He Jiawen

Erosion wear behaviour and model of abradable seal coating

The abradable seal coating for an aircraft turbine engine is mostly composed of metal phase, self-lubricating non-metal phase and many pores. The erosion wear resistance is one of the most important properties of the coating. The erosion wear behaviour and mechanism of several kinds of middle temperature seal coatings were investigated by a CMS-100 self-made vacuum sand erosion machine. The results show that the relationship between the erosion mass loss and the erosion time is linear, the coatings hold a maximum erosion rate at 60° impact angle, and the relation between the erosion rate and the impact speed is an exponential function. The speed exponent increases with the increase of the impact angle. At 90° impact, the abrasive particles impinging on the coating surface produce indentations and extruded lips, and then the lips are work-hardened and fall off; and flattened metal phase grains are impacted repeatedly, loosed and debonded. At 30° impact, the micro-cutting, plowing and tunneling via pores and non-metal phase are involved. The model of the erosion mechanism is advanced on the basis of the above-mentioned erosion wear behaviour.

Thermal effects of residual macrostress and microstrain in plasma-assisted CVD TiN films

Abstract The effects of both the deposition and tempering temperatures on the residual macrostress and microstrain are investigated in plasma-assisted chemical vapour deposition (PACVD) TiN films. Unlike the case of thermal CVD processes, the residual macrostress in PACVD TiN films decreases with the increase of the deposition temperature and shows a dramatic drop once the temperature is above 700 °C. The residual microstrain, indicated by peak broadening and a stress-free lattice constant, exhibits the same variation with the deposition temperature. This implies that the competition for the formation of stress and strain is related to the coating process and that the macrostress in PACVD TiN film is usually of an intrinsic nature. Nevertheless, the thermal stability of macrostress behaves differently from that of microstrain. The tempering treatment tends to release residual macrostress but shows little effect on the microstrain detected by the X-ray method. Different origins may exist for the macrostress and microstrain in the film, although only those films deposited at relatively low temperatures and, hence, involved mainly with intrinsic macrostresses display a significant relaxation rate.

Determination of fatigue cracking direction in composite laminates

The interface plays the central role in the failure analysis of composite laminates, therefore, the interface material properties are taken as the independent parameters. A simple, universal and practicable criterion, i.e. a ratio criterion of strain energy release rate, is proposed to determine the growing direction of a fatigue crack in the composite laminates. The method of arbitrary lines, which is very effective to solve the problems with high gradient feature, is used to analyze the experimental results at the key moments when a crack kinks, turns into the interface, or bifurcates. An approximate method of computing the energy release rate is given. The fatigue fracture tests of composite laminates are carried out, and the numerical predictions of crack growing directions agree well with the experimental results. It is concluded that the methods suggested in this paper are effective to obtain the cracking history and the growing path of a fatigue crack in composite laminates.

X-ray stress constants of tungsten alloys

Abstract This paper studies the X-ray stress constants of tungsten (W) alloys determined experimentally with the X-ray diffraction method. Specimens were designed in the shape of an equi-stress cantilever beam and were machined from materials with different W contents. Their surfaces were ground with abrasive papers to simulate the processing states of real products. The stress constants for different surface states were measured using Co Kα and Fe Kα radiations on (222) and (310) diffracting planes. The effects of W contents and surface states on the stress constant are discussed. It was found that the stress constants did not change much with W content, but decreased significantly with increase of surface plastic deformation. For the purpose of estimating the stress constant, the half width values of the diffraction profiles are used as a measure of the degree of surface deformation. The relations of stress constant with half width value are built up in terms of the experimental results, thus it is possible to determine the residual stresses for different processing states in real production.

EFFECT OF PRE-LOADING ON SHORT FATIGUE CRACK GROWTH AT A NOTCH

ABSTRACT Results indicate that under cyclic tension the growth rate of short fatigue crack from notch root will be lowerd greatly by tensile pre-loading, but only shows a little change by compressive preloading. The effect of tensile pre-loading will decrease with the increase of stress ratio. The variation of short fatigue crack growth rate is related to the residual stress distribution around notch root.