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In this work the failure analysis of a coupled shaft from a shredder is presented. The failure analysis was performed using both experimental methods (visual and microscopic inspection, tensile and hardness tests) and theoretical calculations (stress analysis) aiming to determine the root causes of failure of the coupled shaft. The failure of the shaft occurred due to fatigue, on a perpendicular plane to the rotation axis, in the region of the coupling transverse hole. The shaft fracture surface presents characteristics of fatigue due to torsion combined with bending high loads, with an associated strong plastic deformation. The contribution of bending solicitations related with radial misalignment between the shaft and the shredder module was evaluated. The radial misalignment plays an important role on the failure of the shaft. For the reported life service of the shaft under maximum torsional moments, and considered working conditions, the design guidelines of radial misalignments compromise the structural integrity of the shaft in the shredder system. Recommendation regarding the design and assembly of the shaft are also presented.

A case study of a catastrophic failure of a diesel motor crankshaft is presented. The aim of this failure analysis is to investigate the root cause of this important mechanical component. This crankshaft belonged to a particular vehicle (140 cv, at 4000 rpm, 1968 cm3 displacement) that fractured after 180,000 km and 8 years in service. The motor was disassembled, and the crankpin No. 3 and the main bearing cap No. 4 were broken. Defects of material and machining defects were not found at the crack initiation sites by optical and SEM microscope. Results shown that the crankpin and the main bearing cap clearly failed by fatigue, and the root cause seems to be related with deficient tightening of the main bearing cap No. 4, which fractured due to a crack developed on the its outer side. The main journal No. 4 did run out of support, and the crankthrow No. 3 increased the alternating stress amplitude, whereby an inevitable catastrophic crankshaft failure happened.

Fused deposition modelling (FDM) shows a great potential for the production of complex shapes in the case of several materials including nylon, which is a type of polyamide. In such materials, water absorption is an issue that requires attention and there is interest in studying the application of protective products, with the goal of improving the sealing properties of the parts. The objective of this work is to analyze and evaluate the water absorption, mechanical properties and consequent failure surfaces of nylon parts obtained by FDM, with two raster angles, after being coated with a protective material. Two polymeric products were used to coat the parts, polyurethane elastomer and silicone. The performance of the coated parts was evaluated with water absorption tests, as well as compression and tensile tests. The treatment was found to reduce the open porosity and to decrease the mechanical properties of the specimens in different raster angles. Failure occurs at surfaces making angles of zero or forty five degrees from the applied force, depending on the raster angle, both in treated and untreated samples.

This paper reports a failure mode analysis of a 1900 cm3 turbo diesel engine of a well-known commercial brand. The engine is a supercharged diesel engine with turbocharger, producing a maximum power of 81 kW; it was produced in 1999 and collapsed at 120,000 km without warning. A fracture occurred at the crankpin n°1 of the crankshaft in the region of web-fillet. Crankshafts are mechanical power transmission components with complex geometries and experience multiaxial stress states in main journals and crankpins. The objective of this work is to determine the root cause that led to the crankshaft collapse. A fractographic, metallographic, and numeric analysis were performed to understand the crankshaft failure mode and its mechanical mechanisms. Results show that the crankshaft failure resulted from a fatigue process governed by normal stresses raised by two possible processes, namely, a notch in the crack initiation spot, or the crankshaft misalignment.

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.

The structural integrity of several structures could be determined by their joints strength. Over the years, adhesively bonded joints have been often chosen to achieve a compromise between mass reduction and higher mechanical strength. Among others, the reduction in stress concentrations, the ability of producing smooth surfaces with no discontinuities and the reduced weight penalties are some of the factors that make this type of joints so attractive. Normally, to increase the bond strength, the materials to be bonded must be subjected to a kind of surface treatment. For metals, and more specifically, for aluminium alloys, phosphoric acid anodizing and chromic acid anodizing have been the most used treatments worldwide. However, recent investigations show that these kinds of anodizing are detrimental to health due to the release of carcinogenic substances. With this in mind, it is of the utmost importance to find alternative surface treatments that can ensure an effective bond. In this paper, a vast experimental study was performed based in the single lap joint ASTM D 1002 standard method, with the objective of determining the best alternative surface treatment (Sulfuric Acid Anodizing and Boric-Sulfuric Acid Anodizing), for aluminium-to-aluminium joints, using two types of adhesives, namely the AF 163 and the EA 9658 AERO. Results show that the optimum surface treatment is different for each type of adhesive and this fact has a huge influence on mechanical behavior of this type of aeronautical adhesive joints. info:eu-repo/semantics/publishedVersion

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.

