Multiple random crack propagation using a boundary element formulation

This paper proposes a boundary element method (BEM) model that is used for the analysis of multiple random crack growth by considering linear elastic fracture mechanics problems and structures subjected to fatigue. The formulation presented in this paper is based on the dual boundary element method, in which singular and hyper-singular integral equations are used. This technique avoids singularities of the resulting algebraic system of equations, despite the fact that the collocation points coincide for the two opposite crack faces. In fracture mechanics analyses, the displacement correlation technique is applied to evaluate stress intensity factors. The maximum circumferential stress theory is used to evaluate the propagation angle and the effective stress intensity factor. The fatigue model uses Paris` law to predict structural life. Examples of simple and multi-fractured structures loaded until rupture are considered. These analyses demonstrate the robustness of the proposed model. In addition, the results indicate that this formulation is accurate and can model localisation and coalescence phenomena. (C) 2010 Elsevier Ltd. All rights reserved.

On the analysis of notched concrete beams: From measurement with digital image correlation to identification with boundary element method of a cohesive model

A way of coupling digital image correlation (to measure displacement fields) and boundary element method (to compute displacements and tractions along a crack surface) is presented herein. It allows for the identification of Young`s modulus and fracture parameters associated with a cohesive model. This procedure is illustrated to analyze the latter for an ordinary concrete in a three-point bend test on a notched beam. In view of measurement uncertainties, the results are deemed trustworthy thanks to the fact that numerous measurement points are accessible and used as entries to the identification procedure. (C) 2010 Elsevier Ltd. All rights reserved.

Fatigue behaviour of friction stir welded AA2024-T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024-T3 aluminium alloy have been studied under constant load amplitude (increasing-Delta K), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka`s method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold Delta K values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K-C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non-conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non-conservative crack growth rate predictions next to K-C instability. At threshold Delta K values non-conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.

Internal Residual Stresses in Sintered and Commercial Low Expansion Li(2)O-Al(2)O(3)-SiO(2) Glass-Ceramics

We performed Synchrotron X-ray diffraction (XRD) analyses of internal residual stresses in monolithic samples of a newly developed Li(2)O-Al(2)O(3)-SiO(2) (LAS) glass-ceramic produced by sintering and in a commercial LAS glass-ceramic, CERAN (R), produced by the traditional crystal nucleation and growth treatments. The elastic constants were measured by instrumented indentation and a pulse-echo technique. The thermal expansion coefficient of virgilite was determined by high temperature XRD and dilatometry. The c-axis contracts with the increasing temperature whereas the a-axis does not vary significantly. Microcracking of the microstructure affects the thermal expansion coefficients measured by dilatometry and thermal expansion hysteresis is observed for the sintered glass-ceramic as well as for CERAN (R). The measured internal stress is quite low for both glass-ceramics and can be explained by theoretical modeling if the high volume fraction of the crystalline phase (virgilite) is considered. Using a modified Green model, the calculated critical (glass) island diameter for spontaneous cracking agreed with experimental observations. The experimental data collected also allowed the calculation of the critical crystal grain diameters for grain-boundary microcracking due to the anisotropy of thermal expansion of virgilite and for microcracking in the residual glass phase surrounding the virgilite particles. All these parameters are important for the successful microstructural design of sintered glass-ceramics.

On the finite element modeling of fatigue crack growth in pressurized cylindrical shells

A methodology for the computational modeling of the fatigue crack growth in pressurized shell structures, based on the finite element method and concepts of Linear Elastic Fracture Mechanics, is presented. This methodology is based on that developed by Potyondy [Potyondy D, Wawrzynek PA, Ingraffea, AR. Discrete crack growth analysis methodology for through crack in pressurized fuselage structures. Int J Numer Methods Eng 1995;38:1633-1644], which consists of using four stress intensity factors, computed from the modified crack integral method, to predict the fatigue propagation life as well as the crack trajectory, which is computed as part of the numerical simulation. Some issues not presented in the study of Potyondy are investigated herein such as the influence of the crack increment size and the number of nodes per element (4 or 9 nodes) on the simulation results by means of a fatigue crack propagation simulation of a Boeing 737 airplane fuselage. The results of this simulation are compared with experimental results and those obtained by Potyondy [1]. (C) 2008 Elsevier Ltd. All rights reserved.

2D evaluation of crack openings using smeared and embedded crack models

This work deals with the determination of crack openings in 2D reinforced concrete structures using the Finite Element Method with a smeared rotating crack model or an embedded crack model In the smeared crack model, the strong discontinuity associated with the crack is spread throughout the finite element As is well known, the continuity of the displacement field assumed for these models is incompatible with the actual discontinuity However, this type of model has been used extensively due to the relative computational simplicity it provides by treating cracks in a continuum framework, as well as the reportedly good predictions of reinforced concrete members` structural behavior On the other hand, by enriching the displacement field within each finite element crossed by the crack path, the embedded crack model is able to describe the effects of actual discontinuities (cracks) This paper presents a comparative study of the abilities of these 2D models in predicting the mechanical behavior of reinforced concrete structures Structural responses are compared with experimental results from the literature, including crack patterns, crack openings and rebar stresses predicted by both models

