Failure Analysis of Ductile Iron Crankshaft in Four-Cylinder Diesel Engine

Abstract

Spheroidal graphite cast iron is widely used in automobile crankshafts due to features such as high strength, high toughness, good machinability, low cost, and ductility. The purpose of this paper is first to analyze the fatigue failure in fractured truck crankshafts between 400,000 and 1,350,000 km and then to predict the stress field under the influence of the mechanical loads caused by gas combustion. Several experimental studies including chemical composition, material strength, hardness, and microstructure are performed to evaluate the failure analysis. A nonlinear three-dimensional stress analysis model by the elastic–plastic finite element method is used to estimate the crankshaft stress field under cyclic bending combined with steady torsion. Failure analysis showed that the fracture surface consisted of three zones: fatigue crack initiation, fatigue crack propagation, and static fracture; therefore, the failure is in fatigue. Also, the microstructure results showed that the crankshaft nodularity is about 70%, which crankshaft structures are usually acceptable greater than 80% of nodularity. Besides, hardness gradient showed that the crankshaft crankpin had no hardened surface layer. Finally, numerical stress analysis indicated that the highest stress is in the crankpin–web fillet zone that it is in good agreement with those obtained in the experimental field measurements.

Appendices

Appendix 1: Reviews on the Behavior of Cast Irons

According to the first group of literature reviews on the behavior of cast irons (Table 7).

Table 7 Backgrounds of Research

Appendix 2: Reviews on Failure Analysis

According to the second group of the literature reviewing the investigation of failure analysis (Table 8).

Table 8 Backgrounds of research

Appendix 3: Stress Analysis of the Crankshaft

Stress analysis literature include numerous studies devoted to reciprocating piston engine crankshafts.31,32,^–33,35,37,64 The force applied to the piston $F_{\text{P}}$ is equal to the superimposition of the gas force $F_{\text{G}}$ and oscillating inertial force $F_{\text{os}}$ as follows (Figure 13).

Figure 13

Forces acting in a reciprocating piston engine, offset piston pin.

$$ F_{\text{P}} = F_{\text{G}} + F_{\text{os}} $$

(Eqn 1)

The gas force $F_{\text{G}}$ is obtained the product of the maximum combustion chamber pressure $P_{\hbox{max} }$ and the area of the piston, and the inertial force $F_{\text{os}}$ is obtained the oscillating mass $m_{\text{os}}$ (piston mass and part of the connecting rod mass) and $A_{\text{p}}$ the instantaneous acceleration of the piston $a$.

$$ F_{\text{G}} = P_{\hbox{max} } A_{\text{p}} $$

(Eqn 2)

$$ F_{\text{os}} = m_{\text{os}} a $$

(Eqn 3)

The thrust in the connecting rod $F_{\text{cr}}$ is obtained using dividing the force applied to the piston $F_{\text{P}}$ and the inclination angle of the connecting rod with the line of stroke $\theta$.

$$ F_{\text{cr}} = \frac{{F_{\text{P}} }}{\cos \theta } $$

(Eqn 4)

The tangential force applied $F_{\text{t}}$ and subsequently the torsional torque applied to the crankshaft axis are obtained as follows:

$$ F_{\text{t}} = F_{\text{cr}} \sin \left( {\theta + \emptyset } \right) $$

(Eqn 5)

$$ T = F_{\text{t}} \times r $$

(Eqn 6)

The radial force $F_{\text{rad}}$ acts on the crankpin radially and subsequently the bending moment applied to the crankshaft are obtained as follows:

$$ F_{\text{rad}} = F_{\text{cr}} { \cos }\left( {\theta + \emptyset } \right) $$

(Eqn 7)

$$ M = F_{\text{rad}} \times b $$

(Eqn 8)

where $\lambda = l /r$ is the ratio of the length of the connecting rod to the crank radius, $\emptyset = \sin^{ - 1} (\sin \theta /\lambda )$ is the angle of rotation of the crankshaft and b is the distance between the crankpin bearing center to web center.

The maximum bending and shear stresses are taken into account by considering factors such as shock loads due to abrupt change in performance such as start-up or sudden braking, load dynamic factor of bending moment $C_{\text{m}}$, load dynamic factor of torsion $C_{\text{t}}$.65

$$ \sigma_{ \hbox{max} } = \frac{16}{{\pi d^{3} }}\left[ {C_{\text{m}} M + \sqrt {\left( {C_{\text{m}} M} \right)^{2} + \left( {C_{\text{t}} T} \right)^{2} } } \right] $$

(Eqn 9)

$$ \tau_{\hbox{max} } = \frac{16}{{\pi d^{3} }}\left[ {\sqrt {\left( {C_{\text{m}} M} \right)^{2} + \left( {C_{\text{t}} T} \right)^{2} } } \right] $$

(Eqn 10)