Cracked Components Research Articles

破壊力学 オーステナイト系ステンレス鋼の延性破壊評価法の検討

In the development of Fast Breeder Reactors (FBRs), structural integrity must be assured for components subjected to high temperatures up to 550°C, even though possible defects are presumed. Nonlinear fracture mechanics is one of the most effective approaches to evaluate ductile fracture behavior of cracked components. In this study, ductile fracture tests were conducted at room temperature and 550°C for austenitic stainless steel SUS304 and 316FR, which were candidates for FBR structural material. The applicability of fracture parameters was investigated from tests using small CT specimens, small CCT specimens, and wide CCT specimens. Fracture tests under the condition of combined tension and bending loads were also performed to investigate the effect of additional bending stress due to the temperature gradient through thickness. It was ascertained that fracture load could be predicted based on the net section collapse criterion and was not so affected by an additional bending stress.

CHARACTERIZATION AND DEPOSIT FROMING TENDENCY OF POLAR COMPOUNDS IN CRACKED COMPONENTS OF GASOLINE IDENTIFICATION OF PHENOLS AND AROMATICSULFUR COMPOUNDS

ABSTRACT Acidic compounds in cracked components of gasoline, isolated by extraction with aqueous solutions of sodium bicarbonate and sodium hydroxide, were identified as phenol and its mono- di - tri- and tetramethyl- derivatives. Aromatic sulfur compounds seDarated as the hexanc and benzene fractions by chromatography on an alumina column were identified as thiophenc, its mono- di - and trimethyl- derivatives, ethylihiophenc, benzolhiophene and methylbenzoihiophene. The identification of the phenolic compounds was achieved by IR, hydrogen-1 and carbon-13 NMR spectroscopy and GC - MS. Additionally, for complete identification of sulfur compounds a GC equipped with a selective sulfur detector was used. ISD Induction System Deposit (ISD) tests showed a decreased deposit-forming tendency for phenol compounds and no significant deposit formation for aromatic sulfur compounds or other fractions isolated.

An improved probabilistic model of fatigue crack growth

An improved probabilistic model of fatigue crack growth based on Bogdanoff's cumulative damage model, i.e. the Markov Chain model, is proposed. The basic postulation is that the crack growth process consists of many small uniform growth sections, each of which contains some equal probabilistic jump states. Under unrestricted initial conditions, the analytical recurrence formula of the failure probability for a fatigue crack to reach any state at a given time is found by means of Z transform and its complex inversion. The calculated results agree well with the empirical distribution function but are more conservative. It is possible to apply the improved model to the analysis of practical cracked components and achieve reliable probability information.

On the Applicability of Local Approaches for the Determination of the Failure Behavior of Ductile Steels

The strength and deformation behavior of specimens and components is, on the one hand, influenced by the local state of stress and strain, and on the other hand, by the chemical composition and the microstructure of the material used. Using two different steels, it was investigated how far it is possible to predict the failure behavior of specimens and components qualitatively and quantitatively by means of local approaches. For this purpose, two methods differing considerably from the basic idea were chosen. For the description of the failure behavior, so-called damage models were used. These damage models try to describe numerically the process developing microscopically and finally leading to fracture by means of continuum mechanical approaches in order to calculate the macroscopical failure behavior. The results show that for ductile materials, the damage models allow a very accurate calculation of smooth and notched specimens and components. The efforts presently required for the calculation are, however, still very high. Analyses using fracure mechanics approaches (J-integral) in combination with the local stress states (multiaxiality) were performed to describe the failure behavior. With this approach, it was attempted to calculate crack initiation and maximum load of pre-cracked specimens and components. The fracture mechanics methods are preferred for cracked components if an engineering estimation of crack initiation and maximum load only is required since the calculation efforts of the fracture mechanics methods are much lower than those of the damage models.

Lifetime estimates of cracked high temperature components: Webster, G.A. Int. J. Pressure Vessels and Piping 1992 50, (1–3), 133–145

Lifetime estimates of cracked high temperature components: Webster, G.A. Int. J. Pressure Vessels and Piping 1992 50, (1–3), 133–145

Evaluation of fracture parameters by equivalent domain integrals

In numerical analyses of cracked components often the finite element method is used in combination with special subroutines or post-processor programs for the evaluation of J-integral values. In this paper post-processor programs are described which use a volume integral formulation proposed by ATLURI and coworkers and denoted as equivalent domain integral (EDI). The programs use the results of two or three dimensional linear-elastic or elastic-plastic calculations with the finite element program ADINA. Besides the basic equations the numerical implementation is described. Also some results of test calculations on a compact tension shear specimen are presented. The results show a reasonable agreement with values gained by other methods.

Determination of stresses from stress intensity factor solutions: An inverse fracture mechanics problem

If stresses in uncracked components are known it is possible to determine also stress intensity factors for cracks located in these components. Therefore, the weight function for this problem has to be known. On the other hand, it is also possible to get information on the stress state of a component free of cracks if the stress intensity factor of the cracked component has been determined for the same loading.

