The results of an experimental study of the mechanical properties of carbon fiber-reinforced plastics (CFRP) made by the “wet” winding method in the temperature range of room temperature to −196°C with different reinforcement schemes are presented. The temperature dependences of strength, plasticity and elasticity characteristics of these materials in tensile and three-point bending tests are obtained. The elastic modulus of CFRP with a longitudinal scheme of reinforcement in a given temperature range is at the level of 160 GPa. The ultimate stress of the material at room temperature is 1370 MPa. The temperature dependence of this characteristic is close to linear and practically coincides with that of bending. The low-temperature hardening, estimated by the change in the value of the ultimate stress due to cooling, reaches 39%. The mechanical characteristics of CFRP with a transverse reinforcement scheme both in tension and in bending in the considered temperature range have rather low values and are determined mainly by the matrix properties. When cooling from 20 to −196°C, the elastic modulus of CFRP with a ±45° reinforcement scheme increases by 55%, and its low-temperature hardening is 40%. At the same time, the elastic modulus and ultimate stress are six to eight and ten times, respectively, lower than the similar characteristics of CFRP with the longitudinal reinforcement scheme. Fracture of such a composite at all investigated temperatures occurs locally by shifting at an angle of 45° because of delamination of the material.

Results of experimental investigation of mechanical characteristics of ULTEM 9085 thermoplastic, produced by additive manufacturing, i.e., the method of layer-by-layer application of a molten polymer thread, are presented. Flat specimens were tensile tested within the temperature range of (–40) –150°C. Temperature dependencies of ultimate strength, relative elongation at break, elastic modulus, and Poisson’s ratio are obtained. At a temperature of –40°C, the linear sections of diagrams obtained for various specimens coincide; in the area of elastoplastic deformations, their discrepancy is noted. This caused small variations in elastic characteristics and significant ones in strength and relative elongation at break. Similar features of deformation diagrams were also obtained at a temperature of 50°C. However, at 150°C, tensile diagrams do not coincide even in the area of small elastic deformations; their specific bends are noted. Specimens are fractured by the normal separation mechanism at all temperatures. When the temperature changes from –40 to 150°C, thermoplastic ultimate strength almost linearly decreases; at 150°C it is 26% of the initial value at –40 °C. As temperature increases within the specified range, the relative elongation at break monotonously decreases more than twice (2.9–1.3%). The elastic modulus changes insignificantly within a temperature range of (–40)–20°C; when the temperature rises to 150°C, it decreases to 64% of the value at –40 °C. Poisson’s ratio virtually does not change and is in the range of 0.36–0.37.

The paper presents fatigue test results for 2219 aluminum alloy specimens, cut from a plate 40 mm in thickness, under symmetric and pulsating loading cycle. It is shown that at stresses for above the endurance limit, the fracture of specimens has a multi-site nature and starts from their surface. At stresses close to the endurance limit, the fatigue crack in the specimens fractured at a smaller number of loading cycles is initiated from their surface and at longer lives, from subsurface fracture initiation sites. This accounts for the significant scatter of data on the life of specimens near the endurance limit. Fatigue tests under zero-to-tension loading cycle showed that under the same heat treatment conditions, the endurance limit value of specimens made by resistance butt welding is close to that of specimens without weld and much higher than that of specimens with argon-arc weld. The fractographic investigations of the fracture of specimens with weld show that in both types of welding, the fatigue cracks propagate through pores in the weld or in the heat-affected zone.

The paper considers the processes related to the specimens and loading device in the tensile testing of metals in strain-controlled mode, including under the conditions of cryogenic temperatures up to 4.2 K. It is demonstrated how the potential elastic energy accumulation and its subsequent relaxation in the development of the specimen strain affect the process of kinetics. The strain rate dependencies on the strain value and stiffness factor (relation between the values of stiffness of the specimen and the machine) are obtained. Noteworthy is that the values of initial and nominal strain rates can differ by an order of magnitude in case of the machine compliance. To obtain the strain rates that are similar for different machines at the initial stage of the process, the formula is proposed allowing one to calculate the required nominal rate. At the temperatures lower than 30 K there is a dramatic increase of the influence of the machine stiffness, dimensions and shape of the specimens on the obtained characteristics, which requires the special measures in the process of testing. Some international and national standards are considered. It is shown that, at present, the process of standardization for tensile testing of metals is inadequate, and the requirements of the current regulatory documents are the minimum, which are not in agreement with the development paces of testing equipment. To significantly enhance the level of accuracy of the obtained mechanical characteristics, it is required to employ the urgent measures as the machine compliance limitation, as well as the decrease of the dimensions range for the specimens together with the standardization of the permissible range of the stiffness factor, and the required selection of the nominal strain rate considering the latter.

In this study, the stress concentration effect on the low-temperature strain-hardening of 12Kh18N10T and 03Kh20N16AG6 steels is investigated. It is shown that cooling of notched specimens from these steels to the temperature of liquid helium results in the increase of their ultimate strength values by 165 u 240%, respectively, whereas the structural factor contribution is significantly reduced for the former steel and remains unchanged for the latter one.

A particularly strong hardening of the specimens made of austenitic steels with notches of a large radius of bending has been detected at the temperature of 4.2 K. This phenomenon is attributed to special conditions of realization of the low-temperature discontinuous flow effect. The correlation between the strength characteristic and the number of strain jumps in the zone of stress concentration has been revealed.

