Search

The behaviour of metals under mechanical loading, including shock loading conditions is strongly influenced by effects such as impurity levels, grain size, initial dislocation density and texture. The work discussed here is part of a wider study on the effects of orientation of aluminium single crystals to one dimensional shock loading, including the Hugoniot Elastic Limit and spall strength. In this work, specimens with three principle directions (, and ) parallel to the loading axis have been shock loaded and recovered under conditions of purely one-dimensional strain, with their post shock response monitored by quasi-static compression tests. Results show that the crystal demonstrates a significant degree of post shock hardening, whilst the crystal shows virtually none, and the intermediate between the two. These results are consistent with the ordering of both the HELs and spall strengths observed in a previous paper, and have been explained in terms of the Schmidt factors.

For ductile metals, the process of dynamic fracture occurs through nucleation, growth and coalescence of voids. The stress required to nucleate these voids is inferred from the velocimetry data (using the acoustic approach) and termed as the spall strength. This is a key parameter that is used to evaluate a material’s susceptibility to damage and failure. However, it is also well recognized that the dynamic parameters used to generate the shock state such as pulse duration, tensile strain-rate and peak stress coupled with material microstructure itself affect the material response in a complex manner. Yet, it is impossible to capture all this information by assessing only the spall strength measured from simple one-dimensional Photon Doppler Velocimetry measurements. Although, there exist widely used corrections proposed by Kanel et. al. that allow for the inclusion of some of these complexities into the measured spall strength but still does not take the microstructure into account. In this work, we propose another scheme for normalization of spall strength with a damage area to capture the complexities included in the damage and failure process especially pertaining to microstructure. We will also demonstrate the application of this scheme by applying to examples of materials such as Copper, Copper-24 wt%Ag, Copper-15 wt% Nb and additively manufactured 316L SS.

Spall fracture is a high-rate tensile damage phenomenon associated with impulsive and shock-load events. Typically, the material undergoes a sequence of compression followed by release into high rate (on the order of 104 s-1 and up) tension, causing voids to nucleate and grow, which can then coalesce into a crack and the material fails. We present a series of experiments on high purity, well characterized tantalum samples subjected to shock-loading via gas-gun plate impact. Through careful selection of the flyer-plate velocity and material we have independent control over the peak compressive stress and the tensile strain rate in the sample. At all times, the spall damage remains incipient, i.e. in the early stages of void formation and the material does not fully fracture. Velocimetry was used on the rear of the sample to record the wave-profiles and determine spall strength. Soft recovery and sectioning of the samples allowed the internal damage to be observed, quantifying the damage amount, distribution, and relationship to microstructural features with both optical and electron based microscopy.

The influence of shock prestraining, via direct energetic "Taylor-wave" (triangular wave) loading, on the post-shock structure/property behavior of depleted uranium (DU) was studied. Samples were shock prestrained within a "soft" shock recovery fixture composed of momemtum traps and a spall plate to assure 1-dimensional loading. The DU samples exhibit roughly a 30% increase in yield strength following shock prestraining to ∼45 GPa. The texture evolution in DU was quantified using electron-backscatter diffraction (EBSD). Detailed quantification of the substructure evolution following shock prestraining revealed high volume fractions of {130), '{172}', and {112) deformation twins. The volume fraction of (130) twins was found to be ∼10x the volume fraction of '{172}' and {112} twins. Details of the twin system activation and volume fraction relative to the local Schmidt factor within grains are presented. The influence of HE-driven shock prestraining on the structure/property response of DU is compared and contrasted to that seen in 304SS and 316SS subjected to "Taylor-wave" shock prestraining.

