Metallurgical failure analysis of fasteners in an impeller assembly

This paper examines a failure analysis of the bolts from a failed joint between an impeller blade and a rotating assembly unit. The bolts failed due to poor thread manufacture and installation. Sharpened thread roots led to high stress concentrations that favored crack initiation. An oddly shaped thread profile allowed friction between mismatched thread surfaces. Poor installation procedures allowed for the possibility of overtightening to nucleate cracks in the head-to-shank interface (which had a smaller radius and therefore a higher stress concentration) and possibly also in the thread roots. Each of these influences contributed to crack initiation in the bolts. After cracks had formed, bending fatigue then propagated the nucleated cracks to final fracture. The failure analysis also recommended using bolts with rolled threads, which allow a more complete fit between mating male and female threads, and assuring that an appropriate preload is placed on bolts during installation.     

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.     

Torsion fatigue failure of bus drive shafts

The mystery surrounding high failure rates in the drive shafts of a large municipal transit agency's fleet of 40 newly acquired articulated buses is investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel such as AISI 5046. An examination of the drive shafts on all 40 buses is conducted, and 6 different drive shaft designs are identified among the fleet, but all of the failures, 14 in all, are limited to just one of the identified designs. Microscopic examination of the fracture surface of one of the failed drive shafts under a scanning electron microscope is conducted to determine the failure mode. Evidence of high-cycle fatigue is found, and a finite-element analysis is conducted to compare the maximum stress of the design exhibiting failures with the most common of the other designs that exhibits no failures. A fatigue life prediction is performed to determine just how much longer the expected fatigue life of the surviving design is compared to the design that suffered the early failures.     

Metallurgical failure analysis of a horse trailer: A criminal investigation

Following the deaths of three people involved in a vehicular collision with an ejected horse, criminal charges were brought against the horse’s owner. Failure analysis was performed on the horse’s trailer to determine its roadworthiness. The failure mode of the hinge between the door and frame became the focus of the investigation. The analysis concentrated on proving whether or not the hinge was broken prior to the collision; that the trailer was not roadworthy; and whether the trailer was capable of safely transporting two horses on a public freeway. Analysis using optical microscopy and the scanning electron microscope indicated the hinge was significantly corroded and mechanically deformed. Additionally, there was evidence of paint on an alleged fresh fracture surface. The results were presented at the trial of the defendant.     

新型阻燃β钛合金的断裂行为

测试了Ti-25V-15Cr-0.3Si钛合金的力学性能，并运用金相显微镜（OM）、扫描电镜（SEM)主能量散射谱仪（EDS)其不同状态下的金相组织、热压及拉伸断口进行了形貌观察与成分定量分析。结果表明：该阻燃合金具有一定的耐热性，且在不同试验条件下呈现不同的断裂特征；合金状态可以显著影响断裂行为；断口上存在的Fe、Ni、W、S、O等杂质元素相是导致合金过早断裂的内在原因，设法提高冶金质量是改善合金加工性能及力学性能的关键。     

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.     

椭圆形路径载荷下的微动疲劳失效特征

为了解微动疲劳失效机理,通过柱面对柱面的接触方式,研究了60Si2Mn钢在椭圆形路径、拉扭耦合作用下的多轴低周微动疲劳特性,深入分析讨论了不同轴向循环拉伸应力幅值对摩擦磨损表面和断口形貌的影响.结果认为：磨损区产生的氧化物磨屑对微动区磨擦损伤行为具有显著影响;微动摩擦磨损对试样表面的影响深度只有数十微米;微动疲劳裂纹源...     

Failure analysis of cylinder clamping rods in diesel engines

This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200–250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.     

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 broken tooth

Several days after heart surgery, a patient discovered his upper right canine tooth had broken at the root. Such tooth damage, recognized post-operatively, is usually assumed to be caused by blunt mechanical force from an instrument used by the anesthesiologist during placement of a breathing tube at the start of surgery. In this case, the patient had saved the crown portion of the broken tooth, and it was possible to examine the root fracture characteristics. The curvature and direction of the crack path and natural tooth situation suggested that failure could be described through a cantilever beam model. This was confirmed when a whole extracted sample tooth was embedded and broken by a measured force in a manner consistent with the model. The resulting fracture surface matched that of the patient’s broken canine tooth. However, the high load and force direction necessary to fracture the root was inconsistent with forces applied during the anesthesia procedure. The failure analysis and further investigation indicated tooth clenching on the breathing tube during recovery was the likely cause of fracture. This paper presents an alternate explanation for intubation-related dental injury, demonstrates the practicality of fractographic analysis of biological materials, and introduces a methodology for simulating in vitro tooth settings for mechanical testing.     

Investigation of a Boeing 747 wing main landing gear trunnion failure

A loud noise was heard from the vicinity of the port wing landing gear during pushback of a Boeing 747-300 from the terminal at Sydney (Australia) airport. Inspection showed that one of the wing landing gear trunnion fork assemblies had failed. Detailed investigation revealed that the trunnion had failed by fatigue cracking. Deep machining grooves were found at the root of an internal radius that had not been shot-peened as required, and a chemical surface process during manufacture had resulted in shallow intergranular attack at the bottom of these grooves. It is probable that the critical cracking started from some of these grooves. In addition, the wall thickness at the failure location was significantly less than the minimum required in the drawings.Since the deep machining grooves, the lack of peening and the intergranular attack were all consequences of manufacturing, the fatigue cracking probably started shortly after the component entered service. This implies that fatigue cracking was present during all the trunnion overhauls, but was not detected by non-destructive inspections during the overhauls. Quantitative fractography was used to produce a crack growth curve based on fracture surface markings thought to represent the overhaul timings. The crack growth curve suggested that the fatigue cracking was large enough to be detected by inspection during the last overhaul, if not the one before. However, it was probably not easy to detect the cracking. This investigation therefore highlights the difficulties that can be encountered when inspection is the last (or only) line of defence against failure owing to unexpected manufacturing deficiencies.     

