The characterization of shape memory effect for low elastic modulus biomedical β-type titanium alloy

This work investigates the textures of biomedical TiNbTaZr alloy rolled by 99% cold reduction ratios in thickness. The relationship between textures and superelasticity of the specimens treated at 873 K and 1223 K for 1.2 ks is studied. The microstructure of tensile specimen is investigated by transmission electron microscopy. Textures of cold-rolled and heat-treated specimens are studied. During unloading, the anisotropy of superelastic strain and pure elastic strain in the heat-treated specimens is observed. Superelastic strain along rolling direction and transverse direction is larger than those along 45° from rolling direction while pure elastic strain shows the highest value along 45° from rolling direction in the specimen treated at 873 K. For the specimen treated at 1223 K, higher pure elastic strain is obtained along rolling direction. The maximum recovered strain around 2.11% is obtained along rolling direction.     

Compressive performance of laminated bamboo

This study investigated the compressive performance of 24 laminated bamboo specimens made from three different growth portions of the source bamboo. The cross-section of each specimen was 100 mm × 100 mm. The load–strain and load–displacement relationships are obtained from compression tests, and the detailed failure modes, compressive strength and elastic modulus for all specimens are reported. The results show that the mean compressive strength increases with growth portion height, but that the variation in compressive strength also increases with growth portion height. The net result is that the characteristic strength (typically used in the design process) decreases slightly with growth portion height, but not significantly. In contrast, laminated bamboo manufactured from the middle growth portion exhibits the highest elastic modulus, with the variation again increasing with height. Although the source growth portion has a clear effect on the behaviour of laminated bamboo under compression, the paper concludes that the effect is not significant from a design perspective. The results of all the tests are combined to produce a model stress–strain relationship suitable for predicting the performance of laminated bamboo under compression for design purposes. The stress–strain relationship shows that under compression laminated bamboo fails in a ductile manner. Based on the compressive properties obtained in this research, laminated bamboo is a suitable construction material for engineering structures.     

Experimental study on the dynamic compressive mechanical properties of concrete at elevated temperature

Strain rate effect and temperature effect are two important factors affecting the mechanical behavior of concrete. Each of them has been studied for several years. However, the two factors usually work together in the engineering practice. It is necessary to understand the mechanical responses of concrete under high strain rate and elevated temperature. A self-designed high temperature SHPB apparatus was used to study the dynamic compressive mechanical properties of concrete at elevated temperature. The results show that the dynamic compressive strength and specific energy absorption of concrete increase with strain rate at all temperatures. The elastic modulus decreases obviously with strain rate at room temperature and stabilizes at a level with slightly decrease at elevated temperature. The dynamic compressive strength of concrete at 400 °C increases by nearly 14% compared to the room temperature. However, it decreases at 200 °C, 600 °C and 800 °C with the decrease ratio of 20%, 16% and 48%, respectively. The dynamic elastic modulus decreases largely subjected to elevated temperature. The specific energy absorption at 200 °C, 400 °C and 600 °C is higher than room temperature and decreases to be lower than room temperature at 800 °C. Formulas are established under the consideration of mutual effect of strain rate and temperature.     

Role of microstructural condition on fatigue damage development of AISI 316L at 20 and 300 °C

At 300 °C, when dynamic strain ageing takes place, the fatigue life of AISI 316L for lower strain amplitudes is lower than under equivalent conditions at 20 °C. Exhaustive examination of the changes in: (1) apparent elastic modulus, (2) microstructural condition, and (3) fractographic features has been performed to reveal the reason for the life reduction. The analysis of apparent elastic modulus variations and the results of fractographic observations show that the propagation rates for fatigue cracks at 20 °C are faster than for 300 °C. Crack initiation however occurs earlier at 300 °C, in particular for lower strain amplitude tests, due to the activity of localised deformation bands as a consequence of cyclic loading. In addition to persistent slip bands, a form of ladder-free deformation bands is also present at 300 °C, in particular at low strain amplitudes. When the fatigue life is rather short, the influence of the ladder-free deformation bands on cyclic endurance is negligible. The ladder-free type of localised bands have a strong influence on crack initiation once the material endurance increases with lowering strain amplitude, leading to the relative life reduction at the elevated temperature. In addition, the incidence of secondary cyclic hardening for lower strain amplitude tests at 300 °C partly contributes to the more evident life reduction. The influence of dislocation walls on the propagation of microstructurally short fatigue cracks is also examined.     

