Carlos Antonio Reis Pereira Baptista

Comparison of High Cycle Fatigue in 4340 and 300M Steel Welded with Fiber Laser

The AISI/SAE 4340 steel and 300M have been used in severe conditions where high strength and toughness are necessary. Since there is a particular interest in the aerospace industry, they are being used in aircraft landing gear, rocket engines vessels and satellite launch vehicles. Therefore, the understanding of the weldability of these steels has been the subject of special interest. This study aims to evaluate and compare the microstructure and mechanical strength by tensile and fatigue in steel sheets of 4340 and 300M steels after laser welding and tempering heat treatments. The results showed that the welds had high hardness and small heat affected zones. Nevertheless, there was not a decrease in the uniaxial tensile properties for the 300M and 4340 steels. The decrease in fatigue life due to the welding process, compared to the unwelded condition, was small, indicating that the parameters welding were adequate and that this process is viable.

A Damage Accumulation Model of Fatigue Crack Growth in Titanium

This work introduces a predictive method in which the fatigue crack propagation is treated in terms of cumulative damage of volume elements along the crack path. The development of the work includes considerations about the stress distribution in the cracked body and the stress-life and strain-life relations to be used in the computational procedure. It is assumed that the scattering of the stress-life data can be reproduced in the volume elements ahead of the crack, thus allowing probabilistic predictions to be performed. In order to check the reliability of the model, constant amplitude fatigue crack growth tests with load ratios of R = 0.1 and R = 0.5 were carried out in two sets of 15 commercial purity titanium sheet samples and the results were compared to the computational simulations. K e y w o r d s : fatigue crack propagation, cumulative damage, fracture mechanics, titanium.

Study of Fatigue in AISI 4340 Steel with Different Microstructural Conditions, Submitted to a Surface Treatment of Shot Peening

The AISI 4340 steel has been largely employed for structural purposes, which requires resistance levels with yield strength above 1400 MPa and it attains high levels of resistance in dual phase, bainitic or martensitic microstructural conditions. The samples of AISI 4340 steel with different microstructural conditions (martensitic, bainitic and ferritic/perlitic) have been submitted to fatigue tests on push-pull mode. Subsequently, the new specimens underwent a shot peening surface treatment and new fatigue tests. The results have been discussed in comparison to the three microstructural conditions studied and they were related to a microstructural characterization. The results have showed that a shot peening treatment is not always beneficial to fatigue life, since there is a relationship between the compressive stresses developed on the surface and its roughness formed due to the deformations. Under the three microstructural above studied it was noticed a strong fatigue life reduction in the martensitic condition because such microstructure is considered less ductile.

Study of Fatigue Crack Growth in Al-Mg-Si Alloys Using a Predictive Model under Positive and Negative Load Ratios

The study of fatigue crack growth (FCG) is aimed at residual life estimations in order to apply the damage tolerant criterion. Usual approaches are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics, ΔK, as the governing driving force for crack propagation. An alternative approach is the use of predictive theoretical schemes arising from damage mechanics. Although they havent achieved a reliability level high enough to be used in design, predictive models may be important in some situations like material selection. In the present work, a predictive FCG method based on the cumulative damage of volume elements along the crack path is employed. The development of the work includes considerations about the stress distribution in the cracked body and the stress-life and strain-life relations used in the computational procedure. A previously developed analytical expression for the stress distribution ahead of the crack in a finite width plate, based on the numerical analysis performed by the Finite Element Method, is used in the predictive method. The stress field is determined for both upper and lower limits of cyclic loadings. The fatigue crack growth behavior of three Al-Mg-Si alloys: AA 6005, AA 6351 and AA 6063, tempered and aged for the T6 condition, was analysed for positive and negative R-ratios. In order to check the model results, constant amplitude FCG tests with load ratios ±0,5 were carried out in M(T) specimens. The experimental results, compared to the computational simulations, show that it is possible to obtain predictions of FCG behaviour for both positive and negative load ratios.

Mechanical Behaviour of ZrO2-Bioglass Dental Ceramics under Cyclic Fatigue Loading

The cyclic fatigue life of 3mol%Y2O3-stabilized ZrO2 (3Y-TZP) ceramics doped with bioactive glass has been investigated. Samples of 3Y-TZP containing 3 or 5wt% of bioglass were sintered at 1300oC for 120 minutes. Sintered samples were characterized by relative density, XRD and SEM analysis. Mechanical properties of hardness and fracture toughness were determined using Vickers indentation method and Modulus of Rupture was determined by four-point bending testing. Furthermore, the reliability of the samples was estimated using Weibull statistic analysis. The cyclic fatigue life was estimated using four-point bending testing under frequency of 25Hz and stress ratio, R, of 0.1. Highly dense tetragonal ZrO2 samples were obtained after sintering and presented hardness of 10.5 and 11GPa, KIC of 6 and 7MPam1/2, bending strength of 320 and 450MPa and Weibull’ modulus of 6 and 7 for samples containing 5 or 3% of bioglass respectively. The fatigue behavior indicates that the increasing of stress level leads to decreasing of the number of cycles and the number of run-out specimens. Fatigue behavior was similar for the two compositions tested. Samples containing 3% of bioglass are more resistant (near 10%) to fatigue presenting cyclic fatigue life near to 250MPa while samples with 5% presented fatigue limit near to 220MPa.

