Safety Critical Components Research Articles

Effect of Extreme Climate on Topology of Railway Prestressed Concrete Sleepers

Railway networks are exposed to various environmental conditions. It is thus critical that infrastructure components can tolerate such effects by design. Railway sleepers are a critical safety component in ballasted track systems. Prestressed concrete is currently the most common material for railway sleepers due to its superior advantages in structural performance, low maintenance, sustainability, and construction. In practice, many prestressed concrete sleepers are installed in harsh environments that are subject to various changes in climate. Environmental conditions are, therefore, one of the most critical phenomena affecting the time-dependent behaviour of prestressed concrete sleepers. Hence, the impact of climate changes on the serviceability of railway infrastructure needs to be thoroughly investigated. Temperature and relative humidity are crucial aspects that have not been sufficiently studied so far with reference to prestressed concrete sleepers embedded in track systems. This study aims to investigate the effects of extreme climatic conditions on the performance and time-dependent behaviour of prestressed concrete sleepers using contemporary design approaches. The issue concerning the effects of climate uncertainties on creep and shrinkage is rigorously investigated on the basis of both environmental temperature and relative humidity. The outcome indicates that environmental conditions play a vital role in the time-dependent behaviour of prestressed concrete sleepers. The insights will be essential for assessing the long-term serviceability of prestressed concrete sleepers that have been installed in railway lines and are subjected to extreme environmental conditions.

Experimental methods for ultrasonic testing of complex-shaped parts encased in ice

Detection of damage in safety-critical components with complex geometry represents a long-standing challenge in NDE. The ultrasonic inspection of these parts in immersion is often not feasible due to the large impedance and velocity contrast between water and metals which can severely limit the penetration of the ultrasonic signal inside the part. However, this contrast can be significantly reduced if the water is frozen prior to the inspection leading to what has been referred to as Cryoultrasonic NDE. Since the speed of longitudinal waves in ice is more than 2.5 times greater than the speed in water, ice can be an ideal solid couplant provided that it is devoid of bubbles and cracks and that it is fully bonded to the surface of the part. This paper introduces new experimental methods to encase complex parts in blocks of crystal clear ice and presents the first low temperature scanner for ultrasonic array testing of ice-encapsulated parts. It is shown that by controlling the propagation of the solidification front while water is freezing, it is possible to prevent the formation of cracks inside the ice volume. On the other hand, bubble nucleation can be avoided by continuously forcing water circulation on the solidification front. An analytical model is provided to describe the propagation of the front and predict freezing times. Moreover, high-frequency ultrasonic monitoring experiments confirm the excellent adhesion strength properties of ice to metals. Finally, experiments performed with an additively manufactured Ti6Al4V impeller demonstrate the feasibility of performing low temperature array contact scans on the surface of the ice block encasing the part.

Effect of TiN-Particles on Fracture of Press-Hardened Steel Sheets and Components

It is known that large TiN particles deteriorate the toughness of high-strength steels. The toughness of press-hardened steel (PHS) sheets with full-martensite microstructure for safety critical components of a car body has, however, seldom been studied. Consequently, the effect of TiN particles on either the toughness of PHS sheets or the performance of hot-stamped components have not been investigated. In this research, the size and distribution of TiN particles are quantitatively analyzed for two 22MnB5 PHS sheets containing nitrogen of 29 ppm and 44 ppm in mass. Higher populations of coarse TiN particles have a negligible effect on standard tensile properties, but significantly reduce impact toughness by Charpy V-notch impact tests. Hot-formed components with smaller fractions of TiN particles show greater fracture resistance by a three-point bending test.

