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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DACOMAT (Damage Controlled Composite Materials)

Teaser

Summary

\"Composites are due to their high stiffness and strength combined with low weight, indispensable in structures as wind turbine blades and aircrafts structures. Composites are also frequently used in other applications with demand to strength, light weight and environmental stability as marine structures and pipes. Nevertheless, composites still suffer from being vulnerable to production imperfection and damages from for instance impacts. To increase the use of composites in larger constructions, they need to provide a low life cycle cost based on low cost high throughput production and low need for maintenance. Production of still larger parts at low cost, increase the probability of production imperfections. To mitigate the effects of this and to reduce the need for costly maintenance and repairs, composites should be made more damage tolerant. Damage tolerant in this context meaning that cracks originating from production imperfections or extreme loadings will not develop to a critical scale.
The overall objective of DACOMAT is to develop more damage tolerant and damage predictable low cost laminated composite materials, aimed for use in large load carrying constructions like bridges, buildings, wind-turbine blades and off-shore structures. The developed materials and condition monitoring solutions will enable composite structures to be designed and manufactured as large parts, allowing for more and larger manufacturing defects and the need for manual inspection to be dramatically reduced.
DACOMAT represents a radical new way of thinking for composite materials by changing the philosophy from \"\"As strong as possible to avoid any cracks\"\" to \"\"Cracks can be tolerated but they should be controlled and stabilised through optimal fracture mechanical material design\"\".
Technically this will be achieved by developing composite materials which upon delamination shows extensive interlaminar fibre bridging and close parallel cracks that provide an increasing fracture energy with increasing crack size. In addition, sensor technology and advanced materials and structural modelling will be developed to enable detection and assessment of damages (structural health monitoring).

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Work performed

\"During the first year of DACOMAT there has been a strong focus on developing improved composite raw material constituents. The interplay between fibre surface properties and resin is a key factor for composite mechanical properties. 3B and POLYNT has cooperated closely to develop pairs of fibres and resins that are optimal to promote fibre bridging and high fracture resistance. By contribution from HEXCEL, development fibres have been processed into industrial non-crimp fabrics allowing for production of industrial relevant composite laminates based on a series of development resins and fibres. As a result, fracture resistance three times higher than for a predefined commercial reference has been achieved in small scale lab tests.
In DACOMAT, modelling plays a central role to guide materials development and as a tool in design and engineering. Microscale modelling has been carried out to improve understanding of how fibre -matrix interface mechanical properties affects fibre bridging and fracture resistance for different cracking modes. Specifically, this has led to development of a highly computational efficient semi-analytical fibre bridging model that has been validated towards detailed FEM models with very good results. On macroscale numerical studies have been carried out to understand how lamina interface cohesive law shape and through thickness crack position influence crack initiation and growth, and how layer wise differences in cohesive laws can promote parallel cracks.
DACOMAT utilises two approaches for structural health monitoring: acoustic emission sensing (AE) and fibre optic sensing (FOS) based backscattering that provide continuous strain sensing along the fibre. The FOS activities has focused on Model Assisted Probability Of Detection (MOPAD). Studies has been performed to develop a methodology to establish quantitative confidence on the probability of detection (POD) of cracks of certain sizes based on structure, sensor layup, load and crack location. On the AE side acoustic data from lab tests has been analysed in accordance with mechanical data to separate \"\"signatures\"\" of stable and unstable crack growth.
DACOMAT has as an objective to facilitate more use of composite materials in construction with bridges as one demonstration case. An important part of this is to render probable that the material selection will result in lower environmental impact and lower total lifetime costs, taking into account both direct effects as material cost and CO2 footprint, and indirect effects as environmental and economic consequences of change in traffic pattern during construction. JCH has established a LCA database of environmental impacts to inform the material selection process that will be used further in the project. To compare different material choices, a bridge to be built in Skien Norway has been selected as a study case. For this bridge, design solutions based fully or partly on concrete, steel and composite have been developed and forms a valuable base for comparison. A cooperation with stakeholders from the Skien bridge project (owners, engineers) has been established.
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Final results

DACOMAT will progress beyond state of the art by providing tougher more damage resistant and damage predictable low-cost composite materials. Increased damage tolerance and damage predictability combined with improved structural health monitoring brings on several advantages with significant impacts
- Reduction in design safety margins enabling lighter and cheaper design
- Higher tolerance to production imperfections allowing higher throughput and less production discards
- Less need for manual inspection and better measures to perform condition-based maintenance
- Longer operational lifetime through higher durability and more reliable assessment of remaining lifetime of structures
- Reduced probability of damage to develop to severe or catastrophic scale
In total DACOMAT aims for minimum 30 % improvement in durability and life cycle cost compared with today\'s materials and design solutions.
The wind energy sector has experienced an enormous technological development leading to more productive turbines providing power at decaying prices. Nevertheless, manual inspection & maintenance and downtime still generate large costs. The deployment of expected results in the wind energy sector will reduce these and lead to more reliable wind energy production at lower cost.
Throughout Europe there is a large need for establishment, replacement or refurbishment of bridges. DACOMAT will provide materials and technology to do this in a robust, economical and persistent manner. Besides the advantages of strength and outdoor durability, the light weight of composite materials enables prefabrication of smaller bridges that can be installed in very short time with minimum traffic interruption, which otherwise will influence third parties both economically and in terms of well-being.