Indentation Creep Tests Research Articles

Investigations on the influence of hot rolling on the creep characteristics of additively manufactured Mg-5Ag alloy

The requirement for lightweight materials that can survive increasingly severe conditions for aerospace components has fueled the research on magnesium alloys. The current work investigates the influence of hot rolling on the creep characteristics of the Mg-5Ag alloy that is fabricated through a selective laser melting process. The creep properties of parent material (PM) and hot-rolled specimens (HRA) are determined by indentation creep tests at 250 °C, 275 °C, and 300 °C. The microstructural evolution, phase transformation kinetics, and texture evolution were deduced using instrumental analysis, including SEM, EBSD, TEM, and XRD. The specimen PM features α-Mg grains with intermetallic Mg-Ag particles along grain boundaries and random grain orientations with low-angle boundaries. However, specimen HRA has a more fraction of Mg-Ag precipitates, a dominant (0001) grain orientation, and higher peak intensity from the (002) plane. The HRA specimen also demonstrates higher apparent activation energies, attributed to microstructural changes from hot rolling. The results show that the creep resistance of the hot rolled Mg-5Ag alloy is superior to as-fabricated Mg-5Ag alloy based on the creep velocity, stress exponent, and apparent activation energy.

Exploring nanoscale metallic multilayer Ta/Cu films: Structure and some insights on deformation and strengthening mechanisms

Nanoscale metallic multilayer (NMM) films are systems offering insight into the role of interfaces in metal plasticity, deformation, and strengthening mechanisms. Magnetron sputtering was used to fabricate the Ta/Cu NMM films with a periodicity (equal Ta and Cu layer thickness) from 6 to 80 nm, and with the structure exhibiting immiscible tetragonal β-Ta and face-centred cubic Cu phases. Transmission electron microscopy and X-ray diffraction analyses revealed that, irrespective of the period, all films manifested a polycrystalline structure. The growth direction of both Cu and Ta layers was found to be along 〈001〉 β-Ta || 〈111〉 Cu directions, with the crystallite size constrained by the layer thickness. The studies showed that the Ta/Cu NMMs exhibited compressive residual macro-stress and flow strength, and enhanced elastic recovery at the periodicity of ≤12 nm. Activation volume V⁎ value of 11 to 20 b3 as determined from the indentation creep test under a constant load, may indicate a mixed deformation mechanism. This mechanism likely involves the emission of dislocations from the incoherent Ta/Cu interfaces, as well as the formation of screw dislocations within Cu grains. The high-load indentation test, TEM studies, and the rCLS model collectively demonstrate that all NMM films predominantly undergo plastic deformation. This plastic deformation primarily occurs within the soft Cu layer, while the propagation of dislocations across the incoherent interface is largely excluded.

Deformation microstructures of non-irradiated and proton irradiated zirconium alloy from nanoindentation creep tests characterised by TEM

Indentation creep tests were conducted on non-irradiated and proton irradiated Zircaloy-2 at different temperatures. After indentation creep testing, areas beneath the indentation tip were lift-out by (Focused Ion Beam) FIB, then deformation microstructures of the plastic zone were characterized by TEM. We observed that although it has the highest critical resolved shear stress of available slip modes during room temperature plasticity, under creep deformation 〈c + a〉 dislocations play a significant role in the indentation deformation process. These edge 〈c + a〉 dislocations form dipoles that impede dislocation motion and cause work hardening. For non-irradiated samples, 〈c + a〉 dislocation density in samples tested at 300 °C is significantly lower than those tested at room temperature. This may be due to the formation of {11¯01} <1¯012> twins at 300 °C, which is observed in both irradiated and non-irradiated samples. However, there is no decrease in 〈c + a〉 dislocation density at high temperature in irradiated samples. This is hypothesised to be because irradiation induced loops have more effect in impeding the motion of 〈a〉 type dislocations than 〈c + a〉 dislocations. In addition, annealing of 〈c + a〉 dislocation dipoles at 300 °C during creep of the non-irradiated sample may contribute to the decrease in 〈c + a〉 dislocation density. Irradiation induced 〈a〉 loops were swept away by gliding 〈a〉 dislocations during indentation deformation. Irradiation loops that were close to the indent were significantly eliminated while loop-free channels that were parallel with the basal trace formed at positions further away from the indent.

