The project elaborated in frame of engineering studies branch M-STG describe characteristics of plasticity of non-anneal materials E 235, E 190 and E 220. Materials characteristic was receive by static and dynamical tests. Based on the literature study it was conducted survey of the current state of experimental techniques for high-speed deformation. The materials were designed by Holomon approximation of rigid-plastic material model without hardening. It was conducted force analyses for machine Unison MG 2790 for rewind bending and bend with compressive force considering the identified material model. It was describe the impact passive and active forces to move the neutral axis.

This work analyzes the design and implementation of waveguides used to measure stress waves in solid mechanics via explicit finite element analysis and experimentation. Many areas of physics use waveguides where control of timing, location, or frequency of waves is imperative to functionality of a system. Split Hopkinson pressure bars (Kolsky bars) traditionally utilize straight waveguides during testing. Prior research produced the first bent wave guide for use in such an application, the coaxially embedded serpentine bar (CESB). Explicit finite element analysis (FEA) provides a modeling approach to understand the effects of pass and joint geometry and boundary conditions on the functionality of solid-mechanic waveguides like the CESB. FEA and experimentation also contrasts the functionality of welded joints and threaded joints. Novel waveguide designs that do not feature tubes are also detailed for use in dynamic mechanical testing and dynamic hardness indentation experiments. These designs feature acoustic lengths up to two orders of magnitude greater than their physical lengths.

The object of this study was to investigate the micro-structural response of DP 600 subjected to high strain rate, ballistic impact tests. The ballistic tests were conducted using normal impact of a hardened steel penetrator into a 2 mm thick sheet of DP 600. The average strain rates produced from this test method are on the order of 10^5 s-1. Multiple methods were used to investigate the micro-structure before and after high strain rate deformation including optical microscopy, electron microscopy, and X-ray diffraction. A large variation in material response was observed between tests conducted at 0.8 x 10^5 and 2.5 x 10^5 s-1.

No description available.

No description available.

In the laser-induced cavitation (LIC) technique, a vapor-gas cavity is generated in water, or a soft material by focusing an intense laser pulse into the sample. The high-strain-rate mechanical properties of these samples can be investigated through a real-time size measurement of the expanding cavity bubble. Although this LIC technique has been applied to multiple research fields such as mechanical, biological and medical areas. It is possible to simplify and improve this LIC method by introducing optical-fibers. In this approach, we propose to employ an optical-fiber to deliver the intense laser pulse to an arbitrary position of an optical opaque specimen. At the same time, we also attempt to generate LIC at one end of the optical-fiber. This optical-fiber based LIC is achieved by dip-coating of the laser absorbing film on the fiber end. Thus, the film can absorb the laser pulse and generate LIC within the sample. In this study, the development of the coating material, the introduction of the optical-fiber into the existing LIC system, and the optical-fiber based LIC experiments are performed to characterize high-strain-rate mechanical properties of soft materials. We investigate the coating conditions and verify the consistency of the ablation based on the optimized coating materials. By conducting LIC experiments with gelatin samples, the feasibility of developed LIC method is investigated, LIC events are successfully formed at the fiber end which is inserted into the sample, and the rapid expanding dynamics are imaged with ultrafast stroboscopic microscopy. Using the multiple-exposure images, the expanding speeds and maximum cavity sizes are quantified to provide high-strain-rate characteristics of the soft materials. The inconsistency of the cavitation behavior resulted by the fluctuation of the coating condition and the high power intense laser conducting optical-fiber destruction can be improved by developing new coating method and new protective coating on the fiber end in the future.