Crack detection using electric conductive finite elements

The electric conductive problem is formulated in the form of a Laplace equation. Based on this equation, finite elements are formulated and used to solve the distribution of electric potentials. A finite element computer program which can solve problems with two- and three-dimensional scalar variables (for example, electric potential) is generated. Examples include the analysis and plotting for the distributions of electric potential in conductive components with cracks. The proposed technique can be applied efficiently to non-destructive detection of the cracked components made of electric conductive materials.

Non-periodic inspection by Bayesian method I

This paper introduces a Bayesian analysis methodology to determine appropriate non-periodic inspection intervals of fatigue-sensitive structures, so that their reliability remains above a prespecified minimum level throughout their service life. Fatigue damage is considered to initiate in a structural element when cracks develop, whether or not they are detected. The fatigue process then continues by crack propagation, resulting in strength degradation. If a fatigue crack is detected during an inspection, the cracked component is assumed to be repaired or replaced with a new one, resulting in the renewal of its residual strength and fatigue characteristics and increasing thus the overall reliability of the structure. The Bayesian approach introduced in this paper is unique and novel in that it allows one to utilize judiciously the results of earlier inspections for the purpose of determining the time of the next inspection and estimating the values of several parameters involved in the problem that can be treated as uncertain. Numerical simulations verify the above-mentioned capabilities of the Bayesian method.

Method of determining bending stress intensity coefficients for cracked ferroconcrete components

A method is proposed for calculating the stress intensity coefficient in a ferroconcrete element containing a crack in bending with reinforcement by rods. A three-dimensional solution has been obtained. The attachment of the reinforcement to the concrete is incorporated.

Lifetime estimates of cracked high temperature components

Cracked high temperature components subjected to creep-fatigue loading can fail by crack growth, net-section rupture or some combination of both processes. Models for describing this behaviour are presented. The influence of build up of damage in a process zone at the crack tip, material deterioration in the uncracked ligament and the importance of tertiary creep on the mode of failure are each considered. It is shown how the models can be employed to predict crack growth and for making residual life assessments in high temperature equipment. Finally a simple cumulative damage law is outlined for describing creep-fatigue interaction.

COMPARISON BETWEEN TWO ASSESSMENT METHODS FOR DEFECTS IN THE CREEP RANGE

AbstractIn recent years considerable effort has been made to understand the behaviour of creep defects at elevated temperature. A large number of experimental studies have been devoted to creep crack growth behaviour. In Europe, both in U.K. (CEGB) and in France (EMP), attention is focussed not only on crack growth but also on creep crack initiation behaviour in the assessment of defects at high temperature. This paper describes and applies both methods, first to laboratory test results and then to a cracked component. The comparison with test data is made with three materials, a ferritic 1 Cr‐1Mo‐0.25 V steel and two austenitic stainless steels, while the application to a cracked pressure vessel deals with a ½ Cr‐Mo‐V steel. For these materials which are creep ductile, the relevant load‐geometer parameter for stationary creep cracks is the C* parameter, which relies on the concept of reference stress and reference length. The expressions used for these parameters in both procedures are compared. Then the methods used for calculating the time for incubation prior to crack extension, t1, the time for subsequent growth, tg, and the time to failure, tF=ti+tg are compared. This comparison is made not only for laboratory test data but also for a cracked pressure vessel. It is shown that, in spite of different approaches, especially in the assessment of tg, both methods provide comparable t1 ‐ C* and tF=C* diagrams. The reasons for this situation are briefly discussed.

Evaluation of toughness requirements in cracked components based on fracture-mechanics methods

Different fracture-mechanics based defect assessment procedures have been checked using results of wide-plate tests. The CTOD-Design-Curve-Approach, the CEGB-R6-routine, the EPRI-approach and methods using Finite-Element-Computations to evaluate the crack driving force lead to safe predictions of the failure behaviour of components as long as the limits of application are considered carefully. Reversing the conventional procedure of defect assessment, the CTOD-Design-Curve approach and the FEM based method can be used to derive toughness requirements. The requirements depend on the component and defect geometry, the strength of the material and the safety criterion. For practical application simple diagrams have been evaluated showing the required toughness e.g. as a function of crack length or material strength.

Determination of phenalenes in distillate diesel fuels using high performance liquid chromatography

A liquid Chromatographic procedure has been developed to quantify phenalene and its alkyl homologues directly in fuels, utilizing oxidative electrochemical detection to selectively determine these species and the related benzanthracene. The repeatability of the method was better than 5% and the detection limit was found to be 10 mg 1 −1 phenalenes. A number of fuel components which have undergone known refinery processes have been analysed for phenalenes, including catalytically and thermally cracked stocks. No phenalenes were found in straight run fuel components, but high levels were found in fresh cracked components.

