Huiyang Luo

Fluorescent stereo microscopy for 3D surface profilometry and deformation mapping

Recently, mechanobiology has received increased attention. For investigation of biofilm and cellular tissue, measurements of the surface topography and deformation in real-time are a pre-requisite for understanding the growth mechanisms. In this paper, a novel three-dimensional (3D) fluorescent microscopic method for surface profilometry and deformation measurements is developed. In this technique a pair of cameras are connected to a binocular fluorescent microscope to acquire micrographs from two different viewing angles of a sample surface doped or sprayed with fluorescent microparticles. Digital image correlation technique is used to search for matching points in the pairing fluorescence micrographs. After calibration of the system, the 3D surface topography is reconstructed from the pair of planar images. When the deformed surface topography is compared with undeformed topography using fluorescent microparticles for movement tracking of individual material points, the full field deformation of the surface is determined. The technique is demonstrated on topography measurement of a biofilm, and also on surface deformation measurement of the biofilm during growth. The use of 3D imaging of the fluorescent microparticles eliminates the formation of bright parts in an image caused by specular reflections. The technique is appropriate for non-contact, full-field and real-time 3D surface profilometry and deformation measurements of materials and structures at the microscale.

Measurement of Young’s Modulus of Human Tympanic Membrane at High Strain Rates

The mechanical behavior of human tympanic membrane (TM) has been investigated extensively under quasistatic loading conditions in the past. The results, however, are sparse for the mechanical properties (e.g., Youngs modulus) of the TM at high strain rates, which are critical input for modeling the mechanical response under blast wave. The property data at high strain rates can also potentially be converted into complex modulus in frequency domain to model acoustic transmission in the human ear. In this study, we developed a new miniature split Hopkinson tension bar to investigate the mechanical behavior of human TM at high strain rates so that a force of up to half of a newton can be measured accurately under dynamic loading conditions. Youngs modulus of a normal human TM is reported as 45.2-58.9 MPa in the radial direction, and 34.1-56.8 MPa in the circumferential direction at strain rates 300-2000 s(-1). The results indicate that Youngs modulus has a strong dependence on strain rate at these high strain rates.

Characterization of the Compressive Behavior of Glass Fiber Reinforced Polyurethane Foam at Different Strain Rates

A glass fiber reinforced polyurethane foam (R-PUF), used for thermal insulation of liquefied natural gas tanks, was characterized to determine its compressive strength, modulus, and relaxation behavior. Compressive tests were conducted at different strain rates, ranging from 10 ―3 s ―1 to 10 s ―1 using a servohydraulic material testing system, and from 40 s ―1 to 10 ―3 s ―1 using a long split Hopkinson pressure bar (SHPB) designed for materials with low mechanical impedance such as R-PUF. Results indicate that in general both Youngs modulus and collapse strength increase with the strain rate at both room and cryogenic (―170°C) temperatures. The R-PUF shows a linearly viscoelastic behavior prior to collapse. Based on time-temperature superposition principle, relaxation curves at several temperatures were shifted horizontally to determine Youngs relaxation master curve. The results show that Youngs relaxation modulus decreases with time. The relaxation master curve obtained can be used to convert to Youngs modulus at strain rates up to 10 3 s ―1 following linearly viscoelastic analysis after the specimen size effect has been considered.

A comparison of Young's modulus for normal and diseased human eardrums at high strain rates

The viscoelastic properties of a human eardrum or tympanic membrane (TM) have not been fully characterised in the auditory frequency range, despite the fact that these properties are critical data as input in modelling the acoustic transmission in a human ear. In this paper using a miniature split Hopkinson tension bar (SHTB), we investigated the mechanical behaviour of TM at high strain rates, corresponding approximately to the behaviour at high frequency. The Youngs modulus values of diseased human TMs are determined as 63.4-79.2 MPa in the radial direction, and 33.1-42.8 MPa in the circumferential direction at strain rates 300-2000 s -1 , results are compared with those for normal TMs. The comparison indicates that normal human TMs show stronger dependence on high strain rates. The measured Youngs modulus is converted into complex Youngs modulus in the frequency domain in the frequency range of 300-2000 Hz.

High-Strain Rate Compressive Behavior of Dry Mason Sand Under Confinement

Uniaxial compressive behavior of Mason sand, a poorly-graded local sand supplied by Colorado Materials (Longmont, CO), was investigated on a long split Hopkinson pressure bar. Sand grains were confined inside a hardened steel tube and capped by tungsten carbide rods. This assembly was subjected to repeat shaking to consolidate sand to attain desired bulk mass density, then sandwiched between incident and transmission bar ends for dynamic compression. Unsorted dry sand was characterized at high strain rates to determine the volumetric and deviatoric behavior through measurements of both axial and transverse response of cylindrical sand sample under confinement. Effects of sand mass density on the constitutive behavior were investigated. The stress–strain relationship was found to follow a power law relationship with the initial bulk density. The Young’s modulus and hardness of individual sand grains were determined by nanoindentation. The sand deformation was observed through sapphire tube using ultra-high speed photography to determine the elastic deformation and compaction behavior. The energy absorption density and compressibility were determined as a function of axial stress. These results can be analyzed further for constitutive modeling and for mesoscale simulations to understand the soil behavior under blast subject to high pressure and high rate deformations.

