Matter can be defined as anything that has mass and occupies space. Matter may consist of one element alone, or elements in combinations called compounds. Whether elemental or as compounds, matter can exist as a solid, or in gaseous or liquid forms. Chemical properties involve the ways substances act when the amounts or nature of the substances change. In contrast, physical properties are related to any change in which the amounts or nature of the substances do not change. In contrast to physical properties, the mechanical properties indicate the way a substance reacts when acted upon by a mechanical force. They are tensile properties, hardness properties, and fatigue properties. Properties that play a major role in engineering design: proportional limit, elastic limit, yield point, yield strength, ultimate strength, breaking (rupture) strength, ductility, and modulus of elasticity. The two hardness tests most commonly used in industry are the Brinell and the Rockwell. If a material is subjected to fluctuation or to a number of cycles of stress reversal, failure may occur. This failure may occur even though maximum stress at any cycle is less than the value at which failure would occur under constant stress. By subjecting test specimens to stress cycles and, in turn, counting the number of cycles to failure, fatigue properties may be determined.