Browsing by Author "Fonte, M."
Now showing 1 - 10 of 12
Results Per Page
Sort Options
- Crankshaft failure analysis of a boxer diesel motorPublication . Fonte, M.; Anes, V.; Duarte, P.; Reis, L.; Freitas, M.This paper reports a failure mode analysis of a boxer diesel engine crankshaft. Crankshafts are components which experiment severe and complex dynamic loadings due to rotating bending combined with torsion on main journals and alternating bending on crankpins. High level stresses appear on critical areas like web fillets, as well as the effect of centrifugal forces and vibrations. Since the fatigue fracture near the crankpin-web fillet regions is one of the primary failure mechanisms of automotive crankshafts, designers and researchers have done the best for improving its fatigue strength. The present failure has occurred at approximately 2000 manufactured engines, and after about 95,000 km in service. The aim of this work is to investigate the damage root cause and understand the mechanism which led to the catastrophic failure. Recommendations for improving the engine design are also presented.
- Crankshaft failure analysis of a motor vehiclePublication . Fonte, M.; Li, Bin; Reis, L.; Freitas, M.A case study of a crankshaft catastrophic failure of a motor vehicle and its failure analysis is presented. The crankshaft suffered a mechanical seizure on the crankpin no. 2 after 3 years in service. It was repaired and after 30,000 km the vehicle had a damage again, with a catastrophic failure on the same crankpin. A transversal macrograph of the crankpin revealed that the crankpin was rectified and filled with a metal alloy for the same nominal diameter. Two fatigue cracks growing to the center of the crankpin where the final fracture occurred. The symmetric semi-elliptical crack front profile confirms the effect of a pure mode I under alternating bending. The catastrophic failure was a consequence of the inadequate repairing by a non-authorized manufacturer.
- A damage parameter for HCF and VHCF based on hysteretic dampingPublication . Lage, Y.; Cachão, H.; Reis, L.; Fonte, M.; Freitas, M. de; Ribeiro, A.The fatigue limit of materials, due to the improvement of fatigue life of structures and mechanical components should be extended from the traditional 106–107 cycles up to 109 and more, but with traditional testing hardware this is a difficult technical task due to the length of time needed for the completion of tests. Ultrasonic fatigue testing machines seem to be adequate for very high cycle fatigue (VHCF) tests. We propose here to evaluate the behavior of the hysteretic damping in an attempt to associate that with damage parameter. The approach here presented is based on the fact that the fatigue issue can be understood in terms of the energy available for irreversible process triggering. This nonconservative energy will be involved in micro-structural changes in the material before being dissipated as thermal energy. In fact, the balance between the energy supplied to and returned by the material is positive and the hysteretic damping factor represents the inelastic fraction of energy in each cycle. Aiming at building a model to correlate the hysteretic cycle parameters and the fatigue process, both energy loss and material response of the specimens are measured during the fatigue test. The fatigue tests are carried out with an ultrasonic machine test, operated at 20 kHz with amplitude or temperature control, under HCF and VHCF for copper specimens. The results show the behavior of hysteretic damping parameter during fatigue life, the equivalent dissipated energy per cycle and a good correlation between the hysteretic damping factor parameter and the fatigue process S–N curve, suggesting that factor as a promising fatigue life parameter useful for some cases of fatigue life prediction.
- Failure analysis of cylinder head studs of a four stroke marine diesel enginePublication . Fonte, M.; Reis, L.; Infante, V.; Freitas, M.After about seven years in service, four cylinder head studs, which tight the block and cylinder heads of a marine main engine, have fractured. Meantime, they were changed for new ones without to determine the root cause of failure. The aim of this research work is to carry out a failure analysis in order to avoid recurrent damages in that engine. The fracture morphology and thread roots of fractured studs were carefully observed by optical devices. Thread material defects and corrosion were not found. However, the thread roots, where the stress concentration are higher, can be pointed as the local of fatigue crack initiation. As is well-known, all cylinder head stud bolts are pre-tensioned in order to increase the mean stress σm and reduce the stress amplitude σa. The steel alloy quality and design are of primordial importance for improving the lifetime of studs, and this is supervised by the Classification Societies (CS), as is indicated by an engraved stamp on each stud end, that certifies each one of them. The pre-tightening of the studs was calculated, and results point as root cause of failure a significant high stress concentration mainly at the second thread root of the studs, close to the bottom side of the nut, which are critical stress zones.
- Failure mode analysis of a diesel motor crankshaft.Publication . Fonte, M.; Infante, V.; Reis, L.; Freitas, M.A failure mode analysis of a diesel motor (110 kW) crankshaft from an automobile vehicle is presented. After 120,000 km in service, an abnormal vibration was detected which was increasing with the time. The diesel motor was first disassembled for determining the root cause, however without success. No defect was detected, but since a suspicion of damage was present, and being this failure recurrent in this type of diesel motor series, the crankshaft was disassembled again. Then the crankshaft was subjected to a simple vibration analysis and a preliminary indication of possible existence of a crack was concluded. The crankshaft was then replaced by a new one, and the old was subjected to a failure analysis for determining the root cause. A crack was found at the crankpin web-fillet and after a complete opening of the crack, the failure analysis showed that fatigue was the dominant failure mechanism. Observations were carried out by optical and Scanning Electronic Microscope. Material defects at the crack initiation zone were not found. The root cause of damage seems to be a misalignment of the main journals and a weakness of design close to the gear at the region where the crack was initiated. Therefore, probably a poor design and a deficient assembling of the crankshaft helical gear coupled to the main journal end was the first cause of the failure.
