Arrest of unstable compressive cracks in fibre-reinforced polymers using ply-discontinuities
Topic(s) : Material and Structural Behavior - Simulation & Testing
Co-authors :
Bruno Pereira SANTOS (UNITED KINGDOM), Emile GREENHALGH (UNITED KINGDOM), Silvestre TAVEIRA PINHO (UNITED KINGDOM)Abstract :
Compression performance, particularly in the presence of in-service damage, limits composite design. Not only is the compressive strength of fibre-reinforced polymers significantly inferior to their tensile performance [1,2], but also, if compression failure initiates in a component, there are no means to arrest such fracture. Thus, composites are currently constrained to a 'no-damage growth' design philosophy. Being able to tolerate and manage compression crack growth (‘damage growth tolerance’ philosophy) would enable us to realise the full structural potential for composites.
A key component of this design philosophy are crack-arrest methods, which seek to stop unstable failure growth, isolating damage and extending structural life after an initial failure. Features with these capabilities have already been developed and are widely employed in other fields and materials such as in compressed-gas transmission pipelines [3,4], where ribs are used to contain long tensile cracks in metallic pipes, as well as in airplane fuselages [5], where tear straps are used to stop tear propagation in the composite skin. Nevertheless, little to no study has been done on compression crack-arrest in fibrous composite laminates.
The current work aims to compartmentalize compressive failure in large composite structures by proposing a ply-discontinuity based feature for unstable-crack arrest in multidirectional carbon-fibre reinforced polymer laminates subjected to compression. By replacing 0° oriented plies by off-axis ones, the feature is capable of dispersing and dissipating kink-band propagation energy as well as modifying the laminate’s failure modes. Consequently, the laminate manages to stop fast-growing cracks (1 km/s) and increase the specimen’s ultimate failure strain without external aid. To validate the proposed concept, test specimens were designed and manufactured using IM7/8552 carbon-fibre reinforced epoxy. Through compressive tests, the feature’s crack-arrest capability was shown (Figure 1), and a 15% strain tolerance increase was observed between initial (unstable) and ultimate failures. Strain gauges and digital image correlation were used to evaluate mechanical behaviour (Figure 2). C-Scans, X-rays and optical and scanning electron microscopy are used to study the failed specimens and perform fractographic analysis, providing deeper understanding of the processes through which failure initiates and is subsequently arrested. The analysis results and failure mechanisms’ study are summarized in this paper.
A key component of this design philosophy are crack-arrest methods, which seek to stop unstable failure growth, isolating damage and extending structural life after an initial failure. Features with these capabilities have already been developed and are widely employed in other fields and materials such as in compressed-gas transmission pipelines [3,4], where ribs are used to contain long tensile cracks in metallic pipes, as well as in airplane fuselages [5], where tear straps are used to stop tear propagation in the composite skin. Nevertheless, little to no study has been done on compression crack-arrest in fibrous composite laminates.
The current work aims to compartmentalize compressive failure in large composite structures by proposing a ply-discontinuity based feature for unstable-crack arrest in multidirectional carbon-fibre reinforced polymer laminates subjected to compression. By replacing 0° oriented plies by off-axis ones, the feature is capable of dispersing and dissipating kink-band propagation energy as well as modifying the laminate’s failure modes. Consequently, the laminate manages to stop fast-growing cracks (1 km/s) and increase the specimen’s ultimate failure strain without external aid. To validate the proposed concept, test specimens were designed and manufactured using IM7/8552 carbon-fibre reinforced epoxy. Through compressive tests, the feature’s crack-arrest capability was shown (Figure 1), and a 15% strain tolerance increase was observed between initial (unstable) and ultimate failures. Strain gauges and digital image correlation were used to evaluate mechanical behaviour (Figure 2). C-Scans, X-rays and optical and scanning electron microscopy are used to study the failed specimens and perform fractographic analysis, providing deeper understanding of the processes through which failure initiates and is subsequently arrested. The analysis results and failure mechanisms’ study are summarized in this paper.