Microstructure evolution and mechanical properties of a Ti–35Nb–3Zr–2Ta biomedical alloy processed by equal channel angular pressing (ECAP)

In this paper, an equal channel angular pressing method is employed to refine grains and enhance mechanical properties of a new β Ti–35Nb–3Zr–2Ta biomedical alloy. After the 4th pass, the ultrafine equiaxed grains of approximately 300 nm and 600 nm are obtained at pressing temperatures of 500 and 600 °C respectively. The SEM images of billets pressed at 500 °C reveal the evolution of shear bands and finally at the 4th pass intersectant networks of shear bands, involving initial band propagation and new band broadening, are formed with the purpose of accommodating large plastic strain. Furthermore, a unique herringbone microstructure of twinned martensitic variants is observed in TEM images. The results of microhardness measurements and uniaxial tensile tests show a significant improvement in microhardness and tensile strength from 534 MPa to 765 MPa, while keeping a good level of ductility (~ 16%) and low elastic modulus (~ 59 GPa). The maximum superelastic strain of 1.4% and maximum recovered strain of 2.7% are obtained in the billets pressed at 500 °C via the 4th pass, which exhibits an excellent superelastic behavior. Meanwhile, the effects of different accumulative deformations and pressing temperatures on superelasticity of the ECAP-processed alloys are investigated.     

Effect of strain rate on the dynamic compressive mechanical behaviors of rock material subjected to high temperatures

Strain rate is not only an important measure to characterize the deformation property, but also an important parameter to analyze the dynamic mechanical properties of rock materials. In this paper, by using the SHPB test system improved with high temperature device, the dynamic compressive tests of sandstone at seven temperatures in the range of room temperature to 1000 °C and five impact velocities in the range of 11.0–15.0 m/s were conducted. Investigations were carried out on the influences of strain rate on dynamic compressive mechanical behaviors of sandstone. The results of the study indicate that the enhancement effects of strain rates on dynamic compressive strength, peak strain, energy absorption ratio of sandstone under high temperatures still exist. However, the increase ratios of dynamic compressive strength, peak strain, and energy absorption ratio of rock under high temperature compared to room temperature have no obvious strain rate effects. The temperatures at which the strain rates affect dynamic compressive strength and peak strain most, are 800, and 1000 °C, respectively. The temperatures at which the strain rates affect dynamic compressive strength and peak strain weakest, are 1000 °C, and room temperature, respectively. At 200 and 800 °C, the strain rate effect on energy absorption ratio are most significant, while at 1000 °C, it is weakest. There are no obvious strain rate effects on elastic modulus and increase ratio of elastic modulus under high temperatures. According to test results, the relationship formula of strain rate with high temperature and impact load was derived by internalizing fitting parameters. Compared with the strain rate effect at room temperature condition, essential differences have occurred in the strain rate effect of rock material under the influence of high temperature.     

Mechanical properties of conventional and nanostructured plasma sprayed alumina coatings

The hardness and the elastic modulus measured by microindentation of three different types of plasma sprayed alumina coatings have been compared. Usually, such coatings present porosity and heterogeneity which affect the measurement of the mechanical properties. To take such effects into account along with the indentation size effect which is relevant to all hardness studies, the Proportional Specimen Resistance model is applied. The three alumina coatings show closely similar mechanical properties at indentation loads exceeding 1 N, i.e., macrohardness around 5.7 GPa, indentation size effect parameter around 5.5 MPa mm and elastic modulus around 160 GPa. For loads below 1 N, the extrapolated values of the macrohardness of crushed and agglomerated alumina coatings increased to 8.5 GPa, while the indentation size effect parameter has the same value, and the elastic modulus increased to 320 GPa. However, no significant change in the measured values of hardness and the elastic modulus of the nanostructured alumina coating has been observed. This result is attributed to porosity and the bimodal microstructure of the nanostructured coating where a semimolten phase coexists along with the fully molten phases.     

Syntactic foam core metal matrix sandwich composite under bending conditions

The present work aims at characterizing a metal matrix syntactic foam core sandwich composite under three-point bending conditions. The sandwich comprises alumina hollow particle reinforced A356 alloy syntactic foam with carbon fabric skins. Crack initiation in the tensile side of the specimen causing failure of the skin, followed by rapid failure of the core in the direction applied load, is observed as the failure mechanism. Crack propagation through the alumina particles is observed in the failed specimens instead of interfacial failure. The average maximum strength, flexural strain and stiffness were measured as 91.2 ± 5.6 MPa, 0.49 ± 0.06% and 20.6 ± 0.7 GPa respectively. The collapse load is theoretically predicted using mechanics of sandwich beams. Experimental values show good agreement with theoretical predictions.     

