Strain rate and gage length effects on tensile behavior of Kevlar 49 single yarn

In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10^?4 s^?1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s^?1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.     

Quasi-static uni-axial compression behaviour of hollow glass microspheres/epoxy based syntactic foams

Hollow glass microspheres/epoxy foams of different densities were prepared by stir casting process in order to investigate their mechanical properties. The effect of hollow spheres content and wall thickness of the microspheres on the mechanical response of these foams is studied extensively through a series of quasi-static uni-axial compression tests performed at a constant strain rate of 0.001 s^?1. It is found that strength of these foams decreases linearly from 105 MPa (for the pure resin) to 25 MPa (for foam reinforced with 60 vol.% hollow microspheres) with increase in hollow spheres content. However, foams prepared using hollow spheres with a higher density possess higher strength than those prepared with a lower one. The energy absorption capacity increases till a critical volume fraction (40 vol.% of the hollow microspheres content) and then decreases. Failure and fracture of these materials occur through shear yielding of the matrix followed by axial splitting beyond a critical volume fraction.     

Closed cell ZA27–SiC foam made through stir-casting technique

Closed cell zinc aluminum alloy (ZA27)–SiC composite foam has been synthesized using conventional stir-casting technique and CaH₂ as foaming agent. The synthesized foams are characterized in terms of its micro-architectural characteristics and deformation responses under compressive loading. It is observed that ZA27–SiC foams could be easily foamed without any difficulty. The density of the developed foam ranges from 0.25 gm/cc to 0.45 gm/cc due to the variation of CaH₂ percentage. The plateau stress and energy absorption of these foams follow power law relationship with relative density. Wherein, the densification strain follows a linear relationship with the relative density.     

The superplasticity and microstructure evolution of TC11 titanium alloy

The superplasticity is the capability of some metallic materials to exhibit very highly tensile elongation before failure. The superplastic tensile tests were carried out at various deformation conditions in this paper to investigate the superplastic behaviors and microstructure evolution of TC11 titanium alloy. The results indicate that the smaller the grain size, the better the superplasticity is, and the wider the superplastic temperature and strain rate is, in which the superplastic temperature is ranging from 1023 to 1223 K and the strain rate is ranging from 4.4 × 10^?5 to 1.1 × 10^?2 s^?1. The maximum tensile elongation is 1260% at the optimum deformation conditions (1173 K and 2.2 × 10^?4 s^?1). For further enhancing the superplasticity of TC11 titanium alloy, the novel tensile method of maximum m superplastic deformation is adopted in the paper. Compared with the conventional tensile methods, the excellent superplasticity of TC11 titanium alloy has been found with its maximum elongation of 2300%.     

High temperature compression properties of open-cell Ni–20Cr foams produced by impregnation

The mechanical properties of open-cell Ni–20Cr foams were studied by uniaxial compression tests, which were performed at a strain rate of 10^− 3 s^− 1 at ambient temperature, 250 °C, 400 °C and 550 °C respectively. Then, the effect of temperature on the mechanical properties of these foams was analyzed. It was found that both the compressive strength and energy absorbed by Ni–20Cr foam at elevated temperatures became lower than those at room temperature. The compressive strength changed with temperatures in a form of parabola, which satisfied the model derived from Gibson–Ashby model combined with Kocks' kinetic model. A multi-parameter phenomenological elasto-plastic constitutive model was modified to take the temperature effect into consideration. The relationships between model parameters and temperature were determined. The fitting stress–strain curves of the model were in good agreement with the experimental results.     

Temperature and strain rate dependence of deformation behavior of Zr65Al7.5Ni10Cu17.5

In this work, the effects of temperature and strain rate on the deformation behavior of Zr₆₅Al_7.5Ni₁₀Cu_17.5 bulk metallic glass (BMG) were investigated. It was found that both temperature and strain rate had a significant influence on the deformation behavior of the BMG. The alloy exhibited Newtonian behavior at low strain rates but showed non-Newtonian behavior at high strain rates in the supercooled liquid region. However, the crystallization occurred slightly at a strain rate of 2 × 10^?4 s^?1 at the temperature of 693 K that is lower than the crystallization temperature Tx of Zr₆₅Al_7.5Ni₁₀Cu_17.5 BMG. The deformation mechanisms were discussed in terms of the transition state theory based on the free volume model.     

