%%21 Materials Engineering ass m t 4 Essay Example
- Category:Engineering and Construction
- Document type:Assignment
Table of Contents
Task One 4
Task Two 11
Task Three 15
A drive shaft can be described as a cylindrical shaft where torque is transmitted from the vehicle’s engine into the wheels. Normally, drive shafts are precisely weighted and balanced component, because they always rotate at extremely high torque values as well as speeds with the objective of turning wheels. When the drive shaft experiences an issue, it could result in problems which could influence the vehicle’s drivability. Fatigue fracture of the Toyota drive shaft happened in three stages: initiation (irreversible changes in the drive shaft attributed to recurring shear stresses), propagation (micro-crack changes its direction and start growing perpendicular towards the tensile stress), and final rupture (the fatigue crack propagation). This report has established that the cause of the drive shaft failure was fatigue, which was caused by high stress concentration localized around the machine circlip groove’s sharp corner. In order to prevent the failure from taking place again, this report recommends that the effect of stress concentration must be minimized by making sure that the sharp corners in the high stress region is replaced with rounded fillets. Other ways to prevent failure is by changing the groove shape and the designs. The report has also looked at the failure of the Rolls Royce T56-A15’s turbine parts, which is a turbo prop engine from a hurcules aircraft. Dye penetrant testing was used to confirm the presence of cracks and these cracks were caused by excessive heat. Without a doubt, subjecting the turbine blades to excessive heat stress normally result into a stress rupture type of failures.
The objective of this report is to examine the types of failure as well as their causes. The report explicitly focuses on Toyota Drive shaft, which failed after covering 40,000 miles off road. In this report, the possible types of failures are identified and all are described in details. Additionally, the report examines the failure of the drive shaft with the aim of determining the type of failure and offers reasons and evidence to exhibit why fatigue failure is the most likely. Besides that, the report will examine the turbine blade from the T56-A15 engine which had been removed from service because of a defect. The common causes of failure will be identified in these components with the aim of finding the most likely type of failure for the turbine blade. The report also provides numerous recommendations that would help adjust the design and/or the process of manufacturing for the drive shaft with the aim of trying to prevent future failures.
Report and Analysis
Basically, material failure because of fracture can be divided into ductile failure, brittle failure, and fatigue failure. Each type of these failures several have numerous root cause.
Ductile failure can be identified easily through plastic deformation or gross permanent in the area of fracture, and it is normally caused by overloading the ductile material or when a material is subjected to load at a high temperature. Furthermore, ductile materials normally demonstrate high levels of deformation or plastic buckling as compared to brittle materials. In this type of failure, the pace at which the crack grows is very low and is followed by loads of plastic deformation. More importantly, there is no expansion of crack except when there are high stress levels. In nearly all situations of material design, materials showing ductile fractures or failures are considered ideal for a number of reasons, like: ability to deform plastically, which consequently, slows the process of fracture and creating time for the problems to be corrected.
Brittle failure can be identified easily by cracking or breakage of a material into visible parts, whereby no deformation could be noticed. This type of failure normally happens because of sudden shock loading or impact to an exceedingly hard material. Brittle failure is normally typified by rapid propagation of the crack without major plastic deformation and with low energy release. Basically, the fractures normally have chevron patterns and are flat type. Fractures in brittle crystalline materials happen by cleavage because of a tensile stress that acts normal to crystallographic planes having cleavage planes. In this type of failure, the angle between the applied stress and the path through which the
cracks run is almost perpendicular. At the break, this perpendicular fracture normally leaves behind a surface that is nearly flat.
Fatigue failure can be identified easily by striations, which are microscopic features identifying one propagation fatigue crack cycle. The failure can also be identified by clamshell/beach marks, which are macroscopic features of fatigue that marks an interruption of any form of fatigue cracking development. Both striations and beach marks can be utilised to identify fatigue fractures. Fatigue failures are normally caused by repeated loading as well as unloading of stresses, this could be torsional, compressive, tensile, or a combination of all. As pointed out by Sachs (2012), the roughness or condition of the fracture surface is a crucial point to consider while examining the failure due to the difference between fatigue failures as well as overload failure. The surface in overload failures is normally uniformly rough while the cracks induced by fatigue always travel across the face of the fracture at a high speed. The typical face of fatigue fracture is somewhat smooth close to the origins while the ends are fairy rough final fracture (Sachs, 2012). Fatigue failure can be accelerated easily by prior corrosion on the surfaces of the shaft. In addition, severe corrosion that pre-exist could be a source that contributes to fatigue failure. Normally, fatigue failures happen when the material is made to experience a lot of cycles of the applied stress. Components with fatigue fail when the stress values are less than the material’s ultimate strength.
