Absorption of light and development of a laser crack
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1. The effective threshold for the absorption of laser radiation by a crack on repeated irradiation is ∼15–30 J/cm2.
2. After reaching the threshold, the surface of the crack in the irradiation zone becomes clarified and passes light almost to the same extent as the undamaged material.
3. The absorption observed in the present experiments is due to the opaque products of polymer decomposition. The gray absorbing deposit is probably soot, very finely dispersed. Adhesion takes place at the instant at which the crack jumps, when there is an effective injection of gas into the almost evacuated space of the new break; it occupies a time t ≤10 µ sec. It is by no means impossible that the decomposition process which started in the highly absorbing gray layer initiates and attracts into reaction a thin layer of the pure material as well.
4. The repeated irradiation of the crack through the clarified zone so created may lead to the decomposition of the material, the formation of gas, and the completion of crack development. For this purpose the energy density has to be roughly doubled by comparison with the threshold value corresponding to the nonclarified crack.
KeywordsRadiation Polymer Energy Density Thin Layer Present Experiment
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