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 this force 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.