We introduce a new nanoindentation method to continuously measure the hardness while sweeping through orders of magnitudes of strain rates within a single experiment. While nanoindentation already ...allows the determination of the strain rate sensitivity of materials by means of strain rate jump tests, these are typically limited to few discrete strain rates. With the new method, the strain rate sensitivity can be measured continuously as a function of the strain rate. Applications to fused silica, Zn-22 %Al superplastic alloy, single crystalline aluminum, various nanocrystalline metals and a palladium-based metallic glass are shown. Besides some discrepancy with the reference measurements, the new method seems only affected by the presence of a strong nanoindentation size effect. Provided this indentation size effect is not excessively large and can be corrected for accurately, the method proves robust, with no suggestion that the direction of the strain rate sweep affects the evaluation of the strain rate sensitivity.
In this study, the effects of strain rate on trigger stress (σT) of stress-induced martensitic transformation (SIMT) and mechanical properties of Ti-10V-2Fe-3Al alloy were investigated. The ...experimental results indicated that the SIMT occurs in the β-solution treated Ti-10V-2Fe-3Al alloy at the strain rate ranging from 10−5s−1 to 10−1s−1. The trigger stress rose continuously with the increase of strain rate, which could be explained by the free energy change associated with the SIMT. However, with the strain rate decreasing, both the ultimate tensile strength (UTS) and the uniform elongation (δ) were found to increase, which exhibited the negative strain rate sensitivity (NSRS). Meanwhile, the shallow dimples on the fracture surface became non-uniform and large under high strain rate. The high work hardening rate and three-stage work hardening behavior were closely related to the SIMT serving as obstacles of dislocation motion. It is found that the formation of thin α" plates can divide β grain into smaller domains which responded to the improvement of mechanical properties.
For more than 150 years, it has been considered proven that hydrogen generally degrades the mechanical performance of metals. Nevertheless, there is no consensus on the exact mechanisms, how hydrogen ...affects plastic deformation. The strain rate sensitivity of a material results from a thermally activated contribution to the rate-determining deformation process, e.g. to dislocation slip or dislocation grain boundary interaction. In this study, the extent to which hydrogen affects thermally activated dislocation motion and hence the strain rate sensitivity was investigated. For this purpose, specimens were cathodically charged in situ, and subjected to nanoindentation. In addition, macro-tensile tests with strain rate jumps were performed varying the temperature into the cryogenic range, to inhibit effusion, but also to test the effect of hydrogen on the activation parameters of deformation. Hydrogen was shown to increase the strain rate sensitivity of f.c.c. nickel, whereas it is not affected for a structural steel with a b.c.c. lattice. The activation volume for plastic deformation in a direct comparison between nanocrystalline and coarse-grained f.c.c. nickel and the b.c.c. structural steel shows, that the rate-determining deformation mechanism appears to change for f.c.c. but not for the b.c.c. material.
•Hydrogen increases the strain rate sensitivity and thus decreases the activation volume in coarse and nano-grained nickel.•In b.c.c. structural steel the activation volume is not affected by hydrogen.•Combination of tensile and nanoindentation strain rate jumps tests allows also to test materials that suffer from severe hydrogen embrittlement.
Implementation of Additive Manufacturing (AM) parts in the growing applications within the automotive and aerospace industries encourages further investigations of the material behavior under various ...strain rates, spanning from quasi-static to the high strain rate regimes. Although mechanical properties of AM-Selective Laser Melting (SLM) AlSi10Mg parts under a static regime have been investigated, the strain rate sensitivity of these materials, to the best of our knowledge, has not been discussed in the literature. In this work, the properties of AM-SLM AlSi10Mg material were systematically investigated under a wide range of strain rates, spanning from 2.77×10−6 to 2.77×10−1 S−1. The AM-SLM AlSi10Mg alloy, as opposed to Al alloys fabricated by conventional methods, was found to be strain rate sensitive with significant changes to the flow stress and strain hardening exponents with an increase in strain rate. The fracture mechanisms of these specimens, built in different orientations, are discussed.
