汽车方向盘自挤螺钉断裂分析

采用断口分析、金相检验、显微硬度测试、化学成分分析、氢脆预载荷试验等方法对某型号乘用车方向盘固定螺钉的断裂原因进行了分析。结果表明：螺钉的断裂性质是氢致延迟断裂,裂纹起源于螺钉头下第一牙底。电镀后驱氢不充分致使螺钉表层残留较高含量的氢,表面渗碳层及首牙处的应力集中构成了导致氢脆的3个敏感因素。     

SWRCH22A十字头螺钉断裂分析

SWRCH22A盘条生产的十字头螺钉在进行扭力和攻速测试时发生断裂。通过对盘条和螺钉的组织和断口进行分析,探讨了断裂的原因。结果表明:在螺钉生产的搓丝过程中形成的齿间严重表面裂纹是造成螺钉断裂的根本原因。此外,盘条冷镦前球化退火工艺不良造成表面严重脱碳并形成粗大铁素体,而后表面过度渗碳处理致使螺钉表层严重脆化,并在渗碳层下形成铁素体+马氏体过渡层组织和沿晶渗碳体,螺钉表面裂纹在最大剪切应力作用下通过沿晶断裂最终导致螺钉失效。     

Experimental and Simulative Failure Analysis of AISI 316L Stainless Steel Screw Shaft

This paper presents a study on failure analysis of a screw shaft in a pure terephthalic acid screw conveyor after about 6-year operation. A series of characterization analysis were performed by careful visual observation, mechanical properties tests, chemical content examination, microstructural analysis and microchemical analysis of nonmetallic inclusions. Furthermore, the outer surface topography of the shaft at the fracture position was measured by three-dimensional scanning method. Finite element method considering surface topography was also employed to define the evolution of critical region. Results reveal that the serious stress concentration resulted from structure discontinuity including small radius at the transition and surface roughness was the main reason leading to the crack initiated and propagated, and the latter one was proved to be the biggest driving. To prevent the failure, better controls of surface state and structure size were strongly recommended.     

吊车转盘连接螺栓断裂分析

吊车转盘后部的连接螺栓发生断裂,通过化学成分分析、宏观和微观检验等方法对断裂原因进行了分析。结果表明：螺栓为疲劳断裂,螺纹根部的细小裂纹是导致螺栓发生疲劳断裂的主要原因;螺栓松动后受到弯曲载荷是引起螺栓发生疲劳断裂的诱因。     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Fracture study of a polyacetal resin

Fracture experiments have been carried out with unnotched and notched tensile specimens of a polyacetal resin at room temperature at various rates of extension. An increase of approximately 13% in yield stress was observed in the unnotched tests with increased deformation rates from 1–1000 mm min^–1, whilst strain to failure was reduced from about 85% to approximately 0.05%. In all specimens, failure appeared to have taken place by initiation of a microscopic flaw upon yielding, and this flaw then slowly grew into a critical size when catastrophic fracture set in. In the slow-growth region, the mechanism of failure was by void growth whereby the lamellar texture was transformed into a fibrillar one. However, on the rapid fracture surface, there was evidence of lamellar texture being retained, but with small voids in the stacks of lamellae, In notched fracture specimens containing sharp razor cut, a fracture toughness, K _lc, of approximately 5 MPa m^1/2 was obtained between crosshead speeds of 0.5 and 50 mm min^–1. At speeds of 500 and 1000 mm min^–1, the K _lc was reduced to 4 MPa m^1/2. In the absence of a starter crack, blunt notch fracture toughness of about 6.3 MPa m^1/2 was observed at all test speeds. In specimens with a razor cut, the slow-growth region decreased as test speed increased; this can be used to construct an R-curve.     

Failure analysis in a dental implant

A Ti–6Al–4V dental implant failed catastrophically after 6 months of service. This kind of failures is rare in the short term, 6 months; however in this case, a fracture was presented at the inner screw used for fixation of the abutment. The failure examination included visual inspection, chemical analysis, metallography, microhardness testing, as well as macroscopic and microscopic fractographic observations using scanning electron microscope with EDS (Energy-dispersive X-ray spectroscopy) were made. Additionally finite element analysis (FEA) was undertaken in order to clarify the stress scenario of the failure. Based on the examination results, the failure was promoted by bone re-sorption leading a cantilever condition resulting an overload system with cyclic high level stresses. This condition along with the rough surface finish found in the screw and the concentration factor by geometry change, caused a crack which was propagated until the failure of the component.     