Tuning the mechanical properties of sol-gel monolithic metal-organic frameworks by ligand engineering

The sol-gel monolithic MOFs has come to prominent attention for industrial application owing to the higher powder packing density, enhanced processabilities and mechanical stabilities compared to the powder counterpart. The mechanical properties are particularly important during machine shaping processing because of porous framework structure. We used ligand engineering to design and synthesize monoUiO-66-type materials modified different chemical functional groups (–NH2, –2OH, –2COOH) by sol-gel method, with the aim to assess the impact of different functional groups on the mechanical properties of these monolithic materials based on nanoindentation technology. We observe larger mass and sterically bulky functional groups (–2COOH) can significantly decrease the BET areas and pore volume of monoUiO-66 through N2 adsorption isotherms at 77 K. Hence, the two –COOH groups modified monoUiO-66 tends to exhibit the higher H of 0.589 ± 0.018 GPa and E of 15.471 ± 0.250 GPa compared with monoUiO-66 modified with –NH2 (0.334 ± 0.009 GPa/11.959 ± 0.243 GPa) and –2OH (0.331 ± 0.008 GPa/10.251 ± 0.142 GPa) groups. The creep indentation tests and the jump indentation tests further demonstrate the modification by larger functional groups –COOH on monoUiO-66 could resist irreversible plastic deformation. Furthermore, the monoUiO-66-(COOH)2 has significantly smaller the activation volume of 0.34 ∼ 0.43 nm3, highlighting the introduction of –COOH groups reduced the pore volume and restrict the number of pores involved in one collapse event. Our results demonstrate the larger mass and sterically bulky functional groups have significant influence on the mechanical properties of the monoMOFs materials.

Study on the constitutive model of NEPE propellant based on indentation creep

AbstractThe significance of the mechanical properties of solid propellant during the curing process lies in their contribution to stress‐strain calculations of viscoelastic engineering materials and the analysis of structural integrity of solid rocket motor charges. In order to study the Constitutive equation of the nitrate ester plasticized polyether (NEPE) propellant during the curing process, indentation creep tests were carried out on the NEPE propellant, and the viscoelastic constitutive model of propellant with curing reaction at 50 °C and atmospheric pressure was studied. Firstly, based on the viscoelastic theory, the experimental principle of indentation creep test is deduced, and indentation creep tests are carried out for propellants with different curing degrees. Then, based on 2S2P1D (2 Springs, 2 Parabolic elements, 1 Dashpot) model and time‐temperature equivalence principle, the indentation test data are processed, and the complex modulus master curve of propellant with different curing degrees is established, and the influence of curing reaction on viscoelastic properties of propellant is analyzed, so as to better understand the response characteristics of propellant system during curing. The results show that the 2S2P1D model has a good fitting effect for the master curve in low frequency period, the fitting error is controlled within 10 %, and the correlation coefficients are all above 0.95. With the curing reaction, the complex modulus gradually increases, while the change rate gradually decreases. And it tends to be stable on the seventh day.

Creep Deformation Property of MCrAlY Bond Coatings by High-Temperature Creep Indentation Tests

Creep Deformation Property of MCrAlY Bond Coatings by High-Temperature Creep Indentation Tests

Time-dependent plastic behavior of bacteria leading to rupture

A study of the mechanical response of bacteria is essential in designing an antibacterial surface for implants and food packaging applications. This research evaluated the mechanical response of Escherichia coli under different loading conditions. Indentation and prolonged creep tests were performed to understand their viscoelastic-plastic response. The results indicate that varying loading rates from 1 μm/s to 5 μm/s show an increase in modulus of 182% and 90%, calculated in the loading and unloading cycles, respectively, and a decrease in adhesion force by 42%. However, on varying loads from 5 nN to 25 nN, nominal change is observed in both modulus and adhesion force. The rupture curve at 100 nN load shows elastic and a small plastic deformation accompanied by a sharp peak indicating the cell wall rupture. The rupture force at the peak was found to be 34.38 ± 5.15 nN, irrespective of the loading rate, making it a failure criterion for bacteria rupture. The creep response of bacteria increases (for 6 s) and then remains constant (for 15 s) with time, indicating that a standard linear solid (SLS) model applies to this behavior. This work attempts to evaluate the mechanical properties of E. coli bacteria focusing on its rupture by contact killing mechanism.