Luminescent LaF3:Ce-doped organically modified nanoporous silica xerogels

Organically modified silica compounds (ORMOSILs) were synthesized by a sol-gel method from amine-functionalized 3-aminopropyl triethoxylsilane and tetramethylorthosilicate and were doped in situ with LaF3:Ce nanoparticles, which in turn were prepared either in water or in ethanol. Doped ORMOSILs display strong photoluminescence either by UV or X-ray excitation and maintain good transparency up to a loading level of 15.66% w/w. The TEM observations demonstrate that ORMOSILs remain nanoporous with pore diameters in the 5–10 nm range. LaF3:Ce nanoparticles doped into the ORMOSILs are rod-like, 5 nm in diameter and 10–15 nm in length. Compression testing indicates that the nanocomposites have very good strength, without significant lateral dilatation and buckling under quasi-static compression. LaF3:Ce nanoparticle-doped ORMOSILs have potential for applications in radiation detection and solid state lighting.

Mechanical Characterization of Aerogels

Aerogels are multifunctional porous nanostructured materials (e.g., thermally/acoustically insulating) derived from their vast internal empty space and their high specific surface area. Under certain conditions, aerogels may also have exceptional specific mechanical properties as well. The mechanical characteristics of aerogels are discussed in this chapter. First, we summarize work conducted on the mechanical characterization of traditional aerogels, and second, we describe the mechanical behavior of polymer crosslinked aerogels. In polymer crosslinked aerogels, a few nanometer thick conformal polymer coating is applied on secondary particles without clogging the pores, thus preserving the multifunctionality of the native framework while improving mechanical strength. The mechanical properties were characterized under both quasi-static loading conditions (dynamic mechanical analysis, compression, and flexural bending testing) as well as under high strain rate loading conditions using a split Hopkinson pressure bar. The effects of strain rate, mass density, loading–unloading, moisture concentration, and low temperature on the mechanical properties were evaluated. Digital image correlation was used to measure the surface strains through analysis of images acquired by ultrahigh-speed photography for calculation of properties including dynamic Poisson’s ratio. Among remarkable results described herewith, crosslinked vanadia aerogels remain ductile even at −180°C, a property derived from interlocking and sintering-like fusion of skeletal nanoworms during compression.

Observation of the Microstructural Evolution in a Structural Polymeric Foam Using Incremental Digital Volume Correlation

Polymeric structural foams are widely used in many engineering applications due to their exceptional properties including high specific strength and energy absorption. The mechanical properties depend strongly on their microstructures, which also dictate their load-bearing capability under deformation. However, the mechanical behavior of polymer foams in compression is not well understood, due to the complex local deformation and strain characteristics associated with the cellular microstructure. In this paper, unconfined uniaxial compression of a polymeric structural foam was conducted while its microstructure was determined using micro-computed tomography (micro-CT) subjected to large deformations. The detailed local deformations and strains are obtained by using three dimensional digital volume correlations (DVC) method. This incremental DVC allows the use of intermediate bridging images to determine large nonlinear deformations in the foam under compression. The evolution and deformation mechanism of the microstructure are observed during different compression stages using the incremental DVC techniques.

Mechanical Properties of a Human Eardrum at High Strain Rates After Exposure to Blast Waves

The mechanical properties of a human tympanic membrane (TM) or eardrum were characterized at high strain rates after multiple exposures to blast waves. Human cadaveric temporal bones were subjected to blast waves at first, then TM strip specimens were prepared either along the radial or the circumferential direction. A highly sensitive miniature split Hopkinson tension bar was used for tensile experiments on the human eardrum strip specimens at high strain rates. The mechanical properties of the human TMs before and after exposure to blast waves were compared and discussed. The mechanical properties in the time-domain were subsequently converted to the corresponding properties in the frequency domain to investigate the effect of blast waves on the viscoelastic properties. The results indicate that the blast waves have different effects on the mechanical properties in the radial and circumferential directions. After exposure to the overpressure induced by the blast waves, the mechanical behavior in the radial direction in general becomes stiffened, while it is weakened in the circumferential direction. The results could be analyzed further in an ear simulation model to develop understanding of the effect of blast waves on hearing loss.

Incremental Digital Volume Correlation for Large Deformation Measurement of PMI Foam in Compression

We implemented an incremental digital volume correlation (DVC) technique for large nonlinear deformation measurements, and apply the technique for cell deformations in a closed-cell polymethacrylimide (PMI). The DVC technique previously reported in literature has used the natural state as the reference image to calculate all stages of deformation. It does not converge when large nonlinear deformations are involved, as in the case of PMI foam in compression. In this paper, we implemented an incremental DVC technique by updating the reference volume image at the last former stage, in order to achieve the correlation. The incremental DVC technique is verified using simulated rotation test. An unconfined uniaxial compression experiment on a PMI foam was conducted and in-situ microtomography was acquired. The microstructural evolution of the PMI foam was captured during compression; the displacement fields are obtained using the incremental DVC method. The incremental DVC technique can be used to evaluate the microstructural evolution of materials under large nonlinear deformations on images obtained from different 3D imaging techniques, including those from x-ray, laser, ultrasound, magnetic resonance, and neutron sources.Copyright