Abstract:
A test fixture is provided for mounting a sample to a gas gun. The fixture includes a gun barrel mount including an annular enclosure with first and second axial ends, and a sample platform. The mount connects to the gas gun at the first end. The sample platform includes a tubular component having third and fourth axial ends, a pusher disk, an end plate, and a flange. The disk supports the sample and mounts to the end plate. The flange removably attaches to the component at the third end. The end plate removably attaches to the component at the fourth end and to the enclosure at the second end.

Description:
STATEMENT OF GOVERNMENT INTEREST 
       [0001]    The invention described was made in the performance of official duties by one or more employees of the Department of the Navy, and thus, the invention herein may be manufactured, used or licensed by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to test fixtures for material response to blast waves exposure. In particular, the invention relates to a gas gun barrel attachment to mount a target sample and provide instrumentation for blast measurements. 
         [0003]    Traditional methods of measuring blast wave propagation through materials have involved the use of small explosive charges or a gas gun equipped with a Mylar or other burst diaphragm to generate the blast wave and complex target geometries such as instrumented mannequin heads wearing helmets coated with different test materials. There are three key disadvantages to these techniques: 
         [0004]    1) explosive charges pose safety and environmental hazards, 
         [0005]    2) the repeatability of Mylar burst diaphragms is poor at low pressures (below 100 psi), and 
         [0006]    3) complex target geometries introduce uncertainties in the data due to irregular flow of the blast wave around targets and into the interfaces between the helmets and the instrumented mannequin heads. 
       SUMMARY 
       [0007]    Conventional techniques for evaluating material exposure to blast wave yield disadvantages addressed by various exemplary embodiments of the present invention. Various exemplary embodiments provide adaptation to an existing gas gun with components equipped with a fast-opening valve and greatly simplified target geometry. In particular, such embodiments provide a test fixture for mounting a sample to a gas gun. The fixture includes a gun barrel mount including an annular enclosure with first and second axial ends, and a sample platform. 
         [0008]    The mount connects to the gas gun at the first end. The sample platform includes a tubular component having third and fourth axial ends, a shock absorption disk, an end plate, and a front flange. The disk supports the sample and mounts to the end plate. The flange removably attaches to the component at the third end. The end plate removably attaches to the component at the fourth end as well as to the enclosure at the second end. Other embodiments, alternatively or additionally, provide for pressure gauges for measuring pressure or triggering recordation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    These and various other features and aspects of various exemplary embodiments will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which like or similar numbers are used throughout, and in which: 
           [0010]      FIG. 1  is an isometric assembly cross-sectional view of a muzzle adapter; 
           [0011]      FIG. 2  is an isometric exploded cross-sectional view of the target disposition assembly for mounting a target sample; 
           [0012]      FIG. 3  is an elevation assembly view of the muzzle adapter and target assembly; 
           [0013]      FIG. 4  is a graphical view of pressure response to valve opening; 
           [0014]      FIG. 5  is a graphical view of a pressure blast propagation; and 
           [0015]      FIG. 6  is a graphical view of acceleration results. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
         [0017]    Exemplary embodiments provide an attachment mechanism for testing blast wave propagation from a conventional gas gun. Coupon samples serving as experimental targets can be installed to test the blast mitigation properties of materials subject to planar blast waves with pressures ranging from 5 psia to 100 psia. The embodiments provide a two-part fixture consisting of a muzzle adapter ( FIG. 1 ) and target assembly ( FIG. 2 ). The muzzle adapter attaches to an existing 1.575 inches bore diameter gas gun barrel located in the Shock Physics Facility at Naval Surface Warfare Center (NSWC) Dahlgren Division. Much of this information has been reported in “Versatile Gas Gun Target Assembly for Studying Blast Wave Mitigation in Materials by S. Bartyczak and W. Mock Jr., AIP Conference Proceedings, 1426, 501 (2012) available at http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&amp;id=APCPCS0014260 00001000501000001&amp;idtype=cvips&amp;doi=10.1063/1.3686327&amp;prog=normal. This document is incorporated herein by reference in its entirety. 
