High Integrity Applications Research Articles

High-performance Ni-based superalloy 718 fabricated via arc plasma directed energy deposition: effect of post-deposition heat treatments on microstructure and mechanical properties

Ni-based superalloy 718 fabricated via arc plasma direct energy deposition (IN718 AP-DED) exhibit a limited response to heat treatment due to its coarse primary microstructure and interdendritic segregation, which may prevent its use in high-integrity applications. Thus, dedicated heat treatments for IN718 AP-DED must possess a homogenization temperature as high as possible to significantly dissolve the eutectics and increase the γ′′-former elements in solid-solution. The present work proposed heat treatments for IN718 AP-DED (homogenization – 1050, 1100, 1142, and 1185 °C / 2 h – followed by aging – 718 °C / 8 h, cooling at 56 °C / h, and 621 °C / 8 h). The as-built IN718 AP-DED showed the typical coarse and oriented (cube texture) microstructure with eutectics (Laves and MC-typical carbides) in the interdendritic region, which were significantly dissolved during the homogenization, promoting a high-volume fraction of hardening phase (γ′′ and γ′) and outstanding quasi-static mechanical properties after the aging step. The present work showed that IN718 AP-DED mechanical properties can be optimized through dedicated heat treatments, meeting the ductility and yield strength requirements (room temperature) of AMS 5662. Furthermore, the heat treatments did not alter the grain morphology and texture aspect, inducing a lower Young’s modulus compared to the non-oriented material.

Monitoring of Perception Systems: Deterministic, Probabilistic, and Learning-Based Fault Detection and Identification (Abstract Reprint)

This paper investigates runtime monitoring of perception systems. Perception is a critical component of high-integrity applications of robotics and autonomous systems, such as self-driving cars. In these applications, failure of perception systems may put human life at risk, and a broad adoption of these technologies requires the development of methodologies to guarantee and monitor safe operation. Despite the paramount importance of perception, currently there is no formal approach for system-level perception monitoring. In this paper, we formalize the problem of runtime fault detection and identification in perception systems and present a framework to model diagnostic information using a diagnostic graph. We then provide a set of deterministic, probabilistic, and learning-based algorithms that use diagnostic graphs to perform fault detection and identification. Moreover, we investigate fundamental limits and provide deterministic and probabilistic guarantees on the fault detection and identification results. We conclude the paper with an extensive experimental evaluation, which recreates several realistic failure modes in the LGSVL open-source autonomous driving simulator, and applies the proposed system monitors to a state-of-the-art autonomous driving software stack (Baidu's Apollo Auto). The results show that the proposed system monitors outperform baselines, have the potential of preventing accidents in realistic autonomous driving scenarios, and incur a negligible computational overhead.

Monitoring of perception systems: Deterministic, probabilistic, and learning-based fault detection and identification

This paper investigates runtime monitoring of perception systems. Perception is a critical component of high-integrity applications of robotics and autonomous systems, such as self-driving cars. In these applications, failure of perception systems may put human life at risk, and a broad adoption of these technologies requires the development of methodologies to guarantee and monitor safe operation. Despite the paramount importance of perception, currently there is no formal approach for system-level perception monitoring. In this paper, we formalize the problem of runtime fault detection and identification in perception systems and present a framework to model diagnostic information using a diagnostic graph. We then provide a set of deterministic, probabilistic, and learning-based algorithms that use diagnostic graphs to perform fault detection and identification. Moreover, we investigate fundamental limits and provide deterministic and probabilistic guarantees on the fault detection and identification results. We conclude the paper with an extensive experimental evaluation, which recreates several realistic failure modes in the LGSVL open-source autonomous driving simulator, and applies the proposed system monitors to a state-of-the-art autonomous driving software stack (Baidu's Apollo Auto). The results show that the proposed system monitors outperform baselines, have the potential of preventing accidents in realistic autonomous driving scenarios, and incur a negligible computational overhead.

