Constant Strain Rate Method Research Articles

Comparison of the mechanical properties of Mo films by different nanoindentation techniques

Nanocrystalline Mo films were prepared by magnetron sputtering. Indentation size effect (ISE) on hardness (H) was both studied by constant strain rate method (CSR) and continuous stiffness method (CSM). Inverse ISE appears and H from CSR is higher than that from CSM. Significant dislocation motion barrier effect of increased grain boundary (GB) induces strengthening. Each step of CSM promotes the generation of heterogeneous GBs and enhances GB-emitting dislocations ability, leading to softening.

Local strain rate sensitivity of α+β phases within dual-phase Ti alloys

Using in-situ micropillar compression, the local strain rate sensitivity in Ti6242 and Ti6246 has been investigated to strengthen our understanding on the rate- and slip system-sensitive deformation of dual-phase Ti alloys. Electron backscatter diffraction (EBSD) was used to find target grains anticipating basal and primatic slip activities under compression test. Micropillars with similar α orientation and incomparable β morphology were made by a focused ion beam (FIB). Strain rate sensitivity (SRS) was determined based on the constant strain rate method (CSRM). The marked difference of SRS is found in the α+β of both alloys such that in Ti6242 the SRS in the basal slip is considerably higher than that in the prism whilst both slips in Ti6246 show somewhat similar SRS, inferring that either local chemical effects or the β morphology could affect rate-sensitive deformation behaviour.

Influences of elasticity on the measurement of power law creep parameters by nanoindentation

Indentation testing is a well-known method to measure the rate dependence of the strength of materials at small scales through a variety of different testing methodologies. In many cases, a power-law creep constitutive relationship relating the indentation strain rate to the hardness, or the mean pressure applied to the material by the indenter, is assumed. However, this method of analysis does not explicitly include elastic effects, which for some testing methods may be significant. In this work, a new method of analysis that explicitly considers the elastic effects is presented. The conditions and materials for which the elastic contribution is non-negligible are identified, and simple closed-form analytical expressions are developed to account for the elastic effects. This leads to improvements in the measurement of power law creep parameters such as the stress exponent and creep coefficient. Experimental assessment of the new analysis procedures is presented for amorphous selenium (Se) and calcium fluoride (CaF2). After accounting for the elastic effects, an equivalence of different indentation creep testing methods is observed for Se due to the fact that this material has a very nearly history independent plastic response and no indentation size effect (ISE). On the other hand, significant differences are observed in the case of CaF2 because of indentation size effects. The findings of this work have important implications for several widely used indentation creep measurement methods such as the constant strain rate method (CSR) and the strain rate jump test (SRJ). Based on the results, guidelines for performing indentation creep tests with improved precision and accuracy are presented.

Study on a New Rock Fracability Evaluation Model of Shale Gas Reservoir

Shale gas is an important unconventional energy resource that needs large-scale fracturing to form industrial deliverability. The evaluation of reservoir fracability plays a key role in the optimization of the sweet spot, the design of multistage fracturing, and the prediction of economic benefit. Based on volumetric fracturing, the study proceeded from the fracture complexity of the fractured core, and the bursting pressure experiment technology using the constant strain rate method was established. After the core has fractured, the fracture morphology was extracted and the fracture parameters including fracture area ratio and fracture declination dispersion were calculated to construct the fracture complexity of the pressed core. Combined with the core strength, the fracability index of the core was determined to evaluate the reservoir fracability. This method can represent not only the fracturing effect but also the fracturing difficulty. Compared with the monitoring data of hydrofracture-induced microseism of the sample well, the core fracturing index was found to be in good agreement with the actual fracturing effect. This method is more reasonable than the traditional brittleness index method and rock mechanics parameter method.

