Investigation of strain rate sensitivity of Gum Metal under tension using digital image correlatio

Mechanical behavior of a multifunctional titanium alloy Gum Metal was investigated by conducting tensile tests at various strain rates and applying digital image correlation (DIC) technique. Stress–strain curves confirmed low Young’s modulus and high strength of the alloy. The determined values of yield strength had a tendency to increase, whereas the elongation to the specimen rupture tended to decrease with increasing strain rate. True stress versus strain curves were analyzed using selected lengths of virtual extensometer (VE) placed in the strain localization area. When the initial length of the VE was the same as the gauge length, work hardening was observed macroscopically at lower strain rates, and a softening was seen at higher strain rates. However, the softening effect was not observed at the shorter VE lengths. Evolution of the Hencky strain and rate of deformation tensor component fields were analyzed for various strain rates at selected stages of Gum Metal load-ing. The DIC analysis demonstrated that for lower strain rates the deformation is macroscopically uniform up to the higher average Hencky strains, whereas for higher strain rates the strain localization occurs at the lower average Hencky strains of the deformation process and takes place in the smaller area. It was also found that for all strain rates applied, the maximal values of Hencky strain immediately before rupture of Gum Metal samples were similar for each of the applied strain rates, and the maximal local values of deformation rate were two orders higher when compared to applied average strain rate values

Quasi-static and dynamic compressive behaviorof Gum Metal: experiment and constitutive model

The quasi-static and high strain rate compressive behavior of Gum Metal with composition Ti-36Nb-Ta-3Zr-0.3O (wt pct) has been investigated using an electromechanical testing machine and a split Hopkinson pressure bar, respectively. The stress–strain curves obtained for Gum Metal tested under monotonic and dynamic loadings revealed a strain-softening effect which intensified with increasing strain rate. Moreover, the plastic flow stress was observed to increase for both static and dynamic loading conditions with increasing strain rate. The microstructural characterization of the tested Gum Metal specimens showed particular deformation mechanisms regulating the phenomena of strain hardening and strain softening, namely an adiabatic shear band formed at ~ 45 deg with respect to the loading direction as well as widely spaced deformation bands (kink bands). Dislocations within the channels intersecting with twins may cause strain hardening while recrystallized grains and kink bands with crystal rotation inside the grains may lead to strain softening. A constitutive description of the compressive behavior of Gum Metal was proposed using a modified Johnson-Cook model. Good agreement between the experimental and the numerical data obtained in the work was achieved

Orientation dependence of the elastocaloric effect in Ni54Fe19Ga27 ferromagnetic shape memory alloy

The crystallographic anisotropy of elastocaloric effect (ECE) and relative cooling power (RCP) in Ni54Fe19Ga27 shape memory alloy single crystals are studied via compression tests. Single crystals are studied along the [001], [123], and [011] austenite directions and yield different ECE behaviors and maximum RCPs for various strain levels. A thermodynamic framework using the Helmholtz free energy is employed to analyze the total entropy change as a function of strain. Thermodynamic losses are computed from the mechanical hysteresis of superelasticity experiments to quantify the strain dependent RCP. It is found that the [001] orientation generates the highest maximal RCP of 738 J kg−1 when unloaded from 200 MPa. This is attributed mainly to the large superelastic temperature window of 45 K. However, loading the crystals to stresses higher than 200 MPa causes a multistep transformation in the [011] direction, thus reducing the alloy's overall RCP by 135 J kg−1. This is a consequence of the negative entropy change and large transformation hysteresis generated by the second‐stage transformation in the [011] direction. Interestingly, if only the first‐stage transformation in [011] is employed for the ECE, the [011] direction yields the highest RCP compared to [001] and [123] for any strain up to 3.5%

Thermomechanical aspects of stress-induced martensite transformation in TiNi shape memory alloy

