Abstract: Steel bridges are normally subjected to random loads with different traffic frequencies. They are structures with dynamic behavior and are subject to fatigue failure process, where the nucleation of a crack, growth and failure can occur. After locating and determining the size of an existing fault, it is important to predict the crack propagation and the convenient time for repair. Therefore, fracture mechanics and fatigue concepts are essential to the right approach to the problem. To study the fatigue crack growth, a computational code was developed by using the root mean square (RMS) and the cycle-by-cycle models. One observes the variable amplitude loading influence on the life structural prediction. Different loads histories and initial crack length were considered as input variables. Thus, it was evaluated the dispersion of results of the expected structural life choosing different initial parameters.
Abstract: Residual stress and fatigue crack growth rates are important to determine mechanical behavior of rails. This study aims to make relationship between residual stress and fatigue crack growth values in rails. For this purpose, three R260 quality rails (0.6-0.8% C, 0.6-1.25 Mn) were chosen. Residual stress of samples was measured by cutting method that is related in railway standard. Then samples were machined for fatigue crack growth test and analyze was completed according to the ASTM E647 standard which gives information about parameters of rails for this test. Microstructure characterizations were examined by Light Optic Microscope (LOM). The results showed that residual stress change with fatigue crack growth rate. The sample has highest residual stress exhibits highest crack growth rate and pearlitic structure can be seen clearly for all samples by microstructure analyze.
Abstract: It is necessary to manage the fatigue crack growth (FCG) once those cracks are detected during in-service inspections. In this paper, a simulation program (FCG-System) is developed utilizing the commercial software ABAQUS with its object-oriented programming interface to simulate the fatigue crack path and to compute the corresponding fatigue life. In order to apply FCG-System in large-scale marine structures, the substructure modeling technique is integrated in the system under the consideration of structural details and load shedding during crack growth. Based on the nodal forces and nodal displacements obtained from finite element analysis, a formula for shell elements to compute stress intensity factors is proposed in the view of virtual crack closure technique. The cracks initiating from the intersection of flange and the end of the web-stiffener are investigated for fatigue crack paths and growth lives under water pressure loading and axial force loading, separately. It is found that the FCG-System developed by authors could be an efficient tool to perform fatigue crack growth analysis on marine structures.
Abstract: The fatigue crack growth is stochastic because of the fatigue behavior having an uncertainty and a randomness. Therefore, it is necessary to determine the probability distribution of a grown crack size at a specific fatigue crack propagation life for maintenance of structure as well as reliability estimation. The essential purpose of this study is to present the good probability distribution fit for the grown crack size at a specified fatigue life in a rolled magnesium alloy under different specimen thickness conditions. Fatigue crack propagation experiments are carried out in laboratory air under three conditions of specimen thickness using AZ31 to investigate a stochastic crack growth behavior. The goodness-of-fit test for probability distribution of a grown crack size under different specimen thickness conditions is performed by Anderson-Darling test. The effect of a specimen thickness on variability of a grown crack size is also investigated.
Abstract: In this investigation variation of cyclic loading effect on fatigue crack growth is the studied. This study is performed on 2024 T351 and 7050-T74 aluminum alloys, used in aeronautical structures. The propagation model used in this study is NASGRO model. In constant amplitude loading (CA), effect of stress ratio has been investigated. Fatigue life and fatigue crack growth rate were affected by this factor. Results showed an increasing in fatigue crack growth rates (FCGRs) with increasing stress ratio. Variable amplitude loading (VAL) can take many forms i.e. with a single overload, overload band… etc. The shape of these loads affects strongly the fracture life and FCGRs. The application of a single overload (ORL) decrease the FCGR and increase the delay crack length caused by the formation of a larger plastic zone compared to the plastic zone due without VAL. The fatigue behavior of the both material under single overload has been compared.
