Structural damage accumulation and stable postcritical deformation of composite materials

The macroscopic failure of composite materials is preceded by complex multilevel processes accompanied by accumulation and localization of damaged centers and formation of a failure cluster. Therefore, the study of these mechanisms is one of the basic problems for the mechanics of modern composite materials used in aerospace engineering. The formation of a theory of the stable postcritical deformation of the work-softening media is considered. The pseudo-plastic deformation affected by structural damage of granular composites is investigated within the framework of the considered two-level structurally phenomenological model of heterogeneous media. The stable evolution of the interconnected processes is accompanied by stress redistributions, partial or complete unloading, and strain or damage localization that are one of the main causes of implementation of the postcritical deformation stage. The numerical calculation results of inelastic deformation and failure of the periodic unidirectional fiber-reinforced composites are presented under conditions of the displacement-controlled transverse proportional loading mode. The main mechanisms of the work-softening behavior for the indicated type of materials are described in the macro-homogeneous stress-strain states. Macroscopically, the failure of heterogeneous media as a result of postcritical deformation and the loss of stability of damage accumulation depends on the stiffness of the loading system. When a deformable body is fixed on the closed surface with sufficiently but not infinitely large coefficients of stiffness, it is possible to observe the equilibrium development of the localized volumes of work-softening and damage. The constitutive equations for the work-softening isotropic, transverse isotropic, and orthotropic media are presented. The effect of the loading system on the stability of deformation, damage accumulation, and failure under monotone and nonmonotone triaxial loading was studied. The growth of failure strains with increase in stiffness of the loading system and unequal resistance of heterogeneous body are registered and investigated. A preventive unloading method is offered for the mathematical modeling of the damage accumulation during the testing of the materials on the servo-controlled systems. The displacement-controlled mode is simulated by a series of soft loading and unloading cycles. The detected phenomenon of failure where the unloading leads to stress-strain diagrams with a negative slope of the descending branch was not found either in the displacement or stress-controlled monotone loading mode.Submitted to the 10th International Conference on Mechanics of Composite Materials, April 20–23, 1998, Riga, Latvia.Perm' State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 2, pp. 234–250, March–April, 1998.     

Effect of temperature and strain rate on the adhesion of polyvinyl butyral to glass

The shear strength of silicate glass bonded with polyvinyl butyral has been investigated over a broad temperature interval together with the effect of plasticization of the polymer on the strength of the bond. The _adh ^–t curves have intervals corresponding to adhesion failure, cohesion failure of the substrate, and cohesion failure of the adhesive, respectively. The dependence of adhesion on strain rate has been investigated and a maximum observed at 20 mm/min.Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1104–1107, November–December, 1970.     

Investigation of the mechanical properties of acrylics under conditions of creep,stress relaxation,and harmonic vibration

The dynamic and static (creep, stress relaxation) mechanical properties of various acrylics have been investigated. A relation is established between the structure and the macroscopic mechanical properties. The previous thermal history and the characteristics of the state of stress are shown to affect the nature of the relaxation processes and the mechanical properties of the acrylics. Generalized stress relaxation curves are constructed. The mechanical glass transition temperatures of the acrylics investigated are determined.Lenin Moscow State Pedagogic Institute. Translated from Mekhanika Polimerov, No. 1, pp. 157–159, January–February, 1973.     

Effect of elastic filler on the fatigue failure of thermoplastic polyurethane film at low temperature

Conclusion It was established by mathematical modeling of the curves of spectral transmissivity and by comparing them with experiments that in the mixture of polyurethane with caoutchouc an increase of the volume fraction of filler entails changes of the characteristic dimensions of its particles. With small volume fractions of filler (less than 10%), in consequence of the predominantly small size of the impurities, the mechanism of quasibrittle failure is realized without development of bulk damage to the mixture. When the mixture contains 20–30% filler, satisfactory static elastic and strength properties are retained, and in case of fatigue a considerable amount of damage accumulates and the mechanism of inhibiting macrocracks on the boundaries of impurities begins to act. When the proportion of filler increases further, the elastic and strength properties of the mixture are rapidly impaired, and as a consequence the material becomes practically unusable in operation.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1040–1044, November–December, 1988.     

On the derivation of the velocity and fundamental traffic flow diagram from the modelling of the vehicle–driver behaviors

This paper deals with derivation of the fundamental diagram by modelling the individual driver behavior that adjusts the velocity to the density of vehicles in order to respect the braking distance. A parameter is properly introduced to model the quality of the driver–vehicle subsystem referred to the environmental conditions. Subsequently, it is shown how to use this result in order to model traffic flows by the macroscopic representation and by the kinetic theory.     

