David Roylance

Ballistic Impact of Textile Structures

Previous work on transverse impact of single textile fibers is reviewed and extended to model orthogonal weaves in which fiber crossovers are simplified as pin. joints. A dynamic finite-element computer technique previously developed for single fibers is extended to model the woven panel, and this method is shown to produce results which are in sub stantial agreement with experimental observations of ballistic nylon panels. Impact of a woven textile panel is shown to exhibit substantial differences compared to the equivalent impact of a single fiber, primarily in that the propagating strain waves experience pervasive and complex interactions due to the influence of the fiber crossovers. The vast majority of ballistic energy is seen to be deposited in the orthogonal fibers passing through the impact point, while the other fibers are essentially ineffective, which suggests possible improvements in the design of textile structures intended for dynamic impact applications.

Penetration Mechanics of Textile Structures

Abstract : This report reviews those aspects of wave propagation and dynamic fracture relevant to the penetration mechanics of textile structures intended for use in personnel ballistic protection, and then describes the development and implementation of numerical analyses for use in instances for which closed- form analyses are intractable. These numerical treatments are used to assess the manner in which fiber material properties influence ballistic resistance, and this is done by performing simulations of missile impact on four fabrics of actual interest: ballistic nylon, Kevlar 29, Kevlar 49, and graphite. Following this parametric materials study, the numerical treatment is extended to include the effect of linear and non-linear viscoelastic relaxation on fabric response to impact. Finally, a special purpose computer code is described which was developed to study stress wave effects occurring at fiber crossovers.

Photographic Investigation of High-Speed Missile Impact upon Nylon Fabric Part I : Energy Absorption and Cone Radial Velocity in Fabric

Single layers of nylon fabric have been subjected to high-speed missile impact at velocities ranging from 116 to 537 m/sec. The transient responses of the fabric and missile have been observed by high-speed photography. The photo graphs have shown that fabric deformation was pyramidal before penetration and more conical after penetration. The photographs have permitted measurements of the missile exit velocity from the fabric, the missile energy loss due to interaction with the fabric, the time required to penetrate the fabric, and the size and growth rate of the resultant fabric deformation cones. These results, together with a simplified mechanical model, have indicated that the broken orthogonal yarns within the deformation cone could account for 50 100% of the observed missile energy loss. In addition, during penetration of the fabric, the measured average cone-radial velocities in the fabric ranged from 50 to 80% of the values derived from fiber impact theory.

Ballistics of Transversely Impacted Fibers

The rate-independent theory of transverse impact of textile fibers is reviewed and cast in a form that provides convenient preliminary guidance to designers of impact-resistant textile structures. It is shown that energy-absorption rate increases monotonically with fiber modulus, but that decreased ductility at high modulus may result in an optimum fiber stiffness for transverse critical velocity. A reaction-rate fraction model is suggested as a means of rationalizing the observed variation in experimental transverse critical velocities.

Stress wave propagation in fibres: Effect of crossovers

Abstract A direct numerical solution is described which models the wave propagation dynamics of a system of two crossed fibres, one of which has been subjected to transverse ballistic impact. The model provides a means of assessing the influence of fibre materials properties and fibre-fibre slip on the complex wave phenomena occurring at fibre crossovers. This is a matter of considerable importance in rationalising the performance of textile structures used as ballistic protection devices.

Influence of fibre properties on ballistic penetration of textile panels

Abstract A number of computer simulations have been performed in order to assess the ballistic penetration resistance of a series of textile panels. The results indicate that the rate of energy absorption of the panel increases monotonically with the fibre modulus, but that very high modulus material tends to exhibit poor impact resistance due to its low breaking strain. Aramid fibre seems to exhibit the best combination of high modulus while still maintaining reasonably high breaking strain. A ‘master-curve’ description of impact response has been developed from the computer results, which may be useful in minimising the number of full computer simulations necessary in the design of impact protective devices.

Transverse moisture sensitivity of aramid/epoxy composites

Transverse tensile properties of Kevlar 49 aramid/Fiberite 934 epoxy composites have been measured as a function of moisture content. Moisture effects are substantial: losses of 14% in stiffness, 35% in strength and 27% in elongation were observed at 25‡ C. These losses were caused by the expected degradation mechanisms of matrix plasticization and interface weakening and also by an unexpected mechanism which altered the filaments at higher moisture contents. The filaments appear to become more radially compliant with the introduction of moisture and tend to crack internally. This behaviour may be caused by the moisture interrupting what hydrogen bonding is initially present in the filament defect structure.

