Allowable Crack Length Research Articles

LCD 패널의 응력 해석을 통한 단순화 칩핑 허용균열 길이 예측

LCD 패널 제작과정에서 다양한 칩핑이 발생한다. 이는 초기 균열의 형태로 패널의 내구성에 영향을 미칠 수 있다. 본 연구에서는 칩핑이 LCD 패널의 내구성에 미치는 영향을 확인하고 칩핑 제거를 위해 수행되는 연마가공의 효과를 확인하였다. 이를 위해 외부 하중에 대한 9인치와 4인치 LCD 패널의 응력 변화를 연마 가공된 LCD 패널과 가공되지 않은 패널에서 비교하였다. 결과적으로 LCD 패널에 발생한 칩핑은 국부 응력집중 현상으로 인해 내구성이 약화되는 것을 확인하였다. 칩핑 제거를 위한 연마작업은 손상이 없는 패널과 같이 응력집중 현상이 완화되는 것을 확인하였다.

A Study for Brittle Crack Prevention and Fatigue Life Improvement using a Welded Stiffener

Recently, thickness of steels used in ships is increasing along the increased size of vessels, e.g. container carriers in particular. In container vessels, thick plates are applied to hatch side coaming areas, and safety is also becoming important as steel plate thickness increases. Many studies are conducted on the brittle crack arrest characteristics to prevent catastrophic failure in those areas. A number of experiments have been conducted for investigating propagation characteristics of brittle crack from classification societies. Especially, ESSO test method is commonly employed to analyze the BCA (Brittle Crack Arrest) toughness. In order to prevent brittle crack, arrest holes, weld line shift and insert plates are suggested.Various researches have been carried out to arrest brittle crack in BCA steel. However, the actual structure thickness has to be applied to the experiment, and it is difficult to carry out experiments due to the large scale specimens. Thus, in this study, crack retardation method is researched considering the thickness, length, and location of a welded stiffener. Finite element analysis was conducted on the hatch side coaming to compare the allowable crack length according to the effect of stiffener and to compare fatigue life based on the characteristics of BCA steel. Key words: Brittle crack arrest steel, Brittle crack arrest toughness, ESSO test, Fatigue crack growth rate, Stiffener

Safe life approach considering creep-fatigue interaction and Weibull statistics for crack growth evaluation based on field feedback applied to a GT first stage blade

The increased flexibility that the energy market requires makes the service condition of gas turbine (GT) components more and more critical in terms of creep-fatigue interaction. A new lifing strategy has been developed in order to evaluate the expected life of blades and vanes in this demanding service condition taking into consideration creep-fatigue interaction. The developed lifing methodology has been applied to the first stage blade of an Ansaldo Energia F-class GT and the life map has been compared with the available field feedback. With this procedure crack initiation in the base material is considered as the lifing criteria. So, the critical locations have been correctly identified and the number of predicted cycles correspond to service experience. The available field feedback has been treated by using Weibull statistics. The statistical treatment of crack size for the critical locations provides a crack initiation evaluation that is in good agreement with the lifing strategy. Moreover, the available field feedback on the first stage blade allows the correlation of the linear crack size with its depth. This relationship has been used to define the maximum allowable crack length for safe operation of this component.

Experimental investigation and theoretical analysis of tear propagation of GQ-6 airship envelope

This paper reports the central tearing properties of a new airship envelope fabric, GQ-6, from experimental investigation and theoretical analysis. First, the effects of the load ratio, the initial crack length, and the crack orientation on the tearing mechanical properties of such material were evaluated. The experimental results revealed that the mechanical properties of GQ-6 decrease with the increase of initial crack length. Two fracture modes were observed, e.g. either along the warp or the weft yarns directions, which depend on crack orientation and load ratio. Oriented crack can be represented by non-oriented crack with crack equivalent length under biaxial condition. The toughness KIC for the fabric was determined based on experimental results, as well as critical energy release rate GIC. A comparison of the mechanical behaviors of this textile under uniaxial and biaxial tensile condition is also presented. The allowable crack length for GQ-6 envelope fabric during working condition is estimated.

Tensile reliability analysis for gravity dam foundation surface based on FEM and response surface method

In this study, the limit state equation for tensile reliability analysis of the foundation surface of a gravity dam was established. The possible crack length was set as the action effect and allowable crack length was set as the resistance in the limit state. The nonlinear FEM was used to obtain the crack length of the foundation surface of the gravity dam, and the linear response surface method based on the orthogonal test design method was used to calculate the reliability, providing a reasonable and simple method for calculating the reliability of the serviceability limit state. The Longtan RCC gravity dam was chosen as an example. An orthogonal test, including eleven factors and two levels, was conducted, and the tensile reliability was calculated. The analysis shows that this method is reasonable.

