The deformation and fracture of β-HMX

A study has been made of the mechanical deformation properties of $\beta$-HMX, an important secondary explosive. It is shown that under compressive loading twinning takes place on the (101)-plane. At low loads, this twinning is elastic and usually precedes fracture. Cleavage in $\beta$-HMX takes place on the {011}-planes. The fracture surface energy of 0.06 J m$^{-2}$ has been determined by a micro-indentation technique. This compares with a value of 0.045 J m$^{-2}$ obtained for the thermodynamic surface energy from contact-angle measurements. The values suggest that there is relatively little energy loss by plastic deformation associated with crack propagation in HMX compared with, for example, the secondary explosives PETN and RDX. Despite this brittleness the twin deformation allows $\beta$-HMX to undergo large changes of shape: the significance of this in plastic-bonded explosives is commented on.