Czel, Gergely

Jalalvand, Meisam

Potter, Kevin

Wisnom, Michael

Fragmenting hybrid laminates create a pseudo-ductile response and a striped pattern of fractures and delaminations which can be used as a simple visual strain overload sensor. The mechanism is explained and it is shown how it may be used to detect if a specified strain threshold or a range of different strains have been exceeded. The novel hybrid composite sensor may be used as an indicator patch cocured with composite components or retrofitted/bonded onto the surface of composite or metal components, or be incorporated as an integral load-carrying surface layer with sensing functionality

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                          Wisnom, M., Czel, G., Jalalvand, M., & Potter, K. (2016). Pseudo-DuctileHybrid Composites with Overload Sensing Capability. In Proceedings of theAmerican Society for Composites 2016: Thirty-First Technical Conferenceon Composite Materials [3340] American Society for Composites.Peer reviewed versionLink to publication record in Explore Bristol ResearchPDF-documentThis is the author accepted manuscript (AAM). The final published version (version of record) is available onlinevia Destech Publications at http://dpi-proceedings.com/index.php/asc31/article/view/3340. Please refer to anyapplicable terms of use of the publisher.University of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms                    ABSTRACT   Fragmenting hybrid laminates create a pseudo-ductile response and a striped pattern of fractures and delaminations which can be used as a simple visual strain overload sensor. The mechanism is explained and it is shown how it may be used to detect if a specified strain threshold or a range of different strains have been exceeded. The novel hybrid composite sensor may be used as an indicator patch co-cured with composite components or retrofitted/bonded onto the surface of composite or metal components, or be incorporated as an integral load-carrying surface layer with sensing functionality.   INTRODUCTION  Composite materials have excellent specific strength and stiffness, but a major drawback is their lack of ductility. The HiPerDuCT programme is a collaboration between the University of Bristol and Imperial College to address this challenge by developing new materials and architectures that give a more gradual failure than that of current composites [1]. One of the mechanisms that has been demonstrated to achieve pseudo-ductile response is the fragmentation of thin carbon plies in glass/carbon hybrid composites. Unidirectional carbon plies can be added to unidirectional glass, and if they are thin enough and the ratio of constituents is appropriate the carbon plies will fragment and delaminate stably without catastrophic failure [2].   1                                                             Michael R. Wisnom, Advanced Composites Centre for Innovation and Science, University of Bristol, Queen's Building, BS8 1TR, Bristol, United Kingdom. Gergely Czél, MTA–BME Research Group for Composite Science and Technology, Budapest University of Technology and Economics, Műegyetem rkp. 3. H-1111 Budapest, Hungary and Advanced Composites Centre for Innovation and Science, University of Bristol. Meisam Jalavand, Advanced Composites Centre for Innovation and Science, University of Bristol. Kevin. D. Potter, Advanced Composites Centre for Innovation and Science, University of Bristol.  Pseudo-Ductile Hybrid Composites with Overload Sensing Capability   M. R. WISNOM, G. CZÉL, M. JALAVAND and K. D. POTTER     This creates a plateau on the stress-strain curve analogous to yielding in a metal as shown in Fig. 1. The response can be tailored based on the strain to failure and modulus of the carbon fibres and the absolute and relative thickness of the plies [3]. Because the glass plies are translucent, the fragmentation and associated delamination of the carbon can be seen as a striped pattern on the surface, Fig. 2. As well as providing a pseudo-ductile response, this can be used as an indicator that the damage initiation strain has been exceeded [4].     Figure 1: Pseudo-ductile response of S-glass/high strength carbon hybrid [2]      Figure 2. Striped pattern indicating carbon ply fragmentation triggered by overload in a glass/carbon hybrid specimen [5]   USE OF UD HYBRIDCOMPOSITES AS STRAIN SENSORS  Glass/carbon hybrid composites are black if there is no damage in the hybrid since light penetrating through the glass layers is absorbed by the black carbon fibres. But, if the applied strain exceeds the failure strain of the carbon fibres, they break and local delamination at the carbon /glass layer interface produces a gap between the layers and therefore changes the optical parameters of the material. As a result, the appearance of the hybrid will change from fully black to striped black and bright. Fig. 3 shows this mechanism schematically in a glass/carbon hybrid laminate before and after fracture of the carbon layer. Fig. 4 indicates a specific 0500100015000.0 1.0 2.0 3.0 4.0Extension [%]Stress [MPa]   glass/carbon hybrid before any fracture in the carbon layer where the hybrid is fully black and at increasing extensions where the carbon layer becomes more and more fragmented and the appearance of the hybrid has changed to a striped pattern. This example uses high strength carbon fibres, but there is a whole range of lower strain to failure fibres of carbon and other materials that could be used to modify the threshold strain of the sensor.    