Study Of Theoretical Characterization Method For Temperature-dependent Yield And Ultimate Strength Of Polymer Composites

Polymer composites have become indispensable strategic key materials in aerospace,weaponry,rail transportation and other major science and technology projects due to their high specific strength/modulus,high insulation,corrosion resistance,fatigue resistance,and designability.With the rapid development of national defense construction and cutting-edge science and technology,the need to expand the service temperature range of polymer composites,especially to further increase the service temperature,has become increasingly strong.In order to ensure the service safety and reliability of polymer composites and based on the urgent need to develop materials that can be applied to a wider service temperature range,it is of great theoretical significance and engineering application to study the main control mechanisms of mechanical properties of composites and their evolution with temperature,and to establish a temperature-dependent yield/ultimate strength theoretical characterization model with profound physical background applicable to different temperatures,especially high temperatures.This thesis focuses on polymers and their composites,and the following research has been carried out:(1)A theoretical characterization model of temperature-dependent yield strength without fitting parameters was developed for amorphous polymers based on the Force-Heat Equivalence Energy Density Principle.The model reveals the quantitative relationships between temperature-dependent yield strength and Young’s modulus,specific heat capacity,Poisson’s ratio and glass transition temperature.A new temperature and strain rate dependent yield strength model was further proposed by considering the temperature dependence of the strain rate strengthening effect and the strain rate dependence of the glass transition temperature in conjunction with the Eyring theory.Compared with the commonly used Richeton model,which contains five undetermined parameters and relies on the fitting of experimental data at different temperatures and strain rates,the present model introduces two parameters to characterize the coupling effect of temperature and strain rate.And the present model only requires strain rate-dependent experimental data at room temperature to achieve convenient prediction of yield strength of amorphous polymers at different temperatures and strain rates.The research in this chapter provides a theoretical basis for the evaluation of the mechanical properties of materials applied to high temperatures and high strain rates.(2)By considering the temperature dependence of fiber/matrix properties with the effects of fiber content,agglomeration and orientation,temperature-dependent ultimate tensile strength models were developed for random short fiber and unidirectional fiber reinforced polymer matrix composites,respectively.The model predictions achieved good agreement with experimental data at different temperatures.Compared with the Curtin model and Kelly-Tyson model,the present model prediction is more accurate and does not contain difficult-to-obtain parameters such as the characteristic strength of fibers at different temperature-specific scales and the Weibull modulus at different temperatures,which is more convenient for engineering applications.The effects of interfacial shear strength,residual thermal stress and their evolution with temperature,as well as fiber length distribution and orientation distribution were further considered to establish a theoretical model of temperature-dependent ultimate tensile strength of short fiber-reinforced composites with profound physical background.The quantitative effects of interfacial shear strength,Young’s modulus of matrix,and fiber agglomeration on the ultimate tensile strength of the composites at different temperatures were analyzed using this model.This study provides a theoretical basis for the design and reliability evaluation of high temperature resistant composites,especially for the development of materials with wider service temperature range.(3)A temperature-dependent yield strength model for polymer nanocomposites was developed based on the Force-Heat Equivalence Energy Density Principle and the Halpin-Tasi model.The model requires only the input of nanofiller size,component temperature dependent Young’s modulus,matrix glass transition temperature and an easily accessible composite yield strength at a reference temperature to achieve convenient prediction of temperature-dependent yield strength of nanoparticle,nanofiber and nanoplate reinforced polymer matrix composites.Further for the nanoplate reinforced polymer composites,a theoretical characterization model of the temperature-dependent tensile properties of polymer-based nanocomposites containing macroscopic structural features was developed by considering the interfacial debonding effect with the polymer matrix plasticity and their evolution with temperature as well as the nanoplate orientation.The model predictions are in good agreement with the available experimental data at different temperatures.The model establishes quantitative relationships between the tensile properties of composites and key control elements at different temperatures,deepens the understanding of the mechanical behavior of interfaces and their evolution with temperature,and provides theoretical support for the prediction of high-temperature mechanical properties and reliability analysis of nanocomposites and devices.