اثر توأم نرخ بارگذاري و طول شكاف در مقاومت كششي برزيلي سنگ

Coupled effect of loading rate and notch length on tensile strength of rock

مطالعه‌ي اثر ناپيوستگي‌هاي سنگ در خصوصيات مكانيكي آن به‌منظور ارزيابي پايداري سازه‌هاي در معرض بار ديناميكي ضروري است. در پژوهش حاضر، اثر توأم طول شكاف و نرخ بارگذاري در مقاومت كششي سنگ با مدل‌سازي عددي آزمايش هاپكينسون با استفاده از نرم‌افزار 3C‌A كه يك نرم‌افزار تركيبي المان‌محدود ـ المان گسسته است، بررسي شده است. نمونه‌هاي برزيلي با قطر 50، ضخامت 10 و طول‌هاي متفاوت شكاف: 10، 20،30 و 40 ميلي‌متر آزمايش شدند. نتيجه حاكي از آن است كه اثر طول شكاف در مقاومت كششي ديناميكي با اثر طول شكاف در مقاومت كششي استاتيكي متفاوت بوده است و اگرچه در نرخ‌هاي بارگذاري پايين با افزايش طول شكاف مقاومت كاهش يافته است، اما در نرخ‌هاي بارگذاري بالا بسته به مقدار نرخ بارگذاري، مقاومت با افزايش طول شكاف توانسته است ثابت باقي بماند و يا افزايش يابد. همچنين نتايج نشان داد كه سرعت بازشدگي شكاف، نقش اساسي در موضوع اخير دارد.

Rock masses naturally contain joints and fractures and the effect of these fractures needs to be carefully investigated to ensure the stability of rock structures. This is particularly the case when dynamic loading effects due to earthquakes or rock blasting are involved. In this study, Brazilian synthetic specimens, made of gypsum with initial notches, were loaded in the mode-I fracture. The specimens were 50 mm in diameter and 10 mm in thickness. The pre-existing notch length in the specimens varied from 10 to 40 mm. The nominal tensile strength of the specimens was numerically evaluated using a bonded particle model (BPM) for the synthetic rock material. The dynamic tests were performed using the Split Hopkinson Pressure Bar (SHPB) system which was numerically simulated by the CA3 computer program. CA3 is a computer program for static and dynamic simulation of geomaterials in which a hybrid bonded particle and finite element system can be employed. The rock specimen was represented by the bonded particle model, while the incident and transmission bars in the Hopkinson Pressure Bar system were simulated by the finite element model. The bonded particle system was calibrated to ensure that the elastic properties, uniaxial compressive strength, tensile strength, and fracture toughness of the rock were replicated by the numerical model. The combined effect of loading rate and initial fracture length on the rock tensile strength was investigated. The results, as expected, suggest that the static nominal tensile strength of the specimens was reduced as the notch length increased. Under dynamic loading, the material response is more complicated; depending on the applied stress rate, the tensile strength can decrease, remain constant, or increase as the initial notch length increases. It is shown that the speed of the crack tip opening is responsible for this interesting observation of tensile strength changes under dynamic loading as the notch length varies.