Mechanical performance of glass-based geopolymer matrix composites reinforced with cellulose fibers

Glass-based geopolymers, incorporating fly ash and borosilicate glass, were processed in conditions of high alkalinity (NaOH 10-13 M). Different formulations (fly ash and borosilicate in mixtures of 70-30 wt% and 30-70 wt%, respectively) and physical conditions (soaking time and relative humidity) were adopted. Flexural strength and fracture toughness were assessed for samples processed in optimized conditions by three-point bending and chevron notch testing, respectively. SEM was used to evaluate the fracture micromechanisms. Results showed that the geopolymerization efficiency is strongly influenced by the SiO2/Al2O3 ratio and the curing conditions, especially the air humidity. The mechanical performances of the geopolymer samples were compared with those of cellulose fiber-geopolymer matrix composites with different fiber contents (1 wt%, 2 wt%, and 3 wt%). The composites exhibited higher strength and fracture resilience, with the maximum effect observed for the fiber content of 2 wt%. A chemical modification of the cellulose fiber surface was also observe

Mechanical and fracture performance of cellulose fibers based geopolymeric composite incorporating wastes

Geopolymers drew a lot of attention among scientific communities in the last decades for being a cement-like material, sustainable and eco-friendly. Its mechanical properties are well comparable to those of Portland cement and it has been demonstrated that these materials are so chemically stable that they can yield almost infinite durability compared to concrete [1]. While showing these attractive features, geopolymer nonetheless involves high cost due to refined primary materials utilization and low resistance to crack propagation (fracture toughness). On one hand, cost of production can be sensitively reduced by using wastes as source materials, among all fly-ash, a power plant by-product, and borosilicate glass, recycled glass from pharmaceutical vials. On the other, fracture toughness can be improved by producing composites from geopolymeric matrix. Please download the file below for full content

Shear performance at room and high temperatures of glass-ceramic sealants for solid oxide electrolysis cell technology

To provide a reliable integration of components within a solid oxide electrolysis cell stack, it is fundamental to evaluate the mechanical properties of the glass-ceramic sealing materials, as well as the stability of the metal-glass-ceramic interface. In this work, the mechanical behavior of two previously developed glass-ceramic sealants joined to Crofer22APU steel is investigated at room temperature, 650 \ub0C, and 850 \ub0C under shear load. The mechanical properties of both the glass-ceramics showed temperature dependence. The shear strength of Crofer22APU/ glass-ceramic/Crofer22APU joints ranged from 14.1 MPa (20 \ub0C) to 1.8 MPa (850 \ub0C). The elastic modulus of both glass-ceramics also reduced with temperature. The volume fraction of the crystalline phases in the glass-ceramics was the key factor for controlling the mechanical properties and fracture, especially above the glass-transition temperature

Cold Spray metal powder deposition with 9 %Cr-steel applied for the HCPB First Wall fabrication: Proof of concept and options for ODS steel processing

At the KIT a hybrid manufacturing concept for nuclear fusion First Walls is developed combining aspects of conventional and Additive Manufacturing (AM) technologies. The state of the art for ITER does not cover all specifications of a DEMO relevant First Wall. Thus, additional R&D-work has been initiated in terms of manufacturing. The AM technology basis used in the presented process combination is Cold Spray metal powder deposition applied in alternation with machining including the feature of filling grooves temporarily with a water-soluble granulate for creation of closed channels and cavities. Thus, the technology provides the option to manufacture shells with a thin gas tight membrane on top of previously machined structures. This membrane is used as pressure seal and makes the joining of shells by Hot Isostatic Pressing (HIP) into one monolithic body possible. This paper describes the manufacturing process and recalls differences and common aspects with regard to conventional concepts of First Wall manufacturing. The achievement of Technology Readiness Level TRL 3 by mechanical qualification and comparison of the results to other HIP joint experiments is also demonstrated. Finally, an outlook is given concerning integration options of the technology into manufacturing of shells with cooling channel structures using Oxide Dispersion Strengthened (ODS) materials

