Challenges and solutions of environmental scanning electron microscopy characterisation of biomaterials:Application to hygro-expansion of paper

Most methodologies to measure the moisture-induced deformation (hygro-expansion) of paper microconstituents, including fibres and interfibre bonds, are low resolution or time-consuming. Hence, here, a novel method is proposed and validated to measure high-resolution full-field strain maps of paper microconstituents during hygro-expansion, based on environmental scanning electron microscopy (ESEM). To this end, a novel climate stage enables accurate control of the relative humidity (RH) near the specimen in the ESEM from 0%–100%. The fibre surface, which is decorated a priori with a microparticle pattern, is captured during RH change. Subsequently, correlating the fibre surface using a dedicated global digital image correlation algorithm enables high-resolution hygro-expansion strain maps. Method optimisation involved performing contrast enhancement, scan-correction to reduce ESEM artefacts and a background correction, resulting in a strain resolution of (Formula presented.). Method validation revealed that the fibres' crystallinity is affected by the electron beam, even for minimal invasive electron beam settings. Interestingly, however, the fibres consistently exhibit conventional hygro-expansion behaviour during the drying slopes. Using the optimised procedure, hygro-expansion characterisation of two interfibre bonds and four interfibre bond cross-sections revealed the competition between the low longitudinal and large transverse fibre hygro-expansion in the bonded area.</p

The mechanical behaviour of metastable austenitic steels in pure bending

Metastable austenitic steels exhibit remarkable mechanical properties, that to a large extend can be attributed to the martensite phase transformation taking place during (thermo)mechanical loading. Although the behaviour of these materials under homogeneous monotonic and cyclic deformation has intensively been studied in the literature, non-homogeneous stress and strain states have received significantly less attention. In this work, pure bending experiments have been performed on a metastable austenitic stainless steel to characterise the intrinsic interaction between the through thickness non-homogeneous martensite transformation and the resulting mechanical behaviour and the springback response. The moment-curvature response and the springback have been measured and related to the overall martensite content recorded in-situ using the magnetic induction technique. In addition, micrography has been used to study the through thickness tension–compression asymmetry of the martensite formation, which leads to a complex internal stress distribution resulting in a non-trivial springback behaviour, as compared to non-transforming materials. Three stages in the deformation and transformation under pure bending have been identified, each characterised by distinct moment-curvature loading and unloading responses

In situ observation of the dehydration of Li2SO4.H2O monocrystals

In this study the dehydration reaction of Li2SO4.H2O crystals is investigated at different temperatures with the help of optical microscopy. The nucleation and nuclei growth processes during the reaction were recorded photographically using a camera system. Based on a series of pictures the propagation of the reaction front on the crystal surface was captured and an effective speed of growth (in μm/min) was estimated. It is demonstrated, using this in situ measurement technique, that the effective speed at certain temperature and water vapor pressure is a constant for different nuclei. From the surface observations it appeared that the mode of growth of nuclei is non-isotropic with a preferential direction along the [010] axis. In order to measure the growth rate in-depth, measurements on encapsulated crystals were also carried out. It is observed that the propagation of the interface in the crystal is linear and cracks have a remarkable influence on this propagation speed because of a higher mass diffusivity in the cracks

A multi-loading, climate-controlled, stationary ROI device for in-situ X-ray CT hygro-thermo-mechanical testing