         [0018]      FIG. 1  shows an isometric assembly cross-sectional view  100  of a muzzle adapter  110  that includes an annular cylindrical enclosure  120  with inner wall  125 . A target assembly attaches to a proximal end  130  of the enclosure  120 . The muzzle adapter  110  further includes a triggering pressure gauge PG 4   140  using model PCB Piezotronics 132A31 pressure sensor to initiate the oscilloscopes via a communication conduit  145  (such as an electrical conduction wire or cable). The muzzle adapter  110  also includes an instrumentation pair of dynamic pressure gauges PG 1   150  and PG 2   160  using model PCB Piezotronics 113A31 pressure sensors. A gas gun barrel (not shown) attaches into a recess at a distal (i.e., gun receiving) end  170  of the muzzle adapter. 
         [0019]    The muzzle adapter  110  is fabricated from 6061-T6 Al (aluminum alloy), and has a 05.5 inches outer diameter and a length of 11.25 inches. The recess at the distal end  170  has dimensions of 02.2 inches diameter×0.25 inch deep. The recess includes an O-ring groove 01.78 inches inner diameter and 02.06 inches outer diameter×0.08 inch deep that contains a Parker 2-134 O-ring to seal the barrel-muzzle adapter joint at the distal end  170  for blast pressure. This distal end  170  of the muzzle adapter  110  includes three ½-13 UNC threaded holes spaced 120° apart on a 04.25 inches diameter bolt circle for securing the muzzle adapter  110  to the gun barrel. 
         [0020]    The muzzle adapter  110  has a 2.0 inches long transition region with expanding inside diameter to evaluate materials with a diameter larger than the 1.575 inch gun bore diameter. A larger target diameter enables maintaining one-dimensional strain conditions in the target center for a longer time before release waves from the target edge reach the center. In this transition region the inside diameter of the muzzle adapter  110  on the distal end  170  increases from 1.63 inches to 4.25 inches, corresponding to a 33.2° angle. The continuous 4.25 inches inner diameter extends to the target assembly end of the muzzle adapter  110  (a distance of 9.0 inches). This proximal end  130  of the muzzle adapter  110  has three ¼-28 UNF threaded holes spaced 120° apart on a 4.85 inches diameter bolt circle for securing a target assembly thereto. 
         [0021]    The three pressure gauges installed along the enclosure  120  include the triggering transducer gauge  140  and the measurement transducer gauges  150  and  160  for measuring blast wave velocity and pressure as the blast wave propagates towards the mounted target. These gauges are shown disposed 3, 5, and 6 inches, respectively, from the distal end  170  of the muzzle adapter  110 . Standard PCB transducer mounting techniques can be used to secure the pressure gauges to the enclosure  120  of the muzzle adapter  110 . 
         [0022]      FIG. 2  shows an isometric exploded cross-sectional view  200  of the target disposition assembly  210  for attaching a test sample  220 . The target assembly  210  includes an annular cylindrical poly(methyl methacrylate) (PMMA) tube  230  and a pressure gauge PG 3   240  as a model PCB Piezotronics 113B28 pressure gauge to measure the blast pressure at the target edge. PMMA constitutes a transparent shatter-resistant thermoplastic. 
         [0023]    The PMMA tube  230  terminates at a front flange  250  and a rear flange  260 . These flanges  250  and  260  attach to the tube  230  by screws  270 . The target assembly  210  further includes an accelerometer  280  and a polytetra-fluoroethylene (PTFE) disk  290 . The accelerometer  280  represents a model PCB Piezotronics 352C23. PTFE is a synthetic fluoropolymer of tetrafluoro-ethylene. The front flange  250  attaches to the proximal end  130  of the muzzle adapter  110  by the screws  270 . PTFE rods  300  connect the rear flange  260  by threaded through-holes  310  to the disk  290 . The rear flange  260  also includes arc slots  315 . 
         [0024]    The rear flange  260  attaches to the muzzle adapter  110  with three steel bolts  320  accompanied by plastic sleeves  330 . The bolts  320  insert into through-holes  340 . The flanges  250  and  260  are composed of 6061-T6 Al (aluminum alloy) and are secured to the tube  230  with screws  270 . The target  220  disposed against the disk  290  for exposure to the blast wave pushes against the front flange  250  by tightening the PTFE rods  300 . The front flange  250  includes an annular opening to permit propagation of the blast wave towards the target  220 . 
