Cyclic Elastoplastic Deformation Research Articles

Propagation of the front of fast cracks in hard polymers

3. V. V. Moskvitin, Plasticity under Alternating Loads [in Russian], Moscow State Univ. (1965). 4. V. V. Moskvitin, Cyclic Stresses in Structural Elements [in Russian], Nauka, Moscow (1981). 5. R. M. Shneiderovich, Strength during Static and Pulsating Loading [in Russian], Mashinos- troenie, Moscow (1968). 6. N. G. Bychkov, A. N. Petukhov, and I. V. Puehkov, "Certain features of the kinetics of deformation of structural materials during cyclic elastoplastic deformation," Probl. Prochn., No. ii, 7-11 (1986). 7. I. A. Birger, "Deterministic and statistical models of endurance," in: Problems of the Keliability of Aircraft [in Russian], Mashinostroenie, Moscow (1985), pp. 105-150. 8. I. A. Birger, "Prediction of safe life under low-cycle fatigue," Probl. Prochn., No. 10, 39-44 (1985). 9. Yu. M. Temis, "Evaluating plasticity and creep in engineering calculations," in: Analy- tical and Numerical Methods of Solving Boundary-Value Problems of Plasticity and Visco- elasticity [in Russian], UNTs AN SSSR, Sverdlovsk (1986), pp. 100-106. i0. A. P. Gusenkov, Strength under Isothermal and Nonisothermal Low-Cycle Loading [in Rus- sian], Nauka, Moscow (1978). PROPAGATION OF THE FRONT OF FAST CRACKS IN HARD POLYMERS Yu. A. Kostandov and S. I. Fedorkin UDC 620.117.5:539.219.2 Experimental investigations of the dynamic failure of linear and cross-linked polymers such as polymethyl metacrylate (PMMA) and cured epoxy resins showed that in specimens with more stored elastic energy (this corresponds to higher nominal static stresses) the crack accelerates more rapidly to its limit speed [1-3]. The mean rate of crack growth increases from zero, and the time of acceleration usually amounts to some tens of microseconds. Various solutions in determining the limit crack growth rate obtained within the frame- work of the theory of elasticity practically lead to the same results which are confirmed by some experimental data. There exists nevertheless information contradicting the assertion that the limit crack growth rate is dependent solely on the elastic constants of the material. It is assumed that the limit crack growth rate is also dependent on the nature of the energy supply [i]. In connection with that the dependence of the limit growth rate of fast cracks, propagating under conditions of pulsed effects, on the parameters of the process of failure requires thorough investigation. Numerous experimental data testify to the jumpwise nature of the propagation of the crack tip in dynamic failure of materials. Some researchers [i, 5] ascribe this to the effect of stress waves reflected from the boundaries of the specimen. The same effect is apparently also the cause of the j umpwise motion of a crack in splitting cyrstals, which is accompanied by the transition of the crack into neighboring planes [i, 6]. That is why only some publications [2, 3, 7] contain data on the jumpwise nature of crack propagation not connected with the interaction of the crack with the boundaries of the speci- men. In the present work we investigated the nature and speed of propagation of the front of cracks initiated by the detonation of a conductor by a current pulse [8] in specimens of materials ED-2OMA, ED-20MTGFA (epoxy resins cured by maleic and methyl tetrahydrophthaleic anhydrides) and PMMA that are free of static stresses or are prestressed. The specimens were rectangular plates 4 mm thick; halfway along their length there was a shaped notch with radius of curvature 25 mm and radius at the tip 0.05 mm. Into the notch we placed a strip of tin plate which detonated when capacitors charged to high voltage were discharge on it. The dimensions of the plates were chosen such that the incidence of elastic waves reflected from the boundaries of the specimen into the zone of registration was pre- vented. M. V. Frunze Simferopol University, Simferopol. Translated from Problemy Prochnosti, No. 9, pp. 23-26, September, 1988. Original article submitted November 24, 1986. 1156 0039-2316/88/2009-1156512.50 9 1989 Plenum Publishing Corporation

Analytical description of curves of the cyclic elastoplastic deformation of structural materials

Analytical description of curves of the cyclic elastoplastic deformation of structural materials

Special features of the deformation kinetics of structural materials in cyclic elastoplastic deformation

1. The results show that the elasticity modulus of the VT3-1, KhN73MBTYuVD, and KhN77TYuRVD alloys greatly varies (by up to 40%) during cyclic loading. This fact must be taken into account in evaluating the stress state of components and predicting their life. 2. The existence of the recovery effect in the material after removing the load indicates that the evaluation of the degree of damage of the material of the component in service by verifying its mechanical properties in single-cycle loading is ineffectively. 3. VT3-1 titanium alloy is acceptable to softening in the symmetric load cycle even if the amplitude of the acting stresses is considerably lower than the proportionality limit. 4. The mechanism of cumulation of residual plastic strains in the nickel-based alloys and titanium alloys greatly differs depending on the nature of load variation: in the symmetric load cycle the nickel alloys are characterized by one-side cumulation of residual strain ɛr which is not detected in VT3-1 alloy; on the other hand, for the nonzero cycle cumulation of ɛr was detected only in the titanium alloy.

