Fibers that are aligned parallel to the tensile axis, the strains in both phases are equal (and the same as the composite strain), but the external force is partitioned unequally between the phases. Deformation behavior When the fiber is aligned parallel to the direction of the matrix and applied the load as the same strain case. The fiber and matrix has the volume fraction , ; stress , ; strain,; and modulus , . And here ==. The uniaxial stress-strain response of a fiber composite can be divided into several stages. In stage 1, when the fiber and matrix both deform elastically, the stress and strain relation is In stage 2, when the stress for the fiber is bigger than the yield stress, the matrix starts to deform plastically, and the fiber are still elastic, the stress and strain relation is In stage 3, when the matrix the fiber both deform plastically, the stress and strain relation is Since some fibers do not deform permanently prior to fracture, stage 3 cannot be observed in some composite. In stage 4, when the fiber has already become fracture and matrix still deforms plastically, the stress and strain relation is However, it is not completely true, since the failure fibers can still carry some load. Reinforcement with discontinuous fibers For discontinuous fibers (also known as whiskers, depending on the length), tensile force is transmitted from the matrix to the fiber by means of shear stresses that develop along the fiber-matrix interface. Matrix has displacement equals zero at fiber midpoint and maximum at ends relative to the fiber along the interface. Displacement causes interfacial shear stress that is balanced with fiber tensile stress . is the fiber diameter, and is the distance from the fiber end. After only a very small strain, the magnitude of the shear stress at the fiber end becomes large. This leads to two situation: fiber-matrix delamination or matrix having plastic shear. If matrix has plastic shear: interfacial shear stress . Then there is a critical length that when , after certain , remains constant and equals to stress in equal-strain condition. The ratio, is called the "critical aspect ratio". It increases with composite strain . For the mid-point of a fiber to be stressed to the equal-strain condition at composite fracture, its length must be at least . Then calculate average stress. The fraction of the fiber length carrying