La entrevista

Tercer accésit del Certame

Estudio y mejora de la correlación de propiedades mecánicas con los ensayos miniatura de punzonado

El ensayo miniatura de punzonado, que nació en el sector nuclear como alternativa a los ensayos estándar de caracterización mecánica, ha evolucionado mucho en las últimas décadas y ha expandido su potencial aplicabilidad a amplios sectores de la industria. La presente tesis doctoral ha buscado ahondar en el comportamiento mecánico de este ensayo para esclarecer el origen de la dispersión de resultados que se observa al caracterizar una gama de materiales con propiedades mecánicas dispares. Tras esta primera aproximación, se llevó a cabo la búsqueda de métodos de correlación alternativos en el ensayo miniatura de punzonado para la obtención de las propiedades mecánicas inherentes a un ensayo estándar de tracción: módulo de elasticidad, límite elástico y resistencia a la tracción. Esta nueva metodología ha mostrado durante su desarrollo un nivel de fiabilidad y precisión superior a lo reflejado por el resto de métodos

Viscoelasticity and the Small Punch Creep Recovery Test: Numerical analysis and experimental tests on the applicability for polyvinyl chloride (PVC)

Research on the Small Punch Test (SPT) has been mainly focused on metallic alloys, with limited investigations in polymeric materials. The miniature size of the SPT eases and motivates its use in biomedical applications, like the mechanical characterization of surgical implants made of different polymers. It is noted that the aim of these publications was focused on mechanical properties inherent to the tensile test (yield strength, ultimate tensile strength, Young's modulus, etc.), but the applicability of this miniature test for the estimation of singular polymer properties like viscoelasticity or viscoplasticity has not been addressed. The aim of this paper was the assessment of SPT as a characterization test for the viscoelastic properties of polymers. To analyze this applicability, numerical FEM simulations of hypothetical materials were performed and a novel Small Punch Creep Recovery Test (SPCRT) was designed. These FEM simulations were verified with experimental compressive creep recovery tests and SPCRTs for specimens made of polyvinyl chloride (PVC). The results showed that SPCRTs accurately estimated the viscoelastic properties for materials with non-stress-dependent viscoelastic properties. In the case of materials with stress-dependent viscoelastic properties, the SPCRT would estimate a mean or intermediate value of these stress-dependent viscoelastic properties

Application of the Small Punch Creep-Recovery Test (SPCRT) for the Estimation of Large-Amplitude Viscoelastic Properties of Polymers

The Small Punch Creep-Recovery Test (SPCRT) is a novel miniature test used to estimate the viscoelastic properties of polymers and biomaterials. The current investigation related to the SPCRT is limited to Finite Element Method (FEM) simulations and experimental tests on PVC. The aim of this investigation was focused on: (i) extending the experimental tests to other polymers with dissimilar viscoelastic properties; (ii) deepening the influence of non-linear viscoelastic properties in the estimation capabilities of the SPCRT; and (iii) developing a numerical methodology to estimate and take into account the viscoelastic recovery produced during the unloading step of compressive creep-recovery tests (CCRT) and SPCRTs. The experimental tests (CCRTs and SPCRTs) were done on polyethylene PE 500, polyoxymethylene POM C, nylon PA 6, and polytetrafluoroethylene (PTFE), with a range of creep loads, in the case of CCRTs, in the whole elastic regime and the surroundings of the yield strength of each material. The experimental results confirmed that the SPCRT was an accurate and reliable testing method for linear viscoelastic polymers. For a non-linear viscoelastic behavior, SPCRT estimated the viscoelastic properties obtained from CCRTs for creep loads near the yield strength of the polymer, which corresponded with large-amplitude viscoelastic properties in dynamic creep testing. In order to consider the viscoelastic recovery generated in the unloading step of CCRTs and SPCRTs, a Maxwell-Wiechert model with two branches was used, simulating the different steps of the experimental tests, and solving numerically the differential equation of the Maxwell-Wiechert model with the Runge-Kutta-Fehlberg (RKF) numerical method. The coefficients of the elements of the Maxwell-Wiechert model were estimated approaching the straining curve of the recovery step of the simulation with the same curve registered on each experimental test. Experimental CCRTs with different unloading times demonstrated that the use of this procedure derived in no influence of the unloading step time in the viscoelastic properties estimation

