Downloads & Free Reading Options - Results

This cooperative research program emphasizes collaborative research between The Johns Hopkins University and the US Army Research Laboratory toward well-defined common goals: the understanding and development of advanced lightweight materials for vehicular protection, focusing on high-strain-rate/high-stress testing, determining the operative deformation and failure mechanisms, and developing relevant models to allow for materials design. This report summarizes the research carried out during 1 July 2011 through 30 June 2012 in the following areas: 1) nanomicro aluminum, 2) dynamic failure and damage mechanisms, 3) nanostructured magnesium, 4) modeling of body-centered-cubic nanostructures, 5) high-rate loading of piezoelectric ceramics research thrust, 6) continuum modeling of dynamic deformation mechanisms, 7) density functional theory modeling and transmission electron microscopy characterization of nonstoichiometric oxides and carbides (B6Ox and B4C), 8) administration, education, training, and collaborative structures, and 9) list of theses, publications, and presentations from the various thrust areas.

This cooperative research program emphasizes collaborative research between The Johns Hopkins University and the US Army Research Laboratory toward well-defined common goals: the understanding and development of advanced lightweight materials for vehicular protection, focusing on high-strain-rate/high-stress testing, determining the operative deformation and failure mechanisms, and developing relevant models to allow for materials design. This report summarizes the research carried out during 1 July 2011 through 30 June 2012 in the following areas: 1) nanomicro aluminum, 2) dynamic failure and damage mechanisms, 3) nanostructured magnesium, 4) modeling of body-centered-cubic nanostructures, 5) high-rate loading of piezoelectric ceramics research thrust, 6) continuum modeling of dynamic deformation mechanisms, 7) density functional theory modeling and transmission electron microscopy characterization of nonstoichiometric oxides and carbides (B6Ox and B4C), 8) administration, education, training, and collaborative structures, and 9) list of theses, publications, and presentations from the various thrust areas.

The following report describes a series of research tests to evaluate candidate high temperature materials for static to moderately dynamic hypersonic airframe seals. Pin-on-disk reciprocating sliding tests were conducted from 25 to 843 C in air and hydrogen containing inert atmospheres. Friction, both dynamic and static, was monitored and serves as the primary test measurement. In general, soft coatings lead to excessive static friction and temperature affected friction in air environments only.

Time-dependent properties of uniaxially fiber reinforced materials composed of linear viscoelastic matrix and elastic fibers are investigated. Sample calculations are given for the static and dynamic properties of a viscoelastic fiber reinforced material. In addition, the behavior of fiber reinforced viscoelastic structures is investigated, and a number of practical problems for beams, plates, and shells, subjected to static and dynamic loadings, are analyzed.

The main subject of the research program was the further development of elastic-viscoplastic constitutive equations to represent a wide range of material behavior and loading conditions and application of the equations to static and dynamic structural problems. Associated investigations included generalization of the constitutive equations to multiaxial stress states and studies on anelastic materials. Other topics in the program were investigations of ballistic penetration single target plates under oblique impact and of multi-layered targets subject to normal impact, impact strength of composites, and acoustic emission of composites. The accomplishments of the program are discussed and proposed directions of future research are indciated.

This report presents results of a program to improve understanding of dynamic fracture behavior by investigating both crack tip loading conditions and material properties for fracture at high loading rates. A new test procedure, the one-point-bend test was analyzed and further developed the test a wide range of materials at impact loading rates. The test uses inertial loading to load the crack tip and affords an unambiguous measurement of the dynamic initiation toughness. The test's usefulness in producing small controlled increments of crack extension and in crack propagation studies was also demonstrated. Moreover, the concept of inertial loading was extended to study dynamic mixed- mode crack initiation under controlled mixed mode I and II conditions. These new test techniques were used to establish the dynamic fracture behavior of 4340 steel (HRC 50). It was shown that, for loading times to fracture as short as 20 microsecs, no time-modified criterion is necessary to predict crack initiation. The classical fracture criterion equating the applied stress intensity to the dynamic fracture toughness is adequate, provided the dynamically applied stress intensity is correctly evaluated. Under mixed mode I and II loading with (k sub II)/(k sub I) values between 0 and 0.5, it was demonstrated that dynamic crack initiation is essentially controlled by the mode I stress intensity and by the mode I dynamic fracture toughness.

