Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura
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A nonlinear elasto-plastic analysis of Reissner-Mindlin plates by finite element method
Get PDFIn this paper, a finite element simulation of nonlinear elasto-plastic deformations of Reissner-Mindlin bending plates is described. The previously proposed four-node Q4g element with transverse energy of shearing for thick bending plates is extended to account for isotropic material nonlinearities. An incremental finite element procedure has been used for the elasto-plastic analysis of the thick bending plate. Modified Newton-Raphson method has been used to solve the nonlinear equations. Von-Mises yield criteria have been applied for yielding of the materials along with the associated flow rule. To verify the present element, simple tests are demonstrated and various elasto-plastic problems in which the development of the plastic zone are solved
Numerical simulation of subsurface defect identification by pulsed thermography and improvement of this technique for noisy data
Get PDFPulsed thermography is an active non-destructive technique which uses optical excitation source to stimulate heating of the object under investigation. This work is devoted to the simulation of the pulsed thermography method in a steel plate with the ceramic coating containing artificial defects of various depths and sizes. The simulation has been carried out on the base of the model which takes into account complex heat exchange of the sample with the surrounding by convection, conduction and radiation. Comparison of the temperature contrast with the experimental data has shown that the results are in a good qualitative and quantitative agreement in all stages of the cooling process. Due to the fact that the temperature contrast is often susceptible to the surface noise of various nature the Kalman-based signal processing technique was developed. The comparative analysis has shown that the proposed filtration technique provides better value of signal-to-noise ratio in comparison to the considered well-known techniques of signal reconstruction when proper calibration of the filtration parameters is carried ou
Initial orientation of the fretting fatigue cracks in shrink-fit connection specimens
Get PDFThe bending configuration of shrink-fitted connection fretting fatigue tests causes a fluctuation of the contact pressure and in turn a more favourable condition for shear, rather than tensile, crack initiation. According to the classification Type I (shear) and Type II (tensile) fretting cracks, the type II was only observed for the tests with no surface enhancement, which experienced the lowest fretting strength and then a reduced pressure fluctuation effect. The crack initial direction was predicted as the critical plane orientation undergoing the maximum FS stress parameter, for the shallow type I shear cracks. If the critical plane is searched at the fretting hot-spot, the incorrect outward direction is obtained, since the orthogonal direction, with the same shear stress amplitude, experiences a more compressive maximum normal stress. On the contrary, with the application of the Point Method, taking the stress location at half the critical distance and by following the potential critical plane, a higher shear stress along the inward direction is obtained. The more compressive stress below the contact can therefore be compensated, and then the inward shallow direction is correctly predicted
The crack path in fuselage panel under mixed mode biaxial loading
Get PDFThis paper provides the background for practical applications of nonlinear fracture resistance parameters for structural integrity assessment of aviation structures with initial cracks. The subject of this numerical study is a fragment of airplane fuselage panel with initial central crack under biaxial loading. The plastic stress intensity factor (SIF) concept is here applied to estimate the critical crack size in fuselage panel under biaxial loading. The fracture damage zone influence on crack tip stress state in airplane fuselage panel under biaxial loading is evaluated. The values of plastic stress intensity factor Kp obtained from both elastic-plastic solution for isotropic body and elastic-plastic solution with cohesive zone are compared to the critical values of nonlinear fracture resistance parameter, for fuselage panel under biaxial loading. The curvilinear crack path in the considered fuselage panel under mixed mode biaxial loading is finally evaluated
Characterisation and Evaluation of the Mechanical Behaviour of Endodontic-grade NiTi Wires
Get PDFWith the introduction of new materials and advances in medical science, the endodontic files have changed since the early days of root canal treatments. In the late days, we have seen an increasing use of Nickel-Titanium (NiTi) alloys, to the detriment of more conventional alloys. At body temperature, NiTi alloys present a superelastic behaviour, which allows to be more effective in the removal of the tooth pulp tissue, and in the protection of the tooth structure.
Anyhow, these NiTi instruments will eventually fracture, usually without any visual signal of degradation. Thus, there is a need of studying these alloys, as they present a high hysteresis cycle and non-linearities in the Elastic domain.
Currently, there is no international standard to test NiTi endodontic files, so various authors have attempted to design systems that can test them under fatigue loads, usually based on empirical setups.
Following a systematic approach, this work presents the results of rotary fatigue tests for two Alfa Aesar® Nitinol wires with different diameters (0.58mm and 0.25mm).The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions
Impact response of laminated cylindrical shells
Get PDFComposite laminates subjected to low-velocity impact events on the through-thickness direction are conveniently studied and disseminated in the open literature. However, in terms of laminated cylindrical shells this subject is less common. Therefore, the main goal of the present work is to study the impact response of laminated composite cylindrical shells composed by different type of fibres. For this purpose, laminates with different configurations (6C, 2C+2K+2C and 2C+2G+2C), where the “number” represents the number of layers used and C=Carbon, K=Kevlar and G=Glass fibre layers, were analysed in terms of static and impact strength. It is possible to conclude that both static and impact performance are strongly influenced by the shells’ configuration. In terms of compressive static strength, the Kevlar hybrid shells present values 53.2% higher than the 6C shells, while the glass hybrid shells present values 17.3% lower. The impact analyses shows, regardless the similarity of the maximum loads for all configurations, that Kevlar hybrid shells achieved the highest elastic recuperation and the glass hybrid shells the maximum displacement
Low velocity failure and integrity assessment of foam core sandwich panels
Get PDFImpact resistance and energy absorbing capability are of great interest in the design of composite sandwich structures. This paper experimentally studies damage, failure and energy absorption properties of foam core sandwich panels with aluminum and glass fiber reinforced plastic (GFRP) facesheets subjected to low velocity impact. Tests are performed using a drop weight impact tower at different impact velocities. The energy absorbing capabilities of aluminum and composite facesheet sandwich panels with PUR and PS foam core are evaluated by means of absorbed energy-time histories and by specific parameters as normalized absorbed energy, specific energy absorption, and crush force efficiency. Stiffer panels behave better at lower impact velocities, while more ductile ones do better if impact energy is increased
Fatigue strength evaluation and fracture behavior of joined dual phase steel/AA6061-T6 aluminum alloy
Get PDFThe fatigue strength evaluation and fracture behavior for a dual phase steel-AA6061-T6 bonded joints with three different adhesives (DC-80, Betamate 120 and MP55420) are presented in this paper. Single lap shear tests were used to determine maximum shear loads, for the single lap shear testing for 5.0 mm overlap length were 2 to 3.5 times higher in comparison to the 12.7 mm overlap length specimens. The results for the strain measurement revealed that higher strain-stress were developed in the 6061-T6 aluminum alloy adherend and in all cases they were lower than the adherends yield strength. Fatigue testing was carried out at 30, 50 and 70 % of the maximum shear load, 0.1 of reversibility load ratio (R) and 30 Hz of frequency. After testing, Basquin and Wholer graphs were built for each adhesive at 12.7 and 50.0 mm of overlap length. The results suggested that at higher overlapping, the cyclic maximum load increased. Additionally, the maximum fatigue loading at 10^6 cycles for MP55420 adhesive was 1.3 kN for an overlapping of 12.7 mm and 2.9 kN for 50 mm. For DC80 adhesive was 1.75 kN for overlapping 12.7 mm and 4.8 kN for 50 mm. Finally, for the Betamate 120 adhesive was 1.8 kN for 12.7 mm of overlapping and 6 kN for 50 mm. The post-fracture visual inspection revealed that MP55420 and Betamate 120 adhesives had a cohesive failure, while the DC-80 showed cohesive-adhesive failure. Additionally, the scanning electron microscopy evaluation on the spew fillet revealed resolved striations and a network of small micro-dimples for the Betamate 120 and MP55420 adhesives. On the other hand, DC-80 adhesive exhibited notable facet fragile failure that was confirmed by the shape of stress-strain plot with straight line from the origin to the point of fracture
Damage evolution law on the surface field of argillaceous dolomite based on Brazilian test and 3D digital image correlation
Get PDFThis paper aims to disclose the damage evolution law of the surface field of argillaceous dolomite. For this purpose, the 3D digital image correlation was combined with the Brazilian test into a new analysis method for the surface field damage evolution of the rock. First, the stress-strain curve of argillaceous dolomite was obtained in Brazilian test, and the strain contours of the key points were acquired by 3D digital image correlation. Then, the standard deviations of the x and y direction strains at the key points during the test were calculated using statistical methods. In addition, the dual damage factor was introduced to quantify the law of the strain statistics and plotted into a curve. Finally, the strains in x and y directions on the horizontal axis ox in the disc center were obtained through elastic mechanical analysis and compared with those measured by 3D digital image correlation in the elastic phase. In this way, the following conclusions were drawn: the argillaceous dolomite exhibited obvious non-homogeneity in the surface field damage evolution. The different phases of the Brazilian test can be determined accurately according to the turning points of the damage factor curve. The fluctuations of the damage factor curve also reveal the features of surface field damage evolution of argillaceous dolomite in Brazilian test. This research shows that the traditional assumption of homogeneity cannot reflect the heterogeneity of the surface field damage evolution of argillaceous dolomite in Brazilian test and provides a quantitative research method for rock damage evolution in that test.
 
Nonlinear modeling of unreinforced masonry structures under seismic actions: validation using a building hit by the 2016 Central Italy earthquake
Get PDFA prolonged seismic sequence struck the regions of Central Italy between August 2016 and January 2017, causing several fatalities and widespread damage to the built environment. The main objective of this work was to study the structural and seismic behavior of “Pietro Capuzi” school, located in Visso, in the Marche region, which was severely damaged by the 2016-2017 Central Italy Earthquake. A 3D finite element (FE) model of the entire school was prepared, adopting a macro-modeling approach to represent masonry materials. An eigenvalue analysis was initially performed in order to identify the dominant modes of vibration of the structure and to calibrate the numerical model according to the results of the dynamic identification tests. Afterwards, nonlinear static analyses were performed on the calibrated FE model to evaluate the seismic response of the structure. Finally, the numerical results obtained in terms of crack pattern and failure mechanisms were compared with the damage experienced by the building. The numerical model proved to accurately predict the seismic response exhibited by the structure during the past seismic events