Search

The angle between the line-of-fire and the rotation axis of a projectile is called the yaw angle. For fin and spin stabilized projectiles the yaw angle changes continuously during flight. Normally the yaw of a projectile is measured by orthogonal flash imaging of the projectile. However, this method provides only the yaw at a certain position, often at a certain distance

TNO developed an alternative, more configuration independent ceramic test method than the Depth-of-Penetration test method. In this alternative test ceramic tiles and ceramic based armour are evaluated as target without a semi-infinite backing layer. An energy approach is chosen to evaluate and rank the target materials penetration resistance. The alternative testing method for ceramic based armour uses a high speed camera technique to determine residual velocity of target material fragments and of projectile remains. The residual mass of the projectile is determined after capturing the residual projectile in a water basin. Analysis of the projectile core fragments after the impact event, provides valuable information on the status of the core and mass of its fragments. Subtracting the residual kinetic energy from the initial energy of the core of an AP round, the energy absorption of the projectile-target interaction process is obtained with a limited shot-to-shot variation. This provides an accurate and objective performance parameter for the ceramic or ceramic based armour. The test method and analysis procedure are described in this paper. Results on ceramic and ceramic based armour are presented and discussed.

Several specially designed experimental techniques including an alternative test method have been developed for the evaluation of ceramic based armor. Armor grade ceramics and a range of combined materials have been tested using 7.62 AP rounds. Using the energy method [12] the dwell-time and total energy absorbed from the AP core were determined. In additional tests time-resolved fracturing of the ceramic tile (fragments) was recorded using high-speed video at one million frames per second. Also the particle size distribution of the fragments were measured in order to determine the total fracture surface area. The information provided by the results of all tests has resulted in an energy-based engineering model that allows calculation of the dwell-time, erosion and residual velocity of an AP-core. The model predicts the mass and velocity of residual AP cores rather well assuming a failure period during which the intact ceramic material transfers into a massively broken medium. The model does not require detailed mechanical properties of the ceramic materials. This reflects the difficulty within the ceramic armor research community to find a relation between mechanical properties and ballistic efficiency of armor ceramics. The developed engineering model creates a renewed understanding of the relevant phenomena, that could explain the ballistic efficiency of ceramic armor.

TNO has developed an alternative, more configuration independent ceramic test method than the standard Depth-of-Penetration test method. In this test ceramic tiles and ceramic based armor are evaluated as target without a semi-infinite backing layer. An energy approach is chosen to evaluate and rank the target materials penetration resistance. By measuring the armor material’s energy absorption, subtracting the residual projectile energy after penetration from the projectile energy before impact, an objective performance parameter for the ceramic or ceramic based armor is obtained. However, this parameter is still related to the specific projectile used in the test. The presented alternative testing method for ceramic based armor uses a high speed camera technique to determine residual velocity of target material fragments and of projectile remains. The residual mass of the projectile is determined, capturing the penetrated projectile (and ceramic fragments) in a water basin. Analysis of the projectile remains after the impact event, provides valuable information on the two projectile to target interaction stages, dwell and penetration. The test method and analysis method are described in this paper. Results on ceramic and ceramic based armor are presented and discussed.

At TNO mind sized engineering models are created for specific penetration conditions. The models are energy based and calculate the energy absorbed by target deformation (strain energy) and displacement (kinetic energy). The input parameters are restricted to basic target material properties (density, Young’s modulus), impact velocity, mass and the dimensions of projectile and target. In this paper an engineering model for impact of blunt projectiles on metallic sheets is presented. During blunt projectile impact a shock phase precedes the global deformation of the target. It results in increasing membrane and shear deformation of the target, while the projectile deforms and is retarded. These effects are included in the model. When the conditions for shear- and/or tensile failure of the target material are met, the target fails and is penetrated. Calculation results on projectile deformation and penetration behavior are compared with experimental results.

In order use material models in (numerical) calculations, the mechanical properties of all materials involved should be known. At TNO an indirect method to determine the dynamic flow stress of materials has been generated by a combination of ballistic penetration tests with an energy-based engineering model. For visco-elasto-plastic materials, like clay, not only the dynamic flow stress is strain rate dependent, but also its Young’s modulus. Experiments using 7.62x39 MSC bullets have been performed on Roma Plastilina nr. 1 at several target temperatures. The dynamic flow stress and young’s moduli have been determined using an adjusted ductile hole growth engineering model for weak materials.