Fluoropolymers posses several advantageous properties for load-bearing structural components including higher strength at elevated temperatures and higher toughness at lowered temperatures. Stress-strain studies have been conducted on two homopolymers (Teflon (PTFE) and KEL-F 81 (PCTFE)) and two copolymers (KEL-F 800 and THV 500G) as a function of temperature and strain rate. A split-Hopkinson pressure bar (SHPB) was used to achieve a strain rate of ∼3200/s in compression, while screw-driven testing machines afforded a strain rate of 0.001/s. A tensile split-Hopkinson pressure bar (T-SHPB) was used for testing at strain rates of 800/s in tension. To address the large strains to failure (up to 700% extension) an optical extensometer was employed for a strain rate of0.005/s in tension. The effects of temperature and strain rate for this family of polymers is presented in detail, including an observed shift in glass transition (T g ) to higher temperatures with increased strain rate and rate insensitivity of thermally-induced phase transitions.

Highly-textured, crystal-bar-purity hafnium has been shock prestrained at 4.96 and 11 GPa, below the reported alpha to omega phase transformation of 38 GPa. The specimens were "soft caught" following shock prestraining to facilitate quantification of the substructure and post shock constitutive behavior. The substructure of the shocked materials was investigated using optical and transmission electron microscopy. The preshocked Hf samples were subsequently reloaded quasi-statically in compression. The deformation behavior of the as-annealed hafnium under quasi-static conditions is compared to its response following shock prestraining. The reload response is correlated to differences in the defect density due to shock loading.

A rigorous experimentation and validation program is being undertaken to create constitutive models that elucidate the fundamental mechanisms controlling plasticity in uranium-6 wt.% niobium alloys (U-6Nb). These models should accurately predict high-strain-rate large-strain plasticity, damage evolution and failure. The goal is a physically-based constitutive model that captures 1) an understanding of how strain rate, temperature, and aging affects the mechanical response of a material, and 2) an understanding of the operative deformation mechanisms. The stress-strain response of U-6Nb has been studied as a function of temperature, strain-rate, and thermal aging. U-6Nb specimens in a solution-treated and quenched condition (ST/Q) and after subsequent aging at 473K for 2 hours were studied. The constitutive behavior was evaluated over the range of strain rates from quasi-static (0.001 sec -1 ) to dynamic (∼2000 sec -1 ) and temperatures ranging from 77 to 773K. The yield stress of U-6Nb was exhibited pronounced temperature sensitivity. The strain hardening rate is seen to be less sensitive to strain rate and temperature beyond plastic strains of 0.10. The yield strength of the aged material is less significantly affected by temperature and the work hardening rate shows adiabatic heating at lower strain rates (1/s).

The shock induced mechanical response of two γ-titanium alurninides has been investigated using the technique of plate impact. It has been found that the HEL, spall strength and shear strengths are all dependent upon the initial microstructure, with the material possessed of the duplex microstructure being the stronger. This is in agreement with quasi-static measurements. Shear strength has been observed to increase rapidly with increasing longitudinal stress, more so than in Ti-6Al-4V, indicating that the high degree of work hardening is an important factor in determining the shear strength of metallic and intermetallic materials.

Compression stress-strain measurements have been conducted on two commercial high temperature thermoplastic polymers (polyamideimide (PAI) and partially crystalline polyetheretherketone (PEEK)) as a function of temperature (-55°C to 300°C) and strain rate (0.001 s -1 to 2500 s -1 ). A split-Hopkinson pressure bar (SHPB) was used to achieve strain rates of about 2500 s -1 and conventional testing machines were used for strain rates from 0.001 s -1 to 1 s -1 . A large variation in the mechanical response was observed over the range of temperatures tested for both polymers showing yield and plastic deformation below the glass transition temperature (T g ). Above T, both thermoplastic polymers exhibited increasing loading moduli with increasing strain rate or decreasing temperature. T g appears to shift to higher temperatures as the strain rate increases. Below T g both thermoplastic polymers exhibited a yield-type behavior followed by anelastic deformation. PEEK was seen to be more sensitive to strain rate and temperature than PAI. A range of different flow stress versus strain behaviors was observed at different temperatures and strain rates.

