Biomechanical Behavior of a Variable Angle Locked Tibiotalocalcaneal Construct

This paper examines the mechanics of the tibiotalocalcaneal construct made with a PHILOS plating system. A failed device consisting of the LCP plate and cortical, locking, and cannulated screws was used to perform the analysis. Visual, microstructure, and fractographic examinations were carried out to characterize the fracture surface topology. These examinations revealed the presence of surface scratching, inclusions, discoloration, corrosion pits, beach marks, and cleavage and striations on the fracture surface. Further examination of the material crystallography and texture revealed an interaction of S, Ni, and Mo-based inclusions that may have raised pitting susceptibility of the device made with Stainless Steel 316L. These features suggest that the device underwent damage by pitting the corrosion-fatigue mechanism and overloading towards the end to fail the plate and screws in two or more components. The screws failed via conjoint bending and torsion fatigue mechanisms. Computer simulations of variable angle locking screws were performed in this paper. The material of construction of the device was governed by ASTM F138-8 or its ISO equivalent 5832 and exhibited inconsistencies in chemistry and hardness requirements. The failure conditions were matched in finite element modeling and those boundary conditions discussed in this paper.

Micro-Crack Analyses of Chromium Steel JIS-SCr 420 for Helical Gear Transmission

This research aims to investigate micro-crack on a failed for helical gear transmission which was to adjust the engine to increased horsepower. The helical gears made are from chromium steel JIS-SCr 420. The spectrophotometer test machine was used to detect chemical composition, Mechanical properties were evaluated by Vickers hardness and microstructural analysis with an optical microscope, which the crack of the surface layer and energy dispersive spectroscopy using a scanning electron microscope. The results showed that the fracture characteristic of the helical gear’s surface was expected to beach marks and break away. It can be seen that the mixing failure area of oxide inclusion with carbide surrounding before the liquid state of material will be solidified which caused the failure cause of this helical gear. The summary analysis results can be accorded with the assumption of this research and which help prolong service life of the component.

Failure analysis of a 40 ton crane hook at a Hot Strip Mill

There have been accounts of repeated failure of crane hooks at the coil yard of a Hot Strip Mill which pose a serious threat to safety in the area. More than 4 hooks failed in less than 5 years. The crane hook (rated for 36000 kg) failed from the threaded shank while lifting a load of 18143 kg. The metal in the hook was revealed by chemical analysis to be killed IS: 4367 20C15 steel. The hook rod failed from a step where there was a cross sectional change and the locations were associated with machining and chatter marks. Such cross-sectional changes are the potential sites of stress concentrations leading to crack initiations. Fracture surfaces of broken pieces of hook reveal initiation of beach marks from both sides with granular rough surface at the middle of fracture zone. Beach marks initiated from both sides indicate origin of reverse bending fatigue. Distinct granular rough zone at the middle is due to final brittle fracture. Microstructure of the polished sample revealed numerous inclusions which indicate that the steel was not clean .Such a huge number of inclusions are not desirable as they can act as stress concentration sites and lead to fatigue crack initiation. Etched microstructure of failed hook reveals coarse cast structure having inhomogeneous microstructure with a mixture of ferrite and pearlite (which meant lower fatigue strength). This inhomogeneous coarser cast structure is outcome of lower reduction ratio during rolling followed by improper heat treatment process. The fracture was concluded to have occurred due to stress concentration from the step region due to inferior material (Inclusion and Improper Heat treatment process). Preventive maintenance and condition monitoring procedures should be applied to identify and minimize the risk(s) Establishment of an NDT procedure for regular basis inspection of the structural members (welded joints and hookshaped steel rods during incoming inspection and in-service).

