Failure analysis of connecting bolts in collapsed tower crane

The use of tower cranes can cause critical accidents during large building construction projects. Failure of the tower crane results in critical injuries to crane operators and other workers at the construction site. Issues that often cause the fatal collapse of tower cranes include the damage and fracture of the mast (or tower) and turntable parts, and the tower crane slewing‐unit‐support fleeing. Without systematic investigations and proper prevention procedures in place, this type of crane accident, which often leads to worker fatalities, will continue to occur. Therefore, more proactive, specialized and systematic research needs to be performed in terms of forensic engineering to minimize the occurrence of similar disasters. In this research, the tower crane collapse accidents are thoroughly investigated and the exact cause of the damaged crane parts is also identified using fractography and mechanical/chemical characterizations techniques, including visual inspections, field research, scanning electron microscopes and optical microscopes. Specifically, instrumented indentation technique is employed to evaluate the material properties, and energy spectrum analysis is used to determine the chemical characteristics. Based on thorough investigations, it has been identified that the connecting bolts of the tower crane are very vulnerable. It is recommended to improve the reliability of the bolts and to implement additional regulations to check their maintenance and safety. This research clearly provides guidance for investigating crane collapse accidents and contributes to the prevention of similar types of accidents. Several corrective actions and recommendations are suggested to reduce the risk of the tower crane failures.     

Failure analysis of a high-temperature thermocouple

A solid oxide fuel cell unit operates at high temperatures of 700 to 800°C. The operation of the unit is controlled by a set of instrumentation including sensors, thermocouples, and voltage leads. Two S-thermocouples were inserted in the after-burner where temperature is constantly at 1000°C. During operation of one prototype unit and after 1000 h, the thermocouples started to give erratic readings and led to complete shutdown of the unit. Analysis of the thermocouples revealed a series of events may have occurred due to stress induced by the design, materials and operating conditions, contamination during manufacturing, or a reducing environment caused by sheathing. The specification from S-type to B-type may remove the risk of reoccurrence. A design change in platinum burner thermocouples to ensure an oxygen-rich environment for the platinum was also recommended.     

Failure analysis of a pole gin

Lugs on a cast aluminum/fiberglass pole gin failed while raising an electrical transformer on a power pole. This same system had just lowered a heavier transformer. The pole gin consisted of a cast aluminum base that was strapped to a utility pole by a nylon belt-and-ratchet mechanism. A fiberglass pole was mounted in the base, and a pulley was attached to the other end through another aluminum casting. Rigging for the lift was complex and required a physical simulation to estimate actual lug hole loads and to determine that overall loading was within the manufacturer’s published limits. Possible abuse by hammer blows was evaluated by dynamic testing to measure force attenuation in the system. Results ruled out abuse as a factor. Literature revealed that the heat treatable Precedent 71A, or A771-T7 alloy, used for the base casting was very susceptible to stress-corrosion cracking (SCC). Evidence was observed for features indicative of creep-rupture damage on the fracture surface. Evaluation of all of the evidence led to the conclusion that time-dependent crack growth, most likely by both SCC and creep-rupture, plus the effect of bolt hole loading on crack growth could best explain the failure of the gin under a less severe condition than had just occurred earlier in the day.     

某型航空发动机压气机四级转子叶片失效分析

某型航空发动机压气机四级转子叶片是故障多发叶片.对叶片典型失效件进行了断口分析,发现叶片故障是由于叶片表面发生腐蚀导致疲劳强度降低,使得叶片在振动应力下发生疲劳失效.进一步对显微组织进行分析,发现叶片材质合乎要求,叶片腐蚀主要是由于使用环境因素造成的.研究结果对于叶片的故障分析及预防具有重要的意义.     

Failure analysis of a high-speed pinion shaft

The failure of a high-speed pinion shaft from a marine diesel engine was investigated. The shaft, which had been in service for more than 30 years, failed shortly after a service operation in which the bearings were replaced. Examination of the shaft revealed cyclic fatigue as the failure mechanism, with a substantial distribution of nonmetallic inclusions near the fracture initiation site. Fracture mechanics analysis indicated that the inclusions would be unlikely to have served as failure initiation sites if only stresses acting on the shaft were induced by normal service loads. Further examination of the bearing elements revealed an abnormal wear pattern, consistent with the application of elevated bending loads to the shaft after bearing replacement. The root cause of failure was determined to be an increase in service stresses after bearing replacement along with the presence of significant nonmetallic inclusions in the pinion shaft.     

Metallurgical failure analysis of titanium wing attachment bolts

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.     

