Failure Analysis Report on Fractured Torsion Bar in Suspension of a Vehicle

Abstract

Accidents caused by fractured torsion bars in off-road vehicles (ORVs) have occurred during road tests. Through visual inspection of the fracture in the failed samples, the preliminary conclusion is that the typical torsional fracture occurs according to its ratchet shape and 45-degree cracking direction to the bar axis. The crack initiated at the side surface of the fillet on the spline end. All failed parts during road tests have identical fracture modes and crack initiation sites, which implies that they have identical failure reasons. Techniques such as scanning electron microscopy (SEM), metallographic observation, hardness test and spectral component analysis have been performed to obtain detailed information to diagnose the cause of failure. The result reveals that the fracture was caused by fatigue cracking initiated from the spline fillet. In addition, SEM images of the intergranular feature of the fracture initiation zone demonstrate that the untimely formation of a fatigue core and very quick expansion of the crack directly result from over-brittleness. This behavior can be ascribed to insufficient tempering after the quenching process, which was confirmed by a tempering simulation experiment: after the tempering experiment, the artificial struck fracture of the sample became a 100% dimple fracture, and the hardness distribution became more uniform and reasonable. Thus, the brittleness was eliminated, and the strength was ensured after an expanding tempering time. This investigation can provide reference value for solving and preventing such types of failures.