Towards Affordable, Closed-Loop Recyclable Future Low Carbon Vehicles Supervisors : Dr. L.T. Harper, Dr. M. Johnson, Prof. N.A. Warrior
Moulding issues with CF/PP Now looking to use CF/PA6 consolidation quality much better Will require further study of CF/PA6 at the interface 12 th March 2014 2
12 th March 2014 3
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Fibre Sizing Matrix Coupling Agent T700SC 60E PP 100-GA12 Nitrogen Purge Samples Total N 31 32 Y 16 17 Y 33 37 Pyrolysis Y 25 30 Solvolysis Y 21 25 PP 576P 60E Y 25 86 Pyrolysis MA G3015 Y 16 47 Solvolysis Y 22 51 60E EP DLS 1776 N 17 67 60E PA6 N9602 75 tests to be performed Sizing removal seemed to have little effect on IFSS for CF/PP Trial on unsized fibre (solvolysis method) Acrylamide reported as coupling agent for CF/PA6 Potential to investigate effect on IFSS Samples req Y 0 25 Polyacrylamide Y 0 25 Solvolysis Y 0 25 Total 181 392 75 12 th March 2014 5
Potential collaboration with Exeter Expertise in Raman Spectroscopy (Prof Eichorn) Has been used extensively to characterise stress-strain distribution in single fibres at microscopic level When crystalline materials deform under load, distance between neighbouring atoms change Bond energies and vibrational frequencies change It is also commonly used to identify chemical bonding Has been used to measure activity of surface groups in AS4/PEEK Applications for TARF-LCV Complement AFM work Detect residual sizing from pyrolysis/solvolysis samples Detect degradation in polymers (strong absorption for carbonyl group in PP) Identify whether chemical bonding occurs in CF/PA6 or CF/PP samples Measure strain distribution in microbond tests (relatively little information currently for CF/thermoplastics) 12 th March 2014 6
THE INFLUENCE OF COUPLING AGENT, FIBRE SIZING AND MATRIX DEGRADATION ON THE INTERFACIAL SHEAR STRENGTH BETWEEN CARBON FIBRE AND POLYPROPYLENE D.T. Burn 1, L.T. Harper* 1, M. Johnson 1, N.A. Warrior 1, L. Yang 2, J. Thomason 2 1 School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, The ITRC Building, University Park, NG7 2RD, UK 2 Department of Mechanical Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, UK *Corresponding Author: lee.harper@nottingham.ac.uk Paper based on microbond work performed at Strathclyde Reporting on influence of: Fibre recovery (sizing removal by pyrolysis) Unsized fibres (sizing removed by solvolysis) AFM study to be included Polymer degradation due to processing Addition of maleic anhydride coupling agent Explanation of IFSS exceeding matrix shear strength needed (shear strength at different pressures, shear strength dependency on maleic anhydride content) Paper primarily concerned with PP Also includes epoxy benchmark 12 th March 2014 7
INTERACTIONS BETWEEN MICRO- AND MESOSCALE INTERFACE CHARACTERISATION TECHNIQUES FOR CARBON FIBRE COMPOSITES D.T. Burn1, L.T. Harper*1, M. Johnson1, N.A. Warrior1, L. Yang2, J. Thomason2 1 School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, The ITRC Building, University Park, NG7 2RD, UK 2 Department of Mechanical Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, UK *Corresponding Author: lee.harper@nottingham.ac.uk Aim: To investigate whether individual fibre interface testing methods correlate with bundle level tests Objectives: Characterise IFSS for CF/PA6 Characterise ILSS for CF/PA6 using the short beam shear method or ±45 tensile tests Investigate contact angle between fibre and matrix to calculate work of adhesion as supporting study AFM and SEM to be used to investigate fibre surface topology Hypothesis: IFSS and ILSS tests should show some correlation, Exact values of shear strength will be different due to scale effects and inherent nature of testing ILSS testing methods influenced largely by void content 8
Aim: To model the micro-droplet test to understand the effects of material and geometrical parameters on the variability of the interfacial shear strength between carbon fibre and a range of matrices Task Completed? Simplified FEA model geometry (axisymmetric, no meniscus) Surface-based cohesive contact behaviour in ABAQUS Effects of interface properties (friction, damage criteria etc.) Effects of thermal preload Damage model for matrix (UMAT) Benchmark against experimental data Effects of model geometry (meniscus, droplet shape etc.) 9
Glass fibre / epoxy material data from literature Thermal preload (Δ100 C) has been applied to introduce residual stresses due to shrinkage 1. Linear regime 2. Progressive debonding 3 3. Catastrophic failure 4. Sliding 2 1 4 Non-zero stress state at interface after thermal pre-load Failure occurs at a lower ultimate force for model with thermal preload Larger frictional force for model with thermal preload due to higher normal forces at interface after debonding. IFSS reduced from 35.5MPa to 31.7MPa (12%) due to thermal preload 10
