Dielectrophoresis (DEP) is a physical effect that generates a force on polarisable particles experiencing a non-homogeneous electric field; studying the effect as a function of frequency allows the determination of the electrical properties of that particle, i.e. the electrical permittivity and conductivity. In the past, DEP-based techniques applied to the measurement of one or several cells at a time have been subject to many sources of noise, which result in an ambiguous or inaccurate result. However, improvements are possible by generating more information from the experiments. In this paper, we present a rapid automated system that measures the DEP spectrum from a large population of cells with a low level of noise using the microwell electrodes, based on a method of analysis that provides additional information about the electrical properties of the cells and a new theoretical approach was developed to obtain accurate, bias-free results in
Huebner Y, Mulhall H, Hoettges KF, Kass GEN, Ogin SL, Hughes MP (2007) Dielectrophoresis: A new in vitro approach to measure drug-induced cytotoxicity, TOXICOLOGY 240 (3) pp. 183-184 ELSEVIER IRELAND LTD
Barton EN, Ogin SL, Thorne AM, Reed GT, Le Page BH (2001) Interaction between optical fibre sensors and matrix cracks in cross-ply GRP laminates - part 1: passive optical fibres, COMPOSITES SCIENCE AND TECHNOLOGY 61 (13) pp. 1863-1869 ELSEVIER SCI LTD
Palaniappan J, Wang H, Ogin SL, Thorne A, Reed GT, Tjin SC, McCartney LN (2006) Prediction of the reflected spectra from chirped fibre Bragg gratings embedded within cracked crossply laminates, MEASUREMENT SCIENCE AND TECHNOLOGY 17 (6) pp. 1609-1614 IOP PUBLISHING LTD
Sirichantra J, Ogin SL, Sofocleous K, Jesson DA, Marom G, Galpaz N (2009) Controlled impact tests on wovenfabric/ epoxy resin CFRP panels with and without additional carbon nanotube reinforcement, ICCM International Conferences on Composite Materials
Plain woven CFRP carbon/epoxy specimens, with and without additional carbon nanotube reinforcement, have been impacted in controlled-impact tests and sectioned subsequently for microstructural investigation. The results suggest that the panels with additional CNT reinforcement have a significantly greater ability to resist impact damage than specimens without CNT reinforcement.
OGIN SL, SMITH PA (1987) A MODEL FOR MATRIX CRACKING IN CROSSPLY LAMINATES, ESA JOURNAL-EUROPEAN SPACE AGENCY 11 (1) pp. 45-60 EUROPEAN SPACE AGENCY
© The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.Shape-memory alloy composites are relatively new materials and their behaviour is not yet completely understood. The purpose of this work is to minimise the effect of impact damage on their structural performance. To do that, GFRP woven fabric reinforced epoxy composite panels, with and without additional superelastic shape-memory alloy wires, have been impacted at constant velocities using a servo-hydraulic testing machine, and a digital video camera has been used to monitor the impact event. Single, multiple and partial penetration impact tests have been carried out, and the energy absorption and damage development are similar in all cases for the same material. The benefit of using the superelastic shape-memory alloy wires was seen only at high displacements and when the volume fraction of the wires was high.
HITCHEN SA, OGIN SL (1993) DAMAGE ACCUMULATION DURING THE FATIGUE OF AN INJECTION MOLDED GLASS NYLON COMPOSITE, COMPOSITES SCIENCE AND TECHNOLOGY 47 (1) pp. 83-89 ELSEVIER SCI LTD
Saavedra MS, Sims GD, McCartney LN, Stolojan V, Anguita JV, Tan YY, Ogin SL, Smith PA, Silva SRP (2012) Catalysing the production of multiple arm carbon octopi nanostructures, Carbon 50 (6) pp. 2141-2146 Elsevier
Octopus-like carbon nanofibres with leg diameters as small as 9 nm are reported, with a high yield over large areas, using a unique photo-thermal chemical vapour deposition system. The branched nature of these nanostructures leads to geometries ideal for increasing the surface area of contacts for many electronic and electrochemical devices. The manufacture of these structures involves a combination of a polyacrylonitrile/polysiloxane film covering the surface of cupronickel catalysts, supported on silicon. Acetylene is used as the carbon feedstock. High-resolution electron microscopy revealed a relationship between the geometry of the nanoparticles and the catalytic growth process, which can be tuned to maximise geometries (and therefore the surface area) and was obtained with a catalyst size of 125 nm. The technique proposed for growing these carbon octopi nanostructures is ideal to facilitate a new in situ transfer film process to place high-density carbon structures on secondary surfaces to produce high capacitance all-carbon contacts.
Crocombe AD, Capell T, Ogin SL, Reed GT, Thorne AM, Palaniappan J, Tjin SC, Mohanty L (2007) The use of thermal mismatch stresses to detect disbond initiation and propagation in metal/composite bonded joints using a CFBG fibre optic sensor,
Ogin SL, Sanderson AR, Capell TF, Tjin SC, Lin B (2010) Damage initiation and growth in laminates with ply drop-offs under quasi-static and fatigue loading,
The thickness of a laminated composite structure can be varied by reducing the number of plies through the thickness of the composite material (i.e. using ply drop-offs), where one or more plies are terminated prematurely at some position within the composite, or at the surface. One of the disadvantages of using ply drop-offs is the likelihood of stress concentrations at the terminations of the dropped plies, as these stress concentrations frequently lead to the premature development of damage.
