Materials and Manufacturing Engineering
Materials and Manufacturing Engineering
The research activities of the group are embedded in today ‘s engineering challenges and future engineering vision covering expertise in: bioengineering, computational mechanics, AI and digital twin, additive manufacturing and advanced materials and structures including porous nanomaterials for energy storage.
|Professor Oana Ghita|| High Value Materials and Manufacturing – High Performance Polymers, Composites
and Additive Manufacturing.
|Professor Ken Evans|| Auxetic and other unusual materials, additive and other novel manufacturing methods,
theory and experiment.
|Professor Tim Dodwell|| Bayesian Uncertainty Quantification, Multiscale Methods, Reinforcement Learning,
Computational Mechanics, Material Modelling, Functional Instabilities.
|Professor Chris Smith||Simulation methods for mechanics and multiphysics problems, notably optimised clean powertrains,
fretting wear and vibration dmaping. Director of the Centre for Future Clean Mobility.
|Dr Tim Holsgrove|| Degeneration, injury, and treatment of the musculoskeletal system with a specific focus
on the spine.
|Dr Junning Chen|| Exploring the structure-mechanics-functionality relationship in biological tissues and
biomaterials for medical intervention.
|Dr Mi Tian|| Design, fabrication and characterisation of composite material for hydrogen storage
and transportation, and neutron scattering characterisation for novel material innovation.
|Professor Shaowei Zhang|| Structural Ceramics and Refractories, Nanomaterials for Energy, Ceramic Matrix Composites,
Catalytic Materials; 1-D and 2-D Nanomaterials, Aerospace Materials.
|Dr Yongde Xia|| Porous materials for energy, electrochemical energy storage and conversion, photocatalysis and
|Dr Konstantinos Agathos||Computational fracture mechanics, reduced order modelling, inverse problems, digital twins.|
Our research projects include:
Arkema and CALM are working together on optimising Poly Ether Ketone Ketones (PEKK) for the EOSINT P 800 powder bed process.
Arkema is a French chemical company leader in the field of high performance polymer materials, one of which is PEKK, invented in the 1960s as part of the Apollo space program. PEKK Kepstan®, PEKK from Arkema, has a very high melting point (300°C to 360°C depending on the grade) and provides excellent mechanical properties, excellent resistance to chemicals and abrasion. The PhD aims to gain a deep understanding of the material at the microstructure level in order to optimise it for the manufacturing process.
In this Royal Society supported Research Grant project, Co or Mn-based MOFs with polymeric organic ligands will be chosen and atomic homogeneously dispersed transition metal oxide/sulfides embedded within pure or heteroatom-doped nanoporous carbon materials will be readily formed via thermal treatment of MOFs in controlled gas atmosphere at high temperature. Under optimised experimental conditions, the obtained composites with atomic-level uniform dispersion, highly porosity, heteroatom-doping and unique core-shell structure are expected to be promising highly-efficient electrocatalysts in ORR/OER.
High Temperature Polymer Additive Manufacturing – polymer and composite development, optimisation
CALM in partnership with Bond 3D are co-funding a PhD studentship seeking to investigate polymer behaviour and performance of Poly Ether Ether Ketone (PEEK) polymers. Bond 3D is a fast growing, young and entrepreneurial company based in the Netherlands who are developing a novel, ultra-performance Free Form Fabrication (FFF) printer for printing high performance polymers such as PEEK. The PhD programme is focused on understanding the polymer behaviour from molecular scale to macro-scale performance throughout the new manufacturing process.
The project seeks a quantitative link between cartilage ultrastructure and its biomechanical performance. This structure-mechanics-functionality relationship will extend our understanding of cartilage behaviours and reveal the associated cellular environment, which underlie micromechanical signalling involved in initiating osteoarthritis (OA). It will thereby identify targets for early diagnosis of OA and inform the development of in-vivo monitoring optical systems on OA progression. The project will also contribute to a basis for evaluating current tissue-engineered cartilage constructs, providing biomimetic templates for cartilage regeneration, and facilitating the development of anti-OA drugs.
£2m funding from Heart of the South West to establish facility on Science Park.
Find out more about our research on our designated website.