Free-Fall Penetrometer (FFP) testing is an easy and rapid test procedure for seabed sediment characterization favorable to conventional geotechnical testing mainly due to its cost-effectiveness. Yet, FFP testing results are interpreted using empirical correlations, but difficulties arise to understand soil behavior under the high-strain rate (HSR) loading effects during rapid FFP penetration. The numerical simulation of FFP-soil interaction is also challenging. This study aims to numerically analyze FFP testing of saturated loose sands using the particle-based Material Point Method (MPM). The numerical analysis was conducted by simulating calibration chamber FFP tests on saturated loose quartz sand. The numerical results using quasi-static properties resulted in a reaction of the sand softer than the actual calibration chamber test. This implied the necessity of considering HSR effects. After performing parametric analyses, it was concluded that dilation plays an important role in the response of sand-water mixtures. Comparison of dry and saturated simulations showed that FFP penetration increases when the soil is dry and tends to develop a general bearing capacity failure mechanism. This is because the pore water increases the stiffness of the system and due to the increased strength that develops in saturated dilative sands when negative pore pressures develop. Local bearing failure mechanism is observed in all saturated simulations. Finally, numerical CPT (quasi-static) and FFP tests were used to examine the strain rate coefficient used in practice (K); and a consistent range between 1 to 1.5 was obtained. / Master of Science / Accurate characterization of seabed sediments is crucial to understand sediment mobilization processes and to solve nearshore engineering problems such as scouring around offshore structures. Its portability, low testing effort, and repeatability make FreeFall Penetrometer (FFP) testing a highly cost-effective sediment characterization test. Nevertheless, due to the complex penetration mechanism of FFPs in soils (e.g., high-strain rate effects due to rapid FFP loading), converting FFP output into practical information is complicated, and it heavily relies on empirical correlations. This thesis presents a numerical analysis of FFP testing on saturated sand using the Material Point Method. First, the simulation results were compared with laboratory tests. Later, a parametric study was performed to understand the effect of different material parameters on the FFP response and to highlight in a simplified manner the effects of rapid loading on the sand behavior. Additional simulations in dry sand (without water) revealed that dry conditions provide larger FFP penetrations than saturated ones for the same material parameters. Lastly, the strain rate coefficient, which is a parameter required in one of the most common empirical methods for converting FFP output into geotechnical parameters, was back-calculated. The results were consistent with values used in practice for similar conditions.

The split Hopkinson bar test is the most commonly used method for determining material properties at high rates of strain. The theory governing the specifics of Hopkinson bar testing has been around for decades. It has only been the last decade or so, however, that significant data processing advancements have been made. It is the intent of this thesis to offer the insight of its author towards new advancements. The split Hopkinson bar apparatus consists of two long slender bars that sandwich a short cylindrical specimen between them. By striking the end of a bar, a compressive stress wave is generated that immediately begins to traverse towards the specimen. Upon arrival at the specimen, the wave partially reflects back towards the impact end. The remainder of the wave transmits through the specimen and into the second bar, causing irreversible plastic deformation in the specimen. It is shown that the reflected and transmitted waves are proportional to the specimen's strain rate and stress, respectively. Specimen strain can be determined by integrating the strain rate. By monitoring the strains in the two bars, specimen stress-strain properties can be calculated. Several factors influence the accuracy of the results, including longitudinal wave dispersion, impedance mismatch of the bars with the specimens, and transducer properties, among others. A particular area of advancement is a new technique to determine the bars dispersive nature, and hence reducing the distorting effects. By implementing numerical procedures, precise alignment of the strain pulses is facilitated. It is shown that by choosing specimen dimensions based on their impedance, the transmitted stress signal-to-noise ratio can be improved by as much as 25dB. An in depth discussion of realistic expectations of strain gages is presented, along with closed form solutions validating any claims. The effect of windowing on the actual strains is developed by analyzing the convolution of a rectangular window with the impact pulse. The thesis concludes with a statistical evaluation of test results. Several recommendations are then made for pursuing new areas of continual research. / Master of Science

