Suitability of fatigue crack growth thresholds at negative stress ratios for ferritic steels and aluminum alloys in flaw evaluation procedures

Fatigue crack growth thresholds for ferritic steels and aluminum alloys in flaw evaluation documents are reviewed. The WRC (Welding Research Council) Bulletin, ASME (American Society of Mechanical Engineers) Code Section VIII, IIW (International Institute of Welding) and BS (British Standards) 7910 give constant fatigue crack growth threshold values at negative stress ratios. However, the definitions of the thresholds at negative stress ratios are different between these flaw evaluation documents. From fatigue crack growth tests and collection of fatigue crack growth threshold data, the thresholds are affected by compressive stresses. It can be said that the thresholds at negative stress ratios are not constant. Therefore, the thresholds given by the WRC Bulletin, ASME Section VIII and IIW are slightly un-conservative. The threshold given by BS 7910 is considerably conservative. A suitable definition of the threshold at negative stress ratios for application in flaw evaluation procedures is proposed as the variable threshold expressed by full range of stress intensity factors.Web of Science248art. no. 10767

Thermodynamic analysis on disproportionation process of cyclohexylamine to dicyclohexylamine

This work deals with a study of the effect of temperature on the cyclohexylamine disproportionation to dicyclohexy-lamine, conjointly with the thermodynamic analysis of this process. The laboratory experiments were carried out in a glass tubular continuous-flow reactor in a gaseous phase at the reaction temperature 433–463 K over a nickel catalyst. The results show, that the temperature has a trifing effect on equilibrium conversion of cyclohexylamine. However, temperature affects the formation of hydrocarbons, benzene and cyclohexane, and dehydrogenation products of dicyclohexylamine, i.e. N-cyclohexylidenecyclohexanamine and N-phenylcyclohexylamine. The latter one is the dominant product of dicyclohexylamine dehydrogenation. The disproportionation of cyclohexylamine has slightly exothermic character. At the experimental reaction temperature range, the cyclohexylamine disproportionation is spontaneous reaction and other reactions of this process are non-spontaneous

Failure bending stresses for pressurized pipes with circumferentially part-through cracks

Stress corrosion cracks detected in austenitic stainless pipes were circumferential cracks at inside of the pipes. Failure stresses for the cracked pipes were developed by net-section stress concept. Limit Load Criterion provided by the ASME Code based on the net-section stress concept can estimate fully plastic collapse stress, which is called as failure stress. Local Approach of Limit Load Criteria for estimating stress at crack penetration had developed for a part-through cracked pipe based on the Limit Load Criteria. Advanced Local Approach of Limit Load Criteria for estimating stress at crack penetration is newly developed based on the Local Approach in this paper. In comparing with experimental stresses of pipe crack penetrations, the Limit Load Criteria give unconservative estimation, and the Local and the Advanced Local Approaches are close to the experimental stresses. In addition, the Advanced Local Approach always gives higher stresses than the Local Approach for internal cracks, and it is shown that the Advanced Local Approach is beneficial for thick wall pipes with large angle cracks.Web of Science186art. no. 10414

A review of fatigue crack growth thresholds for metals in fitness-for-service codes: On the uncertainty of using the thresholds at negative stress ratios

Fatigue crack growth thresholds Delta K-th for metals are provided in many fitness-for-service codes. However, fatigue crack growth thresholds at negative stress ratios are not consistently defined. There are two forms of thresholds at negative stress ratios: constant thresholds irrespective of stress ratios, or increasing thresholds with decreasing stress ratios. The definitions of the thresholds at negative stress ratios also take two forms: either Delta K-th = K-max-K-min, or Delta K-th = K-max. ASME Section VIII, Section XI (ferritic steel) and International Institute of Welding (IIW) give constant thresholds expressed by Delta K-th = K-max. American Petroleum Institute (API) 579 and ASME Section XI (stainless steel) give increases in thresholds with decreasing stress ratios and the thresholds are expressed by Delta K-th = K-max-K-min. British Standard (BS) 7910 gives constant thresholds expressed by Delta K-th = K-max-K-min. The fatigue crack growth thresholds differ significantly among different fitness-for-service (FFS) codes. Appropriate thresholds for ferritic steels, stainless steels, and aluminum alloys are demonstrated in the literature survey.Web of Science1441art. no. 01120

Fully plastic failure stresses and allowable crack sizes for circumferentially surface-cracked pipes subjected to tensile loading

Fully plastic failure stresses for circumferentially surface-cracked pipes subjected to tensile loading can be estimated by means of limit load criteria based on the net-section stress approach. Limit load criteria of the first type (labeled LLC-1) were derived from the balance of uniaxial forces. Limit load criteria of the second type are given in Section XI of the ASME (American Society of Mechanical Engineering) Code, and were derived from the balance of bending moment and axial force. These are labeled LLC-2. Fully plastic failure stresses estimated by using LLC-1 and LLC-2 were compared. The stresses estimated by LLC-1 are always larger than those estimated by LLC-2. From the literature survey of experimental data, failure stresses obtained by both types of LLC were compared with the experimental data. It can be stated that failure stresses calculated by LLC-1 are better than those calculated by LLC-2 for shallow cracks. On the contrary, for deep cracks, LLC-2 predictions of failure stresses are fairly close to the experimental data. Furthermore, allowable circumferential crack sizes obtained by LLC-1 were compared with the sizes given in Section XI of the ASME Code. The allowable crack sizes obtained by LLC-1 are larger than those obtained by LLC-2. It can be stated that the allowable crack size for tensile stress depends on the condition of constraint of the pipe, and the allowable cracks given in Section XI of the ASME Code are conservative.Web of Science1441art. no. 01130