The fragmentation of ceramic tiles under ballistic impact has been studied. Fragments and aerosol (respirable) particles were collected and analyzed to determine the total surface area generated by fracturing (macro-cracking and comminution) of armor grade ceramics. The larger fragments were collected and the mass distribution was obtained from sieving. The aerosol particles were collected using a particle impactor device that counts the number of particles of specific ranges in size. From the total surface area of all particles was concluded that fracture energy is small compared to impact energy. The average grain size of the ceramic tiles did not lead to a specific peak in mass or concentration for that particle size. In fact, a large concentration of much finer particles has been obtained. The size of the aerosol dust ranged between 10 micron and 10 nanometers, indicating a lot of particle fragmentation (comminution) to have taken place.

The ballistic behavior of four different armor ceramic materials with thicknesses varying from 3 mm to 14 mm has been investigated. These are two types of alumina Al2O3 armor grades and two types of SiC armor grades produced by different armor ceramic producers. The ballistic study has been performed using the standard DoP technique with two types of AP projectiles. The microstructure and the mechanical properties of the armor ceramics are investigated with SEM (Scanning Electron Microscopy), X-ray microanalysis, impulse excitation methods and Vickers hardness method. The microstructures are characterized by the number and chemical nature of the phases, the grain size, the type of fracture mode identified from SEM investigation of fragments. The measured mechanical properties are hardness, fracture toughness, flexural strength and the speed of sound (transversal and longitudinal). The ballistic tests are performed with two 7.62 mm projectiles AP8 (WC core) at 930 m/s and APM2 (hard steel core) at 800 m/s. The mass efficiency factor has been determined for each of the armor ceramics. The difference in the ballistic performance of the two alumina types is marginal. The two silicon carbide types are indistinguishable from ballistic point of view. The gained knowledge can be used two-fold: first, to understand better the influence of the microstructure on the ballistic performance of armor ceramics and second, as reference material for the development of a new class of armor ceramics.

Damage containment is one of the key factors for optimising operational readiness of warships after an internal warhead detonation. Ship designers currently have no other option than to rely on state of the art solutions applied in vehicle and personal protection; mainly ballistic composites. These solutions are prone to being traded off during the design process due to cost of the bulk material. Areas to protect are in the order of magnitude of hundreds of square meters. This called for development of a cost efficient, but lightweight protection against fragments. TNO has teamed up with industrial partners to develop understanding of the physical phenomena involved in pre-layers on armour steels [1], in order to optimise protective solutions. Goal of the project was to assess the potential of using polymer coated armour steels in combination with blast bulkheads. The project consisted of three iterations of testing and analysis. The first test programme was aimed at understanding the physics explaining why armour steels perform significantly better against blunt projectiles when covered with an additional thin layer of a relatively soft material. Knowledge from the preliminary tests was applied to protective concepts for blast bulkheads. The pre-layer on the armour steel allows it to dissipate the energy over a wider area of the steel. In the experiments extreme stretching of the armour plate occurred and about twice the amount of energy is dissipated in the armour steel compared to the uncoated situation. The work resulted in protective concepts that fulfilled the mass requirements and are based on affordable materials. The project team considers this technology to be promising for future application in warships. Solutions from this project will be engineered further into a prototype concept. There is spin-off possible to other platforms like vehicles and offshore rigs, where mass and cost are equally important.

During the interaction between a projectile and a target material the kinetic energy of the projectile is transferred into elastic and plastic deformation of both the projectile and target materials. Using a rigid penetrator the loss in kinetic energy is fully converted into energy absorbed by the target, both as kinetic and strain energy. Using optical high speed imaging the kinetic energy (e.g. velocity of fragments, plug and residual projectile) can generally be obtained. However, the strain energy remains invisible using high speed imaging. By thermal imaging using an IR video camera, the temperature distribution of the target material (on both the strike and rear face) can be obtained. In order to convert the camera recordings into a temperature distribution, the emissivity of the target material in the infra-red range of wave length needs to be known. For most armour materials the density, thickness and heat capacity are known, while the emissivity can be determined using the IR-camera and the target material at a known temperature. This enables one to calculate the amount of energy in the target that was dissipated by plastic and/or visco-elastic strain. Examples of thermal images will be shown for metallic, polymer and weaves/fabrics. From the temperature distributions the total thermal energy is obtained and compared to the total loss in kinetic energy of the projectiles. Thermal imaging provides a non-contact, remote measuring technique that can be used to directly see where and how much energy was dissipated by target materials. As the dissipated energy can be quantified, this technique is also of use for direct comparison with energy-based projectile-target interaction models.

