Indentation fracture behavior of energetic and inert molecular crystals
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Indentation fracture behavior of energetic and inert molecular crystals Alexandra C. Burch1,2
, John D. Yeager2, David F. Bahr1,a)
1
School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, USA; and Explosive Science and Shock Physics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA 2 Explosive Science and Shock Physics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA a) Address all correspondence to this author. e-mail: [email protected] Received: 31 July 2019; accepted: 29 October 2019
Measuring the elastic and plastic properties with nanoindentation is predicated on the indentation not fracturing the material. In this study, an unloading curve analysis is used to identify indentation-induced fracture in brittle molecular organic crystals to define conditions, where properties measurements are accurate, and for calculating the toughness. Single crystals of cyclotetramethylene tetranitramine (HMX) and idoxuridine were indented from 1 to 300 mN with indenter probes of varying acuity to identify fracture initiation loads. Idoxuridine displayed no fracture up to and at 100 mN, with fracture occurrence then seen at an increasing rate until every indentation made induced fracture at 300 mN. HMX displayed no fracture up to and at 4 mN, with fracture then occurring at an increasing rate until every sample fractured at 8 mN. The toughness of HMX and idoxuridine is z0.28 z 0.4–0.5 MPa/m1/2, respectively.
Introduction Molecular crystalline solids include explosives, such as cyclotetramethylene tetranitramine (HMX), and inert materials, such as pharmaceutical 5-iodo-29-deoxyuridine (idoxuridine). These materials are typically processed in particulate form due in part to their brittle nature and in part to their applications, where high surface area may be desirable (e.g., faster drug dissolution rate). While the mechanical properties of molecular crystals are not the primary concern for many pharmaceuticals, these properties impact compaction [1, 2, 3, 4, 5] and amorphization [6, 7] and contribute to composite mechanical properties when formulated with a binder [8]. While the particulate form of materials may be more difficult to test mechanically than larger samples, a larger sample may not be representative of the mechanical behavior of the particulate, particularly in molecular crystals, where producing a larger crystal often requires altering the growth conditions that are used in precipitation of particulate matter. Idoxuridine is a potential mock for HMX. In this context, a mock is an inert material that has certain properties in common with the explosive and can be used in place of the explosive in experiments when safety is of particular concern.
ª Materials Research Society 2019
Idoxuridine was initially chosen to mock HMX because of its similarity in density, thermal stability, and solubility [9]. Viability as a mechanical mock was evaluated with nanoindentation of single crystals [10]. The results of this were promising, with an 8% difference betwe
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