The failure of the drive shaft as exhibited in the figure below clearly demonstrates that it failed because of high stress concentrations that led to fatigue. Given that the stress is was exceedingly high around the sharp corner of the machine circlip groove as compared to other parts of the drive shaft, fatigue failures initiated in this region. It appears that, when the shaft was being designed, the designers failed to minimize the amount of stress concentration by replacing sharp corners or getting rid of grooves where possible since the shaft was to be subjected to cyclical loading,.
Figure One: A Failed Drive Shaft
a shaft is normally subjected to axial, traverse or torsion loads, which act in combination or singlehandedly. As exhibited in figure one, the shaft’s diameter is not uniform since it was stepped in order to offer shoulders for locating bearings, pulleys and gears. Besides that, there was variance in stress on the drive shaft at different points while rotating; thus, leading to fatigue. When the drive shaft was subjected to loads repeatedly of high magnitude, it propagated a fatigue crack in the region that was highly stressed, until the final fracture happened. By investigating the fracture surface visually, it is evident that the drive shaft failure happened because of torsional-bending fatigue since the crack starts at the groove. It is visible that the cracks grew almost perpendicular to the surface that was subjected to maximum stress. Evidently, the properties of the drive shaft’s section changed when the crack was growing. As evidenced in figure two below, fatigue fracture can happen because of internal pressure, bending, torsion, or tension loads. Marudachalam and Kanthavel (2011),As pointed out by
(Sachs, 2012)Figure Two: Four common fatigue forces
the crack in the material normally initiates at the origin and grow slowly across the fatigue zone (see figure three below). When the crack gets to the instantaneous zone boundary the rate of its growth rate increases extremely leading to changes on how the shaft is loaded. This led to surface changes that surface as progression marks.Sachs (2012), Further closer examination, reveals some geometric discontinuities which made the shaft to experience a localised increase in stress field intensity. This is mainly attributed to the shaft’s sharp corners, which connotes that it must be a low stress high stress concentration type failure. As mentioned by
(Sachs, 2012)Figure Three: A typical bending fatigue
instances of rotating bending that led to failure. The first part shows a single origin of the shaft failure while the other part has multiple origins. The load on the shaft on the first part was extremely high as compared to that in the second part. Clearly, the shaft on the second part of figure four demonstrates that although it was not heavily loaded, it has scores of fracture origins which is a sign of high stress concentration.Sachs (2012), they show The figure below was adopted from
(Sachs, 2012)Figure Four: Analysis of two different failures
(Sachs, 2012)Figure Five: Bending fatigue diagnosis
Figure five demonstrates a fatigue failure in motor shaft with no progression marks; thus, demonstrating that the fatigue load was constant. The second part of the photo is akin to Toyota’s drive shaft which has a smooth surface close to the root of crack and then it became rougher progressively while the crack was growing across the shaft.
Basically, a rough surface finish can lead to a high stress concentration because the radii of the discontinuities at the surface are exceedingly small. Besides that, geometrical discontinuities and other features utilised widely in machinery components are also considered as stress raisers. Therefore, the performance of the shaft could have been affected by high stress concentrations in a number of ways. For instance, if the shaft is statically loaded or experiences a single load application at a scale exceeding the material’s capability, the crack would rapidly initiate and propagate from the region with high stress concentration. When the static load is lower than the yield strength and constant and the shaft is vulnerable to aggressive environment, stress corrosion cracking is likely to happen because of the increase in sustained stress at the discontinuity, which consequently can start at the stress raiser. Sharp corners where the shaft’s diameters change of the surfaces intersects in keyways can result in fatigue failure. At the sharp comer, a single overload can result in a rapid brittle fracture and surfaces that are poorly grounded could also lead to fractures. The drive shaft fatigue cracks initiated at the sharp corner and this could be attributed to poorly fitting of the couplings. The failure in the drive shaft initiate at the at points of metallurgical stress concentration, which normally increase stresses locally or reduce the fatigue resistance of the material. Changes in the geometric shape changes could lead to a higher stress concentration effect and because stress concentration is normally associated with high mechanical stress, the stress concentration effects in the region that are extremely stressed can lead to fatigue failure
(Sachs, 2005). The internal stress redistribution of the shaft from a low value towards a high value happens in area where cross-sectional changes happen. In this case, the redistribution happened at machine circlip groove’s sharp corner where the joining of two geometric forms happened.
