Constant Strain Rate Tests Research Articles

Enhancing the superplastic deformation capabilities of the Ti-6Al-4V alloy using superimposed oscillations

This study investigates the effect of imposing low frequency, low amplitude oscillations during uniaxial tensile tests on the mechanical behaviour of the Ti-6Al-4V alloy under a typical superplastic temperature of 900 °C. Different material models were used to simulate these tests with the finite element software ABAQUS. A creep strain power model fit through an iterative FEA process yielded a high correlation to the force/displacement and thinning results from uniaxial tensile tests. The developed material models with and without oscillation materials were also used to simulate a free-bulge forming process within the strain rate range of 0.0003 to 0.01 s−1 under constant strain rate testing. Within the domain of the parts formed in this study, the improvement in the forming time ranged from a 120 % reduction at elevated strain rates and reduced part complexity to 520 % in the most complex part geometry.

A new nanoindentation approach for determining both indentation size effect and continuous apparent activation volume during loading of nanocrystalline Ni and Ni-20 wt.% Fe alloy

A new nanoindentation approach is developed in this study to determine both indentation size effect (ISE) and continuous apparent activation volume () during loading of nanocrystalline nickel (NC Ni) and nanocrystalline nickel-20 wt.% iron alloy (NC Ni-Fe alloy). This approach involves conducting nanoindentation tests under loading modes controlled by both constant strain rate (CSR) and constant loading rate (CLR). Firstly, a new empirical expression for the parameter in the modified Nix-Gao model is established to study the strain rate dependence of the ISEs created in the CSR and CLR tests. Then, continuous data of each individual sample (indentation) during loading are obtained in the CLR test through the alternative use of continuous stiffness measurement technique. The stress dependence of is analyzed based on Duhamel et al.’s model, which considers the role of vacancies generated during inhomogeneous dislocation nucleation. Finally, based on the above experimental and theoretical work as well as transmission electron microscopy observation, the influences of loading mode, loading rate, and material properties (stacking fault energy) on resulting intrinsic hardness of NC Ni and NC Ni-Fe alloy are analyzed. The nanoindentation approach developed in this study provides profound insights into the mechanical behaviors during loading and underlying mechanisms of materials.

Effect of friction stir processing on grain refinement and superplastic properties of binary Al-8 wt.% Si to Al-30 wt.% Si alloys

ABSTRACT As-cast binary Al–Si alloys containing ∼8, 12, 20, and 30 wt.% Si, representing the hypoeutectic, eutectic (12 wt.% Si), and hypereutectic compositions, were subjected to severe plastic deformation by friction stir processing (FSP) for grain refinement to 2.3–2.7 μm by dynamic recrystallization. Tensile tests conducted by differential strain rate test technique over the strain rates of 10−4 – 10−2 s−1 and test temperatures in the range of ∼840–530 K led to strain rate sensitivity index (m) varying from ∼0.04 to 0.40 depending on temperature, strain rate, and alloy composition. The constant initial strain rate tests conducted at 10−4 s−1 and 840 K exhibited a maximum elongation of 250% in the hypereutectic Al–20Si alloy, but the same increased linearly with m irrespective of alloy composition. Generally, with increasing Si content, the activation energy for deformation increased from 104.7 ± 14.4 kJ/mol for eutectic Al–12Si to 310.4 ± 32.3 kJ/mol for hypereutectic Al–30Si, which increased further to 572 ± 148 kJ/mol over the higher temperature range of 840–800 K. Analysis of observed deformation and microstructure behaviour supports the occurrence of superplasticity, whereby the accommodation of grain boundary sliding by grain boundary migration led to enhanced grain growth or else the local high-stress concentration at the particle-matrix interface led to cavity formation. There was no evidence of dynamic recrystallization during high-temperature tensile deformation but the flow softening observed is ascribed to the occurrence of concurrent cavitation.