The influence of temperature, strain rate history, and shock-wave prestraining on the mechanical behavior of Armco-iron (Fe) is presented. Shock-wave prestraining of iron above the polymorphic α-e transition is seen to result in significant changes in the stress-strain response of Fe. The increase in the constitutive response of Fe is seen to correspond to an increase in hardness in the preshocked Fe. A high degree of correlation in the experimental results independently obtained in Russia and USA is presented. The observed mechanical property effects in preshocked Fe are correlated to the microstructure evolution in iron following explosive prestraining.

It is a pressing objective to understand the mechanical behaviour of polymeric materials at high strain rate for a range of industrial and defence applications. Some are used as the hinder phase in plastic bonded explosives (PBXs) and propellants. Others are used either as components for structures or as the binder phase in various composite systems. Such materials need to be understood so that their response may be understood and constitutive descriptions constructed. This work presents experimental data focused at the evaluation of the equation-of-state (EOS) and strength behaviour of four selected polymers. The equation of state and the shear strength of each polymer were measured as a function of impact stress and this gives insight into the role of the microstructure and its relation to response.

A new specimen design has been developed to measure the shear response of materials. This compact forced-simple-shear specimen (CFSS) has been utilized to measure the shear stress/shear strain response of other materials [1, 2]. Earlier, unpublished work on the shear response of beryllium using a split Hopkinson pressure bar (SHPB) with the shear compression specimen (SCS) [3] had limited success at higher strain rates due to compressive deformation in the web leading to tensile failure in the samples. The CFSS geometry was engineered to produce essentially “pure” simple shear, mode II in-plane shear, in a compact-sample geometry that eliminates the multi-mode loading of other sample geometries and produces direct measurements of shear deformation. The use of digital image correlation (DIC) to capture and calculate shear stress and shear strain when paired with this test geometry will be described. There are two competing mechanisms for the deformation in beryllium were observed, a brittle to ductile response due to increased temperature and a transition of the deformation mechanism from dislocation dominated slip to twin deformation as the strain rate is increased. The advantages of this specimen geometry and test results as a function of temperature and strain rate are discussed for high purity beryllium.

Under sufficient stresses, such as during dynamic loading, titanium experiences a phase transformation from hcp alpha phase to hexagonal omega phase. Omega phase is often retained in the microstructure after unloading, and has a strong influence on subsequent mechanical properties. Simulations suggest there are multiple pathways and underlying mechanisms for this transformation. Due to the incredibly short timescales involved, experimental measurements for model validation have been difficult. However, new capabilities at the Advanced Photon Source have enabled diffraction measurements during plate impact experiments to study the evolution of titanium during transformation. These high-rate data allow us to probe the mechanism and kinetics of phase transformations in new ways. Recent results will be presented and compared to post-mortem characterization of soft-recovered shocked specimens. Comparisons are made with previous tests where material was shock-loaded and soft recovered for microstructural analysis. Together these techniques create a consistent picture of material behavior during the shock-induced ff–! phase transformation in titanium.

For Certification and qualification of an engineering component generally involves meeting engineering and physics requirements tied to its functional requirements. In this paper, the results of a study quantifying the microstructure, mechanical behavior, and the dynamic damage evolution of Tantalum (Ta) fabricated using an EOS laser-powder-bed machine are presented. The microstructure and quasi-static mechanical behavior of the AM-Ta is detailed and compared / contrasted to wrought Ta. The dynamic damage evolution and failure response of the AM-Ta material, as well as wrought Ta, was probed using flyer-plate impact driven spallation experiments. The differences in the spallation response between the AM and wrought Ta were measured using in-situ velocimetry as well as post-mortem quantification of damage in “soft-recovered” samples. The damage evolution of the AM and wrought Ta were characterized using both optical metallography and electron-backscatter diffraction.