The performance and strength of a composite aero-structure is established incrementally through a programme of analysis and a series of experimental tests conducted using specimens of varying size and complexity. The process utilises a so-called 'building block' or 'testing pyramid' approach with tests at each of the following levels:
- Structural detail
- Sub-structure or full structure
The 'building block' approach provides a comprehensive and systematic methodology to demonstrate airworthiness and structural integrity, and as such represents the backbone of the certification processes for composite aero-structures. The vast majority of certification tests are conducted at the coupon level, whereas far fewer certification tests are conducted at the subsequent higher pyramid levels. The complexity, cost and time of each test escalates up through the testing pyramid. The underlying assumption is that the material properties derived from tests at the lower levels can be used to define the requirements and design allowables at higher length scales and component complexity. At the mid-pyramid level, the as-manufactured strength of parts is currently assessed by empirical 'manufacturing knockdown factors', and the uncertainties in this assessment, together with uncertain in-service damage, propagate up the pyramid to the full component and structure levels. At best, this leads to conservative, over-constrained design. At worst, there is risk that potentially unsafe scenarios can develop where combinations of weakening events cascade into premature failure. Thus, the very time consuming and expensive testing at the coupon level, produces conservative strain limits with questionable relevance to the strength of large parts or at the full structure level. Also, innovative material and technology developments, which facilitate lightweighting, safer and more damage tolerant composite design, are only relevant at the sub-structure and component levels, and therefore cannot be incorporated into applications because of the current validation practices. Accordingly there is increasing evidence that the building block approach has severe limitations, particularly the high cost of certification, time to market, and the general inability to characterise and predict limit states that may lead to failure at structural scales. There is increasing awareness that, in its current form, the 'building block' approach prevents the innovative use of composites, and consequently that the potential benefits of using advanced composites in terms of lightweighting and efficiency cannot be fully realised under current certification and regulatory procedures.
Composite materials are becoming increasingly important for light-weight solutions in the transport and energy sectors. Reduced structural weight, with improved mechanical performance is essential to achieve aerospace and automotive's sustainability objectives, through reduced fuel-burn, as well as facilitating new technologies such as electric and hydrogen fuels. The nature of fibre reinforced composite materials however makes them highly susceptible to variation during the different stages of their manufacture. This can result in significant reductions in their mechanical performance and design tolerances not being met, reducing their weight saving advantages through requiring "over design".
Modelling methods able to simulate the different processes involved in composite manufacture offer a powerful tool to help mitigate these issues early in the design stage. A major challenge in achieving good simulations is to consider the variability, inherent to both the material and the manufacturing processes, so that the statistical spread of possible outcomes is considered rather than a single deterministic result. To achieve this, a probabilistic modelling framework is required, which necessitates rapid numerical tools for modelling each step in the composite manufacturing process.
Focussing specifically on textile composites, this project will develop a new bespoke solver, with methods to simulate preform creation, preform deposition and finally, preform compaction, three key steps of the composite manufacturing process. Aided by new and developing processor architectures, this bespoke solver will deliver a uniquely fast, yet accurate simulation capability.
The methods developed for each process will be interrogated through systematic probabilistic sensitivity analyses to reduce their complexity while retaining their predictive capability. The aim being to find a balance between predictive capability and run-time efficiency. This will ultimately provide a tool that is numerically efficient enough to run sufficient iterations to capture the significant stochastic variation present in each of the textile composite manufacturing processes, even at large, component scale.
The framework will then be applied to industrially relevant problems. Accounting for real-world variability, the tools will be used to optimise the processes for use in design and to further to explore the optimising of manufacturing processes.
Close collaboration with the project's industrial partners and access to their demonstrator and production manufacturing data will ensure that the tools created are industry relevant and can be integrated within current design processes to achieve immediate impact. This will enable a step change in manufacturing engineers' ability to reach an acceptable solution with significantly fewer trials, less waste and faster time to market, contributing to the digital revolution that is now taking place in industry.
The development of enriched finite element methods for the modelling of fracture, covering different aspects such as discretization, crack representation, and more recently specialized linear solvers. Applications include non-planar crack propagation for damage tolerance assessment, and the modelling of wave propagation in cracked solids in the context of Non-Destructive Testing (NDT).
The development of reduced order modelling techniques for damaged solids and structures, as well as their combination to data-driven techniques. The main objective here is to develop parametric models, able to represent a wide array of damaged configurations at a reduced computational cost. These models, combined to measurements and data-driven techniques, can be used as ‘digital twins’, in damage detection problems.
