Fracture Mechanics

Beryllium Manufacturing Processes

“…This is illustrated in Figure VI-1, which shows a plot of the number of twins remaining on the etched surface as a function of etch depth for machined S-200 beryllium having a grain size ranging from 35 to 50 µm. 40 The analysis was made using both direct observation and acetate replicas of the surface examined at 400X. In this study it was noted that microcracks, laminations, and smeared metal were completely absent after an etch depth of 0.025 mm.…”

“…The depth of the twinned region typically may range from 0.05 to 0.5 mm, and it appears to be less with a decrease in the grain size. 40 For fine-grained material, deformation (mechanical) twins have been detected to depths of 250 µm below the machined surface. For a grain size of 25 µm, this depth would correspond to 10 grains.…”

“…Number of twins observed on the etched surface of a machined beryllium as a function of depth of etching: σ-direct observation; lreplica observation. 40 Surface damage, if not removed, can result in a large drop in both fracture strength and elongation. Although the yield strength may not be significantly affected, elongation can be reduced to a fraction of the damage-free elongation.…”

“…42 It was generally observed that for tensile-test specimens, etching 0.08 to 0.13 mm off from each surface was sufficient to remove nearly all the detrimental effects of most machining treatments even if etching to that depth range did not remove all the metallographically detectable damage. 40 In fabricating the space-shuttle orbiter, it was determined that the removal of 0.152 mm (0.006 in,) of metal by etching was required to eliminate the presence of all machining-induced twinning; and, this was done without creating excessive dimensional tolerance problems (except for holes in thick sections). 43 The ASM Handbook states that typical machining damage ranges from 0.025 to 0.10 mm for most machining and may extend to a depth of 0.50 mm depending on the machining method.…”

“…This unexpected result was attributed to the possibility that the residual surface compressive stresses outweighed the detrimental effects of the surface twins in the as-machined specimens. 40 FigureVI-3 Effect of etching on the fatigue behavior of machined ingot-sheet beryllium. 40 Note that the results are opposite to what would be expected.…”