Three-dimensional meso-scale modeling of asphalt concrete

An efficient method to address the three-dimensional modeling of the visco-elasto-plastic material behavior, specifically of bituminous conglomerates used in asphalt concrete production, is proposed. The method resorts to one of the most recent formulations for asphalt creep modeling, represented by the modified Huet-Sayegh fractional rheological model. The Grünwald-Letnikov representation of the fractional operator is adopted to treat the operator numerically in an efficient manner. Further, a coupling scheme between the creep model and elasto-plasticity is proposed by adopting the additive decomposition of the total strain tensor. This enables the numerical assessment of the mechanical behavior for bituminous materials under short- to long-term loading. In this context, both constant strain rate tests, and creep recovery tests are numerically simulated.Numerical analyses are conducted at the meso-scale with the aim to evaluate the development of inelastic strains in the binder during creep, due to the local interaction between the different material components.

Data-driven forecasting of embankment settlements on soft soils using constant rate of strain test data

Geotechnical engineers continually seek accurate methodologies for predicting the time-dependent settlement of embankments on soft soils. This paper introduces a novel data-driven approach that utilises constant rate of strain (CRS) test data to forecast embankment settlements. The proposed framework circumvents the requirement to subjectively assign soil parameters used in traditional forecasting models by directly integrating CRS test data into an automated analytical process, thereby significantly reducing manual effort and user-dependent variation in forecast results. The accuracy of the method is demonstrated through applications to real-world trial embankments in Australia and the Netherlands. All data used in this study are freely available in digital format from Datamap.

Development of a Flow Rule Based on a Unified Plasticity Model for 13Cr-4Ni Low-Carbon Martensitic Stainless Steel Subject to Post-Weld Heat Treatment

This study aims to develop a flow rule for evaluating the relaxation and redistribution of residual stresses during the post-weld heat treatment (PWHT) of hydroelectric runners made from low-carbon martensitic stainless steel (13Cr-4Ni composition). During the PWHT, austenite reforms in the filler metal and surrounding areas of the base metal near welded joints. The evolving inelastic strain rate with reformed austenite led to defining two distinct flow rules in the pure martensitic (α′) and austenitic (γ) phases. A linear rule of mixture was then applied to assess global effective stress based on the inelastic strain rate and current austenite fraction during the PWHT. A unified constitutive model incorporating drag stress and back stress, evolving with creep and plastic deformation mechanisms during the PWHT, described the stress–strain behavior. To validate this analysis, a third flow rule was determined in the 18% tempered austenitic microstructure, compared with the rule of mixture’s effective stress contribution from each phase on the inelastic strain rate. Isothermal constant strain rate tests in stabilized crystalline microstructures evaluated constants specific to their respective flow rules. This study demonstrates the stability of reformed austenite at elevated temperatures during slow cooling and its significant influence on the mechanical properties of 13Cr-4Ni steels. The effectiveness of estimating yield stress using the rule of mixture based on individual phase behaviors is also confirmed.

Experimental constitutive relationships for high-temperature deformation of as-cast Al–Si binary alloys

ABSTRACT As-cast binary Al–Si alloys containing ∼2 to 30 wt.% Si were deformed by differential strain rate and differential temperature test techniques at 600–840 K and strain rates 10−4–10−2 s−1 to find the parameters of the constitutive relationships for high-temperature deformation. The maximum strain rate sensitivity index (m) of flow stress was found to be 0.28 in the eutectic Al-12Si alloy. The Arrhenius plot for activation energy for deformation (Q) exhibits bilinear behaviour, with Q being greater at higher test temperatures than at lower temperatures. The magnitude of Q varies as a function of Si content: 95.7 ± 9.8 kJ/mol for Al-2Si to 311.4 ± 98.1 kJ/mol for Al-12Si alloy. Also, the constant initial strain rate tests performed at selected strain rates and temperatures revealed flow hardening at small strains followed by flow softening. Deformation of Al–Si alloys is suggested to be controlled by the dislocation climb mechanism through the participation of both the grain interior and grain boundaries by consideration of effective stress instead of applied stress.