In-situ CT mechanical testing yields a full 3D description of a sample material’s behaviour under specific loads. In the literature various devices are proposed which enable in-situ CT hygro-, thermo- or mechanical testing, each with its own merits and limitations. However none of them is able to perform advanced hygro-thermo-mechanical tests on specimens subjected to multiple loading modes, while accurately controlling and measuring the force, displacement, temperature and relative humidity in real time. Therefore, this work proposes an in-situ CT device which allows such multi-faceted experiments. Improvements to the current state-of-the-art devices include: (1) a compact, lightweight and rotationally symmetric design that enables high-resolution CT scans by minimization of wobble during scanning, in practically all lab-scale CT scanners; (2) a stationary region of interest by loading the sample from both sides, which enables high resolution CT characterization of materials exhibiting a large fracture strain; and (3) improved testing modularity by exchanging clamping methods to allow samples of various sizes (e.g., circular or rectangular) to be inserted in a variety of ways, thereby facilitating complex experiments such as three- or four-point bending tests. Validation experiments demonstrate that stringent requirements on CT resolution, loading and displacement accuracy and climate control are met. Furthermore, the in-situ testing capabilities of the device were validated by CT characterization of the creasing and folding process of multi-layer cardboard under varying (controlled) levels of relative humidity and temperature

Microstructural model for the time-dependent thermo-mechanical analysis of cast irons

In this paper, a multiscale modelling approach is followed for the modelling of time and temperature dependent behaviour of compacted graphite cast iron (CGI) material. Cast irons are often used in heavy duty machinery parts subjected to elevated temperatures for prolonged periods of time. This, in combination with its complex heterogeneous microstructure, plays the crucial role in determining the life time of the material. In this work a 2D microstructural model of CGI is developed. The geometry is based on the micrographs of the material. The pearlitic matrix is modelled with the temperature dependent elasto-visco-plastic model calibrated on the pearlitic steel experiments. The graphite particles are modelled as anisotropic elastic. The results of the simulations of tensile and stress relaxation tests at different temperatures between 20C and 500C show that the macroscopic mechanical behaviour of the material deteriorates rapidly above 350◦C. At the microstructural scale, the anisotropic graphite particles act as stress concentrators promoting the formation of strain percolation paths, that become more critical at higher temperatures

Understanding the properties of silicon solar cells aluminium contact layers and its effect on mechanical stability

As the thickness of silicon solar wafer and solar cells becomes thinner, the cells are subjected to high stress due to the thermal coefficient mismatch induced by metallization process. Handling and bowing problems associated with thinner wafers become increasingly important, as these can lead to cells cracking and thus to high yield losses. The goal of this work to provide experimental understanding of Al rear side microstructure development and mechanical properties as well as correlate the obtained results with fracture behaviour of the cell. It is shown that the aluminium back contact has a complex microstructure, consisting of five main components: 1) the back surface field layer; 2) a eutectic layer; 3) spherical (3 - 5 µm) hypereutectic Al-Si particles surrounded by a thin aluminium oxide layer (200 nm); 4) a bis- muth-silicate glass matrix; and 5) pores (14 vol%). It was concluded that the eutectic layer thickness and waviness depends on Al particle size, amount of Al paste and textured surface roughness of silicon wafers. The Young’s modulus of the Al-Si particles is estimated by nano-indentation and the overall Young’s modulus is estimated on the basis of bowing measurements and found to be ~43 GPa. It was found, that there is a relation between aluminium paste composition, eutectic layer thickness, mechanical strength and bowing of solar cells. Three main parameters were found to affect the mechanical strength of mc-silicon solar cells with an aluminium contact layer, namely the eutectic layer thickness and uniformity, the Al layer thickness (which results from the Al particle size and its distribution), and the amount of porosity and the bismuth glass fraction

Mixed-mode cohesive zone parameters from integrated digital image correlation on micrographs only

