Study On Temperature Dependent Theoretical Characterization Methods For Tensile Strength Of Polymer Materials And Yield Strength Of Stainless Steel

With the rapid development of science and technology,the demand for expanding the service conditions of materials is more and more intense,and the requirements for the service performance of materials are also higher and higher.Polymer materials and stainless steel play an important role in aerospace,automation,construction engineering and other fields,and are more and more widely used in high temperature environment.Therefore,it is urgent to obtain and improve the temperature dependent tensile strength of polymer materials and the yield strength of stainless steel,which are the key indicators of application safety and reliability.It is important to study the key factors controlling the tensile strength of polymer materials and the yield strength of stainless steel at different temperatures and their evolution with temperature,and to establish the temperature dependent strength theoretical models including macro-micro structure characteristics and profound physical background.Meanwhile,this work is of great significance for material performance evaluation,optimization design and developing materials to meet the service temperature requirements.In this thesis,the following research work has been carried out for polymer materials and stainless steel:(1)Based on the Force–Heat Equivalence Energy Density Principle and according to the equivalent relationship between strain energy and heat energy,a temperature dependent tensile strength theoretical model without fitting parameters of polymer was established.The quantitative relationship between temperature dependent tensile strength,temperature dependent Young's modulus,heat capacity and melting temperature(crystalline polymer)/viscous flow temperature(amorphous polymer)was revealed.The model can easily predict the tensile strength of polymer at different temperatures.Further,based on the model and considering the strengthening mechanism of reinforced particulate on polymer and its evolution with temperature,a temperature dependent tensile strength theoretical model of particulate-polymer composites is established,which can take into account the effects of particulate size/content,especially the interfacial bonding strength and polymer matrix strength and their evolution with temperature.The model is well verified by experiments.Compared with the Pukanszky empirical model,this model is more accurate and convenient.In addition,the main influencing factors regarding the tensile strength at different temperatures were analyzed by using the model,which provided useful suggestions for improving the temperature dependent tensile strength.(2)Based on the established temperature dependent tensile strength model of polymer and combining with the statistical fracture theory of composites,a temperature dependent longitudinal tensile strength theoretical model of unidirectional fiber reinforced polymer composites was established.This model is more accurate and convenient than other models for theoretical prediction,and does not include parameters that are difficult to obtain.Afterwards,considering the effect of fiber orientation distribution and fiber length distribution,constituent characteristics,residual thermal stress and their evolution with temperature on the tensile strength of random short fiber reinforced polymer composites at different temperatures,a temperature dependent longitudinal tensile strength theoretical model is established.Further,the interfacial debonding criterion of fiber composites was extended to be a wide temperature range,and the theoretical prediction of fiber debonding length at different temperatures is realized.By introducing the maximum fiber axial stress which includes the interfacial debonding effect with temperature evolution,the classical mixture law is revised,and then a temperature dependent tensile strength theoretical model is established,which can consider the interface bonding properties and its evolution the temperature.In addition,combining with the bridging model and introducing the temperature dependent hardening modulus of matrix,a temperature dependent tensile strength theoretical model is established,which can consider the effects of the matrix plasticity and its evolution the temperature.Compared with K-T model and Curtin model,the model has advantages in prediction accuracy and application range.Meanwhile,the key control factors and its evolution with temperature of temperature dependent tensile strength for fiber reinforced polymer composites are analyzed in detail by using the above theoretical models,which provides theoretical guidance and suggestions for material performance evaluation and optimization design.(3)By establishing the equivalent relationship among elastic deformation energy,heat energy and phase transformation energy,a temperature dependent yield strength theoretical model of stainless steel is established.The model considers the effect of nonlinear stress-strain relationship before yield and phase transformation on the yield strength of stainless steel.The experimental results of temperature dependent yield strength of austenitic and ferritic stainless steels from Europe,Japan and China were used to verify the theoretical model.The theoretical work can easily predict the yield strength of stainless steel at different temperatures,especially at high temperatures,through the temperature dependent Young's modulus and the yield strength at any reference temperature,which provides theoretical support for the fire resistance evaluation of stainless steel in key areas such as building fire protection.