In this work, composite coupons with a double ply drop-off have been fabricated using out-of-autoclave CFRP. Coupons cut from the panels have been subjected to quasi-static and fatigue loading and the development of damage has been monitored using microscopy, and in some coupons, by using a chirped fibre Bragg grating (CFBG) sensor. The first damage appeared as resin cracks within the resin pocket associated with the ply drop-off and this damage developed subsequently into delaminations. Under quasi-static loading, the delaminations grew stably within the ply drop-off region with increasing load; under fatigue loading, the delaminations grew with a constant growth rate. The paper will present observations on the initiation and growth of the damage.
Smith PA, Ogin SL (2000) Characterization and modelling of matrix cracking in a (0/90)(2s) GFRP laminate loaded in flexure, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES 456 (2003) pp. 2755-2770 ROYAL SOC
Arjyal BP, Galiotis C, Ogin SL, Whattingham RD (1998) Residual strain and Young's modulus determination in cross-ply composites using an embedded aramid fibre strain sensor, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 29 (11) pp. 1363-1369 ELSEVIER SCI LTD
Le Page BH, Manger CIC, Guild FJ, Ogin SL, Smith PA (2002) Modelling effect of layer shift on properties of woven fabric composites, PLASTICS RUBBER AND COMPOSITES 31 (9) pp. 385-391 MANEY PUBLISHING
The ballistic performance of "smart-sized" S2-glass fibre reinforced epoxy was evaluated in comparison with matrix compatible, matrix semi-compatible and matrix incompatible sized materials. The smart size is a formulation designed to give rate-dependent behaviour. The smart-sized material was shown to exhibit rate dependent changes in interlaminar shear strength and mode I interlaminar fracture toughness testing; the ballistic performance of the material was improved only slightly over compatible sized materials. This is attributed to reduced fibre tensile strength in the smart-sized fibres, which was a competing effect, limiting energy absorption during ballistic impact. Overcoming the fibre strength degradations, which appears to be caused by frictional handling effects, is likely to result in significant improvement in ballistic limit. A hybrid laminate consisting of interleaved plies of the compatible and incompatible sizings described above showed synergistic improvements to ballistic performance above what might be expected through rule of mixtures when incompatible sized plies were located towards the rear of the laminate.
Gao F, Boniface L, Ogin SL, Smith PA, Greaves RP (1999) Damage accumulation in woven-fabric CFRP laminates under tensile loading: 2. Modelling the effect of damage on macro-mechanical properties, COMPOSITES SCIENCE AND TECHNOLOGY 59 (1) pp. 137-145 ELSEVIER SCI LTD
A surface-mounted chirped fibre Bragg grating (CFBG) sensor has been used for the first time to monitor delamination growth within a composite material (a transparent, unidirectionally reinforced glass fibre/epoxy resin double-cantilever beam (DCB) specimen). The specimens were tested using a constant displacement rate, with the delamination length being measured using complementary techniques: (i) in situ photography, (ii) surface-mounted strain gauges, and (iii) the surface-mounted CFBG sensors. The unidirectionally reinforced DCB specimens showed characteristics typical of such material which complicate the curvature of the beams, i.e. the development of extensive fibre bridging and pronounced R-curve behaviour. To validate the interpretation of the CFBG reflected spectrum, the experimentally determined strains from the surface-mounted strain gauges have been used, together with in situ photographs of the position of the delamination front. Using the CFBG sensor technique, the delamination length was measured to within about 4 mm over the 60 mm sensor length.
Topal S, Baiocchi L, Crocombe AD, Ogin SL, Potluri P, Withers PJ, Quaresimin M, Smith PA, Poole MC, Bogdanovich AE (2015) Late-stage fatigue damage in a 3D orthogonal non-crimp woven composite: An experimental and numerical study, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 79 pp. 155-163
ELSEVIER SCI LTD
Balhi N, Vrellos N, Drinkwater BW, Guild FJ, Ogin SL, Smith PA (2006) Intra-laminar cracking in CFRP laminates: observations and modelling, JOURNAL OF MATERIALS SCIENCE 41 (20) pp. 6599-6609 SPRINGER
Tong J, Guild FJ, Ogin SL, Smith PA (1997) On matrix crack growth in quasi-isotropic laminates - I. Experimental investigation, COMPOSITES SCIENCE AND TECHNOLOGY 57 (11) pp. 1527-1535 ELSEVIER SCI LTD
BONIFACE L, OGIN SL, SMITH PA (1990) COMPLIANCE CRACK LENGTH MEASUREMENTS FOR INDIVIDUAL TRANSVERSE PLY CRACKS IN MODEL ARRAYS, DEVELOPMENTS IN THE SCIENCE AND TECHNOLOGY OF COMPOSITE MATERIALS pp. 913-918 ELSEVIER APPL SCI PUBL LTD
Capell TF, Ogin SL, Thorne AM, Reed GT, Crocombe AD, Tjin SC, Lin B (2010) Monitoring delaminations in ENF specimens using chirped fibre Bragg grating sensors,
A chirped fibre Bragg grating (CFBG) optical sensor has been used to detect delamination growth in an ENF specimen. CFBG sensors, with a sensor length of 60 mm, have been embedded within ENF specimens fabricated from out-of-autoclave CFRP. Delaminations have been propagated within the specimen from an insert embedded at the specimen midplane during manufacture. The sensor has been interrogated with the specimen subjected to four-point loading, and the perturbations detected within the reflected spectra of the sensor are in reasonable agreement with the physical position of the delaminations measured at the coupon edges. The prediction of the reflected spectrum for one delamination length, using a combination of 3-D finite-element analysis and commercial software for predicting FBG spectra, is in qualitative agreement with the experimental results. Better quantitative agreement is expected to require a more refined finite-element model of the ENF specimen.