The effects of fatigue and residual strain on the mechanical behavior of poly(ethylene terephthalate) unreinforced and nanocomposite fibers

Poly(ethylene terephthalate) (PET) control fibers (nominal diameter ～24 ± 3 μm) and PET fibers with embedded vapor-grown carbon nanofibers (PET-VGCNF) (nominal diameter ～25 ± 2 μm) were exposed to cyclic loading and monotonic tensile tests. The control fibers were processed through a typical melt-blending technique and the PET-VGCNF samples were processed with approximately 5 wt.% carbon nanofibers present in the sample. Under uniaxial fatigue conditions, the fibers were subjected to a maximum stress that was approximately 60% of the fracture stress of the sample at an elongation rate of 10 mm/min in uniaxial tension. The fibers were subjected to a frequency of 5 Hz. Subsequent to non-fracture fatigue conditions, the fibers were tested under uniaxial stress conditions for observation of the change in mechanical properties to assess the effects of fatigue loading. The elastic modulus, hardening modulus, fracture strength, work done, and yield strain of both PET control and PET-VGCNF samples in uniaxial tension subsequent to fatigue were shown to be dependent on the residual fatigue strains. Relative mechanical properties were used to quantify the difference in PET and PET-VGCNF samples as a function of residual strain. In most cases, the results indicated a strengthening mechanism (strain hardening effect) in the low residual strain limit for fatigued PET samples and not for fatigued PET-VGCNF samples. In comparison with the unreinforced PET sample, the PET-VGCNF fibers showed greater degradation of mechanical properties as a function of residual strain due to fatigue when cycled at 60% of the fracture stress. The effects of the fatigue process on the change in mechanical properties have been quantified and supported through existing qualitative, quantitative, and scanning electron microscopy (SEM) techniques.     

Effect of waste plastic bottles on the stiffness and fatigue properties of modified asphalt mixes

Nowadays, the use of recycled waste materials as modifier additives in asphalt mixes could have several economic and environmental benefits. The main purpose of this research was to investigate the effect of waste plastic bottles (Polyethylene Terephthalate (PET)) on the stiffness and specially fatigue properties of asphalt mixes at two different temperatures of 5 and 20 °C. Likewise, the effect of PET was compared to styrene butadiene styrene (SBS) which is a conventional polymer additive which has been vastly used to modify asphalt mixes. Different PET contents (2–10% by weight of bitumen) were added directly to mixture as the method of dry process. Then the resilient modulus and fatigue tests were performed on cylindrical specimens with indirect tensile loading procedure. Overall, the mix stiffness reduced by increasing the PET content. Although stiffness of asphalt mix initially increased by adding lower amount of PET. Based on the results of resilient modulus test, the stiffness of PET modified mix was acceptable and warranted the proper deformation characteristics of these mixes at heavy loading conditions. At both temperatures, PET improved the fatigue behavior of studied mixes. PET modified mixes revealed comparable stiffness and fatigue behavior to SBS at 20 °C. However, at 5 °C the fatigue life of SBS modified mixes was to some extent higher than that of PET modified ones especially at higher strain levels of 200 microstrain.     

Effects of sintering temperature on morphology and mechanical characteristics of 3D printed porous titanium used as dental implant

Porous titanium samples were manufactured using the 3D printing and sintering method in order to determine the effects of final sintering temperature on morphology and mechanical properties. Cylindrical samples were printed and split into groups according to a final sintering temperature (FST). Irregular geometry samples were also printed and split into groups according to their FST. The cylindrical samples were used to determine part shrinkage, in compressive tests to provide stress-strain data, in microCT scans to provide internal morphology data and for optical microscopy to determine surface morphology. All of the samples were used in microhardness testing to establish the hardness. Below 1100 °C FST, shrinkage was in the region of 20% but increased to approximately 30% by a FST of 1300 °C. Porosity varied from a maximum of approximately 65% at the surface to the region of 30% internally. Between 97 and 99% of the internal porosity is interconnected. Average pore size varied between 24 μm at the surface and 19 μm internally. Sample hardness increased to in excess of 300 HV0.05 with increasing FST while samples with an FST of below 1250 °C produced an elastic–brittle stress/strain curve and samples above this displayed elastic–plastic behaviour. Yield strength increased significantly through the range of sintering temperatures while the Young's modulus remained fairly consistent.     