Indentation creep behavior of Fe–8Ni–xZr oxide dispersion strengthened alloys

Abstract This study was conducted to understand the creep behavior of two oxide dispersion strengthened alloys containing Zr as the alloying addition by performing indentation creep tests at room temperature. The oxide dispersion strengthened alloys were Fe–8Ni–xZr (x = 1 and 4 at.%, i.e., Zr-1 and Zr-4 alloys, respectively), which had been previously fabricated by mechanical alloying; followed by consolidation via equal channel angular extrusion at 1000 °C. The indentation tests were conducted under a maximum load of 100 mN with the loading rates at 300 and 400 mN min−1. The hardness was calculated by the Oliver–Pharr method, and the creep properties, such as the creep displacement, creep strain rate, creep stress, and stress exponent n, were determined. The results showed that the Zr-4 alloy was harder than the Zr-1 alloy. However, the creep resistance of the Zr-1 alloy was better than that of the Zr-4 alloy. It was further demonstrated that both the hardness and creep resistance depended on the loading rate. Moreover, a possible creep mechanism was proposed. Although the tests were performed at room temperature, they can provide insight into the effect of an oxide dispersion strengthened alloys microstructure on creep at higher temperatures.

Effect of hard magnetic ferrite (Ba0.5Sr0.5Fe12O19) nanoparticles on the mechanical properties of the (Bi, Pb)-2223 phase

To manifest the effect of hard magnetic Ba0.5Sr0.5Fe12O19 nanoparticles on the mechanical performance of the (Bi,Pb)-2223 superconducting phase, nano-(Ba0.5Sr0.5Fe12O19)x/Bi1.8Pb0.4Sr2Ca2Cu3.2O10+δ, with x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05, 0.10, and 0.20 wt%, were synthesized using a conventional solid-state reaction method. The X-ray diffraction (XRD) data revealed that adding nano-(Ba0.5Sr0.5Fe12O19) to the host (Bi,Pb)-2223 phase preserved the orthorhombic structure. The porosity (P%) calculations revealed a decrease until x = 0.04 wt%, which suggests that the addition of nano-(Ba0.5Sr0.5Fe12O19)x reduces the number of voids and improves inter-grain connections, as confirmed by SEM micrographs. The superconducting transition temperature (Tc) increased to 112 K with the inclusion of nano-(Ba0.5Sr0.5Fe12O19) up to x = 0.04 wt%. Vickers microhardness (HV) measurements were conducted at various applied loads (0.245–9.800 N) and a duration time of 45 s. The HV number increased with the addition of x up to x = 0.04 wt% but then decreased with further addition. Various models were employed for analysis and modelling of Vickers hardness (HV) versus test load (F), including Meyer’s law, Hays–Kendall (H–K) model, the elastic/plastic deformation (EPD) model, the proportional sample resistance (PSR) model, the modified proportional sample resistance (MPSR), and indentation-induced cracking (IIC) model. It was found that the PSR model was the most appropriate theoretical model for describing the microhardness of nano-(Ba0.5Sr0.5Fe12O19)x/(Bi,Pb)-2223 composites. Moreover, the elastic modulus (E), yield strength (Y), fracture toughness (K), brittleness index (B), and elastic stiffness coefficient (C11) were estimated as a function of the inclusion of nano-(Ba0.5Sr0.5Fe12O19)x. Furthermore, the indentation creep test (time-dependent Vickers microhardness) revealed that the dislocation creep mechanism exists in composite samples with low concentrations (x < 0.05 wt%), whereas the dislocation climbs creep mechanism was observed for x ≥ 0.05 wt%.

