Fracture analysis of strength undermatched Al‐Alloy welds in edge cracked tensile panels using FITNET procedure

This paper presents a methodology for the assessment of the remaining load carrying capacity of thin‐walled components under tension containing highly strength undermatched welds and edge cracks. The analysis is based on the strength mismatch option of the fracture module, part of the newly developed European fitness‐for‐service (FFS) procedure FITNET. The mismatch option of the FITNET fracture module allows weld features such as weld tensile properties and weld geometry to be taken into account in the fracture analysis of cracked welded components. The methodology described was verified for centre cracked Al‐alloy large tensile panels containing undermatched welds in Ref. [ 1 ] and hence the present work provides validation with experimental results of the single edge cracked (SEC) and double edge cracked (DEC) panels. The material used is an age‐hardening aluminium alloy 6013 in T6 temper condition used in welded airframe components. The welds in the form of butt joints were produced using the CO₂ laser beam welding process. The results show that by using the FITNET FFS methodology with an appropriate selection of the input parameters, safe acceptable predictions of the maximum load carrying capacity of the welded panels can be obtained. It should also be noted that one of the main difficulties that engineers encounter when applying mismatch analysis for first time is its apparent complexity. A step‐by‐step analysis is proposed here in order to provide guidance for this kind of assessments.     

Structural integrity of an X-750 jet pump beam of a BWR by means of FITNET FFS procedure

In this paper, the J–R curves of the X-750 alloy, previously obtained in the rising load test environment, have been implemented in the structural integrity assessment of a jet pump beam of a boiling water reactor manufactured in this material. The analysis was performed with the FITNET FFS procedure considering the in-service loads on the beam. The critical crack lengths for the different scenarios studied have been obtained and, taking into consideration real crack length velocities, the lifespan of the beams were conservatively estimated.     

Failure analysis of a cast steel railway wheel

The failure analysis of a broken railway pearlitic cast steel railway wheel, which was involved in a train derailment, has been investigated. The fractographic results near the fracture origin of the wheel have been compared to the regions of stable (fatigue) and unstable crack propagation of cast steel fracture toughness samples. The macro and microfractographic features of the broken wheel did not show any evidence of the occurrence of a fatigue mechanism, indicating that its fracture was caused by transgranular cleavage fracture, as a consequence of the train derailment.     

Finite element analysis of femoral components paper III – hip joints

Finite element analysis of the hip implant is conducted in this paper for representative femoral cross-section geometries and development of stress in the presence of bone cement is elucidated. Differences in cement stresses generated by varying implant cross-sections were compared with conventional features derived from representative implants. The analysis was performed under idealized implant assemblies by constraining the implant movement in the assembly. The cross-sections and implant geometries used are generic and intended to be representative of available geometries. This paper is a part of student research projects, prepared from a series of activities in biomedical implant research currently underway at Ohio Northern and Arkansas Tech Universities.     

Failure analysis of a crankshaft made from ductile cast iron

This paper describes the failure analysis of a diesel engine crankshaft used in a truck, which is made from ductile cast iron. The crankshaft was found to break into two pieces at the crankpin portion before completion of warranty period. The crankshaft was induction hardened. An evaluation of the failed crankshaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that fatigue is the dominant mechanism of failure of the crankshaft. It was observed that the fatigue cracks initiated from the fillet region of the crankpin-web. The absence of the hardened case in the fillet region and the presence of free graphite and nonspheroidal graphite in the microstructure of the crankshaft made fatigue strength decrease to lead to fatigue initiation and propagation in the weaker region and premature fracture.     

Failure and repair analysis of a runway beam: Influence of the standard applied to lifetime prediction

The structural integrity of a welded structure is guaranteed insofar as the integrity of any particular welded detail is ensured. This is highlighted in this paper which analyzes the premature failure of a runway beam, occurring only two years after being commissioned. Initial fractographic analysis and visual in-situ inspection of the beam suggest that the failure was caused by fatigue. The fracture initiated in an apparently innocuous zone where the beam joins a twin runway beam, the bracing of which is performed by a transverse truss beam with fillet welds.The failure analysis was performed according to two standards specific to runway beam design: the former Spanish UNE 76202:1992 standard, applicable at the time the beam was built, and the current EN 1993-1-9:2005 standard. The main difference between the two standards consists in the load spectrum considered for calculations. While UNE 76202:1992 prescribes a defined combination of the most unfavorable loads, EN 1993-1-9:2005 leaves to the designer’s discretion the choice of more realistic load data directly derived from in-service observations or measurements, which presumably better reflect the loads the beam will have to support throughout its service life.To achieve this goal, once the beam was repaired and operating once again, in-service strain measurements were performed over several days. These measurements allowed us to obtain a real load spectrum, which was taken into consideration in fatigue strength assessment based on the EN 1993-1-9:2005 standard. The results provide a more realistic fatigue life prediction than that obtained using UNE 76202:1992, a standard no longer applicable.     