Figure 3. Overload sensor mechanism based on failure of carbon fibres embedded in translucent glass layers    Figure 4. Greyscale optical camera images of a glass/carbon hybrid laminate at three different extensions of 1.5% (before carbon layer failure) and 1.8% and 2.38% (beyond carbon failure strain).    The described change of appearance can easily be spotted visually, indicating that the applied strain has exceeded the known value of the carbon fibre failure strain. The type of fibre used can be selected to match the desired level of strain to be detected.   ALTERNATIVE CONFIGURATIONS  The sensor can be based on fragmentation of the hybrids, as described, or alternatively it can be based on a single ply fracture followed by delamination. The    latter approach  provides  a  clear  single  event  when  the  threshold  strain  has  been   Figure 5. Alternative forms of sensor by changing the thickness of the layer    exceeded rather than the progressively increasing fragmentation shown before. These two approaches are contrasted in Fig. 5. The difference in response is controlled by the thickness of the low failure strain layer, which determines the energy release rate and hence the propensity to delaminate. It is also possible to combine strips of different types of fibres placed side by side in order to create sensors that monitor a number of different strain overload thresholds. By introducing dye into the glass layer it is possible to colour code the strips depending on the strain experienced, as illustrated in Fig. 6. The sensor can be a discrete patch that is bonded on the surface of a composite or metal structure, as shown in Fig. 6. It can also cover whole components in the form of a sensing outer layer, which carries load and contributes to structural performance. Example applications could for instance include wrapping a gas pressure cylinder so that in case of an overload, the appearance of the sensor wrap changes, revealing that the specified strain has been exceeded and that the cylinder should be withdrawn from service or repaired/refurbished.  The sensor concept has been developed primarily for tension dominated applications, and most of the work so far has focussed on this type of loading. However it has also been demonstrated that a similar fragmentation mechanism occurs in compression [6], indicating the potential to extend the concept for compressive as well as tensile loading applications.                     Figure 6. Multiple sensors bonded to the surface of component   This overload sensing concept provides a robust and low-cost tool for warning end users that excessive strain has been applied to a component or structure. It does not need any electric power, electronics, wiring, data acquisition or processing device, and can be checked by a very simple visual inspection. There is no need for special training and it can be applied in a broad range of applications and situations. The mechanism relies only on the failure strain of the carbon layers which stays unchanged in a wide range of environmental conditions.    CONCLUSION  It has been demonstrated how pseudo-ductile hybrid composites can be used to detect when specified strain thresholds have been exceeded. The concept can be applied in the form of patches bonded to the surface of composite or metallic components or as a load carrying surface sensing layer, and offers a simple, robust and low-cost method for structural health monitoring.    ACKNOWLEDGEMENTS  This work was funded under the UK Engineering and Physical Sciences Research Council (EPSRC) Programme Grant EP/I02946X/1 on High Performance Ductile Composite Technology in collaboration with Imperial College, London. Gergely Czél acknowledges the Hungarian Academy of Sciences for funding through the Post-Doctoral Researcher Programme fellowship scheme, the János Bolyai scholarship and the Hungarian National Research, Development and    Innovation Office - NKFIH, for funding through grant OTKA K 116070. Supporting data can be requested from the corresponding author. REFERENCES  1. Wisnom MR, Czél G, Fuller JD and Jalalvand M. 2015. “High performance pseudo-ductile composites” presented at. ICCM20, the. 20th International Conference on Composite Materials”. Copenhagen, July 19-24, 2015. 2. Czél G, Jalalvand M and Wisnom MR. 2016. Design and characterization of advanced pseudo-ductile unidirectional thin-ply carbon/epoxy-glass/ epoxy hybrid composites, Composite Structures. 143, 362-370. 3. Jalalvand M, Czél G and Wisnom MR. 2015. Parametric study of failure mechanisms and optimal configurations of pseudo-ductile thin-ply hybrid composites. Composites: Part A. Applied Science and Manufacturing; 74, 123-131.  4. Potter KD, Czél G, Jalalvand M, Wisnom MR, 2015. Overload indication device and method for manufacturing thereof, UK patent application 1520988.5.  5. Czél G, Jalalvand M and Wisnom MR. 2013. Demonstration of pseudo-ductility in high performance glass-epoxy composites by hybridisation with thin-ply carbon prepreg. Composites Part A: Applied Science and Manufacturing, 52, 23-30. 6. Czél G, Jalalvand M and Wisnom MR. 2015. Hybrid specimens eliminating stress concentrations in tensile and compressive testing of unidirectional composites. Composites Part A: Applied Science and Manufacturing, under review. 

Pseudo-Ductile Hybrid Composites with Overload Sensing Capability

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Pseudo-Ductile Hybrid Composites with Overload Sensing Capability

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