Polycaprolactam hardening during cyclic loading

It has been shown that two competitive processes occur during the fatigue of polycaprolactam under symmetrical tension—compression: the buildup of defects—weakening—and the hardening of the material as a consequence of the removal of the plasticizer and structural changes.Kaunas Polytechnic Institute. Translated from Mekhanika Polimerov, No. 5, pp. 815–819, September–October, 1973.     

Kinetic nature of polymer microhardness

The dependence of the microhardness of PMMA and PS on indenter penetration time and temperature has been studied. It is shown that the microindentation process is a kinetic process. A relation is established between the kinetic characteristics of the microindentation process and the corresponding characteristics of the fracture process.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 22–28, January–February, 1973.     

Accumulation of submicroscopic damage in polymers under simple loading

The accumulation of microdamage in specimens of polystyrene, Ftorlon, and high-pressure polyethylene loaded continuously to failure has been investigated. It is shown that at small specimen lifetimes the residual damage accumulates primarily in the fracture zone. Outside the fracture zone appreciable damage accumulation is observed at specimen lifetimes close to the long-term strength. The ultimate strain at various lifetimes is determined. It is shown that the strain corresponding to failure depends on the fraction of irreversible damage accumulated and is largely determined by the resistance of the polymers to fracture.Plastopolimer Scientific-Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 519–524, May–June, 1972.     

Relation between the mechanical properties of polymers and the kinetic flexibility of the macromolecules

Nuclear magnetic resonance combined with extension of the specimens in the NMR spectrometer on the reversible deformation range has been used to study the cooperative modes of motion of the macromolecules in polymers, specifically polycaprolactam and polyethylene terephthalate. For each of these polymers there are two temperatures near which the nature of the molecular motion changes sharply. At low temperatures there is a transition from independent hindered vibrations of the repeating units to correlated vibrations, as a result of which the chains in the amorphous regions acquire limited kinetic flexibility within the glassy state; at high temperatures there is a transition to segmental motion. The question of how changes in the modes of molecular motion are reflected in the macroscopic mechanical properties of polymers is examined.Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 24–29, January–February, 1971.     

Experimental investigation of the strength of structural plastics in multiple pulsed bending

Strip-beams of molded fabric-reinforced plastic, unidirectional wound glass-reinforced plastic, and polystyrene and syntactic foam have been tested in pulse bending in a modified electrodynamic testing device. The main stages of the failure process and the nature of the relation between the peak load and the number of cycles to failure have been established. For all the materials tested these relations are similar to the cyclic strength curves obtained at low loading rates. It is shown that for glass-reinforced plastics there is a correlation between the various stages of the failure process.Translated from Mekhanika Polimerov, No. 2, pp. 337–341, March–April, 1971.     

Damage to transparent polymers caused by laser pulses lasting 10−3 and 10−8 sec

A study has been made of the internal damage produced in transparent polymers by laser pulses lasting 10^–3 sec (free-generation mode) and 10^–8 sec (giant-pulse mode). It has been established that in these cases different mechanisms are responsible for the damage. The action of giant laser pulses on transparent polymers evidently provides the first and so far only means of studying fracture effects associated with the truly instantaneous application of load, i. e., when the load application time is much less than the fracture time.Mekhanika Polimerov, Vol. 3, No. 3, pp. 493–497, 1967     

Investigation of the fatigue properties and susceptibility to damage of thin-walled polymer shells

The results of a study of the damage suffered by thin-walled polymer shells are evaluated on the basis of an analysis of the process of crack formation and the variation of the modulus of elasticity under cyclic deformation. The process of fatigue failure has been studied in relation to shells made of three groups of polymeric materials. It is shown that for the polymers investigated there are at least two different fatigue fracture mechanisms. Data on the crack growth kinetics are presented.Plastopolimer Research and Production Association, Leningrad; Lensovet Leningrad Technological Institute. Translated from Mekhanika Polimerov, No. 6, pp. 1019–1026, November–December, 1971.     

Polymer breakdown and fatigue

Problems of the mechanical breakdown of polymer materials are discussed on the basis of kinetic considerations on such breakdown, viewed as a thermofluctuation-induced process of an accumulation of breaks in interatomic bonds (either break or damage). The present state of this problem is analyzed, both for static and for cyclic loads (fatigue). Some approaches for developing a theory of breakdown are reviewed. Theories related to the accumulation of damage in the volume of the sample receive particular attention. Recent studies on fatigue in polymer materials are listed.Paper presented at the 2nd All-Union Conference on Polymer Mechanics, Riga, November 10–12, 1971.A. I. Ioffe Physicotechnical Institute, Academy of Sciences of the SSSR, Leningrad. Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 597–611, July–August, 1972.     