Ring Hoop Tension Test for yield strength estimation: Numerical analysis for a novel correlation method and applicability for mechanical testing of tubes and pipes

The tubes and pipes manufacturing industry characterizes the mechanical properties of their products with a wide selection of standards, but most of them are qualitative testing methodologies. To estimate the mechanical properties from a quantitative point of view there are limited options in standards. In that sense, the standard tensile test is the preferred alternative by the manufacturers, but this option limits the mechanical estimation for the longitudinal direction of the tube–pipe product. Particular efforts have been made to design an alternative mechanical testing procedure to characterize the mechanical properties in the hoop direction of pipes and tubes. The Ring Hoop Tension Test (RHTT) was designed to fill this gap, but it shows limitations related to the required tooling and the influence of the frictional contact between the tooling and the ring specimen. In the nuclear industry, the Small Ring Test (SRT), a miniature test derivated from the RHTT, has been investigated in recent years. In this investigation, a novel RHTT was designed to overcome the limitations of SRT and RHTT, and a new procedure was implemented to estimate the yield strength of tubes and pipes. Numerical FEM simulations were performed to reach an optimum estimation method for the yield strength with the specific geometry of the SRT and a wide selection of pipe geometries with the RHTT. A set of hypothetical materials were designed to perform these analyses, taking into account the influence of Young’s modulus, proportional limit, hardening coefficient (based on the Ramberg–Osgood law), and presence of Lüders bands straining. To verify the results obtained from this numerical FEM analysis, experimental tests (standard tensile tests and RHTTs) and metallographic analysis were performed on aluminum Al 6063 T6 and copper C12200 R360 tubes, showing the capability of this optimized RHTT to estimate the yield strength in the hoop direction for anisotropic tubes and pipes

A new prediction method for the ultimate tensile strength of steel alloys with small punch test

The load–deflection curve acquired from the Small Punch Test (SPT) is used to obtain the mechanical properties of materials using different correlation methods. The scattering level of these regressions tends to be high when a wide set of materials is analyzed. In this study, a correlation method based on a specific slope of the SPT curve was proposed to reduce scattering. Assuming the Ramberg–Osgood hardening law, the dependence of the SPT curve slope on the yield strength and the hardening coefficient is demonstrated by numerical simulations (FEM). Considering that the ultimate tensile strength could be obtained from the hardening coefficient, a response surface of the ultimate tensile strength with the yield strength and SPT curve slope, along with its equation, is presented for steel alloys. A summary of steel mechanical properties, based on the Boiler and Pressure Vessel Code (BPVC) and limited to yield strengths lower than 1300 MPa, is shown to select a set of experimental tests (tensile tests and SPTs) for which the range is completely covered. This experimental analysis validates the previous FEM analyses and the validity of the proposed correlation method, which shows more accurate correlations compared to the current method

