Vibratory Test Research Articles

Influence of running time under service conditions and thermal cycling on the fatigue life of the working blades of gas turbine engines

The working blade of the first stage in a marine gas turbine engine made from the chromium-nickel alloy EI826 was studied. Tests were run under service conditions in the actual engine, and also in a gasdynamic test bed which applied cyclic thermal loading to the blade in a gas flow of variable parameters simulating the most dangerous transient working conditions of the engine. The residual service life was determined by the complete fracture method, using a VL-1 electrodynamic vibratory test machine. The procedure for the test is described.

Response of Embedded Footings to Vertical Vibrations

Field vibratory tests have been carried out to study the effect of embedment on the vertical response of footings. Steady-state vibration tests indicate decrease of maximum amplitudes of motion and increase of resonant frequencies as a consequence of embedment. A lumped parameter analog with the inclusion of Coulomb friction damping is suggested to explain the dynamic behavior of embedded footings. A simple procedure by which the constant frictional force of the Coulomb friction damping can be evaluated has been described. The suggested procedure takes into consideration the relevant physical characteristics of the interface between the foundation walls and the surrounding soil. The field test data obtained by the writers as well as those published by other investigators elsewhere are compared with the values predicted by the proposed analog. The correlation between the available experimental data and the predicted values is satisfactory.

Studies on Cavitation Erosion

Cavitation damage tests of iron and steel have been carried out using two types of experimental facilities. One is a water tunnel system which presumably simulates the field conditions fairly closely and the other is a magnetostriction vibratory unit commonly used for accelerated cavitation erosion testing of metals. A comparison between water tunnel and vibratory tests shows a difference in mechanism of cavitation damage. Consequently, it has been concluded that the application of the test results obtained in a magnetostriction vibratory apparatus should be limited to provide rough data for the primary selection of proper materials to be used in the construction of hydraulic units and other industrial systems, unless damage is caused under similar conditions to that of the vibratory apparatus.

Report 2: Investigation of Cavitation Erosion in Non-Aqueous Liquids

Wear due to cavitation in non-aqueous liquids can be serious, leading to premature failures of valves, bearings, and even pumps. Although it can be shown that the collapse of vapour cavities will cause more damage than those containing some gas it is not possible to predict theoretically and with any certainty the stresses produced in materials. Thus, at present, laboratory erosion tests offer the best means of determining the relative behaviour of materials in a single liquid. However, previous experimental investigations using the vibratory cavitation test with different liquids have also been inadequate because of the many variables in the process and the lack of control over the test liquid conditions. An improved test device of the same type has been built at NEL in which temperature, pressure, and gas content can be varied independently. This has eliminated many of the disadvantages of former equipment and permitted a study of the effects of fluid properties to be started.

A discussion on deformation of solids by the impact of liquids, and its relation to rain damage in aircraft and missiles, to blade erosion in steam turbines, and to cavitation erosion - Practical aspects of cavitation

Damage produced by cavitation under field conditions can be a serious problem. The main causes of this damage and its characteristics are discussed briefly and possible remedial measures are examined. Accelerated laboratory tests are found to play an important part in cavitation erosion research, but interpretation of results needs care. Most past investigators have tended to treat cavitation damage and droplet erosion as unrelated phenomena and only qualitative correlations between the respective simulated tests have been possible. This paper presents an attempt to correlate quantitatively the results of three different erosion tests. A broad correlation between results of the drop impact erosion and constricted tube cavitation tests shows general agreement. A more detailed, but restricted, correlation has been obtained between results of drop impact and vibratory cavitation erosion tests. In both correlations, however, there is evidence of some discrepancies between corrodible and incorrodible materials. A number of factors which govern the rate of damage in the various laboratory tests are of interest. In particular, in the drop impact test the velocity of collision and the jet diameter are shown to have significant effects. There is a marked similarity between the behaviour of materials in this test and in fatigue tests and also evidence of a threshold velocity below which measurable damage ceases. The other laboratory tests were found to have their own particular controlling parameters, but the general phenomenon of cavitation erosion is more complex and is not discussed in detail. By conducting comparative tests under reproducible conditions it has been possible to classify a variety of new and traditional materials in order of relative erosion resistance and thus provide some guide to their selection for service. While the results add to the evidence that hardness is the major attribute controlling erosion resistance other properties such as ductility, elasticity and fatigue strength are seen to be significant.