This is the report of one of the experiments performed by the Boiler Research Committee of the Society of Mechanical Engineers in Japan. The experiment was done under the direction of the late Prof. A. Inokuty, to whose idea the device and method of testing are mostly to be ascribed. The holding strength of the expanded tube joints used in boilers, such as smoke tube or water tube, has already been tested abroad ; but in most of these testings the maximum holding strength of the joints were only measured, and the literatures concerning to it are at hand. The object of our experiment is to know the respective holding strength of the joints at any amount of successive yielding, so that we are enable to draw a curve showing the relation between the yielding and the holding strength of the joints, as that of a stress and strain diagram in tension test. To bring to effect our object, we decided to draw off a tube joint at various steps and to measure the holding strength of the joint at each stage. The apparatus was specially deviced to comply with the purpose of our experiment. We attached this apparatus to a Buck ton's 50 ton Testing Machine and drew off the test pieces of several tube joints step by step, the yielding in each step being perdetermined at 0.05″, 0.10″ or 0.15″. The test results of all joints have been plotted in diagrams from No. 1 to 47 which are given as appendix to the text, and you will find the characteristic feature of the strength of various joints on looking upon them. The following remarks may be given as a summary of our experiment : (i) The holding strength varies greatly even with the same kind of test pieces. This is to be attributed to the difference of their expanding and hence a definite figure as to their strength is not to be obtained. (ii) The point of the maximum holding strength occurs when joints have yielded from 1/2 to 2mm. This shows that most tube joints can bear more than the force or pressure which causes to commence their yielding. (iii) In some of conically flared tube joints their strength is more than the tube body itself; or the tube body breaks down before its end joint slips off.

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