Effect of temperature and strain rate on the compressive behaviour of supramolecular polyurethane

Supramolecular polyurethanes (SPUs) possess thermoresponsive and thermoreversible properties, and those characteristics are highly desirable in both bulk commodity and value-added applications such as adhesives, shape-memory materials, healable coatings and lightweight, impact-resistant structures (e.g. protection for mobile electronics). A better understanding of the mechanical properties, especially the rate and temperature sensitivity, of these materials are required to assess their suitability for different applications. In this paper, a newly developed SPU with tuneable thermal properties was studied, and the response of this SPU to compressive loading over strain rates from 10−3 to 104 s−1 was presented. Furthermore, the effect of temperature on the mechanical response was also demonstrated. The sample was tested using an Instron mechanical testing machine for quasi-static loading, a home-made hydraulic system for moderate rates and a traditional split Hopkinson pressure bars (SHPBs) for high strain rates. Results showed that the compression stress-strain behaviour was affected significantly by the thermoresponsive nature of SPU, but that, as expected for polymeric materials, the general trends of the temperature and the rate dependence mirror each other. However, this behaviour is more complicated than observed for many other polymeric materials, as a result of the richer range of transitions that influence the behaviour over the range of temperatures and strain rates tested

Remote monitoring of vibrational information in spider webs

Spiders are fascinating model species to study information-acquisition strategies, with the web acting as an extension of the animal’s body. Here, we compare the strategies of two orb-weaving spiders that acquire information through vibrations transmitted and filtered in the web. Whereas Araneus diadematus monitors web vibration directly on the web, Zygiella x-notata uses a signal thread to remotely monitor web vibration from a retreat, which gives added protection. We assess the implications of these two information-acquisition strategies on the quality of vibration information transfer, using laser Doppler vibrometry to measure vibrations of real webs and finite element analysis in computer models of webs. We observed that the signal thread imposed no biologically relevant time penalty for vibration propagation. However, loss of energy (attenuation) was a cost associated with remote monitoring via a signal thread. The findings have implications for the biological use of vibrations by spiders, including the mechanisms to locate and discriminate between vibration sources. We show that orb-weaver spiders are fascinating examples of organisms that modify their physical environment to shape their information-acquisition strategy

On the mechanical behaviour of PEEK and HA cranial implants under impact loading

The human head can be subjected to numerous impact loadings such as those produced by a fall or during sport activities. These accidents can result in skull fracture and in some complex cases, part of the skull may need to be replaced by a biomedical implant. Even when the skull is not damaged, such accidents can result in brain swelling treated by decompressive craniectomy. Usually, after recovery, the part of the skull that has been removed is replaced by a prosthesis. In such situations, a computational tool able to analyse the choice of prosthesis material depending on the patient's specific activity has the potential to be extremely useful for clinicians. The work proposed here focusses on the development and use of a numerical model for the analysis of cranial implants under impact conditions. In particular, two main biomaterials commonly employed for this kind of prosthesis are polyether-ether-ketone (PEEK) and macroporous hydroxyapatite (HA). In order to study the suitability of these implants, a finite element head model comprising scalp, skull, cerebral falx, cerebrospinal fluid and brain tissues, with a cranial implant replacing part of the skull has been developed from magnetic resonance imaging data. The human tissues and these two biocompatible materials have been independently studied and their constitutive models are provided here. A computational model of the human head under impact loading is then implemented and validated, and a numerical comparison of the mechanical impact response of PEEK and HA implants is presented. This comparison was carried out in terms of the effectiveness of both implants in ensuring structural integrity and preventing traumatic brain injury.The researchers of the University Carlos III 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 support. A.J. acknowledges funding from the European Union's Seventh Framework Programme (FP7 2007–2013) ERC Grant Agreement No. 306587. MRI data were provided by the Human Connectome Project, WUMinn Consortium (Principal Investigators: David Van Essen and Kamil Ugurbil; 1U54MH091657) funded by the 16 NIH Institutes and Centers that support the NIH Blueprint for Neuroscience Research; and by the McDonnell Center for Systems Neuroscience at Washington University. Finally, we would like to thank Dr. S Barhli and Prof. J Marrow for valuable assistance with the X-ray tomography; the machine used was bought from EPSRC Grant EP/M02833X/1 “University of Oxford: experimental equipment upgrade”. Open Access funded by European Research Counci

A dynamic supramolecular polyurethane network whose mechanical properties are kinetically controlled

We report the synthesis and characterization of a kinetically controlled, thermoreversible supramolecular polyurethane whose mechanical properties depend unusually strongly on the processing history. Materials were prepared by solution casting, quenching and annealing of quenched material, allowing pronounced micro-structural evolution, which leads to rapid increases in modulus as determined by rheological analysis. Tensile tests showed that the quenched material is soft, weak and ductile (shear modulus ~ 5 MPa, elongation ~ 250 %), but after annealing, at 70 °C for one hour, it becomes stiffer, stronger and more brittle (~ 20 MPa, ~ 20 %). FTIR and NMR spectroscopic analysis, coupled with MDSC and SAXS, were performed to investigate the network’s dynamic structural changes. SAXS results suggest the presence of a lamellar structure in the sample when solution cast at high temperature, or annealed. This ordering is unique when compared to structurally-related supramolecular bisurethane and bisurea polymers, and may be the cause of the observed path dependence. These mechanical properties, which can be switched repeatedly by simple thermal treatments, coupled with its adhesion properties as determined from peel and tack tests, make it an excellent candidate as a recyclable material for adhesives and coatings

In‐situ SEM observation of grain growth in the austenitic region of carbon steel using thermal etching

