Connecting rod cap 50,698 10,217 Linear tetraedric

Connecting rod body 25,305 5239 Linear tetraedric

Rigid shell 713 768 Linear quadractic rigid

Table 2

Tightening force simulated and the respective  interferences。

Model Thightening force (kN) Interference (mm)

1 30 0。09

2 50 0。15

3 100 0。30

tightening。 As a second step, external loads of 15, 30 and 60 kN were applied in the axis of symmetry, normal to the shell, in order to separate the cap of the connecting rod。 This procedure increases the stress level at the bolt。 The difference between stresses due to second and first steps results in the stress amplitude imposed to the bolt。

2。4。Fracture mechanics analysis

For the connecting rod bolt stress intensity factor calculation, it was adopted the procedure described in the standard BS 7910:2000 [10]。 The section M。6。2 of the standard is dedicated to geometric factor for cylindrical bars with   semi-elliptic

cracks and direction of propagation perpendicular to the axis。 The stress intensity factor (KI) is related to crack length (a), stress amplitude (r) and geometrical factor (Y) according to Eq。 (2)。 The geometrical factor (Y), was obtained from Eq。 (3), the stress intensity magnification factor (Mn) was calculated from Eq。 (4), while the geometrical factor (g) was taken from Eq。 (5)。 These equations are valid for (a/2r) < 0。6, where (r) is the radius of the cylindrical bar。

Y r ¼ Mmrmax ð3Þ

。 。 a 。 n 。pa。o3。

3。1。Fracture analysis

The camshaft fractured due to overload, without signals of fatigue as can be seen in Fig。 3。 This fracture was a conse- quence of impact between valves and pistons。 One of the fourth connecting rod bolts failed due to overload as well, with evident distinct fibrous and shear zones (Fig。 3)。 The other bolt of the fourth connecting rod shows a flat fatigue fracture sur- face with clear indication of beach marks (Fig。 4)。 The fatigue nucleated at the top of one of the grooves (Fig。 5) and prop- agated along most of the section until final fracture。 Only one nucleation site and a large propagation area are evidences of low applied stresses。 The fracture initiation site located close do the crankshaft。 The SEM analysis highlighted stable crack propagation up to 2/3 of the cross section when dimples characteristics of instable propagation can be observed (Fig。 4)。

3。2。Mechanical tests

Hardness test was performed on the fatigued bolt while hardness and tensile tests were performed in another four selected bolts。 At the first tensile test, it was noted fracture at a low load (61。8 kN)。 The fracture surface analysis of摘要：研究人员做了一个分析实验，来寻找修正后的汽车柴油发动机在仅仅工作六个月就发生故障的根本原因。实验过程中监测了连杆螺栓的扭矩分量，并且将断裂的连杆螺栓送到实验室进行断裂分析。通过对四连杆螺栓中第一个连杆螺栓的疲劳断裂来验证猜想。然后将剩下的连杆螺栓进行拉伸实验。在这个过程中，论文网第二个连杆螺栓的疲劳裂纹的扩展表明第一个已经断裂的螺栓没有受到扭矩松弛。研究人员运用了有限元与断裂力学相结合的分析方法对连杆螺栓进行分析，以判定固紧力和疲劳裂纹扩展之间的关系。实验得出结论，发动机崩溃是由于连杆螺栓上的连杆槽上的仿形线圈。最后,为避免今后发生故障，提出几点改进的设计方案:连杆盖与连杆槽之间要预留足够空隙以避免更高应力幅与干涉同时发生；增加组装连杆螺栓的扭矩，以减小应力幅。