A solid-state joining approach to manufacture of transition joints for high integrity applications

Manufacture of a transition hybrid coupling from two different steels, medium carbon S355J2 and stainless 316L, has been demonstrated using two different solid-state joining routes, forge welding (FW) and rotary friction welding (RFW). An additional manufacturing route, electron beam welding (EBW), was also employed in investigating its feasibility for such application. Mechanical and microstructure properties of the final components manufactured from the three different routes have been compared. Finite element (FE) analysis was utilised to determine optimal geometries for the FW and the RFW preform rings and identify optimal parameters for both processes. Of the three manufacture routes, the FW produced an instant diffusion-like bond with a single forging stroke on a 2100T screw press with the thinnest weld interface and most uniform hardness distribution at either side of the joint. The RFW part, manufactured on a 125T RFW machine, also exhibited a very thin weld interface, yet slightly thicker compared to the FW case, with small variations in the hardness distribution at either side. The EBW produced markedly thicker weld interface compared to the two solid-state routes. The EBW part exhibited significant variation in hardness distribution across the weld exhibiting peak hardness in the weld indicating requirements for a post-weld heat treatment (PWHT). Both the FW and the RFW process routes exhibited very uniform micro-hardness and microstructures across the weld interface in contrast with the EBW process in as-manufactured condition.

Comparative study of eutectic Al-Si alloys manufactured by WAAM and casting

Wire and arc additive manufacturing (WAAM) of metallic materials is expected to become part of the new industrial revolution. The possibilities for complex designs and superior mechanical properties can in many cases replace traditional manufacturing processes such as casting. In order to benchmark the properties of aluminium WAAM components, a comparative study was performed with two different casting techniques: permanent casting with steel mould and sand mould casting. Aluminium-silicon alloys with near eutectic composition were used for the comparison. Porosity levels, secondary dendrite arm spacing, grain size distribution, tensile strength and microhardness were considered for the comparison. The WAAM material exhibited superior mechanical properties originating from a finer dendritic and eutectic microstructure compared with the castings. A slight anisotropy in tensile ductility was observed in the WAAM material, probably due to a coarse microstructural zone between individual beads. All investigated materials had low levels of porosity, < 1% by area fraction. The comparative study has shown that WAAM of aluminium-silicon alloys is well suited for high-integrity applications.

Research on Post-processing Microstructure and Property of Titanium Components with Selective Laser Melting (SLM)

Additive manufacturing of titanium component holds promise to deliver benefit such as reduced cost, weight and carbon emissions during both manufacture and use. To capitalize on the benefits, it must be shown that the mechanical performance of parts produced by additive manufacturing can meet design requirement that are typically based on wrought material performance properties. Of particular concern for safety critical structures is the fatigue property of parts produced by additive manufacturing. Microstructure evolution, and its influence on mechanical properties of the alloy in the as-fabricated condition, has been documented by various researchers. However, fatigue crack propagation and the effects of the directional structure have not been sufficiently studied, imposing a barrier for this technology’s potential extension to high-integrity applications. In this study, fatigue life (S-N) and fatigue crack growth (FCG) both parallel and perpendicular to the build directions was studied. The interaction between the directional as-fabricated SLM microstructure and FCG was investigated and compared to that of the hot isostatic pressing (HIP) specimens with and without the stress relief after fabricating with SLM.

GPS navigation data bit decoding error during strong equatorial scintillation

Strong equatorial scintillation is often characterized by simultaneous fast phase changes and deep amplitude fading. The combined effect poses a challenge for GNSS receiver carrier tracking performance. One of the consequences of the strong scintillation is increased navigation message data bit decoding error. Understanding the rate of the data bit decoding error under equatorial scintillation is essential for high accuracy and high integrity applications. We present the statistical relationship between the data bit decoding error occurrences and the intensity of amplitude scintillation based on the processing of intermediate frequency GPS scintillation data collected on Ascension Island in March 2013. A third-order phase lock loop (PLL) is implemented to process the data and to access the data bit error typically expected in conventional receivers. A Kalman filter-based PLL is also used to process the same data to demonstrate that the data bit decoding error can be reduced through advanced carrier tracking designs.

