To design against fatigue failure, some investi- gators have used the overall operating load range of the connecting rod (i。e。 load range comprised the maximum static tensile and compressive loads), whereas others have used the load range at the maxi- mum power output。 This is illustrated with reference to location 8 and Fig。 6。 At this location, the overall operating stress range obtained using the overall load range is 244 MPa (i。e。 from 2160 MPa due to the maximum gas pressure to 84 MPa from Fig。 6)。 However, the stress range at the maximum speed for  this  location  is  157 MPa  (i。e。  from  273  to 84 MPa in Fig。 6), representing a 36 per cent decrease (when compared with 244 MPa) in the stress range。 A 36 per cent change in the stress range or amplitude can result in more than an order of magnitude change in the fatigue life。 Therefore, using the overall operating load range can lead to an overly conserva- tive design of the component。

The tensile load increases as the engine speed increases as evident from Fig。 8, which shows a plot

Fig。 5 Locations on the connecting rod where the stress variation was  traced  over  one complete cycle of the engine

Dynamic analysis of loads and stresses in connecting  rods 621

Fig。 6 Stress variation over the engine cycle in the shank region (locations 1, 2, 8, and 9) at 5700 r/min。 XX is the sx component of stress

of stress variation at location 8 with engine speed。 The tensile stress increases as the speed increases, as an increase of the inertia load due to the piston mass results in increasing the tensile load on the connecting rod。 Note that the tensile load  consists of both  structural load  and acceleration load  due to inertia。 An important observation from Fig。 8 is that even though maximum and mean stresses increase with increasing engine speed, the stress range (or amplitude, as amplitude is half the range) is independent of speed, as it  remains  constant。 The stress range or amplitude is the primary stress controlling fatigue design of the connecting rod, whereas mean stress has a secondary influence。 Connecting rods in the engine are usually tested with a load sequence typically consisting of different engine speeds [10]。

An aspect of dynamic loading is the bending stress it produces and its  significance。  As  noted  earlier, the  locations  specified  in  Fig。  5  are symmetric with  respect  to  the  centerline  of  the     component。

A difference between stresses at the symmetric locations in the stress-time history plots (Figs 6 and

7) indicates the presence of bending stress, the mag- nitude of which is equal to half the difference。 For example, the maximum bending  stress  is  26  per cent of the maximum stress at the section through location 8, and 22 per cent of the maximum stress at the section through location 1 (Fig。 6)。 The shift in the peak stress  between  points 8  and  9  in Fig。 6 is because of the bending stress。 This suggests that the bending stiffness in the shank needs to be ade- quate to sustain these bending stresses。 Bending stiffness as an important  design  factor  has  also been suggested in reference  [4]。

Another important observation that can be made from the stress-time histories in Figs 6  and  7 is that, however, the state of stress is predominantly uniaxial for some locations, it is multiaxial at other locations such as 6, 7, 10, and 11。 Figure 7 indicates that at locations 10 and 11 the stress component sy is as high as 30 per cent of the stress component  sx

622 P S Shenoy and A  Fatemi