Understanding rate of crystallisation, morphology of crystals within a structure as a function of manufacturing parameters and process conditions is critical in order to obtained structures with optimum mechanical performance.
Reference - https://doi.org/10.1016/j.addma.2021.101843
This project is being completed as part of the QUEX PhD programme, and links the Biomedical Engineering Group at the University of Exeter, with the School of Chemical Engineering at the University of Queensland, to develop a ‘design, optimisation, and fabrication’ pipeline for patient-specific multi-phasic TDRs that can be tailored to the mechanical environment of a particular patient’s spine, and through this mechanical-matching, support the generation of new tissue from a patient’s own stem cells.
Titanium alloy components are strategically important to the future UK aerospace, energy and electric vehicle sectors owing to their high strength-to-weight ratio, excellent fracture resistance and fatigue properties, and compatibility with carbon fibre composites. Current state-of-the-art titanium components are processed through complex non-linear open-die and closed-die hot forging that generates non-uniform microstructure and properties within different regions. This necessitates significantly larger geometries than the final shape to be forged before 70% of the material is machined away to retain the "optimum" microstructure and property set in the final part. This expensive and wasteful approach has led to a sector-wide effort to produce components with more homogeneous microstructures and property distributions from less material. For example, many emerging powder-derived manufacturing routes have been explored extensively. The UK developed, hybrid FAST-forge powder-derived process has shown promise over recent years to produce affordable titanium alloy components. The high-value manufacturing sector now needs the tools to objectively inform which processing route is optimum, be it state-of-the-art or emerging routes, such as FAST-forge, based on key drivers such as cost, volume, energy consumption, resource use, and in-service properties.
The project aims to provide knowledge about the relationship between mechanical performance and the powder particle shape, rheology and sintering behaviour.
Step by step the chosen particle shapes will be investigated for their powder flow and compaction using discrete element methods (DEM) and they will be realised experimentally using laser sintered particles. From simple geometric models to more advanced ones including polymer characteristics (such as viscosity, surface tension) through to new sintering kinetics-driven particle interaction algorithm, the sintering dynamics will be examined and compared with hot stage microscopy results. The testing of manufactured components will be also performed to validate and synthesise the findings in a realistic and complex laser sintering process. The in-depth knowledge created by these results will be used to design and demonstrate new spreading and compaction systems to accommodate better the powder morphology and its flow characteristics. The flow and sintering behaviour of different shape nanoparticles added to a laser sintering polymeric powder will be also explored.
Funding Agencies: Swiss National Science Foundation (SNSF) – Polish National Science Center (NCN)
Project Partners: Prof. Eleni Chatzi, ETH Zurich – Prof. Wieslaw Ostachowicz, Polish Academy of Sciences
The project aims to fuse Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM) approaches for the assessment of existing infrastructure, enhancing the use of typical Vibration-based SHM tools, and further exploring NDE tools, such as Guided Wave (GW), and Electro-Mechanical Impedance (EMI) schemes, for continuous monitoring and online damage assessment. This is accomplished on both the SHM and the NDE side by relying on the fusion of numerical models with data. This merger is possible on the basis of reduced order models that offer high accuracy at a reduced computational time.
Role: Co-supervision of an ETH-based PhD student (main supervisor: Prof. Eleni Chatzi).
This project is about the modelling, manufacturing and testing of novel auxetic yarns. This has led to a practical route to make a range of novel fabrics that can exploit the benefits of having negative Poisson’s ratios with applications across a wide range of areas including military, sport and fashion.
- Zhang G, Ghita OR, Lin C, Evans K. (2017) Large-scale manufacturing of helical auxetic yarns using a novel semi-coextrusion process, Textile Research Journal, pages 1-12, DOI:10.1177/0040517517725125
- Zhang G, Ghita OR, Evans KE. (2016) Dynamic thermo-mechanical and impact properties of helical auxetic yarns, Composites Part B: Engineering, volume 99, pages 494-505, DOI:10.1016/j.compositesb.2016.05.059.
