Profile of a laser crack in polymethyl methacrylate

The profile of a laser crack in a transparent dielectric, polymethyl methacrylate (in the free-generation procedure), has been studied by the equal-thickness interference band method. It has been established that zones of changed material, a zone of slow change of profile, and a zone of rapidly changing profile (center of the crack) exist in the cross section of the crack. Values of thickness of the crack are given. Suggestions are expressed as to a possible mechanism of laser damage.Institute of Problems in Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 921–923, September–October, 1973.     

Absorption of light and development of a laser crack

Conclusions 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.Institute of Problems in Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 777–781, September–October, 1976.     

Deceleration of atoms in an n-photon process by means of a laser-travelling wave in the Dirac representation

A detailed theoretical study is made of the slowing down of the atomic beam by resonant laser radiation. By using the density matrix technique in the interaction representation, employing optical Bloch equations, one can obtain the average radiation force, with respect to the coherent state of the laser mode, represented in the complex plane. It is revealed that all the relevant variables are explicit functions of time via the average photon number. It is shown under the conditions which can be attained in practice that the behavior of the velocity and diffusion coefficients in both the longitudinal and transverse directions is consistent with previous works on such phenomenon.     

Supersonic crack propagation in blocks of polymethyl methacrylate

It is shown that a giant laser pulse can cause supersonic crack propagation in PMMA. The results of the experiments and a microstructural analysis of the fracture surface are presented. A study of the microstructure shows that supersonic crack propagation is associated with the propagation of a shock wave in the focal region.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1027–1029, November–December, 1971.     

Experimental study of the effect of laser beams on transparent dielectrics

The destruction of transparent polymer blocks by laser action is investigated. It is established that microscopic breakdown regions, whose size remains constant in time, are formed in the zone of action of the laser pulse. The intensity of the radiation from these microregions varies with time and corresponds to a temperature of (2–4) · 10³°K. The microregions are located near, but not at the centers of the cracks. The previously irradiated region of the polymer is shown to be in a state of stress. The possibility of thermal explosion with subsequent crack formation is evaluated. The continued development of the cracks as a result of repeated irradiation is studied. The temperature of the gas in the large cracks has been measured and found to exceed room temperature by 15°. The molecular weight has been measured before and after irradiation and found to decrease. The effect of laser beams of various wavelengths is considered. The rate of growth of the disk-shaped cracks has been measured.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 5, pp. 827–835, September–October, 1969.     

Causes of the damage created by laser pulses in transparent polymers

The characteristics of the damage created in PMM, PS, and PC specimens by multiple irradiation are discussed. It is shown that, in addition to the growth of the damage zone, the processes of crack development that determine this zone are also important. The damage created in PMM when the laser beam is interrupted by a cylindrical cavity is considered. In this case, the cavity acts as a diverging lens and the damage beyond it is determined by the scattered flux. The damage produced in specimens of different shapes is investigated. A model is proposed for crack development in a polymer irradiated by a powerful flux. The model is consistent with most of the known experimental facts.Mekhanika Polimerov, Vol. 3, No. 6, pp. 1035–1042, 1967     

Autovibrational development of laser cracks

The authors experimentally investigate the discontinuous ("jumping") character of the development of laser cracks in polymethyl methacrylate. The discontinuities arise in such an order that the crack remains rounded, even if the laser beam is rectangular in cross section. The authors estimate the pressure drop in the crack during a "jump."Institute of Problems in Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, Vol. 9, No. 3, pp. 475–481, May–June, 1973.     

Mechanical damage in polymers caused by laser pulses

The damage caused by Q-switched laser pulses and picosecond pulses in specimens of polymethyl methacrylate and polystyrene is described. The damage criterion depends on both characteristics of the laser pulse, i.e., energy and intensity. Nonlinear absorption effects are not observed when the energy threshold is crossed. It is shown that Q-switched and free-running laser pulses react with polymer specimens in essentially the same way. The induction period and rate of crack development have been established. The emission excited in the specimen by the laser pulse is investigated.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow, Physics Institute, Academy of Sciences of the USSR, Moscow, All-Union Scientific-Research Institute of Optico-physical Measurements, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 436–444, May–June, 1971.     

The origin of embryonic laser cracks

The problem of the initial stage of crack formation in PMMA subjected to laser action is investigated. It is shown that the cracks develop from gas-filled cavities (centers). These centers have been experimentally investigated, and the center-crack transition carefully studied.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 1, pp. 82–89, January–February, 1973.     

Visualization of the simulation results for multi-dimensional interaction of laser radiation with a photonic crystal

The article describes a software interface for the visualization of multidimensional time-dependent intensity distribution of laser radiation propagating in a photonic crystal. The intensity distribution is displayed in a three-dimensional space in color, which makes it possible to model the fourth dimension. The program constructs different sections of the intensity distribution, charts the evolution of the surface with a given intensity increment, and writes avi files. It allows viewing selected regions with different levels of detail and if necessary interpolates the computation results to achieve the required smoothness of the intensity distribution. The software has been validated and tested for the problem of transmission of laser radiation through a photonic crystal formed by polymer-filled spheres.     

Conditions for quasi Cerenkov radiation,generated by optical second harmonic polarisation in a nonlinear cristal

The analogy between classical Cerenkov radiation and quasi Cerenkov radiation, induced by intense laser light in a nonlinear crystal, is described. The effect is compared with noncollinear phase matching. In certain cases of interaction in birefringent crystals only second harmonic light within a restricted angular region with respect to the optic crystal axis can be generated by quasi Cerenkov radiation, whereas for all other directions the noncollinear phase matching condition can be fulfilled. A calculation shows that in addition to fulfilment of the Cerenkov condition there exists for quasi Cerenkov radiation a restriction on the nonlinear crystal. This restriction corresponds to the laser intensity inside the nonlinear crystal. This restriction corresponds to the fact that for quasi Cerenkov radiation virtually only longitudinal momentum is conserved. As an example, second harmonic generation in LiNbO₃ with an Nd: glass laser is discussed.     

