Takeo Yokobori

Estimation of creep crack growth rate in IN-100 based on the Q* parameter concept

Since the high-strength Ni-based superalloy, cast IN-100, is considered to be brittle at high temperatures, the stable creep crack growth region is limited. Therefore, technically, it is very difficult to perform creep tests and there are few experimental results on the creep crack growth behaviour of this material. We performed creep crack growth tests using Ni-based superalloy, cast IN-100, and derived the Q* parameter for this material, which characterizes the creep crack growth rate. Using this Q* parameter, we derived a law for the creep rupture life of this material.

Recommendations for a modification of ASTM E 1457 to include creep-brittle materials

Abstract VAMAS Technical Working Area 19 on High Temperature Fracture of Brittle Materials has developed several new recommendations for determining and analysing the creep crack extension behaviour of creep-brittle materials. These recommendations, most of which are also useful for creep-ductile materials, include: machining narrow starter notches for brittle materials; definition of creep crack initiation; definition of creep toughness; recommendations for conducting creep crack growth tests under the conditions of constant displacement rates or step-wise increments in displacements and/or displacement rates, in addition to the usual constant force method; additional parameters for correlating the creep crack extension rate, such as stress intensity factor, K, Ct parameter, Q∗ parameter, and rate of δ5 type CTOD, δ5; and criteria for the various correlation parameters. These recommendations are presented in a form which should aid development of national and international standardization of creep crack extension test methods.

Evaluation of creep crack growth properties using circular notched specimens

It is important to evaluate the effect of multiaxial stress conditions on initiation and growth of creep cracks, when the laboratory data are subsequently applied to structural components under the same or similar stress state. The round robin tests of creep crack growth using circular notched specimens of 1CrMoV steel at 538 and 594 °C and 12CrWCoB steel at 650 °C were conducted by the Japanese VAMAS TWA25 group. The effect of notch depth and specimen size, i.e. stress multiaxiality on crack growth properties was investigated. The test procedure including criteria for crack length measurement by electric potential drop was established. The circular notched specimens fractured intergranularly and showed different crack growth behaviour from that of a CT specimen due to the multiaxial stress field. The creep crack growth rate for the same C* value increased as the ratio of the notch depth to specimen diameter, i.e. stress multiaxiality increased. The Q* evaluation method based on the thermally activated process can also be applied to the circular notched specimen.

Comparative study on characterization parameters for high temperature creep crack growth with special emphasis on dual value behaviour of crack growth rate

Abstract If the logarithms of experimental values of high temperature creep crack growth rate, log d a d t are plotted against the logarithm of C ∗ or δ (creep displacement rate), the relation or curve reveals in many cases the dual value or the nose part. This dual behaviour appears in terms of various patterns according to the materials or its processing. An analytical model has been proposed for dual behaviour of crack growth rate plotted against C ∗ or δ dot . The methodology is to analyse the relation of creep displacement rate curve and crack growth length curve with respect to applied load time. Q ∗ parameter projection for d a d t predicts that log d a d t is a simple monotonical increasing linear function of Q ∗ , does not reveal dual values, and shows threshold behaviour at an early stage of crack growth.

Characterization of high temperature creep crack growth and creep life from high temperature ductile through to high temperature brittle materials