Mixed-mode loading conditions strongly affect the failure mechanisms of interfaces between different material layers as typically encountered in microelectronic systems, exhibiting complex material stacking and 3D microstructures. The integrated digital image correlation (IDIC) method is here extended to enable identification of mixed-mode cohesive zone model parameters under arbitrary levels of mode-mixity. Micrographs of a mechanical experiment with a restricted field of view and without any visual data of the applied far-field boundary conditions are correlated to extract the cohesive zone model parameters used in a corresponding finite element simulation. Reliable or accurate force measurement data is thereby not available, which constitutes a complicating factor. For proof-of-concept, a model system comprising a bilayer double cantilever beam specimen loaded under mixed-mode bending conditions is explored. Virtual experiments are conducted to assess the sensitivities of the technique with respect to mixed-mode loading conditions at the interface. The virtual experiments reveal the necessity of (1) optimizing the applied local boundary conditions in the finite element model and (2) optimizing the region of interest by analyzing the model's kinematic sensitivity relative to the cohesive zone parameters. From a single test-case, exhibiting a range of mode-mixity values, the mixed-mode cohesive zone model parameters are accurately identified with errors below 1%. The IDIC-procedure is shown to be robust against large variations in the initial guess values for the parameters. Real mixed-mode bending experiments are conducted on bilayer specimens comprising two spring steel beams and an epoxy adhesive interface, under different levels of mode-mixity. The mixed-mode cohesive zone model parameters are identified, demonstrating that IDIC is a powerful technique for characterizing interface properties of interfaces, imaged with a limited field of view, as is typically the case in microelectronic applications

Interfacial characterization of pre-strained polymer coated steel by a numerical-experimental approach

In this paper, the delamination in pre-strained polymer coated steel is quantitatively characterized through a combined numerical–experimental approach. The surface roughening of the steel substrate is analyzed by a correlation technique and coupled to the pre-deformation of the coating. The polymer coating shows a clear stress relaxation after pre-deformation, which requires special attention for the correct numerical characterization of the interface. A large displacement cohesive zone model is extended with an interfacial pre-damage parameter representing the loss of adhesion due to substrate roughening. Finally, peel tests are performed on pre-strained specimens, on the basis of which the interface is characterized by an inverse parameter identification procedure

Quantifying discoloration caused by the indoor fungus Penicillium rubens on building material at controlled humidity

Fungal growth in the indoor environment can affect human health by causing allergic reactions and respiratory symptoms. In the search for a solution to prevent or limit fungal growth in the indoor environment it is important to know how parameters such as temperature, substrate, and humidity affect fungal growth. Up to now, no general accepted method is applied to measure surface discoloration due to fungal growth on building and finishing materials. Reproducibility of measurements is difficult due to the non-transparent nature of most finishing materials, furthermore is fungal growth on porous materials relative slow and sparse.The aim of this paper is to present a novel method to accurately measure fungal growth, resulting in discoloration of a porous building material (gypsum), as a function of time under controlled temperature and humidity conditions. To this end, the "Fungal Observatory Climate controlled aUtomized Set-up" (FOCUS) was developed. The surface discoloration of gypsum, caused by sporulation of Penicillium rubens (formerly Penicillium chrysogenum), was quantified using digital imaging over time. The pixel value intensities of acquired images were used to determine a measure of discoloration of the surface, and discoloration of the gypsum surface was plotted in a curve. In parallel, gypsum samples were studied with environmental scanning electron microscopy to relate discoloration (or its absence) with the developmental stage of the fungus. The method described in this paper is unsuitable to determine fungal growth on site, however the method is suitable to assess fungal growth under controlled conditions on finishing materials

Adaptive isogeometric digital height correlation: application to stretchable electronics

A novel adaptive isogeometric digital height correlation (DHC) technique has been developed in which the set of shape functions, needed for discretization of the ill-posed DHC problem, is autonomously optimized for each specific set of profilometric height images, without a priori knowledge of the kinematics of the experiment. To this end, an adaptive refinement scheme is implemented, which refines the shape functions in a hierarchical manner. This technique ensures local refinement, only in the areas where needed, which is beneficial for the noise robustness of the DHC problem. The main advantage of the method is that it can be applied in experiments where the deformation mechanisms are unknown in advance, thereby complicating the choice of suitable shape functions. The method is applied to a virtual experiment in order to provide a proof of concept. A second virtual experiment is executed with stretchable electronics interconnects, which entail localized buckles upon deformation with complex kinematics. In both cases, accurate results were obtained, demonstrating the beneficial aspects of the proposed method. Moreover, the technique performance on profilometric images of a real experiment with stretchable interconnects was demonstrated