Экспериментальное и теоретическое исследование высокоскоростного проникания длинных стержневых ударников в песок

Results of computations with the use of improved modified Alekseevskii-Tate theory (IMATT) are compared to experimental data on high-velocity penetration of long rod projectiles into sand in the impact velocity range of V0=0.5-3.5 km/s. Projectiles were made of three different metals: M1 copper, WNZh tungsten heavy alloy and 30KhGSA steel. The value of hardening coefficient k in the linear dependence of the projectile material yield on pressure could be determined using IMATT and experimental data on dependence of differential penetration coefficient K on the penetration velocity. At penetration in regime of the hydrodynamic erosion of projectile, differential penetration coefficient K could be approximated just by dependence on the ratio of the impact velocity of penetration to the value of the critical velocity, above which the projectile deforms plastically during penetration. The values of the critical velocity may differ for specific projectile material properties as well as the density and the humidity of sand.

SEBS as an Effective Nucleating Agent for Polystyrene Foams

Different percentages of an elastomeric phase of styrene-ethylene-butylene-styrene (SEBS) were added to a polystyrene (PS) matrix to evaluate its nucleating effect in PS foams. It has been demonstrated that a minimum quantity of SEBS produces a high nucleation effect on the cellular materials that are produced. In particular, the results show that by adding 2% of SEBS, it is possible to reduce the cell size by 10 times while maintaining the density and open cell content of the foamed materials. The influence of this polymeric phase on the glass transition temperature (Tg) and the shear and extensional rheological properties has been studied to understand the foaming behavior. The results indicate a slight increase in the Tg and a decrease of the shear viscosity, extensional viscosity, and strain hardening coefficient as the percentage of SEBS increases. Consequently, an increase in the density and a deterioration of the cellular structure is detected for SEBS amounts higher than 3%.

Increasing the Performance of a Fiber-Reinforced Concrete for Protective Facilities

The use of fiber in cement materials is a promising and effective replacement for bar reinforcement. A wide range of fiber-reinforced concretes based on composite binders with increased impact strength characteristics have been developed. The synthesized composites included the composite binder made of Portland cement, silica, and carbonate additives. Basalt and steel were used as fibers. The nature of the influence of the composition and manufacturing technology of cement composites on the dynamic hardening coefficient has been established, while the growth of these indicators is achieved by creating a denser interfacial transition zone between the cement paste, aggregate, and fiber as a result of improving the homogeneity of the concrete mixture and controlling the consistency. Workability indicators (slump flow up to 730 mm; spreading time up to a diameter of 50 cm is up to 3 s) allow them to be classified as self-compacting concrete mixtures. An increase in the values of the impact strength coefficient by a factor of 5.5, the dynamic hardening coefficient by almost 70% as a result of interfacial interaction between fibers and binder matrix in the concrete composite, as well as absorption of impact energy by fiber, was revealed. The formula describing the effect of the loading rate on the coefficient of dynamic hardening of fiber-reinforced concrete has been refined. The fracture processes of the obtained materials have been established: after the initiation of primary cracks, the structure of the composite absorbs impact energy for a long time, while in the inelastic range (the onset of cracking and peak loads), a large number of secondary cracks appear.

Damage constitutive model of pure copper at different annealing temperatures

Aiming at the problem of damage evolution of pure copper during the plastic deformation, the normalized shape factor is introduced based on the RO model (Ramberg-Osgood model). The mesoscopic damage constitutive model of pure copper at different annealing temperatures is established and the tensile deformation of industrial pure copper at different annealing temperatures is analyzed. The results show that the error between the calculated value and the experimental value of the damage constitutive model, based on normalized shape factor, at different annealing temperatures, is less than 10%. The model can effectively reveal the tensile damage evolution behavior of industrial pure copper and accurately predict the plastic tensile flow stress of industrial pure copper at different annealing temperatures. The hardening coefficient and hardening exponent in the model are closely related to the annealing temperature of the material. The annealing temperature has little effect on the hardening exponent and has a significant effect on the hardening coefficient and the hardening coefficient decreases with the increase in annealing temperature.