Influence of Material Ductility on Fatigue Life under Multiaxial Proportional and Non-Proportional Normal and Shear Stresses

Current experiences show that a non-proportional loading of ductile materials such as wrought steels, wrought aluminium or magnesium alloys, not welded or welded, causes a significant fatigue life reduction under an out-of-phase shear strain or shear stress superimposed on a normal strain or normal stress compared with proportional in-phase loading. However, when ductility, here characterised by tensile elongation, is reduced by a heat treatment or by another manufacturing technology such as casting or sintering, the afore-mentioned life reduction is compensated or even inversed, i. e. longer fatigue life results compared with proportional loading. Some actual results, determined with additive manufactured titanium, suggest that microstructural features such as manufacturing-dependent internal defects like microporosities should be considered in addition to the ductility level. This complex life behaviour under non-proportional loading cannot always be estimated. Therefore, in experimental proofs of multiaxial loaded parts, especially safety-critical components or structures, with real or service-like signals, emphasis must be placed on retaining non-proportionalities between loads and stresses/strains, respectively.

Root Cause Analysis of Low Impact Toughness of Cast Steel Yokes Used in Railway Freight Cars

The yoke is a safety-critical component of the coupler assembly of railway freight cars and is responsible for absorption of impact loads during service. The present study investigates the root cause of low impact toughness of a particular batch of yoke castings produced in Texmaco Steel Foundry. Examination of microstructure and fracture surface of specimens extracted from the castings reveals the presence of a substantial number of macro- and microporosities. The fracture morphology confirms that the presence of such large number of porosities increases stress concentration and lowers the load-bearing area, resulting in brittle failure and low impact toughness. EDX analysis indicates that these porosities were caused by entrapment of gases during solidification. Subsequent examination indicates that the porosities were caused due to gases generated by combustion of resins/binders used in the sand system or improper venting during the pouring operation.

Effect of putrefied quenching oil on the transformation behaviour of railway spring steel.

Bogie springs are safety critical components that also provide a smooth ride to the rail vehicle by absorbing vibrations and shock loads during the movement of the train. This study investigates the cause of premature failure of springs. These springs were manufactured from a high strength steel grade, SAE J404 grade 5160. The investigation process focused on heat treatment processes and failure mechanisms. Oil properties were also scrutinised by conducting quench tests. The overall findings of the investigation highlighted the detrimental effect of a degraded quenching oil on the fatigue life of the springs. Implementing certain measures from these findings will increase the spring lifespan and contribute towards safe operation of the trains.

Develop an effective oxygen removal method for copper powder

At present, one of crucial limitations for the hot isostatically pressed (HIPed) Cu-3Ag-0.5Zr alloy, which is used on the combustion chamber liner of aerospace engine, is the high oxygen content, which easily results in the intergranular fracture under high temperature, pressure, liquid hydrogen and oxygen environment during operation. In this study, a novel effective oxygen control method is developed, for which vacuum degassing process is integrated with a flowing hydrogen reduction reaction at an elevated temperature before HIP. For this technique, a container is designed with two gas pipes for hydrogen inflow and outflow, so the hydrogen circulation can be established. Allowing hydrogen to react effectively with oxygen, the oxygen content of HIPed alloy is found to drop significantly from 140 ppm (raw powder) to 28 ppm, which is equivalent to the oxygen-free copper and copper alloys. As a result of the reduction, no prior particle boundaries could be observed in the low oxygen content material. Although the tensile strength of the materials with and without employing this technique does not vary significantly, the ductility of low oxygen content material has improved by about 70% at 500 °C. This significant improvement of ductility is critical to ensure the safety critical PM components.

Cryo-Ultrasonic NDE: Ice-Cold Ultrasonic Waves for the Detection of Damage in Complex-Shaped Engineering Components.

Recent advances in computational methods, materials science, and new manufacturing processes are resulting in an unprecedented design flexibility which is driving the geometrical complexity of the components found in modern structures and machines. For safety-critical components, the geometrical complexity poses a significant challenge to the sensitivity of the existing nondestructive evaluation (NDE) methods available for the detection of manufacturing defects or damage that develops while a component is in service. Although X-ray computed tomography is the primary NDE method used to test these parts in current industrial practice, it is widely recognized that it has limited sensitivity to critical defects, such as cracks, especially in the presence of large size parts made of dense materials. The lack of sensitive NDE methods represents a major technology gap that could impede the acceptance of rapidly developing technologies, such as 3-D printing, for the production of safety-critical components. This paper attempts to bridge this gap by exploring the possibility of inspecting a complex-shaped part with ultrasonic waves after it has been encapsulated in ice, under the paradigm of what can be defined as cryo-ultrasonic NDE. The underpinning hypothesis is that through ice encapsulation a complex-shaped part can be transformed into a simple-shaped solid whose volume can be probed with ultrasonic waves, which are known to be highly sensitive to both pores and crack-like defects and over a wide range of material properties. Damage detection is then performed by analyzing cross-sectional images of the ice-encapsulated part obtained by applying migration methods to the ultrasonic signals measured by an array of transducers. This paper lays the foundation for cryo-ultrasonic NDE and presents the first experimental results demonstrating the possibility of imaging defects through multiple ice-metal interfaces. This paves the way to the detection of defects in complex-shaped parts containing internal vanes which have so far limited the use of conventional NDE methods.