The High Temperature Strength of Single Crystal Ni‐base Superalloys – Re‐visiting Constant Strain Rate, Creep, and Thermomechanical Fatigue Testing

The present work takes a new look at the high temperature strength of single crystal (SX) Ni‐base superalloys. It compares high temperature constant strain rate (CSR) testing, creep testing, and out‐of‐phase thermomechanical fatigue (OP TMF) testing, which represent key characterization methods supporting alloy development and component design in SX material science and technology. The three types of tests are compared using the same SX alloy, working with precisely oriented <001>‐specimens and considering the same temperature range between 1023 and 1223 K, where climb controlled micro‐creep processes need to be considered. Nevertheless, the three types of tests provide different types of information. CSR testing at imposed strain rates of 3.3 × 10−4 s−1 shows a yield stress anomaly (YSA) with a YSA stress peak at a temperature of 1073 K. This increase of strength with increasing temperature is not observed during constant load creep testing at much lower deformation rates around 10−7 s−1. Creep rates show a usual behavior and increase with increasing temperatures. During OP‐TMF loading, the temperature continuously increases/decreases in the compression/tension part of the mechanical strain‐controlled cycle (±0.5%). At the temperature, where the YSA peak stress temperature is observed, no peculiarities are observed. It is shown that OP‐TMF life is sensitive to surface quality, which is not the case in creep. A smaller number of cycles to failure is observed when reducing the heating rate in the compression/heating part of the mechanical strain‐controlled OP‐TMF cycle. The results are discussed on a microstructural basis, using results from scanning and transmission electron microscopy, and in light of previous work published in the literature.

Yield stress anomaly and creep of single crystal Ni-base superalloys – Role of particle size

In the present work we subject the single crystal Ni-base superalloy ERBO1 (CMSX 4 type) to constant strain rate (CSR) and creep testing at temperatures between 1023 and 1223 K. Three material states are considered which have similar particle volume fractions >60% but differ in γ′-particle sizes (material states S, M and L of particle sizes: 240, 390 and 540 nm). In constant strain rate testing, a yield stress anomaly is observed for all three material states, with a yield stress maximum at 1073 K. This increase of strength with increasing temperature is not observed during creep testing at significantly lower deformation rates in this low temperature high stress creep regime, where different elementary deformation mechanisms govern CSR and creep behavior. In contrast, in the low stress high temperature creep regime, stress/strain rate data pairs from CSR creep tests both show decreasing strength with increasing temperature. It is found that in both types of tests the material state M shows the highest strength (highest yield stress and lowest creep rate). This can be rationalized based on a scenario where both, γ-channel and γ′-particle dislocation activities are important. Diffraction contrast transmission electron microscopy is used to study the relevant elementary deformation processes. Details of dislocation arrangements are discussed with a special focus on the role of Kear Wilsdorf (KW) locks, γ′-particle shearing by superlattice stacking faults (extrinsic and intrinsic) and dislocation climb.

In vitro evaluation of stress corrosion cracking susceptibility of PEO-coated rare-earth magnesium alloy WE43

Controlling material degradation in complex mechanobiological environments is the current challenge in advancing biodegradable magnesium implants for orthopaedic applications. Understanding stress corrosion cracking (SCC) mechanisms of magnesium alloy-coating systems in physiological fluids facilitates the design of magnesium implants with excellent biomedical performance. In this study, we investigated the SCC behaviour of a medical grade PEO-coated magnesium alloy WE43 under different loading conditions. Constant loadings tests (CLT) and slow strain rate tests (SSRT) were conducted to evaluate the SCC susceptibility of this alloy-coating system under in vitro conditions. Fracture surfaces of the non-coated and PEO-coated specimens were characterised by scanning electron microscopy to reveal the SCC mechanisms under different loading configurations. The experimental results showed that the alloy-coating system exhibits a high SCC resistance in SSRT conditions. However, high stress levels and plastic deformations lead to a significant acceleration of the SCC process. Moreover, the fracture surface analysis demonstrated that the brittle nature of the PEO coating deteriorates the mechanical integrity of the alloy under critical mechanical loadings, which results in an increased susceptibility towards stress corrosion cracking of the coated WE43.

Determining SCC resistance of stainless steel claddings in high- temperature water by constant load crack growth tests and slow strain rate tests

Stress corrosion cracking (SCC) behaviors of 309 L and 308 L weld overlays are characterized in oxygen-free and oxygen-bearing high-temperature water using slow strain rate tests (SSRTs) and crack growth rate (CGR) tests with compact tension specimens. There is no apparent SCC indication on 308 L, while local SCC is observed on 309 L after SSRT in oxygen-free water. The SCC sensitivities of 308 L and 309 L increase with the increase of dissolved oxygen (DO) content, while 308 L shows relatively higher SCC resistance than 309 L in water with 200 ppb and 8000 ppb DO. SSRT and CGR test results show a similar effect of DO concentration on SCC of 309 L and 308 L. The fracture surface of 309 L exhibits extensive intergranular SCC, while 308 L shows only local interdendritic SCC after CGR tests. The crack tip blunting decreased the CGR in the low alloy steel. The highly island-shaped δ-ferrites and moderate strain hardening in 308 L contribute to its SCC resistance over 309 L.

