Elastic and viscoelastic characterization of thermally-oxidized polymer resin using nanoindentation

Polymer matrix composites used in high temperature applications experience material property degradation due to thermal oxidation. In the present paper, the oxidation profiles of the isothermally aged PMR-15 specimens are studied using an optical microscope. Results show that the oxidized specimen exhibits distinguished heterogeneous structure. In addition, the spatial dependent elastic and viscoelastic properties of PMR-15 specimens isothermally aged at various environmental conditions are probed through nanoindentation tests. Results indicate that both elastic modulus and creep strain rate sensitivity of the oxidized surfaces are higher than those of the unoxidized interiors. These results are the consequences of chemical and physical changes that occur during oxidation including chemical bond breakage and outgassing of low-molecular weight species.     

High strain rate nanoindentation testing: Recent advancements,challenges and opportunities

Recent advancements in electronics have renewed the interest in high strain rate nanoindentation testing, resulting in the development of new high strain rate nanoindentation test equipment and test methodologies. In this work, the current state-of-the-art in high strain rate nanoindentation testing is critically reviewed, with focus on three key aspects - the testing equipment's dynamic mechanical and electronic response, test methodology, and post-processing of raw data to obtain hardness and strain rate. The challenges in instrument hardware design and post-test data analysis are discussed, along with optimal strain rate window for accurate high strain rate measurements. Specific focus will be on instrumented high strain rate testing using self-similar indenters at strain rates in excess of 100 s^?1, wherein load and depth of penetration into the sample are both measured or applied.     

Alternative methods to determine the elastoplastic properties of sintered hydroxyapatite from nanoindentation testing

This study introduces alternative methods to determine the elastoplastic properties of bovine-derived Hydroxyapatite (HA) porous bone graft through a set of nanoindentation tests with a Berkovich indenter. Generally, experimental data obtained from nanoindentation tests are force displacement, hardness and elastic modulus. However, to determine plastic properties such as strength coefficient and work hardening exponent of bovine HA, analytical or inverse finite element models are required. In this paper, the effect of sintering temperature on these properties of HA is studied for the range of 1000–1400 °C. The direct and inverse Finite Element (FE) simulation models for nanoindentation tests were written in MSC, MARC^® software. A special algorithm for the inverse technique was developed to infer the most suitable elastoplastic material model for HA. A semi-empirical method was adapted to calculate the elastoplastic material properties of HA. The numerical results of harder hydroxyapatite showed better agreement with the experiments while the work hardening exponent, or n-value, and strength coefficient k of hard HA were found to be 0.23 and 8.05 GPa respectively. A comparison between the experimental and predicted load–displacement curves showed that the proposed inverse technique is effective in predicting the elastoplastic material properties from the nanoindentation test with error below 4% at maximum load.     

Development of semi-empirical formulation for extracting materials properties from nanoindentation measurements: Residual stresses, substrate effect, and creep

Nanoindentation is one of the most popular techniques for characterizing the mechanical properties of micro- or nano-structured metals or dielectric thin films. However, the obtained experimental data can only provide the relationship between the applied load and the penetration depth. Mechanics models are therefore required to convert the test data into the corresponding material properties. In this work, the effect of residual stress, the substrate effect, and the creep of materials subjected to the indentation test are discussed in order to establish appropriate conversion formulas or criteria for extracting the interested material properties. Dimensional analyses are firstly performed to find the governing parameters and to obtain scaling relationships for subsequent finite element analysis. With the described procedure, models have been developed to convert nanoindentation test data into the desired material properties. Those models provide useful tools for extracting specific material properties, such as residual stress, creep exponent, and stress relaxation time constant. Specifically, this investigation also shows that for the situation of soft film/hard substrate combination, the indentation behavior is essentially identical if the modulus of the substrate is 10 times higher than that of the corresponding film and the response deviates consistently from that of bulk material with increasing of indentation depth. For penetration depth less than 10% of the film thickness, the deviation could be acceptable. On the other hand, significant deviation is observed for hard film/soft substrate systems. In summary, by integrating the models proposed by this work and data from standard tests, it is possible to obtain the Young's modulus, hardness, and the viscoelastic properties as well as the residual stress for a specific material through indentation characterization.     

