Papers by Keyword: Failure Analysis

Abstract: Characteristics and forming causes of the cracks in welded joint of 15Cr1Mo1V steel serviced 70000h are investigated by mechanical and chemical testing and crack morphology observation. Results show that the cracks initiate from welded metal or coarse grain heat affect zone (CGHAZ) near fusion line, and there are three kinds of defects observed in the crack region, which are macrocracks, microcracks and voids. According to the forming position, process and morphology of the cracks, it is estimated that the cracks are a kind of stress relief crack (SRC). The main reasons of the cracking are because of residual stress caused by improper temperature field during post welding heat treatment, strong stress concentration caused by welding structure, additional stress caused by abnormal hangers & supports and decreased ductility of welded joint in service. The SRC in welded joint can be avoided through optimizing the welding process and postweld heat treatment(PWHT) process to ensure enough critical ductility deformation ability εc and avoiding and reducing stress concentration and additional stress to decrease ductility deformation εP of welded joint which make εc>εp consistently.

Abstract: The gear pump housing in bulldozer, which was made of cast Al-Si alloy, was found to break in two halves during running. In order to determine fracture reason of the gear pump housing failure analysis in detail were conducted by visual inspection, optical microscope and scanning electron microscope (SEM), chemical analysis, hardness inspection. Visual inspection showed that the crack source has a white semicircle on the interior side of the section and propagated toward the exterior surface of the housing with obvious radial state. Meanwhile, the inner wall has the wear trace in depth of 0.1 mm due to boring phenomenon. SEM observation indicated brittle cleavage fracture mechanism with many casting defects such as dendritic shrinkage and inclusion as well as initiated crack in gear pump housing. The casting defects probably promoted the initiation and propagation of the crack and decreased the loading capacity of the gear to impacting force from the inside shaft. The analysis results show that the failure of gear pump housing mainly resulted from serious boring phenomenon, dendritic shrinkage and the lower material performance.

Abstract: A dam is an artificial barrier constructed across a stream channel to impound water. Analysis of stresses and displacements are inevitable for the structural design and failure analysis of dams. This paper deals with the numerical simulation of structural response of gravity dams, duly considering the foundation-structure interaction. The optimum depth and width of foundation extend to be considered in the numerical model is also studied. A parametric study based on the stiffness of the foundation is also exercised. As an application of the developed model, a case study of Peechi gravity dam is presented. This study proved the importance of consideration of foundation-structure interaction in the structural analysis of dams. The developed numerical model can be further improved for performing seismic analysis of gravity dams, considering the foundation-structure as well as fluid-structure interactions.

Abstract: In the present work, critical testing methods are employed in order to assess the formability of a Zn-Ti-Cu alloy, evaluating, therefore, the anisotropic properties of the produced sheet. The determination of plastic strain ratios and the induced combined mathematical expressions, utilizing bi-axial strain measurements for the various test directions (0, 45 and 90 degrees towards the RD), together with the performance of cupping tests are compiled, aiming to rank and interpret the bending and sheet metal roll-forming capability. Moreover, the microstructural characterization is realized to address the influence of grain and phase structure on the sheet metal formability and identify potential optimization routes. Fracture analysis approach elucidated the micro-mechanisms prevailed in damage evolution and accumulation during monotonic loading, signifying the importance of microstructure development during thermomechanical process history.

Abstract: A failure case of the outside coating on the furnace roller is presented in this study. The coating consists of WC-Co, which is deposited on 45 steel by High Velocity Oxygen Fuel (HVOF) spraying. The spalling of coatings has been found in the middle of the roller. Rust was also observed on the coatings of the roller. The failure analysis of the coating was performed in terms of composition analysis, and microstructure observation etc. In addition, the effects of different sandblasting post-treatments were compared. And the effect of substrate material is investigated on the corrosion of coatings. It was concluded that the sandblasting particles remained on the substrate surface have effects on the failure of the coating. The peeling-off coating on the roller is also related with the formation of iron oxides. Less sand can be remained on the substrate surface when air purging and brushing is used after the sand-blasting compared with air purging or non-treatment is applied. The stainless steel shows more excellent corrosion resistance than the low carbon steel when they are used as substrate for WC-Co coatings.