The split Hopkinson pressure bar (SHPB) continues to be one of the most common methods of testing materials at medium rates of strain. Elevated rates of strain, such as those found in impact and explosive applications, have been shown to induce phenomena such as strain hardening and phase transitions that can significantly affect the strength of most materials [14]. Due to its relative simplicity and robustness, the SHPB remains one of the preferred platforms for evaluating mechanical properties of materials at rates of strain up to approximately 104 in/in-s (s-1). At the Naval Surface Warfare Center Dahlgren Division (NSWCDD), research has been conducted in which a semiconductor laser diode has been used to measure the radial strain of a plastically deforming cylindrical test specimen in the SHPB. The SHPB consists of two long, slender cylindrical bars, denoted input and output bars, that "sandwich" a cylindrical test specimen. Utilizing a high-pressure gas gun, a third cylindrical steel bar, known as the striker bar, is fired at the input bar, causing a compressive stress wave to travel through the input bar to the input bar - test specimen interface. At this interface, a portion of the stress wave propagates through the test specimen while the remainder of the pulse reflects back through the input bar as a tensile stress wave. The non-reflected portion of the stress pulse transmits through the test specimen and into the output bar causing the specimen to deform both elastically and plastically. Strain gages mounted to the input and output pressure bars measure both the incident, transmitted and reflected pulses. Specimen stress can be calculated using the transmitted strain signal while specimen strain and strain rate can be computed using the reflected strain pulse. In order to measure the specimen strain directly, a 670-nm wavelength semiconductor laser diode was affixed to the SHPB such that a vertical line of light approximately 250 micrometer (µm) wide was generated across the diameter of the test specimen. A collector lens located aft of the specimen was positioned to collate the light not occluded by the diameter of the specimen and refocus the light to be collected by a 25 MHz photodetector. Thus, changes in specimen diameter due to the impact event would result in more light being occluded by the specimen and less spectral energy being collected by the photodetector. The light collected by the photodetector is then converted to a voltage output before being recorded by a digital storage oscilloscope. With a known voltage-to-diameter calibration relationship, medium strain rate compressive tests were conducted to compare the optically measured strain results with the data gathered with the existing strain gages. It was found that the optical measurement system provided increased bandwidth and greater resolution than the conventional strain gage instrumentation while generating strain and strain rate results within 6.7% of corresponding strain gage data. This increased bandwidth and resolution allows the identification of both the elastic and plastic behavior of the specimen. In addition, the loading and unloading of the specimen can be clearly seen in the optical strain signal. These phenomena are evident in the peak diameter and strain achieved by the specimen, data not previously available with strain gage instrumentation. The plastic modulus, the theoretical relationship between the stress and strain in the plastic regime, also exhibits a significant increase in magnitude due to this ability to measure peak rather than average strain. Finally, by ridding the experiment of the input bar strain gage, input bar dispersion and the electrical and mechanical errors associated with the input bar strain gage were nullified. These conclusions will be validated through the presentation of several sets of experimental data correlated to data gathered previously. / Master of Science

Structural energetic materials (SEM) are a class of multicomponent materials which may react under various conditions to release energy. Fragmentation and impact induced reaction are not well characterized phenomena in SEMs. The structural energetic systems under consideration here combine aluminum with one or more of the following: nickel, tantalum, tungsten, and/or zirconium. These metal+Al systems were formulated with powders and consolidated using explosive compaction or the gas dynamic cold spray process. Fragment size distributions of the indicated metal+Al systems were explored; mean fragment sizes were found to be smaller than those from homogeneous ductile metals at comparable strain rates, posing a reduced risk to innocent bystanders if used in munitions. Extensive interface failure was observed which suggested that the interface density of these systems was an important parameter in their fragmentation. Existing fragmentation models for ductile materials did not adequately capture the fragmentation behavior of the structural energetic materials in question. A correction was suggested to modify an existing fragmentation model to expand its applicability to structural energetic materials. Fragment data demonstrated that the structural energetic materials in question provided a significant mass of combustible fragments. The potential combustion enthalpy of these fragments was shown to be significant. Impact experiments were utilized to study impact induced reaction in the indicated metal+Al SEM systems. Mesoscale parametric simulations of these experiments indicated that the topology of the microstructure constituents, particularly the stronger phase(s), played a significant role in regulating impact induced reactions. Materials in which the hard phase was topologically connected were more likely to react at a lower impact velocity due to plastic deformation induced temperature increases. When a compliant matrix surrounded stronger, simply connected particles, the compliant matrix accommodated nearly all of the deformation, which limited plastic deformation induced temperature increases in the stronger particles and reduced reactivity. Decreased difference between the strength of the constituents in the material also increased reactivity. The results presented here demonstrate that the fragmentation and reaction of metal+Al structural energetic materials are influenced by composition, microstructure topology, interface density, and constituent mechanical properties.