Arjyal BP, Galiotis C, Ogin SL, Whattingham RD (1998) Monitoring local strains in cracked cross-ply composites using an embedded aramid fibre strain sensor, JOURNAL OF MATERIALS SCIENCE 33 (11) pp. 2745-2750 KLUWER ACADEMIC PUBL
PeakForceTM quantitative nanomechanical mapping (QNMTM) is a new atomic force microscopy technique for measuring Young's modulus of materials with high spatial resolution and surface sensitivity by probing at the nanoscale. In this work, modulus results from PeakForce" QNM" using three different probes are presented for a number of different polymers with a range of Young's moduli that were measured independently by instrumented (nano) indentation testing (IIT). The results from the diamond and silicon AFM probes were consistent and in reasonable agreement with IIT values for the majority of samples. It is concluded that the technique is complementary to IIT; calibration requirements and potential improvements to the technique are discussed.
Belmonte HMS, Ogin SL, Smith PA, Lewin R (2004) A physically-based model for the notched strength of woven quasi-isotropic CFRP laminates, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 35 (7-8) pp. 763-778 ELSEVIER SCI LTD
Capell TF, Palaniappan J, Ogin SL, Thorne AM, Reed GT, Crocombe AD, Tjin SC, Wang Y, Guo Y (2009) Detection of defects in as manufactured GFRP-GFRP and CFRP-CFRP composite bonded joints using chirped fibre Bragg grating sensors, PLASTICS RUBBER AND COMPOSITES 38 (2-4) pp. 138-145 MANEY PUBLISHING
Ogin SL, Spendley PR, Smith PA, Clarke AB (2010) The relationship between the strength distribution of unnotched and notched composite laminates as a function of test condition,
Simple analytical models are applied in order to explore the relationship between notched and unnotched strength of CFRP laminates under a range of test conditions ? cold dry, room temperature, and hot/wet. The methodology adopted makes use of a stress-based approach for predicting damage growth, in conjunction with a fracture mechanics model for ultimate failure/fracture. Applying the resulting model to the experimental data enables the fracture toughness of the test material to be determined as a function of test condition. The model is also able to demonstrate that at any particular test condition, a given variability in unnotched strength data translates into a smaller variability in the associated notched strength data.
Mulligan DR, Ogin SL, Smith PA, Wells GM, Worrall CM (2003) Fibre-bundling in a short-fibre composite: 1. Review of literature and development of a method for controlling the degree of bundling, COMPOSITES SCIENCE AND TECHNOLOGY 63 (5) PII S0266-3538(02)00259-2 pp. 715-725 ELSEVIER SCI LTD
Gao F, Boniface L, Ogin SL, Smith PA, Greaves RP (1999) Damage accumulation in woven-fabric CFRP laminates under tensile loading: Part 1. Observations of damage accumulation, COMPOSITES SCIENCE AND TECHNOLOGY 59 (1) pp. 123-136 ELSEVIER SCI LTD
HITCHEN SA, OGIN SL, SMITH PA (1995) EFFECT OF FIBER LENGTH ON FATIGUE OF SHORT CARBON-FIBER EPOXY COMPOSITE, COMPOSITES 26 (4) pp. 303-308 BUTTERWORTH-HEINEMANN LTD
OGIN SL, SMITH PA, BEAUMONT PWR (1985) MATRIX CRACKING AND STIFFNESS REDUCTION DURING THE FATIGUE OF A (0/90)S GFRP LAMINATE, COMPOSITES SCIENCE AND TECHNOLOGY 22 (1) pp. 23-31 ELSEVIER SCI LTD
Polarimetric optical fibre sensors have been embedded within the 0° ply and close to the 0°/90° interface of transparent cross-ply GFRP coupons. The coupons have been subjected to an increasing quasi-static load so that transverse ply cracks initiate and propagate across the coupon. Crack accumulation has been monitored using the optical output signal from the polarimetric sensor, strain measurements from a long gauge-length extensometer, load recordings during the test and by video recording of crack development. These combined observations have enabled a direct correlation to be made between matrix crack growth past the sensor and a step-change in the sensor response. The use of band-pass FFT filtering has demonstrated that such cracks could be detected in real time. © 2005 Elsevier Ltd. All rights reserved.