For additive manufacturing (AM), the certification and qualification paradigm needs to evolve as there exists no “ASTM-type” additive manufacturing certified process or AM-material produced specifications. Accordingly, utilization of AM materials to meet engineering applications requires quantification of the constitutive properties of these evolving materials in comparison to conventionally-manufactured metals and alloys. Cylinders of 316L SS were produced using a LENS MR-7 laser additive manufacturing system from Optomec (Albuquerque, NM) equipped with a 1kW Yb-fiber laser. The microstructure of the AM-316L SS is detailed in both the as-built condition and following heat-treatments designed to obtain full recrystallization. The constitutive behavior as a function of strain rate and temperature is presented and compared to that of nominal annealed wrought 316L SS plate. The dynamic damage evolution and failure response of all three materials was probed using flyer-plate impact driven spallation experiments at a peak stress of 4.5 GPa to examine incipient spallation response. The spall strength of AM-produced 316L SS was found to be very similar for the peak shock stress studied to that of annealed wrought or AM-316L SS following recrystallization. The damage evolution as a function of microstructure was characterized using optical metallography.

Under complex, dynamic loading conditions, damage can occur within a material. Should this damage not lead to catastrophic failure, the material can continue to sustain further loading. However, little is understood about how to represent the mechanical response of a material that has experienced dynamic loading leading to incipient damage. Here, this effect is examined in copper. Copper is shock loaded to impart an incipient state of damage to the material. Thereafter compression and tensile specimens were sectioned from the dynamically damaged specimen to quantify the subsequent properties of the material in the region of intense incipient damage and in regions far from the damage. It is observed that enhanced yield stresses result from the damaged material even over material, which has simply been shock loaded and not damaged. These results are rationalized in terms of stored plastic work due to the damage process.

Manganin stress gauges have been placed in samples of a γ-titanium aluminide alloy in such orientation that renders them sensitive to the lateral component of stress during one-dimensional shock loading. The impact stresses were in the range 1.8 to 6 GPa. The resultant stress histories show clear evidence of a transition from elastic to plastic behaviour, which, through the elastic relations correlates with the Hugoniot Elastic Limit of this material. In combination with the longitudinal stress data, it has been possible to use the lateral stresses from this series of experiments to calculate the shear strength during shock loading. The Hugoniot of the γ-titanium aluminide studied is seen to be significantly different than a standard γ-β Ti-alloy such as Ti-6Al-4V. The results show this titanium aluminide alloy displays a significant degree of hardening with increasing shock stress.

The effect of shock-prestraining high-purity Fe, above and below the 13 GPa phase transition, on post-shock compressive stress-strain behavior and substructure evolution is presented. The degree of shock hardening in Fe is higher for shock strength above the transition than below it. The substructure also displayed more deformation twinning for shocks above the phase transition than below. The shock hardening in Fe varied with propagation distance. Modeling shock propagation in Fe revealed that for the geometry used, the transition wave was overtaken and quenched by the rarefaction release explaining the gradient in shock hardening.

Previous work done to measure spall response of U-Nb(6%) alloy showed a somewhat surprising result. In all experiments done previously no evidence for an elastic wave was observed; that is, no two wave structure was detected. In this paper we report on the results of new experiments done to look at the effect in more detail. Dedicated experiments were done to look for evidence of elastic wave under compression, and preliminary results are reported.

The effect of strain rate, temperature, grain size, and texture on the substructure and mechanical response of high-purity polycrystalline titanium is presented. The compressive stress-strain response of 20 and 240 {mu}m grain size high-purity Ti was found to depend on both the applied strain rate; 0.001 {le} {epsilon} {le} 7500 s{sup -1}, and the test temperature; 77 {le} T {le} 873 K. The rate of strain hardening in Ti is seen to increase with increasing strain rate. The substructure of high-purity Ti deformed at high-strain-rate or quasi-statically at 77K displayed a higher incidence of deformation twinning than during quasi-static deformation at 298K.

