- Failure Analysis, Fractography and Damage Tolerance
- Fracture Toughness and Delamination Growth
- Composite Coupon, Element and Structural Testing
- Impact (Threats, Resistance and Damage tolerance) and Energy Absorption
- Nano Reinforced Composites (Hierarchical Composites)
- Structural Power Materials
- Crack Arrest and Self-Healing Composites
- Modelling of Stone Lofting from Runway Debris (Sang Nguyen, Dr Lorenzo Iannucci, Prof Mike Graham
- Delamination Growth Mechanisms (Carla Canturri Gispert, Dr Silvestre Pinho
- Hierarchical Composites (Tomi Herceg, David Anthony, Sheema Raiz, Prof Alexander Bismark, Prof Milo Shaffer)
- Structural Power Materials (Dr Natasha Shirshova, Dr Sherry Qian, Dr Kingsley Ho, Atif Javaid, Matthew Laffan, Prof Alexander Bismark, Prof Milo Shaffer, Dr Joachim Steinke, Prof Anthony Kucernak)
- Crack Arrest and Self-Healing in Composite Materials (Dr Jesper Ankersen, Spyros Tsampas, Prof Ian Bond, Prof Alexander Bismark, Dr Lorenzo Iannucci, Dr Joachim Steinke, Prof Kevin Potter, Dr Richard Trask, Dr Stephen Hallett)
- Fractography and Failure Mechanisms in Composites (Dr Silvestre Pinho)
Videos and Downloads
Stiffener Debonding Initiation and Damage Growth in Post-Buckled Stiffened Structures
Victoria Bloodworth, Paola Apruzzese, Emile Greenhalgh, Brian Falzon, Robin Olsson
Stringer-stiffened structural components are widely used in aerospace and transport applications as primary load carrying structures. For metallic structures, postbuckling of structures is permitted below design limit loads (DLL), which helps in achieving minimum weight. Now these structures are being replaced by composite components, but designers are wary of allowing these structures to buckle below DLL because there are concerns that the stiffeners may disbond. Although a number of studies have demonstrated that stiffened composite structures can maintain structural integrity beyond initial local buckling, the out-of-plane forces at the relatively weak skin-stiffener interface, induced by buckling deformations, can lead to skin-stiffener detachment. The focus of this research is to understand and model the damage initiation and failure mechanisms in post-buckled stringer-stiffened structures. Because of the high expense associated with testing such large structures, the research focuses upon element tests which are representative of the stress conditions at the skin-stiffener interface. By using a combination of fractography and numerical models to characterise the conditions at the nodes and anti-nodes of post-buckled stiffened panels (the sites where failure initiation are most likely) comparison has been made with the behaviour and failure in representative element tests. In the representative element tests, three and four point, as well as lateral tension tests were conducted to characterise the skin-stiffener interface strength under different stress-states. During these experiments the full-field strain at the interface was characterised using Digital Speckle Photogrammetry. After testing, fractographic analysis of the failed elements was conducted to characterise the failure processes. Based on these results, improved designs, with optimised skin and stiffener stacking sequences are being developed and demonstrated experimentally. This research commenced in 2003, and is funded by EPSRC, dstl and QinetiQ.
Modelling of Stone Lofting from Runaway Debris
Sang Nguyen, Emile Greenhalgh, Robin Olsson, Lorenzo Iannucci
Runway debris lofting by aircraft tyres can lead to considerable damage to aircraft structures, yet there is limited understanding of the lofting mechanisms. This research programme has focused on the development of accurate models to understand and predict the stone lofting processes. Vehicle designers will ultimately be able to use these models to predict the probability of a lofted stone impact exceeding a critical magnitude. The research has entailed finite element (FE) modelling using Ls-Dyna and provided experimental validation of the studies through drop-weight impacts. The investigation has focused on a tyre partially rolling over a stone, causing lofting to the sides of a wheel. Parametric studies have been conducted to characterise the influence of factors such as stone geometry, tyre conditions and aircraft velocity in the lofting processes. An FE model of a drop-weight impactor has also been created to link the rolling tyre model with experimental conditions.
In the experimental studies, a steel impactor was covered with reinforced rubber to simulate a solid tyre whilst aluminium ball bearings represented spherical stones. A high speed video camera was used to observe the loft mechanisms and calculate the trajectories of the bearings. The main mechanism observed was described as ‘grip-hammer’ lofting where the tread held the stone in place before the tyre applied a large downward impulse to the stone. The stone was launched upwards with intense backspin at vertical speeds that were strongly dependant on the local geometry.
The future direction of the research is to further refine the models such that they accurately and reliably predict the stone lofting processes. Although the research is primarily focused on aircraft applications, the models are generic and will provide a tool for transport engineers who need to consider the threat of tyre lofted stone impact in their designs. This research commenced in October 2006, and is funded by EPSRC and dstl.
Delamination Failure Criteria
Charlotte Rodgers, Emile Greenhalgh, Paul Robinson
Hierarchical Fibre-Reinforced Nanocomposites
Michael Tran, Steven Lamorinere, Hui Qian, Angelika Menner, Emile Greenhalgh, Alexander Bismark, Milo Shaffer
There are a number of projects underway to develop hierarchical fibre-reinforced nanocomposites; these are materials in which carbon nanotubes (CNTs) are incorporated as an additional constituent in a continuous fibre/polymer matrix composites. The research is studying both thermoplastic and thermoset matrices, and considering a number of different processing routes to introduce the CNTs to the polymer matrix. Some of the key issues which are being addressed include;
- Ensuring a homogeneous distribution of the CNT reinforcement throughout the material.
