obaiined from this investigation are as follows;
1) All the steel bars used for prestressing the beams are of high strength steel having the ultimate strength of over 100kg/mm².
2) The upper limit of repeating load is larger than the cracking load of the prestressed beams found in static test.
The main results obtained in this experiment are;
1) The fatigue strength at N=6.4×10⁵ is 130% of the cracking strength.
2) The fatigue strength at N=6.4×10⁵ is 60% of the statical ultimate strength.
3) When the upper load of fatigue test is below the fatigue strength at N=6.4×10⁵, the statical ultimate strength is little affected by the repeated loading subejected before the statical test.
4) When the upper load of fatigue test is over the fatigue strength, the dynamic deflection and compressive strain during fatigue test increase with the numbers of repetition, and when the upper load of fatigue test is below the fatigue strength, they are almost constant irrespective of the numbers of repetition.

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It goes without saying that the material of a standard block for hardness test should not fail to be homogeneous and give the same hardness number regardless the measuring points on the surface of the block, thus the hardness number measured on the block, must always agree with the standard number, which is alotted according to the same standard, independent from time and place. It is desirable, therefore, that we may obtain the reliability of hardness number as a standard by the stadard hardness block.
A great deal of studies on the standard block have been made, which seems to lead to succeed in manufacturing the standard block to certain extent, but there is still left to be discussed in details about this problem. The author discusses on the Vickers standard block on market in this paper, from the results obtained by the research on depression, especially on the piling-up. The demestic standard blocks supplied by two makrers (H_V=200-800) are tested on depression ranging from 1 to 50kg with subsequent examination of depression by the Ogoshi Surface Roughness Tester on the surface.
The results are summarized as follows:
(1) Every block shows its homogenuity having no difference in hardness number notwithstanding the measuring points.
(2) The degree of piling-up differs according to the makers, but the aspect of piling-up has the same inclination regardless the makers, and there is the highest piling-up at H_V≈400.
(3) There is no continual relationship between the hardness number measured on each standard block and the degree of each piling-up, and the influence of error on harddness caused by the piling-up is not proportional to the hardness. It seems desirable, therefore, to toke the influence of piling-up into consideration for the standard series of hardness block.
(4) And it is necessary that a standard block manufactured shold be of those materials which have less piling or piling to the equal amount at least.
(5) We can easily obtain the general aspects of the piling-up by means of the Knoop indentator.

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Fibers for fishing industry have been supplied exclusively with natural substarces such as flax, cotton or silk years back. However, of late, with such a remarkable development in the fabrication of man-made fibers, synthetics have gained their popularity in the industry and are challenging with success to the more conventional types of fiber.
Among the various properties any maritime fiber is supposed to have, resistance to abrasion as well as strength is of vital importance.
This reporter would like to give some accounts of a newer type of abrasion testing machine for fishing-net cords, which is featured with an elimination of shortcoming involved in an usual measurement method or in a conventional testor. This model yielded very satisfactory results.
Of a number of factors that every testor of this nature must necessarily be possessed of, the following points were received a special attention in our effort to build up this testor.
1. An abradant is open to unreproducible change in its surface condition with abrasion. Therefore, in its stead, such a mechanism is used as to permit abrasion between two ends of cords possible.
2. To that extent that frictional strength created by rubbing process is added to original tensile strength by dead-weight, real breaking tensile strength is likely to be all the more bigger. Yet, in reality, tension is valued by the amount of dead-weight charged, with a resulting measuremeet errors being aggravated in an area where dead-weight bears only a minor significance. This structure is featured with a mechanism by which friction force at abrading process will not be added, per se, to the strings tested.
3. Abrasion will register a different value according to the angle to the string abraded. This mechanism is designed to measure up diagonal abrasion to the string.
4. Generally speaking, abrasion takes place with flexings, so this mechanism is built by taking that fact into fullest account.
5. With our mechanism, it is as easy to measure abrasion under water as that in air.
6. In order to facilitate simultaneous measurement the mechanism is made small in size, yet is able to handle so many of materials for test purpose.
Outline of the Mechanism:
Ends of fibers are crossed, as are depicted on the Figure, on freely rotatable four (4) round poles (16mm∅) that will reciprocate up and down (51r.p.m) with their relative position remaining unchanged. Then set the round poles in motion upward and downward, keeping the strings stretched bs plummets (weighing 0.5 to 2kg.), with pulleys inbetween. With a passage of time, the fibers undergo wear and tear, followed by final breaking. The larger the dead-weight, the smaller are the reciprocating rounds before the break. The functional relationship between the rounds and dead-weight is, as has been found, that of logarithmic linearity with in a certain limit. A series of tests conducted both in water and in air revealed that resistance to abrasion and tensile strength did not go hand by hand. Tension of Teviron and Saran, in particular, is the same either in water or in air. Resistance to abrasion in air is, however, appreciably weaker as compared with the value in water. This may be accounted for by the fact that these synthetics have essentially very low thermo-softening points and that abrasive process otherwise accelerated by rubbing heat is seemingly checked by cooling-off effect of water. Nylon is known to have higher resistance to abrasion than any other fibers, and the blending of the former with the latter to cement for further resistance to abrasion will exert a considerably favorable effect.