The characteristics of shock-wave propagation in high purity sand of fine (75-150 m) and coarse (425-500 m) particle sizes in dry statat 65% theoretical density, and water-saturated with 35% void space filled with water, were collaboratively investigated, via controlled uniaxial-strain experiments and meso-scale modelling using CTH. The meso-scale features of the sand were resolved by explicitly incorpoprating 3D grains andvoid space into the computational domain. The method involved characterizing the structure and configuration of sand, as a model granular geological material, and explicitly tracking the effects of evolving material heterogeneities and their interactions with shock waves. It was observed that both dry and wet sand follow linear trends, with little difference in overall response between the two sizes of sand particles.Accounting for different strength values into CTH, provided upper and lower bounds to the experimental data. Results suggest effects of microkinetic energy, chipping on grain surfaces, and plastic deformation dominating the experimental response. These are difficult to incorporate in the simulation models. Experiments also reveal significant reduction in bulk wave speeds, which requires incorporation of multiple mechanisms insimulations.

Structures used by the U.S. Air Force must be designed to resist catastrophic fracture when subjected to dynamic loads. For example, aircraft components may experience short stress pulses from airborne debris, military projectiles, or intense bursts of laser or x-ray radiation. Landing gear and aircraft retaining cables on carrier ships experience dynamic loads at the end of each flight. A related dynamic fracture problem concerns rapdily running cracks. For example, it is often desirable to know whether a crack, once initiated, will arrest before it reaches a component boundary and thereby preserve the integrity of the structure. Thus, to ensure safe design of Air Force structures, it is necessary to have a knowledge of the dynamic fracture behavior of the component materials. The research being conducted in this program is aimed at improving our understanding of dynamic fracture. Emphasis is on the accurate characterization of material resistance to crack initiation under dynamic loading (K sub Id measurements) and to rapid crack propatation (K sub Id measurements). This annual report reviews the specific program objectives and summarizes the progress during the third research year.

This report describes the work performed in the 14-month period beginning August 26, 1977, relating to a basic study of the materials response processes primarily of polymethylmethacrylate PMMA, polycarbonate PC, and epoxy resins during a ballistic impact encounter. Experimental impact data on PMMA, PC, and epoxy have been obtained using 19 and 4.5 mm diameter strikers at impact velocities of 1.77 and 2.93 m/sec. The data have been analyzed. The irreversible work absorbed by the specimen for a given impact energy, the depth of penetration, and the pulse duration increase in the order PMMA < epoxy < PC, but the irreversible energy per unit penetration is the reverse of this series. The apparent crater depth at the time of projectile/specimen separation shows less regular relative behavior. Cinematographic documentation of the impact process was made of PC and PMMA impacted by a 4.5 mm steel ball at an impact velocity of 120 m/sec. The camera speed was 2 x 10(exp 5) frames/sec. Densification could be clearly observed developing in the case of PC, and less distinctly so in the case of PMMA. Evidence for crack formation during the projectile rebound phase was noted for the case of PMMA. Some shrinkage of the densified zone appeared to occur subsequent to the impact. In another experiment, an impact pulse was obtained on a PMMA specimen struck by a 4.5 mm steel ball at 85 m/sec. This result indicates that attention must be given to eliminating or correcting the taper-on effect at the leading edge of the sensed pulse. (MM)

To ensure safe design of Air Force structures, it is necessary to understand the mechanics of high-rate fracture and to have a knowledge of the dynamic fracture properties of component materials. In accord with this need, a research program is being conducted at SRI with the goals of developing a test procedure for obtaining reliable dynamic initiation toughness values and establishing the relationship between dynamic initiation and dynamic propagation toughness. This annual report summarizes the progress and results of the fourth research year. The capabilities of the one-point-bend impact test (1PBT) conceived by Kalthoff et al. and furether develped during previous years of this program were extended in two ways during this fourth research year. By adding ballast plates to the ends of a specimen plate we showed that the 1PBT could be applied to lower density materials and smaller specimens than heretofore possible. Second, we developed a model of the post-instability behavior of the impacted specimen that allows the test to be used to assess crack propagation as well as crack initiation behavior. To compare dynamic initiation and propagation toughness, we performed a series of one-point-bend experiments in which KId and KID were measured on the same specimen. The propagation toughness values were dependent on crack velocity and propagation distance, and were much larger than the initiation toughness. Keywords: Dynamic fracture, Incubation time.

