Abstract. 1 Introduction

Transcription

1 Penetration resistance of laminated plates from steel and flbreglass-reinforced polyester A.M. Eleiche*, M.S. Abdel-Kader\ A. Almohandes' "Mechanical Engineering Department, UAE University, United Arab Emirates b * Department of Mechanical Engineering, M.T.C., Cairo, Egypt 'AFTRC, Egypt Abstract Ballistic tests are performed to investigate the effect of target configuration on ballistic performance when struck by standard bullets at different velocities. Firstly, single mild steel plates, 1 to 8 mm thick, are tested, and the effects of thickness and mechanical properties of plate material are explored. Secondly, incontact laminae comprising 8 mm-thick targets, and spaced laminae of the same total steel thickness, are tested, and the effect of number, thickness, and arrangement of laminae determined. In addition, Fiberglass Reinforced Polyester (FRP) is used as afillermaterial for targets with spaced steel laminae, and the influence on the ballistic performance is investigated. In each experiment, the specific velocity drop, defined as the bullet velocity divided by the total travel through the target, is calculated and taken to measure target performance. Results show that single targets are more effective than laminated ones of the same total thickness, regardless of configuration or striking velocity. Moreover, the effectiveness of laminated targets, in contact or spaced, increases as the number of laminae comprising each target decreases Performance of laminated targets is further enhanced by using the thickest lamina as the back lamina. Finally, steel-frp targets show better performance than weight-equivalent steel targets. 1 Introduction Many parameters are found to control the mutual interaction between an impacting projectile and a target. These include impact conditions as well as projectile and target characteristics, and result in many failure modes, such as brittle fracture, ductile hole enlargement, petalling, spalling, and plugging*'2 Target plates may be monolithic or multi-layered. In the latter type, individual layers may be spaced or in-contact. Afillermaterial, metallic or non-metallic, may be used with spaced laminae, thus constituting sandwich substructures.

2 118 Structures Under Shock And Impact Transactions on the Built Environment vol 22, 1996 WIT Press, ISSN In this paper, the ballistic performance of mild steel single and laminated targets, with in-contact and spaced laminae, having a total steel thickness of 8 mm is investigated experimentally. Effects of number, thickness and arrangement of laminae are sought for impact velocities ranging from 706 to 826 m/s. In case of spaced laminae, interspatial distances orfillerthickness are chosen equal to, or multiples of the projectile core diameter (6 mm). In all tests, the standard 7.62 mm bullet is used. The ballistic test program also investigates the effect of the filler material configuration on target resistance to penetration, at 826 m/s. 2 Experimental Work Mild steel sheets (Ix2m) having 1, 2, 4, 6 and 8 mm thicknesses were used to prepare square plates (215x215 mm) with four corner holes <J>16mm/90. Hardness and tensile properties were measured in compliance with DIN specifications, and average values are listed in Table 1 Plate thickness t (mm) 1,2, 4 and 6 8 Table 1. Hardness and tensile properties of mild steel. Hardness Yield stress U. T. S. Failure strain BHN CTv(MPa) o»(mpa) Ef Fiberglass reinforced polyester (FRP) was used asfillermaterial between the steel laminae. This consisted of 4 plies/mmfiberglasswoven-fabric with polyester resin having 0.50% hardener. Density, fiber weight fraction, fracture stress and fracture strain were determined according to ASTM specifications, and average values were found to be 1.83 g/cnr*, 07%, 2.58 MPa, and 4.79%, respectively. Ballistic tests were mainly concerned with the determination of projectile impact and post-perforation velocities. The test setup consists mainly of a 7.62 mm launcher with smooth-bore launching tube, a test stand, and a target plate. Cartridge cases were emptied and then refilled with prescribed charges, in order to vary the impact velocity. Two OEHLER Skyscreens where placed 2m apart behind the target at 2m from the muzzle, and an OEHLER chronograph adjusted to read the post-perforation velocity. Experimental details are given elsewhere^. 3 Ballistic Test Results In the present study, the specific velocity drop, AVp, defined as the drop in projectile velocity divided by the total travel of the projectile through the target, was adopted to represent target resistance. Velocity drop (V -Vr, where V and V> are the impact and residual velocities, respectively) or energy loss have been alternatively used by other investigators for the same purpose*. To account for the effect of target thickness, the "specific energy loss", i.e. the loss in projectile kinetic energy divided b> the total plate thickness, was also adopted before as a representative parameter^. Table 2 lists all the ballistic test results obtained

