Failure Analysis of Induction Hardened Automotive Axles
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of Induction Hardened Automotive Axles C. Kendall Clarke Æ Don Halimunanda
Submitted: 28 November 2007 / Accepted: 16 May 2008 / Published online: 20 June 2008 Ó ASM International 2008
Abstract Rollover accidents in light trucks and cars involving an axle failure frequently raise the question of whether the axle broke causing the rollover or did the axle break as a result of the rollover. Axles in these vehicles are induction hardened medium carbon steel. Bearings ride directly on the axles. This article provides a fractography/ fracture mechanic approach to making the determination of when the axle failed. Full scale tests on axle assemblies and suspensions provided data for fracture toughness in the induction hardened outer case on the axle. These tests also demonstrated that roller bearing indentions on the axle journal, cross pin indentation on the end of the axle, and axle bending can be accounted for by spring energy release following axle failure. Pre-existing cracks in the induction hardened axle are small and are often difficult to see without a microscope. The pre-existing crack morphology was intergranular fracture in the axles studied. An estimate of the force required to cause the axle fracture can be made using the measured crack size, fracture toughness determined from these tests, and linear elastic fracture mechanics. The axle can be reliably said to have failed prior to rollover if the estimated force for failure is equal to or less than forces imposed on the axle during events leading to the rollover. Keywords Brittle fracture Fracture mechanics Critical crack size Intergranular cracking Axle failure Induction hardened axle Cleavage fracture
C. K. Clarke (&) D. Halimunanda Metallurgical Consulting, Mobile, AL, USA e-mail: [email protected] D. Halimunanda e-mail: [email protected]
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Introduction Current practice in automobile and light truck design uses induction hardened axles in which the axle itself serves as the inner race for the outer bearings. Semi-float axles found in light trucks (Fig. 1) use roller bearings for the outer bearings riding on the axle. Stub axles use ball bearings that run on induction hardened axles and are bolt on assemblies. Both designs are usually made from 1050 steel or similar steel with increased manganese for hardenability [1]. Semi-float axles are induction hardened from the axle flange radius and along most of the length of the axle. The axles are direct quenched in the induction hardening process and then low temperature tempered. Examination of axles reveals they usually have a clear martensite outer case depth of at least 2 mm and hardness values in this region of 58–60 HRC. A case hardness of at least 58–60 HRC is considered necessary for the journal to function as an inner race and support the bearing loads [2, 3]. Failure of a rear axle on a vehicle can cause a rollover under certain conditions. These conditions usually occur in SUVs because of their higher center of gravity. The
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