Jorge Alberto Rodriguez Duran

Numerical stress-life curves for the AISI 4340 steel using two sets of materials properties and different bi-axial stress ratios

Fatigue assessments by the more robust strain-based approach demand the determination of the local strain history from nominal stresses. For notched members, a cyclic constitutive relation, the stress concentration factor SCF and a strain concentration rule are used with this aim in some approximate solutions. The plastic part of the cyclic constitutive relations for many materials is well adjusted by a Ramberg-Osgood RO type equation. The parameters in the RO equation are the cyclic strength coefficient and exponent H’ and n’ respectively. These parameters can be experimentally determined or estimated from the condition of strain compatibility between the RO and the Coffin-Manson-Basquin CMB equations. The present paper discusses the influence that the use of both types of parameters, independent or experimentally determined and compatible or estimated , has on the numerical stress-life curves of the AISI 4340 Aircraft Quality steel. By numerical stress-life curves we mean the stress amplitudes and the fatigue-life that result from the numerical solution of both, the strain-life CMB and the stress-strain RO relations, for the same strain amplitude. This would be equivalent to using a linear strain concentration rule notched members with two RO equations, one with independent parameters and the other with compatible parameters, for stress and life calculations. The effects of the stress state are also accounted for in the present investigation since both, stress-life and stressstrain equations are modified in accordance with the total deformation theory of plasticity and through the introduction of a plane stress biaxial ratio. The principal finding of the present paper is that, for the studied material, the numerical stress-life curves that result from the use of compatible and independent parameters are indistinguishable for the same stress state. Consequently, there are no important implications on life time calculations when the cyclic stress-strain curve is estimated in such a way that compatibility conditions for the AISI 4340 aircraft quality steel are ensured.

Selection of Reverse Logistics activities using an ANP-BOCR model

The Reverse Logistics (RL) activities are gaining importance in Brazilian companies. However, a relevant problem is to select RL activities using criteria of immediate and long-term effects for the organization. These situations can be treated as being Multi-Criteria Decision-Making (MCDM). In this work we adopted the Analytic Network Process (ANP) as the analytical technique of a MCDM problem that can be formulated when we have interest in to identify existing relationships between RL activities. The strategic criteria were included into the model to rate benefits (B), opportunities (O), cost (C) and risks (R). Here we present an analysis of sustainable practices identified in publishing companies, operating in Brazil. The results indicated that ANP-BOCR is an adequate method to select alternatives RL.

An Evaluation of the Nominal Stress Method for Life Prediction of Cylindrical Circumferential V-Notched Specimens Tested under Variable Amplitude Loading

An experimental device was constructed with the aim of testing various cylindrical V-notched specimens until fracture and under variable amplitude torsional loads. The specimens had different notch depths resulting in the same number of values for the stress concentration factor. Strain gages directly bonded at the specimens’ surface and using a slip ring system for their communication with the conditioner, allowed the measurement of the actual applied loads. The well-known rain flow cycle counting procedure was then applied on the scaled signal for identifying the frequency of the 64 classes of stress amplitudes and means. The traditional nominal stress-based approach was then evaluated as the most widely used tool for fatigue lifetime calculations. As the occurrence of stress amplitudes above the endurance fatigue limit tends to lower it, the Miner elementary method was used. The results show damage sum ranges between 0.5 and 6.4 with a mean value of 2.0. Despite the small size of the sample used in the present paper (only 13 tests), these significant deviations are in agreement with previous results reported by different researchers.

Observaciones Experimentales sobre la Fuerza Motriz en la Propagación de Grietas de Fatiga

The role of the effective and applied stress intensity factors (∆Kapp and ∆Keff) as driving forces in fatigue crack growth is analyzed. To make a reliable use of ∆Keff in design it is needed to show that this parameter depends only on the same parameters that are included in the expression of ∆Kapp (the nominal stress, the crack size and the geometry function). To verify this behavior experimental K-increasing and K-decreasing tests were done with the same specimen. The closure factor U=∆Keff/∆K showed an accentuated variation with crack size in all the tests, so ∆Keff should not be used for design purposes. The experimental data are also presented in three dimensions finding similar results, although it is not possible to generalize the results with the data available.

Modelado del Proceso Convencional de Colada Continua de Aceros Libres de Interticios

The purpose of this work was to develop a mathematical model able to simulate the continuous casting process of IF steel (Interstitial Free).and to determine the optimum operating conditions. The model formulation involves the momentum equations of liquid and solidified materials coupled with the temperature evolution of the slab. The differential equations and boundary conditions are numerically solved using the finite volume technique with appropriated boundary conditions for each region of the casting machine. The model predictions were compared with industrial data for conventional continuous casting conditions and extended to IF steel.