Simplifying Control Flow in Compiler-Generated Parallel Code

Optimizing compilers for data-parallel languages such as High Performance Fortran perform a complex sequence of transformations. However, the effects of many transformations are not independent, which makes it challenging to generate high quality code. In particular, some transformations introduce conditional control flow, while others make some conditionals unnecessary by refining program context. Eliminating unnecessary conditional control flow during compilation can reduce code size and remove a source of overhead in the generated code. This paper describes algorithms to compute symbolic constraints on the values of expressions used in control predicates and to use these constraints to identify and remove unnecessary conditional control flow. These algorithms have been implemented in the Rice dHPF compiler and we show that these algorithms are effective in reducing the number of conditionals and the overall size of generated code. Finally, we describe a synergy between control flow simplification and data-parallel code generation based on loop splitting which achieves the effects of more narrow data-parallel compiler optimizations such as vector message pipelining and the use of overlap areas.     

Transport of large nuclear power plant components: experiences in mechanical design assessment

Abstract

In the course of decommissioning of power plants in Germany large nuclear components (steam generator, reactor pressure vessel) must be transported over public traffic routes to interim storage facilities, where they are dismantled or stored temporarily. Since it concerns surface contaminated objects or low specific activity materials, a safety evaluation considering the IAEA transport regulations mainly for industrial packages (type IP-2) is necessary. For these types of industrial packages the requirements from normal transport conditions are to be covered for the mechanical proof. For example, a free drop of the package from a defined height, in dependence of its mass, onto an unyielding target, and a stacking test are required. Since physical drop tests are impossible generally due to the singularity of such 'packages', a calculation has to be performed, preferably by a complex numerical analysis. The assessment of the loads takes place on the basis of local stress distributions, also with consideration of radiation induced brittleness of the material and with consideration of recent scientific investigation results. Large nuclear components have typically been transported in an unpackaged manner, so that the external shell of the component provides the packaging wall. The investigation must consider the entire component including all penetration areas such as manholes or nozzles. According to the present IAEA regulations the drop position is to be examined, which causes the maximum damage to the package. In the case of a transport under special arrangement a drop only in an attitude representing the usual handling position (administratively controlled) is necessary. If dose rate values of the package are higher than maximum allowable values for a public transport, then it is necessary that additional shielding construction units are attached to the large component.     

Reactive sintering in Fe-Co system

Abstract

Low porosity powder metallurgy compacts have been manufactured from treated elemental iron and cobalt powders sintered at 1150°C under an H_2(g) atmosphere. Their microstructures consist of an interconnected mixed oxide network which encapsulates both the iron and cobalt phases. The production technique employed is an innovative process termed reacto-thermitic sintering (RTS), which leads to near full density and near net shape parts utilising conventional uniaxial compaction and mesh belt furnace practices. The RTS technique relies on microscale exothermic reaction between small quantities of added elemental Al and oxides present on the surface of the bulk powder, together with the bulk powder itself. This results in the production of a transient liquid phase which freezes rapidly and consolidates the compact without slumping. In order to generate an interconnected mixed oxide network, experiments were designed such that the Al powder reacts with the cobalt and the surface of the iron powder which is artificially doped with Fe and Cr oxides.

Differential thermal analysis (DTA) and energy balance calculations revealed that the Al and the oxide coating reaction does not proceed directly. Instead the main contribution to the exothermic process is the reaction between Al and Co/Fe. The system does not exhibit true RTS behaviour and the interconnected network of mixed Al, Cr, and Fe oxides is created by subsequent reaction of Co-Al and Fe-Al intermetallics with the artificial Fe-Cr oxide coating on the Fe. The microstructure obtained exhibits negligible porosity with the metallic particles on the whole fully encapsulated by the oxide.     

Fatigue crack growth in some PM low alloy steels consolidated by rotary forging and sintering

Abstract

Fatigue crack propagation rates under plane strain conditions have been investigated for three PM low alloy steels consolidated to high densities by rotary compaction followed by sintering and heat treatment. It is shown that the densities and properties are intermediate between those of pressed and sintered materials and of powder forged materials. Threshold stress intensities compare satisfactorily with those for wrought counterparts, but resistances to crack growth are inferior to those of wrought steels. Possible reasons for the properties of the rotary compacted materials are considered in the light of their microstructures and the behaviour of other PM materials.     

