Ryuichi Ohtani

Basic investigation for life assessment technology of modified 9Cr–1Mo steel

Abstract In order to develop life assessment techniques for aged components made of modified 9Cr–1Mo steel, specimens were artificially deteriorated by aging, creep and fatigue tests at elevated temperatures, and associated changes in the microstructure and mechanical properties were examined. It was observed that aging resulted in formation of Laves phase causing a decrease in toughness. The creep damage in base metal could be correlated with decrease in hardness, while creep damage in weldments could be correlated with the area fraction and density of creep voids. Creep rupture in weldments occurred in the fine-grained heat affected zone by the formation and growth of creep voids. The fatigue damage in base metal correlated to the maximum length of a crack among micro-cracks initiated during fatigue cycles.

Delamination behavior of a carbon-fiber-reinforced thermoplastic polymer at high temperatures

Abstract This study focuses on the behavior of delamination crack propagation of a carbon-fiber-reinforced polymer at high temperatures. At first, the elastic properties of unidirectionally reinforced CF/PEEK at high temperatures were examined in order to evaluate the stress intensity factor, K, as a leading parameter of fracture mechanics. Fracture toughness and slow crack propagation in creep and fatigue were then investigated by the use of double-cantilever-beam specimens. The results obtained are summarized as follows. (1) The CF/PEEK shows excellent fracture toughness at temperatures up to 373 K, but the toughness decreases at higher temperatures because of creep. (2) In static creep, the creep deformation of matrix is confined near the crack tip as a consequence of constraint by the fibers, and the crack propagation rate is correlated well with the stress intensity factor, K. (3) Crack propagation in high-temperature fatigue is classified into two types, time-dependent and cycle-dependent. In the former, the crack propagation rate per unit time, da dt is governed by the stress intensity factor, and the relationship agrees well with that in static creep. In the latter, the propagation behavior resembles that in room-temperature fatigue, and the crack propagation rate per unit cycle, da dN is controlled by the stress intensity factor range, ΔK. The transition condition between the two types is given by the characteristic equation represented as a function of K for time-dependent crack propagation and ΔK for cycle-dependent propagation.

High-Temperature Low Cycle Fatigue Crack Propagation and Life Laws of Smooth Specimens Derived from the Crack Propagation Laws

The objectives of this work are (1) to make clear the behavior of high-temperature low-cycle fatigue (LCF) crack propagation, (2) to verify the applicability of the J-integral to the fracture mechanics equations of crack propagation rates, (3) to derive two types of LCF life laws of smooth specimens based on the two types of crack propagation equations (i.e., cycle-dependent and time-dependent), (4) to show a resemblance to or a difference from the Manson-Coffin equation and the strain-range partitioning equations, and (5) to characterize the cycle-dependent and time-dependent fatigue laws. The effectiveness of the proposed failure-life equations was examined using experimental data on isothermal and thermal fatigue of smooth specimens.

Cracking behaviour of heat-resisting steels, alloys and a carbon-fibre-reinforced polymer at elevated temperatures

Abstract Characterization of creep-dominated fatigue cracking or cavitation in smooth specimens of a type 304 stainless steel is emphasized in terms of the stochastic nature of small-crack initiation and growth. As compared with monometallic ductile steels, a somewhat different cracking behaviour is shown for a titanium alloy Ti-17, an oxide-dispersion-strengthened nickel-base superalloy Inconel MA754 and a grey cast iron FC25. The mechanical behaviour of the large-crack propagation of creep and high temperature fatigue in high strength, low ductility materials is discussed in comparison with ductile materials. A quasi-small-scale creep condition dominates for the creep-fatigue crack propagation of the nickel-base superalloy Inconel 718, and a pure small-scale creep condition is established in the creep crack propagation of an aluminium alloy 2014-T6 and a carbon-fibre-reinforced polymer, namely unidirectional carbon fibres-poly(ether-ether-ketone) composite.

Probabilistic inverse analysis for predicting the distribution of multiple internal defects

Abstract A concept and a method of probabilistic inverse analysis are proposed for predicting the distribution of internal defects from their two-dimensional image on a cross-section. In this paper, circular (penny-shaped) cracks are chosen as a subject for analysis and a discussion is made on their statistical distribution. The spatial distribution of internal cracks, i.e. the number of cracks in a unit volume and the distribution of crack radius, is inversely predicted from the areal distribution of cracks observed on a cross-section, i.e. the number of cracks in a unit area and the distribution of crack length. When enough information on the areal distribution is given, for example, when a number of intercepted cracks can be observed on a cross-section for dense internal cracks, the spatial distribution of internal cracks is deterministically analyzed. Conversely, when the information is limited in the cross-sectional observation, a unique distribution of internal cracks cannot be determined. Then, a method of probabilistic inverse analysis is proposed. After the validity of the method is confirmed by numerical simulation, it is applied to actual internal cracks which initiate inside the specimens under a creep-fatigue condition. The distribution of internal cracks is successfully predicted.