At high pressures zirconium is known to undergo a phase transformation from the hexagonal close packed (HCP) alpha phase to the simple hexagonal omega phase. Under conditions of shock loading, the high-pressure omega phase is retained upon release. However, the hysteresis in this transformation is not well represented by equilibrium phase diagrams and currently models that accurately represent such a solid-solid phase transformation coupled with the multi-phase plasticity likely under shock conditions do not exist. For this reason, the influence of peak shock stress on the retention of omega phase in Zr is explored in this study. In-situ VISAR measurements along with post-mortem metallographic and neutron diffraction characterization of soft recovered specimens have been utilized to quantify the volume fraction of retained omega phase, morphology of the shocked alpha and omega phases, and qualitatively understand the kinetics of this transformation. This understanding of the role of peak shock stress will be utilized to address physics to be encoded in our present macro-scale models.

Intermetallics are receiving increasing attention for applications requiring high-leverage materials possessing potentially high pay-offs such as in gas-turbine engines. While the quasistatic deformation response of a broad range of intermetallics is receiving intense scientific and engineering study increased utilization of intermetallics under dynamic loading requires an understanding of their high-rate / shock-wave behavior. In this paper the influence of shock loading on the structure / property behavior of Ti-48Al-2Cr-2Nb and Ti-24Al-11Nb is presented. The reload constitutive response of both shock-loaded intermetallics supports the dynamic deformation of both intermetallics being controlled by a Peierls mechanism. Defect generation and storage in intermetallics is compared and contrasted to that typical to conventional disordered metals and alloys.

The forced shear test specimen, first developed by Meyer et al. [Meyer L. et al., Critical Adiabatic Shear Strength of Low Alloyed Steel Under Compressive Loading, Metallurgical Applications of Shock Wave and High Strain Rate Phenomena (Marcel Decker, 1986), 657; Hartmann K. et al., Metallurgical Effects on Impact Loaded Materials, Shock Waves and High Strain rate Phenomena in Metals (Plenum, 1981), 325-337.], has been utilized in a number of studies. While the geometry of this specimen does not allow for the microstructure to exactly define the location of shear band formation and the overall mechanical response of a specimen is highly sensitive to the geometry utilized, the forced shear specimen is useful for characterizing the influence of parameters such as strain rate, temperature, strain, and load on the microstructural evolution within a shear band. Additionally, many studies have utilized this geometry to advance the understanding of shear band development. In this study, by varying the geometry, specifically the ratio of the inner hole to the outer hat diameter, the dynamic shear localization response of high purity Fe was examined. Post mortem characterization was performed to quantify the width of the localizations and examine the microstructural and textural evolution of shearmore » deformation in a bcc metal. Increased instability in mechanical response is strongly linked with development of enhanced intergranular misorientations, high angle boundaries, and classical shear textures characterized through orientation distribution functions.« less

The quasi-static and dynamic responses of two fluoropolymers - polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE) - have been extensively characterized. Here we investigate the influence of the propensity to neck or not between PCTFE and PTFE on their responses under Dynamic-Tensile-Extrusion (Dyn-TenExt). The Dyn-Ten-Ext technique was originally developed for metals and applied to copper and tantalum spheres by Gray, et al. as a tensile corollary to compressive Taylor Cylinder Impact Testing. Under Taylor Cylinder loading both PTFE and PCTFE exhibit a classic three-diameter ductile deformation pattern. The ductile-to-brittle transition in PTFE occurs at 131 m s{sup -1} with a very tight transition range in impact stress of less than 4 m s{sup -1}. Conversely, the ductile-to-brittle transition in PCTFE occurs between 165 and 200 m s{sup -1} with a gradual transition to stochastic crack formation and ultimately failure. Under Dyn-Ten-Ext loading the onset of extrusion occurs above 164 and 259 m s{sup -1} for PTFE and PCTFE respectively.

