Digital Image Sensor-Based Assessment of the Status of Oat (Avena sativa L.) Crops after Frost Damage

The aim of this paper is to classify the land covered with oat crops, and the quantification of frost damage on oats, while plants are still in the flowering stage. The images are taken by a digital colour camera CCD-based sensor. Unsupervised classification methods are applied because the plants present different spectral signatures, depending on two main factors: illumination and the affected state. The colour space used in this application is CIELab, based on the decomposition of the colour in three channels, because it is the closest to human colour perception. The histogram of each channel is successively split into regions by thresholding. The best threshold to be applied is automatically obtained as a combination of three thresholding strategies: (a) Otsu’s method, (b) Isodata algorithm, and (c) Fuzzy thresholding. The fusion of these automatic thresholding techniques and the design of the classification strategy are some of the main findings of the paper, which allows an estimation of the damages and a prediction of the oat production

Bioinspired composite segmented armour: Numerical simulations

Nature has evolved ingenious armour designs, like the flexible carapaces of armadillo and boxfish consisting of hexagonal segments connected by collagen fibres, that serve as bioinspiration for modern ballistic armours. Here, Finite element modelling (FEM) used to analyze the effect of scale geometry and other impact parameters on the ballistic protection provided by a bioinspired segmented ceramic armour. For this purpose, the impact of cylindrical fragment simulating projectiles (FSPs) onto alumina-epoxy non-overlapping scaled plates was simulated. Scale geometrical parameters (size, thickness and shape) and impact conditions (FSP diameter, speed, location) are varied and the amount of damage produced in the ceramic tiles and the final residual velocity of the FSP after the impact are evaluated. It is found that segmentation drastically reduces the size of the damaged area without significantly reducing the ballistic protection in centred impact, provided the tile size is kept over a critical value. Such critical tile size (∼20 mm, inscribed diameter, for impacts at 650 m/s) is independent of the scale thickness, but decreases with projectile speed, although never below the diameter of the projectile. Off-centred impacts reduce the ballistic protection and increase the damaged area, but this can be minimized with an appropriate tile shape. In this sense and in agreement with the natural hexagonal tiles of the boxfish and armadillo, hexagonal scales are found to be optimal, exhibiting a variation of ballistic protection—measured as reduction of projectile speed—with impact location under 12%. Design guidelines for the fabrication of segmented protection systems are proposed in the light of these numerical results. Keywords: Scaled armour, Ballistic impact, Alumina, Epoxy, Finite element analysis, Bioinspiratio

Strength of aluminium titanate/mullite composites containing thermal stabilizers

[EN]This work contains a study on the room temperature-fracture strength of three aluminium titanate-based materials containing mullite and different thermal stabilizers (namely Fe2O3 and MgO). The highest inert strength was reached by the material sintered without any stabilizer. The MgO-doped material had a comparable inert strength, but a significantly higher Weibull modulus. Finally, the Fe2O3-doped material showed the worst mechanical properties. In all cases, a critical load above which strength degraded was apparent. These behaviours have been analyzed in terms of the type of additives and the particular microstructures. Conclusions about the potential use of these materials are briefly stated

Effect of substrate and bond coat on contact damage in zirconia-based plasma-sprayed coatings

This paper reports a Hertzian indentation study of damage modes in zirconia-based plasma-sprayed coatings on metal substrates, with and without bond coats. The structure of the study is as follows: (i) measurement of Hertzian indentation stress-strain curves, first on individual bulk material components (controls) and then on the composite layer structures, to quantify the degree of plasticity; (ii) micrographic analysis of the corresponding subsurface damage modes, particularly of the yield zones, in both coatings and underlayers; (iii) finite element modelling of the elastic-plastic stress fields in the adjacent layers, again with a focus on the yield zones. It is demonstrated that the substrate can have a profound influence on the damage distribution, depending on the degree of elastic-plastic mismatch relative to the coating. The bond coat, by virtue of its relative thinness, plays a lesser role in the damage intensity, not with standing an apparent improvement in substrate adhesion. Indentation variables followed are applied load, to examine the evolution of damage, and number of cycles, to examine fatigue. The results indicate the power and simplicity of the Hertzian technique as a route to mechanical characterization of coating structures: for identifying damage modes, especially yield (but also fracture, in the present case delamination fracture); for evaluating damage parameters, such as Young's modulus and the yield stress, from FEM analysis of stress-strain curves and yield zone microscopy; and for quantifying design concepts, e.g. maximum sustainable bearing stress and damage tolerance

Contact damage in plasma-sprayed alumina-based coatings

A study of Hertzian contact damage in plasma-sprayed alumina-based ceramic coatings on steel substrates has been made. Presectioned specimens are used to identity subsurface micromechanical damage processes within the coating and substrate layers as a function of increasing contact load, from both postcontact and in situ observations. Damage occurs principally by cracking in the ceramic coating and plastic deformation in the metal substrate, along with delamination at the coating/substrate interface. Coating thickness, cycling loading (fatigue), and processing history (coating microstructure) are shown to be important factors in the damage patterns and ensuing modes of failure. Indentation stress-strain curves are used to measure macroscopic mechanical responses, to quantify the maximum sustainable contact stresses and to determine the relative roles of coating and substrate in the net deformation

Mechanical characterization of plasma sprayed ceramic coatings on metal substrates by contact testing

Hertzian indentation testing is used to generate contact damage in plasma sprayed ceramic coatings on metal substrates. Two basic ceramic/metal coating/substrate systems are examined: alumina on steel and zirconia on superalloy. Macroscopic mechanical responses are measured via indentation stress-strain curves, which quantify the relative role of the coating and substrate in the net deformation and facilitate evaluations of elastic moduli and yield stresses. Micromechanical damage processes within the coating and substrate subsurface layers are studied using a "bonded-interface" specimen. Degradation occurs primarily by delamination and other cracking at the coating/substrate interface or in the coating, but plastic deformation of the metal substrate contributes importantly to the crack driving force