The toucan beak: Structure and mechanical response

The structure and mechanical response of a Toco toucan (Ramphastos toco) beak were established. The beak was found to be a sandwich composite with an exterior of keratin scales (50 μm diameter and 1 μm thickness) and a core composed of fibrous network of closed-cells made of collagen. The tensile strength of the external shell is about 50 MPa. Micro- and nanoindentation hardness measurements corroborate these values. The keratin shell exhibits a strain-rate sensitive response with a transition from slippage of the scales due to release of the organic glue, at a low strain rate (5 × 10^− 5 s^− 1) to fracture of the scales at a higher strain rate (1.5 × 10^− 3 s^− 1). The closed-cell foam consists of fibers having a Young's modulus (measured by nanoindentation) of 12.7 GPa. This is twice as high as the keratin shells, which have E = 6.7 GPa. This is attributed to their higher calcium content. The compressive collapse of the foam was modeled by the Gibson–Ashby constitutive equations.There is a synergistic effect between foam and shell evidenced by a finite-element analysis. The foam stabilizes the deformation of the keratin shell by providing an internal support which increases its buckling load under compressive loading.     

Modeling quasi-static and high strain rate deformation and failure behavior of a (±45) symmetric E-glass/polyester composite under compressive loading

Quasi-static (1 × 10⁻³–1 × 10⁻² s⁻¹) and high strain rate (∼1000 s⁻¹) compressive mechanical response and fracture/failure of a (±45) symmetric E-glass/polyester composite along three perpendicular directions were determined experimentally and numerically. A numerical model in LS-DYNA 971 using material model MAT_162 was developed to investigate the compression deformation and fracture of the composite at quasi-static and high strain rates. The compressive stress–strain behaviors of the composite along three directions were found strain rate sensitive. The modulus and maximum stress of the composite increased with increasing strain rate, while the strain rate sensitivity in in-plane direction was higher than that in through-thickness direction. The damage progression determined by high speed camera in the specimens well agreed with that of numerical model. The numerical model successfully predicted the damage initiation and progression as well as the failure modes of the composite.     

Impact and fracture response of sintered 316L stainless steel subjected to high strain rate loading

This paper describes the use of a material testing system (MTS) and a compressive split-Hopkinson bar to investigate the impact behaviour of sintered 316L stainless steel at strain rates ranging from 10^− 3 s^− 1 to 7.5 × 10³ s^− 1. It is found that the flow stress–strain response of the sintered 316L stainless steel depends strongly on the applied strain rate. The rate of work hardening and the strain rate sensitivity change significantly as the strain rate increases. The flow behaviour of the sintered 316L stainless steel can be accurately predicted using a constitutive law based on Gurson's yield criterion and the flow rule of Khan, Huang and Liang (KHL). Microstructural observations reveal that the degree of localized grain deformation increases at higher strain rates. However, the pore density and the grain size vary as a reversible function of the strain rate. Impacts at strain rates higher than 5.6 × 10³ s^− 1 are found to induce adiabatic shear bands in the specimens. These specimens subsequently fail as a result of crack propagation along the dominant band. The fracture surfaces of the failed specimens are characterized by dimple-like structures, which are indicative of ductile failure. The depth and the density of these dimples are found to decrease with increasing strain rate. This observation indicates a reduction in the fracture resistance and is consistent with the observed macroscopic flow stress–strain response.     

