PT104167A - HOPKINSON TESTING MACHINE FOR ELECTROMAGNETIC IMPULSION - Google Patents

Abstract

The present invention relates to a modification of the HOPKINSON TESTS MACHINE FOR THE INTRODUCTION OF AN ELECTROMAGNETIC IMPULSION SYSTEM IN REPLACING TYPICAL PNEUMATIC IMPULSATION SYSTEMS. THIS INVENTION IS SPECIALLY INDICATED IN APPLICATIONS OF THE HOPKINSON TEST MACHINE WHERE A GLOBAL REDUCED DIMENSION OF THE EQUIPMENT IS INTENDED, ALLOWING A BETTER PERFORMANCE IN TERMS OF THE MAXIMUM ACHIEVABLE SPEED, OF THE ADJUSTMENT AND REPETIBILITY OF THE RESULTS, AND IN THE MINIMIZATION OF NOISE SPREAD PARASITES (TYPICAL OPENING OF PNEUMATIC VALVES, EXPANSION OF GAS AND ROUNDING OF PROJECTILE BAR IN THE SHOCK SYSTEM). THE PROPOSED MACHINE IS SUMMARIZED TO INCORPORATE AN ELECTROMAGNETIC ACTUATOR / TRIGGER (3) IN A STRUCTURE OF A HOPKINSON BAR (1), WITH THE INTENDED TO REPLACE THE TRADITIONAL COMPRESSED AIR TRIGGERS. THROUGH A POWER BANK (2) ELECTRICAL DISCHARGE IS POSSIBLE, WHICH WILL MAKE THE ELECTROMAGNETIC ACTUATOR TRANSMIT MOVEMENT TO THE PROJECTILE BAR (4).

Description

Description Hopkinson Assay Machine by Electromagnetic Drive Technical Field of the Invention

Hopkinson Bar / Electromagnetic Actuators / Impact Testing

The present invention proposes the introduction of electromagnetic drive systems in the Hopkinson test machine, particularly introducing the introduction of magnetic reluctance systems. A functional prototype is presented through which it is possible to prove the applicability and success of this new concept. The concept presented here and its implementation, as far as the authors know, have never been reported in other works related to the Hopkinson test machine. The Hopkinson test machine is used in the mechanical characterization of materials at high deformation speed. In its typical configuration, the machine consists of two cylindrical bars (incident and transmitting bar), between which a test piece of the test material is placed. This machine is driven by an air-compressed impeller system that is used to accelerate a projectile against the incident bar. The compressed air is supplied by a reservoir under controlled pressure to allow adjusting the speed of the test. The air-compressed projectile propulsion system is based on the principle of gas expansion, whose fundamentals are perfectly established and easily implemented experimentally. However, the velocity of the projectile increases at a ratio less than the gas pressure within the reservoir, leading to the use of high pressures to achieve significant firing rates. The present invention consists in replacing the compressed air impeller system in the Hopkinson test machine with a new electromagnetic drive system. Different methods can be used for the electromagnetic acceleration of the projectile bar. This change offers numerous advantages over traditional pneumatic triggers, such as: (a) Reducing the overall length of pneumatically driven equipment. This is due to the high acceleration, by electromagnetic action, allowing to reach the speeds typical of the Hopkinson test in a distance covered by the significantly smaller projectile bar. b) Minimize the vibrations typical of pneumatic actuators. This is due to the elimination of the pressure valves, the pressure waves derived from the expansion of the compressed air and the contact between the projectile bar and the pneumatic firing system. Minimizing vibration is important for the quality of the material characterization test. c) Through a bank of capacitive power, it is possible to control the main electrical parameters, allowing a precise definition of the stored energy. In this way it is possible to precisely and repeatably adjust the velocity and the energy of the projectile bar. d) Using the capacitive bank, it is possible to store the energy which allows the velocity and the energy of the projectile bar to be higher than those achievable in the Hopkinson pneumatic drive test machines.

Historical evolution The idea of using a pressure bar to determine the properties of materials was first presented in 1872 by John Hopkinson (1849-1898) [1]. It was able to determine the properties of the iron wire by transferring the energy from the drop of a weight to the wire and measuring the deformation of the wire before this break. It was not until 1913 that this initial development evolved, when his son, Bertram Hopkinson (1874-1918) [2] introduced a technique to determine the temporal relations of pressure propagation in the bars due to the impact produced by a bullet or explosive. Figure 2 shows a schematic of the first Bertram Hopkinson apparatus developed in 1914 [3]. The method consisted in using a bar (B) suspended by two sets of wires, aligned with a carton (D) which was likewise suspended. A small section of bar (C) was placed at the end of the main bar and held in place by a small magnetic force. A bullet (A) was then fired at the end of the long bar causing a pressure wave which was transmitted to the other bar. The wave traveled along the longer bar to the shorter bar causing the same wave to fly into the box. The displacements of the box and the bar were measured by a displacement system, and although the instruments of measurement of the time presented a low performance, it was possible to estimate the calculation of the linear momentum.

