The behaviour of aluminium alloys under compressive loading does not receive the attention given to tensile properties, perhaps because the strength of structural members is so often limited by buckling, and the actual compression strength of the metal is not approached.

For most engineering purposes it is customary to use the same design strength for compressive work as for tensile. In the testing machine, an aluminium alloy will show an apparently higher strength in compression than in tension, but this can in part be attributed to the changing cross-sectional areas of the specimens, increasing in one case and decreasing in the other, while the stress is based on the original area. Cylindrical specimens of the softer aluminium alloys can be compressed to thin discs before cracking, and even then may sustain the load. The harder alloys show a more definite failure point with pronounced cracking.

A proof stress is therefore usually quoted, and will be roughly equal to the corresponding tensile proof stress; in cast or forged metal it is usually slightly higher. Sheet and extruded products, however, are often straightened by stretching, an effect of which is to lower the compressive proof stress and raise the tensile proof stress by small amounts.

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