The present work investigated the effect of strain rates (10−3 to 103s−1) on the deformation behaviour of a twinning-induced plasticity (TWIP) steel. The strain rate sensitivity was studied in terms ...of instantaneous strain rate sensitivity (ISRS) and strain rate sensitivity of work-hardening (SRSW). While ISRS concerns the instantaneous flow stress change upon strain rate jump, SRSW deals with the subsequent modification in microstructure evolution, i.e. change of work-hardening rate. The present TWIP steel demonstrates a positive ISRS which remains stable during deformation and a negative SRSW, i.e. lower work-hardening rate at higher strain rate. Synchrotron X-ray diffraction experiments indicate that the negative SRSW should be attributed to the suppression of dislocations and deformation twins at high strain rate. This unexpected finding is different to conventional face-centred cubic (fcc) metals which generally show enhanced work-hardening rate at higher strain rate. A constitutive model which is strain rate- and temperature-dependent is developed to explain the stable ISRS and the negative SRSW. The modelling results reveal that the stable ISRS should be attributed to the thermally-activated dislocation motion dominated by interstitial carbon atoms and the negative SRSW should be due to the suppression of the dislocations and deformation twins caused by the adiabatic heating associated with high strain rate deformation.
The mechanical response and texture evolution of rare-earth-containing magnesium alloy sheet, ZEK100, are measured under uniaxial (tension and compression) loading along the rolling direction (RD), ...45° to rolling direction (DD), transverse direction (TD) and normal direction (ND), at the strain rate of 10−4 to 3 × 103 s−1, and temperatures of 22 °C & 150 °C. Texture evolution is measured at strain increments between 2 and 10% in compression and tension at both 10−4 and 3 × 103 s−1, and at 22 °C and 150 °C. Measured pole figures reveal relatively weak basal pole intensity with a spread of basal poles from ND toward TD. Consequently, the yield stress in both tension and compression is the largest in RD and decreases with change in orientation from RD to TD. The material exhibits positive strain rate sensitivity as well as tension-compression asymmetry and anisotropy that are a function of temperature and strain rate. Strain hardening behavior in both tension and compression represents the characteristics of twinning dominated deformation even at elevated temperatures. Crystal reorientation due to extension twinning was observed both in compression along all directions, and in tension along the TD. While the flow stress in compression increases with the increase of strain rate from 10−4 to 3 × 103 s−1, differences in measured textures between the two strain rates are negligible. A reduced-order crystal plasticity model that defines extension twinning, basal slip, and non-basal slip as the deformation mechanisms, is used to model the experimental results and to give an insight in the active deformation mechanism. The model captures the work hardening, anisotropy and tension-compression asymmetry behavior of the material at different strain rates and temperatures.
•Strain rate sensitivity, tension-compression asymmetry and anisotropy are studied.•Texture was measured at different strain rates and temperatures at different strain intervals.•Minimum difference in texture measurements between quasi-static and dynamic loading.•Mechanical response at different strain rates and temperatures are modeled.
•Dynamic splitting-tensile failure of concrete is simulated at the meso-scale.•Size effect on dynamic splitting-tensile strength of concrete is explored.•Dynamic size effect in concrete is obviously ...distinct with static one.•A static and dynamic unified size effect law is proposed.
Under static loadings, concrete has been demonstrated to exhibit obvious size effect due to the heterogeneity of mesoscopic composition. Under dynamic loadings, the strain rate effect has a significant influence on the failure of concrete. It is therefore of great importance to explore the size effect of concrete under dynamic loadings. In this study, the focus is on the size effect in dynamic splitting-tensile strength of concrete. A mesoscopic numerical model for the simulation of the splitting-tensile failure and size effect at different strain rates (10−5/s ∼ 200/s) was established. The mesoscopic simulation results indicate that the size effect on the dynamic splitting-tensile strength of concrete has an obvious discrepancy with the static one. There is a critical strain rate in dynamic splitting-tensile strength of concrete. As the applied strain rate below the critical strain rate the size effect behavior is restrained and weakened gradually with the addition of strain rate. As the applied strain rate exceeds the critical strain rate the dynamic strength enhances linearly as the structural size increases. Furthermore, considering the contribution of strain rate on the dynamic size effect, a Static and Dynamic unified Size Effect Law (i.e. SD-SEL) for splitting-tensile strength of concrete was developed. The proposed SD-SEL was verified by the numerical results and the available test data.
