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

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

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

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

Deep cryogenic treatment of HPDC AZ91 magnesium alloys prior to aging and its influence on alloy microstructure and mechanical properties

Studies of deep cryogenic treatment have mainly focused on ferrous alloys, despite the advantages of this form of treatment when applied to other non-ferrous alloys. This study examines the microstructural behavior of a high pressure die-casting (HPDC) AZ91 magnesium alloy, submitted to T6 and T6 with deep cryogenic treatment (DCT) prior to aging. Differences between continuous and discontinuous β-Mg17Al12 phase precipitation are analyzed by means of TEM, SEM and optical microscopy to explain the mechanical behavior following each type of heat treatment. The mechanical properties were improved by both treatments though yield strength was higher following T6 treatment and elongation was greater following T6 with deep cryogenic treatment. The conclusion is that continuous precipitation is promoted by cryogenic treatment, resulting in an improvement of elongation by 20%. This factor is important in view of the limitations of poor plasticity and its consequences for HPDC magnesium alloy applications.The authors wish to thank Grupo Antolín Ingeniería S.A, for the financial support and the provision of samples for all tests

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

Bayesian network modeling of engineering-student perceptions of success in terms of vocation

La influencia de la vocación en el éxito se puede estudiar a través de diversas encuestas, pero se necesita un modelo para validar los resultados de dichos estudios con una alta probabilidad de representar con precisión la realidad estudiada. Las redes bayesianas cumplen este requisito. Los estudiantes que se matriculan en un grado de ingeniería pueden tener que elegir las disciplinas de ingeniería que difieren de su elección inicial. En algunos casos, estas diferencias pueden perjudicar su compromiso profesional y dejarlos con la sensación de que su elección final era incorrecta. Este estudio pretende cuantificar las percepciones de éxito de los estudiantes de ingeniería en términos de vocación y deficiencias ue puedan aparecer en sus actitudes. En cuanto a la metodología utilizada, las redes bayesianas nos han dado la opción de un modelo probabilístico a través de una serie de variables que validarán este modelo considerando parámetros, como la curva ROC (Receiver Operating Characteristic) que puede calcular las probabilidades de éxito en términos de vocación de nuestros estudiantes de ingeniería. Respecto a las principales conclusiones de este estudio, las relativas a las expectativas profesionales y al aprendizaje tienen más influencia sobre los alumnos, ya sean o no en el campo específico de la ingeniería que refleja su vocación. Las tasas de éxito aumentan considerablemente cuando los estudiantes perciben las variables como "encontrar trabajo útil", "encontrar un trabajo fácil" o "disfrutar del aprendizaje". En pocas palabras, ayudar a los estudiantes a no sentirse frustrados, piensan que encontrarán empleo y que sus trabajos les ayudarán a sentirse útiles en el futuro. Por lo tanto, es esencial hablar desde el nivel universitario sobre la vida profesional desde el primer año hasta la graduación.The influence of vocation in success can be studied through various surveys, but a model to validate the results of such studies with a high probability of accurately represent the reality studied is needed. Bayesian networks meet this requirement. Students who enroll in an engineering degree may have to choose engineering disciplines that differ from their initial choice. In some cases, these differences may impair their professional commitment and leave them with the feeling that his final choice was wrong. This study is intended to quantify the perceptions of success of engineering students in terms of vocation and remedy deficiencies that may appear in their attitudes. As for the methodology used have been the Bayesian networks which have given us the choice of a probabilistic model through a series of variables that will validate this model considering parameters such as the ROC (Receiver Operating Characteristic) curve, can calculate probabilities of success in terms of vocation of our engineering students. As for the main conclusions of this study, it was those relating to professional expectations and learning, having more influence on students whether they are or are not in the specific field of engineering that reflects his vocation. Success rates increase considerably when students perceive variables as "find useful work", "find an easy job" or "enjoy learning". In short, helping students to not get frustrated, you think they will find employment and that their jobs will help them feel useful in the future. Therefore, it is essential to speak from the university level on professional life from the first year until graduation and the students consider their choice of college

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