Light absorption and laser-crack development. 2

Conclusions 1. The gray color of a laser crack in PMMA (black in polystyrene) is caused by carbon-black particles that adhere to its walls.2. Further experimentation under an electron microscope is necessary to determine the size of the carbon-black particles. Making use of these new data and the results described here, we can define their absorption index more precisely.3. The absorption index of the gaseous medium between the walls of the crack in a layer 30–100 µ thick is 20 cm^–1; this is 1000 times smaller than that of graphite, but approximately 1000 times greater than that of the ordinary block polymer.4. As a result of photochemical and photothermal reactions, the laser source creates a gray (PMMA) or black (polystyrene) absorbent layer located in the crack and on its walls. Thus, the energy of the laser beam is absorbed due to "self-inductive" opacity. This leads to an increase in the mass of the "gas wedge," to a pressure increase, and ultimately, to crack propagation.For communication 1, see [1].Institute of the Problems of Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 409–412, May–June, 1977.     

Different Optimization Models for Crack-Free Laser Welding

Conventional laser beam welding of aluminum alloys often leads to hot cracking. This is caused by a complex process where thermo-mechanical and metallurgical aspects are involved; cf. [3], [2]. A possibility to prevent hot crack initiation yields the multi-beam welding technique (cf. [2]), where additional laser beams are led parallelly besides the main laser beam. There by optimal positions, sizes, and powers of the additional laser beams play an important role otherwise hot cracking can even be enhanced. In [1], [4], resp., a mechanical 1D and thermal 2D model of hot cracking was derived. It provides the basis for different formulations of constrained nonlinear programming problems to identify the optimal parameters of the additional laser beams. In the present paper a comparison between these formulations and between two different optimizers for the so far best formulation are presented. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of laser radiation on the mechanical strength of a block of polymethyl methacrylate

The effect of pre-existing microcavities, created by laser radiation, on the mechanical strength of polymethyl methacrylate is investigated. It is established that "seeding" with microcavities does not affect the breaking stress and relative elongation, but does shorten the lifetime.Institute of Problems of Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 2, pp. 247–250, March–April, 1971.     

Fracture under cubic compression of polymethylmethacrylate subject to laser radiation

It has been established that samples of PMMA under cubic compression retain their fracture characteristics when subjected to laser radiation. Only the length of the fracture region is altered. The dimensions of the microdefects and the pressure within them causing the growth of discoidal fracture cracks have been determined.Institute of Problems in Mechanics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 449–452, May–June, 1970.     

On the peculiarities of a crack growth path in anisotropic solid

One of the possibilities to increase the resistance of a structure to catastrophic fracture is to force a main line crack to deviate from its path. In this connection the influence of the elastic moduli of an anisotropic material on the possibilities of crack rotation are studied. In particular a linear elastic problem for a straight Mode I crack, located on a symmetry axis of an orthotropic plane is considered. The strength properties of the material are supposed to be isotropic. For studying a direction of a crack growth path several crack models are considered. It is shown that a thin elongated elliptical hole as a crack model leads to more plausible results concerning crack rotation conditions than an ideal cut model. The maximal tensile stresses are taken as a crack growth criterion. It is shown that for some class of orthotropic materials a crack deviates from the straight path just after it starts to grow even in the conditions of uniaxial normal tension. The problem of the stability of a straight crack path under Mode I loading is also considered. This problem is reduced to the problem of the fracture direction determination for thin elongated elliptical cavity slightly inclined to the initial direction. In the frame of the proposed approach the conditions of instability are obtained. It is shown that for some class of orthotropic materials a straight crack path is unstable in the conditions of uniaxial normal tension. This class of materials is wider than one for which a crack deviates from the straight crack path just after its start. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Destruction of epoxy resin by 10−3-sec laser pulses

High-speed photography is used to investigate the destruction of epoxy resin by laser pulses. The results suggest the following mechanism is at work. Initially, structural changes (gas-bubble formation) take place near the focus of the beam. These lead to the formation of an initial crack of fixed minimum size. At first the crack grows rapidly, then more slowly, as it is screened by other cracks. The discontinuous crack growth predicted by the theory is observed, the jump velocities being an order greater than the mean rate of crack growth on the same interval. The total area of destruction increases linearly with time; at the beginning of the process this increase is primarily associated with an increase in the number of cracks, in the final stages with the further growth of the cracks already formed.Scientific-Research Institute of Mechanics, Moscow Lomonosov State University. Translated from Mekhanika Polimerov, No. 3, pp. 460–464, May–June, 1969.     

Kinetic theory of a super-radiating laser

Statistical properties of the radiation of a super-radiating laser are studied on the basis of the kinetic equation for the density matrix of the generating field. An original derivation of this equation is proposed. Statistical properties of the laser medium are investigated in the stationary generating regime. We analyze the role played by the decay rate of intracavity photon fluctuations and by the statistical properties of the active medium in the suppression of the photocurrent shot noise. We consider the external pump to be weak, which is physically the most interesting case. It is shown that if the active medium is weakly excited, then only a 50% depression of the shot noise is possible. In the case of a strongly excited medium, depression can reach 100%. The spectrum of optical generation is obtained for conditions far from the instability zone.Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 109, No. 2, pp. 250–269, November, 1996.