The Dye Penetrant test showed that the turbine blade had leading and trailing edge cracks, even though they could not be seen by naked eyes. Dye Penetrant test is beneficial because it is extremely sensitive to discontinuities in small surface, can be use to inspect complex shapes, and it is cheap. The test has six steps as shown in figure six below:
Surface Preparation: The surfaces had to be cleaned so as to remove contaminants such as grease, oil or water that could prevent the penetrant from penetrating the flaws.
Penetrant Application: After cleaning the surface thoroughly and drying it, the penetrant was applied by spraying the turbine blade.
Penetrant Dwell: Then the penetrant was left on the surface for 15 minutes in order to make sure that adequate penetrant has seeped into the defect.
Excess Penetrant Removal: The turbine blade was then cleaned in order to remove the excess penetrant from the surface and making sure no penetrant was cleaned from the defects.
Developer Application: After removing excess penetrant, a layer of developer was applied to the turbine blade to ensure that the draw penetrant that is trapped in the cracks comes back to the surface in order to be seen visually.
Observing: Inspection was under adequate lighting to detect flaws that could be present in the turbine blade.
Step One: Cleaning Step Two: Applying the dye Step Three: Waiting for 15 minutes
Step Four: Cleaning Step Five: Spray Developer Step Six: Observing
Figure Six: Dye Penetrant Test
The Images below show the Dye Penetrant test
The dye penetrant test demonstrated that the cause of the turbine blade failure was excess stress that led to stress rupture. Stress rupture, according to Gale et al. (1987) normally leads to the breakage of turbine blades because of excessive exposure to heat and physical stress. Stress rupture is always time-dependent and often leads to centrifugal applied stresses as well as temperature in the operating environment of the turbine, often recognised as turbine internal temperature (TIT). Clearly, prolonged exposure to high temperature in the aircraft’s turbine engines, especially the hot part can result in turbine blade separation through the stress rupture mechanisms which can lead to potential engine shutdown. Due to the excessive temperature, the heat under which the turbine blade was operating led to stress rupture as evidenced by fine hairline cracks. These cracks are evident in the trailing edge which is perpendicular to the edge length surface. Another cause could have been sulfidation, which is normallycaused increased metals oxidation in the presence of sulphur ions, particularly sulphates and sulphides (Gale et al., 1987). When the tailpipe loses the oxidized metal, an eroded vane and blade surfaces is left behind.
In conclusion, this report has established that the stress concentration caused by the machined circlip groove resulted in the early failure of the vehicle’s drive shaft.it is evident that a low stress high stress concentration fatigue failure happened since the crack exhibits the presence of beachmarks, which are normally associated with high stress concentration. It was not a brittle failure because there the shaft did not break into visible parts. If it was ductile failure, the roughness of shaft’s surface could have been relatively uniform. The failure happened at the circlip groove’s shoulder because cracks surfaced from the surface that was subjected to high stress concentrations considering that the position of the shoulder had high bending moment which led to high stresses. The turbine blade most likely failed due to excessive heat .the report believes this because of the increased roughness on the surface of the blade without knowing the exact service life of the part however it is impossible to be more specific.
To prevent this happening in future there a number of design changes have to be made considering that fatigue life can be enhanced greatly by reducing stress concentration, changing the designs or facilitating the introduction of compressive stress on the surface. Stress concentartion can be reduced by drilling an enormous hole at the end of the crack. The hole would bring forth smaller stress concentration as compared to the sharp crack’s end. Failure can also be prevented by reducing sharp corners and treating the surface through case hardening, which results in increased fatigue life as well as surface hardness. Case hardening can be realised by exposing the shaft to high temperatures in a carbon-rich atmosphere. This report believes the easiest and most effective implement without compromising performance is minimising the effect of stress concentration by replacing sharp corners with rounded fillets in high stress area. Additionally, fatigue life can be improved by changing the groove shape in order to alleviate the high stress concentration and also through enhancement of the surface at the groove root by utilising the shot peening.
Gale, C.I., Loftin, J.A. & Rockwood, R.J.R., 1987. Conserving Turbine Life. Service News, vol. 14, no. 1, pp.1-16.
Marudachalam, D. & Kanthavel, R.K., 2011. Optimization of shaft design under fatigue loading using Goodman method. International Journal of Scientific & Engineering Research, vol. 2, no. 8, pp.1-5.
Sachs, N.W., 2005. Understanding the Surface Features of Fatigue Fractures: How They Describe the Failure Cause and the Failure History. Journal of Failure Analysis and Prevention, vol. 5, no. 2, pp.11-15.
Sachs, N., 2012. Failure Analysis Of Machine Shafts. [Online] Available at: http://www.maintenancetechnology.com/2012/07/failure-analysis-of-machine-shafts/ [Accessed 14 June 2017].
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