Quantification of microdamage phenomena during tensile straining of high volume fraction particle reinforced aluminum

Particle reinforced composites are produced by infiltrating ceramic particle beds with 99.99% Al. Resulting materials feature a relatively high volume fraction (40-55 vol. pet) of homogeneously distributed reinforcement. The evolution of damage during tensile straining of these composites is monitored using two indirect methods; namely by tracking changes in density and in Young's modulus. Identification and quantification of the active damage mechanisms is conducted on polished sections of failed tensile specimens: particle fracture and void formation in the matrix are the predominant damage micromechanisms in these materials. The damage parameter derived from the change in density at a given strain is found to be one to two orders of magnitude smaller than the parameter based on changes in Young's modulus. A simple micromechanical analysis inspired by the observed damage micromechanisms is used to correlate the two indirect measurements of damage. The predictions of this analysis are in good agreement with experiment. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. AII rights reserved.

Self-Healing Fiber-Reinforced Composites with Tailored Supramolecular Matrices

Federica Sordo

Fiber Reinforced Composites (FRCs), due to their intrinsic heterogeneity, are generally affected by complex multi-scale failure mechanisms leading to the formation of matrix cracks that are difficult to detect and repair. FRCs with the ability to heal and recover at least part of their initial properties after damage could present a solution to extend their lifetime and reliability. A possible approach is to introduce external healing agents in the matrix of the composite, through capsules or micro-channels, but this is often difficult to achieve in a high volume fraction reinforcement composite. An alternative approach is to rely on intrinsic self-healing polymers as matrices, which alleviates the need for additional phases beyond matrix and reinforcement. The present work investigated this second approach, by exploring for the first time the feasibility of processing glass fabric reinforced composites based on supramolecular hybrid network self-healing matrices and analysing their resulting properties and transfer of the self-healing ability to the composite. Hybrid networks based on epoxy chemistry, combining covalent cross-links and cooperative reversible hydrogen bonds, were selected as benchmark materials. These systems were charac- terized, in particular in terms of processing parameters and self-healing ability, and showed to be a-priori compatible with FRCs manufacturing techniques and demonstrated higher healing efficiency by increasing the H-bonds content in the polymer network, from none to 50%. On the other hand, networks characterized by large number of physical cross-links exhibited limited solvent and creep resistance, hindering their potential application. These issues were addressed by modifying the average functionality of the monomers through the combination of bifunctional and tetrafunctional epoxy resins in the compositions while keeping the use of a catalyst, leading to better controlled networks, both in terms of kinetics and final structure, and to propose a range of materials with several levels of compromise between self-healing efficiency, mechanical strength, creep resistance, and damping properties. In particular, 50% to 100% self-healing recovery of tensile properties were observed after one day, depending on the tetraepoxide content. In parallel, since the Tg of benchmark supramolecular hybrid net- works was in the range from 11-16 âŠC, an aliphatic diepoxide was introduced in the synthetic procedure to propose materials with a Tg down to -10 âŠC, to reach applications closer to those of elastomers. In view of FRCs development, the ability of partially supramolecular self-healing polymers to bond to inorganic reinforcements and to heal interfacial failure was then investigated: two dedicated butt joint-like and tack-like test methods were developed. Adhesion strength of the polymers cured on glass substrates decreased with increasing H-bonds content in the polymer networks (from 0 to 50 %) as the mechanical strength of the polymer also decreased, while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with the glass substrates. Interface restoration through healing of the polymer matrices however increased from none to a strength recovery up to 70 % after 1 h healing time for the 50% H-bond polymer...

EPFL2016

Structural analysis of a composite bridge structure combining UHPFRC and prestressed concrete

Eugen Brühwiler, Cornelius Oesterlee, Hamid Sadouki

Reinforced concrete being the most applied construction material today performs very well in most applications but still lacks durability under severe environmental conditions. Especially existing structures built decades ago show degradation. Using Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) to improve durability is a promising option seen the extraordinary performance of this material when applied in a composite section. Restrained shrinkage of the overlay plays an important role in this type of application. A conceptual bridge cross section combining UHPFRC and reinforced concrete has been numerically analysed. Two main parameters, tensile strength and strain hardening capacity, were varied. The analysis indicates the importance of strain hardening of UHPFRC and the influence of its tensile strength on the structural response under restrained shrinkage (deformation controlled loading) and traffic loads (force controlled loading). The results validate the concept of using a UHPFRC layer to improve the structural durability of concrete constructions.

Uni Kassel2008

Next generation synthetic fibers UHPFRC for sustainable structural applications

Amir Hajiesmaeili

The ever-increasing demand for rehabilitation of existing structures and construction of new ones, along with environmental issues of the construction sites represent a heavy burden for society in terms of economy and environment. Meanwhile, Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) has proved its potential to be one of the solutions to contain the explosion of the maintenance costs, because of its extremely low permeability and outstanding mechanical properties. Even though using UHPFRC already significantly contributes to decreasing the environmental impact of rehabilitation construction sites by up to 50% when compared to using conventional concretes, further optimization of their mix design with replacement of high environmental cost components like steel fibers and clinker, would be another step towards sustainability. The main objective of this study was to further improve the already established concept of UHPFRC in two main directions: firstly, to develop new strain hardening UHPFRC mixes with reduced environmental impact by replacing the steel fibers with synthetic ones and by replacing 50% vol. clinker with cheap and widely available Supplementary Cementitious Materials, and secondly, to investigate the mechanical properties, delayed response, protective function, and environmental impact of the newly developed material for structural applications. An advanced packing density model was developed to optimize the powder mix that consisted of six different powders. An extensive experimental campaign with a focus on the tensile behavior was carried out to investigate the factors influencing the mechanical properties of the UHPFRC mixes and validate their performances. The achieved new mix named PE-UHPFRC has on average a tensile elastic limit of 7.7 MPa, a tensile ultimate strength of 11.7 MPa, a tensile deformation capacity of more than 3.5%, and a compressive strength of 120 MPa, well adapted for cast-onsite applications of rehabilitation, or strengthening associated to rebar. The delayed response of PE-UHPFRC was investigated with a focus on autogenous deformations and associated eigenstresses under full restraint conditions, using a Temperature Stress Testing Machine. The results showed more than 70% reduction in the eigenstresses in a 50 mm thick PE-UHPFRC layer when compared to those observed in conventional UHPFRC with steel fibers, which is highly beneficial for rehabilitation applications. An original capillary absorption setup was developed to investigate the protective properties of the PE-UHPFRC under tensile loading, at different strain levels ranging from 0.15 to 20â°. Before reaching the tensile elastic limit, the capillary absorption coefficient was extremely low, at 24 g/m2âh, however, a sudden increase in the capillary absorption was observed after the elastic limit under tensile load. Finally, the environmental benefits of PE-UHPFRC combined with rebar were characterized at the structural level, for the strengthening of a bridge with a span of 34 m, and compared with those of: (1) a reconstruction with prestressed concrete, and (2) a strengthening with conventional UHPFRC with steel fibers combined with rebar. The LCA results showed on average 55% and 34% decrease in the environmental impact for the strengthening method with R-PE-UHPFRC compared with the reconstruction with prestressed concrete and strengthening with conventional R-UHPFRC, respectively.