Papers by Author: Manuel de Freitas

Abstract: Theobservation of cracks in mechanical parts shows that cracks often initiate oncutting edges. A lot of effort has been done to developing theories to predictfatigue behaviour of welds. However for the cut-edges available data is veryscarce on the fatigue behaviour. Thispaper presents theresults obtained in fatigue tests on DP600 dual-phase steel sheet specimens, underthree types of cutting edges processes: milling with two cutting parameters andabrasive waterjet cutting. The tests were carried out using smooth specimens (K_t=1)and with a fatigue constant amplitude loading with R=0.1. Surface roughness andresidual stresses induced by these different cutting conditions were measuredand analysed. It was found that the fatigue strength of the abrasive waterjetcutting specimens was smaller than the predicted fatigue strength of themilling specimens and these may be attributed to the surface roughness inducedby the cutting process. Finally failure mechanisms were studied with the scanningelectron microscope (SEM) at fracture surfaces, including the identification ofthe fatigue crack initiation region. It was also observed that fatigue crackinitiation takes place on the cut-edges.

Abstract: In this work it is studied the low-cycle fatigue behavior of the magnesium alloy AZ31-B at several total strains under uniaxial and multiaxial cyclic loading conditions. Cyclic tests were carried out in a biaxial servo hydraulic machine under strain control at room temperature. Test specimens were machined in an hourglass shape from extruded rods. The total strain amplitudes started at 0,2% and ended at 1.4% regarding the von Mises equivalent strain. The particular mechanical behavior inherent to this type of materials, hexagonal closed pack microstructures, leads to conclude that it is necessary to have a numeric elastoplastic model based in experimental tests. In this paper is presented a numerical model based on stress-strain experimental data. The objective is to modulate several physical mechanisms inherent to the magnesium elastoplastic behavior. In order to validate the achieved model the numeric estimations were correlated with the experimental data and with the Jiang & Sehitoglu plasticity model. Results show that the implemented model modulation is in agreement with the experimental data. Some differences between the Jiang & Sehitoglu and the implemented model regarding the magnesium hysteresis loop modulation are pointed out.

Abstract: Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

Abstract: The unexpected collapse of engineering structures is often caused by the fatigue phenomenon resulting from degradation of mechanical properties of materials due to multiaxial cyclic loadings. The interpretation of such degradation is a topic of intensive research in multiaxial fatigue. The fatigue strength is commonly evaluated by the equivalent stress based on the shear stress in the octahedral plane. However, the use of this kind of equivalent stress in the multiaxial fatigue criteria has been proven to be inappropriate. The degradation of mechanical properties of materials is dependent on several factors, e.g. the loading path has a strong influence on the fatigue strength. Non-proportional loadings cause higher damage in materials than proportional loadings for the same maximum equivalent stress. The purpose of this work is to study the effect of different multiaxial loadings on the 42CrMo4 steel and to improve the understanding about the relation between the fatigue strength and the sequential loading proportionality. The considered loadings were defined with the same history but with different load sequences and equivalent stress. To implement this work a biaxial servo-hydraulic fatigue machine was used. The fatigue life and crack angle were measured for each specimen. An analysis was made in order to correlate the crack initiation and fatigue life with the theoretical models, some remarks regarding these topics are presented.

Abstract: In this paper, the mechanical behaviour of extruded AZ31 magnesium alloys under multiaxial fatigue loading conditions is studied. The monotonic properties of the AZ31 magnesium alloy were determined by tests on the specimens which were machined from extruded rods. Then, the cyclic deformation under multiaxial loading conditions was simulated by ANSYS and a plasticity program with the Jiang & Sehitoglu plasticity model. The fatigue lives were estimated by the critical plane models coupled with Coffin-Manson rule, such as Findley, Fatemi-Socie, Brown-Miller, SWT and Liu models. Four loading paths were considered with different levels of non-proportionality, the results show significant loading path dependence.

