Characterising Composite Microcracking Behaviour Under Load at Cryogenic Temperature
Topic(s) : Special Sessions
Co-authors :
Huw EDWARDS (UNITED KINGDOM), Rebecca CLARK , Byung Chul (eric) KIM (UNITED KINGDOM), Janice M. DULIEU-BARTON (UNITED KINGDOM), Daniel GALPIN , Marcus WALLS-BRUCKAbstract :
Liquid hydrogen is gaining traction as a potential zero carbon fuel across several transport sectors. One of the benefits of hydrogen as a fuel is a high gravimetric energy density, however when considered in a system, the energy density can be significantly reduced by the mass of the hydrogen storage vessel. Conventionally liquid hydrogen is stored in metallic tanks however, up to a 40% reduction in mass had been predicted by changing to carbon fibre reinforced polymer (CFRP) tanks.
Utilising CFRP is not without challenges, as the materials develop thermal stresses due to the differing coefficients of thermal expansion of the fibres and the matrix, and of the plys at different orientations. These thermal stresses can lead to microcracking, triggering leakage and tank failure. As such there is a growing interest in characterising and developing CFRP materials for cryogenic environments including characterising microcracking resistance at such temperatures.
The commonly used approach for testing CFRPs for microcracking resistance in cryogenic conditions is repeated thermal cycling through immersion in liquid nitrogen followed by warming in air, inspecting for microcracks between cycles using optical microscopy [1]. The process is time consuming, especially when with tough materials that may not crack for hundreds of cycles. The method also does not consider the effects of mechanical loading.
The microcrack fracture toughness (MFT) method is a test that has been used at room temperature on CFRPs to generate quantitative values for microcrack fracture toughness that can be used to directly compare materials and for design [2]. It is carried out by applying a tensile load on [0/90]s laminates and observing the microcracking in the transverse lamina using optical microscopy. Through monitoring the load, the applied laminate stress can be plotted against the microcrack density. A variational mechanics approach is then used to generate a values for the microcrack fracture toughness for the material. Some attempts have been made to carry out MFT testing at cryogenic temperatures [3] however none have been carried out using in-situ imaging at cryogenic temperatures. As cracks can close when the specimens are warmed and unloaded, observations at room temperature may be inaccurate.
A novel experimental set up is presented that enables in-situ monitoring of microcrack growth using optical microscopy at cryogenic temperatures in coupons under tensile loading. The approach is demonstrated on two CFRPs types, one with a thermoplastic matrix and one with a thermoset matrix, chosen for different microcracking behaviour. For the purposes of comparison MFT values are obtained at both ambient and cryogenic temperatures. Alongside the new cryogenic MFT method an automated LN2 thermal cycling rig was also developed. The rig enabled comparison of the evolution of cracks under repeated thermal cycling with those from the in-situ cryogenic MFT observations.
Utilising CFRP is not without challenges, as the materials develop thermal stresses due to the differing coefficients of thermal expansion of the fibres and the matrix, and of the plys at different orientations. These thermal stresses can lead to microcracking, triggering leakage and tank failure. As such there is a growing interest in characterising and developing CFRP materials for cryogenic environments including characterising microcracking resistance at such temperatures.
The commonly used approach for testing CFRPs for microcracking resistance in cryogenic conditions is repeated thermal cycling through immersion in liquid nitrogen followed by warming in air, inspecting for microcracks between cycles using optical microscopy [1]. The process is time consuming, especially when with tough materials that may not crack for hundreds of cycles. The method also does not consider the effects of mechanical loading.
The microcrack fracture toughness (MFT) method is a test that has been used at room temperature on CFRPs to generate quantitative values for microcrack fracture toughness that can be used to directly compare materials and for design [2]. It is carried out by applying a tensile load on [0/90]s laminates and observing the microcracking in the transverse lamina using optical microscopy. Through monitoring the load, the applied laminate stress can be plotted against the microcrack density. A variational mechanics approach is then used to generate a values for the microcrack fracture toughness for the material. Some attempts have been made to carry out MFT testing at cryogenic temperatures [3] however none have been carried out using in-situ imaging at cryogenic temperatures. As cracks can close when the specimens are warmed and unloaded, observations at room temperature may be inaccurate.
A novel experimental set up is presented that enables in-situ monitoring of microcrack growth using optical microscopy at cryogenic temperatures in coupons under tensile loading. The approach is demonstrated on two CFRPs types, one with a thermoplastic matrix and one with a thermoset matrix, chosen for different microcracking behaviour. For the purposes of comparison MFT values are obtained at both ambient and cryogenic temperatures. Alongside the new cryogenic MFT method an automated LN2 thermal cycling rig was also developed. The rig enabled comparison of the evolution of cracks under repeated thermal cycling with those from the in-situ cryogenic MFT observations.