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Tensile Stress
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Tensile stress occurs when a material is subjected to pulling or stretching force. Stress is
defined as a force applied over a cross-sectional area, with typical units of pounds per square
inch (psi) or Newtons per square meter, also known as pascals (Pa). The type of stress that a
material is exposed to will depend on how the force is being applied. The three basic types of
stress are tensile, compressive, and shear. An understanding of tensile stress is important in
selecting materials for mechanical engineering and design applications.
The dimensions of an object under stress will change due to the strain or deformation that
occurs when a force is applied. A material that is under tensile stress will elongate, or stretch,
when it experiences strain. A material exposed to low stress will return to its original
dimensions after the force is removed. At high stresses, a material may not return to its
original state when the force is removed and permanent deformation will occur. The
relationship between the applied stress and the corresponding strain can be used to predict the
behavior of a material when it is exposed to tensile stress.
Testing equipment is available that can accurately measure the stress and strain experienced
by a material, and generate a stress-strain curve. The stress-strain curve typically provides an
understanding of how a material will behave when exposed to applied tensile force, and
determines the maximum allowable stress before permanent deformation and ultimate failure
occurs. To measure tensile stress, a gradually increasing force is applied to a test sample and
the amount of force needed to elongate and ultimately break the sample is measured and
recorded. Materials that are exposed to tensile stress and experience a large amount of
deformation before failure are considered to have high elasticity.
The maximum tensile stress that a material can withstand before it fails is known as tensile
strength or ultimate tensile strength. The value of ultimate tensile strength varies widely for
different materials. Soft, malleable materials — such as many plastics, rubber, and metals —
are considered elastic and will undergo significant deformation before a complete failure
occurs. Hard and brittle materials, like concrete and glass, will have little or no deformation
before a complete failure occurs. The ultimate tensile strength for many different types of
metal, wood, glass, rubber, ceramics, concrete, and plastics is readily available in various
material property reference manuals.