80

Source: Aluminum Association [5].

CD When tested per ASTM Test Method E399 and ASTM Practice B645.

©©Thickness for Kic specimens in the T-L and L-T test orientations: Up thru 2 in. (ordered, nominal thickness) use full thickness; over 2 thru 4 in. use 2-in. specimen thickness, centered at T/2; over 4 in. use 2-in specimen thickness centered at T/4. Test location for Kic specimens in the S-L test orientation: locate crack at T/2.

©T74 type tempers, although not previously registered, have appeared in the literature and in some specifications as T736 type tempers.

©©When tested per ASTM Practice B646 and ASTM Practice E561.

Source: Aluminum Association [5].

CD When tested per ASTM Test Method E399 and ASTM Practice B645.

©©Thickness for Kic specimens in the T-L and L-T test orientations: Up thru 2 in. (ordered, nominal thickness) use full thickness; over 2 thru 4 in. use 2-in. specimen thickness, centered at T/2; over 4 in. use 2-in specimen thickness centered at T/4. Test location for Kic specimens in the S-L test orientation: locate crack at T/2.

©T74 type tempers, although not previously registered, have appeared in the literature and in some specifications as T736 type tempers.

©©When tested per ASTM Practice B646 and ASTM Practice E561.

Elongation values are affected by the thickness of the specimen, being higher for thicker specimens. For example, typical elongation values for 1100-0 material are 35% for a ^--in.-(hick specimen, and 45% for a ^-in.-diameter specimen. Elongation is also very much a function of temperature, being lowest at room temperature and increasing at both lower and higher temperatures.

9.2.2.5 Hardness

The hardness of aluminum alloys can be measured by several methods, including Webster hardness (ASTM B647), Barcol hardness (ASTM B648), Newage hardness (ASTM B724), and Rockwell hardness (ASTM E18). The Brinell hardness (ASTM E10) for a 500-kg load on a 10-mm ball is used most often and is given in Tables 9.14 and 9.16. Hardness measurements are sometimes used for quality assurance purposes on temper. The Brinell hardness number (BHN) is approximately related to minimum ultimate tensile strength: BHN = 0.556Ftu; this relationship can be useful to help identify material or estimate its strength based on a simple hardness test. The relationship between hardness and strength is not as dependable for aluminum as for steel, however.

9.2.2.6 Fatigue Strength

Tensile strengths established for metals are based on a single application of load at a rate slow enough to be considered static. The repeated application of loads causing tensile stress may result in fracture at a stress less than the static tensile strength. This behavior is called fatigue. The fatigue strength of aluminum alloys varies by alloy and temper, but this variation is more marked when the number of load cycles is small, which corresponds with high stress ranges (Fig. 9.3). When the number of load cycles is high, designers often consider fatigue strength to be independent of alloy and temper [28].

The fatigue strengths of the various aluminum alloys can be compared based on the endurance limits given in Table 9.14. These endurance limits are the stress range required to fail an R. R. Moore specimen in 500 million cycles of completely reversed stress. Endurance limits are not useful for designing components, however, because the conditions of the test by which endurance limits are established are rarely duplicated in actual applications. Also, endurance limit test specimens are small compared to actual components, and fatigue strength is a function of size, being lower for larger components. This is because fatigue failure initiates at local discontinuities such as scratches or weld inclusions and the probability that a discontinuity will be present is greater the larger the part.

Fatigue strength is strongly influenced by the number of cycles of load and the geometry of the part. Geometries such as connections that result in stress concentrations due to abrupt transitions such as sharp corners or holes have lower fatigue strengths than plain metal without such details. Therefore, for design purposes, applications are categorized by the severity of the detail, from A (being least severe, such as base metal in plain components) to F (being most severe, such as fillet weld metal). Design strengths in fatigue can be found in Table 9.19 by substituting parameters given there into the equation

N 1/m where Srd = allowable stress range, which is the algebraic difference between the minimum and maximum stress (tension is positive, compression is negative) Cf = constant from Table 9.19

N = number of cycles of load m = constant from Table 9.19

This equation is set so that there is a 95% probability that 97.7% of components subjected to fatigue will be strong enough to withstand the stress range given by the equation [27].

This equation shows that fatigue strength decreases rapidly as the number of load cycles increases. For loads of constant amplitude, however, it is believed that the fatigue strength of aluminum alloys does not decrease once the number of cycles reaches approximately 5 million. The fatigue strength predicted by the above equation for N = 5 million is called the constant amplitude fatigue limit (CAFL, or simply fatigue limit), and is given in Table 9.19. Loads may also

TABLE 9.19 Fatigue Strengths of Aluminum Alloys
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