At TNO several energy-based engineering models have been created for various failure mechanism occurring in ballistic testing of materials, like ductile hole growth, denting, plugging, etc. Such models are also under development for ceramic and fiberbased materials (fabrics). As the models are energy-based they can be directly compared to experimental results of ballistic tests as the mass and velocities of projectiles are regularly measured. This allows the models to be validated, as has been done for the ductile hole growth model. Using AP-rounds on ductile target materials like many metals, clay and polymers, ductile hole growth (DHG) normally is the major failure mechanism during projectile penetration. When the core of the projectile remains rigid (which is often the case in ductile materials) the loss in kinetic energy of the core is easily measured from its initial and residual velocity. In the DHG-engineering model this energy loss is also calculated but requires that the flow stress at high strain rates is known. Using the experimental results in combination with this engineering model the dynamic flow stress of the target has been quantified. This procedure has been done for several material (metals, clay types and polymers) and allows the determination of dynamic material properties that are otherwise not easily measured. This method requires a rigid penetration of a projectile through a (thick) plate of the material to be characterized. Hence, no special sample shape or dimension is required. The dynamic flow stresses that are obtained have been compared to high strain rate (order 1000/s) strength values of the same materials determined by other techniques. As the values are very close to each other, this provides confidence in the approach to use ballistic test results of targets failed by DHG in combination with the engineering model for the characterization of materials at high strain rates.

Open-air explosions are inevitable connected to activities from the Ministry of Defense (e.g. large detonations or EOD operations). And also the industry uses open-air explosions for production processes (e.g. explosion welding). This requires the protection of people and buildings relatively close to the source. But the annoyance, for instance due to rattling, can also be an issue for the surrounding community. Often, mitigation measures are necessary, but due to the high source energy this is not an easy task. Two techniques have been tested for charges ranging from 20 to 500 kg. The first technique applies a porous barrier at less than 10 m from the source. The mitigation at 300 m from the source was predicted with a model that takes into account the non-linear interaction with a barrier as well as the nonlinear interaction with the porous material (here: gravel). The model has been validated in a previous research phase. It is demonstrated that, depending on the barrier construction, a significant reduction of the shock wave can be achieved. The second technique applies water based foam directly on the explosives. Blast measurements have been carried out up to 200 m from the source. Peak level reductions ranging from 20 to 50% were achieved.

The use of elastomers in ballistic protection products (armour) is limited to low threat levels and transparent armour solution components. Often armor is considered a parasitic mass that increases with increasing threat levels. Therefore, low weight solutions are welcomed and bulk polymers, especially visco-elastic elastomers, could play a role soon. Some potential of these materials has been demonstrated in earlier work by TNO, as well as other R&D institutes active in armor development. Full use can be made of the strain rate stiffening and strengthening effects after optimization of the (high strain rate) mechanical properties and an expansion of the temperature range in which they perform to the T-range in which armour is used. Near the glass transition temperature (range) damping peaks, and this visco-elastic response generates an additional energy dump, in which mechanical energy is directly converted into heat, that can be applied (amongst others) for ballistic protection applications. Some routes to achieve optimized mechanical properties of elastomers for armour applications are suggested, but this is not a task for a laboratory for ballistic research.

