بررسي آزمايشگاهي رفتار ديناميكي ماسه سنگ در نرخ هاي كرنش

Experimental investigation of dynamic behavior of sandstone

تعيين دقيق پارامترهاي ديناميكي سنگ‌ها در نرخ‌هاي بارگذاري بالا از جمله موضوعات بسيار مهم و قابل توجه در زمينه‌هاي كاربردي مختلف ديناميك سنگ است. طيف وسيعي از مسايل مرتبط با مهندسي سنگ شامل انفجار، زلزله، زمين لغزش و نفوذ پرتابه‌ها در ارتباط مستقيم با علم ديناميك سنگ و خواص و رفتار ديناميكي سنگ‌ها است. به منظور طراحي و تحليل بسياري از سازه‌هاي نظامي و دفاعي احداث شده روي سنگ شامل نيروگاه‌هاي زيرزميني، پناهگاه‌هاي زيرزميني ومغارها، تشخيص دقيق رفتار ديناميكي سنگ‌ها تحت مقادير مختلف نرخ بارگذاري لازم و ضروري است. مرسومترين و پركاربردترين فناوري مورد استفاده به منظور تعيين رفتار ديناميكي مواد در نرخ‌هاي كرنش بالا آزمايش ميله فشاري هاپكينسون است. در اين فناوري يك موج فشارشي با دامنه زياد به نام موج ورودي حاصل از ضربه مستقيم يك ميله ضربه‌زن، به سمت نمونه روانه شده و سپس امواج برگشتي و عبوري از نمونه همراه با موج ورودي به وسيله‌ي كرنش‌سنج‌هاي نصب شده روي ميله‌هاي دستگاه ثبت مي‌شوند. سپس با به كار بردن معادلاتي مشخص كه حاكم بر آزمايش هاپكينسون است، منحني تنش-كرنش ديناميكي نمونه به دست مي‌آيد. در اين پژوهش، ابتدا پارامترهاي مقاومتي ماسه سنگ در شرايط شبه‌استاتيكي به دست آمده و سپس آزمايش هاپكينسون در نرخ‌هاي بارگذاري مختلف روي ماسه سنگ انجام شده است و منحني تنش-كرنش ديناميكي ماسه سنگ به دست آمده است. نتايج نشان مي‌دهد كه در نرخ‌هاي بارگذاري بالا مقاومت ماسه سنگ به مقدار زيادي افزايش پيدا مي‌كند به‌گونه‌اي كه با افزايش نرخ كرنش نمونه تا s^(-1) 150، مقاومت ماسه سنگ از مقدار حدود MPa 160 در شرايط شبه‌استاتيكي به مقدار MPa 260 در شرايط ديناميكي رسيده است.

Rock dynamics as a branch of rock mechanics deals with dynamic behavior of rocks under high loading rates. Considering that many problems in rock engineering including earthquackes, explosions and projectile penetrations deal with high loading rates, rock dynamics has been of high significance to explore. In order to design and stability analysis of many of defense and military structures constructed on and in rocks, exploring of dynamic behavior of rocks under different loading rates is essential. However, detailed understanding of rock dynamics has been of high challenge due to the additional ‘4th’ dimension of time. The split Hopkinson pressure bar test (SHPB) is the most applicable and famous technique in determination of dynamic behavior of materials under high loadin rates. In this thechnique, a pressure wave with a high domain is dispatched to the specimen and the reflected and transmitted waves of specimen will be captured by means of strain gauges glued on the bars of Hopkinson apparatus. A dynamic stress-strain curve will be obtained for the specimen applying some known equations upon physical conditions of SHPB test. A great majority of studies have been shown that dynamic strength of rocks increases with an increase in loading rate. Also, it has been shown that inertial and heterogeneity effects are the most influential factors on dynamic strength increase of rocks under high loading rates. It is of note that Inertial effect boils down to a sudden increase in inner pressure of rock. Although, heterogeneity causes a more proper dynamic stress equilibrium as well as an increase in strain rate of specimen before the failure relative to those of homogenous ones. The more the loading rate is, the more the strength of rock increases.In the present study, efforts have been applied to explore the effect of loading rate on dynamic behavior of rocks using split Hopkinson pressure bar as the most known and common apparatus in studying dynamic behavior of materials under high loading rates. The specimens have been cored of the same block of sandstone with a diameter of 21.5 mm and aspect ratio of 2. First of all, some quasi- static tests including uniaxial and Brezilian have been done to obtain uniaxial compressive strength, Young’s modulus, poison’s ratio and tension strength. In the meantime, Ultra-sonic test has been applyied to group the specimens of same p-wave velocity before doing Hopkinson test. The dynamic stress-strain curves for the specimen under different loading rates have been gained after capturing incident, reflected and transmitted waves by the strain gauges.Results show that there is an intense dependence of dynamic strength of sandstone to the loading rate so that with imposing the strain rate of 150 on the specimen, the dynamic strength of sandstone has been increased to 260 from 160 in quasi-static conditions. That’s why DIF, as the ratio of quasi-static strength to the dynamic one, has been obtained 1.6 at the 150 strain rate.