Papers by Keyword: Creep Crack Growth

Abstract: A suitable constraint parameter is a key to quantify the creep crack tip constraint levels. In this study, a new more accurate creep constraint parameter As of brazed joint was proposed based on the existing constraint parameters R* and Ac. The CCG rate equation of Cr-Mo-V steel related to the constraint parameter As is obtained. The results show that the constraint parameter As is basically unchanged with the increase of creep time. The crack growth data predicted by the constraint dependent CCG rate equation are in good agreement with the experimental data. The established constraint dependent CCG rate equation can predict the CCG rate at other constraint levels very well.

Abstract: Uniaxial creep tests on micro-regions of P92 steel base metal and welding HAZ specimens were carried out at 923 K and under different stress levels. The creep equation of index of B and n was obtained from the test data fitted by using least square method. Then, creep crack growth tests based on the reference stress method on P92 steel base metal and welding HAZ compact tensile samples were carried out at 923 K and under stress intensity factor K of 18 MPa·m^1/2. From the calculated and experimental data, it was found that a good relationship existed between the creep crack growth rate da/dc and high temperature creep fracture parameter C*. The reference stress method was verified to calculate the C* of compact tension specimens and can predict the creep crack growth rate of the P92 steel base metal and welding HAZ materials under steady state creep conditions. At the same time, high temperature defect evaluation parameters of D₀ and Φ were obtained by fitting the linear parts in the relation curves using the log-log method. This study provides tests data on the defects evaluation and life prediction of P92 steel welding materials.

Abstract: A novel test method is presented in this paper for determination of the creep crack growth threshold. In this method, a wedge is placed in the notch of a standard compact tension (CT) specimen with a pre-crack, which provides an evaluable stress fields characterized with stress intensity factor (K) at the crack tip. Then the specimen is kept in the test temperature for a certain time. The creep crack growth threshold of the material at a given temperature can be obtained by extrapolating the curves of the crack growth rate vs. K in the double logarithmic coordinates. Since it does not need mechanical testing machine and several specimens can be tested simultaneously, the proposed method is economical in both costs and time. Tests on a nickel-based powder metallurgy superalloy FGH97 are conducted using this method at 600°C, 650°C and 750°C, which indicate its feasibility.

Abstract: An important issue to be considered in the life assessment of power plant components is the effects of prior creep damage on subsequent fatigue crack growth and fracture behavior. To examine these effects, creep damage has been introduced into 316H stainless steel material by interrupting creep crack growth (CCG) tests on compact tension, C(T), specimens at 550 °C. During the CCG tests, the specimen is loaded in tension, crept and unloaded after a small amount of crack extension. This process introduces compressive residual stress fields at the crack tip, which may subsequently affect the fatigue crack growth test results. In this work, neutron diffraction (ND) measurements have been conducted on interrupted CCG test specimens, which contain creep damage local to the crack tip, and the results are compared to predictions obtained from finite element (FE) simulations. Reasonable agreement has been found between the FE predictions and ND measurements.

Abstract: Life assessment of Type 316H stainless steel (SS) is of great concern to industry due to the widespread use of this material in power plant components that often operate at a temperature of around 550 °C. An important issue to be understood and considered in the life assessment of 316H components is the creep deformation and crack growth behaviour of this material in short term and long term operation times. Due to the limited long term test data available in the literature (i.e. [1, ), a new technique have been recently developed to estimate uniaxial creep ductility trends and subsequently creep crack growth (CCG) behaviour of the material at low load levels [. In this new method the creep ductility trends have been estimated as a function of the applied stress normalised by the temperature dependent 0.2% proof stress of the material, σ/σ_0.2, to include the plasticity effects on the creep deformation behaviour of the material. These trends have been implemented in finite (FE) simulations to predict the CCG behaviour of 316H in intermediate and long term tests at 550 °C and provisional results are presented in [.

Abstract: A conventional hot-pressing method was used to produce Al/Al-4wt%Cu functionally graded material (FGM). Heat treatment, which included solid solution treatment (T4) and aging treatment (T6), was carried out on hot-pressed (F) specimens. The creep crack growth tests were performed under constant loading of the creep-testing machines. The distribution of copper composition was investigated by line analysis via electron probe microanalysis. Fracture morphology and creep crack paths were studied by scanning electron microscopy. During heat treatment, the thickness of the graded transition layer increased due to copper composition redistribution. Creep crack growth retardation was found when crack propagated from the graded transition region to the Al-4wt%Cu layer. Greater improvement in creep crack growth resistance was achieved by the T4 and T6 states of Al/Al-4wt%Cu FGM. For T4 and T6 state specimens, the micro-cracks and crack kinking in the transition region were observed, which prevented creep crack growth.

Abstract: ATI 718Plus^® alloy is a new, cast and wrought, Ni-base superalloy with a maximum use temperature approximately 55°C higher than alloy 718. The mechanical properties have been well characterized by turbine engine OEM’s and the alloy has been specified for use as static components and blades in gas turbine engines. Broader use of ATI 718Plus alloy in engine disk applications requires detailed understanding of the damage tolerance under creep and cyclic loading conditions. The results of a large testing program to evaluate the crack growth behavior of ATI 718Plus alloy at temperatures between 649°C and 704°C under conditions of fatigue, dwell-fatigue, and creep are presented. Crack growth rates in ATI 718Plus alloy in this temperature range are lower than alloy 718 and comparable to Waspaloy under non-dwell-fatigue conditions, and comparable to alloy 720 in dwell-fatigue tests.

Abstract: In the present study, the residual stress generated in the crack tip of compact specimens was investigated in order to analyze the effect of residual stress on the creep crack growth. Residual stresses were generated using loading in compression beyond yield and then unloading. The maximum region of residual stress in the crack tip was obtained by numerical simulations which calculated the effect of notch radius and penetrated stress. It was found that a 2.5 mm notch radius with the applied stress at 36 KN could generate the largest extent of residual stress ahead of crack tip in the compact specimen. Further, when the specimen was heated to a high temperature, the value of residual stress reduced while the distribution changed little.

Abstract: Tubular structure is extensively used from domestic to aviation kind of applications. Life and safety are most considered in designing tube structure that against failure. For the last 200 years of research output and understanding, it was estimated that about 90% of metal failures were due to the external or surface defect and environmental attacks. The present work had focused on damage tolerant fatigue life prediction on aluminium cylindrical structures. Endurance tests were conducted with a constant amplitude repetitive loading at both, in room and high temperatures. A notch is introduced by wire cut machined on external surface and in a straight line with circumferential orientation to represent an external defects and flaws. Crack growth rates were measured by imaging technique. The experimental results suggested that the creep fatigue life is shorter than conventional fatigue life. The effect of stress ratio is also presented. The fully reversed with high temperature results registered the most severe damage with tremendous of life reduction.

Abstract: Advanced steels are designed and produced to be used in engineering applications in which thermo-mechanical fatigue could be a main factor in causing failure in components operating at elevated temperatures. In this paper thermo-mechanical fatigue properties of these steels are studied under the influence of creep and fatigue damage evolution. Development of different models and simulation techniques are reviewed to predict material behaviour. Numerical simulations are carried out to predict experimental tests on parent material notched bar specimens. Numerical predictions are introduced in advance of experimental test to assess the experimental test procedure. This is usually done to enhance the experimental result integrity and expectations. A local ductile damage development methodology is employed using the kinematic hardening criterion and compared to previously used strain hardening material property. The modelling on notched bar geometries is extended to geometries with cracks in which a local damage criterion will be used to predict virtual crack extension in compact tension specimens.