The Spall Strength of the Polytetrafluoroethylene Irradiated above the Melting Point

The features of the deformation and fracture of polytetrafluoroethylene (PTFE) before and after exposure to gamma radiation above the melting point of the crystalline phase are investigated using shock wave loading methods at various impact velocities from 100 to 600 m/s, which corresponds to shock compression pressures of 0.17 ÷ 1.1 GPa. The acceleration of the impactors was carried out using a 50 mm pneumatic gun. The dependences of the spall strength of the two studied materials on the pressure at the shock wave front are constructed and compared by means of the analysis the velocity profiles of the free surface measured using the VISAR laser Doppler velocimeter. The range of strain rates ranged from 106 to 108 s–1 under compression and 104 to 105 s–1 in the rarefaction wave. PTFE irradiated in a melt has been studied under shock wave loading for the first time. It has been established that the spall strength of irradiated PTFE is constant and equal to 80 ÷ 85 MPa over the entire pressure range studied, which is 1.5–2 times higher than that of conventional PTFE (40 ÷ 45 MPa at pressures up to 0.6 GPa), and 3–4 times higher than the tensile strength of PTFE before and after irradiation at quasi–static tensile strength (15 ÷ 30 MPa). An upper estimate of the maximum tensile stresses showed that a complete rupture of PTFE occurs at stresses of 135–170 MPa, and irradiated PTFE at 220–250 MPa. The obtained estimates of tensile strength in the microsecond range of loading durations exceed the static tensile strength for PTFE by about 5–6 times (20 ÷ 30 MPa), and for irradiated PTFE by 15 times (15 MPa). The minimum pressure at which complete separation of the spall plate and fracture into several fragments is observed increases approximately twice after irradiation of PTFE in the melt and amounts to 0.55 GPa. The fracture into multiple fragments was recorded for irradiated PTFE at velocities and pressures also approximately twice as high as for conventional PTFE: 590 m/s and 1.09 GPa.