Case studies in engineering failure analysis最新文献

In this study, the fracture failure of the 4Cr13 stainless steel linkage components in circuit breakers was studied. The microstructure and morphology of fracture surface were observed by scanning electron microscopy and optical microscopy. A micro-Vickers tester measured the hardness of the components. The tensile strength and strain of the components were determined by a universal testing machine. The results show that fracture failure mode was quasi-cleavage fracture, and some dimples and edges of cleavage were present on the fracture surface. The substandard sample exhibited higher hardness than the standard sample. The high hardness could cause the strong rigidity and less toughness, which was not helpful to bend the samples to form hook structure. The heat treatment influenced the mechanical properties of the 4Cr13 components. With increasing the tempering temperature, the hardness of the component was decreased when the quenching temperature was kept stable.

A temporary threadbar used for precast segmental construction broke during the post-tensioning stage before reaching the service load. Failure analysis showed that the premature failure of the bar was due to the presence of an arc strike. The arc strike effects on the material and threadbar integrity are considered.

This study presents a failure analysis of a femoral orthopedic implant. This implant is a locking compression plate that was fixed to a broken femur using two types of screws: locking and compression screws. The study elucidates the causes of an in situ premature failure of the plate and screws. Chemical analysis, hardness tests, and microstructural analysis confirmed that the implant was manufactured from cold-worked stainless steel 316L. The macro and micro fractographic analyses revealed that the failure mechanism was high-cycle fatigue and that the implant underwent approximately 10⁶ loading cycles before failure. A finite element analysis of the assembly indicated that the crack initiation sites are located in the region where the highest stresses are observed. This numerical analysis confirmed that walking induces the loading condition needed for this specific failure. According to this loading condition and to the material properties, the stress amplitude that initiated and propagated the crack is estimated to be in the range of 400 MPa. Several considerations, both mechanical and medical, are discussed in order to explain the failure and to improve the system durability. From an engineering perspective, implant geometry and installation procedure could be optimized in order to reduce the stress concentrations that developed near the crack origin.

This paper investigates the failure of a rocker arm shaft of a passenger car. The shaft failed by brittle fracture across one of the four holes supporting the shaft into the cylinder head. The running distance of the engine just before failure was 40,626 km. Visual examinations of etched sections of the failed shaft and a new one revealed four distinct zones of darker etching appearance. These zones correspond to the four locations where the rocker arms fit the shaft.

Microscopic observations of the failed shaft revealed that the four dark-etching areas are surface hardened zones of martensitic microstructure. Furthermore, scanning the microstructure along the failed shaft showed that the heat treatment was so mistakenly extended by excessive heating so that the structure of the shaft near the supporting holes contains considerable content of martensite phase. This conclusion has been confirmed by the results of hardness measurements along the surface of the shaft.

Microscopic investigations of the failed shaft revealed the presence of microcracks close to the supporting holes. These cracks may have been induced in the shaft by the non-uniform cooling during quenching in the course of heat treatment, or may be nucleated by repeated loading during service. This premature failure has occurred by the rapid crack propagation because of the lower fracture toughness of the martensite.

The purpose of this study was to investigate the main causes of the co-fracture of sixteen connecting bolts of a filter press cylinder–piston system. Stress state of the bolts during the service conditions has been analyzed and the failure reasons were determined from the fractography analysis and gathered information. According to the obtained results, it was concluded that the bolts had failed by the fatigue mechanism. It seems that insufficient torque was used during assembly.