The program focused on a fundamental study of the dynamic response and failure modes of a broad class of conventional and advanced brittle materials. The initial five-year research program addressed materials of current interest to the Army (titanium diboride, titanium carbide, alumina silicon carbide, aluminum, nitride, zirconia, and silicon nitride), as well as novel materials and concepts (nanocrystalline ceramics, laminated and fiber reinforced ceramic composites, functionally graded materials). However, only funding of the first year and partial funding of the second year were received. Hence, limited specific programs were developed in a multi-disciplinary mode, encompassing experimental, analytical, and microstructural characterization components. Through testing and analysis, physically-based constitutive models were developed.

The goals of this research program are first to develop and apply procedures for obtaining more accurate measures of dynamic fracture initiation and propagation toughness, and then to establish the relationship between them. The dynamic initiation toughness K sub Id will be obtained by applying short pulse loads to SEN specimens and using minimum-time crack instability criteria. The dynamic propagation toughness K sub Id will be obtained by measuring the temperature histories in material near the tip of a fast running crack. During the first research year, a Charpy impact machine was modified to produce a well-defined tensile pulse in a SEN specimen. Flat-topped pulses of a shape suitable for K sub Id determinations were obtained that had rise times of about 18 microseconds and durations of 50 microseconds and 100 microseconds. A high-spatial-resolution temperature measuring instrument, based on laser thermoprobe concepts, was constructed and evaluated for use in K sub ID determinations for fast running cracks. By monitoring the change in intensity of a monochromatic light beam passing through a thin CdS film, we could measure the temperature of the film substrate to 0.25 cover a range of 150 C. Thus the technique appears suitable for dynamic fracture investigations.

The objective of this research is to obtain response and constitutive characterization for Concrete and mortar at strain rates up to 105/s, to analyze the evolution of load-carrying and energy absorption capacities, and to understand deformation and failure mechanisms under high pressures. The high rate failure mechanisms considered include fragmentation, comminution and granular flow. Our investigation has focused on (1) the dynamic response of the G-mix concrete under impact conditions; (2) the effect of composite microstructure and aggregate reinforcement on the material stress-carrying capacity; and (3) the failure wave phenomenon in mortar under uniaxial-strain impact loading. Experiments and numerical simulations have shown that while the quasistatic uniaxial strength for the concrete is approximately 30 MPa, the average compressive stresses carried by the concrete under the conditions of impact experiments involving elastic steel target plates and impact velocities between 290 - 330 /ma is on the order of 1600 MPa. The marked increase in stress is attributed to the effect of higher strain rates which are on the order of 104/s and to the effect of lateral confining stresses. Experiments also showed that the stresses carried by mortar (matrix phase in concrete) under the same conditions are approximately 1200 MPa or 75 % of that for the concrete (approximately 1600 MPa). Due to the composite microstructures and its relatively coarse aggregate size, the deformation and stresses are nonuniform inside the concrete.

Dynamic loading of brittle materials is related to many applications, including explosive excavation of rocks, design of ceramic armor, meteor impact on spacecraft windows, particle damage to turbine blades, etc. When brittle materials are exposed to impact or explosive loading, inelastic deformation, fracture, and fragmentation occur under conditions of three-dimensional stress, where at least one stress component is compressive. Unlike fracture and fragmentation of brittle materials under tensile loading, response to compression is far from understood. Rapid application of loads can also introduce rate dependencies which make the analysis even more complicated. This review report is mainly focused on compressive failure. We have collected, reviewed, and assessed information on the behavior and properties of brittle materials that are of greatest current interest, namely polycrystalline ceramics, glasses, and rocks.

The objective of this work aimed at understanding the dynamic response of brittle materials under high rates of loading which are typical of high speed impact and shock generation. Three areas of investigation are covered, namely. (1) the dynamic strength of ceramics in uniaxial compression but under lateral confinement, (2) the propagation of stress waves through fractured but contiguous materials, and (3) shock-induced gas-producing reactions. The results of these studies indicate: The compression strength of ceramics under high speed impact is systematically and materially increased with progressive lateral constraint or lateral stressing. Wave propagation through fractured solids can be assessed through optical interferometry with high-speed photography. It is suggested that under shock conditions the Rayleigh-Taylor instability of interfacial mixing plays a dominant role in potential chemical processes that are shock induced.