Barton EN, Ogin SL, Thorne AM, Reed GT (2002) Optimisation of the coating of a fibre optical sensor embedded in a cross-ply GFRP laminate, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 33 (1) pp. 27-34 ELSEVIER SCI LTD
Tong J, Guild FJ, Ogin SL, Smith PA (1997) On matrix crack growth in quasi-isotropic laminates - II. Finite element analysis, COMPOSITES SCIENCE AND TECHNOLOGY 57 (11) pp. 1537-1545 ELSEVIER SCI LTD
OGIN SL, SMITH PA (1985) FAST FRACTURE AND FATIGUE GROWTH OF TRANSVERSE PLY CRACKS IN COMPOSITE LAMINATES, SCRIPTA METALLURGICA 19 (6) pp. 779-784 PERGAMON-ELSEVIER SCIENCE LTD
BONIFACE L, OGIN SL, SMITH PA (1991) EFFECT OF TRANSVERSE CRACKS ON LOCAL STRAIN DISTRIBUTION IN CROSSPLY COMPOSITE LAMINATES, FIRST INTERNATIONAL CONFERENCE ON DEFORMATION AND FRACTURE OF COMPOSITES pp. 160-165 PLASTICS & RUBBER INST
This paper presents an experimental study which examines the tensile and compressive response of a quasi-isotropic carbon fibre reinforced polymer (CFRP) laminate subjected to ambient conditions and environmental extremes associated with aircraft design. Specimens with and without holes are considered within the context of structural features and associated design allowables. The data obtained, relating to the effect of sample replicates and specimen geometry on the variability in failure strength, support the concept of a reduced qualification test programme for CFRP. © Institute of Materials, Minerals and Mining 2009.
Chirped fibre Bragg grating (CFBG) sensors embedded within composite materials have been shown to be able to monitor delamination growth in adhesively bonded single-lap joints, whether the sensors are embedded within a composite adherend or within the adhesive bondline itself. The relative ease of interpretation of CFBG reflected spectra with regard to delamination growth is a consequence of the relationship between the spectral bandwidth of the reflected spectrum (typically 20 nm) and physical locations along the sensor length (typically 60 mm). When the sensor is embedded in, or bonded to, a composite material subjected to a tensile uniform strain, all the grating spacings are increased and the entire spectrum shifts to higher wavelengths-just as for a uniform FBG sensor. However, if the strain field is perturbed by damage in the composite (such as a matrix crack or a delamination), so that the smooth linear increase in the grating spacing is disrupted, then a perturbation appears in the reflected spectrum that can be used to determine the physical location of the damage. In this paper, results on monitoring delamination/disbond growth will be discussed with regard to sensor location, together with the possibility of using the sensors to monitor repaired composite structures. © 2012 Taylor & Francis Group, London.
GUILD FJ, OGIN SL, SMITH PA (1993) MODELING OF 90-DEGREES PLY CRACKING IN CROSSPLY LAMINATES, INCLUDING 3-DIMENSIONAL EFFECTS, JOURNAL OF COMPOSITE MATERIALS 27 (7) pp. 646-667 TECHNOMIC PUBL CO INC
Ogin SL, Sanderson AR, Crocombe AD, Gower MRL, Lee RJ, Tjin SC, Lin B (2011) Monitoring crack growth in a DCB test using a surface-bonded chirped FBG sensor,
Delamination growth within transparent, unidirectionally reinforced glass fibre/epoxy doublecantilever beam (DCB) specimens has been monitored using a surface-mounted chirped fibre Bragg grating (CFBG) sensor. The specimens were tested using a constant displacement rate, with the delamination length being measured using (i) surface-mounted CFBG sensors, (ii) in situ photography, and (iii) surface-mounted strain gauges. The DCB specimens showed characteristics typical of such material, with the development of extensive fibre bridging and pronounced R-curve behaviour. A distinct perturbation in the reflected spectra, when monitoring the delamination growth using the CFBG sensors, was indicative of the location of the delamination front during the test. The in situ photographs and the surface strain measurements provided by the strain gauges were used to predict the experimentally determined CFBG reflected spectra, with good agreement between prediction and experiment. These results enabled the delamination length to be measured using the CFBG sensor technique to within 4 mm over the 60 mm sensor length.
Guild FJ, Balhi N, Vrellos N, Ogin SL, Smith PA (2007) Matrix cracking in CFRP laminates, ICCM International Conferences on Composite Materials
Matrix ply cracking is the most common damage to form when a laminate is loaded, and is of considerable significance for the integrity of a composite structure. The overall aim of the present work is to provide validated constitutive relations for crack accumulation in off-axis plies under mixed mode loading. The results presented in this paper include experimental investigations to describe the development of the cracking and the development of finite element-based models of cracked laminates. The effect of matrix cracking on the residual stiffness of various laminates is determined both experimentally and using finite element simulation. The ratio of modes in different angle ply laminates and the associated criteria for matrix crack initiation are explored.