3 Structures Under Shock And Impact 119 Expt No. l Table 2 Ballistic test results. Impact ResiduaJ Target configuration and code velocity vel. V, ' Vr (m/s) (m/s) I. Single and laminated steel targets: CC1:(8S) 8 mm singje steel target CC2: (2S-OA-6S) 2 mm steel - 6 mm steel; in contact (6S-OA-2S) 6 mm steel - 2 mm steel; in contact CC3:(4S-OA-4S) 2 (4 mm steel ); in contact. CC4: (2S-6A-6S) 2 mm steel + 6 mm steel, with 6 mm air gap. (6S-6A-2S) 6 mm steel + 2 mm steel; with 6 mm air gap. CC5: (4S-6A-4S) with 6 mm air gap CC6:(1S-6A-1S-6A-6S) 2 (1 mm steel) + 6 mm steel; with 6 mm air gaps. (6S-6A-1S-6A-1S) 6 mm steel + 2(1 mm steel); with 6 mm air gaps. CC7: (2S-6A-2S-6A-4S) 2 (2 mm steel) + 4 mm steel; with 6 mm air gaps. (4S-6A-2S-6A-2S) 4 mm steel + 2(2 mm steel), with 6 mm air gaps Vel. drop AV (m/s) Specific V-drop AVp

4 120 Structures Under Shock And Impact Expt No Table 2. Ballistic test results (continued). Impact Residual Target configuration and code velocity vet V, Vr (m/«) (m/s) CC8: (1S-6A-1S-6A-1S-6A-1S-6A-4S) 4 (1 mm steel) + 4 mm steel; with 6 mm air gaps. (4S-6A-1S-6A-1S-6A-1S-6A-1S) 4 mm steel + 4(1 mm steel); with 6 mm air gap* IL T-«h»«**«i sted-frp tarfets: (4S-6F24-4S) with 6 mm FRP (24 plies). (4S-12F48-4S) with 12 mm FRP (48 plies). (4S-24F96-4S) with 24 mm FRP (96 plies). (4S-24A-4S) with 24 mm air gap. (4S-9A-6S-9A-4S) 4 mm steel + 6 mm steel + 4 mm steel; with 9 mm air gaps (4S-8A-8S-8A-4S) 4 mm steel + 8 mm steel + 4 mm steel; with 8 mm air gaps (2S-12F48-2S-12F48-4S) 2 (2 mm steel) + 4 mm steel; with 2 (12 mm FRP, 48 plies). (2S-12A-2S-12A-4S) 2 (2 mm steel ) + 4 mm steel; with 12 mm air gaps. (2S-4.5A-3S-4.5A-2S-4.5A-3S-4.5A-4S) 2 (2 mm steel) + 2(3 mm steel) + 4 mm steel; with 4.5 mm air gaps. (2S-4A-4S-4A-2S-4A-2S-4A-4S) 2 (2mm steel) + 3 (4mm steel); with 4 mm air gaps. (2S-12F48-4S-12F48-2S) 2mm steel + 4mm steel + 2mm steel; with 2 (12 mm FRP, 48 plies) (2S-12A-4S-12A-2S) 2 mm steel + 4 mm steel + 2 mm steel; with 12 mm air gap* VeL drop AV (m/s) Projectile seized inside Incomplete penetration Incomplete penetration Incomplete penetration Incomplete penetration Specific V-drop AVp (m/s.mm )

