EXPERIMENTAL IMPACT FACILITIES

The following experimental impact facilities at AEEW were typical of those involved in collaborative agreements with French and German companies. As explained in Section 6.2 the provision of micro-concrete which accurately replicates the bonding between actual concrete, aggregate and steel armatures is of paramount importance. Because commercial suppliers were considered unable to meet the required standards of consistency, a small manufacturing laboratory was constructed with an associated suite of measuring devices to test the starting materials of cured concrete and reinforcing steel [106]. Considerable care was also taken to ensure the precise locations of reinforcements. Some 70 test specimens were taken in the form of cubes, cylinders and beams from each concrete mix, and some 10 to 20 pull-out discs were included in each target to assay the quality of the cured material. These tests generally confirmed a ratio ofcompressive to tensile strength of 10:1, which is typical of a prototype. Material data was

image215

~—I -5 -3 -1 1 3

logjo (Strain rate)

 

E Compressive strength of concrete311

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^7 -5 -3 -1 1 3

log1o (Strain rate)

 

E

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Figure 6.6 Typical Strain-Rate Enhancements for Reinforced Concrete

 

obtained using a 3 MN hydraulic press which could induce constant, ramp, triangular, sinusoidal or step loadings on a variety of test geometries.

To satisfy the velocities for replica scaling shown in Table 6.4, the two compressed-air guns shown in Figure 6.7 were constructed. The earlier and smaller Missile Launcher had the performance specifications:

Подпись: 3MJПодпись:Maximum projectable energy Interchangeable barrel IDs Projectile velocity range Target abutment

Compressed air within a reservoir provided the variable energy source and thereby a variable projectile velocity. A tubular barrel was separated from the reservoir by a thin diaphragm of metal or melamine according to the required operating pressure. Firing was activated as appropriate by an explosively fired metal dart or by electrically fusing an overlaid matrix of thin copper wires. By initiating a penetration longer than the critical length [96], a diaphragm collapsed virtually instantaneously and the missile itself or in a wooden sabot was accelerated by compressed air along the barrel. Missile velocities up to impact were measured by three independent systems: light beams, fine transverse wires and high­speed cine at 10,000 fps. In the event of target penetration, the subse­quent missile velocity was recorded by a similar fine-wire system, high speed cine and two induction loops. As well as visual records of the target’s response, analog tape recorders (3dB at 80kHz) monitored as many as 120 transients from linear displacement transducers and resistance-type strain gauges. Typical diamond sawn cross-sections of damaged reinforced concrete targets are shown in Figure 6.8 where a smooth missile entry occurs because concrete is some 10 times stronger in compression than in tension.

The Horizontal Impact Facility was constructed later to investigate primarily the regulatory compliance of irradiated fuel transport flasks. It had the performance specifications

Подпись: 2MJ 2000 kg 250 m/s 0.5, 1.0, 2.0 m 1000 tonneMaximum projectile energy Maximum projectile mass

Maximum projectile velocity Interchangeable barrel IDs Post-stressed concrete abutment

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Figure 6.7 The Winfrith Missile Launcher and Horizontal Impact Facility [285]

Projectiles were fired from the 0.5 and 1.0 m barrels in the same way as with the Missile Launcher, but firings with the 2 m barrel generally required special arrangements like that shown in Figure 6.9. Here a driver plate guided by four rails first propelled the cradled missile along

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Figure 6.8 Typical Cross Sections of Damaged Concrete Targets

six supporting and aligning rails in the breech. After a short distance the plate and trolley were rapidly arrested by buckling sacrificial lengths of aluminium tubing to release the missile along the barrel. Up to five high-speed cine cameras monitored an impact and an infrared system measured missile velocity. As can be seen by the wires atop of the driver plate in Figure 6.9, transducers were sometimes carried by the missile itself. Any induced pitch, roll or yaw on a projectile evidently reduces the direct energy of an impact, so careful engineering was necessary to restrict these to just ±1°. Drop height during flight was also constrained to within 120mm. Quarter-scale replicas of existing steel transport flasks and proposed reinforced concrete designs were successfully tested.

Though valuable in themselves, the principal benefit of these impact experiments lies in underwriting the development of computer simulations. These can now be confidently applied to a wide range of geometries and situations as outlined next in Section 6.4. In passing, the Missile Launcher and Horizontal Impact Facility found commercial applications such as bird strikes on helicopter blades, the survival of

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Figure 6.9 Large Replica in HIF Before Firing [285]

air-transport containers in extreme accidents and the effects of various projectiles on glass windscreens.

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