Sub-diffraction limit resolution in microscopy

A method and apparatus for visualizing sub-micron size particles employs a polarizing microscope wherein a focused beam of polarized light is projected onto a target, and a portion of the illuminating light is blocked from reaching the specimen, whereby to produce a shadow region, and projecting diffracted light from the target onto the shadow region

Selection of loading profiles in kolsky bar experiments

During a Kolsky bar experiment, stress equilibrium and strain rate constancy conditions need to be satisfied to directly obtain rate-sensitive material properties. Stress equilibrium indicates the stress distribution along specimen thickness is uniform. Constant strain rate means the strain rate is relatively constant during most of the loading duration. Satisfaction of these two critical conditions depends on the bars/sample impedance ratio and the sample loading condition. In this analytical and experimental study, bilinear incident pulses with various shapes are generated with different bars/sample impedance ratios. The effects of loading pulse profile and bar/sample impedance ratio on the time to achieve stress equilibrium and strain rate constancy will be presented and discussed

High Strain Rate Experiments of Energetic Material Binder

Energetic materials, in particular HMX, is widely used in many applications as polymer bonded explosives (PBX) and rocket propellant. However, when damaged, HMX is known to be an unstable substance which renders it a hazardous material and in some cases unreliable. Finding critical mechanical conditions at high rates that render various forms of energetic materials as unreliable would be vital to understand the effects that vibrations and compression forces have on energetic materials. A better understanding would enable the ability to develop improvements in the manufacturing of PBX and rocker propellant. The method utilized to evaluate the mechanical properties of the material involved a compression Kolsky bar where a projectile hits an incident bar at 5 meters per second. The incident bar then compresses a binding polymer specimen composed of Sylgard 184 at the other end. Strain gauges were applied to the incident bar to measure voltage changes due to strain. In addition, a load cell was placed behind the specimen to measure compression force histories. The specimens studied were varied to evaluate correlation between composition and mechanical behavior. The results from the experiments showed that the binders with a lower mixing ratio of base to curing agent made the bonding polymer stiffer and less prone to elastic deformation. The results also unveiled that the stiffer binder experienced a higher compression stress due to it’s limited elastic deformation. The results also show that, at the strain rates studied, none of the binders failed. However, the measured results provide insight to manufacturers to select proper binder for specific loads. Further research of the compression force on HMX within Sylgard 184 is needed to delineate whether a stiff or ductile binder is more reliable for PBX

Characterization of Mechanical Properties Displayed in Body Armor Ballistic Fibers

The current body armor systems manufactured using ballistic fibers are not performing as theoretical results predict and are causing injuries. The actual maximum projectile penetration speed that the body armor can endure is significantly lower than the theoretical maximum speed, thus causing a costly build-test relationship that is not aided with modeling design efforts. The main aim of this research is to determine the maximum penetration speeds for ballistic yarns and fabrics. Secondary aim is to examine the common assumption that during transverse impact, single fiber is under pure tension and shear stress is negligible. To examine aforementioned assumption, fibers are tested under quasi-static state with 810 Material Testing System. Different types of fibers are tested in various angles with different Fragment Simulation Projectiles (FSP). A total of seven types of fibers are tested including Kevlar and Dyneema which are the two of the most commonly used materials in body armor systems. The results from static experiments show that the failure strain is significantly affected by the impact angle which indicates the presence of shear stress during impact. The failure strain decreases with increasing impact angle and rate of decrease is maximum for FSP followed by round projectile, with blade projectile showing the lowest rate of decrease. Therefore, the shear stress in ballistic fibers must be accounted for theoretical predictions of penetration velocity for fabric armor systems

Dynamic Analysis of Granular Battery Assembly

Collisions and impacts are an inevitable reality when it comes to commercial cars and vehicles, but as batteries become more prevalent it is vital to understand how they react dynamically and as a system to better protect both the passengers and toxic chemicals inside of the batteries. One potential solution is to surround these batteries with non-vital tubes that can be sacrificed and help protect the batteries. This study is done to understand the structure of batteries and to test how an assembly of sacrificing cells and batteries reacts in an impact situation to see if there is significant energy absorption to justify their use. Several methods were used to analyze the structures of both batteries and sacrificing tubes. First, the individual batteries and tubes underwent both static and dynamic testing. The force versus deflection was then found and from that the energy absorption could be calculated. It was found that the battery assembly absorbs significantly more energy than the base system; however, the thickness of the sacrificing tube plays a big role in the impact. If the tubes are too thin relative to the forces applied, then they are too easily deformable and act as a solid material. On the other hand, if they are too thick, then they will not deform fully. So it was found that depending on the forces involved, different tube thicknesses should be chosen. Additional work can be done on different assemblies at different forces and energies