Metallurgical analysis of a corrosion failure

A Nicrofer 6025 HT alloy 602 tube in a furnace, used for the recovery of steel pickling liquors, failed during operation at 1100°C. Metallographic investigations—microstructural and energy dispersive X-ray spectrometry (EDS)—of the failed tube showed evidence of corrosion and the presence of chlorine and sulfur in the corrosion products. The failure of the furnace tube was due to the presence of sulfur dioxide in combination with chlorine.     

Steel hardenability and failure analysis

Hardenability evaluation is typically applied to heat treatment process control, but can also augment standard metallurgical failure analysis techniques for steel components. A comprehensive understanding of steel hardenability is an essential complement to the skills of the metallurgical failure analyst. The empirical information supplied by hardenability analysis can provide additional processing and service insight to the investigator. The intent of this paper is to describe some applications of steel thermal response concepts in failure analysis, and several case studies are included to illustrate these applications.     

Applicability of damage models for failure analysis of threaded bolts

Damage models have been applied in engineering applications. In the present work both Gurson–Tvergaard–Needleman (GTN) model and the progressive damage model have been employed to describe plastic deformation and damage behaviour in the tensile bars and threaded bolts. The predictive capabilities of the models are critically assessed. The parameters in the damage models are identified from uniaxial tensile tests. The models are applied to predict failure of the threaded bolts. Numerical results reveal both models agree with experimental data in uniaxial tension and in threaded bolts. The overall results do not show substantial differences. However, damage evolutions from both damage models display very different developments. Whereas the GTN model confirms damage nucleation in the specimen center of the uniaxial tensile bars, as observed in experiments, the progressive damage model predicts damage starts from the specimen surface where the plastic strain reaches its maximum. In threaded bolts both models do not show significant deviations in predicting the ultimate loading. The progressive damage model without stress triaxiality consideration predicts an unrealistic ductile fracture process and cannot be applied to metallic materials.     

Self-sensing time-domain reflectometry for detection of the bearing failure of a CFRP laminate fastener hole: effect of peeling of the insulator

Carbon-fiber-reinforced polymer (CFRP) composites are used in aerospace and automobile structures. For many CFRP structures, mechanical metallic fasteners are adopted. In internal structures such as a wing box, damage to CFRP structures around the fastener holes is difficult to see. A simple method of finding damage around fastener holes is thus required for CFRP structures. This study applies self-sensing time-domain reflectometry to detect bearing failure around fastener holes of CFRP structures. In a previous paper, the effect of fasteners was minimized when the micro-strip line (MSL) was placed apart from the fasteners. This study numerically and experimentally investigates the peeling of glass-fiber-reinforced polymer (GFRP) insulator. A bearing failure results in embossing at the edge of a fastener hole. The embossing produces a wedge that causes the peeling of GFRP, generating a gap between the GFRP and CFRP. The gap strongly affects the characteristic impedance of the MSL.     

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.     

Metallurgical failure analysis of a BWR recirculation pump shaft

This paper presents the metallurgical analysis of a Recirculation Pump from a Nuclear Power Plant. The damaged component was the austenitic stainless steel shaft. As a result of a preliminary visual examination, some axial cracks were found in the threaded area of the shaft in the thermal barrier area. The analyses performed include liquid penetrant testing, metallography and deposits analysis. Based on the obtained results, it was concluded that the origin of the axial cracks was thermal fatigue caused by mixing of cold seal injection water and hot reactor coolant in the thermal barrier area.     

Quantitative fractographic analysis of impact fracture surfaces of steel R73

Macroscopic images of fracture surfaces of Charpy test specimens of steel R73 were studied, where bright spots in images represent cleavage facets or ductile dimples, respectively, both in special orientations. Within image analysis, they may be taken for the most significant textural element. Being the brightest patches in the image, they can be extracted by thresholding. Their counts and area distribution are closely related to temperature and impact energy. __________ Translated from Problemy Prochnosti, No. 1, pp. 77–80, January–February, 2008.     

Retrieving Effective Stress Spectrum of Fatigued Components from Fractographic Quantitative Analysis

Under the constant amplitude and pro-grammed loading conditions the fatigued fracturesurfaces of 2024-T3 sheet are examined by SEMand TEM.A corrected method,in which the effectsof loading sequence are taken into account,hasbeen developed for the retrieval of effective stressspectrum.The parameters including d,a,C,n,R,αand σ_y are evaluated according to their influenceson the retrieved results.Several computer programshave been produced for the fractographic quantita-tive analysis,and the application of image pro-cessing technique in the fractographic analysis is al-so interpreted.     

正方形钛试样表面等离子体基离子注入氮的保留剂量研究

为了研究靶形状对等离子体基离子注入均匀性的影响，应用Auger电子能谱(AES)剖面分析，测量并计算了正方形靶表面排布的Ti试样经氮等离子体基离子注入(PBⅡ)的保留剂量分布．由于N的Auger电子发射能量与Ti的完全重合，而且表面氧化层对Ti的Auger跃迁产生很大的影响，因此应用AES对于氮等离子体基离子注入钛基金属进行化学分析变得较为复杂．本文探讨了一种既解决峰位重叠问题又考虑氧化层影响的方法，从而能够根据，AES溅射剖面分析数据计算浓度深度曲线，并获得保留剂量．正方形靶表面不同位置处的剂量测算结果表明，靶的形状对于保留剂量有着很大的影响，造成剂量呈梯度分布．同时还发现，Ti氧化层随着保留剂量的增大而增厚。