Cyclic elastoplastic deformation and endurance of materials under simple and complex loading

Cyclic elastoplastic deformation and endurance of materials under simple and complex loading

Relationship between the parameters of the crack and strains at its tip in cyclic elastoplastic deformation

1. A close relationship exists between the strain at the crack tip, crack length, and crack opening displacement in elastoplastic deformation. 2. The maximum fracture strain detected at a specific distance from the crack tip can be determined from the plastic strains accumulated prior to fracture in fatigue loading a smooth specimen. 3. The crack tip opening displacement can be determined using the equation of the crack contour. 4. Measurements of the crack opening displacement on the surface of the specimen make it possible to examine the growth of surface cracks whose depth cannot be measured.

Effect of temperature and fatigue loading regimes on the regularities of work-softening of steels 15Kh2MFA and 15Kh2NMFA

1. We worked out a method experimentally investigating the regularities of cyclic elastoplastic deformation of structural steels in the range of low temperatures. 2. It was shown that steels 15Kh2NMFA (I) and 15Kh2MFA are cyclically work-softening materials both at normal and at low temperature. The intensity of cyclic work-softening, evaluated according to the decrease of\(\bar S_p ,\bar S_y ,\bar S_{0.4}\), increases with decreasing test temperature and amplitude of inelastic deformations, and evaluated according to the increase of β, it increases with decreasing test temperature and increasing amplitude of the elastoplastic deformations.

Low-cycle strain with holding of steel 0Kh16N15M3B under conditions of reactor irradiation

1. In the study of the resistance of materials to cyclic elastoplastic deformation with holding on installations that have low rigidity, the specimen at the stage of high-temperature holding is in conditions that are intermediate between relaxation and creep and correspond to real conditions of operation of structural elements. 2. The change of the resistance to cyclic elastoplastic deformation of irradiated steel 0Khl6N15M3B at 923°K occurs on account of the accumulation of radiation defects that are predominantly connected with th epreliminary radiation. 3. The generalized stress-strain diagram does not depend on the holding time if the origin of coordinate is the point of intersection of the stress-strain curve and the magnitude of the maximum stress of the preceding half-cycle. 4. Reactor irradiation stimulates the process of cyclic stress relaxation in the investigated steel.

Influence of hardening rule on the elasto-plastic behaviour of a simple structure under cyclic loading

The modes of cyclic elasto-plastic deformation of a two-bar structure with unequal areas and lengths under the simultaneous action of sustained mechanical load and cyclic thermal history are investigated analytically using three types of elasto-plastic material models: perfectly plastic, linear kinematic hardening and linear isotropic hardening. This simple structure is shown to exhibit much of the behaviour of interest in design of structural components subjected to repeated thermal loads: viz, elastic shakedown, reversed plasticity and ratcheting. The cyclic plastic behaviour of the structure is developed in closed form and the effects of strain hardening, hardening rule and geometrical parameters of the two-bar assembly on the deformation modes are critically examined.

Influence of irradiation on the resistance of steel OKh16N15M3B to cyclic elastoplastic deformation

Influence of irradiation on the resistance of steel OKh16N15M3B to cyclic elastoplastic deformation

Resistance of low-alloy steel to low-cycle deformation at low temperatures

1. A method has been developed which can be used to record elastoplastic deformation diagrams on tubular specimens over a wide range of low temperatures (+20, −50, −100, −150, −196°C) with single and cyclic loadings. 2. The parameters of the deformation diagram that can be used to obtain a deformation curve in any loading cycle have been defined; this is essential for evaluation of the stress — strain state in structures operating at low temperatures and under cyclic loads. 3. The presence in the case of steel 10KhSND of a unified cyclic elastoplastic deformation diagram independent of temperature (from +20 to −196°C) in relative coordinates was established experimentally.

A system of remote measurement of forces and deformations in cyclic tests of irradiated materials in the elastoplastic region

In cyclic tests of design materials the forces and deformations must be measured continuously and recorded in the form of cyclic deformation curves and also curves of the change in the test parameters with time. As a result, the measuring system must be sufficiently rapid-acting. However, this requirement is not filled by the frequency-impuls e method of measuring displacements [4] or by the method of resonant hollows [61. In addition, it should be mentioned that the high sensitivity of the latter is possible only at a constant temperature and with location of the resonant cavity close to the sample, which makes the tests significantly more complex and expensive since the chamber together with the adjacent portions of the waveguides becomes a single-usage element. The simplest and most practical are measuring systems using resistance strain gauges and induction sensors. For the purpose of eliminating the effect of radiation the sensors are removed from the active zone and their connection with the sample is either by special extensions [7] or using design elements of the test machine itself. We should mention that despite the rigidity of test machines within the channel, tests made on the basis of design elements of the machine at a large distance from the sample satisfy the requirements of low-cycle tests since they make it possible to obtain data necessary for treating cyclic elastoplastic deformation curves under conditions of ideal elasticity of the design elements of the loading equipment. At the same time, despite some distortion of the mechanical hysteresis loop introduced by elastic deformation of the machine, the measured parameters of the curves (width of the elastoplastic hysteresis loop, its area, the torsional moment, the one-sided accumulated deformation) maintain their true values. In cyclic loading of a sample by a torsional moment, in contrast to tension-compress ion loading, the measurement of deformations is substantially easier since oscillations in temperature do not have an effect on the transmission of the angular displacements from the sample to the sensor. The use of resistance strain

Effects of liquids on the long-term strength of lucite under cyclic elastoplastic deformation

Effects of liquids on the long-term strength of lucite under cyclic elastoplastic deformation

Studies of the bearing capacity of samples in inhomogeneous stress states during cyclic elastoplastic deformation

Studies of the bearing capacity of samples in inhomogeneous stress states during cyclic elastoplastic deformation