ANÁLISE DA FALHA DE PARAFUSOS CLASSE 10.9 UTILIZADOS EM GUINDASTE OFFSHORE

<p>Neste trabalho foram avaliadas as propriedades mecânicas de dois parafusos que têm como função a fixação de um guindaste utilizado nas plataformas offshore. Foram retiradas duas seções de cada elemento para a realização de análises de composição química, da fratura e microestrutural e ensaios mecânicos de dureza e de tração. Os resultados obtidos nas análises da composição química mostraram-se condizentes com as normas e que o material utilizado para fabricação dos parafusos trata-se de um aço ASTM 4140, sendo apropriado para fabricação de parafusos da classe 10.9. Através das fraturas foi possível verificar que o parafuso 1 provavelmente foi submetido a um esforço por torção, tendo uma alta concentração de tensão na região da raiz do filete, assim como a existência de marcas de catraca, o que possibilitou a criação da trinca e propagação por fadiga. No parafuso 2 foram observadas marcas de praia originadas em cavidades e uma região de fratura final pequena como indicativo de baixa tensão nominal. Identificou-se que ambos possuem uma matriz microestrutural martensítica, contudo foi encontrada a presença de austenita retida e também inclusões não metálicas, as quais apresentam maior concentração no parafuso 2, indicando que provavelmente estariam presentes nas cavidades produzidas na fratura. Os valores de dureza determinados são apropriados para os valores mínimos estabelecidos por normas técnicas. No entanto, verificou-se que o parafuso 2 não apresenta as porcentagens mínimas de ductilidade estipuladas por uma das normas técnicas analisadas.</p><p> </p><p>ABSTRACT</p><p>The purpose of this article was evaluate the mechanical properties of two bolts that has as function the fixing of a crane used at oil offshore platforms. Two specimens were removed of each bolt for analysis such of chemical composition, fracture and hardness and tensile testing. The results obtained from analysis of the chemical composition proved to be consistent with the standards and the material used to manufacture the bolts consists of a steel ASTM 4140, being appropriate for manufacturing class 10.9 bolts. Through the fractures it was found that the bolt 1 was probably subjected to a load of torsion with a high concentration of stress in the region of bolt’s root, well as the existence of ratchet marks, which enabled the creation of fatigue crack. Beach marks were observed originating from cavities and a small final fracture region indicated a low nominal stress on bolt 2. It was identified that both have the martensitic microstructure matrix, however it was found the presence of retained austenite and also non-metallic inclusions, which are more concentrated on the bolt 2, indicating that probably would be present in cavities produced in the fracture. The hardness values are appropriate to the minimum values established by technical standards. However, it was found that the bolt 2 does not show the minimum ductility percentages prescribed by technical standards analyzed.</p>

IDENTIFYING INITIAL DAMAGE OF PALM OIL SCREW PRESS OF DRIVE SHAFT

Status: PostprintInvestigation was carried out because of the common failure at the screw press of drive shaft which made disruption on production activity of palm oil. Visual observation, interview and simulation were conducted. The finding supported by simulation using Nastran statistical software showed that the fracture occurred due to the fatigue. Based on simulation, it was known that the stress concentration occured on the keyway of the tooth wheel holder. It was proved by visual observation which showed the attrition of keyway due to the friction between post and keyway continuously occured in the drive shaft rotation. The continuously friction leaded to the loosening of the keyway. Futhermore,it made the strong impulse to the post keyway which finally caused the fracture at the keyway area. The continuously operating drive shaft caused the final fracture. The several characteristic of fatigue fracture could be seen from the fracture surface, such as the presence of residual fracture area, smooth surface and the presence of beach marks

Fracture Problem of the Valve of Piston Engine

In this paper the fracture problem of exhaust valve of the piston engine was investigated. Visual inspection showed that on the fractured surface of the valve the beach marks, typical for fatigue failure were observed. The crack origin was not covered by corrosion products or material defects. In order to explain the reasons of damage of the valve, the non-linear finite element method was utilized. The numerical model composed of the poppet valve, the guide and the seat face was defined. In the analysis both the mechanical load resulting from the valve spring and also the thermal load arising from a non-uniform temperature field were defined. The loads were at first defined separately in order to check which load component has a dominant influence on the stress level. In third load case (which represents the operational thermo-mechanical engine conditions) the mentioned loads were defined simultaneously. The results of performed computations showed that the operational dynamic stress (in the critical zone of the valve where the crack appeared) is more than 12 times lower than the yield stress of the material. It means that the premature fatigue fracture of the valve was probably caused by any phenomenon concerned with the increase of the operational stress in the valve. The additional observation of the second (non-damaged) valve from the same engine head showed that the carbon deposit was located on the valve face. The results of the stress analysis of the valve with additional carbon particle showed, that in the valve stem a high bending stress was observed.

Visual Inspection on Failure Surface of Constant Force Spring (CFS) Fitted in a Counterweight Balancing Mechanism

A failure characteristic of a fractured constant force spring (CFS) or flat spiral spring fitted in a counterweight balancing mechanism is investigated via series of visual experimentation. Macroscopic examination reveals several beach marks that shows direction of fatigue crack propagation has indicated that the CFS fracture had initiated and propagated due to fatigue from an inner surface origin. Macro cracks resulted from stress concentration were also visible on grain boundaries. The crack was initiated at the center of the CFS which later propagated in the direction perpendicular to the applied cyclic load and finally fractured when it can no longer sustain the applied cyclic load. Inspection via Scanning Electron Microscopy (SEM) has indicatedsign of fatigue striations perpendicular to the fracture propagation which is a characteristic of fatigue failure mechanism. Examination of the fractured surface also pointed porosities that reflects points of crack initiation. Multiple crack initiation points identified shows that the fracture was a result of high stress or high stress concentration.