Computational evaluation of resistance of fracture capacity for SUS304L of liquefied natural gas insulation system under cryogenic temperatures using ABAQUS user-defined material subroutine

A computational method for evaluating the resistance of fracture capacity of austenitic stainless steel (SUS304L) under ambient and cryogenic temperatures was proposed in the present study. To do this, a unified elasto-viscoplastic-damage model which can take into consideration the material nonlinearity under cryogenic temperature was adopted and implicitly formulated. In addition, the developed model is implemented into ABAQUS as a type of user-defined material subroutine UMAT. In order to verify the proposed method, the simulation results of resistance of fracture capacity are compared to a series of crack tests such as double-edge-cracked and center-cracked tension tests of SUS304L plates under ambient and cryogenic temperatures. Namely, the relationships between force and displacement as well as allowable crack length and allowable stress regarding to room and cryogenic temperatures are evaluate/predicted using the proposed method.

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.

Assembly procedures of trunnion–hub–girder for bascule bridges

This work is concerned with avoiding failures during the assembly of a trunnion–hub–girder (THG) for bascule bridges. In the current assembly procedure, AP#1, the trunnion is shrunk fit into the hub, followed by the shrink fitting of the trunnion–hub assembly into the girder. Two separate incidents during assembly prompted this study. The first incident involved the development of cracks in the hub during the assembly process using AP#1. The second incident involved the trunnion getting stuck in the hub before the trunnion could be fully inserted. A complete analytical, numerical, and experimental study was conducted to understand these failures, and the results were used to develop specifications and recommendations for assembly. The causes of failures include the development of high stresses at low temperatures during assembly, while noting that fracture toughness of THG materials decreases with temperature. Recommended specifications included following an alternative assembly procedure that nearly doubles allowable crack length, and that lowers cooling temperatures to avoid trunnion sticking in the hub.

Full-Scale Testing of Procedures for Assembling Trunnion-Hub-Girder in Bascule Bridges

To simulate a trunnion-hub-girder (THG) assembly for bascule bridges, two full-scale laboratory tests were conducted for quantifying stresses at previously observed failure locations and for identifying a favorable assembly procedure. One assembly procedure, AP#1, cools the trunnion for a shrink fit into the hub, followed by cooling of the trunnion-hub assembly to shrink fit it into the girder. Using AP#1, development of cracks on the hub was observed in one THG assembly, and, in yet another assembly, the trunnion got stuck in the hub before full insertion could take place. Large hoop stresses and low temperatures were observed at the trunnion-hub interface when the trunnion-hub assembly was cooled for insertion into the girder. Since fracture toughness of THG parts decreases with temperature, allowable crack lengths were small. In an alternative assembly procedure, AP#2, where the hub is shrink fitted into the girder first, followed by cooling the trunnion and shrink fitting it into the hub-girder assembly, the allowable crack length was determined to be double the allowable crack length of AP#1. Hence, for the given full-scale geometry and interference values, assembly procedure AP#2 was found to be better than AP#1.

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.

Study on Plugging Criteria for Thru-wall Axial Crack in Roll Transition Zone of Steam Generator Tube

The stream generator tubes represent an integral part of a major barrier against the fission product release to the environment. So, the rupture of these tubes could permit flow of reactor coolant into the secondary system and injure the safety of reactor coolant system. Therefore, if the crack was detected during In-Service Inspection of tubes the cracked tube should be evaluated by the pulgging criteria and plugged or not. In this study, the fracture mechanics evaluation is carried out on the thru-wall axial crack due to Primary Water Stress Corrosion Cracking in the roll transition aone of steam generator tube to help the assurence the integrity of tubes and estabilish the plugging criteria. Due to the Inconel which is used as tube material is more ductile than others, the plastic instability repture theory was used to calculate the critical and allowable crack length. Based on Leak Before Break concept the leak rate for the critical crack length and the allowable leak rate are compared and the safety of tubes was given.

On the small crack fracture mechanics

The limit of validity of linear fracture mechanics is specified by the minimum allowable crack length through an ASTM convention. Extension into the non-linear region ought to imply an extension towards smaller allowable cracks. In order to elucidate the question “How short is the smallest crack that fits the methods of fracture mechanics, and how do shorter cracks than that behave?” a pilot investigation is carried out. The process region is modelled as a Barenblatt line region and plastic flow off-side the process region is neglected. Results show that instability occurs before the process region is fully developed (as at large cracks) if the crack is short. This implies large deviations from the large crack fracture mechanics if the crack is very small. Even cracks of infinitesimal length are included in the study.