Constitutive Models for Dynamic Strain Aging in Metals: Strain Rate and Temperature Dependences on the Flow Stress

A new constitutive model for Q235B structural steel is proposed, incorporating the effect of dynamic strain aging. Dynamic strain aging hugely affects the microstructural behavior of metallic compounds, in turn leading to significant alterations in their macroscopic mechanical response. Therefore, a constitutive model must incorporate the effect of dynamic strain aging to accurately predict thermo-mechanical deformation processes. The proposed model assumes the overall response of the material as a combination of three contributions: athermal, thermally activated, and dynamic strain aging stress components. The dynamic strain aging is approached by two alternative mathematical expressions: (i) model I: rate-independent model; (ii) model II: rate-dependent model. The proposed model is finally used to study the mechanical response of Q235B steel for a wide range of loading conditions, from quasi-static loading ( ε˙=0.001 s−1 and ε˙=0.02 s−1 ) to dynamic loading ( ε˙=800 s−1 and ε˙=7000 s−1 ), and across a broad range of temperatures ( 93 K−1173 K ). The results from this work highlight the importance of considering strain-rate dependences (model II) to provide reliable predictions under dynamic loading scenarios. In this regard, rate-independent approaches (model I) are rather limited to quasi-static loading

Mechanical impact behavior of polyether-ether-ketone (PEEK)

This paper deals with the mechanical behavior of polyether ether ketone (PEEK) under impact loading. PEEK polymers are the great interested in the field of medical implants due to their biocompatibility, weight reduction, radiology advantage and 3D printing properties. Implant applications can involve impact loading during useful life and medical installation, such as hip systems, bone anchors and cranial prostheses. In this work, the mechanical impact behavior of PEEK is compared with Ti6AI4V titanium alloy commonly used for medical applications. In order to calculate the kinetic energy absorption in the impact process, perforation tests have been conducted on plates of both materials using steel spheres of 1.3 g mass as rigid penetrators. The perforation test covered impact kinetic energies from 21 J to 131 J, the equivalent range observed in a fall, an accidental impact or a bike accident. At all impact energies, the ductile process of PEEK plates was noted and no evidence of brittle failure was observed. Numerical modeling that includes rate dependent material is presented and validated with experimental data.The researchers of the University Carlos Ill of Madrid are indebted to Ministerio de Ciencia e lnnovación de España (Project DPl/2011 24068) for the financial support received which allowed conducting part of this work

Low temperature effect on impact energy absorption capability of PEEK composites

This paper describes the results of an experimental investigation which analyses the impact behavior at low temperature of polyether ether ketone (PEEK) and its short carbon fiber reinforced composite (SCFR PEEK). These polymer materials are widely employed in aeronautical applications subjected to impact loadings in which the energy absorption capability is an aspect that should be taken into account. The energy absorption capability can drastically decrease if temperatures near to the ductile-to-brittle transition temperature of polymeric matrix are reached. In this work, a set of perforation tests has been conducted covering a testing temperature range from -75 degrees C to +25 degrees C and an impact kinetic energy range from 11 J to 175 1 including typical values considered in impact loadings at aeronautical flight speeds. Energy absorption capability, damage extension and failure mechanisms have been quantified and reported. At low temperatures, a ductile-to-brittle transition was found in PEEK unfilled resulting in a suddenly change of its mechanical impact behavior affecting the energy absorption capability. In case of SCFR PEEK composite, a brittle behavior was observed for the whole temperature range considered and its energy absorption capability decreases drastically at lower temperatures. The brittleness of PEEK and SCFR PEEK at low temperature will limit the application of this composite in aeronautical structures exposed to impact.The researchers of the University Carlos III of Madrid are indebted to the Ministerio de Ciencia e Innovación de España (Project DPI/2011-24068) and to the Ministerio de Economía y Competitividad de España (Project DPI/2014-57989-P) for financial support towards part of this work

Influence of stress state on the mechanical impact and deformation behaviors of aluminum alloys

