Study On Stress Relaxation Properties Of GFRP Based On Molecular Dynamics

Glass fiber reinforced plastics(GFRP)is widely used in various engineering structures.According to the form of reinforcing phase,it can be divided into continuous glass fiber reinforced plastics(CGFRP)and short glass fiber reinforced plastics(SGFRP).GFRP has obvious stress relaxation,creep and fatigue phenomena at room temperature,which affects the long-term durability of GFRP structure.There have been many study on the creep and fatigue properties of GFRP,however,there are relatively few studies on stress relaxation.The existing related studies mainly focus on macro experimental aspect,which can only be characterized by phenomenology and cannot be analyzed in a deeper level.In this thesis,molecular dynamics method is proposed to study the stress relaxation properties of GFRP from micro scale.The main research results are as follows:(1)The modeling and stress relaxation simulation methods of CGFRP and SGFRP based on molecular dynamics were established.Through the simulation and comparative analysis of the initial mechanic properties of GFRP and epoxy resin,the reliability of the models was verified.At the same time,the influence of temperature on the initial mechanical properties of GFRP and epoxy resin was obtained.(2)Stress relaxation of 1 ns was simulated for GFRP and epoxy resin,the basic law of stress relaxation was obtained through analysis of results.Epoxy resin,in the vertical fiber direction of CGFRP and in the vertical fiber direction of SGFRP present obvious stress relaxation under the microscope,and the stress decreases range by more than 20% at 348 K.In the fiber direction of CGFRP has no obvious stress relaxation under the microscope,and the stress decreases range by less than 5% at 348 K.At the same temperature and strain,the stress decrease range of SGFRP is slightly higher than epoxy resin,while the stress relaxation range in the vertical direction of CGFRP is relatively low,which is only about 50% of epoxy resin.With the increase of temperature,the stress decrease range of GFRP and epoxy resin increases.(3)Based on the stress relaxation results,the applicability of the time-temperature superposition principle of epoxy resin,in the vertical fiber direction of CGFRP and in the vertical fiber direction of SGFRP was verified at the atomic scale.The results show that the time-temperature superposition principle is applicable and the stress decrease trend is linear in the semi-logarithmic coordinate(σ-Lg(t)).(4)The micro mechanism of stress relaxation of GFRP and epoxy resin was analyzed through the changes of micro energy and microstructure.In the stress relaxation process of epoxy resin,van der Waals energy in non-bond energy plays a major role in energy aspect;the radius of gyration in the non-tensile direction and the fractional free volume after relaxation are reduced in structure aspect,indicating that the relative distance between epoxy resin atoms is reduced.In the vertical fiber direction of CGFRP has a significant relaxation effect at the interface.The stress relaxation of SGFRP is the joint action of matrix and interace.