Material Failure Mechanisms - Report Example

MATERIAL FAILURE By Name Course Instructor Institution City/State Date Table of Contents Material Failure SUMMARY: Failure mechanisms can be grouped into fatigue, wear, corrosion, and overload. Both wear and corrosion hardly cause failures in the drive shaft, and when they do, they normally leave behind clear evidence. On the other hand, fatigue is more common as compared to the overload failure. In this report, it was established that the cause of the drive shaft failure was fatigue, most likely caused by a high stress concentration localised around the sharp corner of the machine circlip groove. To prevent the failure occurring again this report recommends that the stress raisers should be eliminated or reduced by stream lining the drive shaft, steering clear of the sharp surface tears attributed to processes such as shearing, stamping, or punching. Besides that, the surface discontinuities development at the time of processing should be prevented and the tensile residual stresses brought forth by the manufacturing processes should be reduced or eliminated. More importantly, the fastening procedures and fabrication details should be improved and u-joint kit designs and stress relief grooves should be used. This report also examined the failure of the turbine parts from a Rolls Royce T56-A15 turbo prop engine from a hurcules aircraft. Our investigation using Dye penetrate testing confirmed the presence of cracks; these cracks were caused by fatigue as a result of exposure to excessive heat. INTRODUCTION: The objective of this report is to examine the types of failure and their causes. This report takes a look into a Toyota Drive shaft that failed at 40,000 miles after being used off road. The report seeks to identify possible types of failures and describe each one. Besides that, the report will examines the failure and decide what type of failure is most likely, giving reasons and evidence to show why it is most likely. The report will also look at a turbine blade from the T56-A15 engine that has been removed from service due to a defect. The report will then identify the common causes of failure in these components and suggest the most likely for turbine blade. The report will in its discussion make recommendations for adjustments to the design and or manufacturing process for the drive shaft to try and prevent future failures. Task One REPORT & ANALYSIS: Failure in materials due to fracture can be grouped into 3 types: Ductile, Brittle, and Fatigue. Within each group there are several possible causes of the failure. Ductile Failure: This type of failure is easily identified by extensive deformation of plastic, and normally happened before the actual fracture. The failure of highly ductile materials is commonly referred as ‘ductile rupture’, whereby materials disintegrate rather than cracking. These failures are normally caused by overloading a ductile material or subjecting a material to load at an elevated temperature. In ductile fracture, there is a significant gross plastic or permanent deformation in the area of the ductile fracture. Besides that, the ductile fracture’s surface is not essentially associated with the principal tensile stress direction. The surface of a ductile fracture normally appears as fibrous and dull, which is attributed to fracture surface deformation. As mentioned by Sachs (2012), when grossly overloading the ductile materials in a very rapid manner can make the material to act in a brittle manner. Brittle Failure: This type of failure is easily identified by a surface roughness that is relatively uniform since the rate at which the crack travels is constant and the ‘chevron marks’, surface features become evident (Sachs, 2012). Such failures are normally caused by sudden shock loading or impact to a very hard material. In brittle fracture, there is no necking down or thinning; instead, cracking normally happed without warning, at a times, the material is fractured into many pieces. Brittle fracture normally happens during the low temperatures. When the temperature of the material is below the temperature of its brittle-to-ductile transition, then it becomes vulnerable to brittle fracture. Brittle fractures of fine-grain and extremely hard metals normally have little or no noticeable fracture pattern. In such instance, it becomes extremely challenging to confidently locate the origin of fracture. Fatigue Failure: This type of failure is easy identified by beachmarks and Striations. Beachmarks can be seen in materials that have experienced fatigue failures after being utilised for time period, permitted to rest for a similar period of time and then used again. On the other hand, striations are deemed to be stages in crack propagation, where the stress range determines the distance. Therefore, beachmarks could have thousands of striations. When the crack starts forming, it could be challenging to see much change until its size has reached a critical limit. Beachmarks normally show the failure’s propogation from the first cracks. These failures are normally caused by repeated loading and unloading