Long-term strength of rigid polyvinyl chloride in the flat stressed state

On the basis of experimental studies it has been established that the time to failure of rigid polyvinyl chloride depends basically on the magnitude of the largest stretching stress and at a long duration of testing deviates considerably from the exponential dependence. A refined formula is suggested for calculating the time to failure, which takes into account some characteristics of the stressed state.A. A. Zhdanov Leningrad State University. Translated from Mekhanika Polimerov, No. 3, pp. 453–457, May–June, 1970.     

Fatigue performance of glass/polyester laminates and the monitoring of material degradation

Experiments are performed on glass/polyester composites under fatigue loading. The stiffness degradation is recorded as an indirect measure of material damage. The stiffness decrease rate is described by a power dependence on stress; this relationship allows the fatigue law to be derived. Stiffness-controlled fatigue curves are generated and presented in an S-N-diagram based on normalized stress (equivalent to strain). Such stiffness controlled fatigue curves can be used as a basis for design criteria for components.Presented at the 9th International Conference on Mechanics of Composite Materials, 17–20 October 1995, Riga, Latvia.Materials Department, Risø National Laboratory, Roskilde, Denmark. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 1, pp. 32–41, January–February, 1996.     

On the construction of a variant of the theory of fracture

A theory of long-time strength that takes into account the accumulation of damage in the material in different directions is used as a basis for obtaining isochronous fatigue strength curves in stress space for complex states of stress.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 648–653, July–August, 1970.     

Mechanical properties of blends of liquid crystalline copolyesters with polyprophylene

Conclusions A significant effect of the addition of LCP on the mechanical properties and their anisotropy has been established. Already, if one considers the shape of curves of the stress-strain relationship it can be seen that curves typical for semicrystalline polymers (pure polypropylene) with clearly visible yield point and significant cold drawing leading to an anisotropic stiffening are changing into curves without yielding and with a brittle failure (LC-rich blends). Generally, the tensile elasticity modulus increases with increasing LCP content for both MD and TD. The maximum value of anisotropy of elastic properties was noted for a rather low content of LCP (c = 5%). On the contrary, the stress at yield decreases with increasing LCP content. The same was observed for the strain at yield but in both cases an important increase of anisotropy has taken place. Consequently, the total elongation during drawing (strain at break) showed a drastic decrease for blends with higher LCP content (about 60–80 times). The addition of the LCP to polypropylene has led to a stiffness increase (higher elasticity modulus) but simultaneously to a considerable plasticity decrease. As a confirmation of these observations, there served also the creep test where a decrease of the creep compliance (by two times) for LC-rich blends as compared with pure PP was noted.It also should be emphasized that, generally, a smaller effect of LCP content on the elastic deformation was noted than that on the time dependent effects (nonelastic creep deformation).Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 4, pp. 442–450, July–August, 1994.     

Automated thermomechanograph for investigating polymer microsamples

An instrument has been developed for recording the thermomechanical curves of microsamples of polymers (30 mg) by the penetration method in the continuous heating regime. The thermomechanical curves are recorded by a PDS-021 two-coordinate recorder; the effective temperature range is 20–400°C.Institute of High-Molecular Compounds, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 4, pp. 746–749, July–August, 1968.     

Study of the secondary structures of filled rubbers

It is shown that the secondary structure of rubbers filled with carbon blacks can be expressed in terms of the volume of the relaxation kinetic element (V), calculated from the stress relaxation curves. The effect of the type and concentration of carbon black and the extension of the specimen on the quantity V has been investigated. The volume of the kinetic element decreases with increase in the concentration and dispersity of the carbon black. This is associated with an increase in the rubber-filler contact area, which causes the orientation of groups of molecular chains and reduces the size of the deformation unit. As the degree of extension of the specimen increases, V decreases.Mekhanika Polimerov, Vol. 3, No. 6, pp. 1066–1069, 1967     

Creep of fluoroplastic under the joint action of tension and hydrostatic pressure

Conclusions 1. The effect of hydrostatic pressure on the creep of Teflon has been investigated under isothermal conditions. The applicability of the principle of reduced variables for reduction of creep curves with respect to pressure has been shown.2. It has been ascertained that in the pressure region from atmospheric to 1600 kgf/cm² there are two subregions, which are separated by a transition at a pressure of 700–800 kgf/cm². It has been shown that in these subregions the occurrence of relaxation processes has different characters.3. A direct measurement of the free volume of Teflon has been made in the temperature range from 40 to 120°C. A sharp increase in free volume in the region of the temperature transition has been shown.Leningrad Technological Institute of the Refrigeration Industry. Okhtinsk Scientific and Industrial Union "Plastpolimer," Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 434–438, May–June, 1977.