High-precision CTE measurement of hybrid C/SiC composite for cryogenic space telescopes

This paper presents highly precise measurements of thermal expansion of a “hybrid” carbon-fiber reinforced silicon carbide composite, HB-Cesic® – a trademark of ECM, in the temperature region of ～310–10 K. Whilst C/SiC composites have been considered to be promising for the mirrors and other structures of space-borne cryogenic telescopes, the anisotropic thermal expansion has been a potential disadvantage of this material. HB-Cesic® is a newly developed composite using a mixture of different types of chopped, short carbon-fiber, in which one of the important aims of the development was to reduce the anisotropy. The measurements indicate that the anisotropy was much reduced down to 4% as a result of hybridization. The thermal expansion data obtained are presented as functions of temperature using eighth-order polynomials separately for the horizontal (XY-) and vertical (Z-) directions of the fabrication process. The average CTEs and their dispersion (1σ) in the range 293–10 K derived from the data for the XY- and Z-directions were 0.805 ± 0.003 × 10^?6 K^?1 and 0.837 ± 0.001 × 10^?6 K^?1, respectively. The absolute accuracy and the reproducibility of the present measurements are suggested to be better than 0.01 × 10^?6 K^?1 and 0.001 × 10^?6 K^?1, respectively. The residual anisotropy of the thermal expansion was consistent with our previous speculation regarding carbon-fiber, in which the residual anisotropy tended to lie mainly in the horizontal plane.     

Strain rate dependent plastic mutation in a bulk metallic glass under compression

This paper reported a strain rate dependent plasticity in a Zr-based bulk metallic glass (BMG) under axial compression over a strain rate range (1.6 × 10⁻⁵–1.6 × 10⁻¹ s⁻¹). The fracture strain decreased with increasing strain rate up to 1.6 × 10⁻³ s⁻¹. A “brittle-to-malleable” mutation occurred at strain rate of 1.6 × 10⁻² s⁻¹, subsequently, the macro plasticity vanished at 1.6 × 10⁻¹ s⁻¹. It is proposed that the result is strongly related to the combined action of the applied strain rate, the compression speed, and the propagating speed of the shear band. When the three factors coordinated in the optimal condition, multiple mature shear bands were initiated simultaneously to accommodate the applied strain, which propagated through the specimen and distributed homogeneously in space, dominating the overall plastic deformation by consuming the entire specimen effectively.     

The dynamic mechanical response of SiC particulate reinforced 2024 aluminum matrix composites

The compression properties of the aluminum alloy 2024 metal matrix composites reinforced with 50 vol.% SiC particles were investigated using Instron testing machine and split Hopkinson pressure bar (SHPB) in this paper. The compression stress–strain curves were obtained at the strain rates ranging from 1 × 10^− 3 to 2.5 × 10³/s. The fracture surfaces were characterized by scanning electron microscopy. The results showed that SiCp/2024 Al composites exhibited high strain-rate sensitivity. The strength of composites tended to increase–decrease with increasing of strain rates. The effect of the strain rate on elongation was also discussed.     

Effect of fabrication conditions and Cr,Zr contents on the grain structure of 7075 and 6061 aluminum alloys

The correlation between compression conditions at temperatures in the range of 573–773 K with the strain rate range of 0.002–2 s^?1 and grain size after solution heat treatment of 7075 alloy was investigated, as contrasted with 6061 alloy. The grain coarsening occurred under specific Zener-Hollomon (Z) parameters of 5 × 10¹⁰–10¹² s^?1 for 7075 alloy, 10⁸–2 × 10¹² s^?1 for 6061 alloy, respectively. These phenomena could be explained by crystalline orientation analysis and stored deformation strain evaluation. The site of subgrains with less than 15° misorientation and stored strain after compression increased, but the site of recrystallized grains after solution heat treatment increased with Z parameter. Small Z parameter condition could get low stored strain with fine grain which is stable during SHT. Effect of Cr and Zr on the grain structure of 7075 alloy was also investigated. Cr or Zr addition could inhibit the grain coarsening. The role of Zr addition was confirmed to pinning effect of Al₃Zr dispersoids to subgrain boundaries.     