After this first device developed by Hopkinson, it was only in 1941 that significant advances were made in this technique when Dennison Bancroft solved the Pochhammer and Love equation, which consisted in determining the frequency of the bar for the velocity of the longitudinal waves in the cylindrical bars. Bancroft expressed the velocity of the longitudinal waves in the bars admitting that they would have an infinite wavelength. In 1948, Davies developed a technique using capacitors to measure deformations in the bar. According to Davies, the output of the capacitors is proportional to the displacement-time ratios, which in turn are proportional to the pressure-time ratios, assuming that the bars remain in an elastic regime during the tests. The use of capacitors in the measurements of the Hopkinson Bar provided a great evolution in the precision of the obtained results.

In 1949 Kolsky added a second bar to Hopkinson's original apparatus, which contributed to the mechanical evolution of the apparatus, and introduced the equations for calculating the properties of the specimen of the specimen, based on the history of the extension of the bars. The extent was measured using capacitors similar to those used by Davies. With this change, the Hopkinson Bar was renamed in the English literature by Split Hopkinson Bar. The use of strain gauges for Hopkinson Bar measurements only occurred in 1970, when Hauser added the strain gauges to the bars, with the to measure the 4 displacements of the surface thereof. Currently several researchers continue to study the Hopkinson bar test methods, with a particular focus on the data acquisition methods, which are nowadays used by optical systems and laser systems, among others.

Principle of operation The Hopkinson test is the most common method for testing materials at medium and high strain rates. Figure 3 (a) shows a very simple scheme for the operation of the Hopkinson assay. The operation of this device consists in firing the projectile bar, with speeds between 2.5 and 20 m / s, against the incident bar. This impact causes a compressive elastic pressure wave propagating along the incident bar to the interface with the material being tested. Once there, the pressure wave causes a plastic deformation in the cylindrical specimen, previously placed between the bars, where part of this wave is then transmitted to the transmitting bar and another part is reflected to the incident bar. All these pulses are measured by extensometers A and B.

The signals acquired by these are represented in Figure 3.b). The blue line represents the evolution of the pressure wave in the incident bar, and the red line represents the information regarding the transmit bar. The obtained signals are then amplified and displayed on an oscilloscope. Through the mathematical manipulation of these extension measurements in the bars, it will be possible to calculate the strain σ, the strain ε and the strain velocity ε. 5

Applications of the Hoplcinson assay The Hopkinson assay is generally used in research as a technique of characterization of materials, are an example of this ([5] [6] [7]). Examples of such devices can be found at the Naval Surface Warfare Center (Virginia) and the University of Arizona in the USA, respectively. In the Department of Mechanical and Aerospace Engineering of the University of Notre Dame in Indiana (USA), modified Hopkinson assays have been carried out with the objective of determining temperature fields ([8] [9]). It should be noted that in all research centers the Hopkinson test machine uses an air-compressed impeller system.

Detailed Description

When a body is in the influence of a transient electromagnetic field, forces develop that tend to act it. The response to this request will depend on the physical characteristics of the body and the parameters of the electromagnetic field. In this way, when the parameters are properly calibrated, a precise adjustment of the acceleration that animates the body with high repeatability is possible.

One of the main advantages of introducing the electromagnetic impulse in the Hopkinson test is related to the absence of mechanical systems, avoiding the propagation of vibrations (typical of the opening of the pneumatic valves or of the projectile bar in the firing system). The self-centering characteristics of the magnetic field enable the contact between the projectile bar and the guiding elements to be eliminated, thus avoiding frictional losses or vibration generation. Still, and not least, the velocity of the typical projectile bar of Hopkinson's tests is achieved after relatively short distances due to the high acceleration animating the body, contributing to the reduction of the overall length of this testing machine. Allowing for relatively higher speeds compared to other systems used in these test machines. The proposed Hopkinson test machine consists of the following elements (Figure 4): 1) Structure 2) Power Bank 3) Electromagnetic Actuator 4) Projectile Bar 5) Incident Bar 6) Transmitter Bar 7) Bar Support 8) Power Sink 9) Extensometers The electromagnetic impulse can be implemented through several methods, which resort to different principles of electromagnetism, which are essentially dependent on the properties of the material and the geometry of the projectile to be accelerated. In the literature of the specialty can be identified several methods, such as induction trigger, side disc trigger, Thompson trigger, coil trigger, rail trigger, magnetic reluctance trigger, etc. ([10]). Among these methods, the magnetic reluctance trigger was chosen for the implementation in the Hopkinson test machine due to the geometric adaptability between the system used and the projectile bar geometry, and also due to the greater relative efficiency in the transfer of the electromagnetic energy in kinematics of the projectile bar. This system also presents features that facilitate its development and adaptation in the Hopkinson test machine. In this way it was possible to prove the advantages of the application of an electromagnetic impulse system in a Hopkinson bar.