HITCHEN SA, OGIN SL (1993) MATRIX CRACKING AND MODULUS REDUCTION DURING THE FATIGUE OF AN INJECTION-MOLDED GLASS NYLON COMPOSITE, COMPOSITES SCIENCE AND TECHNOLOGY 47 (3) pp. 239-244 ELSEVIER SCI LTD
Bassam F, Boniface L, Jones K, Ogin SL (1998) On the behaviour of the residual strain produced by matrix cracking in cross-ply laminates, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING 29 (11) pp. 1425-1432 ELSEVIER SCI LTD
During the quasi-static indentation of thin composite panels, well-defined flaps (sometimes called ??petals??)
can develop on the exit face as a consequence of through-thickness penetration of the panel; such
flaps can also be seen in impact tests. The flaps develop as four triangles, with the apex of each triangle at
the point of impact. In this work, thin panels of CFRP with a 0/90 configuration have been subjected to
quasi-static indentation tests and the development of the flaps has been monitored. The results show that
the dependence of the flap compliance is proportional to the square of the flap length, which is in agreement
with theoretical predictions. The determination of the compliance/crack-length relationship
enables a toughness value for fracture of the composite panel to be derived that is directly relevant to
through-thickness penetration of the panel.
Recent work has shown that a simple rule-of-mixtures approach may be used to predict the stress strain behaviour of a cross-ply metal matrix composite (MMC) laminate. However, the low-strain behaviour was not predicted accurately, probably because thermal residual stresses are obviously not included in such an approach. To increase the understanding of the limitations of the rule-of-mixtures approach for predicting the stress-strain response, the residual strain-state of the fibre reinforcement has been determined using an etching technique (henceforth referred to as the ?total etch removal method?), and results have been compared both with finite element modelling and with thermal residual strain measurements derived from stress-strain curves. The results show that the residual strain distribution in a cross-ply composite may be more complex than previously thought, with the fibres in internal 00 plies having considerably higher thermal residual strains than fibres in external plies. The results confirm that the rule-of-mixtures approximation can be used, with some reservations with regard to the low strain behaviour.
Three measurement techniques used to measure the glass transition temperature (Tg) have been subjected to a critical comparison; dynamic mechanical analysis (DMA), thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). A new procedure, whereby different specimens are tested over a range of heating rates, has been used in order to eliminate the effects of thermal lag and determine a Tg independent of heating rate (Tg(0)). It has been shown that for measurements of Tg(0) for composites, the DMA thermal lag ?corrected? method gave the most reliable data. The work has provided additional guidance on these techniques that could usefully be incorporated in future standards, to improve precision, comparisons and consistency of Tg measurement.
This paper presents an experimental and numerical study on the structural health monitoring of composite patch repairs using chirped fibre Bragg grating (CFBG) sensors. The repair consisted of bonding a pre-cured patch of the same, but thinner, material to a parent GFRP panel containing a machined hole, with the sensors embedded in the bond-line. The repaired coupons were subjected to four-point flexural loading in fatigue. Bond-line delamination cracks (disbonds) initiated at both ends of the patch repair and grew towards the centre of the repair. Predictions of the strain distributions enabled the reflected spectra obtained during damage growth to be predicted with good agreement between theory and experiment. For practical applications when using CFBG sensors to monitor patch repairs, the results suggest that two sensors should be included within the bond-line so that both vulnerable edges of the repair can be monitored using the low-wavelength end of a sensor.
Continuous reinforcement metal matrix composites (MMCs) have yet to become widely used within
the aerospace industry. Despite the high stiffness and strength of unidirectional MMCs, the
complexity and inherent cost of manufacture are major factors that have prevented the widespread
uptake of these advanced materials.
Furthermore, the relatively poor mechanical performance of unidirectional MMCs subjected to
transverse loads has largely precluded their use in structures which experience complex loading
(with the notable exception of bladed compressor discs in gas turbines). Recently, TISICS has been involved in projects with two major aerospace companies to investigate the use of titanium and aluminium matrix composites. In both projects it was necessary to examine the properties of less conventional cross-ply composites and the work undertaken by the author in conjunction with these projects forms the basis of this EngD thesis.
A range of MMC layups were tested in tension, compression and shear and detailed microscopy was
undertaken to investigate the failure processes. A simple rule-of-mixtures approach was found to
show good agreement with the unidirectional tensile test results. Subsequently, a more complex
approach taking ?weighted? averages for biaxial and multiaxial laminates at a ply level was developed
and this enabled full tensile response of the composites to be modelled, albeit with some limitations
in low strain response.
Part of the work which was conducted to increase the understanding of cross-ply composites
involved the determination of residual stresses. Although cross-ply MMCs were thought to have
greater axial residual stresses than unidirectional MMCs, the variation of residual stresses
throughout the eight ply laminates was found to change significantly (from 0.37% in the innermost
to 0.23% in the outermost plies) depending on the sub-laminate sequence. Finite element modelling
predictions were compared to the experimental residual stress determination techniques and it was
found that the FE modelling underestimated the strains in cross-ply composites.
Both aluminium and titanium pressure vessels were manufactured and tested; the mechanical data
from pressurisation was compared to simple analytical modelling and macroscale FEA.