Friction Stir Welding of Dissimilar Al/Al and Al/Non-Al Alloys: A Review

Friction stir welding is a solid-state welding technique that has many advantages over traditional fusion welding, and has been widely adopted in the aerospace and automotive industries. This article reviews research developments in friction stir welding of dissimilar alloys systems, including combinations of aluminum alloys with Mg alloys, Cu, and steel. Microstructural evolution, hardness, tensile and fatigue properties, residual stresses, and corrosion behavior of dissimilar welds will be reported. The effects of processing parameters such as tool rotation and traverse speeds, tool position, material position, and tool geometry on the weld quality are also presented. Discussions on future research directions in friction stir welding will also be provided in the context of existing literature and future high-integrity applications.

Fully-Deterministic Execution of IEC-61499 Models for Distributed Avionics Applications

The development of time-critical Distributed Avionics Applications (DAAs) pushes beyond the limit of existing modeling methodologies to design dependable systems. Aerospace and industrial automation entail high-integrity applications where execution time is essential for dependability. This tempts us to use modeling technologies from one domain in another. The challenge is to demonstrate that they can be effectively used across domains whilst assuring temporally dependable applications. This paper shows that an IEC61499-modeled DAA can satisfy temporal dependability requirements as to end-to-end flow latency when it is properly scheduled and realized in a fully deterministic avionics platform that entails Integrated Modular Avionics (IMA) computation along with Time-Triggered Protocol (TTP) communication. Outcomes from the execution design of an IEC61499-based DAA model for an IMA-TTP platform are used to check runtime correctness through DAA control stability. IEC 61499 is a modeling standard for industrial automation, and it is meant to facilitate distribution and reconfiguration of applications. The DAA case study is a Distributed Fluid Control System (DFCS) for the Airbus-A380 fuel system. Latency analysis results from timing metrics as well as closed-loop control simulation results are presented. Experimental outcomes suggest that an IEC61499-based DFCS model can achieve desired runtime latency for temporal dependability when executed in an IMA-TTP platform. Concluding remarks and future research direction are also discussed.

Effects of processing on microstructure evolution and fatigue crack growth mechanisms in cold-spray 6061 aluminum alloy

Owing to their fine microstructures, cold-spray processed alloys possess appealing static mechanical properties, similar to their wrought counterparts, making them potential candidates for structural applications. In spite of their great potential, the lack of knowledge of their cyclic behavior, especially the fatigue crack growth response, limits their use in high-integrity applications. Thus, further advanced characterization of cold-spray processed materials is needed. In this study, fatigue crack growth microstructural mechanisms were established for bulk cold-spray processed 6061 aluminum alloy tested in laboratory air at room temperature. The effects of the material’s characteristic microstructure, stress ratio (R = 0.1, 0.5, and 0.7), and post-fabrication heat treatment (annealing and T6) on the fatigue crack growth behavior were systematically investigated. It was found that the initial powder particle characteristics play an important role in the crack’s propagation through the cold-spray 6061 microstructures, and particle boundaries are critical in the materials’ response, especially at high driving force. To aid the material-process design, two crack tip driving force parameter maps that relate loading conditions to damage at the microstructure scale were also established. Furthermore, a Paris-type, microstructure-based model was successfully developed to predict the fatigue crack growth rates in upper Region II and Region III for the as-fabricated and annealed materials. Discussions related to the use of these materials for fatigue-critical applications are also provided.

Fatigue crack growth mechanisms at the microstructure scale in as-fabricated and heat treated Ti-6Al-4V ELI manufactured by electron beam melting (EBM)

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that fabricates parts by selectively scanning consecutive powder layers with an electron beam. Additive manufacturing technologies are increasing in importance for aerospace and medical applications, where the demand for a fundamental understanding and predictability of static and dynamic material properties are high. Ti-6Al-4V is the most widely used and studied alloy for this technology, and is the focus of this work in its Extra Low Interstitial (ELI) variation. The layered manufacturing of metallic components by EBM creates a unique directional microstructure, and consequently, anisotropic properties. Microstructure evolution, and its influence on mechanical properties of the alloy in the as-fabricated condition, has been documented by various researchers. However, fatigue crack propagation and the effects of the directional structure have not been sufficiently studied, imposing a barrier for this technology’s potential extension to high-integrity applications. In this study, fatigue crack growth (FCG) both parallel and perpendicular to the build directions was studied for different stress ratios and crack growth stages. The interaction between the directional as-fabricated EBM microstructure and FCG was investigated and compared to that of the equiaxed β annealed microstructure obtained by annealing above the β transus temperature. The FCG threshold, ΔKth, was analytically modeled for the two relative crack propagation directions at different stress ratios, and FCG microstructural mechanisms were established for all three regions of crack propagation.