Tempering temperature effects on abrasive wear of mottled cast iron

The effects of different tempering temperatures (300-600 degrees C) on abrasive wear resistance of mottled cast iron were studied. Abrasive wear tests were carried out using the rubber-wheel test on quartz sand and the pin test on Al(2)O(3) abrasive cloths. The retained austenite content of the matrix was determined by X-ray diffraction. The wear surface of the specimens was examined by scanning electron microscopy for identifying the wear micromechanism. Bulk hardness and matrix hardness before and after the tests were measured. The results showed that in the two-body (pin-on-disc test) system, the main wear mechanism was microcutting and high matrix hardening was presented. The wear rates presented higher correlation with the retained austenite than with the bulk and matrix hardness. In the three-body system (sand-rubber wheel), the wear surfaces presented indentations due to abrasive rolling. The wear rates had better correlation with both the bulk and matrix hardness (before and after the wear test) than with the retained austenite content. There are two groups of results, high and low wear rates corresponding to each tribosystem, two-body abrasive wear and three-body abrasive wear, respectively. (C) 2009 Elsevier B.V. All rights reserved.

Particle size effect on abrasion resistance of mottled cast iron with different retained austenite contents

Effects of particle abrasive sizes on wear resistance of mottled cast iron with different retained austenite contents were studied. Abrasive wear tests using a pin test on alumina paper were carried out, using abrasive sizes between 16 mu m and 192 mu m. Retained austenite content of the matrix was determined by X-ray diffraction. The wear surface of samples and the alumina paper were examined by scanning electron microscopy for identifying the wear micromechanism. The results show that at lower abrasive sizes the mass loss was similar for the iron with different austenite contents. However, at higher abrasive sizes the samples with higher retained austenite content presented higher abrasion resistance. For lower abrasive sizes tested, samples with higher and lower retained austenite content both presented microcutting. On the other hand, the main wear micromechanism for the samples with higher retained austenite content and higher abrasive sizes was microploughing. The samples with lower retained austenite content presented microcutting and wedge formation at higher abrasive sizes. Higher abrasive size induced more microcutting in samples with lower retained austenite. The iron with lower retained austenite content presented wider grooves for the different abrasive sizes measured. SEM on the abrasive paper used on samples with higher retained austenite showed continuous and discontinuous microchips and the samples with lower retained austenite showed discontinuous microchips at 66 and 141 mu m. This research demonstrates the relation between abrasive size, wear resistance, groove width and wear micromechanism for mottled cast iron with different retained austenite contents. (C) 2009 Elsevier B.V. All rights reserved.

Abrasive wear study of white cast iron with different solidification rates

The abrasive wear resistance of white cast iron was studied. The iron was solidified using two solidification rates of 1.5 and 15 degrees C/s. Mass loss was evaluated with tests of the type pin on abrasive disc using alumina of different sizes. Two matrices were tested: one predominantly austenitic and the other predominantly martensitic, containing M(3)C carbides. Samples with cooling rate of 15 degrees C/s showed higher hardness and more refined microstructure compared with those solidified at 1.5 degrees C/s. During the test, the movement of successive abrasives gave rise to the strain hardening of the austenite phase, leading to the attainment of similar levels of surface hardness, which explains why the wear rate showed no difference compared to the austenite samples with different solidification rates. For the austenitic matrix the wear rate seems to depend on the hardness of the worn surface and not on the hardness of the material without deformation. The austenitic samples showed cracking and fracture of M(3)C carbides. For the predominantly martensitic matrix, the wear rate was higher at the solidification rate of 1.5 degrees C/s, for grain size of 66 and 93 mu m. Higher abrasive sizes were found to produce greater penetration and strain hardening of austenitic matrices. However, martensitic iron produces more microcutting, increasing the wear rate of the material. The analysis of the worn surface by scanning electron microscopy indicated abrasive wear mechanisms such as: microcutting, microfatigue and microploughing. Yet, for the iron of austenitic matrix, the microploughing mechanism was more severe. (C) 2009 Elsevier B.V. All rights reserved.

Load effect in abrasive wear mechanism of cast iron with graphite and cementite

In this study four irons were casted with different chromium and vanadium contents: 2.66% Cr, 5.01% Cr, 2.51% V and 5.19% V. Their microstructure is composed of: ledeburite, graphite and M(3)C carbides (cementite). Pin-abrasion tests were carried out using fixed alumina abrasive grains at different loads: 1, 2, 4.6 and 10 N. The wear surface and the abrasive paper were examined by scanning electron microscopy for identifying the wear micromechanism. The results reveal that the mass loss increased with the load increase, and the effect of the percentage of chromium on mass loss is inverted when the load is increased from 4.6 to 10 N; for 4.6 N the mass loss decreased when the chromium percentage was increased from 2.66% to 5.01%. Nevertheless, for 10 N the mass loss increased when the chromium percentage was increased. The worn surfaces of the materials tested at 1 N show microcutting caused by the abrasive tip that produces continuous microchips. The worn surfaces and the abrasive paper tested at 10 N show continuous microchips and brittle debris. The results show that high pressures produce a brittle wear mechanism and low pressures produce a more ductile wear micromechanism, for this, the applied pressure defines the dependence between the wear resistance and wear micromechanism. (C) 2009 Elsevier B.V. All rights reserved.