Abstract: Constant amplitude fatigue crack growth (FCG) tests
were performed on dissimilar metal welded plates of Type 316L
Stainless Steel (SS) and IS 2062 Grade A Carbon steel (CS). The
plates were welded by TIG welding using SS E309 as electrode. FCG
tests were carried on the Side Edge Notch Tension (SENT)
specimens of 5 mm thickness, with crack initiator (notch) at base
metal region (BM), weld metal region (WM) and heat affected zones
(HAZ). The tests were performed at a test frequency of 10 Hz and at
load ratios (R) of 0.1 & 0.6. FCG rate was found to increase with
stress ratio for weld metals and base metals, where as in case of
HAZ, FCG rates were almost equal at high ΔK. FCG rate of HAZ of
stainless steel was found to be lowest at low and high ΔK. At
intermediate ΔK, WM showed the lowest FCG rate. CS showed
higher crack growth rate at all ΔK. However, the scatter band of data
was found to be narrow. Fracture toughness (Kc) was found to vary
in different locations of weldments. Kc was found lowest for the
weldment and highest for HAZ of stainless steel. A novel method of
characterizing the FCG behavior using an Infrared thermography
(IRT) camera was attempted. By monitoring the temperature rise at
the fast moving crack tip region, the amount of plastic deformation
was estimated.
Abstract: In this paper fatigue crack initiation and propagation in notched plate under constant amplitude loading through tensile residual stress field of 2024 T351 Al-alloy plate were investigated. Residual stress field was generated by plastic deformation using finite element method (FEM) where isotropic hardening in Von Mises model was applied. Simulation of fatigue behavior was made on AFGROW code. It was shown that the fatigue crack initiation and propagation were affected by level of residual stress filed. In this investigation, the presence of tensile residual stresses at notch (hole) reduces considerably the total fatigue life. It was shown that the decreasing in stress reduces the fatigue crack growth rates.
Abstract: This paper presents the results of corrosion fatigue
crack growth behaviour of a Ni-Cr-Mn steel commonly used in
marine applications. The effect of mechanical variables such as
frequency and load ratio on fatigue crack growth rate at various
stages has been studied using compact tension (C(T)) specimens
along the rolling direction of steel plate under 3.5% saturated NaCl
aqueous environment. The significance of crack closure on corrosion
fatigue, and the validity of Elber-s empirical linear crack closure
model with the ASTM compliance offset method have been
examined.
Fatigue crack growth rate is higher and threshold stress intensities
are lower in aqueous environment compared to the lab air conditions.
It is also observed that the crack growth rate increases at lower
frequencies. The higher stress ratio promotes the crack growth. The
effect of oxidization and corrosion pit formation is very less as the
stress ratio is increased. It is observed that as stress ratios are
increased, the Elber-s crack closure model agrees well with the crack
closure estimated by the ASTM compliance offset method for tests
conducted at 5Hz frequency compared to tests conducted at 1Hz in
corrosive environment.
Abstract: In this paper fatigue crack growth behavior of
aeronautical aluminum alloy 2024 T351 was studied. Effects of
various loading and geometrical parameters are studied such as stress
ratio, amplitude loading, etc. The fatigue crack growth with constant
amplitude is studied using the AFGROW code when NASGRO
model is used. The effect of the stress ratio is highlighted, where one
notices a shift of the curves of crack growth. The comparative study
between two orientations L-T and T-L on fatigue behavior are
presented and shows the variation on the fatigue life. L-T orientation
presents a good fatigue crack growth resistance. Effects of crack
closure are shown in Paris domain and that no crack closure
phenomenons are present at high stress intensity factor.
Abstract: This paper reports the fatigue crack growth behaviour
of gas tungsten arc, electron beam and laser beam welded Ti-6Al-4V
titanium alloy. Centre cracked tensile specimens were prepared to
evaluate the fatigue crack growth behaviour. A 100kN servo
hydraulic controlled fatigue testing machine was used under constant
amplitude uniaxial tensile load (stress ratio of 0.1 and frequency of
10 Hz). Crack growth curves were plotted and crack growth
parameters (exponent and intercept) were evaluated. Critical and
threshold stress intensity factor ranges were also evaluated. Fatigue
crack growth behaviour of welds was correlated with mechanical
properties and microstructural characteristics of welds. Of the three
joints, the joint fabricated by laser beam welding exhibited higher
fatigue crack growth resistance due to the presence of fine lamellar
microstructure in the weld metal.
