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Various types of strength are used in the literature and, thus, it is useful to understand their definitions. As strength is determined from data obtained in various loading modes and is different in each loading mode, the specific type of strength should be stated when appropriate, such as tensile, compressive, shear, or flexural strength. In any loading mode, the strength is the highest stress that the material can support, sometimes referred to as the ultimate strength. The fracture strength (failure strength) is the stress at which the material fractures into two or more pieces. In tension or compression, strength is determined by taking the measured load and dividing it by the appropriate cross‐sectional area of the specimen. In torsion, the measured torque and specimen radius at failure along with Eqs. (4.8) and (4.9) are used, whereas in flexure, the measured load and the initial dimensions of the beam‐shaped specimen are substituted in Eq. (4.11) or Eq. (4.13). Commonly, several nominally identical specimens are tested and the strength is expressed as an average value plus or minus its standard deviation.

For ductile materials, such as metals, an important type of strength is the tensile yield strength, that is, the tensile stress at the yield point. Another type of strength that is often used is the tensile strength, sometimes called the ultimate tensile strength. Depending on the nature of the stress–strain curve, the tensile strength can be equal to or different from the fracture strength, that is, the strength at failure (Figure 4.3). If the fracture strength is the highest stress in the curve, for example, the tensile strength and the fracture strength are the same.