Inferences on plastic properties and coefficient of friction during simultaneous compression deformation of dissimilar sintered powder metallurgical preforms

Abstract

Plastic working of powder metallurgical (PM) material necessitates the development of fundamental data such as flow stress, densification behaviour, coefficient of friction, apparent strength coefficient, apparent strain hardening exponent, plastic Poisson's ratio, etc. In the present work compression and standard ring compression tests have been carried out to generate the fundamental data for simultaneous deformation of sintered steel and copper powder metallurgical preforms. The results reveal that the behaviour of individual materials during simultaneous deformation is strongly influenced by local micromechanical interactions at the metal - metal interface. In addition to this, the test conditions (iso-stress and iso-strain) strongly influence the severity of interaction. The interfacial friction coefficients are less than that of the same material when tested between hard tools. The optimal process parameters with higher interfacial friction, which can enhance the solid state joining of dissimilar materials, have been identified. The flow stress of the composite (steel - copper combination) during simultaneous deformation can be estimated if the flow stress of the individual materials comprising the combination/composite are known. With these studies, it should be possible to extend the inferences to the major deformation processes.     

Micro-gravity experiment of a space robotic arm using parabolic flight

We conducted a micro-gravity flight experiment on a space robotic arm, which is a part of the Reconfigurable Brachiating space Robot (RBR) unit arm developed by the authors. We used a 4-d.o.f. arm and an end-effector in the experiment. The airplane (MU-300) generates the micro-gravity environment for approximately 20 s in parabolic flight operation. After the flight, we conducted the corresponding ground experiments, and obtained the data of the motor current, servo control characteristics and manipulation performances, which were compared with the flight experiment data. Then, we conducted the numerical analysis of the 4-d.o.f. RBR arm based on the experiment results. In the analysis, we investigated feasibility of simulation model and identified model parameters. In this paper, we report the results of the flight experiments and numerical analysis.     

Influence of heat treatment processes on fatigue performance of particle reinforced aluminium alloys

Abstract

The fatigue performance of particle reinforced metal matrix composites improves as the matrix strength is increased. However, the heat treatment required for high matrix strength induces residual stresses into the material, which need to be balanced against potential distortion during machining of components. This paper reports results showing the fatigue behaviour of a 2124 aluminium alloy reinforced with 25 vol.-% of silicon carbide particles. The effect of quench medium on tensile and rotating bend fatigue strength is reported. Results are correlated with residual stress profiles measured in quenched plates of the material.     

12—THEORETICAL CONSIDERATIONS IN OPEN-END SPINNING

Two problems are discussed in this paper. In the first, the effect of the deposition of fibres in open-end-spinning systems on the yarn irregularity produced is considered.

The drafting operations are divided into two types, that is, System I and System II. System I is the type in which fibres are accelerated when the leading ends arrive at a point or a specified area, and System II is the type in which the trailing ends are accelerated in the same way as those of System I. System II is more profitable if the fibres have non-uniform length, and the leading-fibre-end-density function contains most of the irregularity. The rotating-drum spinning method is also considered from the viewpoint of yarn irregularity, and the range of wavelengths that should be decreased by this method is calculated. The conclusion is that the total draft must be smaller than 2πR/[lbar], where R is the inner radius of the drum and [lbar] is the mean fibre length.

The second problem concerns the separation draft. Open-end drafting systems require very high drafts for separating the fibres. The problem is to determine what value of draft is needed. The conclusion reached is that perfect separation is impossible because of the random arrangement of fibres. The probability of separation is presented and calculated theoretically.     

ADAPTIVE RIGID-PLASTIC MESHLESS GALERKIN METHOD AND ITS APPLICATION IN ANALYSIS OF EXTRUSION PROCESSES

Under the hypothesis of the rigid-plastic material,specific efforts are placed on the deve-lopments of the key simulation techniques of the meshless Galerkin method because of the complexity of the deformation process as well as the generality and atomization of the simulation procedures for non-steady state large deformation plastic processes,therefore,an adaptive rigid meshless Galerkin method is developed. The influence domain control method is used in the least square approximation by dynamic evaluation of the magnitude of the influence domain and the effective control of the amount and the positions of the points in the least square approximation in order to improve approxi-mation precision. The amount of the Gauss integration points in the discrete domain is maintained in a considerable magnitude in order to ensure the integration precision in the discrete domain. The length of the frictional boundary of the plastic deformation process may be getting longer when its deforma-tion is getting severe. Thus,the densities of the boundary points of some places get lower. The adap-tive boundary points setting method is employed to improve the approximation precision of the boundary points and enhance the constraint of the boundary condition by adaptive control of boun-dary point density. Some typical extrusion processes are analyzed,detail simulation results such as the deformation field,velocity field,effective strain field,effective strain rate field,the volume loss curve and load-stroke curve are obtained. The effectiveness of the method developed is demonstrated and the precision of the meshless simulation is proved by overall comparison with the results obtained by using the commercial software deform.