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This paper develops a 3D homogenization-based continuum damage mechanics (HCDM) model for fiber reinforced composites undergoing micromechanical damage. Micromechanical damage in the representative volume element (RVE) is explicitly incorporated in the form of fiber—matrix interfacial debonding. The model uses the evolving principal damage coordinate system as its reference in order to represent the anisotropic coefficients. This is necessary for retaining accuracy with nonproportional loading. The material constitutive law involves a fourth order orthotropic tensor with stiffness characterized as macroscopic internal variable. Damage in 3D composites is accounted for through functional forms of the fourth order damage tensor in terms of macroscopic strain components. The HCDM model parameters are calibrated by using homogenized micromechanical (HMM) solutions for the RVE for a few strain histories. The proposed model is validated by comparing the CDM results with HMM response of single and multiple fiber RVEs subjected to arbitrary loading history. Finally the HCDM model is incorporated in a macroscopic finite element code to conduct damage analysis in a structure. The effect of different microstructures on the macroscopic damage progression is examined through this study.

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Since backward whirl was discovered as a severe cause of PDC bit failure, our industry has made great strides toward creating whirl-resistant bits and operating practices. But is whirl still the major cause of PDC bit damage in today's applications? This paper reports on a recent field study in which downhole vibrations were measured using a newly available in-bit vibration-monitoring device. The focus of this study was to understand today's North American vertical conventional rotary applications. In addition, four wells were also drilled using a research drill rig in Oklahoma. In these tests, PDC bits, BHAs, and operating parameters were varied to document their effect on downhole vibrations. In these four wells, vibration measurements from the new in-bit measuring device were validated against a commercially available and industry-proven MWD vibration monitoring service. Also, a computer model that accounts for the coupled response of bits and BHAs was run for selected tests in field wells and the research drilling rig. The models agreed well with the measured cases in both whirl and stick-slip. The model also allowed the authors to confirm that high-frequency torsional oscillations (4-9 Hz) which we observed were, as a previous industry paper suggests, due to BHA torsional resonance. The results of this study indicate that the most common field vibration today in hard rock PDC drilling is stick-slip, not whirl, in the target test region. In field tests, stick-slip was almost exclusively observed. For typical field applications with a surface rotary speed of about 70, we have measured peak downhole RPM during the slip phase as high as 500. This paper will report on these findings, document damage resulting from stick-slip, and suggest potential solutions to mitigate downhole vibrations.

have been compared for both ramped and isothermal conditions. It has been noticed that both the nanofluid velocity and temperature are smaller in magnitude in the case of ramped temperature plate than that of isothermal plate.