Measuring elasto-plastic properties of solid wall materials of metallic foams at elevated temperatures by using indentation hardness technique

To evaluate or design metallic foams at the meso-level for applications at high temperatures, an indentation method is extended to measure the Young’s moduli, the yield strengths and the strain hardening exponents of the cell wall materials. The method was verified against a type of aluminum foam and the elasto-plastic properties of the aluminum cell walls were measured at various temperatures up to 400 °C. An approximate linear temperature dependence is observed for the hardness, the Young’s modulus and the yield strength. The present study provides a feasible way to investigate the meso-mechanical behavior of foam materials at high temperatures.     

Analysis of interactions between damage and basic creep of HPC and HPFRC heated between 20 and 80 °C

The present study concerns the uniaxial compressive creep of High Performance Concrete (HPC) at moderate temperatures, 20–80 °C. The study was conducted on four formulations of HPC including two fibrous concretes envisioned for future storage structures of Intermediate Level Long-Life Nuclear Wastes. These wastes are exothermic and lead to maximal temperatures in the field ranging from 50 to 70 °C (Andra, Référentiel des matériaux de stockage de déchets à haute activité et à vie longue, 2005). Here, we investigate the basic creep under uniaxial compression at 50 and 80 °C and compare it to that obtained on the same HPC at 20 °C. The objective of this research is to contribute to a better understanding of the phenomenon of interaction between damage and basic creep of HPC at moderate temperature, especially with a view to its integration in Thermo-Hydro-Mechanical models dealing with the design of special structures (massive structures, specific serviceability conditions in nuclear or hydroelectric power plants, etc.). This test campaign allowed us to assess the effect of temperature on the magnitude of basic creep of HPC, and also the impact of various temperature and mechanical loading conditions on the Young’s modulus of HPC. Heating to 80 °C damages HPC (instantaneous Young’s modulus decrease) and thereby increases the creep capacity, showing a relation between damage and creep amplitude. Moreover, this study gives global activation energy of basic creep of HPC that should be useful for practitioners dealing with concrete structures sensitive to delayed strains and subjected to moderate temperature.     

The effect of heat treatment temperature and time on the microstructure and mechanical properties of PAN-based carbon fibers

The influence of heat treatment temperature from 1400 to 2840 °C and time from 1.2 to 12.0 min on the structure and mechanical properties of polyacrylonitrile carbon fibers was studied. It was observed that the Young’s modulus increased with increasing temperature and time, but the tensile strength exhibited different variation trends with the different processing methods. For a fixed time of 1.2 min, the strength dropped from 4.6 GPa at 1400 °C to 2.6 GPa at 2840 °C, (~43.5 %) as opposed to a 63.9 % increase in Young’s modulus. However, when the treatment time was increased to 6.0 min at 2500 °C, the tensile strength decreased only by 1.9 %, from 3.71 to 3.64 GPa, versus a nearly 20.0 % increase in Young’s modulus. The same situation was found for treatment at 2000 and 2700 °C. Raman spectroscopy and uniform stress model analysis show that the degree of covalent cross-linking between the graphene planes decreased as temperature increased, while it remained almost constant as treatment time was increased. It is believed that during heat treatment of a carbon fiber, the cross-linking collapses at the beginning but the crystalline size keeps growing with prolonging time, so the tensile strength decreases little with further heat treatment while tensile modulus keeps increasing.     

Effect of loading rate on the creep behaviour of epoxy resin insulators by nanoindentation

In the present work, the effect of loading rate on indentation creep was studied. Indentation creep tests were conducted on epoxy resin to provide creep deformation under constant load, contact creep compliance and cut-off time using a Berkovich indenter. Several loading rates, ranging from 0.25 to 6 mN/s, were used to perform the tests. The results showed that there is a strong loading rate dependence on creep response of the epoxy resin under indentation. Contact creep compliance and cut-off time decreased with increasing loading rate. In contrast, an increase in reduced modulus, hardness, displacement variation and contact creep compliance variation during the holding time was noticed. The loading rate sensitivity on creep response under indentation can be attributed to viscoelastic response prior to holding segment and strain rate effect on yield stress of the epoxy resin. This study provided an insight to understand the loading rate dependence on creep behaviour of epoxy resin under indentation.     

