An experimental investigation on a crack propagating from a geartrain housing in an asphalt milling machine

• A crack developed in a gearbox panel mounted on a cold planer machine.
• An identical not cracked panel was instrumented by strain gauge rosettes for a full estimation of the local load.
• Local states of stress and strain were monitored during panel assembly and machine use.
• A mechanical hysteresis event causes an unexpected stress increasing trend at the cracked area.
• Self-loosening of two bolts, caused by engine vibration, is the primary reason for the described hysteresis phenomenon.

The present paper deals with the analysis of a crack that developed in a gearbox panel mounted on a cold planer machine. The crack was discovered after approximately 950 work hours, after that an oil leakage was detected: at that stage of propagation it was about 40 mm long. A research was therefore tackled to experimentally reproduce the history of load that had led to cracking and to estimate the local amount of stress and strain. For this purpose, a not cracked identical panel was instrumented by three-grid strain gauge rosettes. Then, an experimental campaign was arranged to reproduce loads during assembly and in operating conditions. In particular, the most critical and frequent task, obstacle overcoming, was simulated 16 times in about two hours time. The most interesting result was that stress has always an increasing trend during stepping up and stepping down. This event can be regarded as a mechanical hysteresis: the primary cause, standing in the self-loosening of two bolts connecting the panel to the machine frame, due to engine induced vibrations, was found out. The mechanism leading to failure was therefore identified, as testified also by the maximum principal stress direction being perpendicular to that of crack propagation. The crack criticality at the current propagation stage was finally discussed, estimating its stress intensity factor.