- Zhang G, Ghita O, Lin C, Evans KE. (2016) Varying the performance of helical auxetic yarns by altering component properties and geometry, Composite Structures, volume 140, pages 369-377, DOI:10.1016/j.compstruct.2015.12.032
- Zhang GH, Ghita OR, Evans KE. (2015) The fabrication and mechanical properties of a novel 3-component auxetic structure for composites, Composites Science and Technology, volume 117, pages 257-267, DOI:10.1016/j.compscitech.2015.06.012
The heavy isotopes of hydrogen, deuterium (D) and tritium (T) play essential roles in nuclear energy production. However, as D is not directly obtainable as a pure isotope, methods to separate hydrogen isotopes from the more common, lighter isotope, protium (H), are required. The purpose of the project is to study the H/D separation on the selected porous organic cages, 6MT-RCC3, using a complementary multidisciplinary approach of in-situ neutron diffraction and spectroscopy and the computational simulations combined with high-pressure gas adsorption measurement
Searching for reversible hydrogen storage materials operated under ambient conditions is a big challenge. We recently started a new collaboration with the inventor of MXene, Professor Michael Naguib, to exploit an exciting project on hydrogen storage on 2D MXene at ambient conditions for onboard system.
There is a paradox in aerospace manufacturing. The aim is to design an aircraft that has a very small probability of failing. Yet to remain commercially viable, a manufacturer can afford only a few tests of the fully assembled plane. How can engineers confidently predict the failure of a low-probability event? This research will develop novel, unified AI methods that intelligently fuse models and data enabling industry to slash conservatism in engineering design, leading to faster, lighter, more sustainable aircraft.
Our investigations into molecular hydrogen (H2) confined in microporous carbons with different pore geometries at 77 K have provided detailed information on effects of pore shape on densification of confined H2 at pressures up to 15 MPa. We show via a combination of in situ inelastic neutron scattering studies, high-pressure H2 adsorption measurements, and molecular modelling that both slit-shaped and cylindrical pores with a diameter of ∼0.7 nm lead to significant H2 densification compared to bulk hydrogen under the same conditions, with only subtle differences in hydrogen packing (and hence density) due to geometric constraints. This confirmation of the effects of pore geometry and pore size on the confinement of molecules is essential in understanding and guiding the development and scale-up of porous adsorbents that are tailored for maximising H2 storage capacities, in particular for sustainable energy applications.
Metamaterials are materials where the structure is at a scale larger than the atomic level and hence they may not necessarily be constrained by statistical thermodynamics, nor do they have to be constrained in the geometries of their “unit cells” As a result it is possible to have negative refractive indices, negative Poisson’s ratio, negative thermal expansion coefficients and, under certain dynamic conditions negative mass and negative stiffness. In this project we propose and explore mechanical metamaterials with geometries that lead to negative Poisson’s ratios.
- Mizzi L, Attard D, Evans KE, Gatt R, Grima JN. (2020) Auxetic mechanical metamaterials with diamond and elliptically shaped perforations, Acta Mechanica, DOI:10.1007/s00707-020-02881-7
- Farrugia PS, Gatt R, Zammit Lonardelli E, Grima JN, Evans KE. (2019) Different Deformation Mechanisms Leading to Auxetic Behavior Exhibited by Missing Rib Square Grid Structures, Physica Status Solidi (B) Basic Research, volume 256, no. 1, DOI:10.1002/pssb.201800186.
- Mizzi L, Mahdi EM, Titov K, Gatt R, Attard D, Evans KE, Grima JN, Tan JC. (2018) Mechanical metamaterials with star-shaped pores exhibiting negative and zero Poisson's ratio, Materials and Design, volume 146, pages 28-37, DOI:10.1016/j.matdes.2018.02.051.
- Dudek KK, Gatt R, Mizzi L, Dudek MR, Attard D, Evans KE, Grima JN. (2017) On the dynamics and control of mechanical properties of hierarchical rotating rigid unit auxetics, Sci Rep, volume 7, DOI:10.1038/srep46529.
This EU RFCS awarded project, collaborated with 9 other partners cross Europe, to develop an integrated approach for upgrading methane in ventilation emissions of working shafts as well as those emissions coming from abandoned mines. In this collaboration project, we will contribute to explore and design adsorption processes for methane concentration. Both nanostructured materials like MOFs, porous carbons or zeolites and waste-derived materials such as fly ash derived zeolites adsorbent materials will be developed. The planned work includes materials development, screening and modelling at lab scale and design for full-scale operation. Moreover, making use of the membrane technology for methane concentration, the developed nanostructured materials that are active membrane materials will be introduced into membrane modules, which will be evaluated in methane concentration. These work will provide significant understanding of the application of adsorption and membrane technologies to methane separation and concentration; both in terms of materials development and optimisation of operational strategies.