Simulating the ultrasonic scattering from complex surface-breaking defects with a three-dimensional hybrid model

Modelling is increasingly relied on for the design and qualification of ultrasonic inspections applied to safety-critical components. Numerical methods enable the simulation of the ultrasonic interaction with realistic defect morphologies; however, the computational requirements often limit their deployment. The hybrid simulation technique, which combines semi-analytical and numerical methods, realises the potential of high fidelity numerical modelling without the limiting computational factors. The inspection of thick section components for near-backwall surface-breaking defects results in large propagation distances, making them a key application of hybrid modelling. This work presents a methodology for efficiently simulating the ultrasonic inspection of complex surface-breaking defects using a hybrid model. The model is initially verified against full numerical simulation; further validation is presented by comparison to an experimental scan over an artificially machined surface-breaking notch. The potential of the new hybrid method is then demonstrated by carrying out a Monte Carlo analysis on the scattered field from surface-breaking defects with randomly rough surfaces and the results are compared to the Kirchhoff approximation.

Surface quality monitoring based on time-frequency features of acoustic emission signals in end milling Inconel-718

Surface quality deterioration is prone to be found in Inconel-718 milling process, which leads to the decrease of fatigue life of aerospace critical safety components. Therefore, in-process detection of surface quality plays an important role in guaranteeing the workpiece quality and improving the production efficiency. In this paper, a surface quality monitoring system based on time-frequency features of acoustic emission signals is established. The main characteristic of this system is that a two-step recognition is performed through the binary clustering to realize the monitoring of surface roughness and the precise recognition of the surface defects, respectively. A new feature extraction method by means of the normalized time-frequency matrix is proposed to obtain the sensitive information of surface quality. In addition, to accomplish the clustering automatically, the improved method of clustering by fast search and find of density peaks is utilized. The experimental verification with different cutting depth is designed to test the effectiveness of the system. Validation results show that the accuracy of the two steps are 99 and 72%, respectively.

Plane Wave SH0 Piezoceramic Transduction Optimized Using Geometrical Parameters

Structural health monitoring is a prominent alternative to the scheduled maintenance of safety-critical components. The nondispersive nature as well as the through-thickness mode shape of the fundamental shear horizontal guided wave mode (SH) make it a particularly attractive candidate for ultrasonic guided wave structural health monitoring. However, plane wave excitation of SH at a high level of purity remains challenging because of the existence of the fundamental Lamb modes (A and S) below the cutoff frequency thickness product of high-order modes. This paper presents a piezoelectric transducer concept optimized for plane SH wave transduction based on the transducer geometry. The transducer parameter exploration was initially performed using a simple analytical model. A 3D multiphysics finite element model was then used to refine the transducer design. Finally, an experimental validation was conducted with a 3D laser Doppler vibrometer system. The analytical model, the finite element model, and the experimental measurement showed excellent agreement. The modal selectivity of SH within a 20 beam opening angle at the design frequency of 425 kHz in a 1.59 mm aluminum plate was 23 dB, and the angle of the 6 dB wavefront was 86.