Metallographic samples studied from the failed vessels showed deficiencies in manufacture and a
rudimentary form of acoustic emission showed that damage was incurred at relatively low strains,
but did not propagate to failure during the proof test cycles. This suggests that with adequate
provisions, the development of damage within a cross-ply layup does not need to rule out its use in a
non-critical tank application.
Measurement of the degree of cure of composite materials is vital to both research and manufacture of these materials. The glass transition temperature (Tg) is a measurable material property that can be used as an indicator of the degree of cure. The three most common thermal analysis techniques used to measure Tg are DMA, TMA and DSC (i.e. dynamic mechanical analysis, thermomechanical analysis and differential scanning calorimetry). There is a current need to improve the experimental methods and analysis of data when using these techniques, where issues such as thermal lag can negatively impact data precision. In this work, a method using multiple tests at different heating rates has been applied to these three techniques to eliminate the effect of thermal lag as well as assess other variables that can influence test data; specimen moisture condition, specimen thickness and fibre type. It was shown that while thermal lag can be accounted for, there are remaining slight differences between DMA, TMA and DSC Tg data, which can be expected due to the different response modes involved (e.g. mechanical, thermal expansion, calorimetric). For DMA testing, a simple relationship has been proposed, relating heating rate and specimen thickness, which can account for the effect of thermal lag when comparing data obtained for specimens of different thicknesses or for the same thickness at different heating rates; the relationship is supported by relevant experimental evidence.
It was shown for materials with different degrees of cure that the relationship between Tg and degree of cure followed the same trend regardless of differences in Tg measured by the three techniques. Preliminary experiments indicated that FTIR showed promise for measurement of the degree of cure of composite materials, in addition to measurements by DSC.
A number of studies have been conducted on 3D non-crimp orthogonal woven composites, but their industrial application is still in its infancy. 3D woven composites show increased through-thickness strength, reducing delamination damage, which is often a key failure mechanism for composites under various loading conditions, especially fatigue. This work investigates the fatigue performance and damage development in a 3D non-crimp orthogonal woven composite consisting of three weft tow layers, two warp tow layers, and a through-thickness z-binder that interlaces along the warp-direction. While the properties of carbon fibres are generally superior to glass fibres, they are more expensive. Therefore, it is of interest to see if the fatigue performance of a glass fibre 3D orthogonal weave can be improved via selective hybridisation using a small amount of carbon fibres.
Initial work began on a commercial all-glass 3D orthogonal weave called 3D-78, which was produced by 3TEX. It was found that quasi-static tensile mechanical properties were the same for both warp and weft loading directions, but when loaded in tension-tension fatigue, the warp direction had longer fatigue lifetimes than the weft-direction. The crack density was lower in warp-direction specimens as a result of greater micro-delamination growth blunting stress concentrations around the tips of matrix cracks. The micro-delamination damage in warp-direction fatigue specimens showed a shield-like shape (not previously observed), i.e. wider along one side and narrowing to a point on the other side; where delamination was restricted (at the pointed end), fibre fractures occurred in the adjacent warp tow. The pointed portion of the micro-delamination corresponded to proximity to a z-binder crown. Other damage that was common to both loading directions (warp and weft) included: transverse cracks in transverse tow and resin-rich regions, z-binder debonding, and longitudinal tow splitting cracks. No obvious failure sites were noted for weft-direction fatigue loading.
The second material used, 3DMG, was manufactured by the University of Manchester. This material was produced with two different z-binder tensions. The initial z-binder tension (3DMG-T1) resulted in a higher tensile modulus and strength-to-failure, and lower strain-to-failure, for the warp-direction, while the tensile fatigue properties of both directions were similar. Increasing the z-binder tension (3DMG-T2) reduced the tensile modulus and increased the strain-to-failure of the warp-direction, with these properties now similar in both loading directions; the tensile strength for both loading directions remained similar. However, the fatigue performance of the warp-direction was observed to increase with increased z-binder tension, while the weft-direction remained the same. The damage that developed in both materials was similar to the damage in 3D-78, and remained practically the same regardless of z-binder tension, though the energy dissipated per cycle for warp-direction specimens was higher in 3DMG-T1, which corresponds well with the lower number of cycles to failure.
The final material tested was a University of Manchester hybrid 3D non-crimp orthogonal woven composite, termed 3DMHyb; here the glass fibre z-binder was replaced with carbon fibre; the z-binder tension used here was the same as 3DMG-T2. Generally, the quasi-static properties of this hybrid material were similar in both loading directions, with the exception of the tensile modulus which was approximately 10% higher, indicating that the carbon fibre z-binder may influence low strain properties. Additionally, the properties of 3DMHyb remained similar to 3DMG-T2. For fatigue performance, However, the fatigue lifetime to failure appeared to increase by a factor of just over 2 at lower peak stress/initial peak strains for the hybrid warp-direction specimens. Again, the energy dissipation per cycle was lower for specimens that had larger number of cy
Armour which is manufactured and distributed for personnel, vehicle and structural protection, primarily for military or policing applications, undergoes stringent testing to ensure that it can meet the demands of a range of impact scenarios. However, the effects of repetitive low-level damage are not fully understood and, in order to maintain a given level of protection, armour is recalled and replaced periodically, which is costly, and may be unnecessary. This paper reports preliminary studies on the relationship between minor damage and the resulting impact resistance of a woven fabric reinforced composite laminate (E-glass with epoxy resin). Specimens were subjected to displacement-controlled fatigue tests to introduce dispersed damage before being subjected to quasi-static indentation testing. The results showed that during penetration of the specimens, the peak load was reduced by approximately 10% for the pre-fatigued specimens, compared to the non-fatigued specimens, and there was some indication the energy absorption also reduced. It is proposed that the development of fibre fractures during the pre-fatigue of the specimens is the origin of these changes.
