STRUCTURAL DAMAGE ACCUMULATION AND STABLE POSTCRITICAL DEFORMATION OFCOMPOSITE-MATERIALS

The macroscopic failure of composite materials is preceded by complex multilevel processes accompanied by accumulation and localization of d amaged centers and formation of a failure cluster. Therefore, the stud y of these mechanisms is one of the basic problems for the mechanics o f modern composite materials used in aerospace engineering The formati on of a theory of the stable postcritical deformation of the work-soft ening media is considered. The pseudo-plastic deformation affected by structural damage of granular composites is investigated within the fr amework of the considered two-level structurally phenomenological mode l of heterogeneous media. The stable evolution of the interconnected p rocesses is accompanied by stress redistributions, partial or complete unloading, and strain or damage localization that are one of the main causes of implementation of the postcritical deformation stage. The n umerical calculation results of inelastic deformation and failure of t he periodic unidirectional fiber-reinforced composites are presented u nder conditions of the displacement-controlled transverse proportional loading mode. The main mechanisms of the work-softening behavior for the indicated type of materials are described in the macro-homogeneous stress-strain states. Macroscopically, the failure of heterogeneous m edia as a result of postcritical deformation and the loss of stability of damage accumulation depends on the stiffness of the loading system . When a deformable body is fixed on the closed surface with sufficien tly but not infinitely large coefficients of stiffness, it is possible to observe the equilibrium development of the localized volumes of wo rk-softening and damage. The constitutive equations for the work-softe ning isotropic, transverse isotropic, and orthotropic media are presen ted. The effect of the loading system on, the stability of deformation , damage accumulation, and failure under monotone and nonmonotone tria xial loading was studied. The growth of failure strains with increase in stiffness of the loading system and unequal resistance of heterogen eous body are registered and investigated. A preventive unloading meth od is offered for the mathematical modeling of the damage accumulation during the testing of the materials on the servo-controlled systems. The displacement-controlled mode is simulated by a series of soft load ing and unloading cycles. The detected phenomenon. of failure where th e unloading leads to stress-strain diagrams with a negative slope of t he descending branch was not found either in the displacement or stres s-controlled monotone loading mode.