Breakage of Automotive Spring Washer During Twist Test: A Metallurgical Analysis

A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a screw or nut. The washer used in automotive engine component is of split or spring lock washer. A ring split at one point and bent into helical shape. The benefit of spring lock washers lies in the trapezoidal shape of the washer. When compressed to loads near proof strength of the bolt, it will twist and flatten. This reduces the spring rate of the bolted joint which allows it to maintain more force under the same vibration levels. This prevents from loosening. The steel washer is one of the critical safety components for automobile engine, and its failure may cause severe safety issue. Before subjecting to manufacturing, the washer undergoes different process lines (wire rod → drawing → annealing → pickling → flattening → spring washer → single spring washer). After manufacturing of washer, toughness test is done to find out the metallurgical soundness and surface quality of the washer. In this toughness test the washer in a vice with the split ends free and straight above the vice jaws, a 90° segment of the free end is gripped with a wrench and bent. Washers should withstand twist test through a 90° angle without signs of fracture. The present paper highlights premature failure of automotive washer which failed during twist test. From the analysis, it has been observed that the distribution of spheroidized carbide is more uniform in good samples than that of rejected samples. Hardness profile variation was observed between good and bad spring samples. Overall investigation does not indicate any problem related to supplied wire rod material quality as no wire breakage problem observed. Inhomogeneous annealed structure in flat wire seems to result into variation in washer performance during twist test.

Kick control reliability analysis of managed pressure drilling operation

Offshore drilling involves complex operations and equipment; thus, faces many operational challenges, including well control. Managed pressure drilling has been proved to resolve most of these challenges; however, this technology, for the most part, is still in its infancy. This paper explores the safety and reliability assessment of a managed pressure drilling operation by investigating the kick control operation of constant bottomhole pressure technique of managed pressure drilling. In addition, this study seeks to understand the components interactions in an MPD system and their modes of failure. Failure scenarios are first built using a Fault Tree model and then analyzed using a Bayesian Network model. The reliability assessments of kick control operation are performed in two ways: a basic-components approach and a system-barrier elements approach. The analysis identifies communication related components, including network device damage as the most safety-critical component due to their high-level of influence while flowline and pump line blockages/rupture are ranked second-most critical but with limited-level of influence. However, the system-barrier element approach ranks the managed pressure drilling control system as the most safety-critical equipment. Further detailed analysis confirms that the monitoring equipment are the most safety-critical components of the managed pressure drilling control system with Coriolis flow meter and Rig pump exhibiting the most critical monitoring equipment. Additionally, the managed pressure drilling system's components show high degree of dependencies on one another and exhibit non-sequential modes of failure during kick control operation.

Experimental and modelling techniques for hot stamping applications

Hot stamping techniques have been developed for the production of complex-shaped components since the 1970s, increasingly used for the automotive industry. The application of these techniques includes hot stamping of boron steel for critical automobile safety components, and solution heat treatment, forming and cold die quenching (HFQ®) for forming complex-shaped high strength aluminium panels of automobile bodies and chassis structures. The developed forming techniques need dedicated experimental testing methods to be improved for characterising the thermomechanical behaviour of materials at the hot stamping conditions, and advanced materials modelling techniques to be developed for hot stamping applications. In this paper, requirements for thermomechanical tests and difficulties for hot stamping applications are introduced and analysed. The viscoplastic modelling techniques have been developed for hot stamping applications. Improved experimental methods have been proposed and used in order to obtain accurate thermomechanical uniaxial tensile test data and determine forming limits of metallic materials under hot stamping conditions.

Fatigue properties of AlSi10Mg obtained by additive manufacturing: Defect-based modelling and prediction of fatigue strength

Ability to predict the fatigue resistance of parts produced by additive manufacturing (AM) is a very current and frequently relevant open issue. The qualification of AM structural parts often needs a costly and time-consuming series of fatigue tests, on both samples and full-scale parts. Proper control of the AM process allows obtaining comparable and even better fatigue resistance than those obtained with standard manufacturing. Despite this, the experimental results often show a large scatter, mostly due to the presence of defects. In this framework, the present work summarizes the research activity aimed at modelling the high cycle fatigue (HCF) resistance in the presence of defects, focusing on AlSi10Mg produced by selective laser melting. Three batches of samples were investigated by X-ray micro computed tomography and tested under fatigue. A lower bound resistance curve was obtained, which introduced artificial defects of size corresponding to that of the largest occurring defects.The analysis shows that a combination of defect-tolerant design with well-established and newly proposed fracture mechanics methods is the key to expressing the relationship between the fatigue strength and material quality. This is done through suitable statistics of material defects induced by the AM process.The same concepts are then applied in a fatigue crack growth simulation model based on the maximum defect size, for estimating both the life and scatter of the data in the region of elastic material response. Based on this wide activity, it can be concluded that fracture mechanics-based analysis appears to be the tool needed for supporting the application of additive manufacturing to safety-critical components and their qualification.