         [0025]    Various exemplary embodiments provide an attachment to an existing gas gun to test the blast mitigation properties of materials subject to planar blast waves with pressures ranging from 5 psia to 100 psia. The pressure gauge PG 3   240  measures the reflected blast pressure at the target edge. The exemplary embodiments provide a two-part fixture consisting of the muzzle adapter  110  and the target assembly  210 . The design of the muzzle adapter  110  includes a tapered transition region that enables the blast wave to expand from the 1.575 inches gun bore diameter to 4.25 inches and reform into a planar shock front. 
         [0026]    The target assembly design  210  includes: 
         [0027]    1) attachment points for attaching to the muzzle adapter  110 , 
         [0028]    2) enables adjustable positioning of the blast face of the target  220  along the axis of the muzzle adapter  110 , 
         [0029]    3) can accommodate a variety of sample target thicknesses up to 3.5 inches, and 
         [0030]    4) includes an instrumentation suite designed to record initial material stress, transmitted material stress, material transit time, reflected blast wave pressure, and target acceleration. 
         [0031]    The target assembly  210  includes a PMMA tube  230  with front (blast wave side) and rear 6061-T6 Al flanges  250 ,  260  that secure to the tube with screws  270 . The target to be tested rests against the front flange  250  that is open in the middle to enable the blast wave to impinge directly on the target  220 . A PTFE disk  290  with a front recess contacts the back of the target  220  at its edge only. This ensures that the center rear of the target  220  is a free surface. 
         [0032]    The PTFE disk  290  is held in position with three PTFE threaded rods  300  that screw into threaded through-holes  310  in the rear flange  260 . The threaded rods  300  can be turned by hand, enabling fine-tune adjustment of the pressure that holds the target  220  secure against the front flange  250 . This enables a layered target  220  to be tested without necessarily holding the layers together with epoxy. 
         [0033]    The target assembly  210  attaches to the muzzle adapter  110  with the three steel bolts  320  that pass through clearance holes at the edge of the rear flange  260 . The standoff of the target assembly  210  with respect to the muzzle adapter  110  can be adjusted by using different length bolts  320  with plastic sleeves  330 . Using this procedure the position of the target assembly  210  can be easily changed. 
         [0034]    The exemplary PMMA tube  230  is 4.5 inches long with a 3.0 inches inner diameter and a 3.5 inches outer diameter. There is a 0.375 inch gap between the inside diameter of the muzzle adapter  110  and the outside diameter of the tube  230  to permit blast overpressure to escape from the adapter  110 . A 0.085 inch deep half-moon slot in the outer wall of the tube  230  is used to hold the pressure gauge PG 3   240  flush with the front flange  250 . Several longitudinal grooves  350  in the inside wall of the tube  230  enable the wires of thin film gauges or other instruments to pass through that may (in alternate embodiments) be placed in front of and/or behind the target  220 . These and other dimensions represent exemplary values and artisans of ordinary skill will recognize that they are not limiting. 
         [0035]    The 0.125 inch thick front flange  250  has a 3.5 inches outer diameter, a 2.5 inches inner diameter, and four clearance holes spaced 90° apart on a 3.25 inches diameter bolt circle for 2-56 UNC screws to attach the flange  250  to the PMMA tube  230 . This tube  230  rests in a 0.1 inch deep recess in the rear flange  260  and is secured with four 6-32 UNC screws spaced 90° apart on a 3.25 inches diameter bolt circle. 
         [0036]    Six 0.19 inch wide slots on a 3.88 inch diameter circle in the rear flange  260  ensure release of the blast wave. The three 6-inch long by ½-13 UNC PTFE threaded rods  300  screw into threaded holes  310  spaced 120° apart on a 2.28 inches diameter bolt circle in the rear flange  260 . The 0.75 inch thick by 3.0 inches diameter PTFE target backup disk  290  has a 2.5 inches diameter by 0.125 inch deep recess that contains the accelerometer  280  for measuring the acceleration of the target system due to the blast wave. These dimensions are merely exemplary, and artisans of ordinary skill will recognize that the components described herein can be scaled larger or smaller depending on the target  220  and its test conditions. 