Effect of sintering temperature and cooling rate on the morphology, mechanical behavior and apatite-forming ability of a novel nanostructured magnesium calcium silicate scaffold prepared by a freeze casting method

In this research, sol–gel-derived nanostructured calcium magnesium silicate (merwinite)-based scaffolds were fabricated by water-based freeze casting method. The effect of cooling rate and sintering temperature on pore sizes and mechanical characteristics of the scaffolds was studied. Microstructure and surface morphology of scaffolds were also observed by scanning electron microscopy before and after various time intervals of soaking in simulated body fluid. The results showed that increasing temperature at the constant rate led to increasing the parameters of volume and linear shrinkage, strength (σ), and Young’s modulus (E) but decreasing porosity. This increase was significant for strength and Young’s modulus. In addition, with the increase of rate at the constant temperature, the parameters of volume and linear shrinkage and also porosity decreased whereas strength and Young’s modulus increased significantly. According to the obtained mechanical results, the best mechanical properties were achieved when the scaffold was prepared at cooling rate and sintering temperature of 277.15°K/min and 1623.15°K, respectively (E = 0.048 GPa and σ = 2 MPa). These values were closer to the lower limit of the values for cancellous bone. The acellular in vitro bioactivity revealed that different apatite morphologies were formed on the surfaces for various periods of soaking time when the scaffolds prepared at the freezing temperature of 277.15°K/min and at the two different sintering temperatures. The favorable mechanical behavior of the porous constructs, coupled with the ability of forming apatite particles on the surface of scaffold, indicates the potential of the present freeze casting route for the production of porous scaffolds for bone tissue engineering.     

Mechanical response of a unidirectional graphite fiber/polyimide composite as a function of temperature

In this work, the mechanical response of a unidirectional composite based on T650-35 graphite fibers embedded in a PMR-15 polyimide resin was analytically and numerically predicted as a function of temperature and subsequently compared with the available experimental data. The Eshelby/Mori–Tanaka (E/M–T) method was used to predict the elastic properties of the composite, whereas a finite element unit cell was employed to predict the stress vs. strain curves of the composite under elasto-plastic conditions. It was shown that for the temperature range from 25 to 315 °C the predicted elastic properties of the composite agreed closer with the experiment in the case of the longitudinal and transverse Young’s moduli than in the case of the longitudinal shear modulus. The comparisons for the transverse shear modulus and the longitudinal Poisson’s ratio were uncertain. The agreements between the numerically predicted and experimentally determined stress–strain curves of the composite were found to be dependent on temperature and the type of loading. The experimental and numerical research data and the approaches presented in this work should significantly extend our knowledge of the effect of elevated temperatures on the mechanical behavior of unidirectional high temperature polymer matrix composites.     

The thermal behaviour of glass fibre investigated by thermomechanical analysis

A thermomechanical analysis (TMA) procedure has been developed with the capability of probing the thermal behaviour of glass fibre. A single glass fibre was successfully mounted into TMA fibre configuration and several thermomechanical programmes were carried out over a wide temperature range from 20 to 900 °C. It was found that measured coefficient of linear thermal expansion of boron-free E-glass fibre remained constant below 300 °C and the values had an excellent agreement with that found in the literature. At higher temperatures an abrupt length change in glass transition region allowed for the determination of glass transition temperature. The results from isothermal measurement showed significant fibre length shrinkage, which was a function of both temperature and time. It follows that there exist two mechanisms, thermal expansion and structural relaxation, which together account for overall thermomechanical responses of glass fibre. The former is related to the decrease of Young’s modulus at elevated temperatures and the latter is considered responsible for the observed increase of room-temperature Young’s modulus after thermally conditioning glass fibre at various temperatures.     

Tuning the mechanical properties of glass fiber-reinforced bismaleimide–triazine resin composites by constructing a flexible bridge at the interface

Abstract

We demonstrate a new method that can simultaneously improve the strength and toughness of the glass fiber-reinforced bismaleimide–triazine (BT) resin composites by using polyethylene glycol (PEG) to construct a flexible bridge at the interface. The mechanical properties, including the elongation, ultimate tensile stress, Young’s modulus, toughness and dynamical mechanical properties were studied as a function of the length of PEG molecular chain. It was found that the PEG molecule acts as a bridge to link BT resin and glass fiber through covalent and non-covalent bondings, respectively, resulting in improved interfacial bonding. The incorporation of PEG produces an increase in elongation, ultimate tensile stress and toughness. The Young’s modulus and T_g were slightly reduced when the length of the PEG molecular chain was high. The elongation of the PEG-modified glass fiber-reinforced composites containing 5 wt% PEG-8000 increased by 67.1%, the ultimate tensile stress by 17.9% and the toughness by 78.2% compared to the unmodified one. This approach provides an efficient way to develop substrate material with improved strength and toughness for integrated circuit packaging applications.     