Energetic loading subjects a material to a 'Taylor wave' (triangular wave) loading profile that experiences an evolving balance of hydrostatic (spherical) and deviatoric stresses. While much has been learned over the past five decades concerning the propensity of deformation twinning in samples shockloaded using 'square-topped' profiles as a function of peak stress, achieved most commonly via flyer plate loading, less is known concerning twinning propensity during non-I-dimensional sweeping detonation wave loading. Systematic small-scale energetically-driven shock loading experiments were conducted on Ta samples shock loaded with PEFN that was edge detonated. Deformation twinning was quantified in post-mortem samples as a function of detonation geometry and radial position. In the edge detonated loading geometry examined in this paper, the average volume fraction of deformation twins was observed to drastically increase with increasing shock obliquity. The results of this study are discussed in light of the formation mechanisms of deformation twins, previous literature studies of twinning in shocked materials, and modeling of the effects of shock obliquity on the evolution of the stress tensor during shock loading.

The effects of shock prestrain on the mechanical behavior, microstructure, and substructure of annealed tantalum have been examined. The test material was shocked to 15 GPa for 1 µs and soft-recovered such that deformation that occurred during this procedure was predominantly due to the shock loading. Mechanical characterization of the annealed and shock-recovered tantalum was performed over a wide range of strain rates (10-3 to 7000 s-l) in both tension and compression. Shock prestraining caused an increase in the quasi-static yield strength of tantalum but had virtually no influence on the dynamic stress-strain response. Consistent with this, the dynamic tensile behavior of both the annealed and shock-recovered materials were nearly identical. The effects of shock prestrain on the microstructure and substructure of the test material was examined using optical and transmission electron microscopy. The results of these examinations and how they correlate with the effects of shock prestrain on mechanical behavior are discussed.

Shock recovery experiments on copper have been conducted to investigate the influence of loading rate and stress amplitude on defect storage and post-shock mechanical properties. The shock risetimes varied approximately from one nanosecond for the shock experiments to one microsecond for quasi-isentropic loading experiments. All the experiments had the same peak pressure and pulse duration. Attaining a peak pressure of 10 GPa through multiple shocks or a single shock produced similar results for defect storage and post-shock yield strength. Decreasing the strain-rate of loading is shown to increase both the defect storage and post-shock yield strength of copper. The effect of loading rate on post-shock substructure and mechanical response of impacted copper is postulated to be directly related to the amount of dislocation motion before interaction with other dislocations and to the amount of reversible dislocation motion and resultant annihilation during the rarefaction portion of the shock-release cycle.

Previous experimental research has shown that microstructural features such as interfaces, inclusions, vacancies and heterogeneities can all act as void nucleation sites. However, it is not well understood how important these interfaces are to the damage evolution as a function of the surrounding parent materials. In this work, we present results on three different materials: 1) Cu, 2) Cu-10 wt%Ag, and 3) Cu-15 wt%Nb examined to probe the influence of bi-metal interfaces on void nucleation and evolution. These materials were chosen due to the differences in the stacking fault energy between the two phases. The initial results suggest that when there are significant differences between the bulk properties (for example: stacking fault energy and melting temperature etc) the type of interface between the two parent materials does not influence the damage process. Rather, it is the “weaker” material that dictates the dynamic spall strength of the material.

Time-temperature equivalence is a widely recognized property of many time-dependent material systems, where there is a clear predictive link relating the deformation response at a nominal temperature and a high strain-rate to an equivalent response at a depressed temperature and nominal strain-rate. It has been found that high-density polyethylene (HDPE) obeys a linear empirical formulation relating test temperature and strain-rate. This observation was extended to continuous stress-strain curves, such that material response measured in a load frame at large strains and low strain-rates (at depressed temperatures) could be translated into a temperature-dependent response at high strain-rates and validated against Taylor impact results. Time-temperature equivalence was used in conjuction with jump-rate compression tests to investigate isothermal response at high strain-rate while exluding adiabatic heating. The validated constitutive response was then applied to the analysis of Dynamic-Tensile-Extrusion of HDPE, a tensile analog to Taylor impact developed at LANL. The Dyn-Ten-Ext test results and FEA found that HDPE deformed smoothly after exiting the die, and after substantial drawing appeared to undergo a pressure-dependent shear damage mechanism at intermediate velocities, while it fragmented at high velocities. Dynamic-Tensile-Extrusion, properly coupled with a validated constitutive model, can successfully probe extreme tensile deformation and damage of polymers.

