analysis of an unusual crankshaft failure.pdf

Engineering Failure Analysis 12 (2005) 465–473

www.elsevier.com/locate/engfailanal

Analysis of an unusual crankshaft failure

Changli Wang a,*, Chengjie Zhao a, Deping Wang b

a Analysis and Measurement Center, Harbin Institute of Technology, Heilongjiang, Harbin 150001,Chinab Harbin Science and Technology University, Heilongjiang, Harbin, 150001, China

Received 2 December 2003; accepted 12 January 2004

Available online 31 December 2004

Abstract

A crankshaft cracked in a strange manner when test running was performed for only 20 min. Four cracks were found

on the edge of the oil hole. Using mechanical analysis, microstructure and metallurgy the reason of this event has been

revealed. Force of friction caused by improper crankshaft repair and assembling is main factor of the failure. Why fric-

tion occurs, how the crack initiates and expands and what the process of failure is were studied.

� 2004 Published by Elsevier Ltd.

Keywords: Crankshaft failures; Automotive failures; Bearing failures; Friction; Seizure

1. Introduction

The failure shaft is a new crankshaft; model RD8T, on a bulldozer TCM-175B. After a crankcase repair

process it was assembled to the engine and began test running without any loads. An unusual noise arose

and the test was stopped automatically 20 min later. Disassembly detection was performed and four crackswere found on the fourth main journal. All of them were located on the edge of oil hole (Fig. 1). The lengths

of the cracks were 6–9 mm. Main bush was destroyed seriously (Fig. 2). The copper on inside of bush was

almost torn off totally. Only a little copper was stuck on the surface of shaft and bush randomly. The color

of the surface of main journal was dim and there were blue and black circle zones crossed with cracks. No

other damage can be seen by naked eye.

1350-6307/$ - see front matter � 2004 Published by Elsevier Ltd.

doi:10.1016/j.engfailanal.2004.01.006

* Corresponding author.

E-mail address: [email protected] (C. Wang).

mailto:[email protected]

Fig. 2. Damaged main bush.

Crankshaft

Cracks

Oil hole

Fourth mainjournal

Fig. 1. Schematic illustration of crankshaft and cracks on the fourth main journal.

466 C. Wang et al. / Engineering Failure Analysis 12 (2005) 465–473

2. Chemical analysis

A sample taken from crankshaft was analyzed see Table 1. It is 40CrMnMo alloy. The content of Mn

and Mo was little bit lower than general value, but no serious effect on the properties of material can be

induced.

3. Metallurgy investigation and hardness test

Two samples were taken from shaft. One is cut through and vertical to the crack for the sake of crack

route observation. Another one is made by applying a load to the shaft, leading the crack to expand until

totally cracked in order to carry out metallography investigation.

Microhardness tests found that the hardness of hardened layer near the skin of shaft was reduced be-

cause of heating (Fig. 3).

Table 1

Chemical composition of the material of the crankshaft (wt%)

C Si Mn S P Cr Ni Mo

0.390 0.270 0.830 0.013 0.020 1.170 0.087 0.190

Fig. 3. Distribution of hardness on surface layer of shaft.

C. Wang et al. / Engineering Failure Analysis 12 (2005) 465–473 467

Using electron microscope s-570, the microstructure of material and the contour of fracture were inves-

tigated. The microstructure of the shaft in the center of section was a tempered sorbite (Fig. 4) and in the

hardness layer, about 3.2 mm thick, was lath martensite (Fig. 5). Some tempered sorbite can be seen near

the surface of the shaft(Fig. 6). The size of martensite varied from bulky to fine as the distance increased

from the surface of the shaft.

Fig. 4. Microstructure of center part of the shaft.

Fig. 5. Microstructures in the trasitional zone.

Fig. 6. Microstructure of skin of the shaft.


4. Fractography analysis

It can be identified that the cracks formed in three zones (Fig. 7). Each crack was initiated from the edge

of oil hole and its orientation was parallel to the longitudinal section of the shaft. It is a narrow zone along

the surface of the shaft. The second zone is a small incline area and with dim color. The third zone is a large

area with the same orientation as the first zone.

Microscopy shows that each zone has its respective characteristic. In first zone the contour formed by

multi-slip plane (Fig. 8). The second zone is a dimpled (Fig. 9). The third zone is a fracture (Fig. 10). Grain

Fig. 7. Overall view of the fracture.

Fig. 8. Slip cracks.


size near the surface was bigger than that inside the shaft (Fig. 11). On cross section through the crack path

presents three stages (Fig. 12). The first stage, relative to first zone of fracture, is a straight crack perpen-

dicular to the surface of the shaft. It was only 0.05 mm long. The second stage of crack, 0.6 mm long, sloped

relative to the first stage about 40�. It was a straight one also (Fig. 13). The third stage, about 3.2 mm long,

turned its orientation to the same direction as the first stage. In this stage the crack has a jagged course and

has some branch cracks (Fig. 14).

Fig. 9. Dimple fracture.

Fig. 10. Grain boundary cracks.


5. Failure process analysis

Interfacial friction exists between the surface of shaft and main bush. According to this and combined

with the condition of the shaft and the main bush the analysis was focused on the friction.

It had been clear that the shaft bearing hole had been bored by boring machine. The first and fifth hole

were taken as a basic hole and other three holes were then bored. After boring the alignment was disturbed,so the assembling of shaft was hardly in a good condition. It is a hidden trouble for serious friction.

Fig. 11. Grains size variations.

Fig. 12. The path crack.


Another possible reason for unusual friction is bad lubrication. However, according to the factor that all

principal shafts besides the cracked one are in a good situation, no damage can be found. That means that

the lubrication system of the engine works well. So the fracture was not associated with lubrication.

To sum up it can be deduced that the main factor for shaft cracking was improper repairing and assem-

bling of engine. Because of improper adjustment an exceptional friction between the shaft and main bushwas induced. Thus the temperature of shaft and main bush went up rapidly while test was going on.

The melted copper shows that the temperature had been higher than 600 �C. Under the action of friction,

material below the surface must yield, parallel to the force of friction. see (Fig. 15).

Fig. 13. Outline of first and second stage of crack.

Fig. 14. Microstructure of martensite and third stage of crack.


6. Conclusions

1. The crankshaft cracked by shearing stresses, caused by unusual friction between surface of shaft and the

main bush, due to improper repairing and assembling.

2. Friction can cause slip on the surface of friction boundary, especially when the temperature goes up to a

critical level.

3. To 40CrMnMo 300–500 �C is a special temperature which may lead to temper brittleness.

Fig. 15. Schematic illustration of friction force between shaft and bush and stress distribution in shaft.


4. It is possible that molten copper from the bearing shell caused liquid metal embrittlement in the crack-shaft journal.

analysis of an unusual crankshaft failure.pdf

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