Variations in Mechanical Properties

Tensile properties of structural steel may vary from specified minimum values. Product specifications
generally require that properties of the material as represented by the test specimen meet certain values. With some exceptions, ASTM specifications dictate a test frequency for structural-grade steels of only two tests per heat (in each strength level produced, if applicable) and more frequent testing for pressure-vessel grades. If the heats are very large, the test specimens qualify a considerable amount of product. As a result, there is a possibility  that properties at locations other than those from which the specimens were taken will be different from those specified.
For plates, a test specimen is required by ASTM A6 to be taken from a corner. If the plates are wider than 24 in, the longitudinal axis of the specimen should be oriented trans versely to the final direction in which the plates were rolled. For other products, however,??the longitudinal axis of the specimen should be parallel to the final direction of rolling.

For structural shapes with a flange width of 6 in or more, test specimens should be selected from a point in the flange as near as practicable to 2⁄3 the distance from the flange centerline to the flange toe. Prior to 19971998, the specimens were taken from the web.
An extensive study commissioned by the American Iron and Steel Institute (AISI) compared yield points at various sample locations with the official product test. The studies indicated that the average difference at the check locations was 0.7 ksi. For the top and bottom flanges, at either end of beams, the average difference at check locations was 2.6 ksi.
Although the test value at a given location may be less than that obtained in the official test, the difference is offset to the extent that the value from the official test exceeds the specified minimum value. For example, a statistical study made to develop criteria for load and resistance factor design showed that the mean yield points exceeded the specified minimum yield point Fy (specimen located in web) as indicated below and with the indicated coefficient of variation (COV).


Flanges of rolled shapes 1.05Fy, COV  0.10
Webs of rolled shapes 1.10Fy, COV  0.11
Plates 1.10Fy, COV  0.11
Also, these values incorporate an adjustment to the lower static yield points.
For similar reasons, the notch toughness can be expected to vary throughout a product.
(R. L. Brockenbrough, Chap. 1.2, in Constructional Steel Design An International Guide, R. Bjorhovde, ed., Elsevier Science Publishers, Ltd., New York.)

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