The strength of tubular struts

Abstract

The problem of the strength of a circular tubular strut differs from that of a solid strut of the same material in that there may be a condition of instability in the wall of the tube apart altogether from that type of instability considered in the Eulerian analysis which involves only the bending of the centre line of the strut. It is found that when a thin tube is tested in direct compression up to collapse, it fails by the formation of characterisic folds in the wall, examples of which are given in the photographs, figs. 7,10 and 11 (Plate 11). The folds are of two types depending upon the ratio of the thickness t to the radius R of the tube. Above a certain value of t /R the fold consists of a uniform bulge in which all cross sections remain circular; below this value of t /R the walls “cave in” in several places producing a series of lobes. The phenomenon has been called wrinkling, and also secondary flexure, to distinguish it from the flexure produced by the bending of the centre line of the strut. The problem has been investigated mathematically and experimentally. On the mathematical side the best-known investigations are those of Lilly, Southwell and Dean; on the experimental side those of Lilly, Mason, Barling and Webb, and Popplewell and Carrington. The present investigation is experimental and the greater part of it was carried out in 1915, at which date the only investigations with which the author was acquainted were those of Lilly and Mason. A paper describing the work was presented in 1915 to a Sub-Committee of the War Committee of the Royal Society and was also com­municated to the Air Departments of the Army and Navy. General publica­tion, however, was withheld until the end of the war. A short summary of the conclusions reached was given to the British Association Meeting in 1919. Since the major part of the work was completed and reported upon, experimental investigations have been published by Messrs. Barling and Webb and Messrs.