A novel heat stage, recently developed for use within the Scanning Electron Microscope, has facilitated Secondary Electron imaging at temperatures up to 850°C. This paper demonstrates one of the applications of in‐situ elevated temperature Scanning Electron Microscope imaging: observation and quantification of grain growth within the austenitic region of carbon steels. The resulting Secondary Electron data have used the technique of thermal etching to capture possible ‘abnormal grain growth’ in the austenitic region. Previous ex‐situ and post‐heating results from carbon steels indicate normal, non‐linear grain growth. Therefore, this new dataset provides greater insight into the heat treatment of steels. From comparison of the in‐situ data with the overall grain growth, measured ex‐situ, it is further concluded that abnormal grain growth is representative of the growth at temperature. Thus, the heating and cooling parts of the heat treatment are likely to account for the non‐linearity previously documented in ex‐situ results and, hence, the range of powers recorded when fitting power law models for steel grain growth. The ability of data derived from in‐situ thermal etching to represent the microstructure of the entire surface and the bulk material is also considered

In situ SEM analysis of surface oxidation mechanisms in carbon steel during vacuum heat treatment

Understanding surface effects, such as oxidation, that occur during the heat treatment of steels is vital in producing desirable mechanical properties. To mitigate against thermal oxidation, heat treatments are often conducted in a ‘fine’ vacuum (∼10−4 mbar); however, studies have shown that even at these vacuum levels oxidation occurs. To further understanding of this behaviour, an alternative in situ Scanning Electron Microscopy approach, facilitated by a novel heat stage, has been used to study the surface morphology of carbon steel during thermal oxidation. The data provide insight into the surface formation of oxidation under fine vacuum conditions, demonstrating that the process begins with the generation of thermally etched grain boundaries, where initial oxidation is focussed, followed by the formation of oxide scales across individual grains. Eventually these individual scales agglomerate to form a continuous oxide layer. The data further suggest that these oxide layers are likely to be predominantly wüstite. The surface data also indicate an absence of blistering, commonly observed on the steel surface at the these temperatures and timescales. The lack of blisters both in situ and ex-situ is thought to be attributed to an absence of a gaseous environment preventing blister formation

Experimentally characterising the temperature and rate dependent behaviour of unfilled, and glass microsphere filled, natural rubber

Low-impedance, elastomeric materials are widely used in engineering applications where they are subjected to impact loading leading to high strain rate deformation. The strong rate and temperature dependence that is exhibited by these polymers and their composites provides further motivation for a deeper understanding of this complex behaviour including the interaction between the matrix and filler materials. In this paper, unfilled and glass microsphere filled natural rubbers are used as model materials to better understand the behaviour of low-impedance particulate composites. These materials were characterised experimentally over a range of strain rates and temperatures. The effect of filler volume fraction and mean particle diameter on the rate and temperature dependence of the overall mechanical response was also investigated, as well as damage evolution based on post-deformation analysis of interrupted compression experiments. In this paper, experimental insights and data are presented and discussed, which will influence future constitutive modelling efforts

A novel method for pulse shaping of Split Hopkinson tensile bar signals

Split Hopkinson bar (SHB) techniques are commonly used to experimentally characterise materials at high strain-rates. One important aspect of high-strain rate characterisation using SHBs is the necessity to tailor the input pulse to the needs of the material to be tested. Here, a new method to shape the input pulse, specifically developed for tensile SHBs (SHTB), is presented. The new method overcomes several challenges of existing designs, allows for a controlled adjustment of the pulse rising time, and significantly reduces wave dispersion effects. © 2011 Elsevier Ltd. All rights reserved

An Error Analysis into the Use of Regular Targets and Target Detection in Image Analysis for Impact Engineering

Abstract. This study concentrates on the use of corners targets for photogrammetry in impact engineering. An example of high speed experimentation is presented and the associated difficulties are discussed. The relevant corner detection methods that have been implemented and developed are investigated and their accuracy assessed. This study focuses solely upon the effect of blurring on the accuracy of the detection methods; it is part of a much wider investigation into the use and accuracy of different targets and target detection methods for photogrammetry in impact engineering. A set of tests has been performed and the errors between the true position of the corner and the detected position are compared

An investigation into experimental in situ scanning electron microscope (SEM) imaging at high temperature

This paper presents an investigation into high temperature imaging of metals through the use of a novel heat stage for in situ Scanning Electron Microscopy (SEM). The results obtained demonstrate the benefits and challenges of SEM imaging at elevated temperatures of up to 850 °C using Secondary Electron (SE) and Electron Backscatter Diffraction (EBSD) detectors. The data collected using the heat stage demonstrate good beam, vacuum, and detector stability at high temperatures without the need for shielding or detector modification owing to the heat stage geometry. SE imaging highlighted one possible application: carrying out thermal etching, a process in which surface grooves form along a material’s grain boundaries during heating in situ. The data suggest that using the heat stage to perform imaging during the process gives a more accurate representation of a material’s microstructure at temperature than examining the thermally etched specimen after cooling. This study also highlights some of the challenges of high temperature in situ EBSD imaging in both steel and nickel at a variety of temperatures and time scales. In particular, the data demonstrate the effect of surface roughness on EBSD imaging and how microstructural changes during heating may affect this. Additionally, the ease with which a material can be imaged using EBSD at temperature may be affected by the material’s magnetic properties. For the first time, it is shown that at temperatures close to the Curie temperature of ferromagnetic materials, in this case Nickel, there is a loss of EBSD image quality. Quality was regained when temperatures were further increased. Despite these challenges, good quality EBSD scans were produced, further highlighting the benefits of in situ testing for providing information on grain boundaries, orientations, and phase change at elevated temperatures