Fracture Toughness Of Graphite Research Articles

Determination of tensile strength and fracture toughness of nuclear graphite and prediction of its structural failures

The fracture properties of nuclear graphite and their effects on the structural failure process threaten the integrity and safety of High Temperature Gas-cooled Reactor structures. This study aims to develop close-form solutions to determine the size-independent tensile strength and fracture toughness of graphite from small-sized specimens, and thus to predict its structural failure behaviors. Firstly, the scale coefficient and dispersion coefficient are introduced to reflect the heterogeneity of graphite; these are estimated to be 21.0 and 16.8, respectively, in accordance with the meso-structural characteristics and macro-mechanical responses of the graphite. The fracture parameters of geometrically non-similar IG-11 graphite specimens are statistically analyzed with a normal distribution to determine the true fracture toughness and tensile strength, in the form of analytical solutions. Thus, a complete fracture failure curve is constructed to evaluate its fracture behavior. Then, the proposed method is employed to obtain the fracture responses of geometrically similar IG-11 graphite specimens. Good consistency in the calculation results of two types of specimens are observed, verifying the accuracy and size-independence (including absolute and relative sizes) of the proposed method for determining the fracture parameters of graphite. Finally, the structural failure loads of IG-11 and IG-110U graphite specimens with different sizes are predicted within an error margin of 15%. The minimum size requirements of graphite specimens satisfying the condition of linear elastic fracture mechanics are discussed, and the relationship between the fracture behavior of small-sized graphite specimens in the laboratory and the structural failure of actual large-sized graphite members is established.

Investigation on the mechanical properties and fracture toughness of graphite

AbstractIn the present study, mechanical properties and fracture toughness of graphite as a brittle material were investigated. At first, some specimens were examined in two perpendicular directions to derive Young's modulus and ultimate tensile strength. Then, graphite fracture toughness tests were conducted using some three‐point bending specimens with a sharp machined V‐notch by two different methods. The first method is based on the applied force at the moment of fracture, and the second one uses energy released during the test. Moreover, a technique was adopted to reduce differences between the two methods. It was observed that considering the effect of dehydration of the specimens, the fracture toughness was reduced by about 8%. Finally, crack growth simulation of the experiment was performed and indicated that finite element analysis predicts about 25% lower crack length values when critical energy release rate is utilized as a crack growth criterion instead of fracture toughness. In other words, the required input displacement for crack growth would be overestimated by using the critical energy release rate criterion.

The effect of neutron irradiation on the fracture toughness of graphite

As a part of our irradiated graphite recycle program a small quantity of PCEA grade graphite was irradiated in the High Flux Isotope Reactor (HFIR) at ORNL. The graphite will provide the “raw material” for future recycle experiments. The geometry of the irradiated graphite allowed us to study the effects of neutron irradiation on the critical stress intensity factor, KIc, of graphite. The specimens were irradiated in two groups of 6 at an irradiation temperature of 900°C in “rabbit” capsules to doses of 6.6 and 10.2dpa, respectively. Following a full suite of pre- and post-irradiation examination, which included dimensions, mass, electrical resistivity, elastic constants, and thermal expansion (to 800°C) the samples were notched and tested to determine their KIc using the newly approved ATSM test method for SENB fracture toughness of graphite. Here we report the irradiation induced changes in the dimensions, elastic constants, resistivity, and coefficient of thermal expansion of PCEA graphite. Moreover, irradiation induced changes in the critical stress intensity factor, KIc, or fracture toughness, are reported and discussed. Very little work on the effect of neutron irradiation on the fracture toughness of graphite has previously be performed or reported.

Fracture resistance of zigzag single walled carbon nanotubes

Carbon nanotubes (CNTs) are known to possess extraordinary strength, stiffnessand ductility properties. Their fracture resistance is an important issue from theperspective of durability and reliability of CNT-based materials and devices.According to existing studies, brittle fracture is one of the important failure modesof single-walled carbon nanotube (SWNT) failure due to mechanical loading.However, based on the authors’ knowledge, the fracture resistance of CNTs has notbeen quantified so far. In this paper, the fracture resistance of zigzag SWNTswith preexisting defects is calculated using fracture mechanics concepts basedon atomistic simulations. The interatomic forces are modelled with a modifiedMorse potential; the Anderson thermostat is used for temperature control. Theproblem of unstable crack growth at finite temperature, presumably caused by thelattice trapping effect, is circumvented by computing the strain energy release ratethrough a series of displacement-controlled tensile loading of SWNTs (appliedthrough moving the outermost layer of atoms at one end at constant strain rate of9.4 × 10−4 ps−1) with pre-existing crack-like defects of various lengths. The strain energy release rate,G, is computed for (17, 0), (28, 0) and (35, 0) SWNTs (each with aspectratio 4) with pre-existing cracks up to 29.5 Å long. The fracture resistance,Gc, is determined as a function of crack length for each tube at threedifferent temperatures (1, 300 and 500 K). A significant dependence ofGc on crack length is observed, reminiscent of the risingR curve behaviour of metals at the macroscale: for the zigzag nanotubesGc increases with crack length at small length, and tends to reach a constant value ifthe tube diameter is large enough. We suspect that the lattice trapping effectplays the role of crack tip plasticity at the atomic scale. For example, at 300 K,Gc for the (35, 0) tube with aspect ratio 4 converges to6 J m−2 as the cracklength exceeds 20 Å. This value is comparable with the fracture toughness of graphite and silicon. The fractureresistance of the tubes is found to decrease significantly as the temperature increases. Tostudy the length effects, the computations are repeated for zigzag nanotubes with the samethree chiralities but with aspect ratio 8 at 1 K. The fracture resistances of thelonger nanotubes are found to be comparable to those of the shorter nanotubes.

Neutron irradiation effects on thermal shock resistance and fracture toughness of graphites as plasma-facing first wall components for fusion reactor devices

The thermal shock resistance and thermal shock fracture toughness of graphites to be used as the plasma-facing first wall of fusion reactor devices are quantitatively determined by applying arc discharge heating at the central area of the disk specimens. The graphites used in the measurements included IG-11. a fine grain isotropic mold, HCB-18, a very fine grain mesophase pitch carbon, and C/C-B. a carbon fiber felt reinforced carbon composite. The results obtained showed that the C/C-B composite has an outstandingly high thermal shock resistance and fracture toughness. However, contrary to expectation, HCB-18 was found to have relatively low thermal shock resistance and fracture toughness, although it has higher mechanical strengths than IG-11. The measurements were also carried out for IG-11 and HCB-18 after irradiating them (1.1−1.5 × 10 21 n/ cm 2, > 29fJ at 750–1000°C) in a fission neutron reactor Japanese Material Testing Reactor (JMTR). The thermal shock resistances and the fracture toughnesses of these graphites were found to degrade to about 68–78% of the unirradiated values after undergoing the neutron irradiations. In this study, tile-like plates of HCB-18 and C/C-B graphites were used as the first wall materials at different positions inside a fusion reactor device together with a fine grain isotropic graphite ETP-10, which is similar to IG-11. According to a tentative surveillance following the operations, the tiles of HCB-18 were found to be considerably damaged, but no damage was found for the tiles of ETP-10 and C/C-B. The C/C-B composites, which were placed at comparatively intense positions, showed conspicuous integrity. These results agree with the values of thermal shock resistance and fracture toughness determined in this study.

Dimensional Effects in the Diametral Compressive Strength and Fracture Toughness of Graphites

This paper presents a probabilitic fracture criterion for brittle solids subjected to multiaxial stresses in the diametral compressive testings and shows the application to evaluate the dimensional effects in the diametral compressive strength tests of disk specimens of graphites. The dimensional effects of disk specimens having various sizes from 15mm up to the maximum 240mm in the diameter of four kinds of reactor grade graphites and two kinds of electrode grade graphites are examined as a function of specimen volume.Dimensional effect on the mode I fracture toughness of an electrode grade graphite is also examined.Obtained results of the diametral compressive strengths show to exist a maximum, which is changed by the maximum particle size of the graphite, in each relation between the strength and specimen volume.

Irradiation effects on thermal shock resistance and fracture toughness of HTGR-graphite

This paper describes changes in the thermal shock resistance Δ = σtkEα and the thermal shock fracture thoughness ▽ = KIckEα in addition to the usual mechanical properties including the diametral compressive strength and fracture toughness of four varieties of graphite for the high temperature gas-cooled reactor due to neutron irradiations of (1.6 ∼ 2.3) × 1021 n/cm2 (E > 0.18 MeV) at 600 ∼ 850°C. These experiments are carried out by using small disk specimens which can be conveniently loaded into a capsule for irradiation in the Japanese Materials Testing Reactor. Both the thermal shock resistance and the thermal shock fracture toughness of graphites after irradiation decreased markedly despite of the increase in mechanical strength.

Work of Fracture and Fracture Toughness of Graphite

Abstract Compact tension tests and notched bend tests, according to Tattersall and Tappin, were performed on nuclear graphite in different orientations relative to the extrusion direction. The extension of slow propagating cracks was measured. The critical crack extension compares with the dimensions of defects in the graphite such as shrinkage cracks, pores and crack-like pores which are caused by the extrusion process. Taking these cracks into account results in comparable fracture toughness values regardless whether they are determined by a load criterion or from the work of fracture.