  Membranes of carbon allotropes comprised solely of densely packed pentagonal
rings, known as pentagraphene, exhibit negative Poisson's ratio (auxetic
behavior) and a bandgap of $3.2$ eV. In this work, we investigated the
structural stability, mechanical and fracture properties of nanotubes formed by
rolling up pentagraphene membranes, the so-called pentagraphene nanotubes
(PGNTs). Single-walled PGNT of three distinct configurations: zigzag,
$\alpha$-armchair, and $\beta$-armchair were studied combining first-principles
calculations and reactive molecular dynamics simulations. Our results showed
Young's modulus values of $680-800 GPa$, critical strain of $18-21\%$, and
ultimate tensile stress of $85-110 GPa$. We also observed auxetic behavior.
During stretching at room temperature, we observed a transition between
$\beta$-armchair to $\alpha$-armchair PGNT close to the critical strain. With
relation to fracture patterns, we observed that mechanical failure starts at
bonds mostly aligned to the stretching direction and after tube radial
collapse.