Silicon nitride is a potential candidate for several high temperature structural applications, like heat engine components, cutting tool inserts, bearings, and wear parts. In use, the ceramic component is often subject to dynamic loading conditions at elevated temperatures. Hence, it is necessary to evaluate the performance of the material under dynamic loading conditions over a range of temperatures. This research project focused on the behavior of in situ reinforced silicon nitride, under dynamic compressive loading, over a range of temperatures. Novel experimental (the Split Hopkinson Compression Bar) and analytical (Differential Strain Measurement) techniques have been developed to aid the evaluation of the dynamic properties of in situ reinforced silicon nitride over a range of temperatures. The properties (primarily, the elastic modulus and the failure stress) of the material has been studied from room temperature to 1000 deg C. The experimental work has been supplemented by microstructural studies, both before and after testing, in order to gain an insight into the mechanisms of failure.

The work reported was devoted to making reliable, quantitative impact pulse measurements for free-flight encounters between a spherical projectile and the desired target material. The materials of interest continue to be polycarbonate PC, polymethylmethacrylate PMMA, and epoxy resins. Inorganic glass was also used as a target material because impacts can be confidently modeled from theory in this case. Impact pulse determinations were made on the four target materials with hardened steel strikers of 4.5 mm diameter tips weighing 42 g and of 19 mm diameter weighing 28.3 g usually at 2.5 m per second. The impact strain pulse shapes are increasingly narrower and higher peaked in the order PC, epoxy, PMMA, glass. Semi-quantitative analysis of the general shapes indicates little irreversible deformation work is performed during impacts involving 19 mm striker. However, using the 4.5 mm striker causes irreversible absorption of the net energy received from the striker. The fractional amount absorbed in this series of measurements decreased in the order PC, epoxy, PMMA, and glass. Computer procedures were developed for analyzing and correcting the detailed pulse shapes to reveal how the force and penetration are related and to follow the evolution of the various energy and momentum contributions.

We report a comprehensive computational and experimental program investigating the characteristics and dynamic behavior of nano-size aluminum in novel energetic materials. For the computational portion, we employed (1) our in-house packing code, Rocpack, to generate morphologies of interest for heterogeneous propellants; (2) modified our solid rocket motor CFD code, Rocstar, to include appropriate models for chemistry, radiation, and nano-sized aluminum; and (3) simulated transient behavior of the propellants at rocket motor conditions. For the experimental portion, we (4) used the mixing facility at Purdue University to prepare appropriate propellant samples containing nano-sized aluminum, thus allowing for a full characterization of the propellant (composition; particle size distribution) necessary for a coordinated modeling and experimental program; and (5) carried out both steady and unsteady experiments to fully characterize the propellants. Our R&D program investigated the applicability of the Rocstar simulation framework for novel energetic high-density propellants, sought to establish the feasibility of an experimental validation program for these propellants, and provides a firm a foundation for commercializing the simulation software. The simulations were performed at lllinoisRocstar LLC and the experiments were performed at Purdue.

Results are presented of a three year experimental and theoretical study dealing with wave propagation in particulate materials. The experimental program used dynamic photoelasticity, strain gages and fiber optic sensors to collect information on model granular materials. The theoretical program developed a numerical model using discrete element methods (DEM) to simulate the dynamic behavior of granular media. The general goal of the research program was to determine relationships between granular fabric and wave propagational characteristics. DEM simulations on large two-dimensional systems with circular particles, investigated relationships of branch, wave and void vector fabric with wave speed and attenuation. Both experimental and numerical studies were conducted on model assemblies with elliptical particles.

State-of-the-art technology regarding the dynamic behavior of non- metallic materials and structures is reviewed in sixty conference papers. Topics covered include: loading and testing of the materials and structures under a variety of stress conditions (blasts, shocks, vibrations, thermal, strain, high velocity impacts, compression properties, etc.), and impact, spallation, and fracture resistance of materials. Various applications of the materials and studies for military weapons, vehicles and other structural uses are described. (LS-PL).

