## 4 Results and Discussion
### SHTB Validation  
The dynamic tensile testing at high strain rates was conducted on the SHTB apparatus presented in Section 3.2. Figure 7a shows the raw voltage obtained from the strain gauges. The gauges 1 and 2 were installed on the input bar and the gauge 3 on the output bar. The generated stress pulse had an approximate trapezoidal shape with a duration of \(T~{}\approx\) 1 ms. A final peak was observed in the input signal (gauge 1) due to the overlap with the reflected wave generated at the interface between the input bar and clamp. The clamp, with higher impedance than the hollow tube, induced a reflected wave of same sign and lower magnitude, and transmitted to the specimen a tensile wave of same sign and higher magnitude [46].  
The output bar was sensitive enough to monitor the response of the fabric registering a maximum force of \(\approx\)1 kN. Figure 7b shows the forces at the ends of the input and output bars. The time \(t~{}=~{}0\) ms indicates the onset of the stress propagation from the input grip. The response has been divided in three different stages: (i) onset of propagation of the tensile pulse, (ii) first reflection of the transmitted wave at the output bar interface and (iii) end of the tensile pulse. The wave propagation velocity of the nonwoven fabric was determined from the time lapse between the input and output forces (point (i) to (ii)), resulting in \(c~{}\approx~{}827.5\) m/s, exceptionally slow compared to the wave propagation  
Figure 6: Representative nominal stress vs. engineering strain curves for different gauge lengths.  
Figure 7: (**a**) Voltage vs. time for each strain gauge. The gauges 1 and 2 were installed in the input bar and the gauge 3 was installed in the output bar. (**b**) Forces vs. time monitored at input and output bar ends.  
velocity of bulk semi-crystalline polyethylene (3800 m/s, [47]) or the theoretical wave propagation velocity of ultrahigh molecular weight polyethylene fibres (11,600 m/s, [48]), but in agreement with previous values registered for this fabric, \(\approx\)600 m/s [27]. The low wave propagation velocity and the large gauge length hindered the development of a theoretical force equilibrium state between bars. Input and output forces achieved similar force values (\(\approx\)1 kN) after \(t~{}=~{}0.7\) ms; however, the input bar signal presented significant vibrational noise of same magnitude of the obtained force.  
Digital Image Correlation (DIC) was employed to analyse the wave propagation in the material. The measurements were validated against the velocities monitored by the strain gauges, finding very good correlation; see Figure 8a. The maximum velocity at the input interface was registered as \(v_{inp}\approx 14.5\) m/s, with an output bar velocity virtually zero. Once the tensile pulse was exhausted, the input velocity decreased rapidly. The DIC contour plots registered a gradual transition of imposed velocities along the gauge length; see Figure 8b,c, and the wave end of the tensile pulse; see Figure 8d,e. The maximum input bar velocity was in agreement with the estimated analytical bound obtained by one-dimensional wave propagation theory considering the relationship between bar strains (or stresses) and particle velocity [45]:  
\[\sigma=vc\rho \tag{1}\]  
where \(\sigma\) stood for the incident pulse (0.11 GPa), and \(\rho\) and \(c\) stood for the density and the wave speed in the aluminium material, \(\rho~{}=2800\) kg/m\({}^{3}\) and \(c~{}=~{}5070\) m/s, respectively. The resulting theoretical bar velocity was approximately \(v~{}=7.75\) m/s. Due to the strong impedance mismatch between the aluminium tube and the specimen, nearly all the incident tensile wave was reflected at the specimen/bar interface with same magnitude and opposite sign, duplicating the particle velocity imposed at the input interface (\(14.5\approx 2v=15.5\) m/s).