If the bar. has an original length L and changes by an amount L, the strain produce is defined as follows: Strain is thus, a measure of the deformation of the material and is a nondimensional Quantity i.e. it has no units. It is. simply a ratio of two quantities with the same unit.
http://www.idc-online.com/technical_references/pdfs/mechanical_engineering/Analysis_of_Strains.pdf
Strain is defined as the amount of deformation an object experiences compared to its original size and shape. For example, if a block 10 cm on a side is deformed so that it becomes 9 cm long, the strain is (10-9)/10 or 0.1 (sometimes expressed in percent, in this case 10 percent.) Note that strain is dimensionless.
https://www.uwgb.edu/dutchs/structge/stress.htm
First, we find the mass of the block. or 20 x 15 x 10 m3 x 2700 kg/m3 = 8.1 x 106 kg. Note that we have m3 times kg/m3, and the m3 terms cancel out to leave the correct unit, kilograms. Now the force the block exerts is given by mass times acceleration, in this case the acceleration of gravity, or 9.8 m/sec2.
https://www.uwgb.edu/dutchs/structge/stress.htm
principal strain. principal strain. The elongation or compression of one of the principal axes of strain relative to its original length.
http://encyclopedia2.thefreedictionary.com/principal+strain
Principal strains, principal directions. The principal strains are determined from the characteristic (eigenvalue) equation: The three eigenvalues are the principal strains. The corresponding eigenvectors. designate the direction (principal direction) associated with each of the principal strains: ! In general the principal directions for the stress and the strain tensors do not coincide.
http://web.uni-weimar.de/cms/uploads/media/lecture4_2007-10-24.pdf
Definition of Principal stresses | Principal plane | Principal Axis. These are the maximum normal stress and the minimum normal stress. Maximum normal stress is called major principal stress while minimum normal stress is called minor principal stress. So these planes are also called as major principal plane and minor principal plane.
http://www.civilengineeringterms.com/mechanics-of-solids-2/definition-of-principal-stresses-principal-plane-principal-axis/
Principal stresses may be defined as. ” The extreme values of the normal stresses possible in the material.”. These are the maximum normal stress and the minimum normal stress. Maximum normal stress is called major principal stress while minimum normal stress is called minor principal stress. Principal plane :
http://www.civilengineeringterms.com/mechanics-of-solids-2/definition-of-principal-stresses-principal-plane-principal-axis/
principal strain. [′prin·sə·pəl ′strān] (mechanics) The elongation or compression of one of the principal axes of strain relative to its original length.
http://encyclopedia2.thefreedictionary.com/principal+strain
This shear strain or slide is and can be defined as the change in right angle. or The angle of deformation  is then. termed as the shear strain. Shear strain is measured in radians & hence is non – dimensional i.e. it has no unit.So we. have two types of strain i.e. normal stress & shear stresses.
http://www.idc-online.com/technical_references/pdfs/mechanical_engineering/Analysis_of_Strains.pdf
In traditional geology the unit of pressure is the bar, which is about equal to atmospheric pressure. It is also about equal to the pressure under 10 meters of water. For pressures deep in the earth we use the kilobar, equal to 1000 bars.
https://www.uwgb.edu/dutchs/structge/stress.htm
principal strain defined
The principal strain defined the elongation or compression of one of the principal axes of strain relative to its original length. The principal of strain is defined as the elongation or compression of one of the principal axes of strain relative to its original length. The elongation or compression of one of the principal axes of strain relative to its original length is call principal strain.