Shape Memory Effect and Superelasticity of [001]-Oriented FeNiCoAlNb Single Crystals Aged under and without Stress

The two-step ageing of Fe-28Ni-17Co-11.5Al-2.5Nb (at. %) single crystals under and without stress, leads to the precipitation of the γ′- and β-phase particles. Research has shown that γ–α′ thermoelastic martensitic transformation (MT), with shape memory effect (SME) and superelasticity (SE), develops in the [001]-oriented crystals under tension. SE was observed within the range from the temperature of the start of MT upon cooling Ms, to the temperature of the end of the reverse MT upon heating Af, and at temperatures from Af to 323–373 K. It was found that at γ–α′ MT in the [001]-oriented crystals, with γ′- and β-phase particles, a high level of elastic energy, ΔGel, is generated, which significantly exceeds the energy dissipation, ΔGdis. As a result, the temperature of the start of the reverse MT, while heating As, became lower than the temperature Ms. The development of γ–α′ MT under stress occurs with high values of the transformation hardening coefficient, Θ = dσ/dε from 2 to 8 GPa and low values of mechanical Δσ and thermal ΔTh hysteresis. The reasons for an increase in ΔGel during the development of γ–α′ MT under stress are discussed.

Effect of deformation hardening intensity, contact friction and back tension on indicators of wire drawing process

Dependences on the hardening factor for the drawing stress, the safety factor of I.L. Perlin and the stress state indicator V.L. Kolmogorov at different values of the drawing angle, coefficient of friction and stress of back tension are presented. For the first time, the nature of the dependences for the limiting and permissible values of delta-criterion is shown for the shape of the deformation zone of round solid profile on the coefficients of friction, hardening and safety factor and the back tension stresses. The limiting and permissible values of the delta-criterion increase from increase in the coefficients of friction and safety factor, the back tension stress and decrease from increase in the hardening coefficient. It is shown that for small values of the criterion ∆ < 1.5, the drawing stress can be higher than the yield point at the die output. As result, the wire material may be destroyed and the drawing process cannot be realized. Assessment of the change degree in the limiting and permissible values of the elongation coefficient and delta-criterion is carried out with increase in the friction coefficient from 0.05 to 0.15, the back tension from 0 to 85 MPa, the safety factor from 1.0 to 1.4, the hardening coefficient from 0 to k.

Температурная зависимость автоволновых характеристик локализованной пластичности

The behavior of autowaves of localized plastic flow in the Fe-Ni-Cr alloy at temperatures of 143≤ T≤ 420 K is considered. The temperature dependence of the propagation velocity of the autowave is studied. It was found that for the region of low temperatures the inverse proportionality of the autowave velocity to the strain hardening coefficient and the quadratic dispersion law are fulfilled. The temperature independence of the elastoplastic deformation invariant is established.

EFFECT OF γ′-PHASE PARTICLES ON THE MECHANICAL BEHAVIOR AND DEFORMATION MECHANISM OF (CoCrFeNi)94Ti2Al4 HIGH ENTROPY ALLOY SINGLE CRYSTALS

