Study On The Mechanics Of Adhesion And Failure Of Fiber/Polymer Interface In Composites

The properties of fiber reinforced polymer matrix composites need eagerly to be identified clearly due to their rich in physical attributes and various applications. Mechanical properties and failure modes of composites on the macro scale are affected extremely by fiber/matrix interfacial bonding degree and fracture pattern on the micro scale. For special fiber/matrix system, it is important to examine quantitatively the bonding parameters and to know how the load transfer and fail between matrix and fiber for understanding and controlling the performance of composites and response of composites structure.To explaining theoretically the debonding phenomenon between fiber and matrix in composites, the cohesive zone model for interface is a proper tool, because it can simulate naturally the interface failure as the separation between fiber and matrix. In the present paper,two different interfacial failure modes, that is, plastic yielding and softening failure behavior in the cohesive force ways, were adopted in modelling the interfacial response of fiber/matrix interface under shear or tensile deformation in fiber reinforced polymer matrix composites, in order to identify that how the different function forms and paremeters in cohesive zone model would affect outcomes of interfacial macro performance, which can be used as a theoretical reasoning of selecting suitable interface model and interfacial parameters when researching the composites’ performance and response of composites structure. The following aspects were carried out in this research work:The tangential failure process of fiber/matrix interface in single fiber composite under axial tension was examined. Different interface states in the process of load transferring from matrix to fiber were established by analytical method, in which an elastic-softening cohesive zone model was adopted to simulate the fiber/matrix interface. The stresses distributions and characteristics in both components and interface of sigle fiber composite under different interface states were analyzed. The effects of interfacial parameters on theoretical interfacial stresses, elastic ultimate load and debonding load were studied. By fitting the theoretical results with experimential data, the parameters of cohesive zone model for specific composite system used in the single fiber fragmentation test were obtained.Concerning the case of initial finite interfacial debonding alone arising at the very moment of the fiber initial break in fragmentation test, interfacial Π mode fracture toughness was derived.The energy balance in single fiber composite before and after the fiber initial break was established by analytical method based on an energy scheme. Both the perfect interfacial bonding model and the elastic-softening cohesive zone model were adopted in simulating thefiber/matrix interface, respectively. The concept of critical debonding strain was proposed. The changes of energy distribution and stresses distribution in both components and interface before and after the fiber initial break were calculated. The debonging length alone arising at the very moment of the fiber initial break and its influencing factors were analyzed. The predicted interfacial Π mode fracture toughness for composite ststem used in test was obtained by utilizing experimental data.The effect of curing temperature on properties of interface between fiber and matrix in composites was studied. Firstly, the thermal residual stresses produced in single fiber-polymer matrix composite after high temperature cured were calculated by analytical method based on the elastic-softening cohesive zone model. The relationships between the interfacial states after curing and both the curing temperature and the interfacial cohesive parameters were determined.The interfacial stresses distributions and their influence factores were analyzed. Using linear constitutive laws for the interfacial unloading or reloading, the interfacial stress-displacement functions for different interfacial states after high temperature cured during the process of loading the single fiber composite were analyzed, and the response of interface in cured composite under external mechanical load was researched. The differences between the interfacial properties of composites formed in different curing temperature were analyzed by combineing with the experimental data.The normal tensile failure process of fiber/matrix interface in single fiber composite under transverse tension was examined. By constructing the interface displacement function, the interfacial normal failure in cruciform specimen under transverse tensile load was researched by analytical method, in which the elastic-softening cohesive zone model was adopted to simulate the fiber/matrix interface. The interfacial stress, interfacial displacement, softening angle,debonding load and their influence factores during ? mode interfacial fracture were analyzed.The feasibility of the proposed interface displacement function was verified by compared theoretical interfacial stresses with finite element results.The classical Cox model for interfacial shear stress transfer and the K-T model assuming constant interfacial shear stress distribution along fiber/matrix interface were combined to investigating the interfacial failure and crack propagation during a single fibre pulling out from polymer matrix in microdebond test by analytical method. Both short and long fiber pullout processes were estabished and the relationships between the fiber length and debonding load or interfacial parameters in elastic-plastic yielding cohesive zone model were analyzed. The theoretical force-displacement results of fiber pulling out based on elastic-softening cohesive zone model in literatures were summarized and were compared with the results based on Cox-K-T model in this paper. The similarities and differences of force-displacement curves based on the two types of cohesive zone model were analyzed.