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An Introduction to Materials
Published in Paul J. Hazell, Armour, 2023
Instrumented drop towers are not usually used to measure the dynamic properties of materials, although they can be configured to do so. A single drop tower can be configured for both compression and tensile tests, and the main data collection is usually achieved through a piezo-elastic transducer located in the impactor (or ‘tup’). Equally, strain data are acquired through the use of a strain gauge applied to the sample or by visual means using a high-speed camera. Velocities of impact are quite modest (~1–25 m/s), and the main purpose of the machine is to simulate an impact from a dropped mass at height by either dropping the tup mounted on a carriage from the actual height or from a ‘simulated’ height through the use of spring acceleration or a bungee cord attached to the carriage and base.
Influence of Nanoparticles and Effect of Defects on Mode I and II Fracture Toughness and Impact Resistance
Published in Frank Abdi, Mohit Garg, Characterization of Nanocomposites, 2017
Christos C. Chamis, Frank Abdi, Harsh Baid, Parviz Yavari
The full potential of an advanced composite material system is not realized because of mostly brittle nature of the matrix (epoxy) used to bond the strong fibers together. The brittle behavior is the result of molecular cross-links in the epoxy. Although the cross-link density improves the stiffness, it adversely affects the toughness and delamination resistance properties of the epoxies. Uddin et al. (2008) recently showed experimentally that addition of 15 wt% silica nanoparticles (Nanopox F400) in DEGBA epoxy helped increase the Mode I and Mode II fracture toughness. In addition, reduction in damage footprint was observed for low-velocity impact (drop tower) tests. Similar experimental studies have been performed earlier that show improvement in fracture toughness and impact resistance by enriching the matrix with nanoparticles of different shapes and sizes.
Fracture Toughness Testing of Metals
Published in T.L. Anderson, Fracture Mechanics, 2017
High loading rates can be achieved in the laboratory by a number of means, including a drop tower, a high rate testing machine, and explosive loading. With a drop tower, the load is imparted to the specimen through the force of gravity; a crosshead with a known weight is dropped onto the specimen from a specific height. A pendulum device such as a Charpy testing machine is a variation of this principle. Some servo-hydraulic machines are capable of high displacement rates. While conventional testing machines are closed loop, where the hydraulic fluid circulates through the system, high rate machines are usually open loop, where a single burst of hydraulic pressure is released over a short time interval. For moderately high displacement rates, a closed-loop machine may be adequate. Explosive loading involves setting off a controlled charge which sends stress waves through the specimen [21].
Soft body armour
Published in Textile Progress, 2019
Unsanhame Mawkhlieng, Abhijit Majumdar
Dynamic test is conducted to evaluate the energy absorption capacity of materials during LVI. In its simplest form, the test facility, known as drop tower, consists of a weighted indenter that falls over a mounted sample at a predetermined velocity (or from a fixed height) and the force applied on the specimen, against time and displacement, is recorded by the strain gauge or piezo sensors attached with the indenter. A schematic diagram of a drop tower impact tester is shown in Figure 26. Impact velocities can range from 1–24 m/s [138]. The lower velocities can be achieved by free fall of the striker. However, for higher velocities, a spring launching system is used to propel the striker. The commonly used standards for impact testing are ASTM: D3763 (for fabrics) or ATSM D7136 (for composites/laminates). The principle of operation is based on the conservation of energy, i.e., the energy absorbed by the material is the difference between the incident (potential) energy and the transmitted (kinetic) energy. When a tightly woven high-performance fabric was subjected to this LVI, a dome was formed outward, indicating that there was sufficient time for the stress waves to travel to the boundaries of the fabrics [139], thereby utilizing a larger area of the fabric for energy absorption as shown in Figure 27.