Stresses shown are at the end of the thermal pre-load step, but before mechanical step Normal Mode I: Opening Thermal stress during cooling results in radial and axial compressive forces in fibre as droplet contracts Droplet is subjected to equivalent tensile forces Classic shear-lag response at the ends of the droplet to satisfy equilibrium Onset of damage is mixed-mode Shear Mode II: In-plane shear 11
No thermal preload With thermal preload Magnitude of stresses suggest that crack propagation dominated by Mode II shear 12
Towards Affordable, Closed-Loop Recyclable Future Low Carbon Vehicles Supervisors : Dr. L.T. Harper, Dr. M. Johnson, Prof. N.A. Warrior
Discontinuous isotropic composites display similar stiffness levels to continuous, but strength is reduced (by up to 50%) Characterise failure evolution through a combination of Destructive and NDT techniques Local Volume Fraction (voids, fibre content) Fibre tow size and bundle length Fibre Ends (Stress Concentrations) Fibre Intersections (Cohesive Interface Properties) Surface A Mid Plane 50mm Surface B 14
Establish whether material variability (voids/local fibre vf) are the source of failure initiation No obvious correlation between high strain levels and local fibre Vf or voids Ultrasound Scan Ultrasound Void Map (Scaled) Density Map DIC Strain Map 15
Establish which fibre interactions are key in failure (Ends / Intersections) CT Scan / Surface Scan / DIC Strain Map Large strain concentrations are evident at fibre bundle ends Highest strains found where bundle end coincides with a transverse fibre Aligned Fibre Transverse Fibre CT Scan 20 Microns Depth Surface Scan DIC Strain Map 16
Surface cracks form at end of aligned bundles which cross transverse fibres Multiple examples found within CT images of failed region throughout sample thickness No void where there is no transverse fibre Aligned Bundle Ends 500x magnification Voids at bundle end after loading 17
Linear elastic model, perfect interface, orientation control of Fibre 1 Matrix given homogenised composite properties (ROM) Studying the effect on stress in the matrix at fibre bundle ends 18
Stress Increases at fibre bundle end crossing transverse fibre (Up to 53 ) This may be conservative, since 10% error due to method of fibre orientation 19
Stress tends towards no Fibre 1 result with increasing gap Effect larger at higher orientations 20
Volume fraction has significant effect on the stress at the fibre end 21
FE Analysis predictions confirmed with known architecture samples 22
1. Create Geometry 2. Define Search Criteria 3. Plot Results Studying the effects of fibre geometry (tow, length) on fibre end stress concentrations 23
THE INITIATION OF FAILURE IN DISCONTINUOUS FIBRE COMPOSITES: IDENTIFYING THE CAUSE OF LOCAL STRAIN CONCENTRATIONS K. Johanson, L.T. Harper*, M. Johnson, N.A. Warrior School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, The ITRC Building, University Park, NG7 2RD, UK *Corresponding Author: lee.harper@nottingham.ac.uk Aim: To identify the cause of local strain concentrations in discontinuous fibre composites, to understand how to improve improving ultimate strength and reduce variability in mechanical performance Objectives: Use a combination of destructive and non-destructive tests to identify the cause of strain concentrations C scanning Micro CT scanning Optical microscopy Digital Image Correlation (DIC) Results: No clear correlation between areal density (C-scan) and strain concentrations No clear correlation between voids (micro CT) and strain concentrations Large strain concentrations observed at bundle ends, particularly where a longitudinal fibre crosses a transverse fibre Cracks initiate at bundle tip and then propagate along bundle boundaries 24
THE INITIATION OF FAILURE IN DISCONTINUOUS FIBRE COMPOSITES: THE INFLUENCE OF BUNDLED FIBRE ARCHITECTURES K. Johanson, L.T. Harper*, M. Johnson, N.A. Warrior School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, The ITRC Building, University Park, NG7 2RD, UK *Corresponding Author: lee.harper@nottingham.ac.uk Aim: To use both analytical and numerical models to understand the influence of key mesostructural parameters on the occurrence and magnitude of strain concentrations in discontinuous fibre composites Objectives: To understand the influence of the following parameters: Fibre length Tow (bundle) filament count Bundle end shape Hybrid fibre architectures (combination of fibre lengths and tow sizes) Fibre orientation distribution Hypotheses: Longer fibre lengths and larger tow sizes will minimise the probability of critical tow intersections Contribution of critical tow intersection on final failure will reduce as tow size reduces, as ratio of tow width to sample width reduces Chopping bundles at an angle may reduce strain concentration effects at bundle end at critical intersections 25