Investigation on high temperature fracture properties of amorphous silicon dioxide by large-scale atomistic simulations

Amorphous silicon dioxide exhibits low temperature expansion coefficient and stability of dielectric properties over a wide range of frequencies and temperatures, and plays an important role in integrated circuits and microelectronics. Downscaling of dimensions in there devices means great challenges for thin film reliability and physical characterization. Mechanical failure caused by stresses in thermal conditions is the major reliability issues for electronic devices. As the experiments have limitations in micro/nano-scale characterization of fracture properties at high temperatures, atomistic simulation is a proper way to investigate this particular mechanism. In this paper, the structural and fracture properties of amorphous silicon dioxide (a-SiO₂) were studied at temperatures up to 1,500 K. The simulation results consist with the experiments on pair distribution function, structure factor, angular distributions and temperature-dependent Young’s moduli. The calculated Young’s modulus is close to the simulation and experimental results of 72.5–78.9 GPa for SiO₂, and begin to drop after 900 K With temperature increasing.     

Fabrication of advanced “green” composites using potassium hydroxide (KOH) treated liquid crystalline (LC) cellulose fibers

Advanced green composites having excellent strength and stiffness were fabricated using liquid crystalline (LC) cellulose fibers and soy protein isolate (SPI) resin. Further, LC cellulose fibers were treated with potassium hydroxide (KOH) to improve their tensile strength and Young’s modulus by increase the crystallinity of cellulose. The improvements were significant when the treatment was carried out while keeping the fibers under tension. The Young’s modulus (stiffness) of the LC cellulose fibers increased by about 33 % from 47.8 to 63.7 GPa and the strength increased by about 18 % from 1483 MPa to 1749 MPa. X-ray diffraction (XRD) study of the LC cellulose fibers showed over 50 % increase in crystallinity after the KOH treatment. The mechanical properties of the LC cellulose fiber-reinforced composites were also high and improved further when the KOH treated fibers were used. With 65 % fiber volume it should be possible to obtain composites with strength above 1020 MPa and modulus of over 37 GPa, making them truly advanced green composites that could be used for structural applications.     

The Effect of Shear Mixing Speed and Time on the Mechanical Properties of GNP/Epoxy Composites

The aim of this study was to examine the effect of shear mixing speed and time on the mechanical properties of graphene nanoplatelet (GNP) composites. Shear mixing is cited in the literature as one method of making a good dispersion of nanofillers in a polymer that breaks down agglomerates into smaller particles and in the case of GNP can exfoliate layers of graphene. In this paper 0.1 to 5 wt% GNP was mixed with epoxy at different speeds and for different lengths of time. The composites were then cured and the tensile strength and Young’s modulus was measured. Optical microscopy was performed to examine the dispersion of the GNP in the epoxy. The results show that the shear mixing speed and time affect the size of agglomerates, which has an impact on the mechanical properties of the composite. At 3000 rpm and 2 h of mixing the average size of agglomerate was 26.3 μm (30 % reduction compared to that of 1000 rpm and 1 h duration), the tensile strength of epoxy was not affected by the addition of GNP, while a 12 % increase was recorded for the Young’s modulus. It is also found that functionalisation of the surface of the GNP improves the bond formed between the GNP and the resin that enhances its mechanical properties with no effect on the size of the agglomerates. Acetone was used to improve the GNP dispersion and found that shear mixing 5 wt% of GNP with acetone increases the Young’s modulus up to 3.02 from 2.6 GPa for the neat epoxy, an almost 14 % rise.     

Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs

This paper evaluates the fatigue life properties of low carbon grey cast iron (EN-GJL-250), which is widely used for automotive brake discs. Although several authors have examined mechanical and fatigue properties at room temperatures, there has been a lack of such data regarding brake discs operating temperatures. The tension, compression and low cycle fatigue properties were examined at room temperature (RT) and at brake discs’ working temperatures: 500 °C, 600 °C and 700 °C. The microstructure of the material was documented and analysed. Tensile stress–strain curves, cyclic hardening/softening curves, stress–strain hysteresis loops, and fatigue life curves were obtained for all the above-mentioned temperatures. It was concluded, that Young’s modulus is comparable with both tension and compression, but yield its strength and ultimate strength are approximately twice as great in compression than in tension. All the mechanical properties remained quite stable until 500 °C, where at 700 °C all deteriorated drastically. During fatigue testing, the samples endured at 500 °C on average at around 50% of cycles at room temperature. Similar to other materials’ properties, the cycles to failure have dropped significantly at 700 °C.     