of stresses many times, this could be tensile, compressive, torsional or a combination of these loads. Shaft failure is normally initiated by torsional fatigue forces since most of the crack propagation may be in tension because of the change in torsional resonant frequency and the weakening of the shaft. According to Sachs (2012), the face of a typical fatigue fracture is moderately smooth near the origin(s) and normally ends in a fairly rough final fracture. The drive shaft failure shown below clearly demonstrates that the failure must be of a fatigue type failure. This is because it is moderately smooth close to the origin(s) but fairly rough final fracture at the end. Figure One: Toyota Drive shaft that failed Normally, the drive shafts are considered to be the carriers of torque and for that reason, they are subject to shear stress and torsion, which is equal to the difference between the input torque as well as the load. The Toyota drive shaft was not adequately strong to bear the stress and avoid the extra weight; therefore, inertia increased. Fatigue is the main cause of shaft failure, which was caused by a high stress concentration localized around the sharp corner of the machine circlip groove. The localized damage was attributed to the fluctuating strains and stresses on the material. The fatigue cracks apparently started and propagated in areas where the strain was exceedingly high. As evidenced in figure two below, a motor shaft which had failed did not have progression marks; thus, indicating that the fatigue load had remained constant. Besides that, the instantaneous zone is somewhat big, which is an indication that the shaft was heavily loaded (Sachs, 2012). In Toyota drive shaft case, it appears that the cracking initiated different locations across the shaft, but inspection indicates that the root cause was related to high stress concentration. Figure two also shows that fatigue strength could be enormously be reduced by the fretting corrosion.  Figure Two: Material failure as a result of worn sheaves on the belt drive and fretting corrosion (Sachs, 2012) Further closer examination reveals that the centre of the shaft is the origin where the crack actually initiated before growing gradually across the fatigue zone. When the crack was growing slowly, the load variations led to related variations in the rate of the crack growth which looks like progression marks. Ultimately, the crack got to the point where the other section of the shaft was overstressed leading to overload zone (Sachs, 2005). Sharp corners and point loads are the main basis of numerical singularities; that is to say, they cannot predict accurate results even with a very fine mesh and accurate input data.  Sharp corners result in stress concentrations with a radius that is infinitely small. When the corner is exceedingly sharp, there is high likelihood that the shaft would be damaged because of high strains. A ductile material could yield while a brittle material could crack. This kind of damage could result in a local stresses’ redistribution. It appears that the drive shaft was subjected to very localized and high stresses will since the loading was cyclic; thus, resulting in a fatigue failure. Sharp corners normally lead to premature failure because of large stress concentrations as evidenced by the Toyota drive shaft. Furthermore, sharp corners leads to high stress concentration and since the shaft radius is somewhat small, the stress concentration is increased, which consequently, reduces fatigue life and fracture resistance. Besides that, a change in the shaft’s geometric shape could have resulted in additional stress besides the calculated stress that is normally recognized as stress concentration. The shaft’s internal stress was redistributed from low to high value at the point where there was a change in cross-sectional area. In the drive shaft, the redistribution happened at the grooves radius region where the two geometric forms were joined. Since the changes in cross-sectional area are gradual, the internal stresses achieve adequate space for evenly redistribution. When the geometric shape changes abruptly it results in higher effect of stress concentration. Since stress concentration results in high mechanical stress, the effects of stress concentration at the areas that are critically stressed have to be reduced through improved designs. A cross-sectional analysis of this the fatigue failure demonstrates that the shaft has a small radius close to the outer edge. This consequently results in high stress concentration which, when multiplied by the keyway’s stress concentration, led to the failure. Task Two The turbine blade states that it has leading and trailing edge cracks, although these are not visible to the naked eye. We will carry out a dye penetrant test to establish where these cracks are. The effectiveness of dye penetrant test relies on how clean the part is as well as the procedure utilized to carry out the test. The basic steps utilized to detect cracks in the turbine blade include: 1. Cleaning the turbine blade 2. Apply the dye as well as allowing it to dwell 