In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress–strain curve (curve A, figure 2) this is because the engineering stress is calculated assuming the original cross-sectional area before necking. For many applications, plastic deformation is unacceptable, and is used as the design limitation.Īfter the yield point, ductile metals undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases due to plastic flow. A plastically deformed specimen does not completely return to its original size and shape when unloaded. Beyond this elastic region, for ductile materials, such as steel, deformations are plastic. The elastic behavior of materials often extends into a non-linear region, represented in figure 1 by point 2 (the "yield strength"), up to which deformations are completely recoverable upon removal of the load that is, a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Many materials can display linear elastic behavior, defined by a linear stress–strain relationship, as shown in figure 1 up to point 3. Kilopounds per square inch (ksi, or sometimes kpsi) is equal to 1000 psi, and is commonly used in the United States, when measuring tensile strengths.ĭuctile materials Figure 1: "Engineering" stress–strain (σ–ε) curve typical of aluminum A United States customary unit is pounds per square inch (lb/in 2 or psi). In the International System of Units (SI), the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the SI prefix mega) or, equivalently to pascals, newtons per square metre (N/m 2). For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. Tensile strength is defined as a stress, which is measured as force per unit area. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture. Some materials break very sharply, without plastic deformation, in what is called a brittle failure. However, depending on the material, it may be dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. The ultimate tensile strength of a material is an intensive property therefore its value does not depend on the size of the test specimen. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood. Tensile strengths are rarely of any consequence in the design of ductile members, but they are important with brittle members. The equivalent point for the case of compression, instead of tension, is called the compressive strength. The highest point of the stress–strain curve is the ultimate tensile strength and has units of stress. The ultimate tensile strength is usually found by performing a tensile test and recording the engineering stress versus strain. In brittle materials the ultimate tensile strength is close to the yield point, whereas in ductile materials the ultimate tensile strength can be higher. Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F tu in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. The maximum stress it withstands before fracturing is its ultimate tensile strength. Two vises apply tension to a specimen by pulling at it, stretching the specimen until it fractures. Maximum stress withstood by stretched material before breaking
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