Mechanical properties of amorphous silicon carbonitride thin films at elevated temperatures

The mechanical properties of amorphous silicon carbonitride (a-SiC _x N _y ) films with various nitrogen content (y = 0–40 at.%) were investigated in situ at elevated temperatures up to 650 °C in inert atmosphere. A SiC film was measured also at 700 °C in air. The hardness and elastic modulus were evaluated using instrumented nanoindentation with thermally stable cubic boron nitride Berkovich indenter. Both the sample and the indenter were separately heated during the experiments to temperatures of 300, 500, and 650 °C. Short duration high temperature creep tests (1200 s) of the films were also carried out. The results revealed that the room temperature hardness and elastic modulus deteriorate with the increase of the nitrogen content. Furthermore, the hardness of both the a-SiC and the a-SiCN films with lower nitrogen content at 300 °C drops to approx. 77 % of the corresponding room temperature values, while it reduces to 69 % for the a-SiCN film with 40 at.% of nitrogen. Further increase of temperature is accompanied with minor reduction in hardness except for the a-SiCN film with highest nitrogen content, where the hardness decreases at a much faster rate. Upon heating up to 500 °C, the elastic modulus of the a-SiCN film decreases, while it increases at 650 °C due to the pronounced effect of short-range ordering. The steady-state creep rate increases at elevated temperatures and the a-SiC exhibits slower creep rates compared to the a-SiCN films. The value of the universal constant x = 7 relating the W _p/W _t and H/E ^* was established and its applicability was demonstrated. Analysis of the experimental indentation data suggests a theoretical limit of hardness to elastic modulus ratio of 0.143.     

Fabrication and mechanical characterization of continuous carbon fiber-reinforced thermoplastic using a preform by three-dimensional printing and via hot-press molding

A new 3D printer equipped novel nozzle structure for continuous carbon fiber-reinforced thermoplastics (C-CFRTP) was developed and the suitable printing conditions were studied. C-CFRTP filament and additional matrix resin were supplied independently using each extruder, which is useful for variety printing and precise form control in 3D printing. To measure the mechanical properties, specimens for tensile strength testing were fabricated using C-CFRTP filament (Vf:50%) without additional matrix resin. The experimental results indicate that the tensile strength and Young’s modulus were approximately 700 MPa and 53 GPa, respectively. The recrystallization effect through annealing after 3D printing yielded no drastic improvement. The mechanical properties were considerably improved by a hot-press treatment after 3D printing. The tensile strength and Young’s modulus increased to approximately 1400 MPa and approximately 90 GPa, respectively. These results suggest that one of the useful applications of C-CFRTP 3D printing technology is preforming of small parts in industrial products.     

Creep behavior of nanocrystalline Au films as a function of temperature

Freestanding nanocrystalline Au films, subjected to nominally elastic loads at 25–110 °C, demonstrated high primary (10^?7–10^?4 s^?1) and steady-state creep rates (10^?8–10^?5 s^?1). The deformation mechanisms for creep were strongly temperature dependent: grain boundary sliding-based creep dominated at room temperature and 50 °C, while the contribution of dislocation-mediated creep increased at 80 and 110 °C. The effect of applied stress on primary and steady-state creep strain at different temperatures was captured well by a non-linear model that was based on the kinetics of thermal activation. Multi-cycle creep experiments showed that at room temperature virtually all the primary strain accumulated during each forward creep cycle was recovered upon complete unloading. As the contribution of dislocation-mediated creep increased with temperature, the ratio of strain recovery to primary strain accumulated during each cycle was reduced due to the accumulation of plastic strain at higher temperatures. Notably, at all temperatures, the steady-state creep rate decreased after the first creep cycle. Moreover, the entire creep response remained virtually unchanged in all subsequent cycles, which implies that the first creep cycle resulted in mechanical annealing. This conclusion was further supported by calculations of the activation entropy: A reduction in its magnitude between the first and all subsequent creep cycles at all temperatures pointed out to mechanical annealing of initial material defects during the first loading cycle. The negative values of the calculated activation entropy indicated that entropy changes due to annihilation of defects-dominated entropy changes associated with the generation of new defects. Finally, the activation entropy for steady-state creep was temperature insensitive, but increased with stress, which is consistent with an increase in defect generation at higher stresses.     

Werkstoffverhalten unter zügiger elastischer Beanspruchung

Material properties by continuous elastic straining Within the scope of a common research project of the steel and automotive industry, 20 sheet steels have been investigated to obtain input data for FE‐analysis. In detail, characteristical elastic, plastic and fatique values were determined by several testing institutes for a period of 3 years. Knowledge of dependency of Young’s modulus from temperature and orientation is important for spring back at the press shop and stiffness of parts for automotive. Young’s modulus was determined by tensile tests in delivered state, after prestraining, heat treatment at room temperature and –40 °C and 100 °C. Young’s modulus is dependent from the orientation to rolling direction and can be classified in groups. Young’s modulus of ferritic steels is decreased about 10 % by prestraining of 2 % but recovered after annealing at 170 °C. Temperature dependency well known from non destructive tests are confirmed.     