Experiments on stress-triaxiality dependence of material behavior of aluminum alloys

The paper presents and discusses experimental procedures, visual observations and test results considered important to obtain data that can be used in validation of constitutive relations and failure criteria. The aim is to investigate the combined effects of stress intensity, stress-triaxiality and Lode parameter on the material response and failure behavior of aluminum alloys. Smooth and pre-notched tensile and shear specimens were manufactured from both very thin sheets and thicker plates to cover a wide range of stress triaxialities and Lode parameters. In addition, modified Arcan specimens were designed allowing investigation of the effect of sudden changes in stress states and deformation modes on the material behavior. (C) 2009 Elsevier Ltd. All rights reserved.

Polymer toughening using residue of recycled windshields: PVB film as impact modifier

In this work, poly(vinyl butyral) (PVB) film originated from the mechanical separation of windshields was tested as all impact modifier of Polyamide-6 (PA-6). The changes undergone by PVB film during the recycling process and the blend manufacturing were evaluated by thermal analyses, infrared spectroscopy and loss oil ignition. Blends of PA-6/original PVB film and PA-6/recovered PVB film were obtained in concentrations ranging from 90/10 to 60/40. The mechanical properties of the blends were investigated and explained in light of the blends morphologies, which in turns were correlated to the changes undergone by the PVB film during the recycling process. The original film presented a plasticizer content of 33 wt.%, which decreased to as low as 20 wt.%, after the recycling and blend preparation processes. The PA-6/PVB film blends presented lower values of tensile strength and Young`s modulus than Polyamide-6, but all blends presented a dramatic increase in their toughness, with a special feature for the 40 wt.%(, blend, which resulted in a super toughened material (impact strength exceeding 500 J/m). Similar results were obtained with recovered PVB film and super tough blends were also obtained. The use of recovered PVB resulted in a smaller improvement of the impact strength due to the loss of plasticizer undergone during the recycling process. The morphological observations showed that if the interparticle distance is smaller than around 0.2 mu m (critical value), the notched Izod impact strength values increase considerably and the fracture surface of blends exhibit characteristics of tough failure. (C) 2007 Elsevier Ltd. All rights reserved.

Photodegradation of Thermodegraded Polypropylene/High-Impact Polystyrene Blends: Mechanical Properties

The influence of the addition of high-impact polystyrene (HIPS) on polypropylene (PP) photodegradation was studied with blends obtained by extrusion with and without styrene-butadiene-styrene (SBS) copolymer (10 wt % with respect to the dispersed phase). The concentrations of HIPS ranged from 10 to 30 wt %. The blends and pure materials were exposed for periods of up to 15 weeks of UV irradiation; their mechanical properties (tensile and impact), fracture surface, and melt flow indices were monitored. After 3 weeks of UV exposure, all of the materials presented mechanical properties of the same order of magnitude. However, for times of exposure greater than 3 weeks, an increasing concentration of HIPS resulted in a better photostability of PP. These results were explained in light of morphological observations. This increase of photostability was even greater when SBS was added to the blends. It was more difficult to measure the melt flow index of the binary PP/HIPS blends than that of PP for low concentrations of HIPS; this was most likely due to energy transfer between the blend domains during photodegradation. This phenomenon was not observed for the ternary blends. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 770-779, 2011

Briquetting of steel grit recovered from the ornamental rocks cutting waste

This paper presents the results obtained with the production of briquettes from the steel grit found in the residue of ornamental rocks. The grit recovered by magnetic separation was characterized by titrimetric analysis, EDS (Electron Dispersive Spectroscopy) and X-ray diffraction for the analysis of iron concentration in the residue. The size and distribution of particles were obtained by the granulometric analysis method and scanning electron microscopy (SEM). The process resulted in a concentrate containing 93% metallic iron. The maximum load before fracture of the green briquettes was 1.02kN and of the dry briquettes was 3.59kN.

Effect of precracking method on K(Ic) results for medium-density polyethylene tested under cryogenic condition

The effect of different precracking methods on the results of linear elastic K(Ic) fracture toughness testing with medium-density polyethylene (MDPE) was investigated. Cryogenic conditions were imposed in order to obtain valid K(Ic) values from specimens of suitable size. Most conservative K(Ic) values were obtained by slow pressing a fresh razor blade at the notch root of the specimen. Due to the low deformation level imposed on the crack tip region, the slow pressing razor blade technique also produced less scatter in fracture toughness results. It has been shown that the slow stable crack growth preceding catastrophic brittle failure during K(Ic) tests in MOPE under cryogenic conditions should not be disregarded as it has relevant physical meaning and may affect the fracture toughness results. (C) 2010 Elsevier Ltd. All rights reserved.