Experimental characterisation of the local mechanical behaviour of cellulose fibres: an in‑situ micro‑profilometry approach

The accurate mechanical characterisation of fibres of micrometric length is a challenging task, especially in the case of organically-formed fibres that naturally exhibit considerable irregularities along the longitudinal fibre direction. The present paper proposes a novel experimental methodology for the evaluation of the local mechanical behaviour of organically-formed (aged and unaged) and regenerated cellulose fibres, which is based on in-situ micro-tensile testing combined with optical profilometry. In order to accurately determine the cross-sectional area profile of a cellulose fibre specimen, optical profilometry is performed both at the top and bottom surfaces of the fibre. The evolution of the local stress at specific fibre locations is next determined from the force value recorded during the tensile test and the local cross-sectional area. An accurate measurement of the corresponding local strain is obtained by using Global Digital Height Correlation (GDHC), thus resulting in multiple, local stress--strain curves per fibre, from which local tensile strengths, elastic moduli, and strains at fracture can be deduced. Since the variations in the geometrical and material properties within an individual fibre are comparable to those observed across fibres, the proposed methodology is able to attain statistically representative measurement data from just one, or a small number of fibre samples. This makes the experimental methodology very suitable for the mechanical analysis of fibres taken from valuable and historical objects, for which typically a limited number of samples is available. It is further demonstrated that the accuracy of the measurement data obtained by the present, local measuring technique may be significantly higher than for a common, global measuring technique since possible errors induced by fibre slip at the grip surfaces are avoided

In depths of paper degradation: A microscale experimental methodology

Speaking of conservation of articles in museums and collections there is no question about the importance of better understanding of how paper degrades. Loss of mechanical properties, due to degradation through the ages, dramatically influences the accessibility of books, artworks and documents. The change in these properties starts from atomic levels and travels across scales to result in tangible changes in the scale of the sheets of paper. One of the most relevant changes is the loss of mechanical properties of paper. Advanced measurement techniques make it possible to dive into the depths of these processes in smaller scales than before with impressively high accuracies. The current study focuses on the development of a thorough experimental methodology to study the mechanical behaviour of cellulose fibres. In-situ micro-tensile testing with optical profilometry in combination with Digital Image Correlation (DIC) technique results in high accuracy mechanical characterization of single cellulose fibres. Such detailed assessment of cellulose fibres’ properties can be applied to naturally aged paper samples, or combined with accelerated aging experiments to shed valuable light on the degradation of paper, and provide better guidance for conservators

Experimental Determination of the Mechanical Properties of The Night Watch Canvas using Micro-Tensile Testing

The determination of the structural condition and the mechanical properties of The Night Watch are of high importance when it comes to defining conservation treatments and future preservation policies for the painting. To this end, an advanced minimally-invasive experimental investigation of the mechanical properties of the canvas is performed. Due to its significant contribution to the structural support to the painting, the study principally focuses on the lining canvas, which was applied to the back of the painting in 1975. Considering the limited availability of samples, micro-scale mechanical tests are carried out on single cellulose fibers that have been extracted from the canvas threads. The strength, the stiffness and the fracture strain of the fibers are robustly and accurately measured using a recently developed micro-tensile testing methodology. This information can then be extended to larger scales (thread and thread-network) to provide insight into the long-term stability of the lining canvas and to ensure that the load introduced with the spring-tensioning system will be kept well below the failure threshold of the lining canvas

In depths of paper: A microscale experimental story

Speaking of conservation of articles in museums and collections there is no question about the importance of better understanding of how paper degrades. Loss of mechanical properties, due to degradation through the ages, dramatically influences the accessibility of books, artworks and documents. The change in these properties starts from atomic levels and travels across scales to result in tangible changes in the scale of the sheets of paper. Advanced measurement techniques make it possible to dive into the depths of these processes in smaller scales than before with impressively high accuracies. In this presentation I will introduce the experimental methodology to study the mechanical behaviour of single cellulose fibres, the building block of paper. An optical profilometer, a micro-tensile stage and Digital Image Correlation (DIC) technique come together to result in a deeper understanding of the mechanics of each fibre. Such detailed assessment of cellulose fibres’ properties can be applied to naturally aged paper samples, or combined with accelerated aging experiments to shed valuable light on the degradation of paper, and provide better guidance for conservators

A discrete slip plane model for simulating heterogeneous plastic deformation in single crystals

In small-scale mechanical tests, such as micropillar compression tests, plastic deformation is often localized in narrow slip traces. These slip traces result from a few dislocation sources with relatively low nucleation stresses that are present in the material. In order to accurately simulate such small-scale experiments, the stochastics of the underlying dislocation network must be taken into account, which is usually done by performing discrete dislocation dynamics simulations. However, their high computational cost generally restricts these simulations to small and simple geometries and small applied displacements. Furthermore, effects of geometrical changes are usually neglected in the small strain formulation adopted. In this study, a discrete slip plane model for simulating small-scale experiments on single crystals is proposed, which takes the most important characteristics of dislocation plasticity for geometries in the micrometer range into account, i.e.\ the stochastics and physics of dislocation sources. In the model, the properties of all lattice planes are sampled from a probability density function. This results in a heterogeneous flow stress within a single crystal, unlike the uniform properties assumed in conventional crystal plasticity formulations. Moreover, the slip planes can be grouped together in bands via a weakest-link principle. The resulting equations are implemented in a standard crystal plasticity finite element model, using a finite deformation formulation. Within this setting, only the collective dislocation motion on glide planes is modeled, resulting in a significantly lower computational cost compared to frameworks in which the dynamics of individual dislocations are considered. This allows for simulating multiple realizations in 3D, up to large deformations. A small case study on micropillar compression tests is presented to illustrate the capabilities of the model

Integrated Digital Image Correlation for Multi-Beam Optical Stress Sensor

In a wide variety of technology, thin films are of substantial importance to obtain high performance and reliability. Internal stresses in thin films may cause a loss of functionality. In order to control the internal stresses, first an accurate stress measurement is required. Multi beam optical stress sensor (MOSS) is an extremely reliable and sensitive system for in-situ real-time stress measurements. By measuring the curvature of a thin film substrate system, the film stress can be deduced by applying Stoney’s equation without knowledge of thin film material properties. Traditionally, a two-step method is used to calculate the curvature. First, the centroids of an array of parallel laser beams are fitted with Gaussian profiles. Secondly, the shifts of the centroids are converted in a curvature using a least square method. In this paper, integrated digital image correlation(IDIC) is performed to directly calculate the curvature without making assumptions about the shape of the laser. Analytical formulas, describing the laser beam profile including deformations, are derived to obtain virtual images, mimicking the experiment, to optimize both methods and compare the methods with each other. With these virtual images, it is proven that IDIC outperforms the Gaussian method. <br/