The distinct molecular architecture and thermomechanical properties of polyurethane block copolymers make them suitable for applications ranging from textile fibers to temperature sensors. In the present study, differential scanning calorimetry (DSC) analysis and macroscopic stress relaxation measurements are used to identify the key internal processes occurring in the temperature ranges between −10 °C and 0 °C and between 60 °C and 70 °C. The underlying physical phenomena are elucidated by the small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) study of synchrotron beams, allowing the exploration of the structure-property relationships as a function of temperature. In situ multiscale deformation analysis under uniaxial cyclic thermomechanical loading reveals a significant anomaly in the strain evolution at the nanoscale (assessed via SAXS) in the range between −10 °C and 0 °C owing to the 'melting' of the soft matrix. Furthermore, WAXS measurement of crystal strain within the hard regions reveals significant compressive residual strains arising from unloading at ∼60 °C, which are associated with the dynamic shape memory effect in polyurethane at these temperatures.
Caries, a major global disease associated with dental enamel demineralization, remains insufficiently understood to devise effective prevention or minimally invasive treatment. Understanding the ultrastructural changes in enamel is hampered by a lack of nanoscale characterization of the chemical spatial distributions within the dental tissue. This leads to the requirement to develop techniques based on various characterization methods. The purpose of the present study is to demonstrate the strength of analytic methods using a correlative technique on a single sample of human dental enamel as a specific case study to test the accuracy of techniques to compare regions in enamel. The science of the different techniques is integrated to genuinely study the enamel. The hierarchical structures within carious tissue were mapped using the combination of focused ion beam scanning electron microscopy with synchrotron X-ray tomography. The chemical changes were studied using scanning X-ray fluorescence (XRF) and X-ray wide-angle and small-angle scattering using a beam size below 80 nm for ångström and nanometer length scales. The analysis of XRF intensity gradients revealed subtle variations of Ca intensity in carious samples in comparison with those of normal mature enamel. In addition, the pathways for enamel rod demineralization were studied using X-ray ptychography. The results show the chemical and structural modification in carious enamel with differing locations. These results reinforce the need for multi-modal approaches to nanoscale analysis in complex hierarchically structured materials to interpret the changes of materials. The approach establishes a meticulous correlative characterization platform for the analysis of biomineralized tissues at the nanoscale, which adds confidence in the interpretation of the results and time-saving imaging techniques. The protocol demonstrated here using the dental tissue sample can be applied to other samples for statistical study and the investigation of nanoscale structural changes. The information gathered from the combination of methods could not be obtained with traditional individual techniques.
AISI 316L stainless steels are widely employed in applications where durability is crucial. For this reason, an accurate prediction of its behaviour is of paramount importance. In this work, the spotlight is on the cyclic response and low-cycle fatigue performance of this material, at room temperature. Particularly, the first aim of this work is to experimentally test this material and use the results as input to calibrate the parameters involved in a kinematic and isotropic nonlinear plasticity model (Chaboche and Voce). This procedure is conducted through a newly developed calibration procedure to minimise the parameter estimates errors. Experimental data are eventually used also to estimate the strain–life curve, namely the Manson–Coffin curve representing the 50% failure probability and, afterwards, the design strain–life curves (at 5% failure probability) obtained by four statistical methods (i.e., deterministic, “Equivalent Prediction Interval”, univariate tolerance interval, Owen’s tolerance interval for regression). Besides the characterisation of the AISI 316L stainless steel, the statistical methodology presented in this work appears to be an efficient tool for engineers dealing with durability problems as it allows one to select fatigue strength curves at various failure probabilities depending on the sought safety level.
Abstract Mechanical polishing is commonly used for both surface finishing and metallographic sample preparation for a broad range of materials. However, polishing causes local deformation and induces residual stress, which has an important effect on many surface phenomena. Until recently, it has not been possible to quantify the nanoscale depth variation of polishing‐induced plastic deformation (eigenstrain) and the associated residual stress. In this study, the magnitude and depth of polishing‐induced residual stress are evaluated directly by focused ion beam milling and digital image correlation using the micro‐ring‐core geometry method. Depth‐resolved residual stress profiles are obtained with sub‐micrometer resolution at the surface of a titanium alloy sample that is subjected to various polishing steps. It is found that electrochemical polishing and polishing with colloidal silica do not induce any significant residual stress. However, polishing with diamond slurry leads to the formation of compressive residual stresses of up to 300 MPa, which extend deeper into the material when larger diamond particles are used. This study paves the way for further research on polishing and its effect on surface properties.
The impact of mechanical fatigue on load-bearing metallic components and structures is highly significant, encompassing economy, environment and safety aspects. For nearly 200 years, engineers and scientists have been relentlessly trying to avoid fatigue failures and to understand their causes. The last few decades have seen the prominent advent of a wide range of experimental and computational techniques that allowed us to make once-unthinkable advances in this field. Despite this progress, a significant number of problems remain unsolved. This short note pinpoints the most critical aspect of fatigue failure: the conditions that initiate or allow propagating cracks to form. Specifically, the fundamentals of mechanical fatigue are examined, while acknowledging the crucial role of multiphysics aspects that often are present in real-life engineering applications. The first part frames the problem by introducing essential concepts and exploring mechanistic aspects across different length scales and loading regimes. Subsequently, a brief but wide-ranging review highlights current research trends. This foundation sets the stage for identifying outstanding challenges and exploring potential future research directions in the conclusive part of the article.
A supersaturated γ phase microstructure is produced using laser powder bed fusion (L-PBF) – the cooling rate arising from the process is shown to suppress the solid-state precipitation of the γ' phase. The response of the material to heat treatment therefore requires new understanding at the fundamental level, since the first population of γ' precipitate forms upon heating, in contrast to cooling from homogenisation above the γ' solvus. In this work, we interrogated two new nickel-based superalloys designed for the L-PBF technology, both in situ and ex situ, at multiple length scales using advanced characterisation. First, we conducted in situ synchrotron X-ray diffraction during various heat treatments to trace the evolution of the γ' volume fraction with temperature. The first structural changes were detected at an unexpectedly low temperature of ~445 degC. Second, the temperature for γ' nucleation and its sensitivity to heating rate is studied using an electrical resistivity method. Then, the γ' composition upon heating, isothermal holding and cooling is analysed using atom probe tomography (APT), the result is rationalised by further scanning-transmission electron microscopy and nanoscale secondary ion mass spectroscopy. Finally, static recrystallisation during isothermal exposure was investigated, which occurs within minutes. This work has shed light on a new strategy of tailoring microstructure for additively manufactured superalloys by controlling the γ' precipitate distribution upon heating.
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