Fatigue of Zirconia - Bioglass Dental Ceramics

In this work the cyclic fatigue life of 3mol.%Y2O3-stabilized zirconia polycrystalline ceramics, doped with 5%wt 3CaO.P2O5,-SiO2-MgO, has been investigated. Samples with 5 and 10%wt were cold uniaxial pressed (80MPa) and sintered in air at 1200 and 1300oC for 120 minutes. Sintered samples were characterized by X-Ray diffraction and Scanning Electronic Microscopy. Hardness and fracture toughness were determined using Vicker’s indentation method, and Modulus of Rupture was determined by four-point bending testing. Furthermore, the cyclic fatigue tests were also realized by four-point bending tests, under frequency of 25 Hz and stress ratio, R, of 0.1, for the best condition. In this condition, highly dense samples were obtained and presented values of hardness, fracture toughness and bending strength of 11.3 ±0.1GPa, 6.1±0.4MPa.m1/2 and 320±55MPa, respectively. The increasing of stress level leads to decreasing of the number of cycles and the number of run-out specimens. The stress induced tetragonal-monoclinic (t-m)-ZrO2 transformation, observed by X-Ray diffraction, contributes to the increasing of the fatigue life. Samples 3Y-TZP presents clearly a range of loading conditions where cyclic fatigue can be detected.

Fatigue Crack Growth in an Al-Mg-Si Alloy under Negative Load Ratio

The usual approaches for Fatigue Crack Growth (FCG) assessment are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics, ΔK, as the governing driving force for crack propagation. However, FCG data incorporating compressive loads have brought discussions and controversies. The compressive loads are not taken into account in the usual life prediction codes and the negative portion of the loading cycle is neglected. In the present work, constant amplitude fatigue crack growth tests with load ratios-1 and 0 are performed, for two diferent stess levels, in AA 6005 alloy centre-notched middle tension M(T) specimens and the obtained results are discussed. It is shown that the crack closure concept, that usually work well for positive ratios, is not enough to satisfactorily explain the negative load ratio effects. The results also showed how the negative part of the load affected the propagation rate of the crack, mainly for larger cracks and smaller loads. Finally, it is shown that a previously proposed expression for the stress distribution ahead of the crack can shed some light on this question.

Fracture Resistance of Flash Welded and Laser Welded Butt Joints in a Microalloyed HSLA Steel

High-strength, low-alloy (HSLA) steels are widely used in the automotive and oil industries due to their good mechanical properties and weldability. The selection of the welding process depends on several factors, including the quality of the weld bead and the production capacity. The knowledge of the mechanical performance of the welded joint is essential to ensure structural reliability. In the present work, butt joints were produced from 5 mm thick plates of a microalloyed HSLA steel by flash welding and by laser welding processes, the latter using two different heat input conditions. The microsctructure and hardness of the weld beads were evaluated. The fracture toughness of the welded joints was assessed by means of CTOD tests. The higher heat input laser welded joint presented critical CTOD comparable to that of the flash welded joint, whereas the lower heat input martensitic-bainitic laser welded joint tended to a brittle behavior.

Effect of Shot Peening on Fatigue Behavior of AISI 4340 in Different Loading Conditions

Shot peening is a method widely used to improve the fatigue strength of materials, through the creation of a compressive residual stress field (CRSF) in their surface layers. In the present research the gain in fatigue life of AISI 4340 steel, obtained by shot peening treatment, is evaluated under the three different hardnesses used in landing gear. Rotating bending fatigue tests and alternating tension tests were conducted and the CRSF was measured by x-ray tensometry prior and after interrupted fatigue tests. The evaluation of fatigue life after shot peening in relation to the relaxation of CRSF, of the crack initiation sites and surface roughness is done.

Laser Surface Treatment of SAE 4340 and 300M Steels

The 300M steel has emerged from the high strength SAE 4340 steel, which had been developed in order to obtain increased toughness and weldability in applications requiring better mechanical properties; some minor changes in the alloy elements were introduced: addition of vanadium, and raise in the silicon content. The laser surface hardening is a process in which the steel surface is heated below the melting point, in order to produce solid state phase transformations whithout changing the bulk. This work has evaluated which would be the best parameters for the laser carburizing heat treatment using a low power (50W) CO2 laser, comparing and contrasting the resulting microstructure and hardness of both steels. The following CO2 laser parameters were varied: speed, resolution and number of passes; power was kept constant. Carbon black was used as coating. After the surface laser treatment, the samples were cooled in air and the formation of a hardened layer and heat affected zone in both materials was observed. Samples were prepared for metallographic analysis, followed by optical and scanning electron microscopy observation. Layers presented a Vickers hardness value higher than the base metal and the hardened layer had a higher hardness value compared to the heat affect zone. The formation of martensite in the heat affected zone in both steels was observed, although not to the full extension, making this a multiphasic zone.