Elastic properties of isotropic graphite

Journal of Nuclear Materials

et al. 2016

Laser ultrasonic methods have been used to measure the elastic moduli of various nuclear graphites. Measurements were made to assess wavespeeds for longitudinal and shear waves in different propagation directions and these were used along with density measurements to compute the longitudinal and shear moduli as well as Young's modulus. All moduli decreased with increasing graphite porosity and these variations could be interpreted using models describing the effect of porosity on material stiffness. Extrapolations for these models to zero porosity were used to infer the moduli for theoretically dense graphite; the results were far below predicted values reported in the literature for fully dense, polycrystalline, isotropic graphite. Differences can be attributed to microcracking in the graphite microstructure. Using models for the effects of microcracking on modulus, estimates for microcrack populations indicate that the number of cracks per unit volume must be much greater than the number of pores per unit volume. Experimental results reported in the literature for irradiated graphites as well as for the stress dependence of graphite modulus are consistent with the influence of microcracking on elastic behavior and could be interpreted using concepts developed here. Results in this work for graphite structure-property relationships should allow for more sophisticated characterization of nuclear graphites using ultrasonic methods.

Section: Variation Of Modulus With Bulk Porositysupporting

confidence: 67%

Section: Moduli Of Porous Mediamentioning

confidence: 99%

Laser ultrasonic assessment of the effects of porosity and microcracking on the elastic moduli of nuclear graphites

Spicer

Olasov

Journal of Nuclear Materials

et al. 2016

“…Essentially, elastic constants that depend heavily on microcracks oriented along graphitic planes are significantly modified while others are not altered to any great degree. In addition, a value of r  0.984 was produced for the modified Hill averaging scheme indicating that the measured modulus values were close to those associated with the Reuss boundthe lower limitand this quantitative result is consistent with observations made previously in various graphites [53,54].…”

Section: Resultssupporting

confidence: 89%

Theory and application of laser ultrasonic shear wave birefringence measurements to the determination of microstructure orientation in transversely isotropic, polycrystalline graphite materials

Contescu

Gallego

et al. 2017

Carbon

Laser ultrasonic line source methods have been used to study elastic anisotropy in nuclear graphites by measuring shear wave birefringence. Depending on the manufacturing processes used during production, nuclear graphites can exhibit various degrees of material anisotropy related to preferred crystallite orientation and to microcracking. In this study, laser ultrasonic line source measurements of shear wave birefringence on NBG-25 have been performed to assess elastic anisotropy. Laser line sources allow specific polarizations for shear waves to be transmittedthe corresponding wavespeeds can be used to compute bulk, elastic moduli that serve to quantify anisotropy. These modulus values can be interpreted using physical property models based on orientation distribution coefficients and microcrack-modified, single crystal moduli to represent the combined effects of crystallite orientation and microcracking on material anisotropy. Ultrasonic results are compared to and contrasted with measurements of anisotropy based on the coefficient of thermal expansion to show the relationship of results from these techniques.

“…However, other variations can be assumed and will yield different values for the moduli of theoretically dense graphite. For example, early work by Cost et al on a type of isotropic graphite assumed an exponential variation of modulus with porosity based on prevailing experimental evidence at that time [5]. Using an exponential fit to the data shown here yields modulus values at zero porosity of 32 and 13 GPa for Young's and the shear modulus respectively.…”

Section: B Elastic Moduli Vs Porositymentioning

confidence: 82%

Effects of graphite porosity and anisotropy on measurements of elastic modulus using laser ultrasonics

Spicer

2014 IEEE International Ultrasonics Symposium

Han

et al. 2014

Laser ultrasonic techniques can be used to study the ultrasonic properties of nuclear graphites and can serve as tools in establishing relationships between materials microstructure and the macroscopic stiffnesses of graphite. Establishing structure-property relationships permits improved ultrasonic sensing of graphite microstructural changes related to service-induced degradation. Laser ultrasonic measurements were made using a pulsed Nd:YAG laser source and detection was performed using a Michelson-type interferometer. This source-receiver combination provides for non-contacting, highly linear transduction of broadbanded, ultrasonic pulses permitting simultaneous determination of longitudinal and shear stiffnesses. Measurements show that among the graphites examined, a change in density of 0.21 g/cm 3 (average 1.8 g/cm 3 ) results in a change in the longitudinal elastic stiffness of 7.1 GPa (average 12.2 GPa) and 3.2 GPa (average 4.3 GPa) for the shear stiffness. Larger variations in density were produced by controlled oxidation of IG-110 and NBG-18. Shear wave birefringence measurements using laser line sources in IG-110 and PCEA indicate that IG-110 behaves isotropically while PCEA displays texture characteristic of transversely isotropic materials.