Effects of strain rate on tensile deformation behaviour in Ti-6Al-4V at cryogenic temperature

In this study, we investigated the effects of strain rate on tensile deformation behaviour in Ti-6Al-4V sheet at cryogenic temperature. X-ray diffraction (XRD) was used to identify the crystallographic orientation of rolled Ti-6Al-4V. A series of tensile tests were performed by constant strain rate method (CRS) with variable strain rates (i.e., on the order of 1x10-2 to 10-4•s-1). Liquid nitrogen (LN2) was used to mimic cryogenic environment, and for the thermal equilibrium the specimens were immersed in the vessel containing liquid nitrogen for ~10 minutes before tensile testing, and the temperature condition was continuously maintained during the testing. Microstructure and fracture surface was analysed by polarised light microscopy and scanning electron microscope (SEM). Electron backscatter diffraction (EBSD) was further used to characterise local deformation behaviour. Deformation twinning is occurred at cryogenic tempearture, which is rather different to the deformation at room temperature. It is thought that the twinning induced deformation behaviour may lead to a strength enhancement and a rate dependent ductility improvement. Key words: Ti-6Al-4V, cryogenic, microstructure, deformation twinning, EBSD

Local strain rate sensitivity of single α phase within a dual-phase Ti alloy

We have performed in-situ micropillar compression to investigate the local strain rate sensitivity of single α phase in dual-phase Ti alloy, Ti–6Al–2Sn–4Zr–2Mo (wt%). Electron backscatter diffraction (EBSD) was used to identify two grains, anticipated to primarily activate a slip on the basal and prismatic plane respectively. Comparative micropillars were fabricated within single α laths and load-hold tests were conducted with variable strain rates (on the order of 10−2 to 10−4 s−1). Local strain rate sensitivity exponent (i.e. m value) is determined using two types of methods, constant strain rate method (CSRM) and conventional stress relaxation method (SRM), showing similar rate sensitivity trends but one order higher magnitude in SRM. We thus propose a new approach to analyse the SRM data, resulting in satisfactory agreement with the CSRM. Significant slip system dependent rate sensitivity is observed such that the prism slip has a strikingly higher m value than the basal. Fundamental mechanisms differing the rate sensitivity are discussed with regards to dislocation plasticity, where more resistance to move dislocations and hence higher hardening gradients are found in the basal slip. The impact of this finding for dwell fatigue deformation modes and the effectiveness of the present methodology for screening new alloy designs are discussed.

Participation of the Intestinal Layers in Supplying of the Mechanical Strength of the Intact and Sutured Gut

The tensile properties of the intact and sutured human intestine were examined using Instron 1122 tensimeter on 471 cadaveric and 98 surgically removed specimens. By a constant strain rate method two maxima curves were received, which permit examination of the intestinal wall as a multi-layered structure. Repeatable results were generated for cadaveric and surgically removed intestines, whose mechanical properties were similar under certain storage conditions. The mechanical properties of intact intestine in axial and transversal directions were different. The mechanical strength of the intact intestinal wall was conditioned by the submucosa and muscularis, while the serosa and mucosa showed no significant strength. Comparison of axial specimens’ mechanical properties of the intact intestinal wall and intestinal wall with all layers sutured through determined that only the submucosa supplies mechanical strength to anastomosis. Other layers contribute no significant force to anastomotic strength. The strength applied to thread during knot tying does not change the participation of the intestinal layers in supplying suture-holding capacity for 8 and 12 mm stitch depth. It was shown graphically why sutures that don’t stitch onto the submucosa are unreliable.

Uniaxial compression testing using a linear combination of stress and strain as the control variable

A new control method is proposed in which a linear combination of stress and strain, (ϵ − σ/E′), is used as the feedback signal in a closed-loop, servo-controlled testing machine. In this study, uniaxial compression tests of one Class I and seven Class II rocks were conducted by this new method under loading conditions specified as ϵ − σ/ E′ = C· t , or ϵ = σ/ E′ + C· t , where C is a constant. The results indicate that failure of Class II rock can be controlled by this method and stress-strain characteristics up to the strength failure point coincide with those obtained by the constant strain rate method. Naturally, Class I rock can also be controlled by this method. With this method, if stress decreases by an amount δσ, strain is decreased by an amount δσ/E′, whereas strain is monotonically increased in the constant strain rate method. This is the reason why a Class II rock which has a positively inclined portion of the stress-strain curve in the post-failure region can be controlled by the new method. The method, by which complete stress-strain curves of Class I and Class II rocks can be obtained reproducibly, is an improvement on the constant strain rate method which is currently standard for rock testing.

