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Fatigue Behavior and Life Predictions of Forged Steel and Powder Metal Connecting Ro

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
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
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.

Mechanical components can fail at stresses well below the tensile strength of the material if subjected to alternating loads.
Failure of ductile materials under alternating loads occurs in a quasi brittle manner, i.e. by crack propagation.
Failure is preceded by characteristic changes in the material microstructure
This phenomenon is called Metals Fatigue .
This Figure below shows a laboratory fatigue specimen. They are machined with shape characteristics which maximize the fatigue life of a metal, and are highly polished to provide the surface characteristics which enable the best fatigue life.
A single test consists of applying a known, constant Bending Stress to a round sample of the material, and rotating the sample around the bending stress axis until it fails. The test mechanism counts the number of rotations (cycles) until the specimen fails. The cyclic stress level that the material can sustain for 10 million cycles is called the Endurance Limit (EL).
1. Drop Forging:
2. Powder Forging: Powder forging is a process in which powders such as iron and copper are compacted, heated and forged so that their density increases up to that of wrought steel.
3. Die casting: Die-casting is accomplished by forcing molten metal under high pressure into reusable metal dies.
1. Monotonic Tension Test:
All monotonic tests in this study were performed using test methods specified by ASTM Standard E8.
One specimen was used from each material to obtain the monotonic properties.
Raw material is cheaper than powder metal.
This process provides high strength, ductility, and impact resistance along the grain flow of the forged steel.
The density achieve by this process is uniform.
Fatigue performance of forged steel rods is higher than die cast and powder metal rods.
In conventional forging, flash is produced during forging and requires extra steps of trimming to remove.
To ensure proper weight balance, conventional connecting rods are provided with excess weight, which is later remove during finishing operations.
The rough stock weigh required is more than needed for connecting rod, resulting in some scrap.
Powder metal preform connecting rods start with the net shape, that result in no material waste.
There is no balancing pad used in this Process.
Fracture splitting of cap from rod without subsequent machining of matching surfaces is most popular for powder metal connecting rods.
Raw material of this process is expensive because of operations of powder formation, presintering, and sintering.
Lower mechanical properties because of rapid solidification.
During compacting trapped oxygen in the powder results in porosity, decarb, and oxide penetration.
The density can vary within powder metal connecting rod.
Costly part density modification or infiltration is required to prevent powder metal defect.
From tensile tests and monotonic curves it is concluded that forged steel is considerably stronger than the powder metal.
Yield strength of forged steel is 19% higher than that for the powder metal. Ultimate tensile strength of forged steel is 8% higher than that for the powder metal.
Better fatigue resistance of the forged steel material, as compared with the powder metal material was observed.
Based on strain-life fatigue behavior, the forged steel provides about a factor of 7.
longer life than the powder metal in the high cycle regime.
Fatigue limit (Nf = 106 ) for powder metal is 79% of that for the forged steel.

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