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.     

Initial stage of laser-induced crack development in polymethyl methacrylate

It is shown that the successive stages of development of the damage caused by a laser beam are the same in technically pure polymethyl methacrylate and the same material containing artificially introduced absorbing particles. The formation of the characteristic flat cracks from gas bubbles in the specimen is observed. The mechanism of fracture development in polymethyl methacrylate is further refined.Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 551–552, May–June, 1971.     

Structural characteristics of laser damage in polymethyl methacrylate

Three types of optical defects are present in the starting structure of specimens of PMM: nontransparent inclusions (1 µ and above), small (less than 1 µ) scattering centers, and local elastic-stress fields. The relationship between these defects and the damage centers excited by laser radiation is considered.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 5, pp. 944–945, September–October, 1970.     

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.     

Notch sensitivity of polymethyl methacrylate

It is shown that acrylics have a notch sensitivity determined by the nature of the state of stress produced by the loading.Lenin Riga Komsomol Institute of Civil Aviation Engineers. Translated from Mekhanika Polimerov, No. 3, pp. 562–564, May–June, 1971.     

Effect of continuous gas-laser radiation on polymethyl methacrylate

The effect of continuous powerful (up to 20 W) 10.6- radiation on PMM is described. The relationship between crater growth and time is established. A region of softened polymer with a clearly defined boundary, an isotherm with temperature equal to the softening point, has been detected. Theoretical estimates of the propagation velocity and depth of the crater are obtained starting from the heat balance equation. The agreement with experiment is good.Mekhanika Polimerov, Vol. 4, No. 2, pp. 282–287, 1968     

Accumulation of fatigue defects in polymethyl methacrylate in rotational bending

The results of tests on polymethyl methacrylate for fatigue during polycyclic rotational bending through an angle under thermostatic conditions are analyzed. A method of calculation is given for the determination of the defects. The observed changes in the mechanical characteristics and the fractography of the fractures are taken into account comprehensively. Possibilities for the simplification of the method of calculation, and the acceptable methods for the indication of defects before the formation of the main crack are discussed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 1038–1045, November–December, 1974.     

Analysis of fracture surfaces in polymethyl methacrylate tested in bending with rotation

Typical fracture surfaces are examined for polymethyl methacrylate specimens subjected to multicycle fatigue testing in circular cantilever bending at T=20, 40, and 60°C. It is established that the fatigue life of smooth specimens is very closely correlated with the size of the diffuse fracture zone. The kinetics of damage accumulation with increase in the number of cycles are investigated by rapidly fracturing prefatigued specimens. The decisive role of diffuse fracture in the fatigue-fracture process is demonstrated. A method of estimating the mean main crack propagation velocity in circular bending is described.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 984–990, November–December, 1973.     

Effect of biaxial orientation on the fracture kinetics of polymethyl methacrylate

Biaxially oriented PMM is shown to have important advantages as a structural material over unoriented PMM owing to a difference in fracture kinetics. Under identical conditions primary cracks appear later in the oriented PMM, their growth is impeded, and the rate of crack propagation in the avalanche stage of failure is much lower. This accounts for the greater resistance of biaxially oriented PMM to stress raisers and its higher fracture energy under biaxial loading.All-Union Scientific-Research Institute of Aviation Materials, Moscow. Translated from Mekhanika Polimerov, Vol. 5, No. 2, pp. 274–281, March–April, 1969.     

Fatigue life of polymethyl methacrylate in stationary and stepped nonisothermal cyclic loading regimes

Rotating cantilever experiments have revealed the heating characteristics of PMMA in multicycle fatigue tests. The effect of heating and damage on the fatigue life under stationary and single-step loading is considered. The statistical characteristics of the fatigue life are presented.     

Investigation of the thermophysical properties of polymethyl methacrylate at high hydrostatic pressures

Conclusions 1. The experimentally observed course of heat capacity is explained by competition of contributions from the lattice and defects, on one hand, and from retarded rotators, on the other.2. An equation is proposed which describes the dependence of the thermal conductivity of polymers on pressure and which agrees with experimental data to an accuracy of ±1%.Moscow Chemical Engineering Institute. Translated from Mekhanika Polimerov, No. 5, pp. 845–851, September–October, 1976.     

Nature of the friction deformation of the surface layers of polymethyl methacrylate

The surface deformation of amorphous thermoplastics (polymethyl methacrylate) by a spherical steel indentor has been investigated at various sliding velocities. Small velocities correspond to elastic and forced-elastic deformation of the surface layers and asperities. At temperatures corresponding to the high-elastic state the deformed surface layer completely recovers its shape. As the sliding velocity increases, the forced-elastic deformation is localized in a thinner layer of plastic. Starting from a certain velocity, depending on the temperature and the activation energy for transition of the chain segments from one equilibrium position to another in the process of thermal motion, the deformation of the surface layers and asperities becomes purely elastic. In the event of elastic deformation at pressures above a certain value the surface layer of plastic suffers brittle fracture in the tensile zone behind the indentor.Mekhanika Polimerov, Vol. 4, No. 1, pp. 90–94, 1968.     

Reduced strength of polymethyl methacrylate saturated with carbon dioxide at high pressure

The effect of external pressure on the amount of carbon dioxide sorbed by specimens of polymethyl methacrylate and on the strength of the specimens after removal of the pressure during the initial period of desorption has been experimentally investigated. The experiments were conducted at 20°C, the pressure varied from 20 to 56 bar.S. M. Kirov Ural Polytechnic Institute, Sverlovsk. Translated from Mekhanika Polimerov, No. 2, pp. 243–246, March–April, 1971.     

Removing "silver" type microcracks in polymethyl methacrylate glass by dry friction

"Silver" type microcracks in polymethyl methacrylate are self-removed on aging. Dry friction can assist in their accelerated removal; by intensifying relaxation processes and causing flow in the surface layer of the polymer, it is able to restore both the polymer structure in the zones of "silver" type microcracks and also to age obvious microcracks. The restored polymer quality is stable. Dry friction raises the resistance of polymethyl methacrylate to splitting in failure.Lenin Civil Aviation Engineering Institute. Translated from Mekhanika Polimerov, No. 3, pp. 465–470, May–June, 1970.     

Growth of arterial cracks in polymethyl methacrylate plates containing "silver" cracks

Conclusions 1. We have studied the influence of previously formed surface SC of various orientations on the force and kinetic characteristics of a slowly growing arterial FC. The subject of investigation was industrial glass based on polymethyl methacrylate. Steady growth of the FC was achieved in our experiments by applying torque in order to split lamellar samples.2. We have found that for no mutual disposition of the two kinds of cracks do surface SC ease the conditions of FC growth. On the contrary, in a number of cases the presence of SC having linear dimensions of only 3–5% of the front length interrupt the steady growth of the FC, reduce its average velocity, and sometimes stop it altogether. For the growth of the FC to be renewed the load has to be augmented, so that the limiting specific work of fracture W_lim increases by 10–40%. The growth of an FC is opposed most strongly by SC lying at right angles to it.3. The results of our experiments indicate comparatively easy fracture of the SC, and also show that FC prefer to develop along earlier-formed SC, subject to appropriate mutual orientation.4. By virtue of the presence of elastic interactions between different sections, any action applied to a small part of the leading edge (front) will alter the conditions of material fracture along the whole FC front.Institute of High-Molecular-Weight Compounds, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 4, pp. 609–615, July–August, 1976.