Stress Wave Damage in Graphite/Epoxy Laminates:

A series of unidirectional graphite/epoxy laminates of various fiber moduli have been subjected to uniaxial-strain shock loading to assess their resistance to dynamic loading. All of the laminates were found to exhibit nearly identical values of wave attenuation and shock hardness, and the resistance to impact in the through-thickness direction was ascribed prima rily to the response of the epoxy matrix.

An experimental investigation of some models of polymer fracture

It has recently been shown that the techniques of EPR spectroscopy can be used to monitor atomic bond rupture during fracture of polymeric materials and thus provide important new insights of the molecular mechanisms of fracture in these materials. In particular, this method can be used as a fundamental check of the several atomistically-derived theories of polymer fracture. In this paper such a check is made of two representative such theories, those of S. N. Zhurkov and W. G. Knauss. It is shown that these theories do not provide physically realistic models of the fracture mechanisms in oriented polymeric fibers and it is suggested that their lack of relevance is due to the unique morphological structure of highly oriented polymers.RésuméOn a montré récemment que les techniques de spectroscopie électronique a résonnance paramagnétique peuvent être utilisées avec succès pour la detection des ruptures des liaisons atomiques au cours de la rupture des matériaux polymeres; ces techniques ont permis de jeter de nouvelles lumières sur les mécanismes moléculaires qui interviennent au cours de la rupture de ces matériaux.En particulier, la méthode permet de procéder a un controle, sur une base fondamentale, de quelques théories atomiques de la rupture des polymères.Dans le présent mémoire, on procède à un tel controle de deux théeories des plus typiques, celles de Zhurkov et de Knauss.On démontre que ces theories ne conduisent pas à des modèles physiquement représentatif du mécanisme de rupture des polymères à fibres orientées. On suggère que cette inadaptation résulte de la structure morphologique particulière des polymères à orientation marquée.ZusammenfassungNeuerdings wurde gezeigt, daß die Verfahren der elektronischen Spektroskopie erfolgreich für den Nachweis von Zerstörungen in den atomaren Bindungen während des Zerreissens von Polymeren herangezogen werden können. Sie führen somit zu wichtigen neuen Erkenntnissen über die molekularen Mechanismen der Zerstörung dieser Werkstoffe.Im besonderen kann dieses Verfahren für eine grundlegende Kontrolle verschiedener atomistisch abgeleiter Theorien über den Bruch von Polymeren herangezogen werden. Im vorliegenden Bericht werden zwei dieser Theorien, nämlich diejenigen von S. N. Zhurkov and W. G. Krauss, überprüft.Es wird gezeigt, daß these Theorien kein physikalisch realistisches Modell des Bruchmechanismus von Polymerfasern geben, and es wird vorgeschlagen diesen Mangel an Zutrefflichkeit durch die spezielle morphologische Struktur der streng gerichteten Fasern zu erklären.Major portions of this work were supported by the National Science Foundation and the National Aeonautics and Space Administration.

Influence of Outdoor Weathering on Dynamic Mechanical Properties of Glass/Epoxy Laminate

A commercially obtained glass/epoxy composite material has been exposed to a wide variety of actual outdoor and accelerated climates at different test sites around the world. Portions of this sample population are returned at various intervals for an extensive laboratory characterization aimed at determining the effect of exposure on engineering properties and elucidating the nature of the deterioration mechanism as well. In spite of clear visual and chemical evidence of severe photolytic degradation of the resin surface layers, test values of tensile and flexural strength do not show a clearly defined correlation with this deterioration. Reasoning that the structural effect of photolytic molecular scission might be to interrupt the three-dimensional network of resin bonds and increase the effective molecular weight between crosslinks, the glass transition temperature of the laminates has been determined by torsional pendulum analysis as a function of exposure time. The molecular weight between crosslinks, as determined from these data, is then shown to correlate strongly with laser pyrolysis observations of chemical degradation.