Recently, the interest of researchers has focused on a new FCC class (FCC – face-centered cubic lattice) high-entropy alloys (HEA), due to their unique properties – high values of the strain hardening coefficient, good plasticity, and ductile fracture at low test temperatures. Such a combination of properties in an FCC of HEA is achieved by mixing five or more elements in equal atomic proportions. Due to the strong temperature dependence of stresses at the σ0.1(T) yield point, these alloys have low σ0.1 values at temperatures above room temperature, which hinders their practical application. A precipitation hardening is an effective way to achieve high strength and is successfully used for hardening HEA FCC. The paper studied the influence of ageing at 923 K for 4 hours and at 1073 K for 18 and 30 hours on the mechanical behavior of single crystals of (CoCrFeNi)94Ti2Al4 (at.%) HEA FCC oriented along the [001] direction under tension. Ageing at 923 K for 4 hours and at 1073 K for 18 and 30 hours leads to the precipitation of γ′-phase particles, the size and volume fraction of which depend on the ageing temperature and time. The γ′-phase particles precipitation leads to an increase in stresses at the yield point from 47 MPa (ageing at 923 K, 4 hours) to 226 MPa (ageing at 1073 K, 30 hours) relative to quenched crystals at 296 K. The study identified the dependence of the strain hardening coefficient, plasticity, and the maximum stress level before fracture on heat treatment. The author discussed the reasons for the growth of stresses at the yield point and the strain hardening coefficient upon precipitation of γ′-phase particles.

Semi-brittle transient creep in Carrara marble: Hardening and Twinning-induced Plasticity

&lt;p&gt;Abundant observations of field- and micro-structures in marble rocks in both natural and laboratory settings indicate that these rocks have deformed by various combinations of mechanical twinning, dislocation motion, and dilatant fracturing. To better constrain the systematics of this semi-brittle flow, we performed a set of about 80 experiments at eight different temperatures (20&amp;#176;C&lt;T&lt;800&amp;#176;C). At each T, deformation conditions included different confining pressures (50 &lt; P&lt;sub&gt;C &lt;/sub&gt;&lt;300 MPa) and strain rates (10&lt;sup&gt;-6&lt;/sup&gt; &lt; &amp;#949;&amp;#8217; &lt;10&lt;sup&gt;-4 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;). Under almost all these conditions, both the strength (&amp;#963;) and the hardening coefficient (&amp;#920;=&amp;#8706;&amp;#963;/&amp;#8706;&amp;#949;) are affected by changes in P&lt;sub&gt;C&lt;/sub&gt; and &amp;#949;&amp;#8217;, but the functional relationships of &amp;#963;(P&lt;sub&gt;C&lt;/sub&gt;, &amp;#949;&amp;#8217;) and &amp;#920;(P&lt;sub&gt;C&lt;/sub&gt;, &amp;#949;&amp;#8217;) are unique. For example, at 20&amp;#176;C, &amp;#963; is a non-linear function of both P&lt;sub&gt;C&lt;/sub&gt; and &amp;#949;&amp;#8217;, while &amp;#920; depends on P&lt;sub&gt;C&lt;/sub&gt; alone. In contrast, at 600&amp;#176;C, the dependence of &amp;#963; on P&lt;sub&gt;C&lt;/sub&gt; is very weak, and &amp;#920; depends on &amp;#949;&amp;#8217; alone.&lt;/p&gt;&lt;p&gt;At T&lt;650&amp;#176;C (less than half the absolute melting point of calcite), and P&lt;sub&gt;C&lt;/sub&gt; greater than 50 MPa, the hardening coefficients are substantial (1% or more of the shear modulus), similar to steels and hexagonal metals that deform in a regime called twinning induced plasticity (TWIP). During TWIP, deformation proceeds with &amp;#8220;easy&amp;#8221; mechanical twinning, combined with dislocation glide on several slip systems whose glide planes are at high angles to the twin plane. In the calcite rocks, depending on conditions, the hardening resulting from twinning may be reduced by dilation and failure owing to brittle processes (at low pressures and temperatures), or by recovery and recrystallization (at higher temperatures or slower strain rates). Thus, both microstructural observations and mechanical deformation data are consistent with the interpretation that the hardening coefficient and strength are determined by the relative partitioning of inelastic strain amongst mechanical twinning, dislocation mechanisms, and dilatant fracturing. One important aspect is the nature of the mechanism that accommodates of discontinuous inelastic strain at the termination of twins at grain boundaries.&lt;/p&gt;