Experimental characterization of flat indentation creep behaviour of Inconel 617

To analyse the creep deformation characteristics of Inconel 617, vacuum flat indentation creep test and microstructure analysis were completed in this study. With an increase in the creep load and temperature, limiting primary creep deflection and power law coefficient increased. An apparent indentation primary creep stage was observed and proven to be about 15–25 h in flat indentation creep test here. To describe the macro–deformation of Inconel 617, a modified theta–projection model was proposed and proven to be a feasible method to predict flat indentation creep curve accurately. Subsequently, a creep life prediction method based on deformation control was proposed. Based on the Larson – Miller parameter, reasonable conversion relationship between indentation creep stress and uniaxial creep equivalent stress was established. Finally, the microstructure analysis was completed to investigate the creep deformation characteristic. A hemispherical region along with significant extended grains and increase in hardness were observed. Considerable amount of Cr-rich precipitates and a small amount of Mo-rich precipitates were observed. Meanwhile, the precipitates were found to be discontinuous and granular for short indentation creep times.

Multi-scale creep analysis of cement paste – Indentation prediction and time correspondence of mechanisms

In this study, long-term uniaxial compression creep experiments and indentation tests were performed on cement paste (CP) under four relative humidity (RH) to investigate creep's multi-scale prediction and mechanism. The long-term creep kinetics of CP were predicted by homogenization calculations at two scales. The time correspondence between uniaxial compression creep and indentation creep of CP was studied by the discrete retardation spectrum (DRS). The results show that the multi-scale prediction of creep was more accurate than those measured by microindentation. DRS of macroscopic creep experiment and indentation test show there are two short-term and one long-term creep mechanisms. The peak time of the first mechanism occurs at 1.3 d of macroscopic uniaxial experiment and 1 s of indentation test, while the second mechanism occurs at 14 d and 10 s. The third mechanism is entered after 62 d in the uniaxial compression experiment and 22 s in the indentation test.

Meso-scale modelling of the effect of coarse aggregate properties on the creep of concrete

The effects of coarse aggregate properties on concrete creep were explored in the paper by meso-scale modelling approach, in which concrete was divided into coarse aggregate and matrix. Firstly, the creep constitutive model parameters of matrix were determined based on macroscopic and indentation creep tests, and then the evolution of creep and the influences of aggregate properties were discussed based on 2D meso-scale simulation. The simulation results of macroscopic creep test showed that the creep was unevenly distributed in the concrete, and the creep of matrix in vertical direction (y direction) of aggregate was greater than that in horizontal direction (x direction). Through parametric studies, it was found that: (1) the shape and gradation of course aggregate had little effect on the macroscopic creep, but the creep distribution of the matrix was changed by the aggregate shape; (2) the higher the aggregate content, the smaller the creep of concrete; (3) the larger the elastic modulus, the smaller the creep, and the matrix creep in x direction of aggregate of decreased with the increase of elastic modulus ratio of aggregate to matrix. Based on the simulation results, stress redistribution and continuous retardation time spectrum were discussed. Stress redistribution at meso-scale mainly occurred in the matrix in the x direction of aggregate. Through the retardation time spectrum, the creep process including two short-term creeps and a long-term creep and their transition retardation time were determined. The aggregate content had no effect on the transition retardation time, while transition retardation time between two short-term creep was increased by the increase of elastic modulus of aggregate.

Quantitative Study of Articular Cartilage Based on Greyscale Assessment Using Low-Field MRI

Osteoarthritis is a degenerative disorder that changes the biomechanical properties of articular cartilage in its development phase. MRI has become the diagnosing tool used widely to examine the articular cartilage in the synovial joint since it can provide excellent soft-tissue contrast. However, most diagnoses were conducted using clinical high-field MRI, while the low-field MRI was only used to obtain the geometrical data. This study aims to quantitatively assess the biomechanical properties of cartilage tissue using a low-field MRI system based on the image greyscale assessment. The articular cartilage image of intact bovine hip joints was obtained using 0.18 T MRI. The MRI images were characterized based on the intensity of the greyscale. The biomechanical properties of elastic modulus and permeability of cartilage were subsequently characterized by incorporating the creep indentation test data with the computational finite element model. Further correlation analyses were performed to examine the relationship between the greyscale of MRI images and biomechanical properties of elastic modulus and permeability of the cartilage. The cartilage greyscale was found to be strongly associated with the cartilage biphasic elastic modulus (r = 0.85), while the permeability (r = -0.51) was observed to have a moderate correlation with the greyscale. These findings show the capability of low-field MRI to produce an image that correlates with the articular cartilage's biomechanical properties, which could be adapted as a biomarker to detect osteoarthritis earlier than usual.

