Improving the delamination bridging behaviour of z-pins through material solutions
Lead Research Organisation:
University of Bristol
Department Name: Aerospace Engineering
Abstract
Z-pinning is the most effective method for embedding through-thickness reinforcement in prepeg laminates. Composite Z-pins have been traditionally manufactured employing carbon fibres combined with a bismaleimide (BMI) resin. These materials allow achieving a substantial enhancement of the mode I interlaminar fracture toughness of Z-pinned laminates. Nonetheless, the toughness improvements in mode II are much more modest, due to the inherent brittleness of fibrous carbon/BMI when transversely loaded.
Through-thickness reinforcement elements such as Z-pins act as micro-fasteners, restraining the opening and sliding displacements of delaminations and hence inhibiting interlaminar crack growth. As such, ensuring a balanced mode I and mode II performance will reduce the chance of catastrophic failure of damaged composite structures. Mode II delamination toughness is especially crucial in reducing the damage and crack propagation in impacted composite laminates. This includes applications such as gas turbine blades.
The aim of this PhD project is to address the poor performance of composite Z-pins under mode II delamination conditions. This will be carried out by means of materials solutions, with the objective of understanding the inherent properties that affect the pin's performance. Ultimately, single-type Z-pins that provide a balanced mode I and mode II delamination toughness will be identified.
The key objectives to be completed throughout the length of the project are:
- Identify potential reinforcement/matrix materials to be used in Z-pins to achieve a balanced bridging performance in mode I and mode II regimes.
- Develop, as necessary, resin blends and fibre architectures that increase the mechanical performance of the pins under mode II loading.
- Successfully manufacture candidate Z-pins using the selected materials and architectures, optimising the process to obtain a high-quality output.
- Experimentally characterise the meso-mechanics of the candidate Z-pins for the full range of mode mixity and compare with the performance of commercially available Z-pins.
Most published work on Z-pinning has concentrated on understanding the performance of only the commercially available carbon/BMI Z-pins. These materials have remained almost unchanged for over a decade. Throughout the project, novel resin blends and fibre architectures will be developed to specifically satisfy the requirements of Z-pins and Z-pinned laminates. Additionally, the study will help to better understand the relationship between the mechanical properties of the pins and their ability to supress mode II delamination.
Through-thickness reinforcement elements such as Z-pins act as micro-fasteners, restraining the opening and sliding displacements of delaminations and hence inhibiting interlaminar crack growth. As such, ensuring a balanced mode I and mode II performance will reduce the chance of catastrophic failure of damaged composite structures. Mode II delamination toughness is especially crucial in reducing the damage and crack propagation in impacted composite laminates. This includes applications such as gas turbine blades.
The aim of this PhD project is to address the poor performance of composite Z-pins under mode II delamination conditions. This will be carried out by means of materials solutions, with the objective of understanding the inherent properties that affect the pin's performance. Ultimately, single-type Z-pins that provide a balanced mode I and mode II delamination toughness will be identified.
The key objectives to be completed throughout the length of the project are:
- Identify potential reinforcement/matrix materials to be used in Z-pins to achieve a balanced bridging performance in mode I and mode II regimes.
- Develop, as necessary, resin blends and fibre architectures that increase the mechanical performance of the pins under mode II loading.
- Successfully manufacture candidate Z-pins using the selected materials and architectures, optimising the process to obtain a high-quality output.
- Experimentally characterise the meso-mechanics of the candidate Z-pins for the full range of mode mixity and compare with the performance of commercially available Z-pins.
Most published work on Z-pinning has concentrated on understanding the performance of only the commercially available carbon/BMI Z-pins. These materials have remained almost unchanged for over a decade. Throughout the project, novel resin blends and fibre architectures will be developed to specifically satisfy the requirements of Z-pins and Z-pinned laminates. Additionally, the study will help to better understand the relationship between the mechanical properties of the pins and their ability to supress mode II delamination.
Planned Impact
The chief impacts are twofold:
1. Supply of doctoral level engineers trained to the very highest standards in advanced composites. They will take up positions in industry as well as academia.