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It is the well-known fact from long ago that porcelain insulators are broken by using the cement as binding materials for porcelains and hardwares.
The causes have been considered as follows.
(1) the thermal expansion of cement
(2) the chemical expansion of cement
(3) the abnormal expansion of cement caused by CaO and MgO
(4) the expansion of cement caused by absorption of moisture
(5) the expansion of cement caused by freezing
Writers tried an experimental investigation on some deteriorated insulators and specimens exposed to open air for years. The specimens for the test are bars made of various kinds of cements and mortars, and porcelain cups filled up with cement. From the result obtained by this experiment, we have come to the following conclusion.
Of above-mentioned five causes, comm, (4) is the most significant, in general case, followed by (1) and the others are almost unimportant. But the failure of porcelain insulators can't be fully explained by these causes only.
Hardening and contraction by losing moisture in cement lead to give rise cracks in it. If it takes place these cracks are filled up with dusts and the deposition of soluble salts. In original, if the expansion by absorption of moisture take place on cement, which such a condition, its volume becomes far greater than the leads the porcelain insulators to break, In the process of this kind of action the thermal expansion of cement plays its part in some extent.

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Previously, the authors carried out the static and the fatigue tests on various welded joints of the mild steel and the high tensile strength steel tentatively manufactured by the JSTM, and made a proposal on the allowable stress in the designs of welded steel bridges.
In this experimental research, (1) the authors carrid out the fatigue tests on butt welded joints connecting the thick plates of high tensile strength structural steel, under the repeated stresses of completely reversed type -σ_max-+σ_max, partly reversed type -6kg/mm²-+σ_max. and pulsating type 0-+σ_max., +6kg/mm²-+σ_max., +12kg/mm²-+σ_max., +20kg/mm²-+σ_max..
The results of the present tests were compared with those of previous tests.
(2) Moreover, the authors studied the effect of the treatment of the welded part on the fatigue strengths.
The results obtained are as follows.
(a) In these tests the relation between fatigue strength σ_max and σ_min was found to be expressed by the linear equation at pulsating stress area on one hand, but by the non linear equation at reversed stress area on the other hand. For simplicity the same linear equation was adopted for the latter as it is in safety side.
(b) The fatigue strengths obtained from test (1) above were slightly lower than those obtained from the previous tests of the high tensile strength structural steel (t=12mm) trially manufactured by the JSTM.
The fatigue strength, however, can be raised up by finishing the welded part.
Judging from these test results, the general equation for the allowable stress of the welded steel bridges proposed by the authors previously, can also be applied to that of the welded bridges of the thick plate of high tensile strength structural steel.
(c) The fatigue failures of all specimens at as-welded condition and of those with peening on reinforcement occured at the side of the butt welds, which might be due to the stress concentration at that points.
(d) It was recognized that the fatigue strength of the as-welded specimen could be raised up greatly by the treatment of peening on reinforcement, and the specimens grinding finished after reinforcement was chipped off show the highest fatigue strengths.

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