18-06-2011, 02:21 PM
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This study investigates and compares fatigue behavior of forged steel and powder metal connecting rods. A literature review on several aspects of connecting rods in the areas of load and stress analysis, durability, manufacturing, and optimization is also provided. The experiments included strain-controlled specimen testing, with specimens obtained from the connecting rods, as well as load-controlled connecting rod bench testing. Monotonic and cyclic deformation behaviors, as well as strain-controlled fatigue properties of the two materials are evaluated and compared.
Experimental S-N curves of the two connecting rods from the bench tests obtained under
R = -1.25 constant amplitude axial loading conditions are also evaluated and compared. Fatigue properties obtained from specimen testing are then used in life predictions of the connecting rods, using the S-N approach. The predicted lives are compared with bench test results and include the effects of stress concentrations, surface finish, and mean stress. The stress concentrations factors were obtained from FEA, and the modified Goodman equation was used to account for mean stress effect.
Comparative study of forged steel connecting rods versus powdered metal connecting rods. Validation of fatigue life prediction analytical tools based on baseline material properties and S-N approach. Element size of 1.25 mm (0.05”) was finalized by mesh sensitivity. Tetrahedral Element type was used, Analysis was linear elastic.Specimens from C-70 connecting rods were also tested and results compared with forged steel and powder metal connecting rods. C-70 connecting rod is considered to be an economical alternative to powder metal and conventional steel connecting rods.
Chapter 1
INTRODUCTION
1.1 Background
Connecting rods are widely used in variety of engines such as, in-line engines, opposed cylinder engines, radial engines and oppose-piston engines. A connecting rod consists of a pin-end, a shank section, and a crank-end as shown in Figure:1 Pin-end and crank-end pinholes at the upper and lower ends are machined to permit accurate fitting of bearings. These holes must be parallel. The upper end of the connecting rod is connected to the piston by the piston pin. If the piston pin is locked in the piston pin bosses or if it floats in the piston and the connecting rod, the upper hole of the connecting rod will have a solid bearing (bushing) of bronze or a similar material. As the lower end of the connecting rod revolves with the crankshaft, the upper end is forced to turn back and forth on the piston pin. Although this movement is slight, the bushing is necessary because of the high pressure and temperatures.
The lower hole in the connecting rod is split to permit it to be clamped around the crankshaft. The bottom part, or cap, is made of the same material as the rod and is attached by two bolts. The surface that bears on the crankshaft is generally a bearing material in the form of a separate split shell. The two parts of the bearing are positioned in the rod and cap by dowel pins, projections, or short brass screws. Split bearings may be of the precision or semi precision type.
Figure:1: Parts of Connecting Rod
1.2 Service loads and failures experienced by connecting rods
The function of connecting rod is to translate the transverse motion to rotational motion. It is a part of the engine, which is subjected to millions of repetitive cyclic loadings. It should be strong enough to remain rigid under loading. Connecting rod is submitted to mass and gas forces. The superposition of these two forces results in the axial force, which acts on the connecting rod. The gas force is determined by the speed of rotation, the masses of the piston, gudgeon pin and oscillating part of the connecting rod consisting of the small end and the shank. Figure 2 shows axial loading (Fay) due to gas pressure and rotational mass forces. Bending moments (Mb,xy, Mb,zy) originate due to eccentricities, crankshaft, case wall deformation, and rotational mass force, which can be determined only by strain analyses in engine (Sonsino, 1996).
Failure in the shank section as a result of these bending loads occurs in any part of the shank between piston-pin end and the crank-pin end. At the crank end fracture can occur at the threaded holes or notches for the location of headed bolts.
Figure:2: Origin of Stresses in Connecting Rod
Connecting rod is typically designed for infinite-life and the design criterion is endurance limit. It experiences axial tension/compression with constant amplitude loading and multi-directional bending with variable amplitude, as inertia force, torque and moment are all functions of engine speed (rpm).
Chapter 2
MATERIALS AND MANUFACTURING PROCESSES
2.1 Drop-Forged
A forged steel connecting rod is a production of drop-forged closed die process. The round steel stock as being forged to a connecting rod. Hot working proportions the metal for forming the connecting rods. Fullering, which is the portion of the die, is used in hammer forging primarily to reduce the cross section and lengthen a portion of the forging stock. The fullering impression is often used in conjunction with an edger or edging impression. Bustering converts square section bar into a preform to reduce the cross-section and lengthen it.
Blocking operation forms the connecting rod into its first definite shape. This involves hot working of the metal in several successive blows of the hammer, compelling the work piece to flow into and fill the blocking impression in the dies. Flash is produced, which is the unformed metal around the edge of the connecting rod that was forced away from blocking die impressions by the successive blows of the forging hammer. Flash is removed by different ways with trim dies in mechanical press or in special circumstances by sawing and grinding. The trimmed connecting rod is ready for heat-treating and machining.
Heat treating: After final forging and before machining, proper heat treatment methods are used to acquire optimum grain size, microstructure and mechanical properties.
