24-11-2012, 12:02 PM
DESIGN OF CONNECTING ROD FOR HEAVY DUTY APPLICATIONS PRODUCED BY DIFFERENT PROCESSES FOR ENHANCED FATIGUE LIFE
DESIGN OF CONNECTING.pdf (Size: 659.96 KB / Downloads: 79)
Abstract
The connecting rod forms an integral part of an internal combustion engine. The connecting rod is acted upon by
different types of loads while undergoing its operation. One of the main reasons contributing to its failure is fatigue. The
aim of this study is to redesign the connecting rod by incorporating the manufacturing process effects into the analysis and
obtain a better fatigue performance. The redesign is aimed at reducing the weight of the component.
Heavy duty application’s connecting rod was selected for the study. The analytically calculated loads acting on the
small end of connecting rod were used to carry out the static analysis using ANSYS. A stress concentration was observed
near the transition between small end and shank. A piston-crank-connecting rod assembly was simulated for one complete
cycle (0.02 seconds) using ADAMS to obtain the loads acting on small end of connecting rod. This force vs. time graph
was converted into an equivalent stress vs. time graph. This stress vs. time graph was used as loading graph for fe-safe.
The fatigue life calculated using fe-safe is 6.94×106 cycles and these results are validated with the help of Palmgren-Miner
linear damage rule.
The fatigue life of connecting rod can be further enhanced by incorporating manufacturing process effects in the
analysis stage. Fatigue life was estimated by incorporating the shot peening process effects. An in-plane residual stress for
the selected surface elements were applied for obtaining the beneficial effect of shot peening. There was an increment of
72% in fatigue life cycles). We concude that shot peening can significantly increase the fatigue life of a connecting rod
component.
INTRODUCTION
Connecting rod is an integral component of
internal combustion engine and it is classified under
functional component [1]. It acts as a linkage between
piston and crank shaft. The main function of connecting
rod is to transmit the translational motion of piston to
rotational motion of crank shaft. The function of the
connecting rod also involves transmitting the thrust of
the piston to the connecting rod. Connecting rod has
three main zones. The piston pin end, the center shank
and the big end. The piston pin end is the small end, the
crank end is the big end and the center shank is of Icross
section. Connecting rod is a pin jointed strut in
which more weight is concentrated towards the big end.
Fatigue design requirement
Connecting rod is acted upon by gas loads and
inertial loads during its operation. The forces include
gas forces due to combustion and inertia forces due to
its own weight. In that point of view fatigue is an
important parameter to be considered for estimating the
life of the component.
The magnitudes of inertia forces are constant but
gas forces are varying in nature. Due to fluctuating
nature of these forces the chances of component failure
due to fatigue is very high. Thus fatigue is one of the
significant factors to be taken into account while
optimising an existing design.
EXPERIMENTAL PROCEDURE
Experimental procedure involves modelling of
connecting rod using CATIA software. Static analysis
of connecting rod is done using ANSYS inorder to
understand the fatigue locations in connecting rod. The
maximum load acting on connecting rod was calculated
analytically. The load thus calculated was used as an
axial tensile force at the small end inorder to do perform
static analysis of connecting rod. The transition zone
between small end and shank was selected for this
study. The force acting on the small end of connecting
rod is a combination of gas forces and inertia forces.
Static Analysis using ANSYS
Static analysis of connecting rod was conducted in
order to understand the fatigue locations. The
constraints used for performing the static analysis were
as follows. The big end of connecting rod was
constrained for all degrees of freedom taking into
account the reaction from journal bearings. An axial
force of 78 kN is applied at the small end center by
creating rigids. The model was also provided with
symmetric boundary condition since half model is
considered for analysis.
MANUFACTURING PROCESS EFFECTS
INTO DESIGN
The process selected for analyzing the effect of
manufacturing on components was shot peening. Shot
peening creates a compressive residual stress on the
surface. Due to these stresses there is a crack closure
effect on the surface preventing the cracks to propagate
from the surface. It is possible to achieve a compressive
stress of magnitudes -250 MPa, -480 MPa, -660 MPa
and -870 MPa on the surface of connecting rod made of
AISI 4340 due to shot peening [6].
It is possible to superimpose the compressive
residual stresses in design phase itself. If the peak
stresses in the loading curve being superimposed with
a compressive stress of -250 MPa, the expected
behaviour will be a reduction in the stress and it may
lead to an improved life.
The effective stress loading graph due to shot
peening is given in Figure 11. Superimposed values of
stress shows a reducing trend and are shown in Figure
11. The mean stress and alternating stress values of
new superimposed stress curve are required to
incorporate the effect of shot peening on fatigue life.
The mean, alternating and effective alternating stress
corresponding to effective stresses due to compressive
stresses are given in Table 8.
SUMMARY
The present work was aimed at evaluating the
fatigue life of a heavy duty connecting rod under 2
different conditions namely without considering the
effect of shot peening and with considering the effect of
shot peening. Fatigue life was estimated by
incorporating the shot peening process effects into the
analysis. An in-plane residual stress for the selected
surface elements were applied for obtaining the
beneficial effect of shot peening. There was an
increment of 72% in fatigue life cycles). We concude
that shot peening can significantly increase the fatigue
life of a connecting rod component.