Material Engg. A SEMINAR ON FATIGUEPresented by- Guided by-Mr.Sandip Wanave Prof. Nerkar sir
An Introduction• in materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.• Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure.
Fatigue life• Fatigue life• ASTM defines fatigue life, Nf, as the number of stress cycles of a specified character that a specimen sustains before failure of a specified nature occurs.• One method to predict fatigue life of materials is the Uniform Material Law (UML). UML was developed for fatigue life prediction of aluminum and titanium alloys by the end of 20th century and extended to high-strength steels and cast iron. For some materials, there is a theoretical value for stress amplitude below which the material will not fail for any number of cycles, called a fatigue limit, endurance limit, or fatigue strength.
Characteristics of fatigue• In metals and alloys, the process starts with dislocation movements, eventually forming persistent slip bands that nucleate short cracks.• Fatigue is a stochastic process, often showing considerable scatter even in controlled environments.• The greater the applied stress range, the shorter the life.• Fatigue life scatter tends to increase for longer fatigue lives.• Damage is cumulative. Materials do not recover when rested.
Characteristics of fatigue• Fatigue life is influenced by a variety of factors, such as temperature, surface finish, microstructure, presence of oxidizing or inert chemicals, residual stresses, contact (fretting), etc.• Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit below which continued loading does not lead to structural failure.
The S-N curve• A very useful way to visualize time to failure for a specific material is with the S-N curve. The "S-N" means stress verse cycles to failure, which when plotted uses the stress amplitude, sa plotted on the vertical axis and the logarithm of the number of cycles to failure. An important characteristic to this plot as seen in Fig. 2 is the fatigue limit.• The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Material such as aluminum, copper and magnesium do not show a fatigue limit, therefor they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The stress at which failure occurs for a given number of cycles is the fatigue strength. The number of cycles required for a material to fail at a certain stress in fatigue life.
Crack Initiation and Propagation• Failure of a material due to fatigue may be viewed on a microscopic level in three steps:• Crack Initiation: The initial crack occurs in this stage. The crack may be caused by surface scratches caused by handling, or tooling of the material; threads ( as in a screw or bolt); slip bands or dislocations intersecting the surface as a result of previous cyclic loading or work hardening.• Crack Propagation: The crack continues to grow during this stage as a result of continuously applied stresses• Failure: Failure occurs when the material that has not been affected by the crack cannot withstand the applied stress. This stage happens very quickly.
Crack Initiation and Propagation Figure 3 A diagram showing location of the three steps in a fatigue fracture under axial stress
Crack Initiation and Propagation• One can determine that a material failed by fatigue by examining the fracture sight. A fatigue fracture will have two distinct regions; One being smooth or burnished as a result of the rubbing of the bottom and top of the crack( steps 1 & 2 ); The second is granular, due to the rapid failure of the material. These visual clues may be seen in Fig. 4:
Crack Initiation and Propagation Figure 4A diagram showing the surface of a fatigue fracture. Notice that the rough surface indicates brittle failure, while the smooth surface represents crack propagation
Demonstration of Crack Propagation Due to Fatigue
Demonstration of Crack Propagation Due to Fatigue• The figure above illustrates the various ways in which cracks are initiated and the stages that occur after they start. This is extremely important since these cracks will ultimately lead to failure of the material if not detected and recognized. The material shown is pulled in tension with a cyclic stress in the y ,or horizontal, direction. Cracks can be initiated by several different causes, the three that will be discussed here are nucleating slip planes, notches. and internal flaws. This figure is an image map so all the crack types and stages are clickable.
Infamous fatigue failures• Following the Kings fete celebrations at the Palace of Versailles, a train returning to Paris crashed in May 1842 at Meudon after the leading locomotive broke an axle. The carriages behind piled into the wrecked engines and caught fire. At least 55 passengers were killed trapped in the carriages, including the explorer Jules Dumont dUrville. This accident is known in France as the "Catastrophe ferroviaire de Meudon". The accident was witnessed by the British locomotive engineer Joseph Locke and widely reported in Britain. It was discussed extensively by engineers, who sought an explanation.• The derailment had been the result of a broken locomotive axle. Rankines investigation of broken axles in Britain highlighted the importance of stress concentration, and the mechanism of crack growth with repeated loading. His and other papers suggesting a crack growth mechanism through repeated stressing, however, were ignored, and fatigue failures occurred at an ever increasing rate on the expanding railway system. Other spurious theories seemed to be more acceptable, such as the idea that the metal had somehow "crystallized". The notion was based on the crystalline appearance of the fast fracture region of the crack surface, but ignored the fact that the metal was already highly crystalline.
Factors that affect fatigue-life• Cyclic stress state: Depending on the complexity of the geometry and the loading, one or more properties of the stress state need to be considered, such as stress amplitude, mean stress, biaxiality, in-phase or out-of-phase shear stress, and load sequence,• Geometry: Notches and variation in cross section throughout a part lead to stress concentrations where fatigue cracks initiate.• Material Type: Fatigue life, as well as the behavior during cyclic loading, varies widely for different materials, e.g. composites and polymers differ markedly from metals.