W pracy przedstawiono wyniki badań sprzężeń termodynamicznych zachodzących podczas indukowanej naprężeniami przemiany fazowej w stopach TiNi z pamięcią kształtu. Wykazano, że przemiana martenzytyczna wprost i odwrotna, zachodząca podczas pseudosprężystego odkształcania tych stopów, ma charakter niejednorodny. Makroskopowo rozwija się w zlokalizowanych cienkich pasmach, podobnych do pasm Lüdersa, w dwóch prawie prostopadłych kierunkach, co udokumentowano stosując metodę badań w podczerwieni oraz fotografując relief na powierzchni próbki.Experimental investigations of thermomechanical couplings related to stress-induced martensite transformation in shape memory alloy (SMA) are presented in the paper. TiNi shape memory alloy specimens were subjected to uniaxial tensile tests carried out with various strain rates on an MTS testing machine (Fig. 1). The tem-perature changes, accompanying the process of stress-induced martensite transformation were recorded with use of a thermovision camera. It was shown that the martensite forward and reverse transformation, related to the SMA pseudo-elastic behavior, can occur and develop in inhomogeneous way in Lüders-like bands [3-5]. Nucleation and development of the phase transformation fronts in the SMA recorded by infrared camera were studied. Narrow bands of considerably higher temperature corresponding to the martensitic phase, starting from the central part of the specimen and developing towards the specimen grips, were recorded. Next, the upswing region was observed, manifested by the more advanced and more homogeneous stage of the phase transformation, accompanied by the more uniform temperature distribution. The inclined bands of heterogeneous temperature distribution were observed also during the unloading process of the SMA, while the reverse transformation accompanied by the temperature decrease took place [14]. The martensite transformation heterogeneity was confirmed by infrared technique and by photographs of the SMA specimen surface relief (Fig. 2). Thermomechanical aspects of the exothermic martensitic forward and the endothermic reverse transformations for various strain rates were analyzed

Thermoelastic effect in austenitic steel referred to its hardening

The effect of thermomechanical coupling occuring in metal during consecutive tensile tests, were examined. The temperature, stress and strain characteristic were found both in elastic and plastic ranges. The change of the character of the sample temperature was employed as a criterion for the limit between the elastic and plastic regimes. The thermomechanical coupling effects were interpreted in terms of the material microstructure evolution and referred to its strain hardening degree. A quite precise evaluation unloading seems to be particularly valuable.W pracy wykazano w sposób doświadczalny, że ze wzrostem umocnienia, co wiąże się ze zwiększeniem ilości i ewolucją defektów w materiale, wzrasta efekt termosprężysty oraz efekt termosprężystego odciążania. Umocnienie realizowano drogą kolejnych cyklicznych obciążeń, jak również odpowiednią wstępną obróbką materiału. Badaniom poddano próbki takiej samej stali austenitycznej o dwóch różnych stanach wyjściowych: po wygrzewaniu ujędrniającym oraz po 50% umocnieniu w wyniku walcowania na zimno. Badania przeprowadzono na maszynie wytrzymałościowej, rejestrując jednocześnie zmiany temperatury przy wykorzystaniu zestawu termowizyjnego z komputerową rejestracją i obróbką wyników doświadczalnych. Na podstawie charakterystyk mechanicznych oraz zmiany temperatury materiału próbki zbadano termosprężyste oraz termoplastyczne efekty sprzężeń termo-mechanicznych. Wykazano, że efekty tych sprzężeń wzrastają w miarę wzrostu umocnienia materiału. W próbkach po walcowaniu przeanalizowano rozwój lokalizacji odkształcenia plastycznego. Zaobserwowano, że odkształenie plastyczne zlokalizowane podczas pierwszego cyklu, rozwija się w tym samym miejscu w kolejnych cyklach deformacji. Wyższy wzrost temperatury w próbkach po walcowaniu występuje w miejscu lokalizacji, natomiast w miarę wzrostu odległości od tego miejsca stwierdzono coraz mniejsze przyrosty temperatury

Development of Strain Localization in a Beta-Titanium Alloy Gum Metal Analyzed by Infrared Camera and Digital Image Correlation for Various Strain Rates

Effects of thermomechanical couplings were studied in a new beta Ti alloy by IR and DIC techniques. The obtained stress-strain curves confirmed low Young’s modulus and high strength of the alloy. The determined values of yield strength increases and values of elongation till rupture decreases with increasing strain rate. It was found, by using fast and sensitive infrared camera, that the large limit of the Gum Metal reversible nonlinear deformation originates from mechanisms of dissipative nature, probably exothermic stress-induced transition of ” nanodomains