Abstract Yokobori and colleagues have treated time-dependent fracture, such as fatigue and creep fracture, as a stochastic process and a thermally activated process, extended the studies, and proposed the crack growth rate equations for fracture of various kinds. Among those for high temperature creep, fatigue and creep-fatigue multiplications, the following equation has been proposed for crack growth rate based on stochastic and thermally activated processes: d a d t =A e Q∗ where Q ∗ = −[(ΔH g − θ(σ))]/RT + constant. It has been found that, using the Q ∗ parameter, the high temperature creep crack growth rate is very well characterized. In smooth specimens, final fracture of two types may be realistic. One is where one crack which started from the small original defect extends and the final fracture is caused: by the stress intensity factor for the critical length of the crack attained due to this one-crack extension. The other may be the case where multi-site cracks, say a number of cracks initiated from a number of small defects, grow and join together. In the latter case it was shown previously that the fracture time is determined by the time when each growing crack attains the critical length depending on the distance between each tip of the originally initiated small cracks. Thus, for the final fracture of both types mentioned above, creep fracture time t ƒ can be obtained by integrating da/dt expressed in terms of Q ∗ mentioned above. Using creep strain rate (for steady creep or minimum creep) expressed for a thermally activated process, and calculating t ƒ ϵ , then the equation t ƒ ϵ is found to be given as the equation of activation type. This equation thus obtained for smooth specimen includes, as a special case, the formula experimentally obtained by Monkman and Grant, and nearly coincides with the formula modified experimentally by Wiederhorn et al. for alumina ceramics. For notched or cracked specimens, it has not been found whether the Larson-Miller formula is valid or not. By integrating the equation expressed in terms of Q ∗ , it was found that the master curve for creep fracture time t ƒ was obtained in terms of applied stress and temperature. In conclusion, it may be possible to predict or characterize the creep crack growth rate and the creep life throughout, from high temperature ductile materials such as Cr-Mo—V steel and SUS 304 stainless steel to high temperature brittle materials such as alumina ceramics, using the unified formula based on the Q ∗ parameter concept. The effect of creep ductility on creep life can be discriminated by the material constants.

High temperature creep, fatigue and creep–fatigue interaction in engineering materials

Abstract An attempt has been made to systematize the criterion for the crack initiation, the crack growth and the final fracture in creep, fatigue and creep–fatigue interaction conditions at high temperatures. Reference has been made to the systematically designed studies performed hitherto by the authors and colleagues, including the comparative study on the similarity and the dissimilarity between creep and fatigue. Further research as complexity science and engineering is emphasized.

Influence of notch shape and geometry during creep crack growth testing of TiAl intermetallic compounds

Abstract In order to establish the optimum pre-crack shape and geometry for creep crack growth (CCG) testing of high-temperature brittle materials, several shapes of starter notches were evaluated experimentally on TiAl intermetallic compound. Fatigue pre-cracks are frequently inappropriate in these materials because they tend to promote out-of-plane crack growth in the lamellar structured TiAl materials. Based on the experimental results with several notch shapes and also an analytic assessment of stress distribution around notch-tip, a V-notched specimen is proposed as being optimum for generating creep crack growth data in lamellar structured TiAl. A procedure for machining the notches to minimize its effect on the CCG behavior is also outlined.

The master curve and the constitutive equation for creep deformation and fracture for Cr-Mo-V steel throughout smooth, notched and precracked specimens

It has been shown experimentally that the master curve for creep deformation versus the ratio of time to fracture time, can be obtained for smooth, notched and precracked specimens of Cr-Mo-V steel, a high-temperature ductile material. A simple unified constitutive equation, i.e. a master curve equation, has been proposed. It is suggested that there is some correlation between the creep deformation fracture curve and the creep damage size master curve. Although the range of the applicability of methodology might be rather limited, the development of this concept is needed for improved long-term creep lives and for other creep ductile materials.

Some critical questions and future directions for fracture research

Abstract The present article discusses the critical issues and questions and also the future direction in mechanical properties of advanced materials research. Initially, it is emphasized how different fracture science and engineering are from mainly deductive philosophy, such as elasticity or physics, etc. Then, some schools of fracture research are discussed and in the light of the argument, various critical issues and questions are presented. Furthermore, in the present paper, some concepts for overcoming these difficulties are described for the critical problems.

Visco-elasticity in alumina ceramics at room temperature

The decrease in the fracture strength of alumina ceramics at room temperature has usually been assumed to be caused by the effect of stress corrosion. However, at high temperatures, a viscous deformation will occur due to a glass-like phase which exists in the grain boundary. A study of the relation of the fracture strength of an alumina ceramics smooth specimen at room temperature to the visco-elastic property was performed. Mechanical tests and visco-elastic analysis, has shown that the fracture strength of this kind is controlled by the visco-elasticity.