Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura
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A novel algorithm for crack path identification based on infrared images
Get PDFThe present work aims at developing a new approach towards the analysis of the crack path through the access to the information from the infrared (IR) video imaging of the steady fatigue crack growth. We first test some commonly available infrared image filtering techniques, and then we introduce a new robust algorithm to identify the position of the dynamic crack. The proposed procedure performs the local analysis of the temperature distribution around the fatigue crack tip in the 316L stainless steel. The developed MATLAB framework simplifies the routine processing and permits analyzing multiple experiments reasonably easily. Two filtering procedures - Gaussian and wavelet shrinkage - were implemented and applied successively to denoise the IR images. Both filters produce comparable and good quality results. The new algorithm capable of automatic locating the crack tip position from consecutive filtered IR images is proposed. Remarkably successful results are demonstrated with only small errors caused by materials distortion
The experimental study of heat dissipation during fatigue crack propagation under biaxial loading
Get PDFThe work is devoted to experimental study of heat flow evolution at the fatigue crack tip during biaxial loading. The plane samples of titanium alloy Graid-2 with thick of 1 mm were weakened by notch to initiate fatigue crack at the center of samples. Infrared thermography and the contact heat flux sensor based on the Seebeck effect are used to monitor the dissipated thermal energy. During tests the samples were subjected to cyclic loading of 10 Hz with constant stress amplitude and different biaxial coefficient. The experimental results confirm the previous conclusions of authors about two regime of energy dissipation at fatigue crack tip. The heat dissipation curve can be divided in two stages. Under the first stage the power of heat flux is proportional to the multiplication of the crack rate by its length. In the second stage is characterized by classical linear relationship between crack rate and heat flux rom the crack tip. The qualitative correspondence of the energy approach to the classical representation based on the stress intensity factor was shown
Granite stone subjected to ultrasonic fatigue tests under three point bending modality
Get PDFUltrasonic fatigue tests are carried out under the modality of three points bending on the granite stone. The testing specimens present a prismatic shape with the dimensions: 12 x 3 x 0.8 centimeters, in order to be supported at the two ends (1 centimeter of support each side), and to configure the three points bending modality. Granite specimens are supported on an acrylic structure destined to fix and immobilize the specimens. Vibrating load is applied at the center of testing specimens by a self-designed and fabricated aluminum alloy awl, which dimensions have been obtained by modal numerical analysis to fit the resonance condition, necessary for the ultrasonic fatigue tests.
Experimental results are obtained at room temperature and humidity comprised between 60 and 75%. The fatigue endurance is determined on this material under this modality of loading, and the fracture surfaces are analyzed to identify the principal trends related to crack initiation and propagation on this material.  
Specimen Level and Component Level Simulations of Fatigue Crack Growth Behavior under Cyclic Bending
Get PDFPower plant piping system subjected to many cyclic loading conditions leads to fatigue loading and thereby crack initiation in the piping system. Demonstrating Leak-Before-Break (LBB) in the design stage itself plays an important role in ensuring the safety of the Sodium-cooled Fast Reactors (SFR) system. The design code guidelines as per RCC MRx A16 in LBB demonstration is conservative in nature. Conducting experiments for each size of the piping systems are expensive and time-consuming. In this paper present, the efforts taken to develop reliable numerical methods to demonstrate Fatigue Crack Growth (FCG). The geometries selected for the FCG analysis include both specimen level as well as full-scale component. The results show that KI estimate for the plate specimen increased with the increase of ‘a/t’ ratio up to 0.6 and then decreases, whereas, in case of the pipe bend, it reduced beyond the ‘a/t’ ratio of 0.4. This early reduction is due to the influence of global compressive stress present ahead of the crack growth. Thus, component level FCG simulation is essential for the accurate demonstration of LBB. FCG simulation has numerically demonstrated with better engineering accuracy for both specimen level as well as component level geometrie
The effect of creep damage model formulation on crack path prediction
Get PDFThe stress, strain rate and process zone with respect to the creep-crack growth are analyzed by employing damage-evolution equations. The damage models for the fracture of the process zone are represented using a stress and ductility based formulation. Special attention has been addressed in the present study to the influence of the creep damage model formulation on the on crack path prediction. To evaluate the significance of dominating fracture mechanism a comparison of the cases for pure mode II is considered. It was observed that in the case of stress based model one side of the notch, dominated by tensile stresses, blunts, while the other side, dominated by shear strains, sharpens. In the case of ductility based model, there is a tendency for creep damage to localize only at the blunted part of the notch. This because the highest tensile hydrostatic stress and crack-tip constraint always occur near the blunted part of the notch. In this region, the crack growth direction and general creep damage zone deviate from the initial crack plane. As a result of numerical calculations the consequence of the crack deviation angle values, crack length increments and finally crack path were determined
