Dependence of Joint Strength on the Degree of Expanding in the Tube Expanding Process

The experiments, which were sponsored by the British Shipbuilding Research Association, were designed to study how the strength of an expanded joint depends on the degree of expanding with normal propulsive type expanders. Fourteen tests were made, varying the back extrusion from 0.029 in. to 0.093 in. on a 2 in. O.D. × 11/2 in. bore tube in 1 1/4 in. long seat

There is an optimum degree of expanding for maximum joint strength measured either by the push-out load or mean seat pressure. The position of the maximum in the curve of push-out load is dependent upon surface finish. Within the inevitable variation present in industrial machining finishes the presence of a peak will be masked. Moreover, the peak occurs at a degree of expanding well beyond the values recommended by previous workers for the same size of joint. Bearing in mind the added difficulties of controlling the process in the field, these recommended figures are open to question

Mean seat pressures as high as 5 ton/in² are easily reached at the optimum degree of expanding

In view of the large variations in joint strength that can occur even with carefully controlled expandings, reproducibility tests were carried out. It is shown that, where exceptional care is taken to control all the conditions of the test, good consistency in the characteristics of identically expanded joints is possible