In operation, drill bit tips are subjected to cyclic loads and amplitude vibrations. Such fluctuating load profiles induce cyclic stresses that often result in castrophic fatigue failure.
To ensure that drill bit tips are suitable for purpose and predict the expected service life, the components must be tested under worst-case combined compression and torsional loads.
The specialist loadstring fixture was supplied by the drill bit manufacturer and mounted onto an 8874 table top servohydraulic testing system. The standard system is capable of generating combined axial and torsional loads up to 25 kN and 100 Nm. The lower adapter was mounted directly to the lower t-slot table, and features a chuck to hold the shank of the drill bit. The upper part of the fixture features a clamp to the carbide tip that interfaces directly to the Biaxial Dynacell™ load cell.
After simple auto-tuning to optimize loop gains and set up of inertia compensation in both axial and torsional axes, a compression-torsion test was set up within WaveMatrix®2 Dynamic Testing Software. The test consisted of ramping to a maximum load and torque level on both axes, and subsequently running a cyclic off-set square wave profile repetitively to simulate a worst-case hammering action on the drill bit. After a number of cycles the torque required to unlock the carbide tip from the drill bit shank is measured and the main test is then continued. The unlocking torque is monitored throughout the test and reported.