شرایط حفاری عملی و معیارهای کیفیت سوراخ برای PMR-15 / گرم ورقه کامپوزیت
Abstract: A generalized methodology for machinability-cum-structural integrity-testing of holes drilled in composite structures has been demonstrated in this research, by developing, deploying and validating a common set of quality evaluation criteria. The main objectives of the research were to determine optimum thermoset composite laminate drilling conditions, using PMR-15/gr test samples, and to track and document the effect that anomalies (due to non-optimal drilling) have on hole performance under both, static and dynamic loading conditions. Holes were drilled in PMR-15/gr panels during statistically designed experiments to evaluate three special purpose solid carbide drills versus a high speed steel NAS 907-J drill standard often used for composite repairs, under process conditions widely believed (yet experimentally unproven) to reduce panel damage, such as use of coolant and backup. The machinability test results indicate that in all cases, drill configuration had the greatest effect on hole quality measured reliably in terms of panel damage and thrust force. Over the conditions investigated, the optimal drill design was the dagger, used at high cutting speeds of 500 fpm (156 m/min) and feedrates 0.002 ipr (0.051 mm/rev), dry, without backup; however, this design was more prone to producing out-of-round holes in the composite. By employing innovative image processing techniques, a new index, called the \"damage ratio\", was developed using data on peripheral damaged area measurements from ultrasonic C-scans of drilled panels. After validation, with ultrasonic C-scans, photomicrographic sectioning, and static mechanical and residual strength tests, it was concluded that though composite laminates are more damage tolerant than metals, drill design and choice of drilling conditions are critical to hole performance under cyclic stress. Non-optimal drilling introduced substantial surface and sub-surface level delamination anomalies (accompanied by a high density of microcracks) in the hole periphery, which accelerated the development of fatigue failure in the PMR-15 matrix by reducing the initial buckling strength of the laminate. Finally, a method of reducing subjectivity in classification of hole quality in laminates has been demonstrated for each drill configuration tested. This technique, based on the assumption that the most common panel damage modes are visible to the naked eye, utilizes fuzzy set theory, validated by the damage ratio index from C-scans. It would facilitate greater objectivity in ascertaining hole quality during production drilling of composites.