The NeT Task Group 4 residual stress measurement and analysis round robin on a three-pass slot-welded plate specimen

Accurate prediction and measurement of residual stresses in welds is an important part of assuring their short and long-term structural performance in high value, safety critical engineering components and structures. However, both measurements and predictions of weld residual stresses often exhibit high levels of variability that are not widely appreciated. The mission of the European Network on Neutron Techniques Standardization for Structural Integrity (NeT) is to develop experimental and numerical techniques and standards for the reliable characterisation of residual stresses in structural welds.The NeT Task Group 4 project examined residual stresses in a three-pass slot-welded plate specimen fabricated from AISI 316L(N) austenitic stainless steel plate. Several nominally identical specimens were fabricated under closely controlled conditions, with detailed records kept of the manufacturing history, weld process parameters, transient temperatures during welding, and the resulting geometric distortions. Comprehensive stress-strain material property characterisation was then undertaken, extending to the isothermal cyclic tests necessary to calibrate the mixed isotropic-kinematic material hardening models required for accurate weld residual stress prediction. Parallel residual stress measurement and simulation round robins were performed by a large number of participants from around the world.Residual stresses were measured using neutron and high energy synchrotron diffraction, surface X-ray diffraction, surface and deep hole drilling, the contour method, and ultrasonics. Neutron diffraction measurements were made at eight different instruments. The diffraction measurements database alone is large enough to generate reliable mean profiles, to identify clear outliers, and to establish that there is no statistically significant difference in the residual stress field in the specimens used for the non-destructive measurements. NeT Task Group 4 gives a unique insight into the real-world variability of diffraction-based residual stress measurements, and forms a reliable foundation against which to benchmark other measurement methods.NeT Task Group 4 is also a unique test bed for the development and validation of weld residual stress simulation techniques in austenitic stainless steel. Its combination of extensive materials characterisation, accurately characterized welding temperature transients, and reliable residual stress and distortion measurements is currently unrivalled. About thirty finite element simulations were submitted to the network over the course of the project, giving insights into the required accuracy of welding thermal solutions, the mechanical solution accuracy achievable using optimized material constitutive models, and the level of acceptable error in finite element residual stress simulation results for use in structural integrity assessments of high integrity engineering components.

Ultrasound measurement of texture in bulk polycrystalline materials

Manufacturing of metal components often results in significant texture, that is, to say, preferred orientations of their polycrystals. Since each crystal can be strongly anisotropic, this can give the component orientation-dependent material properties, affecting stiffness, thermal expansion, strength, and fatigue and creep resistance. So, it is important to be able to measure texture, especially for high-value safety-critical components. Typically, the Orientation Distribution Function (ODF) of the crystals can be measured on exposed surfaces using EBSD, or in thin samples using neutron diffraction. But both are expensive, and until recently there has not been a means to measure ODFs internally in bulk materials. However, the authors have recently developed a method to determine internal texture from measurements of wave speeds at selected angles through the volume of the material. This is based on a convolution of wave speeds in a single crystal with the ODF, giving the resultant polycrystal wave speed angular function. The principles of the method have been established and validated for single phase hexagonal and cubic materials. Ongoing work is investigating the use of multiple wave modes (shear and compressional) to identify further information; the extraction of volume fraction and texture of two-phase materials; and the pursuit of further refinement of the texture results by improved knowledge of the single crystal properties.