- Failure mode analysis of two crankshafts of a diesel single cylinder enginePublication . Fonte, M.; Duarte, P.; Reis, L.; Freitas, M.; Infante, V.This paper reports an investigation carried out on two damaged crankshafts of single cylinder diesel engines used in agricultural services for several purposes. Recurrent damages of these crankshafts type have happened after approximately 100 h in service. The root cause never was imputed to the manufacturer. The fatigue design and an accurate prediction of fatigue life are of primordial importance to insure the safety of these components and its reliability. This study firstly presents a short review on fatigue power shafts for supporting the failure mode analysis, which can lead to determine the root cause of failure. The material of these damaged crankshafts has the same chemical composition to others found where the same type of fracture occurred at least ten years ago. A finite element analysis was also carried out in order to find the critical zones where high stress concentrations are present. Results showed a clear failure by fatigue under low stress and high cyclic fatigue on crankpins.
- Failure mode analysis of two diesel engine crankshaftsPublication . Fonte, M.; Infante, V.; Freitas, M.; Reis, L.A failure analysis of two damaged crankshafts are presented: one obtained from a diesel engine of a mini backhoe, and another one from an automobile vehicle. The diesel motor suffered a serious mechanical damage after 3 years and 5000 hours in service: the connecting rod n° 3 broke and, in consequence, the crankcase and motor block suffered damage. The motor was repaired by a non-authorized workshop, but maintaining the same crankshaft without being properly inspected. After 1100 hours working the crankshaft failed on the 3rd crankpin. The second crankshaft failed after 105 000 km in service. In both crankshafts a crack grew from the crankpin-web fillet, and their symmetric semi-elliptical crack front profiles confirms the effect of a pure mode I (reversed bending). Fractographic analyses show the semi-elliptical beach marks and results indicate that fatigue fracture was the dominant failure mechanism of these two crankshafts.
- New approach for analysis of complex multiaxial loading pathsPublication . Anes, V.; Reis, L.; Li, B.; Fonte, M.; Freitas, M.Experimentally, it has been proven that the stress level needed to cause fatigue failure in pure shear is less than the axial one. This fact has led to consider a stress scale factor between shear and axial stress in order to reduce different applied stresses to the same shear stress space or principal stress space to facilitate the yielding analysis or fatigue damage evaluations. In this way most of multiaxial fatigue models use a stress scale factor to consider the fatigue damage contributions from the axial and shear stress components regarding the material strength degradation. Much efforts were made to quantify the effective shear and axial stress amplitudes under a three-dimensional stress state, however, the combined damage resulted from those amplitudes have been reduced to a constant value. In some cases, the approaches used proved to be inadequate, leading to compute the same equivalent stress for different loading paths with different fatigue lives. In this work it is performed a series of multiaxial fatigue tests on a high-strength steel in order to determine the multiaxial fatigue strength under proportional and non-proportional loading conditions. A stress scale factor function was mapped based on the experimental results using as arguments, the axial stress amplitude and the stress amplitude ratio, which has proven to be sensitive to the loading path nature. Using the stress scale factor surface an equivalent shear stress was defined and it was used in the fatigue life correlation. Results indicate that the stress scale factor (ssf) is not a constant value and it is strongly dependent on stress amplitude level and loading path shape. The equivalent stress was successfully applied to proportional and non-proportional loading paths with satisfactory results.
- On the assessment of fatigue life of marine diesel engine crankshaftsPublication . Fonte, M.; Duarte, P.; Anes, V.; Freitas, M.; Reis, L.The fatigue strength and its correct assessment play an important role in design and maintenance of marine crankshafts to obtain operational safety and reliability. Crankshafts are under alternating bending on crankpins and rotating bending combined with torsion on main journals, which mostly are responsible for fatigue failure. The commercial management success substantially depends on the main engine in service and of its design crankshaft, in particular. The crankshaft design strictly follows the rules of classification societies. The present study provides an overview on the assessment of fatigue life of marine engine crankshafts and its maintenance taking into account the design improving in the last decades, considering that accurate estimation of fatigue life is very important to ensure safety of components and its reliability. An example of a semi-built crankshaft failure is also presented and the probable root case of damage, and at the end some final remarks are presented.
- Rotary fatigue testing machine to determine the fatigue life of NiTi alloy Wwres and endondontic filesPublication . Carvalho, André; Freitas, Manuel; Reis, Luís; Montalvão, Diogo; Fonte, M.Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (MemryTM and EuroflexTM). The tests were done on a versatile fully automatic rotary bending testing machine. The formulation is also presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure.