Orientation and loading condition dependence of fracture toughness in cortical bone

The fracture toughness at crack initiation were determined for bovine cortical bone under tension (mode I), shear (mode II), and tear (mode III). A total of 140 compact tension specimens, compact shear specimens and triple pantleg (TP) specimens were used to measure fracture toughness under tension, shear, and tear, respectively. Multiple-sample compliance method was utilized to measure the critical strain energy release rate (G_c) at the a/W=0.55 (crack length, a, to specimen width, W, ratio). The critical stress intensity factor (K_c) was also calculates from the critical loading (P_c) of the specimens at the a/W=0.55. The effect of the anisotropy of bone on its resistance to crack initiation under shear and tear loading was investigated as well. Fracture toughness of bone with precrack orientations parallel (designed as longitudinal fracture) and vertical (designed as transverse fracture) to the longitudinal axis of bone were compared. In longitudinal fracture, the critical strain energy release rate (G_c) of cortical bone under tension, shear, and tear was 644±102, 2430±836, and 1723±486 N/m, respectively. In transverse fracture, the critical strain energy release rate (G_c) of cortical bone under tension, shear, and tear was 1374±183, 4710±1284, and 4016±948 N/m, respectively. An unpaired t-test analysis demonstrated that the crack initiation fracture toughness of bone under shear and tear loading were significantly greater than that under tensile loading in both longitudinal and transverse fracture (P<0.0001 for all). Our results also suggest that cortical bone has been “designed” to prevent crack initiation in transverse fracture under tension, shear, and tear.     

Determination of fracture toughness of 15Kh2NMFA vessel steel by kinetic indentation

A specimen free technique based on kinetic indentation for determination of the fracture toughness of materials is proposed. It is based on the iteration calculation of the specific indentation work which is taken to be equal to the fracture toughness of a material controlled at a defined indentation depth. This technique makes it possible to estimate the degree of embrittlement of materials of long-operating constructions by means of periodic natural nondestructive testing. Examples of the utilization of the technique for steels of nuclear reactor vessels are presented.     

The dependence of the dynamic material toughness on the velocity and on some loading parameters

A recently proposed finite difference code, the SMF2D code, is used to simulate fast crack propagation in double cantilever beams (DCB) and single edge notch (SEN) specimens. The crack length-time history, whether measured experimentally, as in the DCB case, or conjectured, as in the SEN case, serves as input data. Several trial functions were used to simulate the initial acceleration phase since this phase has little available experimental data. Their effect on the overall results are found to be negligible except in the deceleration stage of crack propagation. Results for the cases investigated indicate that the dynamic material toughness is not a single valued function of crack velocity only, but that it also depends on some loading parameters and the specimen geometry.

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Résumé On utilise une méthode aux différences finies récemment proposée, le code SMF2D, en vue de simuler la propagation rapide d'une fissure dans des poutres doubles cantilever (DCB) et dans des éprouvettes à entaille latérale simple (SEN). Comme données d'entrée, on utilise l'évolution de la longueur de fissure en fonction du temps, soit mesurée expérimentalement comme dans le cas de la DCB, soit supposée comme dans le cas de l'éprouvette SEN. Diverses fonctions expérimentales ont été utilisées pour simuler la phase d'accélération initiale, dans la mesure où cette phase n'est décrite que par un nombre limité de données expérimentales. On trouve que leur effet sur les résultats d'ensemble sont négligeables, excepté dans la phase de décélération de la propagation de fissuration. Les résultats obtenus dans les cas analysés indiquent que la ténacité dynamique du matériau n'est pas une fonction simple de la vitesse de propagation seulement mais qu'elle dépend également de certains paramètres de mise en charge ainsi que de la géométrie de l'éprouvette.

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On leave from the Technion—Israel Inst. of Technology.     

Crack-size dependence of fracture toughness in transformation-toughened ceramics

Fracture toughness (K _IC) has been determined for Y₂O₃-partially stabilized zirconia, Y₂O₃-partially stabilized hafnia, CaO-partially stabilized zirconia and Al₂O₃+ZrO₂ composites. It is shown thatK _IC determined using the identation technique may not yield a unique number but may depend upon the crack size (C) (on the indent load). The slope ofK _IC againstC ^1/2 yields the magnitude of the surface stress created by the tetragonal monoclinic transition on the surface induced by grinding.K _IC determined using the double cantilever beam (DCB) technique, on the other hand, is shown to be independent of crack length.     

Low temperature and strain rate dependence of fracture stress and fracture toughness on thin Fe40Ni40B20 amorphous ribbon

The fracture stress and the critical stress intensity factor of the Fe₄₀Ni₄₀B₂₀ amorphous metallic ribbons 20 μm thick were measured in the temperature range 4.2–300 K and at deformation rates from 3.3×10⁻⁶ to 1.25×10⁻³ m⁻¹ with the aim to obtain more information on the condition for the onset and development of the inhomogeneous plastic deformation and fracture.     

The effect of cyclic loading on the apparent cleavage fracture toughness of 1Cr-Mo-V rotor steel

A significant increase in the cleavage fracture toughness of 1Cr-Mo-V rotor steel due to cyclic loading was observed. A new model that is capable of predicting and explaining the observation is proposed. The model combines macroscopic fracture criteria with the assumption that transient flow properties of a material in the cyclic plastic zone can be represented by those of the material subjected to macroscopic low cycle fatigue under fully reversed strain control.     

Influence of the parameters of temperature cycles on the cyclic fracture toughness of 5Kh3V3MFAS steel

We performed experimental investigations of the crack resistance of tool steel under conditions of nonisothermal loading and established the effect of the time of holding at the maximum temperature of thermal cycles on the critical size of a fatigue crack and cyclic fracture toughness. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 34–38, July–August, 1998.     

Nature of scatter of fracture toughness in static loading

The previously proposed model of unstable fatigue crack growth is used to explain a large (in comparison with other mechanical characteristics) scatter of static fracture toughness for 15Kh2MFA and 15Kh2NMFA steels at temperatures below the tactile-brittle transition temperature. The results show that for the materials for which K_fc ¹ < K_Ic the critical stress intensity factor K_Ic depends on the specific energy of inelastic strain W at the tip of the initial fatigue crack in its formation stage. The value of W is a function of the number of load cycles (in the conditions with a constant range of the stress intensity factor K) as a result of irregular fatigue crack growth. Here K_fc ¹ is the minimum cyclic fracture toughness. A method is proposed of evaluating the minimum fracture toughness of the material in static loading based on inspection of the process of irregular fatigue crack growth in the stage of crack initiation.Translated from Problemy Prochnosti, No. 2, pp. 10–16, February, 1990.