- Ensuring high quality and uniformity of the CNT constituents to give well defined composite properties.
- Optimising the alignment of the CNTs, and maintaining it during processing, to maximise utilisation of their properties when in the composite material.
- Providing a good interface between the CNTs and the parent material, to ensure adequate stress transfer between these constituents.
The research on thermoplastic matrices is the most advanced, and is currently characterising the mechanical properties of the composites produced. Fabrication methods include a powder p reging route and spinning of CNT reinforced polymer fibres which can either act as reinforcing fibres, or as nano-reinforced matrices for co-mingling with conventional reinforcing fibres such as carbon.
Preliminary results show considerable improvements in interlaminar toughness can be achieved by the inclusion of modest conce ntrations of CNT reinforcement. The research on thermoset matrices has focussed on a number of processing routes to produce hierarchical composites. Methods includes direct mixing, an emulsion route and grafting of CNTs onto the surface of carbon fibres to form ‘hairy’ fibres. For the latter, the first phase has entailed experimental investigation into chemical vapour deposition of CNT onto carbon fibres to form ''hairy'' fibres.
Ways of incorporating the required catalyst for CNT growth onto the surface of the carbon fibres are being developed. The effect of the processing conditions on the distribution, density and alignment of the CNTs on the fibre surface is being studied, as well as the influence of the grafting process on the microstructure of the parent fibres. The grafting is being characterised using scanning and transmission electron microscopy and Raman Spectroscopy, whilst the increase in surface area is measured using nitrogen adsorption.
If sufficient quantities of ''hairy'' carbon fibres can be successfully produced, the investigation will continue into a second phase to produce composites based on these materials. The composite tows or bars will be fabricated using a liquid resin infusion processing route. Microscopic studies will be undertaken to characterise the microstructure, and assessment will be made on the processability of the material.
This thermoplastic composite research commenced in 2004, whilst the thermoset composite research commenced in 2005 and is funded by EPSRC, industry, MoD and QinetiQ.
Multifunctional Composite Materials
Natasha Shirshova, Emile Greenhalgh, Alexander Bismark, Milo Shaffer, Joachim Steinke
The initial stages of this research assessed the feasibility of a multifunctional material with the capacity to carry mechanical loads and store electrical energy; the key here was not simply to bind two disparate components together, but to produce a single coherent material that inherently performed both roles. We have moved the research from the initial idea to a working proof of concept demonstrator; a patent application has been filed (number 0607957.8) covering both the concept and a number of improvements that require investigation.
Conventional approaches to energy storage include batteries, capacitors and supercapacitors. Batteries have a high energy density, but low power density, due to high internal resistance at high discharge rates associated with the kinetics of the redox process; capacitors offer a limited energy density with a high power density, since the energy is only stored as charge on the electrodes. The focus of our research are supercapacitors, which provide a half-way-house between batteries and conventional capacitors (typical energy and power densities of 5Wh/kg and 0.2-5 kW/kg respectively) and avoid solid state redox reactions. The electrical performance of supercapacitors makes them desirable as short term storage media and high power density energy sources in applications in which fast bursts of energy are inherent.
These components are particularly useful for the load-levelling applications; when used in conjunction with a battery they provide for peak power demands (for example, during rapid acceleration of a vehicle) that cannot be supplied efficiently by the battery, giving substantial improvements in battery life. The most common form of electrochemical double layer supercapacitor consists of two electrodes, a separator, and an electrolyte.
The two electrodes, made of activated carbon, provide a high surface area, and are separated by a layer that is ionically-conducting but electrically insulating. The energy is stored by the accumulation of charges at the boundary between electrode and electrolyte; the nanometre separation of the charges gives rise to a high capacitance. The amount of stored energy is a function of the available electrode surface, the size of the ions, and the electrolyte stability (usually about 3V).
Our subsequent research is developing a proof-of-concept multifunctional structural power storage material. We have investigated development of a carbon fibre reinforced polymer composite which can act as a supercapacitor whilst sustaining mechanical loads. To this end we have investigated multifunctional composites derived from carbon fibres and their activation as mechanically robust electrode materials, polymer gel electrolytes as the ion conducting phase, glass fibres as the insul ator layers and sol-gel derived porous silica as further structural reinforcement.
Extrapolating to the conventional operating range for organic electrolytes, our earliest demonstrator had an apparent energy density of 1.5 J/kg; with the development of the new fibre activation treatment, this figure was boosted by more than an order of magnitude. This energy density exceeds high performance hybrid capacitors used in aerospace (64J/kg), but is still at least an order of magnitude lower than commercial supercapacitors.
This research has commenced in 2004 and is funded by Imperial College and MoD.
Structural Energy Storage Team; Emile, Natasha, Milo and Alexander.
- Funding Funding sources include; EPSRC, dstl (CASE award and Joint Grant Scheme funding), QinetiQ.
Fractography of Composites
Dr Joachim Steinke, Multifunctional Materials
Dr Lorenzo Iannucci, Runway Debris
Dr Robin Olsson, Runway Debris
Dr Brian G. Falzon, Imperial College, Stiffener Debonding in Stiffened Structures
Dr Paul Robinson, Imperial College, Delamination Mechanisms
Research Student Supervision
Ibrahim,S, Active Composite Materials for Energy Absorption
Bloodworth,V, Stiffener Debonding in Post-Buckled Structures
Apruzzese,P, Failure Modelling of Post-Buckled Stiffened Panels
Nguyen,S, Modelling of Stone Lofting from Runway Debris
Rogers,C, Delamination Failure Criteria