DETERMINATION OF DAMAGE ONSET OF THIN-PLY CARBON-FIBER COMPOSITES IN CRYOGENIC ENVIRONMENTS
Topic(s) : Experimental techniques
Co-authors :
Eduardo SZPOGANICZ (GERMANY), Fabian HUEBNER (GERMANY), Uwe BEIER , Matthias GEISTBECK , Holger RUCKDAESCHELAbstract :
Carbon-fiber reinforced polymer (CFRP) composites are the preferred lightweight material for cryogenic storage tanks in advanced reusable launch vehicles and upcoming commercial aircraft due to their impressive strength-to-weight ratio [1–3]. These vessels, intended for storing liquid hydrogen and oxygen, must endure the severe conditions of cryogenic temperatures (down to 20 K), leading to thermal fatigue and overall embrittlement, making microcracking inevitable [2]. In this study, we aim to determine the onset of damage in quasi-isotropic CFRP composite laminates when subjected to tensile testing in cryogenic environments, focusing on the stress-strain signal shift caused by the failure of angled CFRP plies. This study utilized a toughened epoxy matrix CFRP material designed for thin-ply prepreg and cryogenic operation, employing aerospace-grade carbon fibers (HexTow® IM7, Hexcel). The material was characterized through tensile stress-strain tests, both in cryogenic (CT) and room temperature (RT) conditions, following the ASTM 3039 standard. In-situ cryogenic testing, immersed in liquid nitrogen, incorporated a cryogenic-clip extensometer to accurately capture the CFRP material's strain behaviour from start to failure.
Scanning electron microscopy (SEM) was employed to observe and validate the stress-strain shift signal in the CFRP specimens. Images before and after the shift range were analyzed to observe damage progression during the failure process. The stress-strain behavior of CFRP laminates in multiple fiber orientations was tested in both RT and CT. Results obtained herein are summarized in Table 1.
Literature highlights a monotonically increasing trend in stress-failure values at cryogenic temperatures compared to RT, attributed to material stiffening. Matrix stiffening is more pronounced in 90° than QI and least in 0° configurations. Lower temperatures also lead to overall embrittlement and increased sensitivity to defects. Sensitive specimens, such as those with a 90° fiber orientation, often fail due to pre-existing defects and specific stress conditions, while 0° specimens can withstand larger damage, suppressing sensitivity.
Strain-failure values for 90° samples enabled estimating damage onset for the QI configuration. In RT testing, the system exhibited two main shifts at 1.0% and 1.2% strain, corresponding to the expected failure of 90° plies and overall CFRP laminate failure, respectively. Cryogenic measurements aligned with these shifts, with the stress-strain shift during 90° ply failure matching the 0.7-0.8% strain-range observed in cryogenic testing. A representative curve of a QI CFRP laminate measured in CT is shown in Figure 1. Here, the original data collected by the cryogenic-extensometer is shown alongside the normalized data. Fractography experiments within the detected stress-strain range validated the correlation between signal shift and damage onset, observed in both RT and in-situ cryogenic testing.
Scanning electron microscopy (SEM) was employed to observe and validate the stress-strain shift signal in the CFRP specimens. Images before and after the shift range were analyzed to observe damage progression during the failure process. The stress-strain behavior of CFRP laminates in multiple fiber orientations was tested in both RT and CT. Results obtained herein are summarized in Table 1.
Literature highlights a monotonically increasing trend in stress-failure values at cryogenic temperatures compared to RT, attributed to material stiffening. Matrix stiffening is more pronounced in 90° than QI and least in 0° configurations. Lower temperatures also lead to overall embrittlement and increased sensitivity to defects. Sensitive specimens, such as those with a 90° fiber orientation, often fail due to pre-existing defects and specific stress conditions, while 0° specimens can withstand larger damage, suppressing sensitivity.
Strain-failure values for 90° samples enabled estimating damage onset for the QI configuration. In RT testing, the system exhibited two main shifts at 1.0% and 1.2% strain, corresponding to the expected failure of 90° plies and overall CFRP laminate failure, respectively. Cryogenic measurements aligned with these shifts, with the stress-strain shift during 90° ply failure matching the 0.7-0.8% strain-range observed in cryogenic testing. A representative curve of a QI CFRP laminate measured in CT is shown in Figure 1. Here, the original data collected by the cryogenic-extensometer is shown alongside the normalized data. Fractography experiments within the detected stress-strain range validated the correlation between signal shift and damage onset, observed in both RT and in-situ cryogenic testing.