One-Dimensional Compressibility and Creep Characteristics of Unsaturated Compacted Loess Based on Incremental Loading and Constant Rate of Strain Methods

In engineering practice, unsaturated compacted loess is often utilized as a filling material in the loess regions. The loess inevitably undergoes one-dimensional compressibility and creep deformation due to the long-term effects of the upper soil layers and buildings. When the deformation is large enough, it tends to damage buildings and threaten engineering safety. In this regard, the one-dimensional compressibility and creep properties of unsaturated compacted loess based on incremental loading (IL) and constant rate of strain (CRS) methods have been studied. First, soil materials with an initial moisture content of 15% were prepared and then compacted into soil samples with an 80 mm diameter and a 10 mm height. Second, the compressibility and creep properties of the compacted loess samples obtained via the IL and CRS compression tests were compared and analyzed. In this study, several parameters, including the primary compression index Cc and secondary compression index Cα, were derived. Meanwhile, the moisture content of the samples was measured via electrical resistivity methods. Finally, the microstructural characteristics were derived via nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) tests. The results showed that Cc and Cα increased with the increase in moisture content and vertical stress; Cα/Cc ranged from 0.026 to 0.042. Compared with the compression parameters and deformation of the samples, those obtained via the CRS tests are a little larger than those obtained via the IL tests for a given loading and initial moisture content. The electrical resistivity depends on pore water-connected channels, which were deeply affected by the initial moisture content, vertical stress and loading duration (or strain rate). Moreover, as vertical stress increased, the pore size and pore area gradually decreased, the coarse particles were broken, and the fine particles increased. The contacts between particles changed from point-to-point contacts and edge-to-edge contacts to face-to-face contacts. Meanwhile, as vertical stress and loading rate increased, the loess particles were apt to vary from irregular elongated particles to equiaxial circular particles. This investigation can provide a theoretical base and experimental support for improving ground stability and preventing landslide disasters in loess regions.

Study on the compression behavior and strain rate effect of Zhuhai soft clay

As a marine deposit, Zhuhai soft clay generally exhibits poor mechanical and engineering properties. In order to investigate its compression behavior, a series of incremental loading (IL) tests and constant rate of strain (CRS) tests were conducted on both intact and reconstituted samples. The compression index (Cc ), coefficient of secondary compression (Ca ), and the ratio Ca /Cc of Zhuhai clay were determined using the end of primary (EOP) concept. In IL tests, the ratio Ca /Cc was found to be non-constant, but varied with time and vertical stress. Furthermore, CRS tests revealed an unexpected strain rate effect that deviated from the classical stress-strain-strain rate relationships. Specifically, the compression curves obtained at different strain rates were not parallel, but gradually approached each other at high stress levels. Regression analysis results suggested a positive correlation between the strain rate and the compression index, indicating that higher strain rates resulted in a greater compression index. To acquire a comprehensive understanding of this unusual phenomenon, a concise review of several relevant works was conducted. It is postulated that the observed discrepancy between Zhuhai clay and the classical isotache concept may be attributed to the development of microstructure and the evolution of Ca /Cc during the loading.