Abstract: An injection pipe of diesel-hydrogenated unit exploded in a refinery of Shandong Province. In order to analyze the causes of the pipe explosion, the authors measured the pipe wall thickness, sampled the fractured pipe, observed the fracture of the samples by metallographic microscope, surveyed the pipe service environment, and calculated the pipe burst pressure. As the exploded pipe deformed very seriously and no any significant original defect in the fracture was observed, the failure reason of the pipe might be over pressure due to heat tracing by continuous supply of steam during the unit shutdown. Meanwhile, measures were suggested to avoid the similar accidents.

Abstract: Failure analysis was carried out to find out the causes of burst on water-cooled wall tubes elbow for circulating fluidized bed (CFB) boiler.Through macro-observations, chemical composition analysis, macro structure observations，grain size measurements and water quality examinations, it can conclude that the high pH value of boiler water at the fire-facing side of weld dense phase zone will lead to alkali concentration that will cause alkali corrosion. The existence of residual stress in elbow under strong alkaline environment will cause alkali stress corrosion. Research results show that the tube failure is caused by both alkali corrosion and alkali stress corrosion. Strengthen water quality monitoring and elimination of stress can effectively prevent the alkali corrosion and alkali stress corrosion.

Abstract: A method based on structural modelling is developed for failure evaluation and analysis of mechatronics-based production systems. Majority of the elements in production systems are mechatronics-based, which includes various elements such as; electrical, electronic and mechanical. Each of these may have different failure types that may be interdependence/interactive. The reliability of the system mainly depends on how well the failures are taken care of during design stage. In general, individual failures are generalized into probable failure modes and early identification of these helps to reduce their probability. However, consideration of failures and their interdependence / interactions will help to evaluate and analyse the failures of complicated systems in an efficient and effective manner and increase the inherent system reliability. The system structure modeling helps in this regard. Digraph model, in conjunction with matrix method, is employed for failure evaluation and analysis of a mechatronics-based production system based on its structure.

Abstract: Tibial fractures were, still are and will be a challenge for the orthopedic surgeon. In this paper we conducted a case study on a system plate-screw made by titanium that was used in clinical practice for reduction and fixation of a tibial pilon fracture. After eight months the patient returns for the ablation of metallic implants. Clinical and strengthen the fracture callus is found radiologic hypertrophic and degradation plaque.On retrieved plate were made following investigations in order to establish the causes that lead to the failure: determining the chemical composition through spectral analysis, macrostructural analysis using stereomicroscopy, microstructure analysis using optical microscope metallographic, and fractographic analysis using scanning electron microscopy. Complex analysis of the fracture surfaces of the locking plate has led to the ultimate conclusion that the material has been made self-locking plate is inadequate chemical purity satisfactory embedding, which has led to breakage within an area of ​​non-homogeneous structure. Breaking looks fragile behaviour by transgranular cleavage with sharp facets. There were also highlighted intergranular cracks between planes of cleavage sides.

Abstract: Currently orthopedics challenge is the use of intramedullary nails to straighten diaphyseal femoral fractures. In this paper it was conducted a case study on an intramedullary nail used for femoral fracture stabilization, in the case of one young patient. The evolution was unfavorable to the fracture site and the implant failed after three month of first surgical intervention. Intramedullary nail dimensions were 300mm length and 10mm in diameter. The following investigations were made in order to establish the failure causes: determination of the chemical composition through spectral analysis, macrostructural analysis using stereomicroscopy, optical microstructural analysis using optical microscopy, fractographic analysis using scanning electron microscopy. The final conclusions showed that the metallic biomaterial used for manufacturing the intramedullary nail was approximately an austenitic stainless steel AISI 316L, but with major microstructural defects. Macro-structural analysis revealed the presence of two zones of cracking, which are very rare at austenitic stainless steels. Also breaking with a fragile character has radial front tear propagation. In longitudinal section, cracks were evidenced due to the extraction operation and also the presence of corrosion products was shown. From fractographic analysis it was determined that intramedullary nail failure was predominantly through the mechanism of brittle fracture that took place at the point of maximum bending of the implant fragile.