Negative Poisson’s ratio materials (auxetics) are intriguing, unusual and have enormous potential for expanding the range of properties and applications of materials; either by “converting” existing materials or creating new ones. To do this it is essential to understand the relationships between geometry and deformation mechanism at a wide range of length scales. This project explores these aspects and proposes new auxetic materials for synthesis and manufacture.
- Degabriele EP, Attard D, Grima-Cornish JN, Caruana-Gauci R, Gatt R, Evans KE, Grima JN. (2019) On the Compressibility Properties of the Wine-Rack-Like Carbon Allotropes and Related Poly(phenylacetylene) Systems, Physica Status Solidi (B) Basic Research, volume 256, no. 1, DOI:10.1002/pssb.201800572.
- Grima-Cornish JN, Grima JN, Evans KE. (2017) On the Structural and Mechanical Properties of Poly(Phenylacetylene) Truss-Like Hexagonal Hierarchical Nanonetworks (Phys. Status Solidi B 12/2017), physica status solidi (b), volume 254, no. 12, pages 1770262-1770262, DOI:10.1002/pssb.201770262.
- Degabriele EP, Grima-Cornish JN, Attard D, Caruana-Gauci R, Gatt R, Evans KE, Grima JN. (2017) On the Mechanical Properties of Graphyne, Graphdiyne, and Other Poly(Phenylacetylene) Networks, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, volume 254, no. 12, article no. ARTN 1700380, DOI:10.1002/pssb.201700380
- Siddorn M, Coudert F-X, Evans K, Marmier ASH. (2015) A systematic typology for negative Poisson's ratio materials and the prediction of complete auxeticity in pure silica zeolite JST, Physical Chemistry Chemical Physics, DOI:10.1039/C5CP01168J
F4PAEK -Multifunctional materials based on PAEKs (funded by Innovate UK) - see our website for further information.
Combining nano-particles with PAEK polymers, this project is looking to develop new bespoke lightweight multifunctional materials that can be 3D printed using powder bed fusion technology and FFF. PAEK polymers are temperature resistant, tough, and corrosion resistant. They are increasingly being used for metal replacement within aviation and military application.
Boron nitride and graphene have been selected as the nano-materials most suited for the intended applications here. These new materials will offer multifunctional capabilities including lightweighting, thermal and electro-magnetic properties. Incorporation of nanomaterials with different particle sizes and shape which will significantly affect powder flow, polymer viscosity and subsequently sintering mechanisms are very important to the success of this project.
The project will study the surface chemistry of the nanoparticles for good interface bonding with PAEK; fabrication of the composite powder (encapsulation of the nanoparticles either on the surface or within the bulk of the PAEK particles) and laser sintering of these new powders. Powder properties (bulk density, compaction, shape - roundness and circularity, aspect ratio, viscosity, surface tension) are key parameters for a good sintering process.
Partners: Qioptiq Ltd, Thales UK Ltd, Victrex Manufacturing Ltd, Hosokawa Micron Ltd, Airbus Operations Ltd, 2-DTech Ltd, Haydale Ltd
Contact: Prof Oana Ghita (firstname.lastname@example.org), Research Fellow – Dr Yaan Liu (email@example.com)
ICure – MOF-3D: 3D Printing with super absorbent materials
University of Exeter have developed a range of MOF polymer materials that can be used in additive manufacturing to produce complex structures. In these materials the MOF crystals are nucleated on the surface of the polymer granules preserving the inherent porosity of the MOFs. When the polymer granules are fused by selective laser sintering during additive manufacturing the MOF’s remain substantially on the surface of the polymer structure. The effectiveness of this technique has been demonstrated by nucleating ZIF 67 on the surface of PA2200 (polyamide 12) and then 3D printing a porous cubic structure. The 3D printed porous structure was found to have a CO2 adsorbtion capacity approximately 5x greater than alternative MOF polymer blends such as HKUST 1/PA12 where the MOF is incorporated as a filler material. The fabrication technique has also been found to work for a range of polymer materials including polyaryletherketones (PAEKs) and polyoelfins as well as polyamides and with a range of different MOFs.