Strain rate sensitivity of unequal grained nano-multilayers

Nanoscaled bimetallic Cu/Ta multilayers were fabricated and their deformation behaviors characterized under nanoindentation. The individual Cu and Ta layers had equal thickness (∼30 nm) but quite different grain sizes. By evaluating the hardness of the bi-metal system at various strain rates, a transitional point of its strain rate sensitivity at the strain rate of 10⁻³ s⁻¹ was observed. Contributions from dislocation and grain boundary (GB) motions to plastic deformation are found to be strongly dependent upon strain rate as well as grain size in alternative constituent layers. Whilst dislocation-mediated motions take up the majority of deformation in a Cu/Ta multilayer at high strain rates, GB motions occurring mainly in the Ta layers take over at low strain rates.     

A constitutive model for metals over a large range of strain rates Identification for mild-steel and aluminium sheets

A phenomenological model is presented to describe the mechanical behaviour for metals and alloys over a large range of strain rates. It is an elasto-plastic model with a strain rate and temperature-sensitive yield stress. This model partially relies on physical considerations and is specially developed for an easy application in explicit finite element method (FEM) codes. The process for identifying the constants from experimental data is presented, taking into account the exact testing conditions such as temperature increase and strain rate variations during the loading. Applying the model to data obtained for mild steel and commercial aluminium sheets yields satisfactory results.     

材料动态力学性能实验研究技术进展

材料高应变率实验技术是应用于研究材料加载应变率在10^2-10^8S^-1。范围内力学响应的实验基础,在对材料高应变率实验技术综述的基础上,从基本理论、加载方法和测量技术等方面详细论述了SHPB实验技术的进展,分析了SHPB实验技术在实际应用中存在的问题,并提出了相变材料、贵金属及特殊材料等SHPB实验研究值得深入探索的方向。     

Comparison between the Methods of Determining the Critical Stress for Initiation of Dynamic Recrystallization in 316 Stainless Steel

Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization （σc） on the basis of mathematical methods. One＇ of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the （-dθ/dσ vs σ） through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01^-1 s^-1 and strain of I on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained uc=σc/σp=0.92.     

Progressive Failure Modeling for Dynamic Loading of Woven Composites

A three parameter constitutive model was developed for representing tensile progressive damage of the nonlinear large-deformation rate-dependent behavior of polymer-based composite materials, which was characterized using off-axis composite specimens. A strain based failure criterion was proposed that reduces data for different loading directions and strain rates to a single representation. A method of combining the nonlinear constitutive theory and the failure strain methodology for different strain rates is suggested. The strength of the material was successfully represented with a single material constant, for all strain rates and tensile loading directions.     

Compression testing of a sintered Ti6Al4V powder compact for biomedical applications

In this study, the compression deformation behavior of a Ti6Al4V powder compact, prepared by the sintering of cold compacted atomized spherical particles (100–200 μm) and containing 36–38% porosity, was investigated at quasi-static (1.6×10⁻³–1.6×10⁻¹ s⁻¹) and high strain rates (300 and 900 s⁻¹) using, respectively, conventional mechanical testing and Split Hopkinson Pressure Bar techniques. Microscopic studies of as-received powder and sintered powder compact showed that sintering at high temperature (1200 °C) and subsequent slow rate of cooling in the furnace changed the microstructure of powder from the acicular alpha () to the Widmanstätten (+β) microstructure. In compression testing, at both quasi-static and high strain rates, the compact failed via shear bands formed along the diagonal axis, 45° to the loading direction. Increasing the strain rate was found to increase both the flow stress and compressive strength of the compact but it did not affect the critical strain for shear localization. Microscopic analyses of failed samples and deformed but not failed samples of the compact further showed that fracture occurred in a ductile (dimpled) mode consisting of void initiation and growth in phase and/or at the /β interface and macrocracking by void coalescence in the interparticle bond region.     

Identification of optimal deforming parameters from a large range of strain,strain rate and temperature for 3Cr20Ni10W2 heat-resistant alloy

The intrinsic workability of 3Cr20Ni10W2 was investigated using processing maps constructed from the stress–strain data generated by isothermal compression tests with a height reduction of 60% over a temperature range of 1203–1403 K and a strain rate range of 0.01–10 s⁻¹. As the true strain was −0.3, −0.5, −0.7 and −0.9, the responses of strain rate sensitivity (m-value), power dissipation efficiency (η-value) and instability parameter (ξ-value) to temperatures and strain rates were evaluated respectively. By the superimposition of power dissipation and instability maps, the stable regions with higher power dissipation efficiency (⩾0.3) and unstable regions were clarified clearly. As the true strain was −0.3, −0.5, −0.7 and −0.9, respectively, the area of instability regions decreased with increasing true strain from −0.3 to −0.7, while it increased with increasing true strain from −0.7 to −0.9. In further, in the stable area, on the basis of determination for domains with dynamic recrystallization (DRX) microstructural evolution, the DRX-predominant regions with higher power dissipation efficiency were identified and recommended. Then not only DRX-predominant domains were validated by the stable microstructures refined by DRX, but also the regimes of flow instabilities were validated by the microstructures involving cracks. The identification of optimal deforming parameters from a large range of strain, strain rate and temperature for 3Cr20Ni10W2 heat-resistant alloy contributes to designing reasonable hot deforming processes without resorting to expensive and time-consuming trial-and-error methods.     