Influence of thermal conditions on the tensile properties of basalt fiber reinforced polypropylene–clay nanocomposites

In this paper, a comparative study on the tensile properties of clay reinforced polypropylene (PP) nanocomposites (PPCN) and chopped basalt fiber reinforced PP–clay nanocomposites (PPCN-B) is presented. PP matrix are filled with 1, 3 and 5 wt.% of nanoclays. The ultimate tensile strength, yield strength, Young’s modulus and toughness are measured at various temperature conditions. The thermal conditions are included the room temperature (RT), low temperature (LT) and high temperature (HT). The basal spacing of clay in the composites is measured by X-ray diffraction (XRD). Nanoscale morphology of the samples is observed by transmission electron microscopy (TEM). Addition of nanoclay improves the yield strength and Young’s modulus of PPCN and PPCN-B; however, it reduces the ultimate tensile strength. Furthermore, the addition of chopped basalt fibers to PPCN improves the Young’s modulus of the composites. The Young’s modulus and the yield strength of both PPCN and PPCN-B are significantly high at LT (−196 °C), descend at RT (25 °C) and then low at HT (120 °C).     

Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers

The influence of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers was studied. It was observed that the Young’s modulus of the treated fibers increased with heat treatment temperature (HTT) and hot stretching stress, to 173 GPa by 158.2 % through hot stretching at 2700 °C under stress of 270 MPa compared to that of the as-received carbon fiber. Meanwhile the tensile strength increased to 1.75 GPa by 73.3 % through hot stretching at 2700 °C under 252 MPa. The field emission scanning electron images showed markedly increased roughness on the external surface and bigger and more compacted granular morphologies on the cross section of the treated fibers with increasing HTT. The preferred orientation of graphitic layers was improved by hot stretching, and the higher the HTT, the stronger the effectiveness of the hot stretching. The crystallite sizes grew and the crystallite interlayer spacing decreased obviously with increasing HTT but changed just slightly with increasing stretching stress. The analysis based on uniform stress model and shear fracture theory proposed that the improvement of tensile strength and Young’s modulus for rayon-based carbon fiber was mainly due to the increased preferred orientation and nearly unchanged shear modulus between planes with increasing HTT during hot stretching graphitization, which was much different from polyacrylonitrile-based carbon fibers.     

Evaluation and verification of the virgin–recycled mixing ratio of polypropylene plastic

This study has analyzed the properties of blended polypropylene (PP) specimens and employed statistical analysis to develop a method for determining the virgin–recycled mixing ratio of a specimen. Morphological observations and analyses of thermal and mechanical properties were conducted to examine specimen properties. The results were incorporated into regression analysis to create relationship equations. The results revealed that the melt temperature ranged between 167 and 169 °C, melt index (MI) ranged between 7.59 and 18.36 g/10 min, viscosity decreased when the amount of recycled PP and the rotation speed increased, the maximum decomposition temperature decreased with an increase in recycled PP content and increased with the heating rate, activation energy (E_a) ranged between 39.91 and 12.07 kcal/mol, Young’s modulus ranged between 1121.1 and 1910.2 MPa, and impact strength ranged between 37.94and 49.41 J/m (no significant trends). Scanning electron microscopy showed unbroken fibrils distributed on the fracture surface of Specimens 1–3. Additionally, the tensile strain of these specimens was comparatively high. The fracture surfaces of the specimens showed favorable compatibility after undergoing impact tests. The results of regression analysis indicated that the mixing ratio achieved significant correlations with E_a, MI, and Young’s modulus. Thus, regression and multiple regression analysis were performed to create relationship equations.     

基于纳米压痕技术的碳纤维/环氧树脂复合材料各组分原位力学性能测试

基于纳米压痕技术对碳纤维/环氧树脂复合材料各组分的原位硬度、 弹性模量和蠕变性能进行了测试, 实验得到了基体、 纤维和微小厚度界面层的力学性能。结果表明, 从环氧树脂基体到碳纤维过渡过程中, 硬度和弹性模量有明显的梯度变化, 并且纤维和树脂基体的原位弹性模量平均值与其非原位性能有一定的变化, 实验得到纤维的原位弹性模量有所下降, 环氧树脂的弹性模量有所增加。试件制备过程中的机械研磨对其表面产生的残余应力和复合后两种材料的相互影响是组分材料原位性能变化的主要原因。各组分的蠕变性能呈现出明显的差异。