Photoacoustic methods of investigating fibre materials

Abstract

The possibility of measuring the mechanical, particularly the elastic, plvperties of fibre materials by a photoacoustic method with a sufficient degree of accuracy was investigated. Thin monofilaments based on polymers with d(([erent maclvmolecular chain rigidity were investigated.New construction and composite materials and fabrics are used in complex conditions involving extreme physical effects: impact loads on bullet-proof vests, powerful laser irradiation on the shell of moving objects, the flame blowing effect on different containers in fires, etc. [1][2][3][4]. In these conditions, the dynamic characteristics of the materials are extremely important, since they determine the resistance to the articles to dynamic loads. Fibre materials have been increasingly used in such articles recently. For this reason, determining the dynamic characteristics ofmonofilaments and the fibre material as a whole is becoming increasingly important. Most standard methods of detemaining such characteristics are static or quasistatic.The photothermoacoustic method, which can be used to study the thermal, optical, and mechanical characteristics of samples, is in our opinion the most universal method of determining the dynamic properties of a material [5][6]. The essence of this method is that thermal and acoustic fields identified by the corresponding detectors are formed in the material under the effect of light. Photothermoacoustics (PTA) examines not just any transfomlations of light into heat but only those related to absorptio n of light in a medium modulated in time. They cause thermal fluctuations in the medium and as a consequence, acoustic waves, and this takes place in absorption of light modulated by intensity in time in the medium, usually for a comparatively low intensity of irradiation where the aggregate state of the medium does not change. As for the tennphotothermoacoustics, the termphotoacoustics (PA) is used relatively frequently in place of it, and optoacoustics (OA) is used less frequently, especially with respect to recording the effect of acoustic waves without evaluating intermediate temperature changes.In a substance in which a PTA effect takes place, three types of waves can simultaneously be propagated: light, themlal, and acoustic -and two types of transformations are observed: modulated light into thermal modulations with the appearance of thermal fields and thermal modulations into pressure modulations with the appearance of acoustic fields.Modulation of the temperature at a certain point of the medimn by the absorbed part of the energy of the modulated light beam is not only a source of thermal waves, but also results in modulation of the thermal change in the density of the medium, which creates a density gradient in the substance and causes modulation of the pressure at this point and consequently the appearance of acoustic waves. If the medium is a spatially distributed source of themaal waves, then it simultaneously also serves as a spatially distributed source of acoustic waves.The PA effect is...

Photoacoustics: a historical review

“…The pulsed variant of photoacoustics may be described as direct photoacoustics, while the gas-microphone detection of photothermal effects may be described as indirect photoacoustics, or even as photothermoacoustics, which would then accentuate the role of thermal waves in the mechanism responsible for the generation of acoustic waves. The term photothermoacoustics as introduced by Sidorov et al [173] is arguably the best suited to describe the phenomenon, in terms of the processes involved.…”