2. Development of next generation advanced composite materials and applications for wealth creation in the UK.
Other important impacts are:
3. Enhanced UK reputation as a world leading centre in advanced composites that attracts inward investment and export opportunity.
4. Attracting elite overseas students, enhancing the UK's global reputation for excellence in Advanced Composite materials and their applications and widening the pool of highly skilled labour for UK industry.
5. Engaging with local schools and media, to disseminate, enthuse and raise the profile of Engineering to school children and to the wider public.
1. Supply of doctoral level engineers trained to the very highest standards in advanced composites. They will take up positions in industry as well as academia.
2. Development of next generation advanced composite materials and applications for wealth creation in the UK.
Other important impacts are:
3. Enhanced UK reputation as a world leading centre in advanced composites that attracts inward investment and export opportunity.
4. Attracting elite overseas students, enhancing the UK's global reputation for excellence in Advanced Composite materials and their applications and widening the pool of highly skilled labour for UK industry.
5. Engaging with local schools and media, to disseminate, enthuse and raise the profile of Engineering to school children and to the wider public.
| Description | The project aim was to develop composite-based Z-pins that exhibit excellent interlaminar toughening regardless of the mode mixity of the interlaminar crack. Additionally, it was key to understand the effect of altering the individual material properties of the Z-pin components on their interlaminar bridging effectiveness. Through materials selection, manufacturing and experimental mechanics, the aim of the project together with a number of individual achievements were met: - Several novel Z-pins were manufactured in-house through pultrusion and braiding, including hybrid fibre composites Z-pins. - A method of testing individual Z-pins whilst being able to observe the failure mechanism during the test was developed. - The effect of increasing the ductility of the Z-pin matrix was evaluated. The use of a ductile matrix resulted in a significant improvement in the delamination bridging behaviour of carbon fibre Z-pins, pushing the mode-mixity at which the pins are effective closer to mode II. - The effect of employing tougher and stronger fibrous reinforcement was evaluated, creating novel pseudo-ductile composite Z-pins. These novel pins are able to pull out of the laminate under mode II delamination. Their development were the key outcome of the project, as they satisfied the aim of creating a Z-pin with a balanced delamination toughening behaviour and led to the filing of a patent with the industrial sponsor (Rolls Royce plc). A number of new important research questions can been derived from this work: - How effective are these novel pseudo-ductile Z-pins under more complex loading conditions such compression after impact (CAI)?. - What is the effect of environmental degradation on the performance of the new Z-pin materials? - How will the interlaminar toughening effect seen in the coupon-sized samples be translated into larger structures which more representative of real applications? |
| Exploitation Route | The research outcomes will help design composite structures more efficiently, with lower risk of catastrophic interlaminar failure. Sectors which rely on pre-impregnated lamination will benefit the most, as Z-pinning is a well established method of through-thickness reinforcement used in combination with it. The use of the novel Z-pins can be introduced into the manufacturing processes already in place. Preventing interlaminar failure of composite laminates is essential to the successful use of composites in large structures in the aerospace and automotive industries. The outcomes of the project will help engineers make the correct decision with regards to what material to employ as a method of through-thickness reinforcement. |
| Sectors | Aerospace, Defence and Marine,Energy,Transport |
| Description | Industrial Sponsorship |
| Organisation | Rolls Royce Group Plc |
| Country | United Kingdom |
| Sector | Private |
| PI Contribution | The collaboration was founded through the well established Composites University Technology Centre (UTC) at the University of Bristol. The key contributions made towards the collaboration are monthly reports about the work carried out, frequent update meetings with an industrial supervisor and attendance to Rolls Royce led conferences. |
| Collaborator Contribution | As the UTC has been well established for over 15 years, the contribution goes beyond that specific to this project. They have provided us with a network of expertise and intellectual input as well as access to data, monetary contribution towards equipment and intellectual property support. |
| Impact | Research Paper: 10.1016/j.compositesa.2022.107241 Conference Paper: Improving the mode II delamination bridging performance of fibrous composite Z-pins (ECCM20, Lausanne), 10.5075/epfl-298799_978-2-9701614-0-0 Patent application: GB 2118257.1 |
| Start Year | 2018 |