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.

The working principle of the Wells turbine, used in the ocean energy extraction, can sometimes lead to stall in the turbine blades and stators’ vanes, which is an unintended action considering that creates potentially damaging conditions to the mechanical components, plus reducing the efficiency of the take-off system. This work focused on the cracking damage caused to the earth-side guide vanes of the Pico Island Wave Power Plant due to the loading from turbulent air flow and consequent vibrations, as a result of turbine stall. Comprehensive analysis of the design and manufacturing phases, as well as of the constitutive metal properties (AISI 316L stainless steel), including the consequences of welding, was carried out. Strain gages were used to evaluate the loading profile in real-sea working conditions and different suggestions were made in order to postpone and prevent crack nucleation and propagation. The appearance of cracks seems to have been caused by mistakes made at both design and manufacturing phases, eliminating from the guide vanes the ability to endure the aggressive loads which they were subjected to. The lack of an active control strategy on the power plant resulted in consistent turbine stall events, thus contributing to the excessive loadings imposed to the vanes.

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.

This paper presents a detailed analysis of two in service failures concerning trunnions of landing gears of military aircrafts. Several tools were used in order to find out the primary cause of failure, namely, visual examination, chemical analysis and macroscopic, microscopic and microstructural examinations. The progress of the fatigue cracks were determined based on the observation of beach marks caused by the loading history and by striation marks caused by cyclic plastic deformation mechanisms related with events occurring in the loading history of the component. Additionally, the topology of the fracture surfaces provided an indication of the stress system that produced failure, which is closely dependent on the mechanical behaviour of the material. In both examples the failures occurred at the transition of two sections. The visual observation of the fractured surface immediately indicates that these failures occurred by a fatigue process. A main area of the fracture surfaces present high stress failure facies. A more detailed analysis of the fractured surfaces concludes that the surfaces that failed by fatigue is clearly smaller than the one that failed by an overload; this measuring capacity allows the investigator to consider that failures occurred by fatigue but in the presence of high stress levels at the crack initiation zone. In the first case multiple crack initiation was detected while in the second case the cracking occurred at the corner of the lateral surface of the trunnion structure. Any type of initial defect was not observed in the fracture surfaces. Therefore it can be concluded that failure probably occurred by fatigue but in the presence of higher loads in the components than those that were expected.

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 failure analysis of two helical gear wheels of a ducted azimuth thruster is presented. The research work consisted of a fracture examination of the material in order to determine the damage root causes. The samples for the failure analysis were obtained from two broken teeth of two helical gear wheels. An analysis through the scanning electron microscope (SEM) was carried out close to the crack initiation. It was found that the damage in the bevel gears were by the fatigue fracture mode. The SEM analysis showed that the gear teeth were under severe contact stress during the operation aggravated by an inappropriate lubricating. A possible misalignment between the pinions and the gear wheels teeth could also contribute for the premature failure.

In spite of the high levels of reliability of modern aeroengine components resulting from rigid standards and practices, failures of compressor and turbine blades during normal operational environments are common situations which compromise the flight safety. The investigation of real failures affecting these components allows gaining a deeper knowledge concerning the mechanisms of crack initiation and propagation which, in turn, can be used in order to prevent future incidents or accidents. This paper presents the analysis of two in service failures involving blades breakage belonging to different compressor stages. Crack growth mechanisms were evaluated based on the visual inspection of the affected components and with both macroscopic and microscopic observations of the fracture surfaces. The origins of crack initiation were evaluated through the examination of crack path and beach marks on the fracture surface. Mechanical analyses were carried out to identify the possible causes of the failures by examining anomalies in the mechanical behaviour of the materials such as hardness tests, chemical composition and surface coating analysis through SEM observations. The analysis of the different fracture surfaces shows that crack propagation is mainly related with fatigue mechanisms whilst crack initiation can be attributed to distinct causes, either the presence of defects in the surface of the blade due to impact of debris or intrinsic material defects or some degrading mechanisms affecting the internal microstructure of the material. In some cases the blades were in service after having being object of an overhaul procedure, which can justify some crack initiation cases that caused in service failures.