5 Structures Under Shock And Impact 12J Expt No Table 2. Ballistic test results (continued) Impact Residual Target configuration and code velocity vcl. V, Vr (m/s) (m/s) (2S-4.5A-3S-4.5A^S-4.5A-3S-4.5A-2S) 2(2 mm steel) + 2 (3 mm steel) + 4 mm steel; with 4.5 mm air gaps. (2S-4A-4S-4A-4S-4A-4S-4A-2S) 2 (2 mm steel) + 3 (4 mm steel), with 4. 5 mm air gaps (6S-OA-8S) 6 mm steel + 8 mm steel; in contact (8S-OA-8S) 2 (8mm steel); in contact (2S-24A-6S) 2 mm steel + 6 mm steel; with 24 mm air gaps Vel. drop AV (m/s) Incomplete pent:tration Incomplete pene trati on Specific V-drop AVp (2S^-24A-6S^) 2 mm steel + 6 mm steel); with 24 mm air gap (4S^-24A-4S^) with 24 mm air gap (2S -12A-2S^-12A-4S*) 2 (2 mm steel ) + 4 mm steel; with 1 2 mm air gaps Results for laminated steel targets This set of results constitute 12 different configurations, in addition to the monolithic one (cf. Table 2, Expts. 1-13). Because no discernible differences in performance were found, configurations of experiments 2 and 3 were given the same code, CC2. Similarly, experiments 5 and 6 were coded CC4, experiments 10 and 11 coded CC7, and experiments 12 and 13 coded CCS. The effects of target configuration and impact velocity have been discussed in detail elsewhere^. In summary, configurations consisting of in-contact steel plates have higher ballistic resistance than those having the same number of laminae, but spaced. Also, within the test conditions reported herein, the specific velocity drop first increases, at a moderate rate, in the impact velocity range from 706 to 756 m/s. Then, it rapidly decreases in the range from 755 to 805 m/s, and continues to decrease, but at a much slower rat:, in the range from 805 to 826 m/s.

6 722 Structures Under Shock And Impact Effect of number and thickness of laminae Results of different target configurations are shown in Fig. 1 for a group of targets having a total steel thickness of 14 mm. Target effectiveness, expressed by AVp decreases with an increase in the number of laminae. This may be due to the entirely independent deformation of each lamina, since the structural deformation taking place during penetration acts as an energy absorbing mechanism^; in case of relatively thin plates, less specific energy is required to cause target failure. For the same number of laminae, targets with thicker back plates seem to exhibit higher resistance to penetration. The same phenomenon was also observed by previous investigations. Abdel-Kader^, for instance, attributed this response to the increase in amount of energy absorbed in bulging the thick back plate. When this plate is relatively thin, it fails in tension due to lack of structural rigidity Effect of the mechanical properties of the target Figure 2 compares the effectiveness of two groups of laminated target configurations consisting of spaced steel laminae. The first group was prepared from as-received plates of thickness 2 and 4 mm (Expts. 30, 17, 21). The second group, however, was prepared by machining 8-mm thick plates (having higher strength and toughness) into the required dimensions (Expts. 31, 32, 33). It is seen from this figure that, for the same configuration, AVp is lower for the as-received plates than for those prepared from the 8-mm thick plates. Normalizing AVp with respect to toughness (which combines both stress and strain), both groups were found to give almost identical results, as may be seen also in Fig. 2. Toughness of a laminated target was calculated as: U = Uj tj /.1 tj, where U is the target toughness, Ui is the toughness of the i* lamina, ti is its thickness, and n is the number of laminae constituting the target. 3.2 Results For Steel-FRP Laminated targets The second set of ballistic tests includes experiments number for laminated targetsfilledwith FRP composites Effect of FRP thickness Figure 3 shows the dependence of AVp on FRP thickness. As expected, AVp increases withfillerthickness at a decreasing rate Conversely, Gupta and Davids^ found the relation between energy loss and target thickness for steel and FRP of varying density to be nearly linear with a slope increasing with density Effect of target configuration Three target configurations with a total steel thickness of 8 mm and FRP thickness of 24 mm were tested (expts. 16, 20, 24). In each case, the projectile was unable to complete target penetration successfully, as shown in Fig 4.

