非线性超声技术检测P92耐热钢蠕变试验研究

深度调峰工况导致大容量高参数火电机组关键部件耐热钢材料性能劣化加速。作为一种大量用于火电机组的新型高温材料,P92钢的老化状态评估对于机组安全稳定运行具有重要意义。非线性超声信号能够填补传统无损检测方法在微尺度检测领域的空白,在材料早期损伤检测方面具有极大的应用潜力。利用非线性超声信号对微纳米尺度的微小缺陷和微裂纹比较敏感的特点,研究了P92钢耐热钢高温蠕变条件下微观组织形貌、析出相形态和成分的变化情况,试验得到P92钢蠕变时间与超声非线性系数之间的关系曲线,并根据材料的显微组织演变特征进行分析非线性超声系数的变化规律。试验表明,随着蠕变时间的增加,马氏体板条不断碎化,向等轴晶粒转变,碳化物不断沿晶界析出,少量在晶内析出,P92钢析出相颗粒平均尺寸在1000 h左右达到极大值490 mm后有所减小,析出相面积分数在2000 h左右增加到最大值0.089%后趋于稳定,析出相成分主要为块状的Cr23C6和碳化物;P92钢的二阶超声非线性系数随蠕变老化时间的变化曲线的拟合优度为0.94,比一阶超声非线性系数对蠕变老化更为敏感,并且与蠕变时间成正比关系,变化趋势与蠕变曲线类似。结果证明,非线性超声对于P92钢蠕变老化状态评估的可行性,为高温工况下P92钢非线性超声检测提供了理论依据。

Correlation of Cooperatively Localized Rearrangement on the ＂Fluidized Domain＂ of Polymers to Their Nonexponentially Viscoelastic Behaviors at Double Aging Processes （I）： A Set of Reduced Universal Equations on the Stress Relaxation Modulus and Creep Comp

Based on the structure of glass （or liquid） polymers consisting of α-domain, β-co-domain, and entanglement constituent chain networks, and the nonexponentially viscoelastic behavior, a “heterophase fluctuation” model was proposed. It was found that the dynamics of cooperative rearrangement on the “fluidized domain” has a great shear rate, domain size, and temperature dependences. When the shear rate, domain size, and temperature dependences were taken account into the cooperatively localized rearrangement on the fluidized domain by the degradation of primary α-domain and the reformation of secondary β-co-domain constituent chains. A new dynamic theory of cooperatively localized rearrangement on the fluidized domain constituent chains with different size and different network chain length during physical and mechanical aging was established. The total viscoelastic free en-ergy of deformation resulting from the change in conformations of α-domain, β-co-domain, crytallite, crosslinked, and trapped entanglement constituent chains during aging processes was calculated by the combining method of kinetics and statistical mechanics. The constitu- tive equations and reduced stress relaxation modulus and creep compliances for three types of polymers were also derived. Finally, two reduced universal equations on creep compliance and stress relaxation modulus with a non-linear and two nonexponential parameters α and β were theoretically derived from the dynamic theory and a statistically extended mode coupling theory for double aging effects of polymers was developed. Results show that the two reduced universal equations have the same form as Kohlraush-Williams-Watts （K-W-W） stretched exponential function. The nonlinearity and the nonexponentiality are, respectively, originated from the memory effects of nonthermal and thermal history. The correlation of nonlinearity, α and β to the aging time, aging temperature, and the mesomorphic structure of fluidized domains was also established.

Correlation of Cooperatively Localized Rearrangement on the ＂Fluidized Domain＂ of Polymers to Their Nonexponentially Viscoelastic Behavior and Lifetime at Double Aging Processes II： Estimation of Long-term Mechanical Behavior and Lifetime of Polymeric Materials from Short-time Creep and Stress Relaxation Tests

Three kinds of polymeric materials are taken as example for the verification of linear ex-trapolation method from unified master lines with reduced universal equations on creep and stress relaxation tests. The theoretical values of long-term mechanical behavior and lifetime for a cured epoxide, polypropylene, poly（methyl-methacrylate）, and SBR rubber are directly evaluated with the universal equations on reduced creep compliance and reduced stress relax-ation modulus and are compared with their predicted values by the linear extrapolation from the unified master lines of creep and stress relaxation. The results show that the theoretical values of dimensional stability, bearing ability and lifetime are in an excellent agreement with the predicted values, it shows that the linear extrapolation method is more simple and reliable. The dependences of long-term mechanical behaviors and lifetime on the different aging times are discussed.

An elastic-viscous-plastic model for overconsolidated clays

The influences of time on clays are discussed first,and the concept of the instant normal compression line is proposed by analyzing the existing theories and experimental results.Based on the creep law,the relationship between the aging time and the overconsolidation parameter is built.With the reloading equation of the UH model(unified hardening model for overconsolidated clays) used to calculate the instant compression deformation,a one-dimensional stress-strain-time relationship is proposed.Furthermore,the evolution of this relationship is analyzed,and the characteristic rate that is a function of the overconsolidation parameter is defined.Then a three-dimensional elastic-viscous-plastic constitutive model is suggested by incorporating equivalent time into the current yield function of the UH model.The new model can describe not only creep,rate effect and other viscous phenomena,but also shear dilatancy,strain softening and other behaviors of overconsolidated clays.Besides,compared with the modified Cam-clay model it requires only one additional parameter(the coefficient of secondary compression) to consider the creep law.Finally,because the proposed model can be changed into the UH model under instantaneous loading,the elastic-plastic and elastic-viscous-plastic frameworks are unified.