The durability analysis has been performed, for each test condition, with two different methods:
- Quasi-static superposition analysis;
- Transient (multistep) analysis.
Quasi-static superposition analysis
This analysis has been performed considering the effective stress tensors time histories during the engine cycle, obtained combining the results of a multibody simulation and another FEM analysis. In particular, It has been implemented a simple model, with the same geometry of the buckling analysis ( without piston pin, piston and crankshaft), used in order to compute unit load/stress transfer functions.
The structural continuity has been assumed between the components due to the linearity request of the model. Each transfer function has been computed applying a unit (bearing) load at con-rod interfaces without constraining the system (it has been exploited the Ansys/Workbench’s Inertia Relief feature); it has been considered also the stress field due to angular velocity field. As output, it has been obtained the i-th stress field σi (x,y,z) associated to each load channel. The same model, only the variation to consider “frictional” the contacts, it has been used to compute the static stress field σs (x,y,z) due to interferences and bolts pretension. Therefore, the stress tensors time histories σ(x,y,z,t) have been obtained with a linear combination of the transfer functions with the associated load time histories Fi (t) (computing and extracting by multibody simulations), added to the (constant) static stress field: σ(x,y,z,t) = σs (x,y,z) + Σi(Fi(t) • σi(x,y,z))
Figure 6.- Diametrical deformation of the small eye
The stress tensors time histories have been obtained considering only the maximum tensile and compressive conditions in the transient durability analysis. Therefore, the same FE model of deformation analysis, with the only variation of considering linear elastic materials’ behaviour, has been used to estimate the stress field required (possible local plasticity has been taken into account with Neuber’s rule, setted in the durability solver parameters). This analysis allows to consider the contacts’ nonlinearity and consequently it enables to estimate the performance near connecting rod’s eyes area in most severe conditions.
Both the analysis have been performed using the strain life approach and the same solver parameters. The material properties have been defined using the internal Piaggio’s procedures. The overall fatigue safety factor has been computed and compared with the minimum allowable value according to Piaggio standards.
For confidentiality reasons, the numerical values of the followings results cannot be provided.
Figure 7 -Diametrical deformation of the big eye
By the assessment of the critical buckling conditions Pc and by the evaluation of the maximum compressive load Pmax acting on the connecting rod, It has been possible to compute the buckling safety factor:
The minimum of buckling safety factor has shown inline values compared with a similar connecting rod currently operating without criticalities.
If the frequency of excitation nears with any of the natural frequencies, resonance could occur and it that may result in the mechanical failure of the system. Therefore, once the first frequencies of the first modes has been computed, it has been verified that there is no chance of resonance while operating.