NDTnet 1998 Aug, Vol.3 No.8

Abstract

The paper deals with the problem of the determination polymeric material's mechanical characteristics with a dynamic indentation method. In this method the material under test is subjected to the impact loading with a rigid indenter and a set of its current velocity during the impact is picked. The obtained dependence of the indenter's velocity versus time during the impact is the initial data for construction of the following functions: the dependence of the indenter's penetration into material tested versus time and the dependence of the contact force versus time. From these two dependencies the dependence of the mechanical stress, which is proportional to the ratio of the contact force to the penetration squared, is computed , and the rigidity is computed as the ratio of the contact force to the penetration of the indenter into material tested. The viscosity is calculated from the assumed third element mechanical model consisting of a spring and a Voight element in series, where the Voight element includes a spring in parallel with a dashpot.

The principle scheme including the magnetic inductive transducer, the preamplifier of the analog signal, the synchronization scheme, the 12-digit analog-to-digital converter with 0.1 MHz frequency of sampling and PC is presented. The indenter was made from the nickel alloy and had a conical tip with angle 40° at the vertex. The indenter's mass was I g and kinetic energy of the impact was 0.5 mJ.

The materials considered were a polyvinilchloride (PVC) outer coating of cable after irradiation at room temperature by hard gamma rays with 10 rad/sec dose power and thermal ageing at 90°C. The total irradiation doses were 7, 30, 50, 100 Mrad. The total times of heating fell within a range of 83 to 321 days.
The maximum contact force fell within a range of 7 to 10 N and the maximum penetration ranged between 50 and 90 mcm. The rigidity ranged from 80 to 160 N/mm. The viscosity was of the order of 5 N s/m. During the test, indenter vibration may cause the noise in initial data to appear. This noise interferes with processing of the initial data. To overcome this problem, the computer program filtration and the least square method of data smoothing was applied. The application of these procedures improves the accuracy of rigidity calculations up to 5%. The results on statistical analysis of the error of rigidity are presented.

The calculated rigidities of cable's PVC outer coating are compared with elongations at break obtained from the experiments on tension strain which were carried out on tensile testing machine like INSTRON. It is shown that there is an exponential relation between the material's rigidity, which change is caused by radiation and temperature, and elongation at break. This results give us the base to propose the method of evaluation polymeric material's state.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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