Study of the relationship between fracture toughness (J IC) and bulge ductility

A theoretical model relating fracture toughness expressed as J _IC and bulge ductility {ie71-1} for a material exhibiting linear elastic behavior at low temperature and elastic-plastic behavior at higher temperatures is proposed. This model shows a variation of J _IC with {ie71-2} for linear elastic behavior and J _IC with {ie71-3} for elastic-plastic behavior. The model contains three constants to be determined experimentally for a given material, specimen geometry and testing conditions. A case study on 1045 steel in the temperature range ?60 to 25°C confirms the validity of the model. The experimental results help in determining the size of the fracture zone ahead of the crack as well as the mechanisms for crack blunting and crack growth.     

Temperature dependence of fracture stress in the brittle fracture region

The temperature dependence of the fracture stress of bcc metals in the brittle fracture region is examined. A model taking into account the effect of local plastic deformation of the crack tip on transition to brittle fracture is proposed. The model is then used to derive analytical expression for the temperature dependence of the fracture stress in the material with an initial crack or notch.Translated from Problemy Prochnosti, No. 11, pp. 57–62, November, 1991.     

Temperature dependence of the fracture stress of molybdenum in the transition region (above Tc)

The authors examined the temperature dependences of the fracture stress S_k of TsM6 molybdenum alloy in the cold brittleness range (above T_c) on specimens with different stress concentration factors. They propose a physical model which combines the detected dependences (curves with a minimum) and which is based on the mechanism of sharpening the crack as a result of coalescence with microcrack formed in the zone of intensive plastic deformation (the prefailure) at the crack. The effect of plastic deformation on the variation of the crack dimensions (length and curvature radius at the tip) is examined and an analytical equation in the form of an inequality, describing the condition of transition to cleavage macrofailure, is derived.Translated from Problemy Prochnosti, No. 6, pp. 20–25, June, 1992.     

Temperature and strain-rate dependence of fracture toughness of phenolphthalein polyether ketone

A strong strain-rate and temperature dependence was observed for the fracture toughness of phenolphthalein polyether ketone (PEK-C). Two separate crack-blunting mechanisms have been proposed to account for the fracture-toughness data. The first mechanism involves thermal blunting due to adiabatic heating at the crack tip for the high temperatures studied. In the high-temperature range, thermal blunting increases the fracture toughness corresponding to an effectively higher test temperature. However, in the low-temperature range, the adiabatic temperature rise is insufficient to cause softening and J _IC increases with increasing temperature owing to viscoelastic losses associated with the -relaxation there. The second mechanism involves plastic blunting due to shear yield/flow processes at the crack tip and this takes place at slow strain testing of the single-edge notched bending (SENB) samples. The temperature and strain-rate dependence of the plastic zone size may also be responsible for the temperature and strain-rate dependence of fracture toughness.Key project of the National Natural Science Foundation of China.     

Fracture toughness in the transition region

In order to study the origin of the scatter in the fracture toughness in the transition region and to specify the lower bound of the scatter, approximately 100 pieces 0.5T-CT NiCrMoV steel specimens were tested in the transition region, and their fracture surfaces were investigated. Major portion of the scatter was caused by the scatter in the length of preceding dimple crack which was generated at the fatigue precrack before conversion to final cleavage fracture. A method to predict the lower bound of the scatter in the small-specimen fracture toughness was proposed. It employs Weibull plot of new parameter J₁ which represents the scatter in the cleavage strength of material. The cleavage fracture origin may be associated with micro-stress-concentration in the microstructure which may not be caused by the micromechanism in terms of grain size unit.     

Temperature dependence of fracture toughness of epoxy resins cured with diamines

The fracture toughness (critical stress intensity factor, K _Ic) of epoxy resins cured with four diamines has been measured as a function of temperature over the range from –35° C to above T _g. It was found that K _Ic for each epoxy-diamine system did not vary below room temperature, and in the higher temperature range K _Ic increased with increasing temperature to a maximum and then decreased. The temperature which maximized K _Ic, agreed with the temperature at which the flexural modulus of the epoxy resins abruptly dropped. This temperature was therefore considered as T _g. This temperature was found to be about 20° C lower than the heat deflection temperature under load (1.82 M Pa) of the resins.     

Interlaminar fracture toughness for composite materials

A new equation for the energy release rate for a double cantilever beam specimen is proposed within the framework of the higher order shear deformable plate theory. The interlaminar fracture toughnesses (IFTs) found using the present theory, the ASTM round robin test method and the acoustic emission method are compared for thermoset graphite/epoxy and thermoplastic AS4/PEEK composites. As a result, IFTs by the present theory show agreement within 5% when compared with those found using the ASTM method and it is shown that IFTs found by the acoustic emission method are lower than those found by the ASTM method. It is observed that the IFT of the thermoplastic AS4/PEEK composite is about 10 times larger than that of the thermoset graphite/epoxy composite.     

