If the component of the resultant force along the connecting rod length suggested a tensile load to act on the connecting rod, the resultant load was applied with cosine distribution, based on exper- imental results [2]。 The cosine distribution was applied 908 on either side of the direction of the resultant load, totally 1808。 But if the  component of the resultant force along the connecting rod length suggested a compressive load to act on the connecting rod, the resultant load was applied with uniform distribution [2]。 The uniformly distributed load was applied 608 on either side of the direction of the resultant load, totally 1208 on the contact surface。

The application of boundary condition is illus- trated in Fig。 4, for a random crank angle of 4328。 The computed direction of the resultant load at the crank end is 73。18。 Therefore, 1208 of the surface of the crank end (608 on either side of this direction) carries a uniformly distributed  load。  The  direction of the resultant load at the piston pin end is 39。48。 Therefore, 1208 of the surface of the pin end   carries a uniformly distributed load。 As the axial com- ponents of the loads are compressive, loads were applied with  uniform  distribution,  rather  than cosine  distribution。

Stresses in the regions near the ends of the con- necting rod are sensitive to the type of load distri- bution applied (uniformly distributed or cosine distribution)。 Away from these regions however, for example at the crank end transition to the shank (typical critical or failure region), stresses differ only by 7 per cent at the crank angle of 4328, when

Fig。 4 Illustration of the way in which boundary conditions were applied when solving the quasi-dynamic FEA model。 The illustration is for a crank angle of  4328

620 P S Shenoy and A  Fatemi

load distribution is changed from cosine to uniformly distributed load。

To account for the dynamic motion of the connecting rod and the resulting inertia loads, the acceleration boundary conditions were imposed。 Translational acceleration in the direction of the crank towards the crank center, angular velocity, and angular acceleration were imposed on the con- necting rod。 The crank end center was specified as the center of rotation。

A way to simulate the pin joint is to apply all the loads acting on the connecting rod that keeps the connecting rod in dynamic equilibrium at the instant under consideration (i。e。 at a specific crank angle) and then solve the model。 The FE model was solved by eliminating the rigid body motion, achieved by specifying kinematics degrees of freedom, and speci- fying elimination of rigid body motion while solving, as opposed to applying restraints。 Not applying restraints and using loads at both ends of the con- necting rod permits better representation of the loads transferred through the pin joints。

4 RESULTS AND DISCUSSION OF STRESS ANALYSIS

A few geometric locations were identified on the con- necting rod at which stresses were traced over the entire load cycle to obtain the stress-time history。 Some of these locations are shown in Fig。 5 and include high stressed regions of the crank end (locations 5, 10, and 11) the pin end at the oil hole (locations 6 and 7), the shank (locations 8 and 9), and at transitions to the shank at the crank and piston pin ends (locations 1, 2, 3, and 4)。 Locations

1,  3,  6,  8,  and  10  are  symmetrically  located from

locations 2, 4, 7, 9, and 11, respectively, with respect to the centerline of the connecting rod。 The points selected cover the typical critical (i。e。 failure) locations of connecting rod  [8]。

Figures 6 and 7 show the stress-time histories for the shank region (locations 1, 2, 8, and 9) as well as the transitions of the shank to the crank and pin ends (locations 3, 4, 10, and 11) at a crank speed of 5700 r/min。 von Mises stress variation under service loading condition is also plotted。 The von Mises stress carries the sign of the principal stress that has the maximum absolute value。 Clearly, not one instant of time can be identified as the time at which all the points on the connecting rod experi- ence the maximum state of stress。 However, the stress-time histories indicate that all the critical locations identified in Fig。 5 undergo maximum ten- sile stress at the crank angle of 3608, except locations 2 and 9 where the maximum stress occurs at the crank angle of 3488。 The transitions to the crank and pin ends are the critical regions with high tensile stresses。 In the shank region, the compressive stress is higher in magnitude than the tensile stress。 There- fore, adequate buckling strength is also required。