Assessment of three oxide/oxide ceramic matrix composites: Mechanical performance and effects of heat treatments

Mechanical performance of three oxide/oxide ceramic matrix composites (CMCs) based on Nextel 610 fibers and SiOC, alumina, and mullite/SiOC matrices respectively, is evaluated herein. Tensile strength and stiffness of all materials decreased at 1000 °C and 1200 °C, probably because of degradation of fiber properties beyond 1000 °C. Microstructural changes in the composites during exposure at 1000 °C and 1200 °C for 50 h reduce their flexural strength, fracture toughness and work of fracture. A literature review regarding mechanical properties of several oxide/oxide CMCs revealed lower influence of fiber properties on composite strength compared with elastic modulus. The tested composites exhibit comparable stiffness and strength but higher fracture toughness compared with average values determined from a literature review. Considering CMCs with different compositions, we observed an interesting linear trend between strength and fracture toughness. The validity of the linear relationship between fracture strength and flexural toughness for CMCs is discussed.     

Zr–Si biomaterials with high strength and low elastic modulus

In order to develop new biomaterials for hard tissue replacements (HTR), the Zr–8.8Si–xNb (x = 0.0, 0.3, 0.6 and 0.9) alloys with required properties were designed and prepared for the first time. Experimental results show that these alloys can provide excellent mechanical compatibility for the special demands for substitution of human bones. The highest compression strength of the alloys is 1189.30 MPa, while the highest yield strength of alloys is 850.25 MPa. The elastic energy is determined to be 5.001–12.01 MJ/m³, and the Young’s modulus is in the range of 25.08–29.63 GPa. The composition of high strength and low elastic modulus of Zr–8.8Si–xNb alloys offer potential advantages for biomedical applications.     

Nanoscale characterization of natural fibers and their composites using contact-resonance force microscopy

Contact-resonance force microscopy (CR-FM) has been used for the first time to evaluate the mechanical properties of the interphase in natural fiber-reinforced composites and of cell wall layers of natural fibers. With CR-FM, quantitative images of the spatial distribution in nanoscale elastic properties were acquired. The images were calibrated with nanoindentation values. From the modulus images, the average interphase width was found to be (49 ± 5) nm for composite without any treatment, and (139 ± 21) nm for one with a maleic anhydride polypropylene treatment. There was a gradient of modulus across the interphase that ranged between the values of fiber and the polymer. The average values of indentation modulus obtained for different cell wall layers within a fiber were 22.5–28.0 GPa, 17.9–20.2 GPa, and 15.0–15.5 GPa for the S₂ and S₁ layers and the compound middle lamellae, respectively.     

Pseudo-ductility in intermingled carbon/glass hybrid composites with highly aligned discontinuous fibres

The aim of this research is to manufacture intermingled hybrid composites using aligned discontinuous fibres to achieve pseudo-ductility. Hybrid composites, made with different types of fibres that provide a balanced suite of modulus, strength and ductility, allow avoiding catastrophic failure that is a key limitation of composites. Two different material combinations of high strength carbon/E-glass and high modulus carbon/E-glass were selected. Several highly aligned and well dispersed short fibre hybrid composites with different carbon/glass ratios were manufactured and tested in tension in order to investigate the carbon ratio effect on the stress–strain curve. Good pseudo-ductile responses were obtained from the high modulus carbon/E-glass composites due to the fragmentation of the carbon fibres. The experimental results were also compared with an analytical solution. The intermingled hybrid composite with 0.25 relative carbon ratio gave the maximum pseudo-ductile strain, 1.1%, with a 110 GPa tensile modulus. Moreover, the initial modulus of the intermingled hybrids with 0.4 relative carbon ratio is 134 GPa, 3.5 times higher than that of E-glass/epoxy composites. The stress–strain curve shows a clear “yield point” at 441 MPa and a well dispersed and gradual damage process.     

