ارزیابی سه آزمون رایج جهت تعیین چقرمگی شکست مود I استاتیکی سنگ ها

نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه مکانیک سنگ، دانشکده ی مهندسی معدن، دانشگاه تهران، تهران، ایران

2 گروه مکانیک سنگ، دانشکده‌ی مهندسی معدن، دانشکده فنی، دانشگاه تهران.

چکیده

چقرمگی شکست مبین مقاومت در برابر گسترش ترک می باشد که یکی از مهمترین پارامترهای مکانیک شکستی سنگ ها و سایر جامدات می باشد. چقرمگی شکست مود I سنگ ها یکی از خواص ذاتی بسیار مهم آن ها جهت پیش بینی گسترش ترک در کشش و در فرآیند شکست هیدرولیکی می باشد. تاکنون، روش های متنوعی جهت تعیین چقرمگی شکست مود I سنگ ها ارائه شده است که نتایج آن ها در عمل انطباق چندان مناسبی با یکدیگر ندارند. در این مقاله سه آزمون نمونه ی استوانه ای با شیار چورون، آزمون برزیلی و آزمون برزیلی مسطح شده، جهت تعیین مقدار چقرمگی شکست مود I برروی نمونه ی سنگی بازالت انتخاب شدند. پس از بررسی نتایج آزمایشگاهی و مدلسازی عددی با روش المان محدود سه بعدی، نهایتا، آزمون نمونه ی استوانه ای با شیار چورون به دلیل رشد پایدار یک ترک طبیعی، کم تر بودن حجم ناحیه ی پلاستیک نسبت به حجم کل نمونه و در نتیجه صحت بیش تر فرض الاستیک خطی در این نمونه، همچنین عدم تاثیر شرایط مرزی بر مقادیر چقرمگی شکست و آماده سازی نسبتا مناسب آن بهترین آزمون در بین سه آزمون اشاره شده جهت تعیین چقرمگی شکست مود I سنگ ها تشخیص داده شد.

کلیدواژه‌ها


عنوان مقاله [English]

Evaluating three conventional tests for determining mode I static fracture toughness of rocks

نویسندگان [English]

  • Ali Mohamad Pakdaman 1
  • Mahdi Moosavi 2
1 School of Mining Engineering, University of Tehran, Tehran, Iran
2 School of Mining Engineering, University of Tehran, Tehran, Iran
چکیده [English]

Summary
Fracture toughness is defined as the material resistance against crack extension. Different tests have been presented for determination of mode I fracture toughness of rocks, but results of these tests have considerable differences with each other. In this paper, amongst various methods for determining mode I fracture toughness of rocks, Chevron Bend, Brazilian and Flattened Brazilian tests were chosen for investigation of main causes of difference of fracture toughness values. For all tests, 3-dimensional finite element analysis also was used to find the best test for determining this parameter. Finally, it was concluded that amongst three mentioned tests, the Chevron Bend test is the best for determination of mode I fracture toughness because of the least fracture process zone around the crack tip, fairly good sample preparation, a stable crack extension and the least influence of boundary conditions on fracture toughness values.

Introduction
Fracture toughness represents the material resistance against crack extension and is one of the most important parameters in hydraulic fracturing test. Because this parameter is one of inherit characteristics of rocks, values of fracture toughness of different tests seems to have a good agreement with each other, but in practice it doesn’t happen. In this paper amongst various methods of mode I fracture toughness determination, Chevron Bend, Brazilian and Flattened Brazilian tests have been selected in order to find main reasons for difference of fracture toughness values.

Methodology and Approaches
tests were evaluated based on sample preparation, crack extension, crack type, effect of boundary condition and volume of fracture process zone around crack tip according to experimental results and numerical modeling.


Results and Conclusions
Results showed that Brazilian test is not suitable test because of assuming infinite plate in its equations, crack initiation near loading plates, non-uniform loading condition and effect of boundary conditions on fracture toughness values. In Flattened Brazilian test crack is initiated at the center of sample, fracture toughness is numerically determined based on finite plate assumption and loading conditions is uniform, but fracture toughness values is also affected by boundary conditions. In general, Chevron Bend test is chosen as the best test for determining fracture toughness of rocks according to stable crack growth, which makes natural crack at the point of calculating the fracture toughness, and lower size of the fracture process zone around the crack tip. Also, boundary conditions have no considerable influence on fracture toughness values in this test.

کلیدواژه‌ها [English]

  • Fracture Toughness of Rocks
  • Experimental Tests
  • finite element analysis
  • Fracture Mechanics of Rocks
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