This paper outlines preliminary work developing graphene modified thermoplastic inserts to be used for the toughening of CFRP. The paper outlines laminate
manufacture, mechanical testing and fracture analysis of graphene modified CFRP.
The choice of the materials used for the core and skin of a sandwich structure plays an extremely important role in the skin-core interfacial behaviour. In
this paper, three PMI foams are used as core material and the effect of foam type in the skin-core interfacial response is examined.
The range of applications for composite materials is growing, but understanding of the effect of defects is limited, as is the ability to detect them. Versatile non-destructive testing (NDT) techniques, that can be deployed rapidly and reliably, to detect and monitor damage in composite components are vital to the continued growth of this sector.
This research investigates the application of Digital Image Correlation (DIC) as an NDT technique for monitoring delamination defects in Fibre Reinforced Polymer (FRP) composites. The research has matched physical experiments with Finite Element (FE) modelling. Four types of glass-fibre reinforced epoxy matrix composite structural elements were designed and manufactured to assess this application of DIC. The first two types of structural elements were flat coupons containing fully embedded delaminations, artificially introduced using two different methods. It was found that by placing these specimens in three-point bending, near surface delaminations, one ply below the surface being monitored, would cause a plateau in the surface strains. This plateau in strains was used to measure the embedded defect sizes. The size of the delamination was consistently overestimated from the interpretation of the strain fields. This was improved with the assistance of FE modelling to identify the relationship between the feature and the delamination. Pulse thermography was found to be a better technique for measuring the size of these defects.
The third type of specimen was a flat coupon containing a milled-slot, which was fatigued to grow a delamination at the foot of the milled slot, and the delamination measured visually. For this specimen, the DIC results showed good correlation with the visually determined delamination lengths with an empirical fit applied to the strain results. Both lock-in thermography and pulse thermography were used to measure the delamination size of the same specimens and showed reasonable correlation with the visually determined delamination lengths.
Finally, tubular specimens containing embedded PTFE delamination-defects were fatigued at different ratios of tension and torsion. DIC of the specimens loaded at the fatigue load ratio at which the delaminations were grown could not be used to quantify the size of the delaminations.
The work has shown that DIC can be used to monitor delaminations in some structural elements, however the type of loading needs to be considered to ensure sufficient influence on the surface strains to enable strain features that can be used to measure the size of delamination
This paper considers the effects of an atmospheric plasma treatment (APT) on the surface properties of an amine cured carbon fibre/epoxy resin composite, and how those effects manifest over time. In particular, the ability of the APT to remove a thin layer of silicone-containing, proprietary, release agent (Chemlease® 41 EZ), typically used in the production of composite components, has been investigated. It was concluded that the reduction in water contact angle (WCA) after APT for both the solvent wiped and contaminated surfaces was as a result of an increase in oxygen containing species at the surface, as determined through X-ray photoelectron spectroscopy (XPS). Further, it was found that the APT slightly reduced the failure strength of lap shear specimens for solvent wiped surfaces, whereas an increase in failure strength was observed for silicone contaminated samples. WCA and XPS results suggest that the contaminant layer was not removed, but instead transformed to a more stable inorganic form.
Debonding between the skins and the core in a sandwich structure is a critical failure mode in automotive applications; once debonding occurs, the load carrying capacity of a sandwich structure drastically decreases. In the present paper, the effect of using three core materials, with different cell characteristics, on the interfacial strength between the foam cores and a CFRP skin is investigated through mechanical testing and numerical modelling. A key finding is that foams with a coarse cellular structure favour a high resin uptake at the interface during the manufacturing process, which results in a stronger interfacial bonding between the foam and the CFRP. During Mode I loading, the thick resin layer at the interface postpones crack initiation and kinking in the core, whilst under Mode II, this resin layer delays the collapse in compression of foam cells under the crack tip. Thus, the importance of including this thick resin layer in the FE modelling was demonstrated. Finally, as the CZM was shown to be unable to predict the unstable crack propagation within the core, an alternative approach was suggested which has the significant benefit of not requiring experimental testing of the interface between the skin and the core.
Debonding between the skins and the core in a sandwich structure is a critical failure mode in automotive applications where the energy absorption capability is a key property for crashworthiness. Once debonding occurs, the load carrying capacity of a sandwich structure, and therefore its energy absorption capability, drastically decreases. Debonding between a Carbon Fibre Reinforced Polymer (CFRP) skin and a foam core depends on many factors, including skin layup, foam material properties and skin-core thickness ratio.