While numerous studies have investigated the low-strain-rate constitutive response of beryllium, the combined influence of high strain rate and temperature on the mechanical behavior and microstructure of beryllium has received limited attention over the last 40 years. In the current work, high strain rate tests were conducted using both explosive drive and a gas gun to accelerate the material. Prior studies have focused on tensile loading behavior, or limited conditions of dynamic strain rate and/or temperature. Two constitutive strength (plasticity) models, the Preston-Tonks-Wallace (PTW) and Mechanical Threshold Stress (MTS) models, were calibrated using common quasi-static and Hopkinson bar data. However, simulations with the two models give noticeably different results when compared with the measured experimental wave profiles. The experimental results indicate that, even if fractured by the initial shock loading, the Be remains sufficiently intact to support a shear stress following partial release and subsequent shock re-loading. Additional “arrested” drive shots were designed and tested to minimize the reflected tensile pulse in the sample. These tests were done to both validate the model and to put large shock induced compressive loads into the beryllium sample.

Widespread research over the past five decades has provided a wealth of experimental data and insight concerning the shock hardening, damage evolution, and the spallation response of materials subjected to square-topped shock-wave loading profiles. However, fewer quantitative studies have been conducted on the effect of direct, in-contact, high explosive (HE)-driven Taylor wave (unsupported shocks) loading on the shock hardening, damage evolution, or spallation response of materials. Systematic studies quantifying the effect of sweeping-detonation wave loading are yet sparser. In this study, the shock hardening and spallation response of Ta is shown to be critically dependent on the peak shock stress and the shock obliquity during sweeping-detonation-wave shock loading. Sweeping-wave loading is observed to: a) yield a lower spall strength than previously documented for 1-D supported-shock-wave loading, b) exhibit increased shock hardening as a function of increasing obliquity, and c) lead to an increased incidence of deformation twin formation with increasing shock obliquity.

The cooperative nature of the molecular motions in the crystal smectic B and smectic G phases of 50.7 have been investigated using coherent neutron scattering from a fully deuterated sample. It is concluded that the incommensurable transverse modulation of the smectic B layers, which develops as the smectic G phase is approached, is a static deformation of the layers. New excitations in the energy spectrum should therefore be associated with the layer modulation. Reasons why such excitations were not observed are discussed Etude de la nature cooperative du mouvement moleculaire dans les phases smectique B et smectique G sur un echantillon completement deutere. La modulation transverse incommensurable de la couche smectique B, qui croit lorsqu'on approche de la phase smectique G, est une deformation statique des couches. Dans le spectre d'energie de nouvelles excitations doivent etre associees a la modulation des couches

The smectic properties of four series of esters — the methyl to n-decyl esters of the 4'-n-hexyloxy, 4'-n-heptyloxy-, 4'-n-octyloxy- and 4'-n-nonyloxy-biphenyl-4-carboxylic acids — are discussed. The effects on the smectic phases of increasing the alkyl chain lengths are quite striking, particularly in the last three of these series, where SC properties are introduced for the midde members. The SC-SA transition temperatures in each series lie on two parabolae with the result that the earlier homologues exhibit A, B, E sequences and the higher homologues form only SA phases. Thus, the tendency of these series to exhibit SC phases is a function of alkyl chain length and is not determined simply by the more rigid central part of the molecule containing the permanent dipole moments. Evidence for the SC properties of these esters is given in detail. Other cases in which increasing the alkyl chain length leads to marked changes in smectic polymorphic behaviour, including the sudden injection of different smectic polymorphic phases into particular sequences of smectic phases, are reviewed.