Methods for mechanical testing based on analysis of elastic waves in rods

University dissertation from Luleå : Luleå tekniska universitet

Abstract: This thesis summarizes six papers on development and applications of a method to evaluate the normal force and particle velocity at any cross-section of a nonuniform rod from measurements of strain at two different cross-sections. In Paper A the method is developed. It is an extension of that developed by Lundberg and Henchoz for uniform rods. Theoretical results, based on onedimensional elastic wave theory, are provided for the general case as well as for cases of piecewise constant characteristic impedance. In Paper B an extension of the method has been used to determine the characteristic impedance function for an initially uniform rod heated at one end. The results show that force and particle velocity at the heated end can be evaluated with the same accuracy as for a uniform rod. In Paper C the method has been used to extend the split-Hopkinson pressure bar technique to testing of high-strength materials such as ceramics. The technique gives an accurate value of the fracture stress and a fair approximation of the strain rate. A modified Hopkinson pressure bar has been used in Paper D to determine the dynamic fracture toughness from the load-point displacement of a three-point bend specimen. The latter quantity was assumed to be proportional to the stress intensity factor. The results agreed well with those obtained from strain measurement near the notch tip. From the penetration tests made in Paper E it was concluded that the method established allows force-penetration data to be determined satisfactorily for standard threaded rod-bit configurations. Paper F presents a rod transducer for rotational point mobility. The tests showed that the transducer works satisfactorily in frequency and mobility ranges of approximately two decades.

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