Maximum Allowable Crack Research Articles

Rapid risk assessment using probability of fracture nomographs

ABSTRACTTraditional risk‐based design process involves designing the structure based on risk estimates obtained during several iterations of an optimization routine. This approach is computationally expensive for large‐scale aircraft structural systems. Therefore, this paper introduces the concept of risk‐based design plots that can be used for both structural sizing and risk assessment for fracture strength when maximum allowable crack length is available. In situations when crack length is defined as a probability distribution the presented approach can only be applied for various percentiles of crack lengths. These plots are obtained using normalized probability density models of load and material properties and are applicable for any arbitrary load and strength values. Risk‐based design plots serve as a tool for failure probability assessment given geometry and applied load or they can determine geometric constraints to be used in sizing given allowable failure probability. This approach would transform a reliability‐based optimization problem into a deterministic optimization problem with geometric constraints that implicitly incorporate risk into the design. In this paper, cracked flat plate and stiffened plate are used to demonstrate the methodology and its applicability.

海洋環境下におけるステンレス鉄筋の適用性に関する検討

For the application to port concrete structures under marine environment, corrosion-resistant property of three types of stainless steel bars (SUS430, SUS304 and SUS316) in concrete was studied by immersing each steel bars in the simulated pore solution in concrete and exposing concrete specimens, in which each steel bars was embedded, under marine environment. Results obtained are as follows. 1. In the sound concrete region without crack, no corrosion was observed in all stainless steels (SUS430, SUS304, SUS316) under high chloride ion content which was around 15 kg/m3 in concrete. Therefore, it can be judged that the chloride threshold level for corrosion of these stainless steels is more than 15 kg/m3. 2. In the cracked concrete region, no corrosion was observed in two types of stainless steel (SUS430 and SUS316). However, very small corrosion was observed in the only crack region on SUS304. This reason is considered to be larger crack width than SUS430 and SUS316. This indicates that some large crack width can not be allowed in case of the stainless steel which has similar corrosion-resistant property to SUS304. It is necessary to study on the maximum allowable crack width. 3. In case that stainless steel bar is applied to port concrete structures under marine environment, the life cycle cost can be lower than the case of carbon steel bar in some specific cases.

Eurocode design of underground metro structures

Eurocodes were used for the design of a new metro, for which the contractor's design was undertaken largely between 1996 and 1999, when the Eurocodes were in their infancy. This paper discusses the experience, the difficulties encountered and the benefits gained by the project, with particular reference to the geotechnical design of the underground stations. Eurocodes 1, 2 and 7 were used for the design of the retaining walls for the deep stations. Two design parameters created challenges for the structural design of the reinforced concrete embedded retaining walls: a restriction on the maximum allowable crack width, and a large cover to the reinforcement, required by the construction method. Benefits were gained by using the observational method.

Effect of specimen type and size on fracture resistance curve determination for CuCrZr alloy

The precipitation hardened copper alloy CuCrZr is one of the candidate materials for the first wall and divertor components of ITER. In order to validate the applicability of copper alloys in the structural components of ITER, the effect of neutron irradiation on the fracture toughness behaviour has to be studied. Fracture toughness testing of irradiated materials requires the use of miniaturised specimens and the verification of specimen size effects on fracture toughness test results. In the present study the effects of specimen type, size as well as temperature on the ductile tearing fracture resistance behaviour of CuCrZr alloy will be shown. The applicability of miniaturised specimens for fracture resistance curve determination is considered by studying the allowable crack extension with which the miniaturised specimens still have similar fracture resistance curves as full-size specimens. The crack length and J-integral values will be compared to ASTM standard requirements for the maximum allowable crack extension and J-integral values in the fracture resistance test.

Reliability of Steam Generator Tubes With Axial Cracks

An approach for estimating the failure probability of tubes containing through-wall axial cracks has already been proposed by the authors. It is based on probabilistic fracture mechanics and accounts for scatter in tube geometry and material properties, scatter in residual and operational stresses responsible for crack propagation, and characteristics of nondestructive examination and plugging procedures (e.g., detection probability, sizing accuracy, human errors). Results of preliminary tests demonstrated wide applicability of this approach and triggered some improvements. The additions to the model are extensively discussed in this paper. Capabilities are demonstrated by results of analysis of steam generator no. 1 in Slovenian nuclear power plant located in Krsˇko after the 1992 inspection and plugging campaign. First, the number of cracked tubes and the crack length distribution were estimated using data obtained by the 100-percent motorized pancake coil inspection. The inspection and plugging activities were simulated in the second step to estimate the efficiency of maintenance in terms of single and multiple-tube rupture probabilities. They were calculated as a function of maximum allowable crack length. The importance of human errors and some limitations of present nondestructive examination techniques were identified. The traditional wall thickness and crack-length-based plugging criteria are compared. The crack-length-based criterion is shown to be more efficient and more safe, especially because of strong suppression effect on probability of multiple-tube rupture. The results are considered to be important for safety and maintenance of existing plants and for further research.

Nondestructive estimation of residual stress in welded pressure vessel steel by means of remanent magnetization measurement

A new technique is described which makes use of measurements of remanent magnetization in a weldment to estimate the suitability of post-weld heat treatments in reducing weld residual stress. The technique has been assessed by comparison with the value for macroscopic residual stress obtained by a stress relaxation method. An attempt is also made to estimate a maximum allowable crack size in the weld HAZ region taking the presence of weld residual stress into consideration and using a relationship between the remanent magnetization and the fracture toughness parameter K r corresponding to the weld residual stress.

The influence of fibres upon crack development in reinforced concrete subject to uniaxial tension

Synopsis This investigation was designed to assess the possibility of incorporating steel fibres in reinforced concrete to retard the development of cracks and thus facilitate the use of steels of higher strength. To obtain this information, 75 × 75 × 500 mm concrete prisms were tested in uniaxial tension applied to the protruding ends of the concentrically placed reinforcing bars. Variables that were studied included type, size and quantity of fibre and type of main reinforcing bar. The inclusion of steel fibres in the concrete significantly increased the stress in the reinforcement corresponding to the development of the maximum allowable crack width in the concrete.