On the fracture toughness of low- and medium-strength steels (BCC metals)

Proofs are given of the validity of the K_μ model proposed by G. S. Pisarenko and A. J. Krasowsky and to the hypothesis of the exponential dependence of the fracture toughness K _Ic on the plastic strain activation energy. The obtained dependences, which relate the material fracture toughness to the main physicomechanical properties as well as to its grain structure, described to sufficient accuracy the experimental results of the determination of K _Ic as a function of temperature, strain rate, and ductile-to-brittle transition temperature with the plane strain conditions for low-and medium-strength steels being met. Translated from Problemy Prochnosti, No. 2, pp. 141–146, March–April, 1997.     

Generalized fracture toughness of brittle materials

A generalization of the concept of fracture toughness of brittle materials, when subjected to multiaxial loadings, is presented. The theory characterizes the fracture strength of materials under any combination of the three basic modes of crack surfaces displacement.With reference to three-dimensional loading systems, the fracture toughness may be represented, in theK ₁ K ₂ K ₃ Cartesian orthogonal space, by a surface Fracture Envelope characteristic for a specified material, whose equation is determined by the (symmetric) fracture toughnessK _1c and Poisson's ratio .It is shown that the most general fracture process, resulting from the combination of the opening mode of the tangential stress component and the tearing mode of the antiplane shear, may be conveniently analyzed with the aid of the generalized fracture toughness concept. From the knowledge of the Fracture Envelope relative to a structural material, a simple fracture criterion permits forecasting crack propagation for any combination of loads and geometries.The theory is applied to mixed-mode problems to define the analytic threshold of fatigue crack growth.

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Résumé On présente une généralisation du concept de ténacité à la rupture de matériaux fragiles soumis à des contraintes multiaxiales. La théorie proposée caractérise la résistance à la rupture des matériaux sous toutes les combinaisons possibles des trois modes de base des déplacements des surfaces d'une fissure. Par rapport à un système de mise en charge à deux dimensions, la ténacité à la rupture peut être représentée dans un espace orthogonal cartésienK ₁ K ₂ K ₃ par une Enveloppe de Rupture caractéristique d'un matériau donné dont l'équation est déterminée par la ténacité à la rupture symétriqueK _c et le module de Poisson .On montre que le processus de rupture le plus général qui résulte de la combinaison d'une ouverture sous l'effet de la composante tangentielle de la contrainte et d'un arrachement sous l'effet du cisaillement antiplanaire peut être analysé d'une manière satisfaisanteà l'aide du concept de ténacité à la rupture généralisée. A partir de la connaissance de l'Enveloppe de Rupture relative à un matériau de construction déterminé, un critère simple de rupture permet de prévoir la propagation d'une fissure pour toutes combinaisons de contraintes et de géométries.La théorie est appliquée à des problèmes de fissure suivant des modes mixtes en vue de définir de manière analytique le seuil de propagation d'une fissure de fatigue.

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Operated for the U.S. Department of Energy, Contract No. DE-AC12-76N00052.     

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.     

Modelling of fracture toughness data in the ductile to brittle transition temperature region by statistical analysis

A fracture toughness database for a ferritic 22NiMoCr37 steel forging for 12.5, 25, 50 and 100 mm thick specimens tested at nine different temperatures has been analysed statistically. The method employed uses a statistical procedure based on competing risks to evaluate the fracture toughness and quantify the probability of cleavage fracture as a function of temperature, specimen thickness and ductile crack growth. This paper describes the application of the competing risks statistical methods to the fracture toughness database obtained from the joint European Project.     

Acoustic emissions during fracture toughness tests of steels exhibiting varying ductility

This investigation is primarily aimed at examining steels with varying ductility using characteristics of acoustic emission (AE). Four steels (AISI 1060, AISI 1080, SA333 grade 6 and AISI 304LN) were selected and their structure property relations were characterized using standard metallographic examinations, hardness and tensile properties. Fracture toughness of these steels was determined as per the guidelines of ASTM standard E1820 with simultaneous recording of AE signals. The results of these investigations have been used to demonstrate that: (a) nature of the variation of AE cumulative counts with time is different for linear and non-linear load–displacement plots, (b) synergistic analysis of the rate of change of cumulative energy, cumulative counts and intensity of AE signals provide the point of crack initiation in a material, and (c) fracture toughness of a material estimated using AE parameters is lower compared to that obtained by ASTM standard procedure.     

An assessment of fracture toughness in the ductile to brittle transition regime using the Euro fracture toughness dataset

The effect of specimen size on the fracture toughness of a ferritic steel in the transition regime has been investigated in a joint European Project. The project involved the testing of 25, 50, 100 and 200 mm wide compact specimens over the temperature range −154°C to 20°C with the aim of evaluating techniques for assessing the fracture toughness data.This paper evaluates the data at, or close to, the onset of stable tearing instead of at cleavage. The approach, which is applicable to structural assessment procedures, results in a temperature shift of less than 12°C between the specimen widths. The approach also enables simplified recommendations to be made for fracture toughness testing in the transition regime and the onset of upper shelf behaviour to be quantified.