Analysis of strain rate dependence of tensile elongation for a mechanical milling Al–1.1Mg–1.2Cu alloy tested at 748 K from a dislocation dynamics viewpoint

Tensile deformation was carried out for a mechanically milled and thermo-mechanically treated Al–1.1Mg–1.2Cu (at.%) alloy at 748 K and three nominal strain rates of 10⁻³, 10⁰, and 10² s⁻¹. Despite the prevailing belief that superplasticity occurs by grain boundary sliding which requires slow strain rates at high temperatures, the maximum elongation was observed at the intermediate strain rate of 10⁰ s⁻¹, neither at the lowest nor the highest strain rates. In order to explain this phenomenon, the true stress–true strain behaviors at these three nominal strain rates were analyzed from a viewpoint of dislocation dynamics by computer-simulation with four variables of the thermal stress component σ*, dislocation immobilization rate U, re-mobilization probability of unlocked, immobile dislocations Ω and dislocation density at yielding ρ₀. It can then be concluded that the large elongation (>400% in nominal strain) at the intermediate strain rate is produced by a combination of a very large Ω and a moderate U, resulting in a large strain rate sensitivity m value.     

基于测量噪声确定传感器动态补偿的理想指标

测量噪声是影响传感器动态补偿效果的主要因素,对在噪声环境下传感器动态补偿所能达到的理想效果进行了研究。提出根据测量噪声确定传感器动态补偿后理想工作带宽的方法,使用该理想带宽可进一步确定补偿后时域性能指标中调节时间与超调量的理想值。将该方法应用于应变天平的动态校准实验,验证了其有效性。进而采用该方法评定应变天平经动态补偿后能够满足飞行器舱门风洞动态试验的要求,表明该方法可用于确定传感器动态补偿后是否适用于动态试验。     

The SEĖ method for determination of behaviour laws for strain rate dependent material: Application to polymer material

The need to model fracture in crashworthiness by means of finite element codes is a real challenge for research. Before implementing fracture criteria, an excellent knowledge of the stress and strain states in the material just before the crack appearance is the first condition necessary to ensure the model development. At present, most of the material behaviour laws, for example for steel, are only defined until the maximum force when necking occurs. For polymers, the early occurrence of the diffuse necking leads to an experimental technique in which the speed loading is controlled in real time to maintain a constant strain rate during the test. This technique is not however used, due to technical limitations, for high strain rate behaviour laws. In this paper, the authors propose to use the heterogeneity of the displacement field on the surface of the tensile specimen as an initial condition to identify behaviour laws. The method developed uses the information in all the surface zone of the specimen by using digital image correlation. Stresses, strains and strain rates are then obtained to build a surface behaviour called the SE? surface. By cutting it, the experimental behaviour laws for a range of large strains and strain rates are then defined for model identification.     

Comparative study on mechanical behavior of low temperature application materials for ships and offshore structures: Part I—Experimental investigations

Austenitic stainless steels (ASSs), aluminum alloys, and nickel alloys are potential candidate materials for cryogenic applications owing to their superior mechanical properties under low temperatures. In the liquefied natural gas (LNG) industry, these materials are widely used in the construction of thermal barriers for the insulation systems of storage tanks and LNG equipment. Among the typical material nonlinearities of ASSs, phase transformation induced plasticity (TRIP)-related nonlinear hardening characteristics have been experimentally and numerically reported in a number of detailed studies [1] and [2]; however, to the best of our knowledge, quantitative studies on aluminum and nickel alloys are not available for reference. Moreover, although these materials are used under various temperatures and strain rates, their temperature- and strain rate-dependent properties have not been determined thus far.In this study, a series of tensile tests is carried out under various temperatures (110-293 K) and strain rate (0.00016-0.01 s⁻¹) ranges as a preliminary step in the overall process for understanding the material characteristics of ASSs, aluminum alloys, and nickel alloys. On the basis of the experimental results, the essential mechanical properties are summarized in a quantitative manner in terms of the temperature and strain rate. The strain-hardening rate and strain sensitivity, which can be used to describe cryogenic temperature dependent material nonlinearities, are also proposed for the selected materials.     