Failure mechanisms of friction stir spot welds of AA6061‐T6/DP590 steel during tensile‐shear testing

The tensile‐shear test was conducted for the evaluation of shear load and failure mechanisms of dissimilar friction stir spot weldments of AA6061‐T6/DP590 dual‐phase steel sheets. The joints were fabricated using a penetrated pin into the steel sheet (lower member). Such design resulted in the formation of a hook at the joint interface and an intermetallic compound layer (IMC) between the upper part of the hook and Al‐side. A maximum tensile‐shear load of ~2950 N was measured for the joint fabricated using a heat input of ~10 kJ; a lower strength was noted by varying the heat input. Partial plug was the failure mechanism in all joints. However, three different types of crack propagation paths were found depending on the heat input: along the interface between 6061 Al and IMC layer, thru 6061 Al near the joint interface, or within the IMC layer using relatively low, moderate, and high heat inputs.     

Comparison of transmission and scanning electron fractography

The transmission electron microscope, with suitable replication techniques, was used extensively during the 1950s and 1960s to study the surfaces of metal fractures, both to understand the mechanism of fracture and for practical failure analysis. When the scanning electron microscope was introduced in the 1960s, these studies were greatly simplified by the lack of necessity for replication. The appearance of the fracture surfaces, however, was different in some cases from the appearance the fractographer was accustomed to seeing from his or her negative replica. This work was done to help translate from one technique to another.     

Failure Analysis of a Steel Motorcycle Kickstand

A fractured steel motorcycle kickstand was metallurgically investigated using a range of failure analysis tools [visual examination, energy dispersive X-ray (EDX) analysis, electron microprobe analysis (EPMA), scanning electron microscopy (SEM), fractography, optical microscopy, hardness testing and non-destructive testing (NDT)]. The steel kickstand’s composition, its microstructure, electron fractographs, and mechanical test results have been critically interpreted. Some evidence of wear damage, in the failed kickstand, was observed. The microstructural and fractographic analyses showed pre-existing micro-cracks which were believed to have grown to result in ductile failure followed by acceleration of corrosion. Recommendations have been made to avoid the failure of the motorcycle kickstand.     

SEM fractography and failure analysis of nonmetallic materials

This paper analyzes a collection of SEM fractographs compiled from 12 years of undergraduate and graduate level courses on fractography and failure analysis. Each nonmetallic material studied (glass, plastic, FRP, and wood) was fractured under controlled conditions so that the fracture was due mainly to one mode of loading such as tension, torsion, unidirectional bending, fatigue by reverse bending, and impact. With the aid of the stereomicroscope and the SEM, the fracture features of each sample were analyzed and fractographs obtained at a wide range of magnifications. The features and direction of crack propagation were correlated with the mode of loading which induced the fracture. From this study, correlations among the main fracture modes, micromechanisms, microfracture features, and loading conditions were compiled. Such correlations are invaluable for the proper interpretation of fracture features during failure analysis especially where little is known about the cause of the fracture. Reprinted from Microstructural Science, vol. 21, Metallographic Characterization of Materials Behavior, Proc. of the Twenty Sixth Annual Tech. Meeting of the International Metallographic Society, C.R. Brooks and M.R. Louthan, Jr., ed., The International Metallographic Society, Columbus, Ohio, and ASM International, 1994, pp. 121–134.     

Failure analysis in a vehicle accident reconstruction

A metallurgical and mechanical failure analysis was applied as part of a vehicle accident reconstruction of a multi-vehicle collision. One of these vehicles was a coal-hauling tractor-trailer. Examination of the trailer involved in the incident revealed a fatigue fracture to a primary lateral stiffener, along with a significant misalignment of the stiffener. Stress and fatigue analysis indicated that the misalignment severely degraded the fatigue life of the stiffener. Evaluation of the structural dynamics of the trailer after the fatigue fracture indicated decreased lateral stability. The decreased stability caused by fracture of the lateral stiffener allowed rollover of the trailer to occur while negotiating a curve. The failure sequence developed in this investigation proved consistent with all physical damage observed on the trailer and with witness accounts of the incident. The failure scenario developed in this investigation is compared with other conclusions made by other investigators to show that those conclusions are not consistent with all of the available evidence.     

Failure analysis of a pilot scale melter

Failure of the pilot-scale test melter resulted from severe overheating of the Inconel 690 (690) jacketed molybdenum electrode. Extreme temperatures were required to melt the glass during this campaign because the feed material contained a very high waste loading. Metallurgical evaluation revealed the presence of an alloy containing nickel and molybdenum in several ingots found on the bottom of the melter and on a drip which had solidified on the electrode sheath. This indicates that a major portion of the electrode assembly was exposed to a temperature of at least 1317°C, the nickel/molybdenum eutectic temperature. Small regions on the end of the 690 sheath showed evidence of melting, indicating that this localized region exceeded 1345 °C, the melting point of 690. In addition to nickel, antimony was found on the grain boundaries of the molybdenum electrode. This also contributed to the failure of the electrode. The source of the antimony was not identified but is believed to have originated from the feed material. Metallurgical evaluation also revealed that nickel had attacked the grain boundaries of the molybdenum/tungsten drain valve. This component did not fail in service; however, intergranular attack led to degradation of the mechanical properties, resulting in the fracture of the drain valve tip during disassembly. Antimony was not observed on this component.     