Pull-out strength of screws from cortical bone in the maxillo-facial region

The fixation of maxillofacial fractures is an important clinical procedure, which may be achieved by the attachment of plates across the fracture. The stability of the fracture will depend on the stiffness of the fracture fixation plates and the security of the fixation screws to the thin maxillofacial cortical bone. The design of screws, manufactured by Champy and AO were tested from both mini-and micro-fixation systems. Pull-out tests were conducted on cortical bone plates, ranging in thickness from 0.6 to 3.5 mm. No significant differences were observed in the ultimate pull-out forces achieved for both mini-systems of 2 mm outer diameter. However, these pull-out forces were generally greater than those obtained for the micro-screws, even at the lower bone thicknesses. Two models were developed which attempted to predict the behaviour of screw pull-out failure. The failure mechanism was primarily dependent on the thickness of the bone, with secondary influences related to the shear strength of the bone and a geometrical factor of the screw.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Ermittlung von Referenztemperaturen nach dem Master‐Curve‐Konzept

Application of the Instrumented Impact Test for the Determination of Reference Temperatures Using the Master Curve Concept The instrumented impact test is suitable for the determination of fracture mechanical parameters. In this paper the determination of the dynamic fracture toughness values in the lower ductile‐to‐brittle transition region is presented. The fracture toughness is determined at the onset of cleavage fracture and evaluated by the Master Curve (MC) concept. The MC concept allows to quantify the variation of fracture toughness with the temperature within the lower ductile‐to‐brittle transition region. Limit curves of fracture toughness for defined failure probabilities and a reference temperature can be determined using this method. This paper presents the application of the master curve concept to the reference temperature determination through the thickness of reactor pressure vessel (RPV) steel plate. The reference temperatures determined dynamic fracture toughness values (T₀^dy) are compared with quasi‐static reference temperatures (T₀^st) and Charpy‐V transition temperatures (TT). T₀^dy, T₀^st and TT increase from the surface to the middle of the RPV steel plate. Compared with T₀^st, the T₀^dy values are higher approximately 70 to 90 K.     

氢气压缩机缸盖螺栓断裂失效分析

某石化公司氢气压缩机缸盖螺栓发生断裂,采用宏、微观断口分析、化学成分分析、金相检验以及力学性能测试等试验手段分析了该氢气压缩机缸盖螺栓的断裂原因。结果表明：断裂起源于螺杆与螺栓头部连接螺纹末端的应力集中处,由于应力集中,加之螺栓强度等级偏低,使螺栓在长期工作过程中的往复交变应力和扭转应力共同作用下发生了低应力高周疲劳断裂。     

Some aspects of the fracture behaviour of Mg65Cu25Y10 bulk metallic glass during room-temperature bending

The fracture behaviour of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass (BMG) during room-temperature three-point bending was investigated. The BMG was initially produced by casting into a wedge-shaped mold which generated an amorphous structure below the ∼4 mm thickness zone of the wedge. Three-point bend testing was then carried out on the BMG with the fracture angles and salient features of the fracture surfaces examined by scanning electron microscopy. Observations indicate that this type of deformation mode results in fracture via crack propagation from both surfaces of the samples where the tensile and compressive stresses are greatest. The direction of crack propagation was also found to deviate considerably from 45° to the length direction of sample. A scanning electron microscopy (SEM) study of the fracture surfaces indicated that deformation banding was a feature of crack propagation within compressive zone whereas the tensile zone generated a featureless surface characteristic of brittle failure. The mechanism of failure of the present alloy is discussed on the basis of the observed features on the fracture surfaces and the direction of propagation of cracks during failure and compared with the failure mechanism of samples fractured under both simple tension and compression.     

Failure Analysis of a Pellet-Mill Die

One of the most important parts for pellet-mills is the die, since the die transforms raw materials in small cylinders called pellets. In this paper, a failure analysis was conducted for a pellet-mill die that had not reached its service life expectancy. The failure analysis consist of a characterization of the material using atomic emission spectroscopy, hardness measurements, optical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy by SEM. Additionally, visual inspection and fractography of the fracture surfaces and FE analysis were performed. It was found that the die material was a CA40 alloy, which microstructure consists of a martensitic matrix with finely dispersed carbides of the type M₂₃C₆. Also, a non-common level of inclusion type Al₂O₃ was found on the microstructure of the die material. According to the fractographic analysis, the crack initiation was located in a high-stress concentration region on the counterbored holes of the die, and also aided by the inclusions on the structure. Crack propagated along the material as an intergranular brittle fracture.     

Effect of hydrogen on stress corrosion cracking of copper

The effects of hydrogen on electrochemical behavior and susceptibility of stress corrosion cracking (SCC) of pure copper were studied. SCC susceptibility of pure copper in a 1 M NaNO₂ solution was increased by pre-charged hydrogen. The effect of hydrogen on the susceptibility is more obvious in the low stress region due to the longer fracture time, which resulted in a longer time for more hydrogen to diffuse toward the crack tip. Synergistic effects of hydrogen and stress on corrosion and SCC processes were discussed. The results showed that an interaction between stress and hydrogen at the crack tip could increase the anodic dissolution rate remarkably.     