Ever since WWII transparent armour consists of a multi-layer of glass panels bonded by thin polymer bond-films using an autoclave process. TNO has worked on the development of innovative transparent armour concepts that are lighter and a have better multi-hit capacity. Two new transparent armour concepts are described; glass-clad polyurethane and pellets-in-polyurethane. Both make use of conventional glass and a castable, transparent polyurethane resin (PUR), which mechanical properties at high strain-rate are significantly improved over its quasi-static properties. The (glass) pellets-in-PUR concepts shows remarkable good multi-hit capacity, but the transparency requires a very good match of optical properties of the two components. The glass-clad-PUR concept has intrinsic good optical properties and combines a good blast resistance with (soft core) bullet and bomb fragment protection. The level of protection of both concepts can be scaled by the thickness of the glass strikeface.

Current tools describing behavior of ballistic materials under impact seem to be less suitable to assist armor developers in their efforts. They are insufficiently detailed or need comprehensive material properties and material failure description, unavailable in the initial stages of the development of new solutions. Therefore TNO is building a series of calculation tools, capturing the majority of the energy absorption processes during projectile penetration of materials. These tools must finally cover the main ballistic materials (metals, fiber materials and ceramics) with their various failure mechanisms. In this work, two failure mechanisms for the ballistic behavior of metals are described. The “ductile hole growth” mechanism for relatively thick material and “bulging” for thin sheets are described by a series of simple energy conservation. The thinking process generated by the development and use of these tools, supports armour engineers to create new solutions for projectile defeat.

Against the manufacturing requirement for both lower lead time and reduced machining time for titanium components, a new concept was conceived assembling sheet material and other stock into semi finished parts by (explosive) impact welding. It is believed that this concept (which we named SLAM) could be especially beneficial for titanium alloy and nickel alloy. The present investigation centered on the feasibility of this technology for the widely used Ti6Al4V alloy. Impact process parameters were established and mechanical properties were investigated. In general, static properties were good. Fatigue strength reduced however, although much less so for notched specimens compared to un-notched material. Fatigue crack initiation could be linked to complex features within the (wavy) weld interface associated with the 'first generation' impact welding parameters. Next steps are foreseen to improve the fatigue performance. Two manufacturing demonstrators were manufactured showing the application to a T-bar manufactured from plate and sheet, as well as a tool-cladding application. Copyright © 2006 SAE International.

A method was developed for making dense hydroxyapatite-polymer composites. Hydroxyapatite (HA) is a type of calcium phosphate, which is a bioactive material. The polymer used in this work was Polyactive™ 60/40, a block copolymer from polyethylene oxide (PEO) and polybutylene terephtalate (PBT) in a weight ratio of 60/40. It is a biodegradable polymer with bone bonding properties. In this study the submicron HA starting powders were coated in such a way that a chemical bonding existed with the polymer. Polymer composites with a high fraction of HA (70-80 wt %) are of interest as a temporary replacement for bone, but are hard to densify by conventional means. Here it is shown that they can be densified up to about 87-88 % of the theoretical maximum density. When this is followed by a heat treatment in which the polymer chains are allowed to entangle, a sound compact will result. The compact thus obtained is stable in water, contrary to those samples that do not receive a heat treatment. Furthermore, the limitations in strength that can be obtained for such composites are being addressed here. These findings can be generalized for the densification of polymer (composites) by shock densification means.

Shock compaction was used in the fabrication of high temperature ceramic-based materials. The materials' development was geared towards the fabrication of nozzles for rocket engines using solid propellants, for which the following metal-ceramic (cermet) materials were fabricated and tested: B4C-Ti (15 vol.-%). B4C-Al, and TiB2-Al, with an Al content typically between 15-20 vol.-%. Here, the B4C-Ti was only shock-compacted, while the other two cermets were shock compacted followed by melt infiltration with Al. The materials were subjected to gradually more severe testing conditions. Slabs of the materials were first tested for thermal shock resistance in an acetylene flame, followed by testing in the exhaust gas stream of a rocket propellant, and thereafter as a cylindrical insert in a nozzle of TZM alloy. The B4C-Ti composite showed erosion and cracking after the first test in the propellant flame, while the B4C-Al composite failed the insert tests. The TiB2-Al composite performed well under all conditions. A venturi nozzle of that material was formed during compaction. This real, shaped nozzle was shown to function well, even during repeated 3-6 s tests. This could be explained by the resistance of TiB2 to molten Al, the high thermal conductivity of the TiB2-Al cermet and the in situ formation of a protective layer, consisting mainly of Al2O3.