بررسي خستگي سنگ بكر بلوري تحت بارگذاري كاملاً معكوس شونده

Evaluating Fatigue of Intact Crystalline Rocks under Completely Reversed Loading

در طي پديده خستگي جسم به علت تجمع آسيب و گسترش ترك ناشي از تكرار روند بارگذاري، در تنشي كمتر از مقاومت استاتيكي خود به‌صورت ناگهاني و ترد مي‌شكند. سازندها و سازه‌هاي سنگي ازجمله گسل‌ها، درزه‌ها، صفحات لايه‌بندي، مخازن زيرزميني، ديواره تونل‌ها، ديواره حائل پل‌ها، پي راه‌ها و سدها و ... مي‌توانند تحت تأثير بارهاي ديناميكي ناشي از زمين‌لرزه‌هاي بزرگ، انفجار سنگ، آتشباري، حفاري، ترافيك قرار گيرند. در اين كار تحقيقاتي رفتار سنگ‌ها تحت تأثير بارگذاري كاملاً معكوس شونده (كشش – فشار) و به كمك دستگاه آزمايش خستگي ساخته‌شده بر اساس آزمايش رايج خستگي فولاد بررسي شده است. از دو نمونه سنگ بلوري گابروي نطنز و مرمر سبز با تركيب كاني‌شناسي و منشأ ساخت متفاوت جهت بررسي پديده خستگي با اين روش استفاده شد. نتايج به‌دست‌آمده نشان مي‌دهد كه در نمودار S-N براي هر دو نوع سنگ با كاهش مقدار تنش اعمالي به نمونه، تعداد دوره تحمل شده به‌صورت لگاريتمي افزايش پيدا مي‌كند و نمودار هر دو سنگ از رابطه وهلر پيروي مي‌كنند. همچنين امكان ارزيابي حد دوام به كمك اين نوع آزمايش وجود دارد، درحالي‌كه در تحقيقات قبلي اين امكان وجود نداشته است. نتايج نشان مي‌دهد كه حد دوام براي گابروي نطنز در حدود 14/7 مگا پاسكال (53% مقاومت كششي در حالت استاتيكي) و براي مرمر سبز حدود10/8 مگا پاسكال (60% مقاومت كششي در حالت استاتيكي) است. مقايسه نتايج حاصل‌شده با نتايج ساير مصالح تأييد مي‌كند كه دستگاه ساخته‌شده و روش مورداستفاده در اين تحقيق براي برآورد خستگي در سنگ‌هاي بلوري مناسب است.

Summary The fatigue behavior under completely reversed loading for two crystalline quarry stones in Iran; Natanz gabbro and Green onyx, which have different mineral compositions and formation conditions, were evaluated using a new developed apparatus. The obtained S-N curves followed common “Wöhler relationship” in the variation of applied amplitude stress versus loading cycle number. In addition, the endurance limit is perceived for both tested rocks as well as observed in ferrous metals. The endurance limit for Natanz gabbro and Green onyx were evaluated 14.7MPa (0.53 of its tensile strength) and 10.8Mpa (0.6 of its tensile strength), respectively. Introduction During fatigue phenomenon cyclic loading causes a material to fail prematurely and suddenly like a brittle failure at a stress level less than determined strength under monotonic condition. Rock formations and structures such as faults, joints, bedding planes, underground reservoirs, tunnels walls, bridge abutments, roads and dams’ foundations, etc. can be subjected to dynamic loadings resulting from vibrations of the earth’s crust, through major earthquakes, rock bursts, rock blasting and drilling and also, traffic. Reaction of rocks to cyclic and repetitive stresses resulting from dynamic loads has been generally neglected with the exception of a few rather limited studies. In this research work, the fatigue behavior of two crystalline quarry stones in Iran; Natanz gabbro and Green onyx which have different mineral compositions and formation conditions were studied under completely reversed loading. Methodology and Approaches The stress-life method was applied and fatigue behavior and the existence of the endurance limit for two crystalline quarry stones in Iran; Natanz gabbro and Green onyx which have different mineral compositions and formation conditions were evaluated using a new developed apparatus. The developed fatigue testing machine was inspired by rotating beam fatigue testing machine (R. R. Moore) which is commonly used for laboratory fatigue test in metals. Results and Conclusions The obtained results (S-N diagram) followed common “Wöhler relationship” in the variation of applied amplitude stress versus loading cycle number. In addition, results showed that both rocks have endurance limit. The results also illustrated that the endurance limits for both tested rocks are ranged between 0.4 and 0.6 of their tensile strengths. The endurance limit for Natanz gabbro and Green onyx were evaluated 14.7MPa (0.53 of its tensile strength) and 10.8Mpa (0.6 of its tensile strength), respectively. The agreement between the obtained results and presented results in fatigue of other materials showed that the offered method can be applied for studying the fatigue of rocks.