This research investigates the effect of using various core materials, with different cell characteristics, on the interfacial strength between a cellular foam and a CFRP skin. A key finding was that foams with a coarse cellular structure favoured a high resin uptake at the interface during the manufacturing process. The role of this resin layer was well-defined with the experimental tests, where it was shown to postpone crack initiation and kinking on samples subjected to Mode I loading and to delay the collapse of foam cells under the crack tip in compression on samples under Mode II loading.
The research has matched physical experiments with Finite Element (FE) modelling. Initially, the work presented in this thesis focused on gaining a deep understanding of two of the most used numerical techniques to model interfacial failure: the Cohesive Zone Method (CZM) and the Virtual Crack Closure Technique (VCCT). However, limitations of these two techniques were found when modelling skin-core debonding in sandwich specimens. Experimentally, debonding in a foam-cored pre-cracked specimen was seen to initiate at the interface but to propagate within the foam core, growing in an unstable manner under both Mode I and Mode II loading. Both CZM and VCCT were not capable of capturing the stick-slip behaviour resulting from the unstable crack propagation. Therefore, a new method was suggested in order to accurately simulate debonding; this included the modelling of foam failure with an element deletion mechanism.
This was the first time a failure criterion-based method was used to model debonding in foam cored sandwich specimen. The suggested modelling technique was also validated comparing the FE predictions to the experimental results obtained performing three-point bending tests on sandwich panels with three different configurations. As a result, the work has shown that debonding in foam-cored sandwich structures can be modelled as foam failure instead of as interface failure, decreasing the computational time and the pre-processing efforts.
Composite airframes are designed to be lightweight and robust. In order to avoid the regions of enhanced stress around mechanical fasteners, composite airframes also make use of adhesives to bond components. In doing so, the stress concentrations arising from joining are distributed over a larger area. When joining composites with adhesives, it is important to consider the surface condition prior to bonding; any contaminant present at the surface may result in a reduction in performance. If contaminants are present, they must be appropriately removed before the adhesive is applied and this is one major area of interest in this work. A second major area relates to understanding how bonded composite structures perform under loading. Advancing up the design tree, mechanical testing becomes more expensive due to the costs of specimens and testing, and thus there is an onus to produce finite element (FE) models that can accurately predict how these structures may perform. FE models require experimental testing to be undertaken for model validation, and the mechanical testing of such a structural element and its behaviour in relation to an FE model of the test specimen is the second major area of this work.
With regard to the characterisation of surfaces to be bonded, two aspects were considered. The first aspect considered a limited investigation into two surface inspection techniques for pre-bond qualification that were considered to show potential for automation. Whilst both techniques were able to detect a release agent contaminant on all surfaces tested, both techniques were unable to discern a hydraulic oil or barrier cream contaminant on peel-ply finished surfaces; it was therefore concluded that further development, beyond the scope of this thesis, was required before they could be integrated into a production line.
The second aspect of work on surfaces to be bonded considered the atmospheric plasma treatment (APT) of a carbon fibre/epoxy resin composite surface. The removal of a silicone-based release agent using APT, characterised using surface analytical techniques, was coupled with adhesive bond strength measurements. Failure surfaces of adhesively bonded lap-shear joints that had been bonded at various intervals in time after treatment were also investigated. It was concluded that the contaminant was not removed, but rather converted to a more stable silica form.
The work on the bonded composite structure investigated the mechanical testing to failure of adhesively bonded composite T-Joint structural elements. The full-field strain contours exhibited by the T-Joints during the load cycle were obtained through digital image correlation and compared to the predictions of an existing finite element model. Good agreement was found between the model predictions and the measured strains. Regions of high strain indicated by both the digital image correlation results and the finite element analysis were indicative of potential failure initiation sites in the fractured specimens and there was some evidence of these initiation sites on the specimen fracture surfaces.
A need for light-weighting in the automotive industry is leading to more widespread adoption of composite materials. The high cost of these materials, however, is leading to a search for lower cost solutions. Carbon fibre-reinforced sheet moulding compound (C-SMC) is proposed as a possible cost-effective solution but has several barriers to use; namely warpage of components, and high variability of mechanical properties. This thesis investigated possible factors affecting the distortion of components and found that the distortion, measured by a laser scanner on a ROMER arm, was related to the orientation distribution of the reinforcement affected by the flow, characterised by X-ray computed tomography. It was found that use of cooling jigs could control this warpage to produce in-tolerance components. These findings have already been implemented by the sponsor company to enable use of this material in multiple products.
The effects of coupon width, thickness, and mould coverage on apparent tensile strength and Young?s modulus were investigated. This found that strength increases when the coupon width reaches twice the length of the reinforcement. The strength was found to increase with thickness according to a logarithmic relationship, with non-significant differences occurring after the thickness had reached 4 mm thickness. The strength was found to decrease with increasing mould coverage for the mould coverages tested. The modulus was found to not correlate to coupon width, although there was a decrease in variation at 50 mm width. Similarly, the Young?s modulus was not found to change with increasing thickness, but the variance did decrease. The Young?s modulus increased with decreasing mould coverage. This allowed definition of the geometry of a tensile test coupon to increase the confidence in the determined mechanical properties for future test programmes.