Investigation of structural,optical and micro-mechanical properties of (NdyTi1 − y)Ox thin films deposited by magnetron sputtering

TiO₂ and (Nd_yTi_1 − y)O_x thin films were deposited by reactive magnetron sputtering process from mosaic Ti–Nd targets and characterised by X-ray diffraction (XRD), Raman optical spectroscopy and nanoindentation technique. XRD measurements revealed that as-prepared titanium dioxide and TiO₂ thin films with 4 and 7 at.% of Nd had nanocrystalline rutile structure, while coatings with larger amount of Nd were amorphous. Raman spectroscopy investigations showed that the increase of the neodymium concentration caused amorphisation of the coatings and hindered their crystal growth. All as-prepared coatings were transparent in the visible wavelength range with a transmittance of approximately 80%. The refractive index and extinction coefficient of the thin films gradually decreased with the increase of the neodymium concentration. Micro-mechanical properties, i.e. hardness and elastic modulus, were determined using traditional load-controlled nanoindentation testing and continuous stiffness measurements. The highest hardness and elastic modulus values were obtained for thin films with 7 at.% of Nd and were approximately 14.8 GPa and 166.3 GPa, respectively.     

Mechanical properties of normal strength mild steel and high strength steel S690 in low temperature relevant to Arctic environment

The development of Arctic oil and gas fields requires low temperature high strength steel materials that can resist critical loads in extreme environments. This paper investigates the mechanical properties such as stress–strain curves, elastic modulus, yield strength, ultimate tensile strength, and fracture strain of normal mild steel and high strength S690 steel to be used in low temperatures relevant to arctic environment. Tensile tests are carried out on steel coupons at different temperatures ranging from −80 °C to +30 °C in a cooling chamber. The influences of the low temperatures on the mechanical properties of mild steel and high strength steel are compared and their differences are discussed. Regression analyses are also carried out on the test data to develop empirical formulae to predict the elastic modulus, yield strength, and ultimate strength of the steels at ambient low temperatures. Finally, design formulae are recommended and their accuracies are further confirmed by the test data including those from the literature.     

Innovative flax tapes reinforced Acrodur biocomposites: A new alternative for automotive applications

Flax Acrodur biocomposites are elaborated with an innovative flax reinforcement consisting of long technical fibers unidirectionally arranged without any weft and twist. The fibers cohesion is performed by using a new process consisting by reactivating the pectin cement. A polyester thermoset matrix (Acrodur) is used to impregnate the flax reinforcement and to produce unidirectional (UD) laminates. The relationship between the main process variables (drying, fibers content, densification and curing parameters) and the properties of the biocomposites is investigated. The optimized biocomposites have an elastic modulus of 18 ± 1 GPa with 55% wt.% flax fiber content and a low density of 0.93 g/cm³. The thermal stability of the developed biocomposites is also investigated by Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and Dynamic Mechanical Analysis (DMA). DMA results show a slight change of the storage modulus in a range of temperature from 23 °C to 160 °C. The appropriate processing parameters for the biocomposites are established. The developed flax tapes reinforced Acrodur biocomposites have a potential to be integrated for automotive applications thanks to their high stiffness/weight ratio and environmental advantages.     

Demonstration of pseudo-ductility in unidirectional hybrid composites made of discontinuous carbon/epoxy and continuous glass/epoxy plies

A new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860 MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately.     

Recent developments in local concepts of fatigue assessment of welded joints

Several lately proposed modifications or variants of the structural stress or strain concepts, of the notch stress or strain concepts (also termed ‘local stress or strain concepts’) and of the fracture mechanics concepts of fatigue assessment of welded joints are reviewed, whereas the wider context is presented in a recently republished and actualised standard work. The structural stress concepts described first are based on a linearisation of the stress distribution across the plate thickness or along the anticipated crack path and, alternatively, on the structural stress 1 mm in depth below the weld toe. The structural stress is defined and set against design S–N curves. A further structural stress concept is presented for welded joints in thin-sheet steels and aluminium alloys. Among the elastic notch stress concepts, the variant with the reference notch radius, ρ_r = 1 mm, recently verified also for welded joints in aluminium alloys with plate thicknesses t ? 5 mm and the variant with a small-size reference notch radius, ρ_r = 0.05 mm, applicable to welded joints in thin-sheet materials, are outlined. The elastic–plastic notch strain concept is applied to a spot-welded tensile-shear specimen starting from a small-size keyhole notch at the nugget edge. The novel notch stress intensity factor (NSIF) approach relating to crack initiation and extrapolated to final fracture of seam-welded joints in steels and in aluminium alloys is reviewed. A more recently developed crack propagation approach for spot welds is finally described.