The technology is currently at TRL 3 4 with proof of concept demonstrated in the laboratory. 3D MOF polymer materials have been manufactured at a kilogram scale but the methodology is easily scalable to production volumes.
A fundamental limitation to the study and application of dense, solid phases of molecular hydrogen (H2) is that they generally only form at exceedingly low temperatures or extremely high pressures. Confinement of H2 in nanoscale pores of carbon nanomaterials, however, has enabled formation of solid-like H2 at atmospheric pressure at temperatures above the critical point. In this project, we uncovered the different dense, crystalline phases of H2 confined in both meso- and microporous carbons using neutron scattering, demonstrating that confinement in sub-nanometre pores leads to preferential stabilisation of face centred cubic (FCC) solid H2 at temperatures higher than possible in the bulk. These results suggest ways in which potentially higher pressure phases of solid hydrogen may be accessed by nanoconfinement.
This is a Leverhulme Trust awarded Research project grant. To achieve the carbon emission goal set in the Paris Agreement, innovative technologies to reduce carbon emission are urgently required. Carbon capture and utilization offers a sustainable long-term low-carbon approach to mitigate carbon dioxide. This programme aims to develop innovative catalysts and catalytic membrane reactors to effectively convert “waste” carbon dioxide into energy product dimethyl carbonate. The multifunctional membrane reactor, featured with catalytic active porous oxides on a polydimethylsiloxane-based matrix, could enable carbon dioxide conversion to dimethyl carbonate and simultaneously separate dimethyl carbonate from the product stream in one-step, symbolling a cutting-edge technology advancement with huge economic and social impacts.
The exploration and evaluation of new composites possessing both processability and enhanced hydrogen storage capacity are of significant interest for onboard hydrogen storage systems and fuel cell based electric vehicle development. In this project, we demonstrate the fabrication of composite membranes with sufficient mechanical properties for enhanced hydrogen storage that are based on a polymer of intrinsic microporosity (PIM-1) matrix containing nano-sized fillers: activated carbon (AX21) or metal–organic framework (MIL-101). This is one of the first comparative studies of different composite systems for hydrogen storage and, in addition, the first detailed evaluation of the diffusion kinetics of hydrogen in polymer-based nanoporous composites.
Industry funded research on high performance polymeric powder development for Powder Bed Fusion processes, known as Laser Sintering. Investigations into powder flowability characteristics, particle morphology, particle coalescence and crystallisation kinetics. The CALM centre led by Prof Ghita works closely with the material manufacturers in developing tailored grades for AM processes and specific applications.
Recent advances in 3D printed stainless steel have garnered great interest, but many questions remain about its material properties, due to its inherent variability, and how to guarantee standards for its safety and manufacture. This project is using statistical techniques in conjunction with material science to address these challenges.
This EPSRC funded project will develop a novel system to understand the effects of mechanical loading on the cells, nutrition, and structures of the intervertebral disc, and implement the system to evaluate a novel regenerative therapy developed by collaborating academic Professor Justin Cooper-White from the University of Queensland.
In-vitro models of intervertebral degeneration are valuable tools to understand how structures of the spine change during natural processes such as ageing, and injuries due to overloading. They are also critical in evaluating if new treatments are effective in restoring the biomechanics of the spine. While there are already many different models used in both in-vitro and in-vivo animal models to investigate specific aspects of degeneration or injury, there is still a need for a clinically relevant model, that replicates the degenerative process as a whole.
Building on the long term collaboration between CALM and Victrex, a multimillion partnership over 5 years has been set up in 2018 between the two organisations. The partnership is looking to develop new PAEK grades of materials for additive manufacturing (AM), explore new AM technologies and processes for advanced manufacture. Find out more about this on the Victrex - Exeter Partnership Partnership website.
Exposure to whole-body vibration increases the risk of low back pain, spinal degeneration, and injury. Cycling can expose participants to WBV, but there are limited data available. This project aims to acquire generalisable data about whole-body vibration exposure during cycling, and develop tools to assist in how cyclists can do to minimise their WBV exposure, while still enjoying the many other health benefits that cycling provides.