Experimental Study of Breakage of Particles under Compression

Granular materials are used widely and can be seen in natural and industrial applications such as sand bags or pharmaceutical pills. During their manufacturing, processing, transport and use, granular materials are subjected to various kinds of loadings. If the amplitude of the loading is above the strength threshold, particles constituting granular materials may fracture. It is very important to understand the failure of particles under these loading conditions to prevent or control their failure during all stages of their manufacturing and use. Better characterization of the fracture behavior of particles composed of different materials and sizes will allow more precise application and better maintenance of granular materials in commercial usage. The effects of size and material properties on the deformation and fracture behavior of granular particles are studied by investigating particles from three different size ranges for three different materials. The mechanical behavior is characterized by force-displacement and stress-strain plots under quasi-static compression (strain rate = 10-2s-1). Along with the deformation behavior, the strengths of particles are also recorded and Weibull distribution is fitted to the fracture stresses. It was observed that the smaller particles break at lower forces but actually withstand higher stress at fracture. The calculated Weibull moduli for different size range and materials show that the flaw population from the manufacturing process is different for different sizes and materials. This study shows that size and material properties alter the fracture stresses. Future experiment can be performed for the same particles under dynamic compression to better understand effects of strain rate on the fracture of particles

Dynamic Response of Textile Material under Transverse Impact

Textile materials, such as Dyneema and Kevlar, are the major raw materials for state of the art military or personal security armor vests. However, in impact experiments, actual observed penetration speed is much lower than theoretically predicted penetration speed. Each armor vest is composed of high performance yarns which are woven together to form fabrics, which when stacked together form a vest. Understanding penetration behavior of yarns is essential to evaluate the performance of fabric, which will be useful for the design of better vests. The project is composed of three parts: static experiments, dynamic yarn experiments and dynamic fabric experiments. In the static experiments, several types of textile materials will be loaded onto MTS testing machine under slow and constant speed by different projectiles, such as Fragment Simulating Projectile, Hemispherical Nose Projectile and Blade Projectile. Secondly, a powder gun will be used to provide high speed impact conditions. Several yarns will be impacted at high velocities and imaged simultaneously using a high speed camera. Finally, aforementioned experimental conditions will be utilized for fabrics experiments. At this preliminary phase of the investigation, only expected results are being reviewed. In the yarn experiments, impact angle, between impacted region (shear wave propagation region) and impacting region (transvers wave propagation region), is expected to be approximately constant. In the fabric experiments, the goal is to acquire the range of the penetration speeds for different types of textile materials with different number of layers. The acquired data will yield a strong background database for further improvement and adjustment in personal vest design

Experimental assessment of fracture of individual sand particles at different loading rates

Failure mechanisms in individual sand particles under compressive loading at different loading rates were -investigated using X-ray imaging. High speed X-ray phase contrast imaging was utilized to study the damage mechanisms in dry and wet sand under dynamic compressive loading. A modified Kolsky bar setup was used to apply controlled dynamic compression on two contacting sand particles. One of the particles was observed to pulverize, whereas other particle remained intact for dry sand particles with average failure load of 34.344 N. In wet conditions, one of the particles was observed to break into large subparticles which pulverized upon further loading. Other particle was observed to stay intact. The failure load was observed to increase to 65.466 N for wet particles. 3D X-ray tomography was used to assess the failure of dry sand particles under static compressive loading. One particle broke into large subparticles which subsequently pulverized under static compressive loading. Even under static loading, second particle did not fail until first particle was completely pulverized. The pulverization load under static compressive loading was observed to be 42 N. The order of pulverization for the particles was observed to be random in all experiments

Simultaneous X-ray diffraction and phase-contrast imaging for investigating material deformation mechanisms during high-rate loading

Using a high-speed camera and an intensified charge-coupled device (ICCD), a simultaneous X-ray imaging and diffraction technique has been developed for studying dynamic material behaviors during high-rate tensile loading. A Kolsky tension bar has been used to pull samples at 1000 s(−1) and 5000 s(−1) strain-rates for super-elastic equiatomic NiTi and 1100-O series aluminium, respectively. By altering the ICCD gating time, temporal resolutions of 100 ps and 3.37 µs have been achieved in capturing the diffraction patterns of interest, thus equating to single-pulse and 22-pulse X-ray exposure. Furthermore, the sample through-thickness deformation process has been simultaneously imaged via phase-contrast imaging. It is also shown that adequate signal-to-noise ratios are achieved for the detected white-beam diffraction patterns, thereby allowing sufficient information to perform quantitative data analysis diffraction via in-house software (WBXRD_GUI). Of current interest is the ability to evaluate crystal d-spacing, texture evolution and material phase transitions, all of which will be established from experiments performed at the aforementioned elevated strain-rates