Papers by Keyword: Crack Growth Rate

Abstract: Three CT specimens from stainless steel AISI 304L were subjected to constant amplitude cyclic loadings with various asymmetries. Crack growth was recorded in detail. Fracture surfaces were documented by 3D maps in about 110 locations in the crack growth direction. 3D maps and their local gradients were represented by 2D wavelet decompositions in 10 levels resulting in 60 textural features. Statistical models expressing crack growth rate as a function of textural features were optimized. Training and testing approach, a high ratio of overfitting, and testing of significance of components ensured model's robustness. Quality of results is documented by graphs confronting model outputs with real data known from experiment. Results are acceptable in all cases.

Abstract: The use of S355 high strength steel in civil engineering to design bridges, its elements or simple engineering parts allows material and economical savings meeting the strict construction requirements. The knowledge of the fatigue resistance of material plays the key role during design and maintenance of the bridge structures. This contribution brings a comparison of the fatigue crack growth resistance of two standard S355 J0 and S355 J2 steel grades. Differences in chemical composition and the texture of material structure could generally play a role in the fatigue crack growth. This study shows that in the case of studied steels the chemical composition has an influence on material fatigue behaviour, whereas the texture of material structure is irrelevant.

Abstract: Multi-scale Fatigue crack growth behavior of titanium alloy TC4 under different maximum stress was identified in the present work by using an in-situ fatigue testing system. Results showed that specimen tested at lower maximum stress exhibited longer fatigue life, since the small fatigue crack stage accounted for a larger fraction of the total fatigue life. Local deformation around fatigue crack was measured by digital image correlation (DIC) technique. The evolution of multi-scale fatigue crack was identified. Relationship between size of plastic zone and crack growth rate was identified.

Abstract: Linear-elastic fracture mechanics based technique was used to measure the fracture toughness in terms of K1C of a solid solution super alloy. Due to thermal fatigue and high temperature exposure for various application of Alloy 617, it was demanded to measure crack growth behaviour of this alloy. Pre-cracked compact tension (CT) specimens ware used to determine the crack growth rate (CGR) of alloy 617 by direct current potential drop (DCPD) in-situ crack monitoring technique. Artificial Neural network (ANN) statistical model computed different fracture parameters from experimental inputs by feeding information to the network. This feed-forward network calculated the threshold fracture toughness, number of cycle to failure, slope of the Paris curve for the alloy at different temperatures and load ratios. The computational model correlates and converges with the experimental results with a maximum deviation of 6%. Thus, the model is recommended for complex and stochastic application of the nickel base super alloy 617.

Abstract: When loaded over their yield strength, materials suffer significant plastic deformations. In such zones, the degree of plastic deformation depends on the characteristics of the material and also on the nature of the loading. As known, the degree of plastic deformation may be correlated with the value of hardness, determined in the plastically-deformed zone. The present paper discusses the three types of testings applied, which led to the fracture of some specimens and, consequently, to plastic deformations in the immediate vicinity of the fractured surfaces. The testings, performed on Compact Tension Specimens, aimed at determining the following characteristics developed by the Mechanics of fracture: the crack growth rate, da/dN, fracture toughness for materials with brittle behavior, K_Ic, and fracture toughness for the materials with a preponderantly ductile character, J_Ic. Along the surfaces fractured during these testings, plastic deformations occur, differentiated exactly by the load differences. Under such conditions, testings were made upon the 1C45 (1.053) steel, with the characteristics established by the manufacturer, as well as upon the same steel, yet subjected to an annealing thermal treatment, and the variations of Vickers hardness were recorded both along the fractured surfaces and from one specimen to another.

Abstract: This paper reports a new way to conduct fracture mechanics experiments of welded thin-walled structure for aircraft fuselage applications made of the Al 6156 ally using lock-in infrared thermography. The heat wave, generated by both the themo-mechanical coupling and intrinsic energy dissipation was detected by an infrared camera. The crack growth rate measured by the lock-in infrared thermography is found to be consistent with that measured by conventional method. It is observed that the crack can either grow underneath the stiffener or grow into the stiffener web with the crack length measured at the same time. In addition, the crack tip plastic zone was observed before the specimen was about to be destroyed.

Abstract: Determination of fatigue crack growth characteristics under shear-mode loading is a rather complicated problem. To increase an efficiency and precision of such testing, special specimens enabling simultaneous propagation of shear cracks under II, III and II+III loading modes started to be used rather recently. K-calibration of these specimens was performed and, after unique pre-crack and heat-treatment procedures, effective thresholds in several metallic materials could be measured. However, a description of crack growth rate in terms of appropriate fracture mechanics quantities demands a precise assessment of plastic zone size under various shear-mode loading levels. This contribution is focused on the numerical elasto-plastic analysis of stress-strain field at the crack tip in specimens made of a pure polycrystalline (ARMCO) iron. The results reveal that the small scale yielding conditions are fulfilled in the near-threshold region. Starting from ΔK values approximately two times higher than the threshold, however, the ΔK_J or ΔJ approach should already be utilized. Probably the most interesting result of the analysis lies in a simple procedure that enables us to separate individual loading components ΔK_J,II and ΔK_J,III, applied in the mixed-mode II+III part of the specimen, by comparing elasto-plastic and elastic solutions.

Abstract: Corrosion damage (pit) is a stress raiser that can have deleterious effects on the fatigue life of airframe structural components. A better understanding and method for modeling the corrosion pit to fatigue crack transition would advance the fidelity of aircraft structural integrity estimates and fleet management decision making. Here, the focus is on developing a standardized fatigue test method for investigating the transition of a corrosion pit to fatigue crack in aluminum alloy AA 7075-T651 specimens. The standardized test method requires the development and validation of two sub-protocols (1) a pitting protocol to create a corrosion pit less than 200 μm diameter at the intersection of the central hole bore and planar surface of sheet and (2) a spot welding protocol for attaching the direct current potential drop (dcPD) probes on either side of the corrosion pit for fatigue crack growth measurement. A dcPD fatigue test method coupled with a unique 10-4-6 marker load sequence is used to measure the fatigue crack growth. The crack shape evolution and crack growth life are predicted using AFGROW.

Abstract: The reference texture is a subset of the image texture in SEM fractographs of fatigue fractures. It is common to all fractures caused by loadings in which significant events occur sufficiently regularly and frequently. The reference crack growth rate is unambiguously related to the reference texture. A particular loading is characterized by the ratio of the reference and conventional crack growth rates called reference factor. Its value may be related to the sequence of successive sizes of cyclic plastic zone, while the mechanism of the effect of overloads follows the models of Wheeler and Willenborg. Application to a set of three test specimens from stainless steel AISI 304L loaded by various loading regimes is shown.

Abstract: This letter describes the work conducted at our laboratory for the implementation of an automated vision system for fatigue crack growth measurement. The system relies on a dedicated illumination system with grazing incidence and optimized feature extraction by morphological image processing and continuous calculation of the crack growth, for adjustment of the optimal time interval for image registration.