         [0037]    The firing event begins when the fast-acting ball valve in the gas gun is opened, releasing high pressure gas from the gun breech. The released gas forms a planar blast wave that travels down the 1.575 inch bore diameter barrel until it reaches the distal end  170 . A 2.0 inches long transition region in the muzzle adapter  110  causes the blast wave to expand to 4.25 inches diameter, slightly larger than the diameter of the target  220  to permit the gas to escape. 
         [0038]    This expansion of the blast wave causes turbulence in the flow of the blast pressure. The 4.25 inches bore in the muzzle adapter  110  is 9.0 inches long to enable the turbulence to subside and the planar blast wave to reform prior to impact with the target assembly  210 . The position of the target assembly  210  can be adjusted along the length of the muzzle adapter  110  in order to tailor its location for optimum flow characteristics. 
         [0039]    The muzzle adapter  110  is equipped with a trigger gauge  140  to start the data acquisition system and two pressure gauges, PG 1   150  and PG 2   160  to record incident pressure and velocity of the blast wave prior to impact with the target assembly. The target assembly  210  is equipped with one pressure gauge PG 3   240  to record reflected pressure and an accelerometer  280  to record acceleration of the test sample  220 . As the blast wave impacts the target assembly  210 , its pressure front expands around the target  220  and escapes through a 0.375 inch gap between the target assembly  210  and the muzzle adapter  110 . 
         [0040]      FIG. 3  shows an elevation schematic view  360  of an experimental setup with the muzzle adapter  110  and target assembly  210  connected together for investigating blast wave reduction in a layered target system. The blast wave propagates in a direction  370  from the distal end  170  towards the target  220 . The gauges PG 1   150  and PG 2   160  measure the blast wave velocity along direction  370  and accompanying pressure. The gauge PG 3   240  measures the reflected blast pressure at the exposed face of the target  220 . The accelerometer  280  on the rear of the target  220  measures the damped vibration of the target assembly  210 . 
         [0041]    Initially a series of checkout experiments was conducted without a target assembly  210  to determine the largest ball valve opening time without reducing the blast pressure appreciably.  FIG. 4  shows a graphical view  400  of several transient pressure profiles. The abscissa  410  represents time in micro-seconds (ps) and the ordinate  420  represents blast pressure in psig. The time is recorded with respect to initiation from the trigger gauge  140 . The pressure responses for gauge PG 1   150  are plotted for valve opening times: 10 ms as line  430 , 20 ms as line  440 , 30 ms as line  450  and 310 ms as line  460 . 
         [0042]    For a very long valve opening time (on the order of many hundreds of milliseconds), the compression wave in the gun barrel would not be expected to form into a blast wave at the target  220 . For these tests, the blast pressure was measured with gauge PG 1   150  for selected valve opening times for a 60 psig breech pressure. As the valve opening time increases, the blast wave slope, amplitude, and velocity decrease. Based on these results a 20 ms valve opening time  440  was chosen for the blast wave experiments since the 10 ns and 20 ms profiles,  430  and  440  respectively, are very similar. 
         [0043]    A series of experiments was subsequently performed to determine blast wave planarity for different target standoff positions in the muzzle adapter  110 . For these experiments, the target  220  was removed, and a supplemental pressure gauge PG 4  (not shown) was mounted in the middle of a modified PTFE backup disk  290  such that this gauge was in the same plane as gauge PG 3   240 . To determine blast wave planarity, the arrival time of the blast wave at this gauge PG 4  was compared to that of gauge PG 3   240  for selected standoff positions of the rear flange  260  from the proximal end  130  of the muzzle adapter  110 . At  0  mm standoff, the rear target flange  260  attaches directly to the rear of the muzzle adapter  110 . In this position, the blast wave releases through the arc slots  315  in the rear target flange  260 . At this standoff position, the front of the target  220  was disposed  19  mm from gauge PG 2   160 . 
         [0044]    At 115 mm standoff the front of the target  220  is flush with the proximal end  130  of the muzzle adapter  110 . In this configuration, long bolts  320  with plastic sleeves  330  are used to stand off the rear target flange  260  a distance of 115 mm from the distal end  130  of the muzzle adapter  110 . The blast wave planarity experiments were performed for 60 psig breech pressure. Satisfactory blast wave planarity was achieved for the 115 mm standoff position. In this position, the blast wave arrived at gauge PG 4  about 1 ps prior to arriving at transducer gauge PG 3   240 . The measured blast wave velocity was 397 m/s (1302 ft/s) for this test. 
