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Under the conditions of drilling in gravel-bearing and heterogeneous stratas, the movement and force of the PDC bit during drilling are highly unstable. Irregular impact loads often cause fatigue failures such as tooth fracture, tooth breakage and delamination of the composite sheet. Dynamic impact load is the main cause of fatigue failure of cutting-tooth, which seriously affects the rock-breaking performance of PDC bits. This paper proposes a flexible cutting tooth unit consisting of a central tooth, an elastic element, and a base. The technical concept of flexible-cutting rock breaking is to reduce the impact load amplitude suffered during the cutting process to a certain threshold range by setting elastic elements or reducing the support stiffness of the cutting tooth, so as to inhibit the expansion of micro defects caused by the impact dynamic load of cutting teeth and prolong the service life of drill bits. The finite-element models of flexible cutting teeth for rock cutting were established. The influence of cutting depth, front rake angle and stiffness of elastic elements on the cushioning and vibration-absorption effect of flexible cutting was analysed. The results show that flexible cutting can reduce the peak and average value of von Mises stress at the cutting edge. Under different cutting-depth conditions, the decline rates were 21.45–49.02% and 9.42–17.8%, respectively. Then, under different front-rake-angle conditions, the decline rates were 20.51–24.02% and 9.41–17.8%, respectively. There is a suitable stiffness of the elastic element, which makes the peak and average value of von Mises stress at the cutting edge of the flexible cutting-tooth unit perform better and the effect of improving the uneven stress distribution at the cutting edge better. Flexible cutting technology can effectively improve the adaptability of the PDC bit in heterogeneous formations and reduce the occurrence of abnormal failure of cutting teeth. The research results of this paper can provide theoretical support for the drilling speed of PDC bits in hard formations. [ABSTRACT FROM AUTHOR]

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Residual stresses are induced in the material during manufacturing operations, which considerably affect the fatigue performance and the lifespan of a mechanical work piece. The nature, magnitude, and distribution of residual stresses decide their beneficial or detrimental effects. Past research efforts concluded that mechanical process parameters influence residual stress nature, distribution, and the magnitude. Nevertheless, how residual stress generation depends on the process parameters, is not well investigated especially in the case of a drilling operation. In fact, the residual stress field is required to be regulated near drilled holes to improve the fatigue strength of structural joints, especially in the aircraft industry. Accordingly, this work attempts to estimate the drilling-induced micro-residual stress distribution near the drilled hole. In addition, the effect of drilling speed on residual stress distribution has also been studied. A nanoindentation technique is used to follow-up precise distribution of micro-residual stresses near the holes drilled at three different drilling speeds of 700, 900, and 1100 r/min. The outcomes indicate the presence of compressive residual stresses near the hole. In addition, an increase in residual stress level is noticed with an increase in the drilling speed up to 900 r/min. A uniform distribution of residual stresses is observed near the hole when drilled at a higher drilling speed of 1100 r/min. These findings may be useful in planning an improved drilling operation to produce beneficial residual stress distribution. This may ultimately improve the fatigue strength and the service life of mechanical components or structures with drilled holes. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)