Periodic Reporting for period 4 - DACOMAT (Damage Controlled Composite Materials)
Berichtszeitraum: 2021-01-01 bis 2022-10-31
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 imperfections 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 has been 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. In addition, advanced materials and structural modelling, and structural health monitoring methods to enable detection and assessment of damages has been developed.
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".
DACOMAT has focused on laminated glass fibre composites build layer by layer. The interface between the layers/lamina are “weak zones” and are vulnerable to cracks along the interface, so called delamination. In DACAMONT the resistance to delamination, technically called interlaminar fracture resistance, has been significantly improved.
Technically, the approach has been to utilize the concept of interlaminar fibre bridging and close parallel cracks. When a crack forms between two lamina, fibres can remain anchored in both crack surfaces. This is fibre bridging. As the crack grows the fibres are strained and gradually pulled out until they fail. This requires energy and increase the interlaminar fracture toughness. In addition, if close parallel cracks in adjacent interfaces are formed and advances simultaneously the fracture resistance will add up to nearly the sum of the fracture resistance of both cracks.
The overall objective of DACOMAT has been 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. In addition, advanced materials and structural modelling, and structural health monitoring methods to enable detection and assessment of damages has been developed.
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".
DACOMAT has focused on laminated glass fibre composites build layer by layer. The interface between the layers/lamina are “weak zones” and are vulnerable to cracks along the interface, so called delamination. In DACAMONT the resistance to delamination, technically called interlaminar fracture resistance, has been significantly improved.
Technically, the approach has been to utilize the concept of interlaminar fibre bridging and close parallel cracks. When a crack forms between two lamina, fibres can remain anchored in both crack surfaces. This is fibre bridging. As the crack grows the fibres are strained and gradually pulled out until they fail. This requires energy and increase the interlaminar fracture toughness. In addition, if close parallel cracks in adjacent interfaces are formed and advances simultaneously the fracture resistance will add up to nearly the sum of the fracture resistance of both cracks.
The interplay between fibre surface properties and resin is a key factor for composite mechanical properties. 3B and POLYNT have cooperated closely to develop pairs of fibres and resins that are optimal to promote 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. In addition, it has been explored how fabric architecture characteristics as stitching and backing fibre pattern and density influences fibre bridging. As a result, fracture resistance two times higher than for a predefined commercial reference has been achieved.
Parallel cracks can form spontaneously. However, to exploit the additional fracture resistance their formation needs to be controlled. In DACOMAT such damage control has been achieved by plasma treatment of fibre fabric surfaces to locally reduce crack initiation resistance.
Modelling has played 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 affect fibre bridging and fracture resistance for different cracking modes. 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.
Testing and test methods have both been a tool for materials characterization and a development area in DACOMAT. Among many developments, the project has developed an advanced single fibre peel test to measure fibre – matrix bonding energy in mixed mode, a moment-based mode III delamination test method and a method to measure peak traction with curved beam tests.
DACOMAT utilises two approaches for structural health monitoring: acoustic emission sensing and fibre optic sensing based on backscattering that provide continuous strain sensing along the fibre. Studies has been performed to develop a methodology to establish quantitative confidence on the probability of detection of cracks of certain sizes based on structure, sensor layup, load and crack location. Data processing algorithms based on principal component analyses and Recurrent Neural Networks have been established with significant improvement in probability of detection. Acoustic data from lab tests has been analysed in accordance with mechanical data to separate "signatures" of stable and unstable crack growth.
JCH has established a LCA database of environmental impacts to inform the material selection process and a LCA calculator to assess the impact of the different materials. To exemplify the effect of material choices, a bridge to be built in Skien Norway has been selected as a case study. Design solutions based on concrete, steel and composite have been developed. For wind turbine blades and other current composite applications, the contribution to increased sustainability from damage tolerance and control is directly proportional to the increased lifetime.
Parallel cracks can form spontaneously. However, to exploit the additional fracture resistance their formation needs to be controlled. In DACOMAT such damage control has been achieved by plasma treatment of fibre fabric surfaces to locally reduce crack initiation resistance.
Modelling has played 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 affect fibre bridging and fracture resistance for different cracking modes. 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.
Testing and test methods have both been a tool for materials characterization and a development area in DACOMAT. Among many developments, the project has developed an advanced single fibre peel test to measure fibre – matrix bonding energy in mixed mode, a moment-based mode III delamination test method and a method to measure peak traction with curved beam tests.
DACOMAT utilises two approaches for structural health monitoring: acoustic emission sensing and fibre optic sensing based on backscattering that provide continuous strain sensing along the fibre. Studies has been performed to develop a methodology to establish quantitative confidence on the probability of detection of cracks of certain sizes based on structure, sensor layup, load and crack location. Data processing algorithms based on principal component analyses and Recurrent Neural Networks have been established with significant improvement in probability of detection. Acoustic data from lab tests has been analysed in accordance with mechanical data to separate "signatures" of stable and unstable crack growth.
JCH has established a LCA database of environmental impacts to inform the material selection process and a LCA calculator to assess the impact of the different materials. To exemplify the effect of material choices, a bridge to be built in Skien Norway has been selected as a case study. Design solutions based on concrete, steel and composite have been developed. For wind turbine blades and other current composite applications, the contribution to increased sustainability from damage tolerance and control is directly proportional to the increased lifetime.
DACOMAT has progressed 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 potential for 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 addition to the materials development DACOMAT have made progress on testing and modelling required to qualify materials and use them in design and engineering of damage tolerant structures.
However, an important outcome is, that it has been realised that wide implementation and acceptance of damage tolerant composite materials in industry and society, will require establishment of standards for both materials qualification and structural design which are beyond the scope of DACOMAT. DACOMAT has initiated this through development of a recommended practice based on our findings as a starting point to engage the composite society for a broad cooperation to develop common standards.
- 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 addition to the materials development DACOMAT have made progress on testing and modelling required to qualify materials and use them in design and engineering of damage tolerant structures.
However, an important outcome is, that it has been realised that wide implementation and acceptance of damage tolerant composite materials in industry and society, will require establishment of standards for both materials qualification and structural design which are beyond the scope of DACOMAT. DACOMAT has initiated this through development of a recommended practice based on our findings as a starting point to engage the composite society for a broad cooperation to develop common standards.