7 Structures Under Shock And Impact 12 Specific Velocity Drop, avp (rasvmm) -4 m o? Et, *U mm 17 1 ~\JJ ^ - H H n! Configijration *r R$$ 33 ^,p 1 - iprri "{ me ^ "*Pd d w.! ^J -^ mp ^3 1 m^^ d 52J r^ I ys^ 23 Figure 1: Effect of number of steel laminae on target performance. E c fluo CL 2 a *0-2 "u Ol a. in M Otfo-waled from 6mm-plates o i ; i COI 31 %l w I rccffwo PIQIM f* A* r-<"- ^ V, =626.2 m/sec a 4i iljj 4Vp 4Vu P cn - b r- en Figure 2: Effect of mechanical properties of steel laminae on target performance Normalized Velocity Drop, avy (m.s^/j.mm"^)

8 124 Structures Under Shock And Impact Three experiments were run with the FRP filler removed (expts. 17, 21, 25), thus enabling the projectile to perforate the three configurations although at different residual velocities. It was found that the configuration consisting of two 4-mm steel plates is more effective than the other two (each having three laminae). It was also observed that the 3-lamina target with the 4-mm back plate is more effective than the one having a 2-mm back plate 7. This behaviour of steel plates was also reported by Abdel-Kader^. Next, a steel plate of weight equivalent to that of the 24-mm thick FRP was found to be 5.5 mm thick. Since such a steel plate was not available, a 6 mm steel plate was used instead. Thus three equivalent configurations were tested (expts. 18, 22, 26). It was found that these configurations were less effective than those with the FRP filler. The equivalent steel thickness was further increased to 8 mm and three more configurations were tested (expts. 19, 23, 27). The projectile could hardly perforate any of these configurations. Furthermore, two laminated steel configurations with total thickness 14 and 16 mm, respectively, were tested (expts. 28, 29); thefirstconfiguration was perforated while the second was not. Based on these results, it can be concluded that the steel plate thickness equivalent to the 24 mm FRP lies between 6 and 8 mm, thus possessing more weight. Therefore, from a practical point of view, especially when considering light-weight requirement, it seems advantageous to use steel-frp composite rather than steel alone. E = m/sec Q. O w Q >% 100 Of Q. Figure FRP Thickness, t, (mm) Effect of FRP thickness on target performance.

9 Projectile is seized inside the backjamina GROUP J U SECOND GROUP J U THIRD GROUP : 4: Effect of steel-frp laminated configuration on target performance

10 126 Structures Under Shock And Impact 4 Conclusions (a) Single steel target plates are more effective than laminated targets (consisting of in-contact or separated plies) of the same total thicness. The difference in effectiveness diminishes with impact velocity. (b) In terms of specific velocity drop, the ballistic resistance of steel targets first increases slightly, then decreases with impact velocity. (c) Resistance of laminated steel targets increases as the number of laminae decreases, and as the thickness of the back plate increases. (d) Use of fiberglass reinforced polyester asfillerin spaced laminated steel targets improves the ballistic resistance. 5 References 1. Backman, M. & Goldsmith, W The Mechanics of Penetration of Projectiles into Targets, Int. J. Engng. Sci., 1978, 16, Zukas, J.A., Nicholas, T, Swift, H.F., Greszcuk, L.B. & Curran, D.R. Impact Dynamics, pp , John Wiley & Sons, New York, Almohandes, A A Ballistic Resistance of Steel-Fiberglass Reinforced Polyester Laminated Plates, M.Sc. Thesis. Faculty of Engineering, Cairo University, Egypt, Recht, R.F. & Ipson, T W Ballistic Penetration Resistance and its Measurements, Exp. Mech., 1975, 15, Abdel-Kader, M.S. The Penetration Capability of High-Speed Projectiles Fired Against Brass and Steel Plates, M.Sc. Thesis. M.T.C., Cairo, Egypt, Eleiche, A.M. & Abdel-Kader, M.S. Effect of Shot Peening on the Ballistic Performance of Thin Plates, Structural Impact and Crashworthiness, ed. J. Norton, Vol. 2, pp , Elsevier Appl. Sci. Publ., London, U.K., Almohandes, A.A., Abdel-Kader, M.S. & Eleiche, A.M. Ballistic Performance of Single and Laminated Mild Steel Plates, Proc. 6th Applied Mechanics and Mechanical Engineering Conf. (2nd Intl.), M.T.C., Cairo, Egypt, Marom, I. & Bodner, S.R. Projectile Perforation of Multi-Layered Beams, Int. J. Mech. Sci., 1978, 21, Gupta, B P. & Davids, N. Experiments with Fiberglass-Reinforced Plastics, Exp. Mech., 1966, 6,