STRESS

STRESS

Stress is the internal resistance of a material to the distorting effects of an external force or load.

Stress, s = f/a

When a metal is subjected to a load (force), it is distorted or deformed, no matter how strong the metal or light the load. If the load is small, the distortion will probably disappear when the load is removed. The intensity, or degree, of distortion is known as strain. If the distortion disappears and the metal returns to its original dimensions upon removal of the load, the strain is called elastic strain. If the distortion disappears and the metal remains distorted, the strain type is called plastic strain.

Stress is the internal resistance, or counterforce, of a material to the distorting effects of an external force or load. These counterforces tend to return the atoms to their normal positions. The total resistance developed is equal to the external load. This resistance is known as stress.

Although it is impossible to measure the intensity of this stress, the external load and the area to which it is applied can be measured. Stress (s) can be equated to the load per unit area or the force (f) applied per cross-sectional area (a) perpendicular to the force as shown

Stress, s = f/a

Where:

s = stress (psi or lbs of force per in.2)

F = applied force (lbs of force per in.2)

A = cross-sectional area (in.2)

Types of stress

Stresses occur in any material that is subject to a load or any applied force. There are many types of stresses, but they can all be generally classified in one of six categories

1) Residual stress

Residual stresses are due to the manufacturing processes that leave stresses in a material. Welding leaves residual stresses in the metals welded

2) Structural stress

Structural stresses are stresses produced in structural members because of the weights they support. The weights provide the loadings. These stresses are found in building foundations and frameworks, as well as in machinery parts.

3) Pressure stress

Pressure stresses are stresses induced in vessels containing pressurized materials. The loading is provided by the same force producing the pressure. In a reactor facility, the reactor vessel is a prime example of a pressure vessel.

4) Flow stress

Flow stresses occur when a mass of flowing fluid induces a dynamic pressure on a

conduit wall. The force of the fluid striking the wall acts as the load. This type of

Stress may be applied in an unsteady fashion when flow rates fluctuate. Water hammer is an example of a transient flow stress.

5) Thermal stress

Thermal stresses exist whenever temperature gradients are present in a material.

Different temperatures produce different expansions and subject materials to internal stress. This type of stress is particularly noticeable in mechanisms operating at high temperatures that are cooled by a cold fluid

6) Fatigue stress

Fatigue stresses are due to cyclic application of a stress. The stresses could be due to vibration or thermal cycling.

Types of applied stresses

These are known as tensile, compressive, and shear

As illustrated in figure, the plane of a tensile or compressive stress lies perpendicular to the axis of operation of the force from which it originates. The plane of a shear stress lies in the plane of the force system from which it originates.

a) Tensile stress

Tensile stress is that type of stress in which the two sections of material on either side of a stress plane tend to pull apart or elongate

b) Compressive stress

Compressive stress is the reverse of tensile stress. Adjacent parts of the material tend to press against each other through a typical stress plane

c) Shear stress

Shear stress exists when two parts of a material tend to slide across each other in any typical plane of shear upon application of force parallel to that plane

Assessment of mechanical properties is made by addressing the three basic stress types. Because tensile and compressive loads produce stresses that act across a plane, in a direction perpendicular (normal) to the plane, tensile and compressive stresses are called normal stresses.

Two types of stress can be present simultaneously in one plane, provided that one of the stresses is shear stress. Under certain conditions, different basic stress type combinations may be simultaneously present in the material. An example would be a reactor vessel during operation. The wall has tensile stress at various locations due to the temperature and pressure of the fluid acting on the wall. Compressive stress is applied from the outside at other locations on the wall due to outside pressure, temperature, and constriction of the supports associated with the vessel. In this situation, the tensile and compressive stresses are considered principal stresses. If present, shear stress will act at a 90° angle to the principal stress.