A peculiar type of edge discoloration defect on the surface of some galvanized fuel tank was observed, causing significant appearance problems. In the present study, the surface defect was characterized by visual inspection, optical microscopy, scanning electron microscopy and energy dispersive spectroscopic analysis to understand the source and mechanism of the defect. In the visual inspection, these peculiar surface appearances were observed in fuel tank at three distinct locations. The SEM examination exhibited two distinct regions on the surface apart from the normal galvanized surface: (1) galvannealed, (2) mixture of galvanized and galvannealed texture. The energy dispersive spectroscopic analysis of galvannealed region indicated enrichment of Zn and Al whereas in the region of galvanized majorly Zn was observed. Surface texture of galvannealed region showed majorly zeta crystals along with skin pass marks; whereas no such zeta crystals were observed in case of galvanized regions. Based on the preliminary results, the following hypothesis was made: Coil processed during galvanizing to galvannealing transition. Thickness and width changed to wider and thicker section, which resulted into lower line speed. Due to the lower Al content, lower speed and thicker section combination resulted in formation of partial GA in the coil owing to the internal heat content of the coil. This paper presents the results of the investigation.

In this work we present a technical description related to the rupture of a titanium alloy pedicle screw and connecting bar implanted in dorsal vertebras of a patient. Only metallurgical facts are described, with no attempt to identify any imperfections in the clinical aspects related to the rupture. The results described here are based on extensive analysis of the broken materials in a material sciences specialized laboratory. Excluding an incorrect prosthesis implantation in the surgical procedure and a possible low bone density, an information not available to the research team, with high probability the rupture of metallic pieces used in the prosthetic implant, was produced by the low fatigue resistance resulting by an improper machining process and excessive bending of the connecting bar prior to implant.

Tyre bead grade wire is used for tyre making application. The wire is used as reinforcement inside the polymer of tyre. The wire is available in different size/section such as 1.6–0.80 mm thin Cu coated wire. During tyre making operation at tyre manufacturer company, wire failed frequently. In this present study, different broken/defective wire samples were collected from wire mill for detailed investigation of the defect. The natures of the defects were localized and similar in nature. The fracture surface was of finger nail type. Crow feet like defects including button like surface abnormalities were also observed on the broken wire samples. The defect was studied at different directions under microscope. Different advanced metallographic techniques have been used for detail investigation. The analysis revealed that, white layer of surface martensite was formed and it caused the final breakage of wire. In this present study we have also discussed about the possible reason for the formation of such kind of surface martensite (hard-phase).

A failure analysis investigation was performed on a fractured polymer impeller used in a respiratory blower. Light microscopy, scanning electron microscopy and finite element analysis techniques were utilized to characterize the mode(s) of failure and fracture surfaces. A radial split down the impeller center was observed with symmetric fracture faces about the impeller bore. Fractographic analysis revealed brittle fracture features including Wallner lines, mirror, mist and hackle features stemming from the impeller bore, emanating radially outward. Crazed fibrils and faint fatigue striations suggest that intermittent load cycling led to initiation, and rapid propagation of multiple crack fronts originating along the impeller lip. Finite element analysis revealed a flexural condition induces localized stresses along the impeller lip. Significant wear features were also observed within the impeller bore, which may have contributed to premature failure of the impeller. The brittle fracture morphology and defects within the impeller bore suggest that premature failure occurred because of multiple interacting factors including: intermittently high centrifugal velocities, imbalance bore and shaft conditions, defects within the bore caused by machining, and stress concentrations along the circumference of the impeller lip.

In general technical applications, truck heavy duty engine flywheel starter ring gears teeth is conventionally induction hardened and tempered in order to meet metallurgical aspects on specification. Especially for large sectioned starter ring gears (in this case diameter of the gear is 470 mm) the critical issue is to maintain a stable hardness distribution on gear cross sections. These hardness variations in process could yield to:

• 1.

  High scrap costs due to out of specification parts.

• 2.

  Increase in tact time in process, decrease in labor efficiency due to scrap rate and excessive hardness measurements.

• 3.

  Potential risk of failure on engine in low mileages.

• 4.

  100% hardness measurements in ongoing process which leads to time and cost waste.

This technical paper summarizes the case study conducted for truck flywheel starter ring gears with diameter of 470 mm in order to reduce hardness variation by optimization of induction hardening and tempering processes.