Superplasticity of fine-grained magnesium alloys for biomedical applications: A comprehensive review

The superplastic behavior of medical magnesium alloys is reviewed in this overview article. Firstly, the basics of superplasticity and superplastic forming via grain boundary sliding (GBS) as the main deformation mechanism are discussed. Subsequently, the biomedical Mg alloys and their properties are tabulated. Afterwards, the superplasticity of biocompatible Mg-Al, Mg-Zn, Mg-Li, and Mg-RE (rare earth) alloys is critically discussed, where the influence of grain size, hot deformation temperature, and strain rate on the tensile ductility (elongation to failure) is assessed. Moreover, the thermomechanical processing routes (e.g. by dynamic recrystallization (DRX)) and severe plastic deformation (SPD) methods for grain refinement and superplasticity in each alloying system are introduced. The importance of thermal stability (thermostability) of the microstructure against the grain coarsening (grain growth) is emphasized, where the addition of alloying elements for the formation of thermally stable pinning particles and segregation of solutes at grain boundaries are found to be major controlling factors. It is revealed that superplasticity at very high temperatures can be achieved in the presence of stable rare-earth intermetallics. On the other hand, the high-strain-rate superplasticity and low-temperature superplasticity in Mg alloys with great potential for industrial applications are summarized. In this regard, it is shown that the ultrafine-grained (UFG) duplex Mg-Li alloys might show remarkable superplasticity at low temperatures. Finally, the future prospects and distinct research suggestions are summarized. Accordingly, this paper presents the opportunities that superplastic Mg alloys can offer for the biomedical industries.     

Cryogenic mechanical behavior of 5000- and 6000-series aluminum alloys: Issues on application to offshore plants

The mechanical behavior of aluminum alloys was investigated in terms of four aspects: temperature, strain rate, material type, and fracture shape. The candidate materials were 5000- and 6000-series alloys. The material characteristics were investigated and summarized as a function of low temperature (110–293 K) and quasi-static strain rate (10⁻⁴ and 10⁻² s⁻¹). The results confirmed that the strength and ductility of aluminum alloys improved with a decrease in the temperature. The aluminum alloys showed a strain rate effect only in terms of the ductility of the 5000-series alloys. In addition, fractography analyses were performed on the fracture specimens to explain the material behavior at cryogenic temperatures.     

The influence of a threshold stress for grain boundary sliding on constitutive response of polycrystalline Al during high temperature deformation

A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminum alloy AA5083 when subjected to plane strain uniaxial tension at 450 °C. Under these conditions, AA5083 deforms by dislocation glide at strain rates exceeding 0.001 s⁻¹, and by grain boundary sliding at lower strain rates. The stress–strain rate response can be approximated by , where A and n depend on grain size and strain rate. We find that a threshold stress less or equal to 4 MPa has only a small influence on flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2 MPa increases n from 4.2 to 4.5), but substantially increases both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2 MPa increases n from 1.5 to 2.7). In addition, when the threshold stress is included, our model predicts stress versus strain rate behavior that is in good agreement with experimental measurements reported by Kulas et al. [M.A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, T.R. McNelley, Metall. Mater. Trans. A 36 (2005) 1249].     

Removal of metals from landfill leachate by sorption to activated carbon, bone meal and iron fines

Sorption filters based on granular activated carbon, bone meal and iron fines were tested for their efficiency of removing metals from landfill leachate. Removal of Al, As, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Mo, Ni, Pb, Sr and Zn were studied in a laboratory scale setup. Activated carbon removed more than 90% of Co, Cr, Cu, Fe, Mn and Ni. Ca, Pb, Sr and Zn were removed but less efficiently. Bone meal removed over 80% of Cr, Fe, Hg, Mn and Sr and 20-80% of Al, Ca, Cu, Mo, Ni, Pb and Zn. Iron fines removed most metals (As, Ca, Co, Cr, Cu, Fe, Mg, Mn, Pb, Sr and Zn) to some extent but less efficiently. All materials released unwanted substances (metals, TOC or nutrients), highlighting the need to study the uptake and release of a large number of compounds, not only the target metals. To remove a wide range of metals using these materials two or more filter materials may need to be combined. Sorption mechanisms for all materials include ion exchange, sorption and precipitation. For iron fines oxidation of Fe(0) seems to be important for metal immobilisation.