Under impact loading conditions, the stress state derived from the contact between the projectile and the target, as well as from the subsequent mechanical waves, is a variable of great interest. The geometry of the projectile plays a dertermining role in the resulting stress state in the targeted structure. In this regard, different stress states lead to different failure modes. In this work, we analyze the influence of the stress state on the deformation and failure behaviors of three aluminum alloys that are commonly used in the aeronautical, naval, and automotive industries. To this purpose, tension-torsion tests are performed covering a wide range of stress triaxialities and Lode parameters. Secondly, the observations from these static tests are compared to failure mode of the same materials at high impact velocities tests with the aim of analysing the role of stress state and strain rate in the mechanical response of the aluminum plates. Experimental impacts are conducted with different projectile geometries to allow for the analysis of stress states influence. In addition, these experiments are simulated by using finite element models to evaluate the predictive capability of three failure criteria: critical plastic deformation, Johnson-Cook, and Bai-Wierzbicki.The researchers of the University CarlosIII are indebted to the Ministerio de Economía y Competitividad de España (Project DPI2014-57989-P) and Vicerrectorado de Política Científica UC3M (Project 2013-00219-002) for the financial suppor

Investigation of mechanical impact behavior of short carbon-fiber-reinforced PEEK composites

This paper describes the results of an experimental and numerical investigation of the impact behavior of short carbon fiber reinforced polyether-ether-ketone (SCFR PEEK) composites. The biocompatibility of PEEK and its short fiber composites, their rapid processing by injection molding and suitability for modern imaging have supported technological advances in prosthetic implants used in orthopedic medicine. Surgical implants, including hip and cranial implants, can experience clinically significant impact loading during medical installation and useful life. While the incorporation of short fibers in a thermoplastic matrix can produce significant improvements in stiffness and strength, it can also cause a marked reduction in ductility, making study of their energy absorption capability essential. In this work, the mechanical impact behavior of PEEK composites reinforced with polyacrylonitrile (PAN) short carbon fibers 30% in weight is compared with unfilled PEEK. The perforation tests conducted covered an impact kinetic energy range from 21 J to 131 J, equivalent to the range observed in a fall, the leading cause of hip fractures. Energy absorption capability, damage extension and failure mechanism have been quantified and reported. A numerical modeling that includes homogenization of elastic material and anisotropic damage is presented and validated with experimental data. At all impact energies, SCFR PEEK composites showed a brittle failure and their absorption energy capability decreases drastically in comparison with unfilled PEEK. (C) 2015 Elsevier Ltd. All rights reserved.The researchers of the University Carlos III of Madrid are indebted to the Ministerio de Ciencia e Innovación de España (Project DPI/2011-24068) and to the Ministerio de Economía y Competitividad de España (Project DPI/2014-57989-P) for financial support towards part of this work. The researchers are indebted to LATI Company for PEEK material supplie

Perforation mechanics of 2024 aluminium protective plates subjected to impact by different nose shapes of projectiles

This paper focuses on the mechanical behaviour of aluminium alloy 2024-T351 under impact loading. This study has been carried out combining experimental and numerical techniques. Firstly, experimental impact tests were conducted on plates of 4 mm of thickness covering impact velocities from 50 m/s to 200 m/s and varying the stress state through the projectile nose shape: conical, hemispherical and blunt. The mechanisms behind the perforation process were studied depending on the projectile configuration used by analyzing the associated failure modes and post-mortem deflection. Secondly, a numerical study of the mechanical behaviour of aluminium alloy 2024-T351 under impact loading was conducted. To this end, a three-dimensional model was developed in the finite element solver ABAQUS/Explicit. This model combines Lagrangian elements with Smoothed Particle Hydrodynamics (SPH) elements. A good correlation was obtained between numerical and experimental results in terms of residual and ballistic limit velocities

Material and structural behaviour of PMMA from low temperatures to over the glass transition: Quasi-static and dynamic loading