Failure analysis of brass bolt from mausoleum

Brass is usually considered to be a good candidate material for use in a nonaggressive corrosion environment. Brass 260 was used in a rural environment (at a mausoleum) as fixturing hardware that holds etched marble slabs that cover the interment region. After 12 years of service, the bolts began breaking, and the marble slabs crashed to the ground. Failure analysis investigation showed that “seasonal cracking” occurred due to interaction of the bolt with moisture and fertilizer. The company that designed the mausoleum hardware believed that the bolt manufacturer was at fault. However, the only specification given to the bolt manufacturer indicated alloy and dimensions. The bolts were replaced at considerable cost, proving once again that even a seemingly simple component can prove to be costly if not designed by the proper engineer.     

Failure analysis of a polysulfone flow sensor body — A case study

A failure analysis was conducted on a flow-sensing device that had cracked while in service. The polysulfone sensor body cracked radially, adjacent to a molded-in steel insert. This article describes the investigative methods used to conduct the failure analysis. The techniques utilized included scanning electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermomechanical analysis, and melt flow rate determination. It was the conclusion of the investigation that the part failed via brittle fracture, with evidence also indicating low cycle fatigue associated with cyclic temperature changes from normal service. The design of the part and the material selection were significant contributing factors because of stresses induced during molding, physical aging of the amorphous polysulfone resin, and the substantial differential in coefficients of thermal expansion between the polysulfone and the mating steel insert.     

空间斯特林制冷机失效分析

针对空间用的"牛津型"斯特林制冷机的结构特点,对其早期失效模式和正常失效模式等主要失效模式进行了较深入分析,提出了改进措施.通过结构设计、工艺等改进,可以消除早期失效模式、延长正常失效的时间.     

Ti6Al_4V人工膝关节的腐蚀疲劳破坏

针对一实际病例,通过具体人工膝关节的受力分析和断口SEM观察及金相观察,说明和解释了该人工膝的失效型式和原因,估算了寿命并提出了改进措施。     

Failure analysis of automotive battery parts

In the injection molding process defective pieces are sometimes detected, showing weak points which break easily. In this work, the possible causes of these failure points are evaluated. Thermal analysis techniques were applied to the material to better understand the degradation that takes place with relation to the transformation process. Simulations were made to determine the possible causes of deterioration and at the same time reproduce the effects of this fragility. The creation of the weak points was simulated and their origins were determined.     

How to best smash a snail: the effect of tooth shape on crushing load

Organisms that are durophagous, hard prey consumers, have a diversity of tooth forms. To determine why we see this variation, we tested whether some tooth forms break shells better than others. We measured the force needed with three series of aluminium tooth models, which varied in concavity and the morphology of a stress concentrating cusp, to break a shell. We created functionally identical copies of two intertidal snail shells: the thicker shelled Nucella ostrina and the more ornamented Nucella lamellosa using a three-dimensional printer. In this way, we reduced variation in material properties between test shells, allowing us to test only the interaction of the experimental teeth with the two shell morphologies. We found that for all tooth shapes, thicker shells are harder to break than the thinner shells and that increased ornamentation has no discernible effect. Our results show that for both shell morphologies, domed and flat teeth break shells better than cupped teeth, and teeth with tall or skinny cusps break shells best. While our results indicate that there is an ideal tooth form for shell breaking, we do not see this shape in nature. This suggests a probable trade-off between tooth function and the structural integrity of the tooth.     

Failure analysis of a gear wheel of a marine azimuth thruster

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.     

突跳式温控器的失效分析

突跳式温控器失效会导致电茶壶、电饭煲等家电产品控温和过热保护功能失效.采用FMECA分析方法,确定导致突跳式温控器失效的关键元件,通过关键元件失效分析,得到突跳式温控器的失效机理,提出提高突跳式温控器工作可靠性的技术措施.FMECA分析表明,突跳式温控器的主要失效模式为触头粘结、簧片断裂和温度漂移,引起突跳式温控器失效的关键元件为触头、簧片、热双金属片.关键元件失效分析表明,触头粘结现象是由于触头燃弧引起的,簧片断裂现象是由于簧片受到的疲劳应力超过疲劳强度引起的,温度漂移现象是由于热双金属片尺寸参数改变或热双金属片层间分离引起的.