Revista de educación

Anexo: p. 90-95Título, resumen y palabras clave en español e inglésResumen basado en el de la publicaciónDurante gran parte del siglo XX se ha escrito mucho sobre la fiabilidad de los test multirrespuesta como método para la evaluación de contenidos. En concreto son muchos los estudios teóricos y empíricos que buscan enfrentar los distintos sistemas de puntuación existentes. Se ha diseñado un algoritmo que genera estudiantes virtuales con los siguientes atributos: conocimiento real, nivel de cautela y conocimiento erróneo. El primer parámetro establece la probabilidad que tiene el alumno de conocer la veracidad o falsedad de cada opción de respuesta del test. El nivel de cautela refleja la probabilidad de responder a una cuestión desconocida. Finalmente, el conocimiento erróneo es aquel conocimiento falsamente asimilado como cierto. El algoritmo también tiene en cuenta parámetros de configuración del test como el número de preguntas, el número de opciones de respuesta por pregunta y el sistema de puntuación establecido. El algoritmo lanza test a los individuos virtuales analizando la desviación generada entre el conocimiento real y el conocimiento estimado (la puntuación alcanzada en el test). Se confrontaron los sistemas de puntuación más comúnmente utilizados (marcado positivo, marcado negativo, test de elección libre y método de la respuesta doble) para comprobar la fiabilidad de cada uno de ellos. Para la validación del algoritmo, se comparó con un modelo analítico probabilístico. De los resultados obtenidos, se observó que la existencia o no de conocimiento erróneo generaba una importante alteración en la fiabilidad de los test más aceptados por la comunidad educativa (los test de marcado negativo). Ante la imposibilidad de comprobar la existencia de conocimiento erróneo en los individuos a través de un test, es decisión del evaluador castigar su presencia con el uso del marcado negativo, o buscar una estimación más real del conocimiento real a través del marcado positivo.ES

Fractional viscoelastic models for the estimation of the frequency response of rubber bushings based on relaxation tests

Estimation of the viscoelastic properties of rubber bushings at very high frequencies (up to 2 kHz) is a challenge for many damping component manufacturers in the design stage of a quality monitoring procedure. This investigation is focused on the capability of lower strain rate testing procedures, such as relaxation tests, to estimate and extrapolate the dynamic behavior of rubber bushings from low to moderate frequencies. Fractional Zener models are employed to approach bushing behavior in experimental relaxation tests, thus leading to a linear viscoelastic model which is employed to estimate the dynamic behavior of rubber bushing under harmonics loads up to 150 Hz. The validation of this extrapolation procedure is performed by comparing these analytical results with experimental dynamic harmonic tests applied to the same rubber bushings. The deviation between both curves demonstrates that it is difficult to compare the behavior from very small deformation rates (relaxation tests) to higher deformation rates (harmonic dynamic tests) due to the nonlinear behavior of the rubber and its amplitude dependence. However, this investigation demonstrates that the relaxation tests contain enough data to define the frequency behavior of linear viscoelastic materials up to moderate frequencie

Change of mechanical properties of AM60B alloy with heat treatments and its correlation with small punch tests

AM60B alloy, injected by high pressure die casting process (HPDC), is one of the most widely used alloys for its ease of processing and low price. There is an industrial interest in the use of heat treatments in order to increase the elongation before failure of the alloy. This paper aims to correlate the results of tensile test of heat treated specimens with small punch tests (SPT’s). It is also intended to find out if the different characteristic values of such tests for different mechanical properties are sensitive enough to detect changes produced by heat treatments or injection process parameters and if the correlation factors are sufficiently stable. In addition, this study is also focused on the validation of the applicability of such miniature tests for high pressure die casting magnesium alloys, since this process introduces a significant number of defects and thus a variability of the mechanical properties is expected

Cold Expansion Process with Multiple Balls—Numerical Simulation and Comparison with Single Ball and Tapered Mandrels

Cold expansion technology is an extended method used in aeronautics to increase fatigue life of holes and hence extending inspection intervals. During the cold expansion process, a mechanical mandrel is forced to pass along the hole generating compressive residual hoop stresses. The most widely accepted geometry for this mandrel is the tapered one and simpler options like balls have generally been rejected based on the non-conforming residual hoop stresses derived from their use. In this investigation a novelty process using multiple balls with incremental interference, instead of a single one, was simulated. Experimental tests were performed to validate the finite element method (FEM) models and residual hoop stresses from multiple balls simulation were compared with one ball and tapered mandrel simulations. Results showed that the use of three incremental balls significantly reduced the magnitude of non-conforming residual hoop stresses and the extension of these detrimental zone