3. Waiting for 15 minutes 4. Cleaning it to remove the excess Penetrant 5. Apply a developer and giving it time to develop 6. Observing Cleaning Applying the dye waiting for 15 minutes Cleaning Spray Developer Observing The turbine blade most likely failed due to exposure to excessive heat. When the turbine blade is exposed to excessive operating temperatures, the maintenance cost is increased and operational efficiency is reduced (Gale et al., 1987). The failure was caused by excessive heat because the high rotational speed of the turbine results in an elevated temperature. The blade damage can be attributed to high temperature corrosion. One of the heat-related factors that can result in turbine blade failure is stress rupture, which is the breakage of turbine blades because of exposure to physical stress. The factor is time-dependent and is attributed to centrifugal force stresses as well as the turbine’s operating environment’s temperature. The failure can be attributed to hot gas erosion, which normally happens because of thermal cycling and localized overheating because of a coating breakdown or intermittent loss of cooling. After numerous cycles, damage happens and the erosion (increased roughness) exacerbates the issue. The failure can also be attributed to creep, which normally happens under elevated temperature, which is often beyond 40 percent of the material’s melting temperature. Failures because of creep normally appear in brittle or ductile manner. Task Three CONCLUSION: A low stress high stress concentration fatigue failure is most likely the cause because the crack shows nucleation along the high shear stress planes which is normally 45o to the direction of loading. Basically, a single origin normally shows a failure that has low overstress and existence of multiple origins could be attributed to high stress concentrations (Sachs, 2005). Basically, the presence of beachmarks shows relatively high total stresses and multiple origins. The beachmarks of the drive shaft can be attributed to high stress concentrations. If it was a brittle failure it would have markings commonly recognized as ‘herringbone’ or as ‘chevron’ marks. Basically, brittle fracture normally happens when there is little plastic deformation and the brittle fracture’s surface is normally perpendicular to the principle tensile stress. And if it was ductile, the surface roughness would be relatively uniform. In ductile fracture, there is significant plastic deformation in the area of the fracture. Furthermore, it cannot be a high stress low cycle failure because it does not show relatively high loads which generate both elastic strain and some plastic strain in all cycles. Still, the failure occurred at the shoulder of the circlip groove because fatigue cracks normally happen when there is an abrupt or sharp change in the shape of the component. Clearly, there was abrupt change in the shaft’s diameter, which is commonly referred as a shoulder. This resulted in considerable high stress concentrations since the position of the shoulder was a region that had high bending moment; thus, resulting in high compressive and tensile stresses. The turbine blade most likely failed due to exposure to excessive heat. The failure was caused by excessive heat because the high rotational speed of the turbine results in an elevated temperature. The blade damage can be attributed to high temperature corrosion. DISCUSSION: The report concluded that the stress concentration caused by the machined circlip groove contributed to the early failure of this part. In order to prevent this happening in future there are several design changes that could be made, these include careful machining as well as assembly, torsional variations’ measurement and correction, and ensuring that the torsional vibration is prevented from entering the drive system by making the angles at all the drive shaft’s ends the same so as to eliminate torsional vibration.  Besides that, the rotating components should be balance, mass of reciprocating members should be reduced, and start-up shock should be minimized by incorporating a soft-start device. Besides that, improvements can be made using U-Joint Kit Designs so as to allow for lubrication. The drive assembly should be balanced by using a Two Plane balancer. Stress relief grooves can be used to improve the fatigue life and lessen the stress concentration of the drive shaft. The stress relief groove would help extend the shaft’s fatigue life, if a suitable location and size of the groove is selected. This report believes the easiest and most effective implement without compromising performance is making sure that the components are installed correctly to lessen driveline vibrations as well as parts failures. REFERENCE: Gale, C.I., Loftin, J.A. & Rockwood, R.J.R., 1987. Conserving Turbine Life. Service News, vol. 14, no. 1, pp.1-16. 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: HYPERLINK "http://www.maintenancetechnology.com/2012/07/failure-analysis-of-machine-shafts/" http://www.maintenancetechnology.com/2012/07/failure-analysis-of-machine-shafts/ [Accessed 14 June 2017]. Read More