A Constitutive Model of Time-Dependent Deformation Behavior for Sandstone

Considering sandstone's heterogeneity in the mesoscale and homogeneity in the macroscale, it is very difficult to describe its time-dependent behavior under stress. The mesoscale heterogeneity can affect the initiation and propagation of cracks. Clusters of cracks have a strong influence on the formation of macroscale fractures. In order to investigate the influence of crack evolution on the formation of fractures during creep deformation, a time-dependent damage model is introduced in this paper. First, the instantaneous elastoplastic damage model of sandstone was built based on the elastoplastic theory of rock and the micro-heterogeneous characteristics of sandstone. A viscoelastic plastic creep damage model was established by combining the Nishihara model and the elastoplastic damage constitutive model. The proposed models have been validated by the results of corresponding analytical solutions. To help back up the model, some conventional constant strain rate tests and multi-step creep tests were carried out to analyze the time-dependent behavior of sandstone. The results show that the proposed damage model can not only reflect the time-dependent viscoelastic deformation characteristics of sandstone, but also provide a good fit to the viscoelastic plastic deformation characteristics of sandstone's creep behavior. The damage model can also reproduce the propagation process of mesoscopic cracks in sandstone upon the damage and failure of micro-units. This research can provide an effective tool for studying the propagation of microscopic cracks in sandstone.

Viscous Behavior and Constitutive Modeling of Peaty Soil in Kunming, China

The Kunming peaty soil exhibits distinct physical and mechanical properties from the inorganic clay due to the special soil-forming process and material composition. To better understand the viscous behavior of peaty soil, a series of laboratory tests, including four types of oedometer tests (one- and multistage loading creep tests, constant rate of strain tests, and constant rate of stress) and constant rate of strain undrained triaxial compression tests, are conducted on the Kunming peaty soil. The experimental results demonstrate that the Kunming peaty soil shows significant secondary compression during oedometer creep tests. Meanwhile, the variations of the measured preconsolidation pressure and undrained shear strength with the strain rate are essentially linear in log-log plot for the examined range of strain rate. Based on the generalized power law overstress viscoplastic theory, it is confirmed that the rate-sensitivity and time-dependency of the Kunming peaty soil can be uniformly described using the sensitivity parameter involved in the theory, regardless of the applied loading conditions. The average value of 0.06 for the sensitivity parameter is generally greater than those for inorganic clay, indicating that the Kunming peaty soil exhibits more significant viscous behavior. Finally, by using the yield surface of the modified Cam–clay model, an overstress elastic-viscoplastic model is established and numerically implemented. The numerical model with a unique sensitivity parameter can well predict the rate-sensitive and time-dependent response of Kunming peaty soil under different loading conditions.

Prediction of Post-Yield Strain from Loading and Unloading Phases of Pressuremeter, Triaxial, and Consolidation Test Curves for Sustainable Embankment Design

Exponential development of post-yield strain (Ԑpost) is a pivotal indicator of failure in embankments constructed on soft saturated clays. This paper characterizes saturated clay stratum comprising very soft to very stiff stratigraphy, with plasticity index (PI) ranging from 19% to 31%, by performing widely used geotechnical engineering tests, i.e., the prebored pressuremeter (PMT) test, the triaxial (TXL) test, and constant-rate-of-strain (CRS) consolidation. PMT, TXL, and CRS tests were performed at a strain rate range of 0.18%/min to 0.21%/min to explore the yield stress (σ′y), the pre-yield strain (Ԑpre), and the post-yield strain (Ԑpost). Results indicate that Ԑpost/Ԑpre for PMT, TXL, and CRS stress–strain curves range from 2.7 to 19 in the loading phase and 2 to 21 in the unloading phase. An exponential increase in Ԑpost/Ԑpre is observed in the range of 10 to 21 for very soft to soft clay which is congruent with the realistic sustainable range of 4 to 30 for embankment failure on soft clays worldwide. The evaluated Ԑpost/Ԑpre can be applied for sustainable prediction of post-failure evolution of strains in embankments on soft clays. Simplistic correlations are developed for approximation and prediction of Ԑpost as a function of σ′y, Ԑpre and maximum applied pressure (Pmax) for loading and unloading phases with reasonable accuracy. The intuitive zone of critical ℇpost is quantified for impending failure in embankments for maximum applied pressure (Pmax), ranging from 36 kPa to 100 kPa for very soft to soft clay for use in sustainable embankment design and construction. Variation in predicted versus measured results of an individual site is observed to be within ±10% of line of equality.