Kinetic characteristics of discontinuous yielding at a temperature of 4 K as functions of a number of factors are obtained using numerical simulation and experimental data for austenitic steel and aluminum alloy. During the development of a strain jump, the deformation rate and acceleration are 19 s(-1) and 5000 s(-2), respectively, for steel specimens and are much lower for aluminum alloy. The jump duration is mainly determined by the characteristics of the loading system. An equation relating the strain jump and the critical stress for low-temperature ductile materials is derived. The energy balance and the mechanism of low-temperature discontinuous yielding of metals are discussed. Its dynamic and thermally activated components are estimated taking into account the strain hardening of the material. (C) 2015 Elsevier Ltd. All rights reserved.

The influence of machine compliance on the standard mechanical characteristics of metallic materials is investigated. It is shown that with an increase of compliance percent elongation and reduction of area after fracture decrease by several times. At cryogenic temperatures ultimate strength can decrease to the level of yield strength due to the effect of jumplike deformation of metals.

Tensile tests of titanium alloy and austenitic steel specimens have been conducted in the range of temperatures 300–4 K. Under deep-freezing conditions, there occurs a new type of scale effect – variation of low-temperature jumplike deformation, which causes a sharp variation of the plasticity characteristic (the relative elongation). The difference between jumplike deformation values in the maximum and minimum cross-sectional areas of standard cylindrical specimens is introduced as the scale effect measure. Via mathematical modeling performed for 03Kh20N16AG6 steel, we have obtained dependencies of the scale effect on the most critical factors – the trigger stress of deformation jump and specimen–test machine system stiffness. The scale effect nature is studied, and options of its minimization, in reference to the standardization of mechanical tests, are discussed.

Numerical results on the discontinuous yielding of austenitic steel and aluminum alloy at a temperature of 4 K are presented. The method used allows qualitative and quantitative predictions of the influence of eight determining factors, including characteristics of the material and parameters of the loading system, on the low-temperature discontinuous deformation of metals. Recommendations are formulated on how to improve standards on tensile testing of metals in liquid helium as regards machine rigidity, standard specimen sizes, and new mechanical characteristics. (C) 2012 Elsevier B.V. All rights reserved.

A nonlinear model of the process of low-temperature discontinuous yielding of metals has been constructed, which allows the totality of strain jumps to be described as a function of the mechanical properties of material and dynamic characteristics of the loading system. The adequacy of the model has been experimentally verified for austenitic steel and an aluminum alloy.

Force, deformation, velocity, energy, and other criteria of limiting states at low-temperature serrated yield of metals are examined. The feasibility of their use for setting norms and standards of strength and mechanical tests is demonstrated.

The regularities of repeated necking have been studies on flat specimens of alloys for cryogenic engineering in the case of effect of low-temperature discontinuous yield. It has been shown that in contrast to cylindrical specimens, in this particular case there is a considerable deviation of the stressed state from linear stress, which affects greatly the mechanical characteristics determined and does not allow flat specimens to be recommended for standard mechanical tensile tests of metals at temperatures below 30 K.

In relation to the problem of determining reliable values of mechanical characteristics of metallic materials at temperatures below 30 K, the peculiarities of repeated neck fomation have been studied on cylindrical specimens of structural alloys for cryogenic engineering in the case of occurrence of the effect of low-temperature discontinuous yield. The evaluation performed of the rigidity and inhomogeneity of the three-dimensional stressed state of typical necks showed that it differs only slightly from linear stress, and that the use of such specimens for standard mechanical testing under deep freezing conditions is reasonable.

The investigation results for the effect of stress concentration on the mechanical behavior of titanium alloy 3M and aluminum alloy AMg5 within the temperature range of 293 to 4.2 K Under conditions of deep cooling, its character is shown to change considerably due to a manifestation of the effect of low-temperature jumplike deformation.

The effect of stress concentration on the mechanical behavior of 03KH20N16AG6 steel over a temperature range of 293–4.2 K is studied. The flow discontinuity effect under cryogenic cooling is shown to appear with a decrease in theoretical stress concentration factors, which gives rise to anomalous changes in strength and plasticity characteristics of the material.

The paper presents the results of an investigation into the low-temperature jumplike deformation of BrKh08Tsr bronze and the effect of pulsed magnetic cycling on its regularities. The authors established the regularities of build-up of strain with increasing magnetic field strength and applied load. The possibility of experimental modeling of the phenomenon of intermittent yielding by the method of superposition of magnetic field pulses was demonstrated and a corresponding mathematical model of the deformation process was developed.

The given study shows that, in subzero cooling, relatively small changes in specimen dimensions can have a considerable influence on the mechanical properties and intermittent yielding by changing qualitatively the low-temperature deformation mechanism. The authors propose a new criterion of energy similarity of mechanical test conditions to study the size-scale effect manifestations and to obtain comparable characteristics of various materials.

We present the results of investigations into the deformability and strength of conical and tubular specimens of 03Kh20N16ÀG6 and 12Kh18N10Ò steels in uniaxial tension over a temperature range of 293–4.2 K. It is demonstrated that the deformability of the above steels is related to the geometry of the test object, whereas the character of this relation is determined by the test temperature and class of steel. At a temperature of 4.2 K, a qualitative change in the deformation mechanisms occurs, as a result of which the influence of the design factors changes significantly. A drop in the plasticity for both types of the specimens and a considerable hardening of the 03Kh20N16ÀG6 steel thin-walled tubular specimens are noted.

The results of investigating the effect of loading system stiffness on the parameters of low-temperature intermittent yield and the mechanical characteristics of the steels 03Kh20N16AG6 and 12Kh18N10T and the titanium alloy 3M at 4.2 K at different development stages of plastic strain instability are presented. Criteria of fracture by the adiabatic-shear mechanism are considered. An analysis of the stiffness components of an electrohydraulically driven testing machine is given. The necessity of laying down stiffness regulations for conventional mechanical tests of materials has been shown.