Fig。 7 Stress variation over the engine cycle at the transitions to the pin end (locations 3 and 4) and to the crank end (locations 10 and 11) at 5700 r/min。 XX is the sx component of stress, YY  is the sy  component of stress, XY  is  the txy    component  of  stress,  and  XZ   is  the  txz

component of stress

Fig。 8 Minimum stress, maximum stress, mean stress, and stress range at location 8 on the connecting rod as a function of engine speed

This equation also captures the beneficial effect of the compressive residual stresses produced through the common practice of surface peening of connect- ing rods, as residual stresses can be treated as mean stresses in fatigue analysis。

The stress ratio (minimum to maximum stress ratio), and therefore mean stress, varies not only with location in the connecting rod, but also with engine speed at a location。 For example, for location 8 shown in Fig。 8, the stress ratio changes from 219

at 2000 r/min to 21 at 5700 r/min。 The combination of tensile mean stress and stress amplitude  results in higher fatigue damage at locations 2, 4, and 9, as compared with the corresponding symmetric locations 1, 3, and 8, respectively。 To account for the mean stress, completely reversed von Mises equivalent stress amplitude, Sf, can be computed based on the commonly used modified Goodman equation

at the crank angle of 3608, which is significant。    The

 sqa

Sf  þ

 sqm

Su   ¼

1 (2)

stress multiaxiality is proportional (or in-phase), and results from stress concentrations such as in locations 10 and 11。 Equivalent stress approach based on von Mises criterion is commonly used for multiaxial proportional stresses to compute equival- ent stress amplitude,  sqa。

For multiaxial mean stresses, it has been observed

that mean shear stress does not affect cyclic bending or cyclic torsion fatigue behaviours, whereas mean hydrostatic stress influences fatigue life [11, 12]。 As a result, using the following equation, which is insen- sitive to the mean shear stress, but accounts for mean hydrostatic stress can be used to compute an equivalent mean stress, sqm, based on inpidual mean  stress  components  smx,  smy, and smz

sqm ¼ smx þ smy þ smz (1)

where  Su   is  the  ultimate  tensile  strength  of   the

material [12]。

5COMPARISON OF QUASI-DYNAMIC AND STATIC ANALYSES

Most investigators have used static axial loads for the design, analysis, and testing of connecting rods。 In this study, FEA was also carried out under axial static load to compare the results with the more realistic quasi-dynamic analysis results discussed in section 4。 Quasi-dynamic FEA results differ from the static FEA results because of the time-varying inertia load of the connecting rod, which is respon- sible for inducing bending stresses and varying axial load along the length, as discussed earlier。

Dynamic analysis of loads and stresses in connecting  rods 623

Fig。 9 FEA model of the connecting rod with static tensile load at the crank end with cosine distribution over 1808 and piston pin end fully restrained over 1808

The static FEA model is shown in Fig。 9。 Note that, under tensile static load, half of the piston pin inner sur- face (1808) is completely restrained。 Similarly, when the connecting rod is under axial compressive load, 1208 of contact surface area is totally restrained。 The stress range used for fatigue design based on a static analysis is obtained from the difference between the maximum stress corresponding to the maximum static tensile load, and the minimum stress corresponding to the maximum static compressive load。