http://hdl.handle.net/10016/7177 2013-05-18T16:29:11Z http://hdl.handle.net/10016/15849 Title: A dislocation-based constitutive description for modeling the behavior of FCC metals within wide ranges of strain rate and temperature Author(s): Rodríguez-Martínez, José Antonio; Rodríguez-Millán, Marcos; Rusinek, Alexis; Arias, Ángel Abstract: In this work a dislocation based constitutive description for modeling the thermo visco plastic behavior of FCC metals has been developed. The constitutive description, which is founded on the concepts of thermal activation analysis and dislocation dynamics, assumes the plastic flow additively decomposed into internal stress and effective stress. The internal stress represents the applied stress required for the transmission of plastic flow between the polycrystal grains and it is defined by the Hall Petch relationship. The effective stress formulation, which is the main innovative feature of this work, represents the thermally activated deformation behavior. This is defined taking into account the interrelationship between strain rate and temperature, and gathers structural evolution dependence. This structural evolution is described as a function of dislocations density, which acts as internal state variable in the material deformation behavior. A systematic procedure for identifica tion of the material parameters is developed and the model is applied to define the behav ior of annealed OFHC copper. The analytical predictions of the constitutive description are compared with the experimental data reported by Nemat Nasser and Li (Nemat Nasser, S., Li, Y., (1998). Flow stress of FCC polycrystals with application to OFHC Copper. Acta Mater. 46, 565 577). Good correlation between experiments and analytical predictions is found within wide ranges of strain rate and temperature. 2011-11-30T23:00:00Z http://hdl.handle.net/10016/15847 Title: Dynamic fracture-initation toughness determination of AI 7075-T651 aluminum alloy Author(s): Loya, José Antonio; Fernández-Sáez, José Abstract: We present values of the static and dynamic fracture-initiation toughness at different loading rates of the aluminum alloy Al 7075-T651 obtained on three-point bend specimens with different thicknesses and initial crack lengths. In static conditions, the critical stress intensity factor was measured according to well known procedures. The methodology of dynamic fracture initiation toughness is not yet standardized, and the dynamic tests were done with a modified Hopkinson pressure bar coupled to high-speed cameras to measure the crack mouth opening displacement CMOD of the specimens. The critical stress intensity value was obtained from the CMOD at the time that crack starts to grow. This instant was detected by means of crack gauges at the crack tip. 2006-12-31T23:00:00Z http://hdl.handle.net/10016/15810 Title: Experimental study on the martensitic transformation in AISI 304 steel sheets subjected to tension under wide ranges of strain rate at room temperature Author(s): Rodríguez-Martínez, José Antonio; Pesci, R.; Rusinek, Alexis Abstract: In this work, the martensitic transformation occurring in AISI 304 steel sheets subjected to tension at room temperature has been experimentally studied. Tensile tests performedonAISI304specimens are split into two different types; in situ tensile tests and macroscopic tensile tests. The former are conducted mounting the sample in a tensile micromachine originally developed in ENSAM/Metz within the range of strain rates 10−5 s−1 ≤ ε˙ ≤ 10−3 s−1. The latter are performed under static and dynamic conditions of deformation within the range of strain rates 10−3 s−1 ≤ ε˙ ≤ 102 s−1. Using X-rays diffraction technique, in situ tensile tests have allowed measuring the stress of the phases during loading by placing the tensile micromachine under the range of a PROTO goniometer. Additionally, the martensitic transformation has been recorded via mounting the tensile micromachine into a scanning electron microscope. The volume fraction of martensite has been measured in the post mortem specimens, V˛ ≈70%. Moreover, the static macroscopic tensile tests, 10−3 s−1 ≤ ε˙ ≤ 10−1 s−1, have been recorded using a high speed infrared camera. It has been proven that martensitic transformation takes place in AISI 304 steel for a temperature increase over T > 140 K. The macroscopic dynamic tensile tests are performed within the range of strain rates, 1 s−1 ≤ ε˙ ≤ 100 s−1.Aconsiderable amount ofmartensitehas beendetectedinthepostmortemsamples,V˛ ≈35%. This proves that plastic deformation is the dominant mechanism responsible for the transformation phenomenon in this steel. The free energy supplied by straining the sample relegates to a secondary role the rise in temperature taking place during the course of plastic deformation. 2011-07-14T22:00:00Z http://hdl.handle.net/10016/15809 Title: Natural frequencies for bending vibrations of Timoshenko cracked beams Author(s): Loya, José Antonio; Rubio, Lourdes; Fernández-Sáez, José Abstract: The natural frequencies for bending vibrations of Timoshenko cracked beams with simple boundary conditions have been obtained. The beam is modelled as two segments connected by two massless springs (one extensional and another one rotational). This model promotes discontinuities in both vertical displacement and rotation due to bending, which are proportional to shear force and bending moment transmitted by the cracked section, respectively. The differential equations for the free bending vibrations are established and then solved individually for each segment with the corresponding boundary conditions and the appropriated compatibility conditions at the cracked section. The problem is also solved by the perturbation method and both procedures are applied to the case of a simply supported cracked beam. The results show that the perturbation method provides simple expressions for the natural frequencies of cracked beams and it gives good results for shallow cracks. 2006-03-06T23:00:00Z