Effect of strain rate on cushioning properties of molded pulp products

Molded pulp product is widely used in distribution chains as a cushioning packaging of industrial products due to its favorable cushioning capability. How to evaluate the cushioning capability of molded pulp product is the key issue many scholars are interesting in. The load carrying capacity and energy absorbing of the molded pulp products used in the cushion packaging of mobile phones both in the static compression and dynamic impact were investigated in this paper by applying the experiment and finite element analysis. The static compression was conducted with the compression speed of 12 mm/min corresponding to the nominal strain rate 3.8 × 10⁻³ s⁻¹, and the dynamic impact tests were conducted with three drop heights of 25, 50 and 80 cm corresponding respectively to the nominal strain rates 4.2 × 10¹, 6.0 × 10¹ and 7.5 × 10¹ s⁻¹. The high speed camera was used to record the dynamic impact process and deformation. The finite element model of molded pulp product was built, and the stress and displacement nephograms, the dynamic impact deformation process, the load–displacement curve and the energy absorption curve of the molded pulp product were archived. The comparison between the finite element analysis and the experiment was made. The load–displacement curve of the finite element analysis is in agreement with that of the experiment in the static compression, and the energy absorption curves of the finite element analysis with different nominal strain rates are in agreement with that of the experiment in the area of the point of optimum energy absorption. However, a growing gap between the finite element analysis and the experiment appears with the nominal strain rate increasing, which may be induced by the use of the static stress–strain curve of the material in the finite element analysis of dynamic impact. The molded pulp product experiences the process from structural deformation, local stress concentration, first local buckling, redistribution of stress, global buckling, to structural dilapidation and densification. Two obvious buckling processes occur because of its complicated structure and two layers in structure. However, some additional local buckling also occur before the global buckling of structure in the case of dynamic impact with higher nominal strain rate. The deformation processes of molded pulp product from the finite element analysis and the experiment recorded by high-speed camera are coincident. With the nominal strain rate increasing, the yield stress of molded pulp product increases obviously, and the shoulder point of the energy absorption curve moves upward to the right. The yield stress under the dynamic impact at a drop height of 80 cm increases 59.4% compared with that under the static compression, and the corresponding optimum energy absorption increases 85.4%. The effects of strain rate on the load carrying capacity and the energy absorption of molded pulp product are remarkable. The results can be applied to the design of molded pulp products.     

Relationship between mechanical properties and microstructural response of 6061-T6 aluminum alloy impacted at elevated temperatures

The high temperature impact properties and microstructural evolution of 6061-T6 aluminum alloy are investigated at temperatures ranging from 100 to 350 °C and strain rates ranging from 1 × 10³ to 5 × 10³ s⁻¹ using a compressive split-Hopkinson pressure bar (SHPB) system. It is found that the flow response and microstructural characteristics of 6061-T6 aluminum alloy are significantly dependent on the strain rate and temperature. The flow stress and strain rate sensitivity increase with increasing strain rate or decreasing temperature. Moreover, the temperature sensitivity increases with both increasing strain rate and increasing temperature. The flow stress–strain response of the present 6061-T6 alloy specimens can be adequately described by the Zerilli–Armstrong fcc model. The grain size and dislocation cell size increase significantly with a decreasing strain rate or an increasing temperature. The higher flow stress is the result of a smaller grain size and smaller dislocation cell size. The stacking fault energy of the deformed specimens has a value of 145.78 mJ/m².     

Optimization of hot working parameters for thermo-mechanical processing of modified 9Cr–1Mo (P91) steel employing dynamic materials model

Intrinsic workability of modified 9Cr–1Mo steel has been studied in a wide range of temperatures (1123–1373 K) and strain rates (0.001–10 s^?1). Using the experimental data obtained from isothermal hot compression tests, processing map at 0.5 true strain has been developed employing dynamic material model (DMM) approach. The activation energy map has been developed to substantiate the results obtained from processing map and to finalize the optimum processing parameters. Microstructural studies have been carried out to validate the domains of the processing map. The material shows localized deformation bands in the temperature range of 1150–1373 K at strain rates above 1 s^?1 and exhibits abnormally elongated martensite laths at higher temperature (1373 K) and lower strain rates (0.001–0.01 s^?1). The optimum domain for the hot deformation is found to be in the temperature ranges of 1250–1350 K and strain rate ranges 0.015–0.3 s^?1 with a peak efficiency of 38%. In this domain, apparent activation energy is found to be 400 kJ/mol. The microstructure of the specimens deformed in this region exhibits defect free equiaxed grains.     

