a b s t r a c t   An analysis was performed to asses the failure root cause of an automotive diesel engine which experienced collapse only 6 month after revision。 The connecting rod bolts torque disassembly was monitored and fractured parts were selected to laboratory fracture anal- ysis。 It was verified with fatigue rupture of one of the fourth connecting rod bolt。 Tensile tests were performed in four of the remaining connecting rod bolts。 During this procedure, it was verified another bolt with fatigue crack propagation an indication that the first fati- gued bolt did not have suffer torque relaxation。 A finite element analysis was performed in connection with an analytical fracture mechanics approach aiming to evaluate the relation between tightening force and fatigue crack propagation in connecting rod bolts。 The engine collapse occurred due to forming laps in the grooves of the bolt shank。 Finally, some design improvements were suggested for avoid future failures: a gap in the groove length at the connecting rod cap interface, enough to avoid combination of forming laps and higher stress amplitude; increase of the bolt torque assembly to reduce stress   amplitude。85046

© 2008 Elsevier Ltd。 All rights reserved。

1。Introduction

Failure of bolts can result in catastrophic events in an overall complex system [1]。 Despite the wide understanding regard- ing bolt mechanics, cases of failure are still reported。 The most common factors which contribute to failure are low tighten- ing force, design defects specially concerning thread root radius, material or fabrication defects which can be achieved in material selection, heat or mechanical treatment [2–6]。 In this paper, a failure analysis was performed in a 6。6 automotive diesel engine which experienced collapse in service only 6 months after revision。 It was reported that at the time of revision, the engine was opened and some parts such as seals, bushings and connecting rod bolts were replaced。 The failure analysis methodology was evaluated, based on the experimental sequence suggested by the ASM [7]。 For this analysis the engine dis- assembly was performed and preliminary visual analysis of damaged parts was conducted to select the fractured parts that could be the failure root cause。 Laboratory fracture analysis, mechanical tests, metallographic analysis and numerical mod- eling with an analytical fracture mechanics approach were    performed。

2。Materials and methods

2。1。Disassembly of the engine

The first observation showed fracture of the engine block, due to impact of the fourth connecting rod, whose cap was dis- assembled as a result of bolts failure。 The engine head was then disassembled showing camshaft fracture in three parts and some other resulting damages such as: piston marks due to valve contact, secondary cracks in the fourth piston, as well as

*  Corresponding author。 Tel。: +55 51 3308 4251; fax: +55 51 3308  3565。

E-mail address: sandro@demet。ufrgs。br (S。  Griza)。

1350-6307/$ - see front matter © 2008 Elsevier Ltd。 All rights reserved。 doi:10。1016/j。engfailanal。2008。10。002

warping of valves, valve lifters and valve rods。 No abnormal scratch marks in the cylinder liners were observed after block and pistons disassembly。 The fourth connecting rod bolts fractured at the plane of the cap interface。 The bolts used were Whitworth UNF7/16 with 20 threads per inch a 12 mm shank diameter and 68 mm tightening length。 Those bolts had 18 shank grooves lengthwise (Fig。 1), which provide bolt assembly alignment with low contact surface, to enhance fretting and corrosion resistance。 The bolt manufacturer recommends 100 N m assembly torque。 Disassembly of all connecting rods was performed using a ratchet torquimeter as a way to verify the residual torque present at the bolts。