Stéphane Corn

Coupled hydro-mechanical aging of short flax fiber reinforced composites

One of the challenges in the widespread use of biocomposites for engineering applications is the influence of environmental conditions on their mechanical properties, particularly for a combination of aging factors such as temperature, moisture, and mechanical stresses. Thus, the purpose of this paper is to study the influence of coupled aging factors by focusing on a 100% bio-based and biodegradable composites made of flax/poly(lactic acid) with several fiber contents. The development of a specific testing setup enabled continuous in-situ measurements and allowed comparing the effects of combined aging factors to those of uncombined aging factors. It was confirmed that the aging temperature in wet conditions led to a loss of elastic properties, especially for higher fiber fractions. While creep tests in dry conditions resulted in little decrease of elastic properties, it was observed that mechanical loading of the materials combined with water immersion resulted in a strong synergistic effect on the loss of stiffness. Finally, the presence of fibers reduced environmental stress cracking mechanisms and increased the time to failure.

Caractérisation mécanique de géomatériaux par micropénétration: Calibration et applications

ABSTRACT The micro-indentation test is currently an effective method to characterize the mechanical properties of materials. The objective of this work is to provide a comprehensive methodology for the characterization of geomaterials by micro-indentation. The study concerns the calibration of the device and its application to the characterization of rocks. A finite element (FE) model of the test is also proposed to complete the calibration. After validation of the numerical model EF on the basis of tests solved analytically, a comparison between numerical simulation and experimental data is discussed.

Sodium alginate adhesives as binders in wood fibers/textile waste fibers biocomposites for building insulation

Alginate derived from seaweed is a natural polysaccharide able to form stable gel through carbohydrate functional groups largely used in the food and pharmaceutical industry. This article deals with the use of sodium alginate as an adhesive binder for wood fibres/textile waste fibres biocomposites. Several aldehyde-based crosslinking agents (glyoxal, glutaraldehyde) were compared for various wood/textile waste ratios (100/0, 50/50, 60/40, 70/30 and 0/100 in weight). The fully biomass derived composites whose properties are herewith described satisfy most of the appropriate requirements for building materials. They are insulating with a thermal conductivity in the range 0.078-0.089 W/m/K for an average density in the range 308-333 kg/m3 according to the biocomposite considered. They are semi-rigid with a maximal mechanical strength of 0.84 MPa under bending and 0.44 MPa under compression for 60/40 w/w wood/textile waste biocomposites with a glutaraldehyde crosslinking agent.

Glass Ceramic CAD/CAM crowns and severely altered posterior teeth: a three levels study

For many practitioners, longevity of full glass ceramic crowns in the posterior area, molars and premolars, remains a real challenge. The purpose of this article is to identify and evaluate the parameters that can significantly influence their resistance when preparing a tooth. The analysis proposed in this article relies on interrelated studies conducted at three levels: in vitro (mechanical tests), in silico (finite elements simulations) and in vivo (clinical survival rates). The in vitro and the in silico studies proved that an appropriate variation of the geometric design of the preparations enables to increase up to 80% the mechanical strength of ceramic reconstructions. The in vivo clinical study of CAD/CAM full ceramic crowns was performed in accordance with the principles stated within the in vitro and the in silico studies and provided a 98.97% success rate over a 6 years period. The variations of geometric design parameters for dental preparation allows for reconstructions with a mechanical breaking up to 80% higher than that of a non-appropriate combination. These results are confirmed in clinical practice.Graphical Abstract

Characterization of the Interface/Interphase in Natural Fibre Based Composites

There are currently no standardized methods to assess the quality of the interface/interphase in natural fibre reinforced composites. Nevertheless different approaches have been developed and are widely used by the scientific and industrial communities. This last chapter proposes a review of the experimental techniques modelling approaches used to investigate the interface/interphase in natural fibres composites and quantify the interfacial adhesion.

Modification of the Interface/Interphase in Natural Fibre Reinforced Composites: Treatments and Processes

The modification of surface properties of synthetic reinforcement fibres to modify composite interphase performance is mostly achieved by chemical functionalization techniques in aqueous media, and in some cases in organic media. In particular, surface treatments of glass fibres are carried out by the use of complex aqueous chemical systems, known as sizings, including one or more organofunctional silane coupling agents, a film former and other additives, i.e. cationic or non-ionic lubricants, anti-static agents, surfactants, wetting agents, chopping aids, and antioxydants). Natural fibres does not yet benefit from such a technological and scientific background. Thereby, many strategies of bulk and surface modifications are currently developed to implement natural fibres in composite materials applications. In this chapter, the different pre-treatments and functionalization treatments and related processes developed to modify natural fibres and interfacial properties in biocomposites will be exposed.

Introduction on Natural Fibre Structure: From the Molecular to the Macrostructural Level

Natural fibres are complex hierarchical bio-assemblies built-up of several biopolymers. In this chapter, the main features related to biopolymers organization within natural fibres are described. Then, the specific surface properties and porous structure of natural fibres that are key parameters as regard to fibre and interface modifications are detailed.

Interfaces in Natural Fibre Reinforced Composites: Definitions and Roles

Natural fibres are a real opportunity to replace conventional synthetic fibres in composite applications. However, even if they have several advantages in comparison to synthetic fibres (lightness, carbon balance, price…), their moisture sensitivity, their poor compatibility with polymer matrices and their low thermal stability and flammability makes their modification by chemical or physical treatments essential. In this chapter, the multiple and multi-scale interfaces in natural fibre based composites is described. The importance and role of the fibre/matrix interface on mechanical performances and durability, and main strategies to enhance the interfacial adhesion is discussed. Finally, an opening towards new functionalities that could be achieved by fibre and interface modifications is addressed.

Characterization of the Fibre Modifications and Localization of the Functionalization Molecules

As pointed out by George et al. (Polym Eng Sci 41(9):1471–1485, 2001), a clear understanding of the complex nature of surfaces in lignocellulosic substrates is needed to optimize modification procedures and thus to increase the usefulness of lignocellulosic biomass as a constituent of composite materials in technical applications. Surface chemistry and topographical features of the fibres are key parameters that influence chemical bonding and mechanical interlocking with polymer matrices, and hence govern the wetting and adhesion/adherence processes in natural fibre reinforced composites. This chapter proposes a comprehensive description of the different approaches used to characterize natural fibres modifications.

High speed imaging for assessment of impact damage in natural fibre biocomposites

The use of Digital Image Correlation has been generally limited to the estimation of mechanical properties and fracture behaviour at low to moderate strain rates. High speed cameras dedicated to ballistic testing are often used to measure the initial and residual velocities of the projectile but rarely for damage assessment. The evaluation of impact damage is frequently achieved post-impact using visual inspection, ultrasonic C-scan or other NDI methods. Ultra-high speed cameras and developments in image processing have made possible the measurement of surface deformations and stresses in real time during dynamic cracking. In this paper, a method is presented to correlate the force- displacement data from the sensors to the slow motion tracking of the transient failure cracks using real-time high speed imaging. Natural fibre reinforced composites made of flax fibres and polypropylene matrix was chosen for the study. The creation of macro-cracks during the impact results in the loss of stiffness and a corresponding drop in the force history. However, optical instrumentation shows that the initiation of damage is not always evident and so the assessment of damage requires the use of a local approach. Digital Image Correlation is used to study the strain history of the composite and to identify the initiation and progression of damage. The effect of fly-speckled texture on strain measurement by image correlation is also studied. The developed method can be used for the evaluation of impact damage for different composite materials.