Papers by Keyword: Crack Growth

Abstract: The understanding of crack growth and therefore failure of components in geothermal energy production could lower maintenance costs tremendously. During in-situ corrosion fatigue experiments of high alloyed steel in geothermal brine the electrochemical potential was measured as well as temperature, frequency and pH of the surrounding geothermal brine. The decline of the electrochemical potential is in good agreement with estimated increasing linear crack propagation velocity. During initial crack propagation the electrochemical potential is in good agreement with the stress amplitude applied revealing that a steeper decline of potential indicates faster crack propagation. However, towards the end of propagation, this potential becomes independent of the stress amplitude.

Abstract: This paper presents a finite element modeling procedure to determine of crack growth behaviour of butt welded joints under the subject of load for mode I. This paper presents a computation procedure to determine the ratio of fatigue crack growth in butt welded plates for mode I fracture mechanics loading conditions. The presence of residual stresses in welded structures can significantly affect the material’s resistance to fatigue under cyclic loading. The presence of tensile residual stresses adversely affects the fatigue crack growth rate increased it. Change of microstructure and hardening material as a result of the welding process also has a negative impact on the growth of the crack. Accurate prediction and reliable assessment of the residual stress are important for the structural integrity and residual life assessment of welded parts design. Although there are several techniques for the determination of residual stresses, the finite element method (FEM) is one of the most convenient and useful. This paper presents a finite element modeling procedure to determine of crack growth behaviour of butt welded joints under tensile load for mode I. Keywords: Welding, Residual stress, Residual life, Crack growth, FEM, Butt welded joint

Abstract: Characteristic features of fatigue damage of 316L austenitic stainless steel cyclically strained axially in tension-compression, reversed torsion and combined axial-torsional mode were studied. All loading modes resulted in the formation of persistent slip markings (PSMs). Predominantly one slip system was activated in the case of axial and torsional loading while at biaxial loading, activation of several slip systems was involved. PSMs acted as sites of multiple fatigue crack initiation. The path of subsequent crack growth at a macroscopic scale differed considerably in dependence on loading mode and applied amplitude. The hardening-softening curves and fatigue life curves were evaluated and results were compared and discussed in terms of the type of applied loading.

Abstract: The paper is aimed to the study of the corrosion-fatigue behavior of high strength steels for offshore pipelines. Tests have been performed in order to study fatigue crack growth in synthetic seawater under cathodic protection. The tests have been carried out on three different steel grades from 65 to 85 ksi with tempered martensite and ferrite-bainite microstructures. The effect of stress intensity factor, cathodic protection potential and cyclic loading frequency is shown.

Abstract: The fatigue limit and fracture behavior of epoxy carbon fiber reinforced composites and hybrid composites were investigated. An aluminum foil and thin aluminum plate were incorporated with the carbon fiber to make the hybrid reinforced composites. Several specimens were prepared, and a series of tests were carried out to investigate the fatigue life and fracture behavior. Then the results were compared among epoxy carbon fiber reinforced composites and the hybrid composites. It is discussed whether the S–N curves shows almost the similar characteristics of these two types of hybrid composites or not. As far as the present result, the stress ratio and the type of hybrid composites effect on the fatigue life. The interfacial bonding plays an important role in the strength and fracture behavior of notched specimen of the fabricated composites.

Abstract: Fatigue plays a significant role in the crack growth of the fuselage skin structures. In addition, the fuselage may suffer also from the corrosion damage, and the wear defects. The proper maintenance and scheduled test intervals can avoid the sudden skin failure. Therefore, the inspection interval has to be shortened. Nevertheless, the young machines may be also suffering from the unexpected skin rupture. The cracks are emanating from the rivets and the holes under cyclic loading. The stress concentration around the notch has an effective role under the effect of cyclic loading. The cracks propagate toward the high stressed area such as the notches or other crack locations. The propagation into a critical crack size is rather fast and causes a sudden aircraft fuselage cracking. Hence, the number of cycles to failure will be decreased dramatically. During the last decades, the fracture toughness, design, and the new alloying element have been enhanced. The previous fuselage failures show that the inspections against the cracking are recommended even after a few thousand of cycles. To prevent the crack extending, the crack arresting is recommended to use around the fuselage.

Abstract: Since the industrial revolution when a German mining engineer August Wohler first studied the frequent breaking of chains causing several casualties and developed the concept of what we now know as the S-N curve, many experimental, theoretical and software-aided simulation techniques have been developed to study ageing material behaviour and to design new materials. Over time the demands placed on new materials have required operation under more severe temperatures and loads in order to conserve natural resources and minimise emissions. Fracture mechanics based finite element algorithms to simulate 3D cracks in components / structures have proved very useful in assessing the residual life and developing repair and maintenance strategies as mandatorily required by various licensing authorities for the continuous operation of infrastructure projects in Aerospace, Power, Transportation, Oil and Chemical industries under the ever more demanding operating conditions. Here one such software tool for crack simulation of industrial applications is presented with examples including combined fatigue and time dependent crack growth under thermo-mechanical loading including hold-time and weld defect assessment with inclusion of dis-similar materials.

Abstract: The initiation and propagation behaviors of small fatigue crack in TC4 were investigated in the present work. Surface replication on the basis of a two-part silicon mixture and confocal laser scanning microscope were used to record and observe the small crack initiation and growth processes at room temperature in air. Results showed that surface cracks initiated from the interfaces between α and β phases. When the crack lengths were below ~200 μm, the crack growth rates exhibited large oscillations and temporary retardations due to the presence of α/β interfaces. The corner crack propagated much faster and might have shielding effect on the surface crack.

Abstract: The fatigue limit and crack growth behavior of slit specimens of carbon fiber reinforced epoxy composites were investigated. The fatigue limit was defined by the maximum stress amplitude that the specimen endured 10⁶ times repeated stress when S-N curve was used. The highest fatigue limit was obtained when all the fiber directions were parallel to the load axis. The fatigue limits were evaluated in the cases of composites using alternately parallel and perpendicular to the load axis and compared with the result of the specimen having all the carbon fiber orientations were parallel to the load axis. When the measured value of the fatigue limit was lower, shear damage to the epoxy resin and peeling of fiber from epoxy resin occurred clearly. According to those results, it was expected that the fatigue limit of smooth specimens of carbon composites with long fibers can be evaluated from the results of the slit specimens.

Abstract: In this work, a method for determining the Equivalent Initial Flaw Size (EIFS) distribution using the Boundary Element Method (BEM) is proposed. Maximum Likelihood Estimation (MLE) is used to infer the EIFS distribution of a cracked stiffened panel under multiple sources of uncertainty, including uncertainty in the loading conditions, fatigue crack growth model parameters, and in the measurement of crack size found from routine inspections. Results suggest that MLE is an effective tool for estimating the parameters of an EIFS distribution when no prior knowledge is available regarding the EIFS distribution or its parameters.