Estimating the importance of cyclic thermal loads in thermo-mechanical fatigue

A method for evaluating the effect of cyclic thermal loading on crack tip stress fields is developed. In its development, advantage is taken of the periodic nature of fatigue loading and only harmonic loadings are evaluated. Formulating the problem in this way permits the extraction of time as an explicit variable and replaces its role with a dependence on the frequency of the thermal loading. The means for evaluating the effect of periodic loadings on crack tip stress fields is the stress intensity factor which is calculated from numerically defined stress and displacement fields using a path independent integral. Results obtained indicate that stress intensity factors of cracked components exposed to thermal fatigue conditions have a significant dependence on the frequency of the thermal cycle and the crack geometry. Numerical estimates for mode I thermal stress intensity have been obtained using thermal fatigue test data for a titanium alloy and can be as high as 25 percent of the critical mode I mechanical stress intensity.

On the importance of direct resistance heating in thermo-mechanical fatigue

A computationally efficient method of determining the temperature distribution and stress intensity factor induced by the resistance heating method of cycling temperature in thermo-mechanical fatigue is presented. It is further used to evaluate the importance of the singularity of the heat source caused by the electric current. Results indicate the thermally induced stress intensity factor of cracked components cycled by the direct resistance method of heating increases with the frequency of the thermal loading and may become quite large at higher frequencies.

Composite patch reinforcement of cracked aircraft upper longeron: analysis and specimen simulation

The use of composite patches on cracked portions of metallic aircraft structures is an accepted means of improving fatigue life and attaining high structural efficiency. As more and more advanced composite materials are beng developed, the wider use of the repair technology is anticipated even for the reinforcement of primary aircraft structure. The objective of this work is to illustrate how the composite patch repair technology can be successfully applied to restore the structural integrity of cracked components. The Phosphoric Acid Anodize (PAA) surface treatment on aluminum when applied in conjunction with the AVI13/HV998 adhesive were essential for achieving the appropriate patch bonding strength. Such a process was done without immersing the component into the PAA tank; dismantling the component from the aircraft was not necessary. Boron/epoxy and carbon/epoxy patches were applied at room temperature to the 7075-T6511 cracked specimens and tested under fatigue simulating the load spectrum for the upper longeron attached to the access door of the electronic equipment bay. Considerable improvement in the fatigue life was observed after the repair. Equivalent flight test hours were increased from approximately two thousand hours at which the component fractured completely when not repired to twelve thousand hours when the repair was made with only a small amount of crack growth. A six times increase fatigue life is obtained. The laboratory developed technique has been applied to several in-service aircraft which have now been flown for more than 700 h without detection of crack growth.

Modified zeolite-based catalyst for effective extinction hydrocracking

The shape selectivity of zeolites makes them generally ineffective for extinction hydrocracking of polycyclic aromatic feeds. To overcome this problem, the zeolite can be modified with an amorphous cracking component to form a composite catalyst. This composite catalyst will be effective for extinction hydrocracking and retain the superior performance characteristics of a zeolite catalyst at the same time because the zeolite and the amorphous components of the catalyst operate complementarily. To illustrate this principle, NiW/REX-NiW/SiO/sub 2/Al/sub 2/O/sub 3/ composite catalyst was tested in the pilot plant. It was active, low in aging rate, resistant to nitrogen poisoning and high in selectivities for naphthas. The aged catalyst could be oxidatively regenerated to fully recover the activity and the product selectivities. This composite catalyst was superior to both individual (zeolite and amorphous) components for extinction hydrocracking. Catalysts similar to this have been used commercially for many years.

A RELIABILITY ANALYSIS OF FATIGUE CRACK GROWTH UNDER RANDOM LOADING

Abstract— The fracture mechanics approach to the growth of fatigue cracks under random loading has been used to predict both the distribution of crack lengths after a given number of cycles, and the distribution of cycles to grow a crack to a given length. The analysis ignores interaction effects between cycles, and its predictions agree closely with the results of Monte‐Carlo simulations. These results allow a simple method of assessing the reliability of cracked components subjected to narrow or broad band random loading and the introduction of material variability allows a quantitative description of the parameters which affect the reliability of a structural component.

High temperature fatigue crack growth in superalloy blade materials

The main factors influencing high temperature fatigue crack growth in superalloy blade materials are discussed. The significance of temperature, frequency, environment, and minimum to maximum load ratio R are considered in turn. Transgranular cycle dependent and intergranular time dependent controlled cracking processes are identified. Conditions favouring each mechanism are clarified and it is shown that cumulative damage concepts can be applied to predict interaction effects. It is found that cycle controlled processes are most likely to dominate at high frequency and low values of R. Creep and oxidation mechanisms, which are favoured by low frequencies, high temperatures, and high values of R, have been identified as contributing to the time-dependent component of cracking. It is shown that creep and oxidation can enhance crack growth/cycle significantly and reduced blade lifetimes.MST/517