         [0045]    After performing the impact planarity experiments, layered target experiments were conducted for a breech pressure at 60 psig. The layered target  220  consists of a 3.18 mm thick Sorbothane disk (50 durometer, shore 00) sandwiched between two 3.1 mm thick 6061-T6 Al coupons. Sorbothane is a commercially available synthetic viscoelastic polymer used for shock attenuation and vibration isolation. For these tests, the Sorbothane material was covered with the aluminum coupons to preclude or mitigate non-uniform deformation under blast. The aluminum coupons consist of 2.3 mm and 0.8 mm thick aluminum disks with a polyvinylidene fluoride (PVDF) thin film polymer stress gauge (not shown) between them. The PVDF gauges from Dynasen, Inc. of Goleta, Calif. measure the input and output stresses in the sample material of the target  220 . 
         [0046]      FIG. 5  shows a graphical view  500  of pressure-time profiles for a selected experiment. The abscissa  510  represents time in microseconds (ps) and the ordinate  520  represents pressure in psig. Measurement lines for gauges PG 1 , PG 2  and PG 3  are shown, with an exponential decay trace  530  being highlighted for gauge PG 3   240  beginning at 520 ps. Time is measured with respect to the trigger pulse as determined by the trigger gauge  140 . The breech pressure within the enclosure  120  was 60 psig for these experiments. The incident peak pressures measured by respective gauges PG  1   150  and PG 2   160  are 7.0 psig and 6.1 psig. A 403 m/s (1322 ft/s) blast wave velocity was calculated using these profiles. The gauge PG 3   240  measures a 7.7 psig exponential decaying reflected blast wave with a 250 ps duration. 
         [0047]      FIG. 6  shows a graphical view  600  of acceleration-time profiles for the accelerometer  280  for selected valve opening times. The abscissa  610  represents time in microseconds (ps) and the ordinate  620  represents acceleration in equivalent earth gravitational acceleration at sea-level. The accelerometer  280  measures the damped vibration of the natural frequency of the target system. 
         [0048]    A damping coefficient of 0.074 (using the first two periods of the plot) was calculated from these data. The damped natural frequency of the system was 12 kHz (83 ps period). For the purposes of analysis, the target  220  responds to a long-duration pulse as measured by the gauge PG 3   240  due to the 250 ps blast pulse duration being three times the 83 ps system period. Because the damping coefficient varies between zero and unity, this result suggests that minimum damping is achieved with this target configuration and low (7.7 psig) reflected pressure. A damping coefficient can be obtained for the system from this damped sinusoidal vibration. Time is measured with respect to the trigger pulse as determined by the trigger gauge  140 . 
         [0049]    Due to the recent conflicts in the Middle East and the threat of improvised explosive devices (IEDs), the incidence of blast related injuries is increasing as is the need for research to develop blast mitigating materials. Various exemplary embodiments have the potential to be used commercially by other facilities using gas guns to characterize blast wave attenuation of the new materials developed. 
         [0050]    The purpose of various exemplary embodiments is to provide a capability for using an existing gas gun to test the blast mitigation properties of materials with the end goal of identifying materials suitable for military armor to protect war-fighters from blast-related injuries. The advantages of various exemplary embodiments include: 
         [0051]    1) the design adapts to an existing gas gun that uses a fast-opening valve and non-explosive nitrogen and helium gases to generate the blast wave thereby eliminating the safety and environmental hazards associated explosive charges and the poor repeatability issues associated Mylar burst diaphragms, 
         [0052]    2) a unique target assembly design that enables: a) a test sample  220  to be located at any position in the muzzle adapter  110  in a continuous manner by simple screw adjustment, b) test samples  220  of different thicknesses to be easily inserted into the target assembly  210  for measurement, and c) a sandwich test sample target  220  to be held together in the target assembly  210  by simple screw adjustment. 
         [0053]    While certain features of the embodiments of the invention have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments.