| Fiber-reinforced composites have been used increasingly in aviation,aerospace, shipbuilding,automobile,building,sports,medical instrument,and so on due to their good mechanical properties.The multivariable and designable features make composites possess many good and special performances,which result in their complex strength prediction and failure process.Composites have been seen as homogenous materials based on traditional strength criterion,which can not reveal the influences of the components,internal defects,interface debonding and sliding on the mechanical properties of the whole composites.The optimum design of the material can not be conducted without clear failure mechanisms.Thus,it is very important to study the strength prediction,multi-level/-scale damage forming mechanism,damage evaluation in service and the controlling mechanism of composites.There are complete composite components in the single-fiber composite,including fiber,matrix and interface,which is the basic composite model and has been widely used in studying mesoscopic failure behaviors of composites.And the relevant theoretical model-shear-lag model has been improving.However,the main researches were concentrated on stress disturbances of the broken fiber on the surroundings,and influences of fiber,matrix and interface on mechanical properties and failure process of composites in the case of quasi-static loading.The multi-fiber composite investigations were incomplete due to the difficult specimen preparation.As being financed by the National Major Fundamental Research Program of China (973 Project),meso-mechanical failure performances of glass fiber/epoxy matrix composites were studied using single-/multi-fiber composite models by both numerical and experimental methods.Further more,the strain rate dependence has been investigated.As a result,the relation between mesoscopic failure evolution and macroscopic failure behavior were established,and the important influencing factors on failure mode were concluded.The present author participated in developing some of the used equipments,such as the fiber-positioning apparatus and the tensile tester. Typically,the fiber arrays are controlled much more precisely and the operation is more convenient by conducting the fiber-positioning apparatus.And the tensile tester is equipped with a microscope attached with a polarizer,which brings off real-time monitoring the mesoscopic failure process and further capturing stress concentration images during testing.Interface plays a vital important role in mechanical properties of composites and the failure behavior.Based on the experimental result that there are different crack propagation modes in composites with different interfaces,a meso-mechanical finite element model with a defect was constructed to simulate the crack propagation routes and local stress distributions affected by the broken fiber under different interfacial strengths by using the ANSYS/LS-DYNA soft and the designed program.The failure criterion is a certain effective plastic strain.The local and whole resistance abilities to failure of the composite under different interfacial strengths have been evaluated,and the simulation results agreed well with the experimental results.The results show that, with the interfacial strength varying from weak to strong,there are five kinds of representative crack propagation modes observed in the composite in tension.The stronger the interface is,the better the local resistance to crack propagation,but the worse the resistance to the whole destroyed is.The resistance to crack propagation and the whole destroyed will not be improved any more if the interfacial strength increases to some certain values.During the curing process of composite,some kinds of the molecules in matrix will be adsorbed to fibers preferentially,which results in non-stoichiometric cured epoxy matrix at the interphase around fiber.So,the effect of curing agent ratio to epoxy on mechanical properties of matrix,interface properties between fiber and matrix (interface failure behaviors and the interfacial shear strength),and the wettability of matrix to fiber were studied experimentally.It was found that,excluding the initial elastic modulus,the mechanical properties of epoxy matrix such as yield strength, yield strain and fracture strength were all dependent on the curing agent ratio to epoxy, which reached their maximum values at the stoichiometric ratio.The interface property and the wettability between fiber and matrix performed the best around the stoichiometric ratio also.Further more,it has been found that the interfacial shear strength is linear to the work of adhesion approximately by varying the curing agent ratio.However,there are variations in the mechanical properties of matrix with varying curing agent ratio which do have effects on the interfacial shear strength as well.In order to quantify the relationship between the interfacial shear strength and the work of adhesion definitely,the investigation of changing fiber surface treatment with the curing agent ratio to epoxy invariable has been carried out.First,self-assembled hydrophobic monolayers were prepared on the glass fiber surface to normalize the glass fiber surface property.The hydrophilicity of the glass fiber surface was controlled by the oxidation time in ozone.According to the comprehensive analysis of the experimental data,it is indicated that the interfacial shear strength does be linear to the work of adhesion in the current researching system.That is to say,the mechanical parameter—the interfacial shear strength and the thermodynamical parameter—the work of adhesion are indeed in close agreement with each other when characterizing the interface cohesive property.Additionally,the equilibrium interatomic/intermolecular distance at the interface has been evaluated to be approximately within 0.3-0.6 nm,which is linear to the interfacial shear strength and the work of adhesion,respectively.Concretely,the stronger the interface is,the smaller the equilibrium interatomic/intermolecular distance at the interface will be.Based on the current experimental conditions,the effect of tensile strain rate on mechanical properties,stress relaxation and failure behaviors of single-/multi-fiber composites and pure epoxy matrix were investigated at low strain rate level(range from 2E-5 s-1 to 2E-2 s-1).It was found that,the dependence of the initial elastic modulus of the pure epoxy matrix and fiber composite on strain rate is indistinct, however,the ultimate stress and the fracture strength increase with strain rate increasing.The fracture modes of both pure epoxy matrix and fiber composite change from ductile fracture to brittle fracture with strain rate increasing,but the strain rate of changing fracture mode for them are different.The higher the pre-tensile strain rate is, the weaker the birefringence induced by residual shear stress around fiber break is and the faster it fades,to which the conclusion obtained "the relaxation time decreases with pre-tensile strain rate increase" and the interphase-chain model introduced in the thesis can give a reasonable explanation.By studying the mesoscopic failure modes of multi-fiber composites,it was found that fiber-fiber interactions showed a sign of decrease while increasing tensile strain rate.The influences of strain rate on different composites cross-compounded by three kinds of fibers and two kinds of matrices were investigated systematically in this thesis.The results of general effects of strain rate on mechanical properties were obtained,including the rate independence of the initial elastic modulus,the rate dependence of the ultimate stress and fracture mode.Further more,the combined effects of strain rate and components on fracture mode of composites were summarized.For example,the released fracture energy of the fiber(is related to fracture strength/elastic modulus/fracture strain of the fiber),the capacity of the surroundings to absorb the released energy(is related to yield strength/yield strain/elastic modulus of the matrix and the interfacial strength),the dimension of the breakpoint(is related to fiber diameter),the stage of fiber breakpoint generates(in elastic or plastic stage of the matrix or the interface),and the external loading rate (tensile strain rate) all have effects on the fracture mode of composite.
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