Effect of Spray Method and Sprayed Material on Creep Behavior of MCrAlY Coating

The blade of gas turbines used in combined cycle power plant are exposed to a high-temperature combustion gas. In order to protect it from such a harsh environment, a thermal barrier coating (TBC) is applied to the surface of the blade. TBC consists of a top coat (TC), which is a thermal barrier layer, and a bond coat (BC), which increases the adhesion strength between the TC and the substrate. Thermal stress is caused by the thermal expansion mismatch between the TC and substrate, which leads to delaminating in TC. In order to reduce such thermal stress in TC, creep deformability of BC is essential. In this study, we attempted to evaluate the creep deformation property of BC by creep indentation test method developed by our group. CoNiCrAlY and CoCrAlY are selected as BC materials and APS and HVOF are also as thermal spraying methods, and the creep indentation tests were conducted at various temperatures. It was found that material differences had little effect on creep deformation properties, and that creep properties were strongly affected by the thermal spraying method even if the material systems had the same chemical composition.

Application of PSO Method to Identification of Creep Material Constants Using Indentation Load Relaxation Curves

In the design of high-density mounting boards, the strength reliability of solder joints should be evaluated by conducting finite element analyzes (FEAs) considering the distribution of creep properties in the joints. To conduct such FEAs, a testing method that can evaluate the creep properties of the solder joints in-situ must be developed. The indentation creep test is an effective method for the evaluation, however, there are some problems with the method such as the creep law obtained by this method does not coincide with that obtained by tensile creep tests. Therefore, we previously proposed a new indentation test including the indenter depth maintenance process in which creep properties are evaluated by using the load relaxation curve given by the maintenance process. However, we have not developed a systematic method to estimate material constants in creep law based on the test results yet. Then, in this study, we proposed a method using the particle swarm optimization (PSO) method to estimate creep material constants from the load relaxation curve due to maintaining indenter depth. To discuss the availability of the proposed method, elastic-plastic-creep FEAs were conducted as the numerical experiments of the indentation test including the indenter depth maintenance process for an Sn-Ag-Cu (SAC) solder.

Comparison of biphasic material properties of equine articular cartilage estimated from stress relaxation and creep indentation tests

Abstract Mechanical parameters of hard and soft tissues are explicit markers for quantitative tissue characterization. In this study, we present a comparison of biphasic material properties of equine articular cartilage estimated from stress relaxation (ε = 6 %, t = 1000 s) and creep indentation tests (F = 0.1 N, t = 1000 s). A biphasic 3D-FE-based method is used to determine the biomechanical properties of equine articular cartilage. The FE-model computation was optimized by exploiting the axial symmetry and mesh resolution. Parameter identification was executed with the Levenberg- Marquardt-algorithm. Additionally, sensitivity analyses of the calculated biomechanical parameters were performed. Results show that the Young’s modulus E has the largest influence and the Poisson’s ratio of ν ≤ 0.1 is rather insensitive. The R² of the fit results varies between 0.882 and 0.974 (creep model) and between 0.695 and 0.930 (relaxation model). The averaged parameters E and k determined from the creep model yield higher values in comparison to the relaxation model. The differences can be traced back to the experimental settings and to the biphasic material model.