Post- Impact Fatigue Damage Analysis of Quasi Isotropic CFRP Laminates through Infrared Thermography
Get PDFCarbon fiber reinforced plastic (CFRP) materials used for lightweight aircraft structures are susceptible to low velocity accidental impacts during manufacture and while in service. In this study the CFRP laminates subjected to low velocity impact conditions of three different energy levels were tested under various fatigue loading conditions till it reached the failure state. The infrared thermography NDT technique was used for the damage analysis of these post-impact fatigue loaded CFRP specimens to understand the damage distribution across the specimen. The cooling response curves obtained from the active thermography of the post impact fatigue specimens under four different combinations (transmission or reflection mode with impacted or un-impacted surface facing camera) for each of the specimens were analyzed. The rate of cooling in the temperature response curves is different for each specimens subjected with various post impact-fatigue loading conditions and it seems to be correlated with the extent of post-impact fatigue damage present in each specimen quantified through X-ray CT image processing. The temperature rise observed in the passive thermography at the fiber/ply breakage during static strength tests were proportional to corresponding load drops and the non-homogeneous spread of damage at the onset of tensile failure was seen from the thermographs
Damage detection in a reinforced concrete bridge applying wavelet transform in experimental and numerical data
Get PDFIn recent years, one can observe a constant worry of the scientific community on the search for demage detection techniques in structures. Such worry contributed to the progressive development of Structural Health Monitoring. This paper purpose a methodology using Continuous Wavelet Transform associated with interpolation and regularization techniques applied in the experimental and numerical mode shapes of the Dogna Bridge using only the demaged responses. The results of the analyses are presented and discussed in this paper
Effect of machining parameters on the mechanical properties of high dosage short –carbon- fiber reinforced composites
Get PDFThe machinability of polymer matrix composites with fibers strongly depends on the type of fiber and dosage in question, having a high influence on the selection of tools and cutting parameters. The cutting temperature depends of rotation and feed cutting tools and is significantly influencing on the quality of the machined surfaces and tool life. This paper presents the results of a current study concerning the effect of the rotation-cutting speed on the cutting temperature, roughness and tensile strength of short carbon fiber reinforced epoxy composites, potentially used in automotive and aeronautic industries. Composite plates were manufactured by compression molding, using short carbon fibers with length from 0.5 mm and 6 mm. The increasing of the rotation-cutting speed increases significantly the temperature generated in the tool and slightly increases surface roughness. Tensile strength and Young´s modulus are little sensitive to drilling speed. However, above 3000 rpm it was observed significant loss of stiffness, associated with the developed temperature in the machining process
Stress Intensity Factor Solutions for CTS Mixed Mode Specimen
Get PDFThe Compact Tension Shear (CTS) specimen is used to study fracture and fatigue under mixed mode I/II loading conditions. Existing K solution was developed for fracture studies and it does not consider the effect of crack deflection. The aim of present work is to develop KI and KII empirical solutions for cracks with different crack length, loading angle and crack orientation. A total number of 1120 cracked geometries were studied numerically by the finite element method and solutions were fitted to numerical predictions. An average difference of 0.53 % was found between numerical predictions and the solution proposed for KI. For KII the difference is higher, but the equivalent stress intensity factor showed a difference of only 1% because KII is lower than KI. Experimental work was developed to study fatigue crack growth in CTS specimens. The cracks always adopted a direction approximately normal to loading direction, i.e., tend to propagate under mode I
Analysis of the presence of bonding defects on the fracture behavior of a damaged plate repaired by composite patch
Get PDFThe repair technique has presented its effectiveness in the reduction of the stresses at the level of stresses concentration areas. The search for a patch and suitable adhesive for a good transfer of load has pushed researchers to develop many ideas, which relate the form, the nature, the stacking sequence of the patch and the adhesive type to give a better combination of choice between the patch and the adhesive. Our work fits in this context; the objective is to analyze by the method of finite elements the behavior in the rupture of a damaged plate in the presence of defect of bonding. The analysis of J-Integral and stresses in the tow substrates adhesive and patch shows clearly that their values depends strongly on the position of the default essentially when it’s located close to the free edge of the free edge of the adhesive or the crack