Structural integrity for DEMO: An opportunity to close the gap from materials science to engineering needs

It is clear that fusion demonstration devices offer unique challenges due to the myriad, interacting material degradation effects and the numerous, conflicting requirements that must be addressed in order for in-vessel components to deliver satisfactory performance over the required lifetime. The link between mechanical engineering and materials science is pivotal to assure the timely realisation and exploitation of successful fusion power. A key aspect of this link is the verification of structural integrity, achieved at the design stage via structural design criteria against which designs are judged to be sufficiently resilient (or not) to failure, for a given set of loading conditions and desired lifetime. As various demonstration power plant designs progress through their current conceptual design phases, this paper seeks to highlight key shortfalls in this vital link between engineering needs and materials science, offering a perspective on where future attention can be prioritised to maximise impact.Firstly, issues in applying existing structural design criteria to demonstration power plant designs are identified. Whilst fusion offers particular challenges, there are significant insights to be gained from attempts to address such issues for high performance, high integrity applications in other demanding environments. Therefore case studies from beyond fusion are discussed. These offer examples where similar shortfalls have been successfully addressed, via approaches at the design stage and through service lifetime in order to deliver significant insight for structural materials and improved design solutions for first-of-a-kind engineering endeavours where the consequences of failure were of similar concern.Finally, drawing inspiration from these case studies, the current state-of-the-art is explored to propose how both materials science and engineering should be aligned in order to address the issues we face in realizing effective fusion demonstration power plants.

Testing-based translation validation of generated code in the context of IEC 61508

Production code generation with Model-Based Design has successfully replaced manual coding across various industries and application domains. Furthermore, code generated from executable graphical models is increasingly being deployed in high-integrity embedded applications. To validate the model-to-code translation process, generated software components and its precursory stages (i.e. models) should be subjected to an appropriate combination of quality assurance measures. For high-integrity applications, compliance with safety standards such as IEC 61508 needs to be demonstrated as well. On principle, translation validation of generated code could be carried out in the same manner as for manually written code. However, this would not leverage the advantages of Model-Based Design and w.r.t. process efficiency this would leave something to be desired. Therefore, engineering methods and tools for effective and efficient translation validation of generated code are highly desirable. As a step towards this goal, a workflow for verification and validation of models and generated code will be proposed and as far as possible mapped onto the objectives of IEC 61508-3. A cornerstone of this workflow is testing for numerical equivalence between models and generated code.

Verification and Validation According to IEC 61508: A Workflow to Facilitate the Development of High-Integrity Applications

Verification and Validation According to IEC 61508: A Workflow to Facilitate the Development of High-Integrity Applications

Microstructure Formation in AlSi4MgMn and AlMg5Si2Mn High-Pressure Die Castings

Understanding microstructure formation during high-pressure die casting (HPDC) is important for the effective quality control of high-pressure diecast aluminum-alloy components for high-integrity applications. In this study, two HPDC-specific aluminum alloys, AlSi4MgMn and AlMg5Si2Mn, were cast into tensile test bars by cold-chamber (CC) HPDC. The microstructures of the tensile bar specimens were characterized at different length scales, from the scale of the casting to the scale of the eutectic interlamellar spacing. The results show that the salient as-cast microstructural features, e.g., externally solidified crystals (ESCs), defect bands, the surface layer, grain size distribution, porosity, and hot tears were similar for both alloys. The formation of these features can be understood by considering the influence of flow and solidification during each stage of the HPDC process.

