Abstract：Large bore four stroke diesel engines are widely employed in power demanding applications such as large ships and terrestrial power plants. Due to the extremely high number of cycles they are subject to (above 108), components are designed according to the infinite-life approach. Although fatigue cracks are well below the propagation threshold, concurring damage phenomena such as corrosion, wear, fretting, and overloads can trigger them, leading to failure in relatively short times and posing great challenges to an accurate root cause failure analysis.

In this paper, the fatigue failure of connecting rods initiated by fretting damage is described. Fracture mechanics and Hertz theory were employed to assess the main parameters controlling the damage and to define thus appropriate corrective actions. Connecting rods which were considered to be susceptible to fatigue crack propagation were subsequently monitored through a tailored ultrasonic testing procedure.

Keywords: Fretting fatigue; Fracture mechanics; Connecting rod; Four stroke engine.

1. Introduction

Fatigue is a ubiquitous problem which causes failures in numberless applications, examples being automotive, aerospace, chemical, and process industry. Due to the materials of construction (steel and cast iron), high loads (maximum cylinder pressure up to 250 bars), and expected lifetime (108-109 cycles), large bore four stroke engines can be seriously challenged by fatigue. The extremely long life of the engine in conjunction with the requirements of high reliability and safety for human operators makes the infinite-life approach the only viable way for fatigue

design. Anyhow, even though design stresses are low enough to keep fatigue cracks well below the propagation threshold, concurring damage phenomena such as corrosion, wear, fretting, and overloads can trigger them, leading to failure in relatively short times and posing great challenges to an accurate root cause failure analysis.

In this paper, a specific case study, the fretting fatigue failure of connecting rods, is described. It is shown how simple calculations based on Hertz theory and fracture mechanics can be of help in the choice of appropriate corrective actions, nondestructive testing being a fundamental support for monitoring their effectiveness.

2. Fretting fatigue crack initiation in connecting rods

In the period from 2009 to 2010, three installations were affected by failure of connecting rod caps in a rapid sequence. The features of the failures were remarkably similar, as shown in Fig. 1.

Fig. 1. Overview of failed connecting rod caps.

Already at a visual inspection level, fretting damage was noticed on the big end bearing housing, mainly located in the area of the lube oil channel (Fig. 2a). The crack which led to failure was clearly starting from this area. The final resisting section was found to be 15–20% of the initial section, suggesting that the applied nominal stress was quite low (Fig. 2b). The metallographic inspection of the cross-section showed several cracks, 0.1 to 0.3 mm deep, inclined by 45° (Fig. 2c). This feature can be attributed to high under-skin shear stresses due to adhesion spots produced by fretting on mating surfaces. Smaller cracks were found also in the rest of the upper part of the housing.

Fig. 2. (a) Fretting damage on big end bearing housing; (b) fracture surface, fatigue crack initiation pointed by red arrow; (c) fretting cracks.

Table 1. Chemical composition of the connecting rod material. C         Si         Mn      P S Cr        Ni        Mo 0.40     0.26     0.74     0.007     0.003     0.80     0.79     0.18

Table 2. Mechanical properties of the connecting rod material.

Rp0.2 [MPa] Rm [MPa] A [%] Z [%] KV at 20 °C [J] HBW 820 940 20 64 93 277

The material of the connecting rods was found to fulfil the requirements of the quality instructions. Table 1 and 2 report respectively chemical composition and mechanical properties of the quench hardened and tempered steel.