9224 Fracture Toughness and Elongation

Fracture toughness is a measure of a material's resistance to the extension of a crack. Aluminum has a face-centered cubic crystal structure and so does not exhibit a transition temperature (like steel) below which the material suffers a significant loss in fracture toughness. Furthermore, alloys of the 1xxx, 3xxx, 4xxx, 5xxx, and 6xxx series are so tough that their fracture toughness cannot be readily measured by the methods commonly used for less tough materials and is rarely of concern. Alloys of the 2xxx and 7xxx series are less tough and when they are used in fracture critical applications such as aircraft, their fracture toughness is of interest to the designer.

The plane strain fracture toughness (KIc) for some products of the 2xxx and 7xxx alloys can be measured by ASTM B645. For those products whose fracture toughness cannot be measured by this method (such as sheet, which is too thin for applying B645), nonplane strain fracture toughness (Kc) may be measured by ASTM B646. Fracture toughness limits established by the Aluminum Association are given in Table 9.18. Fracture toughness is a function of the orientation of the specimen and the notch relative to the part, and so toughness is identified by two letters: L for the length direction, T for the width (long transverse) direction, and S for the thickness (short transverse) direction. The first letter denotes the specimen direction perpendicular to the crack, and the second letter the direction of the notch.

Ductility, the ability of a material to absorb plastic strain before fracture, is related to elongation. Elongation is the percentage increase in the distance between two gage marks of a specimen tensile tested to fracture. All other things being equal, the greater the elongation, the greater the ductility. The elongation of aluminum alloys tends to be less than mild carbon steels; while A36 steel has a minimum elongation of 20%, the comparable aluminum alloy, 6061-T6, has a minimum elongation requirement of 8 or 10%, depending on the product form. An alloy that is not ductile may fracture at a lower tensile stress than its minimum ultimate tensile stress because it is unable to deform plastically at local stress concentrations. Instead, brittle fracture occurs at a stress raiser, leading to premature failure of the part.

The elongation of annealed tempers is greater than that of strain-hardened or heat-treated tempers, while the strength of annealed tempers is less. Therefore, annealed material is more workable and able to undergo more severe forming operations without cracking.

TABLE 9.18 Fracture Toughness Limits

Alloy and Temper

Thickness

Kic,

ksiVnTmin

(in.)

0 0

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