This work aims at characterizing the mechanical behaviour of polymethyl-methacrylate (PMMA) under high velocity impact conditions over a wide range of testing temperatures. To this end, the mechanical response at uniaxial compression is studied for both quasi-static and dynamic conditions covering testing temperatures below, at and above glass transition. A pseudo-brittle to ductile transition in the failure of PMMA is observed at a threshold that depends on testing temperature and strain rate. This analysis allows for the interpretation of the perforation impact tests and to explain the principal deformation and failure mechanisms. To complete the study, the Richeton model to predict yielding is revisited. Finally, we provide a new constitutive model for finite deformations to further identify the deformation mechanisms governing the mechanical behaviour of PMMA and the influence of temperature and strain rate on them.D. Garcia-Gonzalez acknowledges support from the Talent Attraction grant (CM 2018 - 2018-T2/IND-9992) from the Comunidad de Madrid

Conductive 3D printed PLA composites: On the interplay of mechanical, electrical and thermal behaviours

Additive manufacturing (AM) techniques represent a real challenge to manufacture novel composites with cou- pled multifunctional properties. This work focuses on the mechanical, electrical and thermal behaviours of 3D printed polymeric composites of polylactic acid (PLA) filled with carbon black (CB) conductive particles. The incorporation of conductive particles within the polymer matrix allows for programmable conduction paths via the printing process, whose electric properties are intimately coupled to thermo‐mechanical processes. In this study, samples were prepared using a fused deposition modelling (FDM) printer, controlling the filament ori- entation to manufacture three different types: longitudinal (0°); transverse (90°); oblique (±45°) printing ori- entations. Different types of multifunctional characterisation have been made: (i) electro‐thermal tests, evaluating the influence of electrical conductivity on the sample temperature due to Joule’s heating; (ii) thermo‐electrical tests, analysing the influence of temperature on the DC resistance of the samples; (iii) mechano‐electrical tests, analysing the effect of mechanical deformation on the specimens’ electric resistance. The results show a strong dependence of printing direction on the material properties of 3D printed conductive‐ PLA and identify strong thermo‐electro‐mechanical interplays. The results of this work will contribute to the AM progress in functional electro‐mechanical components with potential applications in biosensing devices, composite sensors, 3D electrodes and soft robotic industry.The authors acknowledge financial support from Ministerio de Ciencia, Innovación y Universidades, Spain, Agencia Estatal de Inves- tigación y Fondo Europeo de Desarrollo Regional, Spain, under Grant number RTI2018‐094318‐BI00. D.G.‐G., S.G.‐H. and A.A. acknowledge support from Programa de Apoyo a la Realización de Proyectos Inter- disciplinares de I + D para Jóvenes Investigadores de la Universidad Carlos III de Madrid (BIOMASKIN‐CM‐UC3M). D.G.‐G. acknowledges support from the Talent Attraction grant (CM 2018 ‐ 2018‐T2/ IND9992) from the Comunidad de Madrid, Spain. Some tests were made in the High Voltage Research and Test Laboratory (LINEALT) at UC3M

What are the evolutionary constraints on larval growth in a trophically transmitted parasite?

For organisms with a complex life cycle, a large larval size is generally beneficial, but it may come at the expense of prolonged development. Individuals that grow fast may avoid this tradeoff and switch habitats at both a larger size and younger age. A fast growth rate itself can be costly, however, as it requires greater resource intake. For parasites, fast larval growth is assumed to increase the likelihood of host death before transmission to the next host occurs. Using the tapeworm Schistocephalus solidus in its copepod first intermediate host, I investigated potential constraints in the parasite’s larval life history. Fast-growing parasites developed infectivity earlier, indicating there is no functional tradeoff between size and developmental time. There was significant growth variation among full-sib worm families, but fast-growing sibships were not characterized by lower host survival or more predation-risky host behavior. Parental investment also had little effect on larval growth rates. The commonly assumed constraints on larval growth and development were not observed in this system, so it remains unclear what prevents worms from exploiting their intermediate hosts more aggressively