A comprehensive and multidisciplinary research program centered on the fundamental study of the dynamic response and failure modes of a broad class of conventional and advanced ductile materials. This research program is founded on four important areas: a) the unique and novel experimental facilities for dynamic testing established at UCSD; b)a balanced multidisciplinary program with investigators of combined expertise and prominence in the fields of materials, mechanics and computational and experimental methods; c) a continuing interaction with U.S. Army research laboratories (primarily, but not exclusively, BRL, MTh, and ARDEC) which serves to maintain the relevance of the program, transfer to the Army its most significant accomplishments, and provide a vehicle (forum) for the continuing training of U.S. Army scientists; d) a vital and productive educational environment for future scientists in the field of dynamic behavior of materials. The research program addresses materials of current interest to the Army (tungsten, tungsten composites, tantalum, advanced steels and copper), as well as novel materials and concepts (nanocrystalline metals, and metal-matrix composites.

The goals of this project are to develop and apply procedures for obtaining more accurate measures of dynamic fracture initiation and propagation toughness and then to establish the relationship between them. We obtain the dynamic initiation toughness, K sub Id, by impacting unsupported edge-cracked bend specimens and using minimum-time crack instability criteria, and we obtain the dynamic propagation toughness, K sub ID, by measuring the temperature histories in material near the tip of a fast running crack. The research efforts on dynamic crack initiation and crack propagation will continue in the next two years and several structural alloys will be investigated.

An experimental study of the breakup of aerated liquid jet in subsonic crossflow was carried out. The test conditions were as follows: jet exit diameter of 1 mm, GLR (gas to liquid ratio) of 8%, and jet-to-free stream momentum flux ratio of 0.74. Digital double-pulsed holograms were recorded at x/do = 0 to 25 in the cross stream direction, y/do = 0 to 27 in the stream wise (injection direction), and z/do = (-13) to 13 in the span-wise direction. Digital double-pulsed holographic microscopy (DHM) was utilized using double exposure 2048x2048 pixels CCD sensor. The field of view of all holograms was 9 mm x 9 mm, and the spatial resolution was 5 m. To overcome this small field of view, three-dimensional spray maps were constructed by patching several high resolution holograms. Measurements include droplets locations, drop sizes and sphericity, and three-dimensional velocities. The distributions of the drop sizes could be fully described by the SMD alone and followed Simmons' universal root-normal distribution.

A novel interfacial joint was developed for reducing the interfacial stress levels. The proposed design, inspired by the shape and mechanics of trees, effectively removed the stress singularity at the interfacial joint for most engineering materials through an integrated theoretical and experimental investigation. Significant tensile loading capacity increase was obtained (up to 81%) using this new joint, while the material volume of the new joint actually was reduced. Dynamic tension experiments showed that the new convex joint yielded an increase in final failure strength (22%). This new joint can be employed to accurately evaluate the interfacial strength improvement of dissimilar material joints. Nanofiber-reinforced epoxy bonding with linker molecules was synthesized and tested for metal/metal and polymer/polymer joints. Mechanical properties including tension and shear bonding strengths showed very low increase or even decrease of nanocomposite bonding over that of pure epoxy bonding.

Dynamic, high strain rate loading characterization of two composite materials that are being used today in the design of military and commercial aircraft was accomplished. Of particular concern was the response of such materials to nuclear blast and thermal environments. Primary emphasis is placed on the graphite epoxy designated AS/3501-6, as would be used in body or wing panels. Of secondary emphasis is the quartz polyimide designated F178/581, a radome material. Both materials were tested quasistatically and dynamically (strain rates up to 18 inches/in/sec), and from -65F to above resin cure temperature (Cure temperatures of 350F for the graphite epoxy and 475F for the quartz polyimide). Test results indicated that both materials were stronger under dynamic loads than quasistatic loads. Additionally, at elevated temperatures the responses were dramatically different with the dynamic properties exhibiting little or no degradation due to temperature effects, while quasistatic properties decreased significantly with temperature. (Author)

The goal of the program was to characterize and quantify physical processes involved in failure of brittle materials under impact loading conditions, and to synthesize results to identify properties which determine the effectiveness of a system (material and configuration) in resisting failure. This was accomplished by developing new methodologies which integrate concepts in mechanics and materials science. Among the accomplishments are: determining the pressure-shear response of alumina powders and of soda-lime glass at very high strain rates; investigating failure waves in glasses by using pressure-shear impact; measuring the pressure-shear response of pure tungsten carbide at temperatures up to 700 degrees centigrade; developing an analytical/computational theory of fragmentation based on a cohesive zone model; developing an experimental method to characterize the fragmentation response of ceramics; measuring dynamic fragmentation characteristics for alumina and silicon nitride; developing an experimental method to study biased damage evolution and projectile deflection in penetration processes.