Face-centered-cubic (FCC) Fe40Mn20Cr20Ni20 high-entropy alloys (HEAs) with a weigh of 50 Kg were fabricated by industrialized vacuum-induction melting. This alloy consists of a tetragonal σ phase and ...minority M23C6 carbides embedded in the continuous FCC matrix after thermomechanical treatments. A heterogeneous structure composed of a phase distribution and grain size is formed. The yield strength and ultimate tensile strength are increased from 398 MPa to 679 MPa at 10−4 s−1 to 743 MPa and 1412 MPa at 3000 s−1, respectively. Meanwhile, the elongation is slightly improved as the strain rate rises. The strain rate sensitivity under quasi-static tension is 0.0172, in contrast to 0.3978 under dynamic deformation. Upon dynamic tension, the simultaneous enhancements of both strength and ductility are attributed to the joint activation of multiple strengthening mechanisms. Deformation-induced twinning further improves the strain-hardening ability of the alloy. Besides, short range order may seriously hinder the dislocation movement, especially when the thermal activation of dislocations gradually fails at high strain rates, which limit the dislocation slip to a smaller scale and result in deformed sub-grains. In contrast, under quasi-static tension, only dislocation slip dominates, accompanied by dislocation entanglement and massive pile-ups. Moreover, a typical Johnson-Cook model was employed to predict the dynamic-flow behavior. This study sheds lights on the mechanical performance superiority from heterogeneous HEAs under dynamic tension and might open new insights for developing high-performance HEAs to resist dynamic impacts.
•The Fe40Mn20Cr20Ni20 high-entropy alloys (HEAs) with a weigh of 50 Kg.•Heterphase and heterostructure induced by different phases and grain sizes.•The deformation twins and subgrains lead to higher strain-rate sensitivity under dynamic tension.•Stack fault energy was calculated from thermodynamics and molecular dynamics simulation.•Critical twin stress was determined for the target HEAs.
On the Notch Tip Strain Rate Toribio, Jesús
Procedia Structural Integrity,
2024, 2024-00-00, Letnik:
59
Journal Article
Odprti dostop
This paper studies the notch tip strain rate (NTSR), local strain rate at a notch tip or local strain rate in the vicinity of a notch tip. It is the really governing variable in environmentally ...assisted fracture (EAF) processes involving notched samples. Therefore, when notched specimens are used in EAF testing and analysis, e.g. using the slow strain rate testing (SSRT) technique, it is the local strain rate at the notch tip or its vicinity (notch tip strain rate NTSR) – instead of the global strain rate, remote strain rate; applied displacement rate or crosshead speed– the kinematic variable governing the environmentally assisted process.
On the Crack Tip Strain Rate Toribio, Jesús
Procedia Structural Integrity,
2024, 2024-00-00, Letnik:
59
Journal Article
Odprti dostop
This paper studies the crack tip strain rate (CTSR), local strain rate at a crack tip or local strain rate in the vicinity of a crack tip. It is the really governing variable in environmentally ...assisted cracking (EAC) processes involving cracked samples. Therefore, when cracked specimens are used in EAC testing and analysis, e.g. using the slow strain rate testing (SSRT) technique, it is the local strain rate at the crack tip or its vicinity (crack tip strain rate CTSR) –instead of the global strain rate, remote strain rate, applied displacement rate or crosshead speed– the kinematic variable governing the environmentally assisted process.