Factors that affect fatigue-life• Residual stresses: Welding, cutting, casting, and other manufacturing processes involving heat or deformation can produce high levels of tensile residual stress, which decreases the fatigue strength.• Size and distribution of internal defects: Casting defects such as gas porosity, non-metallic inclusions and shrinkage voids can significantly reduce fatigue strength.• Grain size: For most metals, smaller grains yield longer fatigue lives, however, the presence of surface defects or scratches will have a greater influence than in a coarse grained alloy.• Environment: Environmental conditions can cause erosion, corrosion, or gas-phase embrittlement, which all affect fatigue life. Corrosion fatigue is a problem encountered in many aggressive environments.• Temperature: Extreme high or low temperatures can decrease fatigue strength.
Stopping fatigue• Fatigue cracks that have begun to propagate can sometimes be stopped by drilling holes, called drill stops, in the path of the fatigue crack. This is not recommended as a general practice because the hole represents a stress concentration factor which depends on the size of the hole and geometry, though the hole is typically less of a stress concentration than the removed tip of the crack. The possibility remains of a new crack starting in the side of the hole. It is always far better to replace the cracked part entirely.
Stopping fatigue• Material change• Changes in the materials used in parts can also improve fatigue life. For example, parts can be made from better fatigue rated metals. Complete replacement and redesign of parts can also reduce if not eliminate fatigue problems. Thus helicopter rotor blades and propellers in metal are being replaced by composite equivalents. They are not only lighter, but also much more resistant to fatigue. They are more expensive, but the extra cost is amply repaid by their greater integrity, since loss of a rotor blade usually leads to total loss of the aircraft. A similar argument has been made for replacement of metal fuselages, wings and tails of aircraft.[15
Fatigue Test• A method for determining the behavior of materials under fluctuating loads. A specified mean load (which may be zero) and an alternating load are applied to a specimen and the number of cycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with identical specimens and various fluctuating loads. Loads may be applied axially, in torsion, or in flexure. Depending on amplitude of the mean and cyclic load, net stress in the specimen may be in one direction through the loading cycle, or may reverse direction. Data from fatigue testing often are presented in an S-N diagram which is a plot of the number of cycles required to cause failure in a specimen against the amplitude of the cyclical stress developed.
Fatigue TestFATIGUE TEST• OBJECTIVE:• The main objectives of this experiment are:•• 1) Perform the fatigue test on the given specimens using the Fatigue tester MT 3012 to predict the fatigue life.•• 2) Determine the safe stress level for the specimens if a fatigue life of 1,000,000 reversals had to be withstood.•• APPARATUS REQUIRED:• Fatigue tester MT3012, Vernier caliper Aluminum specimens.,
: DESCRIPTION OF THE APPARATUS • fatigue tester MT 3012 shown in Fig.1.is driven by an induction squirrel cage motor at 3000rpm. Power supply provided is 220V single phase. The motor is connected on one side to a counter mechanism, which can record 7 figure numbers. Attached to the shaft at the other end is a fixture. The loading device consists of a spherical ball bearing and a micro switch, which automatically switches off the motor when the fracture occurs. • The apparatus is supplied with a recommended standard specimen. The bending stress for a load P (N) is: • Where, L…distance from neck to specimen’s contact point with bearing • d…Diameter of the neck • P…Load applied (measured by digital read out) • • By turning the loading wheel clockwise the loading on the test piece can be increased. A cell load which a digital read out measures the loading value. The fatigue tester, which is designed to be placed on a bench, is very stable on 8 feet, weighing 24kg. Dimensions are 980x280x460 mm.
Figure 4. MT3012 Fatigue tester with the load cell integrated.
EXPERIMENTAL PROCEDURE:• As fatigue fracture experiments may run on for half an hour or so the usual procedure is for each group in a class to set up and start two aluminum specimens and for all the results to be shared at the end. The load sets will be provided in the lab session.• 1. Measure the diameter at the neck of the specimen and inspect the surface roughness.• 2. Slide one end of the specimen into the adapter at the shaft end and slide the other end into the adapter at the load end.• 3. Measure the distance from the neck to the specimen’s contact surface with the bearing.• 4. Apply the given load. Check with the lab instructor about loading the specimen in order to have a precise bending loading condition.•• Dont put any excessive force on the loading arm!!! It will damage the specimen.•• Results from other load cases will be collected an made available to each group after all groups have completed the experiment.•• 4. Set the revolution counter to zero and start the motor.•• 5. Normally the test terminates itself through the fracture of the specimen opening the micro switch and hence stopping the motor. As the onset of fracture approaches the specimen will bend more, and this may open the micro switch before complete fracture occurs. In this case move the micro switch down slightly and restart the motor.•• 6. Collate the results and plot them as they occur on a graph of stress range, s, against logl0 number of reversals N. Note that in the case of a rotating cantilever the stress range is twice the applied bending stress.
RESULTS:• After obtaining the results for your load cases and getting the results of the remaining cases from other groups, plot σ against logl0 N on a suitable graph paper and look for best-fit lines and also determine the safe stress level if a fatigue life of 1,000,000 reversals had to be withstood. Also discuss the ruptured cross- section and identify the cause of the rupture and analyze the factors, which will affect the results.
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