High Integrity Applications Research Articles

Fatigue crack growth characteristics in cast Al–Si–Mg alloysPart I. Effect of processing conditions and microstructure

Abstract 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.

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The Ravenscar Tasking Profile—experience reporting

The Ravenscar Profile was defined at the 18 th International Real-Time Ada Workshop as a simple subset of the tasking features of Ada, in order to support efficient, high integrity applications that need to be analysed for their timing properties. Ada compiler vendor Aonix subsequently implemented the Profile via its Raven product line, and is currently engaged in producing the formal certification material for qualifying the runtime system to the most stringent set of guidelines --- DO-178B Level A. This paper describes some of the experiences in implementing and qualifying the Profile, and some early indications from the user community of its value to address the application domains that it was intended for.

The Ravenscar tasking profile for high integrity real-time programs

The Ravenscar profile defines a simple subset of the tasking features of Ada in order to support efficient, high integrity applications that need to be analysed for their timing properties. This paper describes the Profile and gives the motivations for the features it does (and does not) include. An implementation of the profile is then described in terms of development practice and requirements, run-time characteristics, certification, size, testing and scheduling analysis. Support tools are discussed as are the means by which the timing characteristics of the run-time can be obtained. The important issue of enforcing the restrictions imposed by the Ravenscar profile is also addressed.

Preprocessing of GNSS signals subject to interference

Increasingly, interference effects on GNSS receivers are becoming important as more safety-critical applications such as civil aviation are employing the system. The inherent interference rejection capability of the GNSS spread spectrum modulating technique is now no longer considered adequate to remove the effects of very strong jammers. These can be generated intentionally, as in jamming situations, or unintentionally by systems such as mobile satellite systems. Continuous satellite tracking through this interference is essential for high-integrity applications. This study describes a new hardware-based GNSS interference mitigation scheme which is particularly effective against CW and pulsed CW interference. Interferers with some degree of frequency or amplitude modulation can also be reduced. The core hardware can remove two CW interferers from anywhere within the GPS P code bandwidth or the GLONASS frequency spectrum and further interferers can be removed by cascading modules. Simulation and actual data are shown. Owing to the wideband nature of the signal processing, the module introduces very little additional phase distortion and thus group delay to the GLONASS band, which has been shown to be advantageous to differential GLONASS processing. © 1997 John Wiley & Sons, Ltd.

Control and instrumentation of large nuclear power stations a review of future trends

The basic requirements of control and instrumentation (C & I) for large nuclear power stations are reviewed with special reference to the advanced gas-cooled reactor (AGR) and the pressurised water reactor (PWR), emphasising current developments and trends rather than established techniques. The extensive instrumentation, now provided for condition monitoring, particularly of the chemistry of both primary circuit coolant and the secondary steam and water circuits, is reviewed, and the way some of this instrumentation provides indication of failure of boiler tubes and fuel element cladding in AGRs is described. The power station is controlled automatically by a series of closed-loop controllers, and the current trend is discussed of employing direct digital control with redundancy, when it is justified to meet reliability requirements. However, the need for an effective operator interface is regarded as most important, and the current increased concern about ergonomics is identified, particularly in the design of the main control room, with a strong emphasis on formalised design procedures and reviews of existing designs. The increased use of computer-based techniques for control, protection, interlocks, alarm handling and display and data processing is discussed with special refer-ence to the use of colour visual-display units for data and alarm display. The special problems of high integrity applications, with the need for special design and validation of software, and the basic components of computer systems are described, and the current trend towards distributed systems and high-level user-orientated computer-programming languages is identified. Reliability aspects of C & I systems are discussed, including estimation, correlation with field experience and consideration of common mode effects. The need for evaluation and type testing and its cost are discussed and the results of such testing are reported. The overall structure of national and international C & I standards is described. The organisation and management of a C & I design group is discussed, and the requirements for a systems approach are identified with the use of a matrix structure in a multiproject environment. The rapid obsolescence of modern electronic equipment is discussed and the need for total life cycle support considerations to be incorporated in the design of both hardware and software is identified. An indication is given of current trends in instrumentation, computers and the improvement of operator interface systems.

The development of multiple redundant flight control systems for high integrity applications

Multiple redundant flight control systems have been the subject of continuous development since the 1950s. Early examples include the automatic landing systems of the BAC 1–11 and the VC10; these were followed by the full flight envelope control systems fitted to the Concorde and Tornado. These examples all use analogue computing techniques, with redundancy levels appropriate to the individual fail-passive or fail-operational requirements of each system.However, recent advances in digital technology have enabled highly sophisticated flight control systems to be implemented which would be wholly impractical using analogue computing methods. Furthermore, digital computing is amenable to self-monitoring techniques which can be used not only to improve system maintainability, but also to augment the cross comparison or majority voting algorithms which have been developed on analogue systems.