Stochastic Modeling of Crack Initiation and Short-Crack Growth Under Creep and Creep-Fatigue Conditions

A simplified stochastic model is proposed for crack initiation and short-crack growth under creep and creep-fatigue conditions. Material inhomogeneity provides the random nature of crack initiation and early growth. In the model, the influence of microstructure is introduced by the variability of (1) damage accumulation along grain boundaries, (2) critical damage required for crack initiation or growth, and (3) the grain-boundary length. The probabilities of crack initiation and growth are derived by using convolution integrals. The model is calibrated and used to predict the crack density and crack-growth rate of short cracks of 304 stainless steel under creep and creep-fatigue conditions. The mean crack initiation lives are predicted to be within an average deviation of about ten percent from the experimental results. The predicted cumulative distributions of crack-growth rate follow the experimental data closely. The applicability of the simplified stochastic model is discussed and the future research direction is outlined.

Creep life prediction based on stochastic model of microstructurally short crack growth

A nondimensional model of microstructurally short crack growth in creep is developed based on a detailed observation of the creep fracture process of 304 stainless steel. In order to deal with the scatter of small crack growth rate data caused by microstructural inhomogeneity, a random variable technique is used in the model. A cumulative probability of the crack length at an arbitrary time, G(bar a, bar t), and that of the time when a crack reaches an arbitrary length, F(bar t, bar a), are obtained numerically by means of a Monte Carlo method. G(bar a, bar t), and F(bar t, bar a) are the probabilities for a single crack. However, multiple cracks generally initiate on the surface of a smooth specimen from the early stage of creep life to the final stage. Taking into account the multiple crack initiations, the actual crack length distribution observed on the surface of a specimen is predicted by the combination of probabilities for a single crack. The prediction shows a fairly good agreement with the experimental result for creep of 304 stainless steel at 923 K. The probability of creep life is obtained from an assumption that creep fracture takes place when the longest crack reaches a critical length. The observed and predicted scatter of the life is fairly small for the specimens tested.

Behavior of initiation and growth of creep and creep-fatigue small cracks in austenitic stainless steels

ABSTRACT Creep tests under constant load and creep-fatigue tests under slow-fast and slow-slow strain waveforms were conducted on austenitic stainless steels. Small cracks in the fixed area of the surface of smooth specimens were observed continuously during tests or intermittently by interrupted tests. The cracks initiated successively at many grain facets being nearly perpendicular to the stress axis, and the surface length or the depth of small cracks covered a wide range from 20μm to about 1mm. The cracks were classified into two types; the propagating cracks with the coalescence and the non-propagating cracks blocked by grain boundary triple points. The propagation rates were then distributed widely for small cracks shorter than a few hundred μm. A statistical treatment showed that the mechanism of small crack growth may be the same with that of initiation and that the upper bound of the small crack propagation rate is higher than the rate predicted on the basis of the large through-crack propagation equation correlated with the creep J-integral.

Substance of Creep-Fatigue Interaction Examined From the Point of View of a Crack Propagation Mechanics

The crack propagation rate under creep-fatigue interaction condition could not be predicted by the rule of a linear summation of crack propagation rates of pure creep and fatigue. The acceleration of the crack propagation rate was found to be due mostly to the increase in the accumulation of creep strain and plastic strain in the vicinity of the crack tip. In consideration of the interaction between creep deformation and elastic-plastic deformation, non-steady state creep J-integral range and fatigue J-integral range were evaluated as true driving forces of the time-dependent and the cycle-dependent crack propagation rates, respectively. The interacted creep-fatigue crack propagation was then devided into either the time-dependent creep type correlated with the creep J-integral range or the cycle-dependent fatigue type correlated with the fatigue J-integral range. Same results were obtained for small crack propagation, though it was suggested that another kind of interaction between cracking and grain boundary cavitation may possibly bring the acceleration of early crack growth.

Transition from Small Crack to Large Crack on Creep-Fatigue Crack Propagation

Experiments of creep fatigue crack propagation on a Type 304 stainless steel indicated that there appeared two regimes called “the microstructurally small crack” and “the mechanically large crack” before and after the transient regime from about 100 μm (twice mean grain diameter or four grain boundary lengths) to 500 μm. The former, small cracks were characterized by a great deal of scatter of propagation rates and a number of cracks arrested, while the latter large cracks were represented by a deterministic law of nonlinear fracture mechanics. The numerical simulation based on a one-dimensional model of random fracture resistance of grain boundaries led to the conclusions that the mean value of the driving force for crack propagation was equal to that of the large crack irrespective of the crack length, and that the scatter of crack propagation rates could be predicted based on the distribution of grain boundary resistance represented by uniform random numbers.