Ingenieurwissenschaften und zugeordnete Tätigkeiten
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The use of novel multi-scale correlative methods, which involve the coordinated characterisation of matter across a range of length scales, are becoming of increasing value to materials scientists. Here, we describe for the first time how a multi-scale correlative approach can be used to investigate the nature of ductile fracture in metals. Specimens of a nuclear pressure vessel steel, SA508 Grade 3, are examined following ductile fracture using medium and high-resolution 3D X-ray computed tomography (CT) analyses, and a site-specific analysis using a dual beam plasma focused ion beam scanning electron microscope (PFIB-SEM). The methods are employed sequentially to characterise damage by void nucleation and growth in one volume of interest, allowing for the imaging of voids that ranged in size from less than 100 nm to over 100 mm. This enables the examination of voids initiated at carbide particles to be detected, as well as the large voids initiated at inclusions. We demonstrate that this multi-scale correlative approach is a powerful tool, which not only enhances our understanding of ductile failure through detailed characterisation of microstructure, but also provides quantitative information about the size, volume fractions and spatial distributions of voids that can be used to inform models of failure. It isfound that the vast majority of large voids nucleated at MnS inclusions, and that the volume of a void varied according to the volume of its initiating inclusion raised to the power 3/2. The most severe voiding was concentrated within 500 mm of the fracture surface, but measurable damage was found to extend to a depth of at least 3 mm. Microvoids associated with carbides (carbide-initiated voids) were found to be concentrated around larger inclusion-initiated voids at depths of at least 400 mm. Methods for quantifying X-ray CT void data are discussed, and a procedure for using this data to calibrate parameters in the Gurson-Tvergaard Needleman (GTN) model for ductile failure is also introduced.
Corrosion is the major degradation pathway affecting reinforced concrete structures. In the long-term, the rust layer might become thick and slow down the diffusion of oxygen from the atmosphere to the steel. With this in mind, the mineralogy, microstructure and diffusion properties of an old and thick corrosion product layer were investigated. Despite the presence of macropores, the gas diffusion coefficient was found to be low. The impact of the layer on the oxygen diffusion was evaluated using Fick’s law. The results showed that the layer could reduce the flux of oxygen only in a specific configuration.
Although concrete itself is not a combustible material, concrete mixtures with high density, such has high-performance concretes (HPCs), are susceptible to significant damage during fires due to explosive spalling. Past research has shown that the inclusion of polymer fibers in high density concrete can significantly mitigate fire damage due to the contribution of the fibers to increased permeability levels at high temperature. This allows vapor pressures caused by the evaporation of internal water during fire to escape from the material without causing significant spalling. Recent microscopic investigations have also shown that the addition of polypropylene (PP) fibers to high-density HPC with a high amount of fine-aggregate has a considerable influence on the nature and character of crack formation due to autogenous shrinkage. Initial cracks, which originate from the fiber beds, undergo further expansion and propagation when concrete is subjected to thermal exposure in excess of 170 °C. It is thus of interest to determine whether the resulting cracks join the adjacent fiber beds and therefore contribute to a significant increase in the permeability of the concrete, which is directly correlated to lower pore pressures and reduced spalling during fire.
In this study, X-ray Computed Tomography (CT) was applied to provide a clear demonstration of the interaction between polymer fibers and cracking during thermal exposure. For this purpose, two concrete samples containing different polymer fiber types were subjected to incremental application of a defined thermal exposure. CT images were acquired before and after each thermal exposure and powerful image processing tools were used to segment the various material components, such as polymer fibers, cracks, aggregates and cement matrix, in each image. This enabled a detailed analysis of crack formation and propagation as well as the visualization and quantification of polymer fiber characteristics within the concrete. This paper will provide a description of the distribution and orientation characteristics of the polymer fibers within each sample obtained through the CT-based analysis. Using these results, the impact of fiber distribution and orientation characteristics on actual cracking geometries have been measured and visualized. This paper will also provide recommendations for further optimization of the selected materials and propose improved methods for future CT-based analysis techniques.
One of the great strengths of X-ray computed tomography over conventional inspection methods (ultrasound, thermography, radiography) is that it can image damage in 3D. However for curved ordeformed composite panels it can be difficult to automatically ascribe the damage to specific plies or inter-ply interfaces. An X-ray computed tomography (CT) data processing methodology is developed to extract the through-thickness distribution of damage in curved or deformed composite panels. The method is applied to [(0°/90°)2]s carbon fibre reinforced polymer (CFRP) panels subjected low velocity impact damage (5 J up to 20 J) providing 3D ply-by-ply damage visualisation and analysis. Our distance transform approach allows slices to be taken that approximately follow the composite curvature allowing the impact damage to be separated, visualised and quantified in 3D on a ply-by-ply basis. In this way the interply delaminations have been mapped, showing characteristic peanut shaped delaminations with the major axis oriented with the fibres in the ply below the interface. This registry to the profile of the panel constitutes a significant improvement in our ability to characterise impact damage in composite laminates and extract relevant measurements from X-ray CT datasets.