Strain profiles and defect structure in 6H–SiC crystals implanted with 2 MeV As+ ions

Highly perfect (00.1) oriented 6H–SiC wafers were implanted with 2 MeV As⁺ ions to a number of fluencies in the range from 5 × 10¹² cm^?2 to 1 × 10¹⁴ cm^?2 and examined with synchrotron X-ray diffraction methods and RBS/channeling method. The X-ray methods included the investigation of rocking curves recorded with a small 50 × 50 μm² probe beam and white beam Bragg-Case section and projection topography.The implanted layers provided distinct interference maxima in the rocking curves and interference fringes in Bragg-Case section topographies (strain modulation fringes). A good visibility of interference maxima enabled effective evaluation of the strain profile by fitting the theoretical rocking curves to the experimental ones. The evaluated strain profiles approximated by browsed Gaussian curves were similar to the distribution of point defects calculated with SRIM 2008 code. The profiles were similar to the defect distribution determined from the channeling measurements.     

Fabrication and characterization of boron nitride bulk foam from borazine

Boron nitride (BN) ceramic bulk foams larger than 10 × 10 × 1 cm are fabricated by the pyrolysis of polyborazine foams that are prepared by fast curing of borazine. The principle of the self-foaming phenomenon is the emitting of H₂ in condensation polymerization during the curing of borazine. The as-fabricated BN ceramic foams exhibit closed cells with sizes ranging from 50 to 100 μm and controlled bulk densities varying from 0.08 to 0.25 g cm⁻³ depending on the different cured pressures. Studies on the microstructure and properties of the porous material show that the BN ceramic foams exhibit high specific strength, low dielectric constants and loss tangents, and good thermal conductivity. These characteristics make them useful for high temperature radomes and thermal insulation.     

A visco-hyperelastic constitutive description of elastomeric foam

This study focuses on the constitutive modelling of finite deformation in elastomeric polyurethane foams—in particular, PORON-4701-59-25045-1648 (0.4 g/cm³ density) and PORON-4701-59-20093-1648 (0.32 g/cm³ density). Their mechanical properties under compression, for engineering strains up to about 80%, are characterized over a range of strain rates between 10⁻² and 10³/s. Dynamic compression is applied using a split Hopkinson pressure bar device. Experimental results show that the behaviour of elastomeric foam is sensitive to strain rate and can be described by a visco-hyperelastic material model. In this model, the quasi-static response is defined by compressible hyperelasticity, whereby the strain energy potential is assumed to be representable by a newly proposed polynomial series with three independent parameters. Strain rate sensitivity is characterized by incorporating a nonlinear Maxwell relaxation model with four parameters. The (seven) material parameters in the constitutive model are determined from high-speed mechanical testing methods tailored for high-compliance materials. A comparison of predictions based on the proposed frame-independent constitutive equation with experiments shows that the model is able to describe the rate dependent behaviour of the elastomeric foams examined.     

The impact response of motorcycle helmets at different impact severities

Helmets reduce the frequency and severity of head and brain injuries over a range of impact severities broader than those covered by the impact attenuation standards. Our goal was to document the impact attenuation performance of common helmet types over a wide range of impact speeds. Sixty-five drop tests were performed against the side of 10 different helmets onto a flat anvil at impact speeds of 0.9–10.1 m/s (energy = 2–260 J; equivalent drop heights of 0.04–5.2 m). Three non-approved beanie helmets performed poorly, with the worst helmet reaching a peak headform acceleration of 852g at 29 J. Three full-face and one open-face helmet responded similarly from about 100g at 30 J to between 292g and 344g at 256–260 J. Three shorty style helmets responded like the full-face helmets up to 150 J, above which varying degrees of foam densification appeared to occur. Impact restitution values varied from 0.19 to 0.46. A three-parameter model successfully captured the plateau and densification responses exhibited by the various helmets (R² = 0.95–0.99). Helmet responses varied with foam thickness, foam material and possibly shell material, with the largest response differences consistent with either the presence/absence of a foam liner or the densification of the foam liner.