Automatic framework generation for hard real-time applications

The communication and tasking infrastructure of a real-time application makes up a significant portion of any modern embedded control system. Traditionally, the tasking and communication constructs are provided by a full-fledged real-time operating system. We present an approach to automatically generate a robust framework for an application from its architectural description. This framework sits atop a Ravenscar-compliant runtime as opposed to a standard real-time operating system. We also present an extension of our approach to support code generation for high-integrity distributed applications.

Pressure Assisted Processes for High Integrity Aluminum Castings

Low pressure is perhaps the longest-running pressure assisted process for manufacturing high integrity structural aluminum castings and recent variations on that process have enhanced its capability. Still, high pressure die casting has long dominated the manufacture of aluminum castings and variants like high-vacuum, squeeze and semi-solid now make die casting quite suitable for even the most demanding high integrity applications.

Correctness by construction

This presentation will look at the many problems of designing and building high-integrity software and the processes, languages and tools that are currently used. The need to "engineer" our software implies that discipline and reasoning are required, yet most processes and languages seem to obstruct rather than assist such attempts. To help understand why, we consider the history of programming language design, and how this has shaped the way we think about software and constrained our ability to engineer it. In this generally gloomy picture some good news can be found: the emergence of design-by-contract facilities in languages such as Eiffel and SPARK has made a strong impact in some niche areas. In those areas, we have made a rather unusual observation: customers (some having "flirted" with this week's fashionable approach) are "coming back to Ada", recognizing the language's strengths, particularly for high-integrity applications. The presentation will close by considering why Praxis still uses (and chooses) Ada, and the role that Ada has to play in the future of high-integrity software.

Integrity-directed sequential state estimation: Assessing high reliability requirements via safe confidence intervals

This study deals with the problem of dynamic state estimation of continuous-time systems from discrete-time measurements in the context of high-integrity applications. The objective of integrity-directed estimation is to provide confidence intervals for the state with an extremely low risk of error. We suppose that the process noise can be modelled by Gaussian sequences, and that the measurement noise is Gaussian in the normal operating mode of the sensors. The evolution of the posterior probability distribution of the system’s state is deduced from recursive linear MMSE estimation. The estimation scheme presented here is equivalent to the Kalman filter, with the difference that the data is not processed directly, but collected in sets in preparation for an ulterior, slightly delayed, grouped processing. This strategy is particularly suitable for fault detection, because the estimator naturally takes into account the cross-correlations of close-in-time measurements and the decisions can be based on more data. Next, we introduce dynamic tools for detecting faults and sensor failures. A full Bayesian modelling of the sensors leads to the derivation of a dynamic multiple-model estimator performing the linear MMSE state estimation under various fault hypotheses. This estimator provides estimates of the posterior density function of the state, on which safe confidence intervals certifying very high-integrity levels can be fixed. In practice, the exponential growth of the complexity of the multiple-model estimator requires the simplification of the posterior mixture distributions. A new method for limiting the complexity of the posterior distributions in an integrity-oriented context is presented. Unlike the known mixture simplification strategies (GPB, IMM), the present method has the property to quantify and minimize the loss in integrity of the process of simplification of the distributions. Finally, the estimator is tested on a typical rail navigation problem.

Fatigue crack growth characteristics in cast Al–Si–Mg alloys

Fatigue strength and crack propagation behavior are critical attributes when designing structural components for high-integrity applications. Significant efforts have been directed in the past decade towards determining the effects of casting defects on fatigue response and many causal relationships are available in the literature. What has been missing, however, is a fundamental knowledge of the contribution of various constituent phases and features, specific to the alloy being considered, in governing fatigue crack initiation and propagation. In this work, model systems of Al-based cast alloys were investigated for fatigue crack growth under constant stress ratios using compact tension specimens. Grain size, secondary dendrite arm spacing (SDAS), modification, amount of Al–Si eutectic phase, α-Al phase, and other phases were controlled (kept constant/varied as needed to decouple effects) to determine their impact on fatigue crack growth. In addition, heat treatment effects and the influence of quenching residual stresses (macro-stresses) on fatigue crack growth have also been studied. The effects of critical casting parameters on the fatigue properties of cast Al components used for low-cycle fatigue as well as high-cycle fatigue applications will be critically reviewed and discussed.