High-temperature creep behavior and hot-pressing consolidation of NiAl

AbstractThe steady-state creep behavior of NiAl was studied by constant strain-rate tests in the strain-rate regime 1.5 × 10−4≤ ɛ̇ ≤ 1.0 × 10−2s−1.Primary creep of NiAl was investigated at different stresses (10 MPa−40 MPa) in the temperature range from 1200 to 1400°C. The steady-state creep rate was fitted to a temperature-compensated power-law expression ɛ̇ =B(σ/μ)nexp(−Q/RT), from which a stress exponent n≈3.5 and an activation energy Q ≈ 398 kJ/mol were derived, respectively. The obtained activation parameters indicate that dislocation creep dominates the creep behavior of NiAl at temperatures above 1200°C up to 1400°C. The transient creep behavior at 40 MPa could be fitted to a power-law equationε=ε0+btmwithbα σ3:5and 0.52<m<0.68. The creep data of NiAl obtained in this study were utilized for modeling and simulation of the consolidation process of NiAl composites during hot pressing by finite-element analysis.

Determination of Consolidation Parameters of Geomaterials Using Modified CRS Consolidation Testing System

Nowadays, the constant rate of strain (CRS) consolidation phenomenon has gained more attention as an alternate testing method to evaluate the consolidation characteristics of finegrained soils based on strain rate criterion. In the present study, a modified consolidation cell has been fabricated indigenously and carried out a series of CRS tests using a conventional triaxial loading frame at four different strain rates on reconstituted samples of marine clay at 0.002, 0.003, 0.004 and 0.005 mm/min, black cotton soil at 0.0025, 0.003, 0.005 and 0.0075 mm/min, red mud at 0.10, 0.125, 0.25 and 0.50 mm/min, and Varanasi local soil at 0.05, 0.075, 0.10 and 0.125 mm/min. Based on the pore pressure ratio (PPR) criterion, the best suitable strain rates for marine clay, black cotton soil, red mud, and Varanasi local soil were found to be 0.002, 0.0025, 0.1 and 0.05 mm/min, respectively. Further, the consolidation parameters namely primary compression index (cc), recompression index (cr), coefficient of consolidation (cv), coefficient of axial compressibility (av) and permeability (k) obtained through proposed CRS consolidation method are compared and validated successfully with the parameters obtained through Incremental Loading (IL) tests using both conventional and modified oedometer cells.

Strength and Compressibility of Ammonia-Soda Residue from the Solvay Sodium Plant

This work presents the discussion of the results for an experimental study conducted to characterise the mechanical behaviour of ammonia-soda residue (ASR). The calcareous sludge is an alkaline waste formed during the production of soda ash and deposited at the area of the former Solvay Sodium Plant factory in Krakow, Poland. Isotropically consolidation drained (CID) triaxial tests and constant rate of strain (CRS) consolidation tests include the full saturation with water, completion of the consolidation, and the loading/strain rate choice. For this purpose, ASR undisturbed samples were collected from the ground and submitted to laboratory experiments. These samples show a distinct difference in the initial bulk density, the initial level of compaction, initial void ratio, and the natural water content. The CD triaxial tests were conducted under three different levels of confining pressure; in turn, CRS tests were run with two appropriate input strain rates. According to the physical state of ASR and the depth of sampling, two different evolutions of the critical state in the stress–strain space were observed. In the light of the assessed stress–strain–strength behaviour, key design engineering parameters of ASR were calculated.

Dynamic strain aging in the intermediate temperature regime of near-[formula omitted] titanium alloy, IMI 834: Experimental and modeling

A dynamic strain aging (DSA) regime is established for a near-α titanium alloy with a microstructure consisting of 80% equiaxed α, 15% lamellar α and 5% β with silicon in solid solution through a series of constant strain rate tests carried out in tension from 5 × 10−2 to 5 × 10−6 s−1 in the temperature range of 623–823 K. Transmission electron microscopy shows jogged screw dislocations within slip bands dominating the dislocation structure in this domain. Strain accumulation occurs by conservative jog glide along the length of screw dislocations due to line tension forces. We estimate the solute concentration accumulating at these edge jogs on arrest during their thermally activated glide over static solute obstacles. The domain of DSA is then predicted using Friedel's model to obtain the stress required to break the jogs free from the solute atmosphere. There is a good agreement between model and experimental data showing a DSA peak in the temperature range of 673–723 K. The solute species responsible for DSA from the model are C and Si, but dominated by C in this temperature and strain rate regime. Atom probe tomography demonstrates C and Si segregation at the dislocations, strongly supporting the model.