Influence of Loading Direction and Environment on the SCC Susceptibility of Aluminium 7075–T651

The influence of the loading direciion on the SCC susceptibility of aluminium alloy 7075–T611 has been investigated, the specimens having been tested by the constant strain rate method. The influence of the strain rate on the mechanical properties of the material was also investigated.The results confirm that this material is very susceptible to SCC when it is loaded in the ST-direction. In the LT- and L-directions it is susceptible only in acid environments (in the particular conditions of these tests). The influence of the environment (acidity, and chloride ion concentration) was also investigated, and a comparison made between several failure criteria which can be used to describe the SCC susceptibility in various environments.

Evaluation of the Constant Strain Rate Test Method for Testing Stress Corrosion Cracking in Aluminum Alloys

Abstract In this paper, a general description is given of the use of the constant strain rate method for testing stress corrosion cracking (SCC). A comparison is made between the test results obtai...

Effect of Strain Rate on Stress Corrosion Cracking of Austenitic Stainless Steel in MgCl2 Solutions

Abstract The effect of strain rate on the stress corrosion cracking (SCC) of an austenitic stainless steel in MgCl2 solutions has been investigated by using a constant strain rate method over the range of strain rates from 4 x 10−3 mm/min to 6 mm/min. Crack propagation mode (intergranular vs transgranular) was a function of strain rate and temperature. At low strain rates, the rate determining step of the SCC corresponded to the formation of slip steps, but at higher strain rates, the rate determining step appeared to be a corrosion process on the slip steps. SCC was most prevalent when the formation rate of the slip steps was equal to the corrosion rate of the steps. The mechanism of SCC in this system has been discussed by considering both the formation rate of slip steps and the corrosion rate of these steps.

定速ひずみ法による軟鋼の硫化物割れの研究

Effects of strain rate, applied potential, pH, temperature and H2S concentration on the sulfide cracking of mild steel were studied at constant strain rates by means of an Instron-type tensile test machine. The results obtained are as follows:(1) The susceptibility of sulfide cracking is highest in the strain rate region of 10−6∼10−5 sec−1 and decreases in the higher strain rate region.(2) In both solutions with and without H2S, the lower the pH of solutions the higher is the susceptibility to cracking, which varies almost linearly with pH. The H2S addition to the solution is most effective for the embrittlement at pH 4.7 and above pH 5 the embrittlement index decreases rapidly. The embrittlement indices below pH 4 are almost identical but slightly decrease with the lowering of pH.(3) The susceptibility to the sulfide cracking varies linearly with applied potential, decreasing with anodic polarization and increasing with cathodic polarization.(4) The embrittlement index is highest at 50°C.(5) The change in H2S concentration has little effect on the susceptibility to cracking.(6) The cracking susceptibility is not directly related to the presence of an iron sulfide film on the steel surface.(7) The dissolved species which promotes hydrogen embrittlement in hydrogen sulfide solutions has turned out to be an undissociated, molecular H2S.(8) The sulfide cracking of mild steel is caused by hydrogen embrittlement.(9) The constant strain rate method is suited for quantitative comparison of changes in susceptibility to the hydrogen embrittlement of steel.

酸性硫化水素水溶液中における軟鋼の水素吸収と水素脆性

Relation of hydrogen embrittlement of mild steel with hydrogen electrode reaction and hydrogen absorption in acidic hydrogen sulfide solutions at room temperature was studied by means of a constant strain rate method. From the current passed and the amount of hydrogen absorbed in mild steel during potentiostatic cathodic polarization, the hydrogen electrode reaction mechanism and the effect of hydrogen sulfide on the reaction have been elucidated. An expression has been derived which relates the amount of absorbed hydrogen QH to the fracture stress σf as follows, σf+KQH=constant. Consequently, the absorbed hydrogen contributes to hydrogen embrittlement of mild steel as the sum of applied stress and apparent stress arising from absorbed hydrogen. The failure will occur even at a very low applied stress if the amount of absorbed hydrogen may increase considerably. The activation energy for hydrogen embrittlement of mild steel under a corrosive environment was estimated as 8800 cal/mol.