Peierls–Nabarro stresses of dislocations in monoclinic cyclotetramethylene tetranitramine ( β -HMX)

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The molecular crystal cyclotetramethylene‐tetranitramine (β‐HMX) is the active ingredient in some broadly used plastic bonded explosives. Plasticity is thought to be essential for reaction initiation and detonation. In this work we use molecular simulations to investigate the conditions under which dislocations nucleate homogeneously in β‐HMX. We evaluate the resolved shear stress leading to homogeneous nucleation of dislocation loops in the planes currently considered to be most important for plasticity in this crystal, (011) and (101), and at pressures up to 20 GPa. The importance of homogeneous nucleation for plastic deformation is discussed in relation to the other plasticity mechanisms operating in HMX. It is concluded that homogeneous nucleation is unlikely to happen at pressures above 5 GPa, while at pressures below this threshold homogeneous nucleation competes with shear localization. The article also presents γ‐surfaces for the (011) and (101) planes, which are used here to interpret the nucleation process and critical stresses, and have broader importance in other contexts. A discussion of the procedure required to compute γ‐surfaces in molecular crystals is included.

“…As reported in the literature and reviewed in the introduction,

((), 101)

and

((), 011)

are the most active slip planes [9, 15]. Based on this observation, we focus here on slip in these planes caused by loading in the [010] and

[10 \overline{1}]

for (101), and along

[01 \overline{1}]

and

([], 100)

for (011).…”

Section: Methods Sectionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 98%

[10 \overline{1}]

for the (101) plane, and [100] and

[01 \overline{1}]

for the (011) plane.…”

Section: Introductionmentioning

confidence: 99%

“…Systems

((), 011)

and

(01 \overline{1})

([], 011)

(01 \overline{1})

([], 100)

system.…”

Section: Introductionmentioning

confidence: 99%

“…Molecular packing indicates that the negative slip direction in the (011)[100] system is identical to the positive slip direction in the

(01 \overline{1})

([], 100)

system. The Peierls‐Nabarro stress [9], the temperature dependent dislocation mobility [27] and the pressure sensitivity of the flow stress [10] of these systems have been determined.…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Homogeneous Dislocation Nucleation in Molecular Crystal Cyclotetramethylene‐Tetranitramine (β‐HMX)

Zhang

Picu

2022

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Molecular Dynamics Predictions of Shock‐Induced Pore Collapse in (010)‐Oriented β‐HMX: Effects of Sample Thickness and Transverse Orientation, and Run‐To‐Run Variability among Statistically Equivalent Samples

Zhao

Perera

Sewell

2022

Using shock‐induced quasi‐2D pore collapse in β‐HMX as a specific case study, we address three practical questions that arise when designing and interpreting molecular dynamics (MD) simulations of explicit shock wave propagation in crystals. How sensitive are the overall results to sample thickness perpendicular to the (quasi) plane of the problem? For impacts on a given crystal surface, how much does the transverse orientation of the sample matter? And, for a given sample size and orientation, how much run‐to‐run variability exists among results for independent but statistically equivalent realizations of the shock? The first and second questions are interrelated but pertain individually to assessing the roles of finite‐size and crystal anisotropy effects, respectively, on the simulated collapse mechanisms and associated local thermo‐mechanical states in the sample during and after collapse. The third addresses the confidence with which the results of individual simulations can be regarded as representative. All three questions become particularly important if the MD predictions are intended to serve as “ground truth” for validation of continuum mesoscale models. Here, quasi‐2D samples of (010)‐oriented single‐crystal β‐HMX (β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane) containing a cylindrical pore were subjected to reverse‐ballistic impacts, resulting in explicit, supported shock propagation initially along [010], for impact speeds up =1.0 and 0.5 km s−1. The individual samples differed in either the thickness perpendicular to the sample plane or the transverse crystal orientation normal to [010] in the quasi‐2D cell. Three independent realizations of the shock were performed for a selected case to assess run‐to‐run variability. Comparisons of qualitative features of the collapse, temperature and pressure distributions in the samples, and time scales for pore collapse suggest that there is little sensitivity to sample thickness for the same crystal orientation and moderate sensitivity to transverse crystal orientation for samples of the same thickness. Run‐to‐run variability is evident to the eye in side‐by‐side system observations. However, overall mechanisms of collapse, distributions for temperature and pressure in the samples, and time scales for collapse are in near‐quantitative agreement among the realizations.

Effects of crystallographic orientation and strain‐hardening on nanoindentation response of cyclotetramethylene tetranitramine (‐HMX) single crystals

Paliwal

Picu

2023

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Mechanical behavior of cyclotetramethylene tetranitramine (HMX) is complex and insufficiently understood, but it is imperative as it affects the initiation of the decomposition reaction though hot spots formation. We present a single‐crystal plasticity model for β ${\beta }$ ‐HMX which accounts for pressure dependence of both the elastic constants and the yield surface. We evaluate the relative importance of pressure sensitivity of the elastic constants and plasticity, and of strain hardening, to the quasi‐static mechanical behavior of HMX, in general, and to indentation, in particular. Further, we study the effect of the in‐plane crystallographic orientations of the substrate with respect to the Berkovich indenter tip on the indentation force‐displacement curve. We conclude that pressure sensitivity of plasticity makes the largest contribution to defining the response to indentation, followed by strain‐hardening, sensitivity of which depends on hardening modulus, while the pressure sensitivity of elastic constants play a smaller role. Both the indentation plane normal orientation and the in‐plane indenter tip rotation may influence properties determined from nanoindentation test (e. g. hardness).