A METHOD TO DETECT PROGRESSIVE MATRIX CRACKING USING X-RAY COMPUTED MICROTOMOGRAPHY (μCT) UNDER STATIC LOADING FOR CRYOGENIC TEMPERATURES
Topic(s) : Experimental techniques
Co-authors :
Mayerlin SALGADO (SPAIN), Jose Manuel GUERRERO (SPAIN), Jordi RENARTAbstract :
Recent investigations[1], [2] have highlighted the use of liquid hydrogen (LH2) as a fossil fuel alternative in the aeronautical industry, because its combustion does not generate carbon dioxide (CO2) emissions and presents excellent energy mass density. Nevertheless, current pressure vessels are not designed to fulfil the high storage requirements in the aeronautical sector. In seeking a suitable material for LH2 tanks, Carbon Fibre Reinforced Polymer (CFRP) composites are believed to be good candidates because they present enhanced stiffness and strength with decreasing temperature [2], [3]. However, there is a lack of understanding of their performance at low cryogenic temperatures (under 20 K) and under thermo-mechanical loading.
Regarding the design of the tank, not only the ultimate values of mechanical properties are a concern, but also other aspects are required, such as the capability of the tank wall to prevent hydrogen leakage to the outside. In this regard, a recent investigation by Eberman et al. [4] concluded that matrix cracking in a composite pressure vessel could directly affect leak tightness, even if the crack density is minimal. Besides, as hydrogen molecules are tiny, they can permeate through the tank wall. In addition, matrix cracking can lead to damage modes such as fibre breakage and delamination, which could trigger composite failure [5] and reduce the permeability of the hydrogen tank [4].
This research evaluates the progression of transverse matrix cracking in cross-ply [0/90] CFRP laminates under static loading with two different stacking sequences, 〖[0/90/0_2/90_2]〗_s and 〖[90/0/90_2/0_2]〗_s, through image processing methods from images captured by X-ray tomography. The two stacking sequences are evaluated to find the crack density in the transverse plies presented in the specimens. The method consists of extracting the position from X-ray images and counting the number of matrix cracks in each ply through the width using an in-house routine developed in the free software ImageJ. In this way, a crack density can be measured and related to a strain value and matrix cracking damage in the specimen.
The results show that X-ray tomography is a powerful non-destructive technique to inspect matrix cracking in CFRP specimens, locate it, and count it. Thus, it can be used as a structural health monitoring tool in applications where damage is not visible to the naked eye. Besides, X-ray tomography allows a 3D reconstruction of a specimen where it is possible to differentiate the damage mechanisms and its location, something that cannot be done with X-ray radiography.
Regarding the design of the tank, not only the ultimate values of mechanical properties are a concern, but also other aspects are required, such as the capability of the tank wall to prevent hydrogen leakage to the outside. In this regard, a recent investigation by Eberman et al. [4] concluded that matrix cracking in a composite pressure vessel could directly affect leak tightness, even if the crack density is minimal. Besides, as hydrogen molecules are tiny, they can permeate through the tank wall. In addition, matrix cracking can lead to damage modes such as fibre breakage and delamination, which could trigger composite failure [5] and reduce the permeability of the hydrogen tank [4].
This research evaluates the progression of transverse matrix cracking in cross-ply [0/90] CFRP laminates under static loading with two different stacking sequences, 〖[0/90/0_2/90_2]〗_s and 〖[90/0/90_2/0_2]〗_s, through image processing methods from images captured by X-ray tomography. The two stacking sequences are evaluated to find the crack density in the transverse plies presented in the specimens. The method consists of extracting the position from X-ray images and counting the number of matrix cracks in each ply through the width using an in-house routine developed in the free software ImageJ. In this way, a crack density can be measured and related to a strain value and matrix cracking damage in the specimen.
The results show that X-ray tomography is a powerful non-destructive technique to inspect matrix cracking in CFRP specimens, locate it, and count it. Thus, it can be used as a structural health monitoring tool in applications where damage is not visible to the naked eye. Besides, X-ray tomography allows a 3D reconstruction of a specimen where it is possible to differentiate the damage mechanisms and its location, something that cannot be done with X-ray radiography.