اثر فرضيات سخت‌شدگي چرخه‌يي فولاد در رفتار اتصالات فولادي

Effect of Steel Cyclic Hardening Hypotheses on the Behaviour of Steel Joints

امروزه مهندسان به منظور بررسي رفتار چرخه‌يي سازه‌ها و اجزا سازه‌يي فولادي از فرضيه‌هاي مختلف سخت‌شدگي خميري در مدل رفتاري فولاد استفاده مي‌كنند، كه اغلب آن‌ها با ساده شدگي‌هاي قابل توجهي همراه است. از سوي ديگر، نتايج آزمايشگاهي نشان مي‌دهند كه سخت‌شدگي فولاد تركيبي غيرخطي و به نسبت پيچيده از دو نوع سخت‌شدگي پويا و همسان‌گرد است. از اين رو در اين نوشتار سعي شده است تا ميزان اثرگذاري فرضيات مختلف سخت‌شدگي چرخه‌يي فولاد در برآوردهاي تحليل‌هاي عددي بررسي شود. براساس اين هدف، يك اتصال فولادي خمشي انتخاب و پاسخ آن تحت دو نوع بارگذاري لرزه‌يي دور و نزديك گسل براي 6 نوع فرضيه‌ي سخت‌شدگي و 6 نوع فولاد با ويژگي خميري متفاوت استخراج شده است. نتايج حاصل براي كميت‌هاي مقاومت، ظرفيت جذب انرژي و زمان شروع ترك مورد مقايسه و ارزيابي قرار گرفته‌اند.

Exact determination of loading conditions and the mechanical behavior of constituent materials are among key elements in the safe and economical design of structures. In practice, economic considerations do not allow structures to be designed for their elastic range of behavior. That is, under imposed loads, elements of a typical structure normally undergo irreversible deformation. Thus, accurate numerical estimation of structural responses crucially depends on the accuracy of the nonlinear constitutive models of the materials. Nowadays, the conventional plasticity approach, if carefully employed, can confidently estimate the nonlinear response of various materials. Steel, as the most commonly used metal in the building industry, has been shown to have complex cyclic strain hardening properties, including cyclic creep (ratcheting), relaxation, cyclic hardening or softening, and etc. However, it is frequently observed that elastic-perfectly plastic assumptions or very simple linear isotropic or kinematic forms are adopted in practice. Hence, in order to evaluate how these incorrect hypotheses influence the numerical results of cyclically loaded steel connections, various simplified hardening models used by engineers are identified. These include three kinds of isotropic hardening and two kinds of kinematic hardening model. Then, a moment-resisting beam-to-column connection, previously tested under near fault excitations, is nominated. Using a reliable combined nonlinear isotropic-kinematic model (reference model), a representative FE model is generated and validated against the empirical results. In the next stage, the analysis has been carried out for six different types of steel with different plastic properties, under both near and far fault excitations, whose results are compared to those of the reference models (models with the combined hardening hypothesis). Three variables of maximum tolerable load before failure, plastic energy dissipated, and the critical equivalent plastic strain are introduced for the strength, energy dissipation, and onset of cracking criteria, respectively. It needs to be noted that the micromechanically based cyclic void growth model (CVGM) is employed to calculate the critical equivalent plastic strain associated with the onset of cracking. The results show that, regardless of the type of excitation, considerable errors may arise if the combined hardening rule is not employed. Moreover, some recommendations are provided regarding a steel with specific cyclic hardening behavior.