Papers by Keyword: Multiple Cracks

Abstract: The interaction behavior of twin collinear through-wall cracks in tension loaded plate under elastic-plastic condition is investigated by the finite element method (FEM). The fracture parameter J integral for interacting cracks is calculated and compared to the J integral for a single crack the same size. In this way, the interaction factor of cracks under elastic-plastic condition is defined. This interaction factor is compared to the results of analytical solution of the interaction factor under linear elastic condition. The results show that interaction factor of cracks under elastic-plastic condition is higher than interaction factor of same cracks under linear elastic condition. Also the interaction effect of cracks under elastic-plastic condition is influenced not only by the crack configurations but also by the material properties, especially the strain hardening exponent n.

Abstract: Structural failure of aircraft wings due to nucleation and propagation of cracks is one of the main reasons for failure of aged aircrafts. Reported studies on aircraft failures indicate that the main cause of wing failure is due to fatigue cracks which nucleate from the wing root region. Thus, determination of residual life of the cracked wing structure using fracture mechanics approach becomes important. In the present work an attempt has been made to estimate the SIF of single and multiple cracks in an aircraft wing subjected to lift force. Crack depth ratios ranging between 0.1 and 0.4 and aspect ratios of 0.6 and 1.0 have been considered. Single and multiple cracks are introduced at the wing rib region and the lift force is applied at the bottom surface of the wing. Geometric correction factor (Y) is estimated with the additional consideration of mode II and mode III fracture. The effect of crack depth ratio and number of cracks on SIF is determined. Non symmetric SIF distribution is observed with increase of crack depth ratio. It is also noted that SIF values are always higher at the crack surface region compared to crack middle region irrespective of crack depth ratio.

Abstract: In this paper, an efficient simulation program (FCG-System) is proposed to simulate 2D fatigue crack growth under mixed-mode loading conditions. The simulation is basically an incremental crack extension procedure. An object-oriented modeling frame is proposed for simulating fatigue crack growth of complex structures. The modeling frame is developed in the context of the commercial FE code ABAQUS, utilizing Python language and ABAQUS Scripting Interface (ASI). The highly automatic finite element simulation method is not only used for a single crack tip, but also has been extended to the system of interactive multiple cracks. The robustness and the accuracy of the new simulation code will be shown by two examples, including single crack growth and multiple cracks growth. Those applications indicate that the implementation of the FCG-System, as proposed herein, can be a useful tool for this class of fatigue crack growth.

Abstract: This paper focuses on the multiple crack detection of steel pipelines using PZT-based guided waves. Numerical simulations of cracked pipes based on ultrasonic guided-waves are conducted by using the ANSYS finite element software. Based on the analysis of the reflected signal, the arrival time of the crack reflection waves are determined and the crack positions are accurately evaluated by the calculation of the travel time and group velocity of the PZT-based guided waves. The crack parameters are numerically altered to determine how the parameters impact the sensitive degree of the pipe crack damage. To validate the efficiency of the numerical simulation, an experiment of the multiple crack detection for the same parameter pipe with the numerical model is performed in the laboratory, and the results match well with the numerical simulation.

Abstract: The vibration analysis of a multi-cracked beam using discrete element technique (DET) was investigated in this study. Undamped simply supported beam was traversed by moving mass with constant speed and Euler Bernoulli beam theory was considered. Cracks are located in different positions and maximum deflection of mid-span was derived and compared. The results showed that increasing numbers of cracks in the beam causes more deflection while maximum deflection of beam takes longer time to build up. The results were validated by solving the equations generated using finite element method (FEM) and their comparison with already established results from previous similar studies (literatures) showed good agreement.

Abstract: Multiple flaws are often found in many high-temperature structural components. The interaction and coalescence of multiple cracks may significantly influence the service lives of these high-temperature components. In this paper, the interaction effect on multiple cracks is investigated under creep conditions. At the same time, interaction is considered during crack growth calculation by including the appropriate magnification factors in the calculations of creep fracture parameters. Based on these analyses, the assessment method of high-temperature structure containing multiple flaws is developed.

Abstract: The methods of Green's function, complex function and multi-polar coordinates are applied here to report interaction of an elliptical inclusion and a crack in half-space under incident SH-waves. Based on the symmetry of SH-waves scattering, the "conformal mapping" technology was developed to construct a suitable Green's function, a fundamental solution to the displacement field for the elastic half space containing elliptical inclusion while bearing out-plane line source load at arbitrary point, for creating a beeline crack with arbitrary length at any position combined with crack-division technology. The displacement field and stress field were then deduced while the inclusion coexists with the crack Lastly, numerical examples are presented to discuss the dependence of dynamic stress concentration factor (DSCF) around the elastic inclusion on different parameters.

Abstract: Under cyclic loading, the material weakening processes in structural members inevitably involve multiple cracking originating from some of the spatially-distributed initial flaws and imperfections, and hence diverse cracking behaviors can be expected. It is known from previous studies on multiple cracks that, the cracking behavior in a structural member can abruptly change as a crack or a number of cracks reach a critical value of crack propagation, causing sudden strength degradation. In this study, by applying sequential loads at different locations of the same FE model of a notched beam, it is shown that this unique strength degradation mechanism can repeatedly occur as cracks propagate under sequential loads, leading to multistage strength degradation of the member. This result is in line with early experimental findings that the load-carrying capacity of a notched concrete beam under bending decreases in a similar fashion as the sizes of multiple initial notches are arbitrarily increased. This study has important implications for understanding the fundamental fatigue mechanisms of various engineering materials.

Abstract: Fatigue flexural fracture test is taken out in this paper, to study the fatigue crack propagation rate of ultra-high toughness cementitious composites (UHTCC), as well as the effect of fiber volume fraction on fatigue crack propagation rate. Three fiber volume fractions are adopted: 1.5%, 2.0% and 2.5%. Similar to Paris law, a fatigue crack propagation equation of UHTCC is introduced, as dA/dN=C(△J)^m, with the two parameters A and △J are defined as the covering area of multiple fatigue cracks and the fatigue amplitude of J integral. Through experiment and analysis, the fatigue crack propagation rate slows down with the increase of PVA fiber fraction. Furthermore, the influence of PVA fiber on the propagation rate was found to become obvious with the increase of J integral.

Abstract: This paper presents the implementation of fatigue crack growth power law equations based on ΔK, ΔJ-integral and ΔCTOD fracture mechanics parameters determined in an FE analysis, to plates with multiple site damage (MSD). Results of fatigue tests with constant amplitude tensile loading carried out on mild steel plate specimens damaged with a single central crack and with three collinear cracks are presented. A relatively larger plastic zone occurred in the crack tip region at higher fatigue crack growth rate (FCGR), from 10^-7 to 10^-6 m/cycle. The crack growth models based on the elastic-plastic fracture mechanics (EPFM) parameters describe better fatigue crack growth in this range as compared to the liner elastic models.