Abstract: This study reports an empirical investigation of
fatigue crack initiation and propagation in 2024 T351 aluminium
alloy using constant amplitude loading. In initiation stage, local
strain approach at the notch was used and in stable propagation stage
NASGRO model was applied.
In this investigation, the flat plate of double through crack at hole
is used. Based on experimental results (AFGROW Database), effect
of stress ratio, R, is highlights on fatigue initiation life (FIL) and
fatigue crack growth rate (FCGR). The increasing of dimension of
hole characterizing the notch effect decrease the fatigue life.
Abstract: The knowledge of the nature of loading is very
important in order to hold account on the total behavior such as
vibration, shock, fatigue, etc. Fatigue present 90% of failure when
loadings fatigues are very complex. In this paper a study of double
through crack at hole for plate subjected to fatigue loading is
presented. Various modes loading are studied where the applied load
is the same one. The fatigue life is given where the effect of stress
ratio is highlighted. This work is conducted on aluminum alloy 2024
T351 used for much aerospace and aeronautics applications. The
fatigue crack growth behavior with constant amplitude is studied
using the AFGROW code when Forman model is applied. The
fatigue crack growth rate and fatigue life for different loading modes
are compared with variation of others geometrical parameter such as
thickness and dimensions of notch hole.
Abstract: Stainless steel has been employed in many
engineering applications ranging from pharmaceutical equipment to
piping in the nuclear reactors and storage to chemical products. In
this attempt, simulation of fatigue crack growth based on
experimental results of austenitic stainless steel 304L was presented
using AFGROW code when NASGRO mode laws adopted. Double
through crack at hole specimen is used in this investigation under
constant amplitude loading. Effect of mean stress is highlighted.
Results show that fatigue crack growth rate (FCGR) and fatigue life
were affected by maximum applied load and dimension of hole. An
equivalent of Paris law for this material was estimated.
Abstract: In recent years, a new numerical method has been
developed, the extended finite element method (X-FEM). The
objective of this work is to exploit the (X-FEM) for the treatment of
the fracture mechanics problems on 3D geometries, where we
showed the ability of this method to simulate the fatigue crack
growth into two cases: edge and central crack. In the results we
compared the six first natural frequencies of mode shapes uncracking
with the cracking initiation in the structure, and showed the stress
intensity factor (SIF) evolution function as crack size propagation
into structure, the analytical validation of (SIF) is presented. For to
evidence the aspects of this method, all result is compared between
FEA and X-FEM.
Abstract: Environment-assisted cracking (EAC) is one of the most serious causes of structural failure over a broad range of industrial applications including offshore structures. In EAC condition there is not a definite relation such as Paris equation in Linear Elastic Fracture Mechanics (LEFM). According to studying and searching a lot what the researchers said either a material has contact with hydrogen or any other corrosive environment, phenomenon of electrical and chemical reactions of material with its environment will be happened. In the literature, there are many different works to consider fatigue crack growing and solve it but they are experimental works. Thus, in this paper, authors have an aim to evaluate mathematically the pervious works in LEFM. Obviously, if an environment is more sour and corrosive, the changes of stress intensity factor is more and the calculation of stress intensity factor is difficult. A mathematical relation to deal with the stress intensity factor during the diffusion of sour environment especially hydrogen in a marine pipeline is presented. By using this relation having and some experimental relation an analytical formulation will be presented which enables the fatigue crack growth and critical crack length under cyclic loading to be predicted. In addition, we can calculate KSCC and stress intensity factor in the pipeline caused by EAC.