Size And Temperature Effects On Interfacial Shearing Behavior Of Fiber Reinforced Polymer Matrix Composites

With the increasing application of the composite materials in the engineering structures,there is a new challenge to study the structure and properties of composites to meet their designs requirements.The interface,as the bridge to the optimal combination of the components of the composites,plays a critical role in the durability and stability of these materials,and the mechanism of stress transfer at interface is also key to optimize the performance of composite materials.Therefore,the interfacial properties and structures shouldbe well characterized and studied.The mechanical properties of the interface,stress transfer behavior and failure mechanism between the fiber and the matrix are hot topics in the field of composite engineering.They are also the key in the composite manufacturing.However,although a lot of researches in this area has also been done,there is still no unified understanding of the experimental characterization of properties,stress transfer behavior and failure mechanism for the interface of composite at mesoscale levelThere are many failure mechanisms including brittle fracture of fibers,plastic yielding and failure of matrix,fiber/matrix debonding occurring at micro-scale However,it is often difficult to deduce from macrosocpic experiments which ones are dominant in the failure process of a macro specimen.This is because complex stresses including shearing and tensile behaviors occur at the interface of the composite during the process of curing and testing.And the bond failure of the interface is often obscured by the accompanying damage such as matrix cracking and fiber breakage.In addition,numerical or analytical analyses using different interface models,which typically use different parameters,for the same physical interface often yield different mechanisms These predicted results are sometimes not in good agreement with the experimental data.Therefore,the development and improvement of the experimental method to characterize the interface mechanics at meso-scale is still an effective way to study the interface mechanics.In addition,establishing new theoretical or numerical model is worthy of studying the interfacial properties and behavior.In this paper,the following work on these issues has been conducted(1)For the fiber reinforced polymer composites,the microbond test is used to directly and quantitatively characterize and study the interface properties and behaviors of the single fiber composite system.The experimental platforms at room temperature is built.The devices,the preparation of experimental samples,the experimental procedures and the factors affecting the experimental results are analyzed in detail.The experimental specifications and standards are summarized.The interface performance is quantitatively characterized,and the bonding and failure mode of the interface are adequately characterized.(2)A nonlinear interface mechanics model combining interfacial debonding,frictional sliding and coupling behavior between decohesion and fricition has been developed.It has been implemented into the ABAQUS.Compared with the results of the microbond test,the interface parameters required for the interface mechanics model are determined.A detailed analyses of the stress transfer process,bonding mechanism and failure mode of the interface has been carried out and the numerical results are in good agreement with experiment.In addition,the size effects on the interfacial shear strength(IFSS)and mechanical behavior are studied.It is found that the length of microdroplet embedded fiber(le)affects IFSS.As the size of the microdroplets increases,the stress has a trend of transiting from a nearly uniform distribution along the interface to a localized stress concentration zone which propagate from the up to the bottom of the interface.The evolution of the shear stress distribution during pull-out shows a transition between the two modes as the size of the microdroplet increases.(3)To study the effects of the testing temperature(Tt)on the bond strength and failure mode of the interface,which will change the macroscopic properties of the composite material,an in-house developed apparatus for microbond test under controlled temperature is built.The microbond tests under different Ti are carried out,and its effects are quantified.(4)Based on the bilinear Traction-Separation law considering the temperature factor,a nonlinear interface mechanics model with interface debonding,frictional sliding and coupling behavior between decohesion and fricition has been derived.Compared to the conventional cohesive zone model,the boundary problem is introduced to the shear traction in this modified model.Combined with the finite element method,the effects of the temperature on the interfacial properties and behavior are studied using this model.It is found that IFSS and the frictional stress becomes smaller with the increment of Tt at the interface,but the failure mode of the interface is not barely affected by the temperature.