Aluminium Design and Construction John Dwight

Figure 4.2 Construction for obtaining 0.2% proof stress � o from the stress-strain curve.
These are checked in mill tests, using coupons pulled in tension. The
testing machine in effect generates a stress-strain curve, and the proof
stress is determined by means of the well-known construction shown in
Figure 4.2. The tensile strength is taken as the fracture force divided by
the original section area of the coupon. Sometimes the standard fails to
give specified values and one has to make do with ‘typical’ ones, which
are not binding on the manufacturer.
The compressive proof stress is never recorded, and a designer would
normally assume it to be the same as in tension. This is often a reasonable
assumption, but may be incorrect for non-heat-treatable extrusions which
are hot-finished and then stretch-straightened. Because of the Bauschinger
effect, the tensile proof stress is likely to be enhanced and the compressive
proof depressed by so doing, the difference between the two values
being as much as 20% in some cases. This need not normally worry a
designer, because the f o -value quoted for such material tends to be
conservative.
Standards also call for a minimum percentage elongation el that any
batch must achieve. This is typically measured on a gauge-length of 50
mm and gives a crude indication of ductility.
4.3.2 Specific alloys and their properties
It is impossible in a book such as this to provide strength data for all the
aluminium materials that exist. Not only are there scores of registered
compositions in each alloy series, but their properties vary according to
form of product, temper and thickness. It is essential for a designer to refer
to an appropriate standard to find precise strength values for the materials
he/she is planning to use. Relevant standards, covering a total of some
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