A Mechanism of Hot‐spots Formation at the Crack Tip of Al‐PTFE under Quasi‐static Compression

Abstract

Generally, the Al‐PTFE (polytetrafluoroethylene) is thought to be inert under quasi‐static or static loads. However, it was found that Al‐PTFE would initiate under quasi‐static compression after a specific heat treatment procedure and the opening fracture plays a crucial role in the initiation. A unique micrographic fracture pattern which showed unstable crack propagation and a ductile‐to‐brittle transition was observed at openning cracks by SEM. Combining the observed microstructure with the stress distribution at the path of crack propagation derived from numerical simulation, a mechanism was proposed to explain the formation of “hot‐spots” at the crack tip. The temperature rise at the crack tip was estimated to be at least 612 °C, which is high enough to ignite the Al‐PTFE composite.

Sensitivity of Al‐PTFE upon Low‐Speed Impact

“…The much longer ignition time indicates that the reaction occurred long after the passing of shock wave, consequently the SIR process is unlikely the controlling reaction mechanism in low-speed impact. On the other hand, the experimental evidence shows the ignition was originated from the mode-I cracks, which is very similar to the crackinduced ignition mechanism reported in quasi-static compression of Al-PTFE [18].…”

Influence of Temperature on the Mechanical Properties and Reactive Behavior of Al-PTFE under Quasi-Static Compression

“…As expected, the mechanical response showed significant temperature dependence. At low temperatures (−18, 0, and 16 °C), a stress-drop phenomenon was observed before the failure of samples, which was not manifested at high temperatures (22,35, and 80 °C). As can be seen from Figure 7, Al-PTFE underwent a gradual transition from brittleness to ductility with increased temperature.…”

“…As expected, the mechanical response showed significant temperature dependence. At low temperatures (−18, 0, and 16 • C), a stress-drop phenomenon was observed before the failure of samples, which was not manifested at high temperatures (22,35, and 80 • C). as an example, the stress-strain curves of the three Al-PTFE samples almost overlapped together ( Figure 5), which provided confidence that the data obtained in this paper are reliable.…”

“…In brief, sufficient toughness (the ability of material to absorb energy during deformation, which could be obtained by calculating the area under the stress-strain curve) and an abrupt energy release are two decisive factors for the reaction of the Al-PTFE sample under quasi-static compression. As shown in Figure 14, due to the temperature-induced crystalline phase transformation of the PTFE matrix, the toughness of Al-PTFE showed a 10% increase, from 90.68 J/cm 3 at 16 °C (phase II) to 99.31 J/cm 3 at Feng et al [35] thought that the reaction phenomenon occurs due to the abrupt energy release along the opening crack in the Al-PTFE sample. The abrupt release of energy absorbed during deformation could cause the temperature to rise high enough to induce a chemical reaction between Al and PTFE at the opening crack tip.…”

“…The whole process lasted about 2360 ms (Figure 13d). Feng et al [35] thought that the reaction phenomenon occurs due to the abrupt energy release along the opening crack in the Al-PTFE sample. The abrupt release of energy absorbed during deformation could cause the temperature to rise high enough to induce a chemical reaction between Al and PTFE at the opening crack tip.…”