Abstract: One of the major disadvantages of laminated composites is their tendency to delaminate. Unidirectional glass/epoxy laminates have been tested under static conditions by the use of fracture mechanics. Mode I, mode II, mixed mode I-II, mode III and mixed mode II-III tests were performed. Double cantilever beam (DCB), end-notched flexure (ENF), mixed-mode bending (MMB) and edge crack torsion (ECT) specimens were used. Scanning electron microscopy technique was used to identify distinguishing fractographic features and to establish the differences between the various modes of fracture after specimens testing. The propagated orientation of the delamination could be specified from the patterns of fracture surface. Scanning electron micrographs of fractured surfaces showed that the most predominant fractographic features in mode I and mode II are the large amount of fibre pull-out and the cusps markings respectively. In the MMB specimen the fracture surfaces are characterized by fibre breakage under shearing with fractures localized in the resin with cusps having an orientation of 90º (mode II) and also fractures localized in the resin and along the resin/fibre interface (mode I). Mode III characterization concluded that some limited mixed mode II-III seems to be present for ECT specimen on delamination initiation and growth, but a large majority of mode III delamination is present.

Abstract: The environment is a prominent issue today. Designing environmentally sustainable products is an attempt to address this question. In many cases, natural materials are environmentally friendly for product design manufacturing. The goal of this work is to study the mechanical behaviour of NL10 and NL30 cork agglomerates. Compression, shear and bending tests in sandwich specimens made of glass or jute fiber in facings and cork agglomerates as core were carried out. The sandwich specimens were manufactured by Resin Transfer Moulding (RTM) process. Results show that NL30 has a higher compression strength and shear resistance than NL10 agglomerate due to its manufacturing process, which originates superior density, but the NL30 agglomerate superior density is undesirable. Sandwich test specimens that presented failure by rupture of the core in both types of tests, core shear tests and three point bending tests, showed that the failure is mainly adhesive occurring between the adhesively joined cork grains. Since grains are unaffected and remain intact, it is possible to improve these materials by using better agglutinants and new bonding techniques with the intent of getting cork agglomerates with higher shear and flexural strength.

Abstract: In real engineering components and structures, many accidental failures are due to unexpected or additional loadings, such as additional bending or torsion, etc. Therefore, it has attracted more research attentions to study the mechanical behavior of materials under complex loading conditions. Two typical structural materials are studied and compared in this paper: AISI 303 stainless steel and 6060-T5 Aluminum alloy. The objective is to study the effects of multiaxial loading paths on the crack initiation and orientation of the two materials studied. Fatigue tests were conducted in a biaxial testing machine. Fractographic analyses of the fracture surface were carried out by optical microscope and SEM approaches. In addition to the experimental studies, theoretical predictions of the damage plane were made using critical plane approaches. Comparisons of the predicted orientation of the damage plane with the experimental observations are shown. The applicability of the multiaxial fatigue criteria for the two materials is discussed. It was shown that the two materials studied have different crack orientations under the same loading path. This observation appears to show that the applicability of the fatigue models is dependent on the material type and multiaxial microstructure characteristics.

Abstract: The main objective of the present work is to present and discuss the effects of the residual stresses generated in the matrix. Both, elastic and elastic-plastic models using explicit finite element code Abaqus were developed to investigate the thermal residual stresses generated around a diamond particle embedded in a metal matrix (a binder) during the sintering process for obtaining a diamond tool. The first part of the work consists in examining the thermal residual stress field generated by using three sintering temperatures from which the bodies are cooled from (T1=800°C, T2=700°C and T3=600°C) and two diamond shapes modelled with 2D axisymmetric elements, one from a circle the other from an octagon, respectively. In this case only one type of binder showing typical elastic behaviour is considered. The second part of this work consists in comparing the finite element solutions using binders with different tensile behaviour (elastic vs. elastic-plastic behaviour). This last study has used a particle shape generated from the octagon, since this shape allows, in a great number of real situations, the simulation of a particle with a larger number of facets, in line with what it is observed when looking at a conventional diamond tool.

Abstract: Under constant amplitude loading, a single variable (K) or Kmax are required in crack growth relationships. The transferability of fatigue laws, obtained under constant amplitude loading to variable amplitude fatigue, requires at least an additional variable, whose evolution with crack length accounts for the interactions effects between cycles of different types. This paper presents an analysis of fatigue crack growth tests on M(T) specimens made of a medium carbon steel. The specimens are subjected to repeated blocks of cycles made up of one or several overloads separated by a variable number of baseline cycles and two baseline stress ratios. The main objective of this study is to better understand the mechanisms at the origin of interactions effects due to the presence of overloads (or underloads) at different locations of each block loading. Results have shown that the interaction effects are closely related to the cyclic plastic behaviour of the material and also the so-called Bauschinger effect.