A biharmonic function of two variables whose second partial derivatives give the stress components of a body subject to a plane strain.
Related Terms 
FORM FACTOR

PLATE MODULUS
The ratio of the stress component Txx in an isotropic, elastic body obeying a generalized Hooke's law to the corresponding strain component Sxx, when the strain components Syy and Szz are 0; the sum of the Poisson ratio and twice the rigidity modulus.

PLASTICOVISCOSITY
Plasticity in which the rate of deformation of a body subjected to stresses greater than the yield stress is a linear function of the stress.

PLASTIC DEFORMATION
Permanent change in shape or size of a solid body without fracture resulting from the application of sustained stress beyond the elastic limit.

PLANE STRESS
A state of stress in which two of the principal stresses are always parallel to a given plane and are constant in the normal direction.

OPERATING STRESS
The stress to which a structural unit is subjected in service.

OFFSET YIELD STRENGTH
That stress at which the strain surpasses by a specific amount (called the offset) an extension of the initial proportional portion of the stressstrain curve; usually expressed in pounds per square inch.

NORMAL STRESS
The stress component at a point in a structure which is perpendicular to the reference plane.

NEUTRAL FIBER
A line of zero stress in cross section of a bent beam, separating the region of compressive stress from that of tensile stress.

NEUTRAL AXIS
In a beam bent downward, the line of zero stress below which all fibers are in tension and above which they are in compression.
