Breakage of a wheel shaft

The "failure analysis" in the automotive industry, interests a lot of difference types frequently related with individual physical safety problems. In the following analysis the subject is the breakage of a wheel shaft of a scooter.
TESTING PROCEDURE
Visual examination
The shaft, in this case is shown in macrograph n.1. the fracture surface is transversal oriented  to the samples axes, and develops with helical course.

The aspect is dull and composed by two regions: the first one is circumferential and plane, while the second one contains a "circular" core that is not located in the centre of the section. It has a dull and granular aspect with squeezing traces (photo 2).

In the external part of the shaft a lot of sliding rotational marks (photo 3).
Photo No.1
Photo No.2
Photo No.3
Chemical analysis 
The material of the component could be classified as 39NiCrMo3 according with the UNI EN 10083 specification.

Fractographic test
The fractographic examination showed that both the regions with different aspects have the same morphology.

The region 1 in fact has a ductile morphology with dimples elongated in the tangential direction (photo 4).
The region 2 has a ductile morphology with equiaxial dimples and cavities (photo 5).
Metallografic examination
Micrograph 6 show the structure in the centre of the shaft. It consists in quenched and tempered martensite, typical of this heat treated material.

Micrografia / Micrograph                    : 6
Attacco / Etching                                 : NITAL
Ingrandimenti / Magnification          : 600X
Campione / Sample                          : WHEEL SHAFT
Posizione / Position                           : CORE
Sezione / Section                               : CROSS SECTION
Photo No.6
Micrograph n. 7 show the fracture surface profile, the structure has no anomalies and defects, and the profile is jagged, and irregular.

Micrografia / Micrograph                    : 7
Attacco / Etching                                 : NITAL
Ingrandimenti / Magnification          : 150X
Campione / Sample                          : ASSE RUOTA
Posizione / Position                           : SUPERFICIE DI ROTTURA
Sezione / Section                               : LONGITUDINALE
Hardness and case hardening depth
The hardness of the hardened surface is 61HRC, while in the core is 332HV
In the hardened section the depth of the layer is 1mm.

Conclusions
Every point of the fracture surface, borders included, show a morphology with dimples, typical of a ductile fracture mechanism. The most probable cause of the damage is the mechanical block of the wheel.

Consequently the wheel shaft broke by overload. The material and its characteristics comply with the specification required and are not directly involved with the causes of the fracture.

Tag: Railways and transportation Automotive - Racing

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