Situation Awareness Offshore: Relevant Influencing Factors and Risks

Offshore operations are an inherently hazardous activities that can result in catastrophic outcomes. The amalgamation of different hazards, constraints, and demands on offshore platforms can presents a substantial threat to the performance, productivity, and safety of workers. For most industrial accidents, there is a causal chain between human errors and organizational conditions. An investigation into the Macondo incident identified failures of situation awareness (SA) and risk perception as root causes of the disaster. Improving worker SA has become an important objective for the Oil and Gas industry. SA is a distinct, safety critical component for workers operating in complex, high-risk, and interactive work environments. Possession and maintenance of good quality SA is important for personnel whose work can be hazardous, complex, and pressured by time constraints. The drilling environment can change suddenly and for a drill crew, with an improper decision or inattention, the end result can be loss of life, severe injury, and cost millions of dollars in production loss. Offshore workers need to acquire and integrate information under operational conditions while contending with competing sources of information for their attention. SA can serve as a predictor of performance and has been particularly important where technical and situational complexity impacts the decision making efforts of the driller. A scoping literature review was conducted in order to identify how the influencing factors of elements of human factors on personnel and environmental safety, operational costs, and loss of time. Numerous databases were searched (e.g., EBSCOhost, Medline, PsychInfo, Science Direct) in combination other databases search using key terms: human factors or ergonomics and offshore with all of the following: drilling, production, fatigue, situation awareness, cognitive, oil and gas, as well as a search for human factors offshore and ergonomics offshore and human error offshore. Finally, our primary studies search was supplemented with a search of papers and abstracts within conferences. The fields of interest included stress, fatigue, interface design, human machine interaction, automation, safety culture and safety climate, risk perception and awareness. The purpose of this research was to present a summary of the current literature on the status of the oil and gas industry with regard to the adoption and integration of Human Factors methods, principles, and processes. Specific objectives were to summarize the state of the science regarding situation awareness for offshore operations, understand the importance of situation awareness for this environment, and determine key influencing factors that could affect drillers’ performance.

The imaging of failure in structural materials by synchrotron radiation X-ray microtomography

The initiation and propagation of short cracks are critical to the detailed understanding of the failure mechanisms of engineering materials and structures. Therefore, for a safety critical component, it is beneficial to be able to follow the progress of cracks in loaded structures and real environments ideally at sub-micron spatial resolution and/or sub-second time resolution. Here we review the great progress that has been made in the application of synchrotron radiation X-ray computed microtomography (SR-μCT) to the study of internal damage accumulation and evolution in structural materials including cast irons and steel, nickel superalloys, lightweight titanium and aluminum alloys as well as metallic, ceramic and polymer composite materials together with additatively manufactured or three-dimensional (3D) printed metallic materials. SR-μCT is shedding light on the complex interactions of cracks with intrinsic porosity, precipitates and intermetallic inclusions and grain structure, as well as macroscopic features such as holes and joints. Furthermore it is enabling qualitative and quantitative relations between the microstructural features and engineering performance to be established and validated under realistic service loading and environmental conditions. Here we introduce the basic 3D and time-lapse (4D) tomographic methods and discuss their strengths and limitations across a range of the applications, as well as identify opportunities for the future.

Application of multiple analysis methods in optimising complex residual stress characterisation

An accurate characterisation of residual stress plays an important role in the structural integrity assessment of an engineering component. Several techniques and tools are available for measuring and predicting residual stresses. For example, neutron diffraction (ND) and X-ray diffraction (XRD) are non-destructive techniques used for measuring through-thickness and surface residual stresses respectively, while the deep-hole drilling (DHD) and the incremental centre-hole drilling (ICHD) are semi-destructive techniques and measure through-thickness and sub-surface residual stress respectively. In most open literature, a more favoured method is traditionally used over others, with some degree of validation using finite element analysis (FEA) predictive tool. In this paper it will be shown that the different methods and tools available are not contradicting or more superior to the others, but rather, the use of more than one available technique complementary to each other can improve the quality and the confidence in the characterisation of the residual stress state in an engineering component. In particular, the accurate knowledge of the residual stress field for a safety critical component plays a vital role for subsequent structural integrity assessment.