ZrB2-SiC Composites with Rare-Earth Oxide Additives

The effect of different content (2, 5, and 10 wt.%) of two different types of rare earth (RE) oxides (Eu2O3 and Lu2O3) on the sintering, microstructure, room temperature mechanical properties, and ablation resistance of ZrB2-25vol.% SiC ceramics were investigated. The materials were prepared using non-reactive Field Assisted Sintering Technology (FAST) in the temperature range of 1950°C – 2050°C, with a pressure of 70 MPa and a dwell of 7 min. No significant effect of the addition of 2 and 5 wt.% RE2O3 on the microstructure and the room temperature mechanical properties (hardness, indentation fracture toughness, and flexural strength) were observed. On the other hand, the coarser microstructures led to the deterioration of flexural strength and the hardness of the composites sintered with 10 wt.% RE2O3. The ablation resistance of the materials (tested up to ~ 2670°C) gradually increased with the increasing amount of RE oxides. The material with 10 wt.% Lu2O3 showed the best ablation resistance among all of the investigated compositions, with more than two times a lower ablation rate than that of the reference ZrB2-25vol.%SiC

Micromechanical Aspects of Transgranular and Intergranular Failure Competition

Quantification of characteristics that govern intergranular fracture initiation and propagation of this fracture micromechanism in competition with cleavage one is main aim of the paper. A NiCr steel of commercial quality and the same steel with an increased content of impurity elements, Sn and Sb, were used. Step cooling ageing was applied in order to induce intergranular embrittlement. Standard and pre-cracked Charpy type specimens were both tested in three-point bending to determine fracture toughness characteristics. In order to characterise the quantitative differences in fracture surfaces roughness a fractal analysis was applied. A boundary level of fractal dimension has been determined to be 1.12: fracture surface roughness with a higher value reflects high level of intergranular embrittlement and thus fracture resistance degradation

Modelling of the stiffness evolution of truss core structures damaged by plastic buckling

A finite element study based on 1D beam element model is performed in order to investigate the mechanical behavior of an elasto-plastic beam loaded in axial compression over its buckling limit. The mode of loading is related to the damage of truss-cored beams in truss-cored laminates. The analysis takes into account the effects of geometry and material properties. The results of the FEM analysis are used for developing a simple mechanical model based on the basic Euler-Bernoulli beam theory and accounts for the beam compressibility. The model uses phenomenological functions containing parameters related to the basic material and geometrical properties. The presented model is developed in the form of closed solution which does not require complex numerical methods or extensive parametric studies. Predictions of the compressive stiffness degradation of truss-cored composites are made with the proposed model and compared with the results of FEM simulations. The error of the stiffness prediction with respect to the FEM results is within 10% over a 5 fold range of stiffness

Effect of chemical composition on the optical properties and fracture toughness of transparent magnesium aluminate spinel ceramics

Polycrystalline transparent magnesium aluminate "spinel" ceramics were fabricated by hot-pressing and hot isostatic pressing (HIPing) using commercially available MgO and Al2O3 Powders. Al2O3 content of spinet was systematically changed that can be expressed as MgO.nAl(2)O(3) with n = 1.0, 1.5 and 2.0. UV/visible and near-IR wavelength region light reflection and transmission behaviors of the spinet ceramics were quantitatively correlated to their microstructure to account for the optical quality of the fabricated materials. The stoichiometric spinet ceramic with n = 1.0 revealed a relatively poor optical transparency due to pronounced light scattering at the microcracked grain boundaries with a specular light transmission of similar to 20-40% in the visible wavelength range. On the other hand, Al2O3 rich compositions revealed a specular transmission of -40-60% in the same wavelength range with a high degree of transparency. Additionally, effect of chemical composition on the fracture toughness of spinet ceramics was investigated applying indentation and chevron notched specimen fracture toughness measurement techniques. The spinet ceramic with n = 2.0 revealed the highest fracture toughness with a mean value of similar to 2.02 MPa-m(1/2). Based on their optical and mechanical properties, potential of Al2O3 rich non-stoichiometric polycrystalline spinet ceramics for engineering applications requiring high optical transparency and improved fracture toughness was addressed

Stiffness and strength degradation of damaged truss core composites

Truss core laminates display stiffness and strength/density ratios superior to those seen in foam cored laminates. However, this superiority is held only for ideal shaped struts. If the truss core is damaged, its performance rapidly decreases towards that of a foam. The present study investigates the stiffness and strength degradation with imposed core deformation/damage. This is done for a pyramidal core structure made by electro-discharge machining from AA5083 alloy. The experiments are compared with finite element predictions. The effect of the strain rate sensitivity is studied by performing the tests at different temperatures and by FE simulations with different material data sets. The results show reasonable agreement between experiments and modeling. The stiffness of a damaged truss core rapidly degrades and reaches the performance levels seen in foams after ≈8% of deformation. The results show that a high strain rate sensitivity significantly influences post-buckling core behavior and is able to decrease the stiffness and strength degradation rate