Fracture assessment of damaged square hollow section (SHS) K-joint using BS7910:2005

A pre-cracked square hollow section K-joint was tested under static loads up to failure. It is found that the load-displacement curves are in good agreement with the finite element results. Ductile tearing was observed to initiate from the crack front parallel to the chord side wall where fracture toughness is smaller. Using plastic collapse load obtained via twice elastic compliance technique and fracture toughness obtained from crack tip opening displacement, the two fracture parameters K_r and L_r are plotted on the standard failure assessment diagram. It shows a conservative assessment for the cracked K-joint subjected to brace end axial loads.     

人工耳蜗使用者音乐感知评估系统的设计

该文的目的是设计一套适用于我国人工耳蜗使用者的音乐感知评估系统。所选评估内容包括听辨测试和问卷调查,用以考察人工耳蜗使用者对音乐要素的辨识能力和其聆听感受。听辨测试包括5种不同音色和3个频率范围的音高最小可辨差阈、音高方向分辨和旋律轮廓识别测试;调查问卷包括音乐背景和听赏水平问卷及音乐体验问卷。3名人工耳蜗使用者和6名正常听力者参加了系统验证测试,听辨测试结果与国外类似研究具有可比性。问卷调查的结果显示了音乐教育背景、主观聆听感受与听辨测试之间的联系。     

Experimental analysis of differences in mechanical behaviour of cracked and notched specimens in a ferritic–pearlitic steel: Considerations about the notch effect on structural integrity

This paper analyses the experimental differences found between the mechanical behaviour of cracked and notched specimens. These differences are defined in terms of local strains, global strains, stresses and CTOD values during testing, and also through the correlations established between these variables. The experimental programme has been performed at a wide range of temperatures (from −100 °C to −20 °C), providing an outline of the notch effect throughout the transition region. The corresponding effects on structural integrity assessments (through failure assessment diagrams) are also analysed. From the results obtained, a clear notch effect has been defined in all the mentioned parameters and in the assessment itself for temperatures below the transition temperature (T < T₀) of the material. The effect is less pronounced or even negligible at higher temperatures (T > T₀).     

Investigation of Failed Pin Carrier Plate for the Anti-Rotation Device in a Motor

Parts of the anti-rotation device expelled from a power plant motor were analyzed. The “anti-rotation” device prevents the pump from rotating backwards when there is pressure in the discharge line and the motor is off. Parts that were recovered include about 2/3 of the spring compression ring and an “ear” that apparently came from the missing segment of the spring compression ring. Results of this investigation revealed that a nearly simultaneous overloading occurred on both sides of the pin carrier plate as indicated by almost equal size fracture. At least two crack initiation sites were identified on both sides of the pin carrier plate. Under tensile loads, surface irregularities can lead to surface cracks resulting in the fracture of the plate. Areas rich in aluminum were identified (using energy dispersive spectroscopy) in the vicinity of surface irregularities. A number of suggestions on avoiding such failure are proposed.     

Assessment of notched structural steel components using failure assessment diagrams and the theory of critical distances

When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. The proposal of this paper consists, on the one hand, in the application of the theory of critical distances for the estimation of the notch fracture toughness and, therefore, for the conversion of the notched situation into an equivalent cracked situation in which the material develops a higher fracture resistance. On the other hand, once the notch fracture toughness has been defined, the assessment is performed using the failure assessment diagram methodology, and assuming that the notch effect on the limit load is negligible. The methodology has been applied to 336 CT notched fracture specimens made of two different structural steels, covering temperatures from the corresponding lower shelf up to the upper shelf, providing satisfactory results and a noticeable reduction in the overconservatism derived from the analyses in which the notch effect is not considered.     

Influence of inclusion content on rolling contact fatigue in a gear steel: Experimental analysis and predictive modelling

Rolling contact fatigue tests were carried out on ring specimens made of quenched and tempered SAE 5135 gear steel with three different steel-production processes, through a bi-disc machine under pure rolling condition and water lubrication. Early formation of micro-pits then coalescing into macro-pits was observed on the rolling surface, while the final failure was caused by subsurface originated spalling phenomena. Microscope analysis of specimens section highlighted the complex surface and subsurface crack layout, and permitted to recognise sulphides as preferential sites for cracks initiation. The inclusion content was analysed throughout the extreme value statistics and the maximum expected inclusion in the Hertzian contact zone was introduced in a failure assessment diagram recently proposed, which resulted effective in predicting the specimen failures.     

Failure analysis of a machine support for fique fibre processing

Replacement cast steel supports for a machine consisting of bars that straighten out fique fibres have presented recurrent fracture, beyond 10⁶ cycles of service and their failure surfaces show features typical of fatigue under alternating bending conditions. Differences between original supports and replacement supports related to geometry roughness, hardness, chemical composition and microstructure were identified. To obtain a longer service life selected modifications of the geometry and use of different material were proposed based on load state analysis, finite element analysis, and fatigue strength calculations.     

Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress. Part I: Methodology using the equivalent CTOD ratio

This part I of a two-part paper presents a method of assessing the effects of welding residual stress and constraint loss on the cleavage fracture of a wide plate subjected to membrane stress based on the Weibull stress criterion. It has been found that the Weibull stress criterion is efficient for evaluating the fracture instability of wide plates with and without a welding residual stress field. The concept of an equivalent crack-tip opening displacement (CTOD) ratio β_r under a welding residual stress field is introduced for assessment of constraint loss effects on CTOD fracture toughness of wide plates. The equivalent CTOD ratio β_r is defined as the ratio of the CTOD in the standard fracture toughness specimen to the CTOD in a wide plate with a welding residual stress at the same level of the Weibull stress. Fracture assessment procedures using β_r for wide plates are shown within the framework of the failure assessment diagram. It has been found that the excessive conservatism observed in the conventional procedure can be reduced reasonably by applying the proposed method. The companion part II of the paper presents applications of the CTOD toughness correction method using β_r to the fracture test data of welded joints and verifies the proposed method in the ductile-brittle transition temperature region.     

Failure analysis of bursting on the inner pipe of a jacketed pipe in a tubular heat exchanger

In order to identify the causes of a bursting incident that occurred on the inner pipe of a jacketed pipe in a tubular heat exchanger for synthesis of high pressure polyethylene, series of characterization analysis were conducted. Metallurgical structure and chemical composition of the pipe’s metal matrix were inspected by metallographic microscope (MM) and photoelectric direct reading spectrometer; scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) were applied to observe the microscopic morphology and micro-area composition on the ruptured surface; compositions of the coolant, i.e. the circulating cooling water were examined by inductively coupled plasma-atomic emission spectrometry (ICP-AES), ion chromatography (IC) and Fourier transform infrared spectroscopy (FTIR). In addition to these, finite element analysis (FEA) was employed to study the erosive effect of on the pipe. Analysis results revealed that interaction between corrosion and erosion both led by scaling, was the main cause that accelerated its thinning and eventually resulted in its premature failure. Finally countermeasures and suggestions were proposed.     

A procedure for determining the true stress-strain curve over a large range of strains using digital image correlation and finite element analysis

A procedure for determining the stress-strain curve including post-necking strain is proposed. Hourglass type specimens were used for tensile tests, and the stress-strain curves were identified through an iteration process using finite element analysis. The strain at the position of minimum diameter was measured by digital image correlation. This procedure was applied to carbon steel of various degrees of cold work. The radius of the minimum section of the hourglass type specimen was changed in order to investigate the effect of stress triaxiality on the failure strain. The procedure could derive the stress-strain curve including the post-necking strain. From the obtained curve, it was shown that the stress-strain curves for different degrees of cold work were almost identical when the plastic strain by the cold working was added to the strain. Furthermore, it was revealed that the true stress-strain curve could be approximated well by the power law equation and the curve could be estimated by using the stress-strain relation for before-necking strain.     

Failure analysis of the draft tube connecting bolts of a Francis-type hydroelectric power plant

In this paper, the failure of the bolts that fasten the draft tube of a 95-MW Francis turbine is presented. The fracture of the bolts is especially frequent when the machine operates at partial load. Fracture surface analysis and stress measurements under several power levels were done. Stress was also measured, while pressure relief in the spiral case was performed. Finally, stress measurements were conducted while pressured air was injected at the stay vanes and the machine was operating at partial load. With the strain measured, stress on the bolts was calculated.The fracture surface analysis showed that fatigue is the failure mechanism. Stress measurements revealed that strong vibration and broad stress variation is present when the machine operates at powers below 80 MW. Air injection effectively decreases vibration and stress fluctuation, but pressure relief in the spiral case did not show any beneficial effect.     

Proposed extension of the SINTAP/FITNET thin wall option based on a simple method for reference load determination

A “correct” limit or yield load is an essential element of flaw assessment procedures of the R6 or SINTAP/FITNET type. In this paper the authors propose a definition of this quantity which is based on the SINTAP option 3 failure assessment function. This “reference load” can be determined for any component geometry by finite element analysis. The method is applied to two kinds of thin walled structures (notched plates and curved stiffened panels) and the results demonstrate that the approach is a suitable extension of the existing thin wall module.     

无缝钢管覆膜穿孔顶头的失效分析EI

从宏观断口形貌出发，讨论了无缝钢管覆膜穿孔顶头的膜层结构、顶头的钢基体成分、显微组织及热处理工艺，认为顶头覆膜层结构不致密，不能有效地抑制顶头钢基体出现裂纹，甚至可能会促使裂纹加速扩展。