A magnetic reluctance trigger is essentially characterized by a coil of suitable geometry (Figure 5) and by a capacitive electrical power unit. The passage of an electric current impulse through the coil allows the generation of a high intensity electromagnetic field, which promotes the acceleration of ferromagnetic materials. The coil is generally supported on a tubular guide of high magnetic permeability, which also allows the initial support of the projectile bar (the self-centering effect eliminates contact during firing).

When a projectile of ferromagnetic material is in the zone of interference of the electromagnetic field generated by the coil it acquires an inverse polarization, the induced pole being attracted by the opposite inductor pole, resulting in the acceleration of the projectile in the direction of the center of the coil (independently of the direction of the current) . The force that accelerates the projectile can be described as a function of the strength of the magnetic field and the relative position 8 between the projectile bar and the coil (Equation 1). The force felt in the projectile bar can be increased by increasing the number of turns, the current intensity that travels them or increasing the density of the magnetic field lines. This latter parameter can be controlled by the introduction of a magnetic shield, where a material of high magnetic conductivity allows to conduct the lines of magnetic flux in order to concentrate them more adequately. F =

άφ dx

The novel concept presented in this patent application was implemented by the development of a functional prototype of the Hopkinson test machine with electromagnetic actuation (Figure 6). The tests performed have been able to prove the statements presented in this document.

Caption of Figures

Figure 1 - Schematic of the Hopkinson Electromagnetic Impulse Machine with the nomenclature of the various constituent components; 1) Structure, 2) Power bank, 3) Electromagnetic actuator, 4) Projectile bar

Figure 2 - Schematic of the first Hopkinson apparatus of 1914 [3];

Figure 3 - Hopkinson's Essays (a) Hopkinson Bar Scheme; (b) Signals obtained in data acquisition [4]; 9

Figure 4 - Schematic of the Hopkinson Electromagnetic Impulse Machine with the nomenclature of the various constituent components; 1) Structure, 2) Power Bank, 3) Electromagnetic Actuator, 4) Projectile Bar, 5) Incident Bar, 6) Transmitter Bar, 7) Bar Support, 8) Power Sink, 9)

Figure 5 - Magnetic reluctance trigger scheme [10];

References [1] http://www-g.eng.cam.ac.uk/125/noflash/1875-1900/hopkinson j ohn.html; [2] http://www-g.eng.cam.ac.uk/125/noflash/1900-1925/ hopkinson .html; [3] SALISBURY, Christopher, MSC Thesis, Spectral Analysis of Wave Propagation Through aPolymeric Hopkinson Bar, Ontario (Canada), 2001; [4] http://www.tut.fi/units/ms/elm/laitteet/hopkinson(eng).htm [5] http://www.ame.arizona.edu/research/wchen/; [6] KAISER, Michael Adam, MSc Thesis, Advances in the Split Hopkinson Bar Test, Virginia (USA), 1998; [7] http://www.tut.fi/units/ms/elm/laitteet/hopkinson(eng).htm; [8] http://www.nd.edu/~kvernaza/mason poster2.pdf; [9] K. M. Vernaza-Pena, J. J. Mason, M. Li, Experimental Study of the True Temperature Generated During Orthogonal Machining of an Aluminum Alloy, Experimental Mechanics, 2002, Vol. 42, No. 2; [10] http://www.coilgun.eclipse.co.uk

Lisbon, September 01, 2008. 10

Claims (5)

Hopkinson Electromagnetic Impulse Testing Machine, characterized in that it has an electromagnetic drive system, which is used to actuate the projectile bar. Hopkinson Electromagnetic Impulse Testing Machine according to Claim 1, characterized in that it has a reduced overall length with respect to the high acceleration electromagnetic drive equipment to achieve the speeds typical of the Hopkinson test at a distance traveled by the significantly smaller projectile bar. Hopkinson Electromagnetic Impulse Testing Machine according to Claim 1, characterized in that it minimizes the vibrations typical of pneumatic actuation machines by eliminating the pressure valves, the pressure waves derived from the expansion of the compressed air and of the contact between the projectile bar and the pneumatic firing system Hopkinson Electromagnetic Impulse Testing Machine according to Claim 1, characterized in that it has a capacitive power bank for the control of the main electrical parameters and precise definition of the stored energy, thus accurately and repeatably adjusting the speed and energy of the projectile bar. Hopkinson Electromagnetic Impulse Testing Machine according to Claim 1, characterized in that it has a capacitive power bank where the energy storage raises the speed and energy of the projectile bar to values greater than those achievable on the test machines of Hopkinson of pneumatic impulsion. Lisbon, September 01, 2008. 2