Investigation of Coarse Grain in Brabazon Spar Booms

Abstract

OCCURRENCE of considerable coarse grain toward the back ends of the prototype Mark I spar booms had been known at an early stage in the Brabazon development programme, the result of which had necessitated the selective installation of the most suitable extrusions for the Mark 1 construction. In consequence of this experience, the material manufacturers were consulted at the commence of design on the Mark II spar booms, with the object of producing extrusions to the highest standard of efficiency. Resulting from this close collaboration, the Bristol Aeroplane Co. Engineering Development Laboratory were supplied with offcut lengths from the deliveries of Mark II spar booms, identified at the factory as B.S. 3345 and B.S. 3346. These specimens were subjected to exhaustive tests of etching and tensile tests of the material (D.T.D. 363) to determine the selection of booms for the Mark II spar structure. In all, ten lengths were reviewed: four lengths of B.S. 3345 and six lengths of B.S. 3346, an offcut specimen 8 in. long being taken from each end of the delivered extrusions. From each offcut, a cross‐section was cut approximately ½ in. thick for etching purposes. Resulting from the etching test, further specimen pieces were extracted for tensile tests, one from the front end offcut specimen, and two from the rear end offcut specimen. In the case of the two rear end test pieces, one was extracted from the coarse grain region and one from the fine grain region. The position of these extracted specimens is shown in fig. 1 and is determined entirely from the results of the etching tests carried out on the ½ in. thick cross‐sections.