The Effect of Ultragrain Refinement on the Strength and Strain Rate Sensitivity of a ZK60 Magnesium Alloy

Herein, the relationship between the grain size and the strength in the magnesium alloy ZK60 in the ultrafine‐grained region is clarified. Discs are processed by high‐pressure torsion to refine the grain size and some samples are subjected to annealing to increase the grain size. Microhardness and indentation creep experiments are used to determine the strength and strain rate sensitivity. Also, the grain structure below the tip of an indentation creep test is compared with its counterpart before testing to show that deformation‐induced grain growth is very limited in this alloy. The results show different trends in grain refinement hardening at different grain size ranges. Grain refinement hardening is observed in the coarse‐grain (&gt;1 μm) region, there is a change in slope in the ultrafine grain range and negligible hardening and possible grain refinement softening in the range of ≈100 nm. It is also shown that samples with finer grain sizes display higher strain rate sensitivities, suggesting a change in the deformation mechanism. The current results agree with a model of grain boundary sliding deformation.

Indentation-derived creep response of cast and laser powder bed fused AlSi10Mg alloy: Air temperature

Compared with time-consuming conventional uniaxial tensile/compressive creep experiments, depth-sensing indentation testing is considered a reliable, and convenient testing technique to assess the time-dependent plastic deformation of materials in a reasonable time scale. In the present study, we report the ambient (room) temperature indentation creep properties of additively manufactured (i.e., laser-powder bed fused) and cast AlSi10Mg alloy at as-fabricated and different post-fabrication heat treatment states. The indentation creep testing parameters (i.e., dwell time, peak indentation load, and indenter shape) were optimized to adequately represent the creep response (time-displacement) variations for different material conditions. To this end, dual-stage, constant loading rate followed by constant-load holding, pyramidal indentation experiments were performed at a loading rate of 10 mN/s, a peak load of 200 mN, and a dwell time of 400 s. Besides, electron backscattered diffraction was performed to evaluate the manufacturing process (selective laser melted versus cast)/ post-fabrication heat treatment/ texture/ creep properties relationships for the studied AlSi10Mg alloy. Also, the indentation hardness, indentation strain rate sensitivity, indentation creep exponent, and activation volume were analyzed to study and confirm the mechanism of indentation creep. The calculated high values of creep stress exponents (i.e., >10) are attributed to dislocation-reinforcing particle interaction as the controlling mechanism of the creep which agrees with this assumption that AlSi10Mg is indeed an in-situ metal matrix composite with eutectic silicon as the reinforcing particles.

Ambient-Temperature Indentation Creep of Shape Memory NiTi Alloys: Additively Manufactured versus Cast

In this study, depth-sensing indentation creep response of cast and additively manufactured (laser powder bed fusion) NiTi alloys in heat-treated conditions have been investigated at ambient temperature. Indentation creep tests were evaluated with the help of a dual-stage approach comprising a loading segment with a subsequent constant load-holding stage and an unloading phase afterward. The investigation was carried out at a maximum load of 50 mN along with a holding time of 600 s. Different creep parameters comprising indentation creep displacement, creep strain rate, creep stress exponent as well as the indentation size effect have been analyzed quantitatively for the employed materials. In addition, microstructural analysis has been performed to ascertain the processing–microstructure–creep property correlations. A substantial indentation size effect was seen for both cast and printed NiTi samples in heat-treated conditions. According to the creep stress exponent measurements, the dominant mechanism of rate-dependent plastic deformation for all NiTi samples at ambient temperature is attributed to the dislocation movement (i.e., glide/climb). The outcome of this investigation will act as a framework to understand the underlying mechanisms of ambient-temperature indentation creep of the cast and printed NiTi alloy in conjunction with heat-treated conditions.

Redox reaction in a Mg/Nb2O5 nanocomposite processed by high-pressure torsion

Severe plastic deformation by high-pressure torsion can induce solid-state reactions, including redox reactions, which can affect the mechanical properties of metal matrix composites and hybrids. The present paper shows evidence of a redox reaction between magnesium and nanostructured niobium pentoxide. These chemical reactions release metallic niobium and magnesium oxide. Accordingly, aglomerations of nanocrystalline magnesium oxide is found in the magnesium matrix and a supersaturated solid solution is formed with niobium. Hardness and indentation creep tests show a minor increase in strength and decrease in strain rate sensitivity in the processed material, compared to data for pure magnesium in the literature.