Fracture analysis for ceramic ball in backflow valve

Ceramic balls have been used as components of devices, such as those found in high‐pressure pumps for automobiles and industrial machines. In the backflow valve, for example, a ceramic ball is in contact with a conical surface. Fractures of ceramic balls are extremely rare. It is important to investigate the cause of these rare fractures to guarantee higher reliability in the backflow valve. In this paper, the fracture mechanism and strength are discussed for an equivalent normal stress σ_eq beneath the contact region and the maximum principal stress σ_p near the contact boundary using stress intensity from fracture mechanics. The fracture surface of the ceramic ball was formed perpendicular to load direction. We assumed that fracture origins (defect/crack) existed on lines through three high stresses that analysed by finite element method. Actual fracture of a ceramic ball was found to be caused by the equivalent normal stress beneath contact region and not to be caused by the Hertz principal stress. Stress intensity factor (SIF) was clarified to depend on pressure, taper angle, CrN‐coating thickness and the friction factor of the inside of the valve hole. A pre‐existing defect size involved in failure was estimated by the SIF using three‐dimensional elliptic defects and equivalent normal stress. Therefore, the actual fracture of a ceramic ball, which rarely occurs, could be evaluated by considering three‐dimensional elliptic defects and the Weibull distribution of defects.     

轮胎螺栓断裂失效分析

某汽车轮胎螺栓在使用过程中发生断裂。采用宏、微观检验和化学成分分析等方法对失效件进行了检测。结果表明，其断裂形式为弯曲疲劳断裂。疲劳裂纹的形成是由于螺纹牙底存在脱碳和折叠裂纹等缺陷，这些缺陷的存在使该材料产生了较大的应力集中，从而导致螺栓开裂失效。     

集装箱旋转锁轴失效分析

铸钢锁轴在加工螺纹时轴杆端头断裂。对断裂件进行了化学成分和宏、微观分析,发现断裂锁轴的碳含量超标,并存在中心缩孔、严重区域偏析和局部共晶组织。据此对生产现场作了工序调查,发现浇注温度过高和直浇道欠浇是引起成分偏析及组织缺陷并导致断裂的主要原因。     

Effect of crosshead speed on the fracture toughness of Soda-lime glass, Al2O3 and Si3N4 ceramics determined by the surface crack in flexure (SCF) method

The fracture toughness of soda-lime glass, Al₂O₃ and Si₃N₄ specimens was measured by the surface crack in flexure method. For the soda-lime glass specimens, the fracture toughness was calculated from the initial crack size and flexure strength, and the value increased with increasing crosshead speed. This trend seems to be related to the difficulty in determining the critical crack size at fracture, since slow crack growth occurs during bending test. For the Al₂O₃ specimens, a halo region (stable crack growth region) was formed around the initial precrack during bending test. The halo size increased and the resultant flexure strength decreased with decreasing in the crosshead speed. The halo region, however, could not be observed in the Si₃N₄ specimens. Despite of the difference in the appearance of halo region, the fracture toughness of the Al₂O₃ and Si₃N₄ specimens was constant irrespective of the crosshead speed when the values were calculated with the critical crack sizes at fracture (halo incorporated crack sizes). The constant fracture toughness with the crosshead speed could be explained by the relation between the changes of halo size (thus critical crack size at fracture) and resultant flexure strength.     

Effect of composition on fracture behavior of polypropylene–wollastonite–polyolefin elastomer system

The fracture behavior of polypropylene (PP)–wollastonite–polyolefin elastomer (POE) in the mixed mode region was studied using the essential work of fracture (EWF) method. The relationship between the microstructure and the fracture parameters was analyzed. The effect of wollastonite content on the essential work of fracture and the work of plastic deformation was discussed. The energy dissipation during a double-edge-notched tension (DENT) test was calculated with the EWF method. It was found in the mixed mode region that σ_n increases with shortening of the ligament length region as plastic constraint effect rises and variation of the specific total work of fracture with ligament length was still reasonably linear within the mixed mode region. With increasing wollastonite content, w _e (specific essential work of fracture) increases, while the βw _p (specific non-essential work of fracture) decreases. The measurements of energy dissipation show that improvement in the fracture toughness of PP–wollastonite–POE is mainly due to the increase in crack propagation resistance during the necking and tearing processes after yielding, while the plastic deformation capability of the material depends mainly on the properties of fracture behavior before yielding. It is also found that the impact strength of the material decreases with increasing wollastonite content. However, the composition with high impact strength has lower specific essential energy of fracture and lower long-term fracture resistance, indicating that EWF is a better indicator of long-term fracture properties than the impact strength. DSC results show that the presence of wollastonite hinders crystallization of the PP.     

Interaction between tensile strength failure and mixed mode crack propagation in concrete

Crack size and structure size transitions are illustrated which connect the two limit-cases of ultimate tensile strength failure (small cracks and small structures) and mixed-mode crack propagation (large cracks and large structures). The problem of mixed-mode crack propagation in concrete is then faced. By increasing the size-scale of the element the influences of heterogeneity and cohesive crack tip forces disappear and crack branching is governed only by the linear elastic stress-singularity in the crack tip region. It is proved in this way that the fracture toughness of the material is measured by a unique parameter (G_IF, G_IC or K_IC) even for the mixed-mode condition. The ratio of the sliding or Mode II fracture toughness (G_IIF, G_IIC or K_IIC) to the opening or Mode I fracture toughness depends only on the crack branching criterion adopted and not on the material features. Eventually, very controversial experimental results recently obtained on the shear fracture of concrete are explained on the basis of the above-mentioned size-scale transition.