Naoya Tada

Analysis on the applicability of direct current electrical potential method to the detection of damage by multiple small internal cracks

The direct current electric potential around a number of penny-shaped cracks was analyzed by Crack-Flow Modification Method (CFMM) in order to investigate the applicability of the Direct Current Electrical Potential Method (DC-EPM) to the detection of damage due to multiple small internal cracks. The potential difference was calculated for the cracking process in a smooth round bar specimen of a Type 304 stainless steel under a creep-fatigue condition. It was clarified by the analysis that, in the case of the present creep-fatigue test, damage could be detected at about a half of the lifetime assuming that a change of 1 percent in the potential difference was measurable by a voltmeter.

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.

Characterization of Creep Fatigue Cracking in Type 304 Stainless Steel

The behaviour of initiation and growth of small cracks in high-temperature push-pull low-cycle fatigue (LCF) was investigated on a Type 304 stainless steel. The characterization of cracking was done both on the basis of the precise observation on the surface of as well as inside the specimens which were subjected to a wide range of isothermal loading conditions, and on the basis of the numerical simulation for initiation and growth of small cracks. Similar fundamental properties must be found in thermal fatigue and in other kinds of polycrystalline steels and alloys.

Analysis of direct current potential field around multiple random cracks

A method for evaluating the distribution of electrical potential around multiple through cracks is proposed. The method gives accurate potential values for cracks with different lengths at random positions by combining theoretical potential functions for a single crack in an infinite plate. The validity of the method is numerically confirmed with respect to problems of a single and three cracks existing in a square plate subjected to a current flow, and the method is applied to a problem of random cracks in an infinite plate.

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.

Analysis of the Intergranular Cracking Process Inside Polycrystalline Heat-Resistant Materials Under Creep-Fatigue Conditions

In order to investigate the intergranular failure process in polycrystalline materials under creep-dominant fatigue, numerical simulation of inner small cracks was conducted on the basis of a simple probabilistic model. Although the simulation condition is determined from experimental observation of the cross section of specimens interrupted at different fatigue cycles, the simulation is carried out continuously and reproduces inner cracking behavior throughout the fatigue life. This enables one not only to extract the spatial distribution of inner cracks at arbitrary creep-fatigue cycles, but also to calculate the propagation rate of each inner crack.

METHODOLOGY OF REMAINING LIFE ASSESSMENT IN HIGH TEMPERATURE APPLICATIONS BASED ON A MONTE CARLO SIMULATION OF GRAIN BOUNDARY CRACKING

ABSTRACT Microstrueturally small cracks along grain boundaries are focused as a substance of creep and creep dominated fatigue damage in steels and alloys. A method of computer aided simulation is proposed for remaining life assessment of high temperature components, based on the results of detailed observation of small cracks and a stochastic model on random behavior of crack initiation and growth.

Characterization of Creep-Fatigue Failure Process Based on Stochastic Behavior of Small Crack Initiation and Growth

In creep-fatigue conditions of usual ductile steels and alloys, multiple small cracks initiated on the specimen surface and their growth and coalescence brought about the failure. The process of crack initiation and growth was highly dependent on the microstructure of materials and the test conditions. A stochastic model was developed in order to simulate the random behavior of small cracks. Two variables, F and K, were used to characterize and simulate the initiation and growth behavior, respectively. A normalization and characterization method of failure process was proposed on the basis of the ratio of K to F. The failure process was dominated by the initiation and coalescence of multiple small cracks for a small value of K/F, while it was governed by the growth of restricted number of cracks for a large one.

Stochastic Simulation of Initiation and Early Growth of Small Cracks in Creep-Fatigue

The creep-fatigue failure of polycrystalline steels and alloys is caused by multiple intergranular cracks which begin to initiate from the early stage of life. The initiation and the growth of small cracks show random behavior due to the local inhomogeneity of materials. In order to investigate the mechanics hidden behind the stochastic nature, a method of inverse analysis based on a numerical simulation is proposed. As a result, the characteristics of the failure process is successfully drawn out in terms of the cracking behavior by the analyses.