PSI - Issue 14

Kishora Shetty et al. / Procedia Structural Integrity 14 (2019) 849–854

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Kishora Shetty et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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properties of CFRP composites is a useful to designers and application engineers. Composite undergoes changes in materials properties under hot-wet conditions. It is known that composite structures are very sensitive to hygrothermo -elastic stress as composite materials have heterogeneity, related to the difference in hygrothermo-elastic properties of adjacent entities -A. Benkheddaet al. (2008). Design of composite structures need to take into proper account hygrothermo-elastic stresses. Polymeric basedcomposite materials with basic constituents as fibres and matrices, exhibit different swelling strains when exposed to the same amount of moisture and temperature. Normally we consider fibres are insensitive to moisture. Evaluating a CFRP for its behaviour at hygothermal conditions start with weight gain studies by subjecting that composite to different hot-wet conditions. Weight gain happens as moisture gets absorbed by composite. Studies were carried out till the samples completely get saturatedby moisturecontent. This represents theworst-case condition for a component, as the ingress of moisture combinedwith a high operating temperature, significantly weakens thematerials mechanical properties. Morphological characteristics of Fibre Reinforced Composite haveclose correlationwith its mechanical properties - Dany ArnoldoHernandezet al. (2017). Laminated Composites are susceptible to interlaminar failures. M. Li et al. (1999) investigated the interlaminar failure in carbon/carbon laminated composite and found that three point bend test results in laminar shear failures in this composite . As aircraft will have primary loaded CFRP materials, it becomes essential that the use of hot-wet design allowables and effect on properties are needed to be investigated in order that weight savings canbe optimized. This paper deals with experiments carried out to access the effect of moisture absorption on Inter Laminar Shear Strength (ILSS) of an Aircraft grade CFRP composite at different hot – wet conditions. Three-point bend tests were conducted as per ASTM test standard specifications using a 25 KN servo-hydraulic test machine todetermine ILSS properties for aged or conditioned and unagedmaterial. 2. Experimental An aircraft grade CFRP HexPly® M21E/34%/UD/194/IMA used here for this study of moisture absorption and ILLS tests. Laminate was prepared fromprepreg using standard autoclave process. Inter Laminar Shear Strength (ILSS) test specimens were cut and prepared fromtheses laminates. Standard ILSS specimens were preparedof size 20mmX 10 mm X 2 mm from laminates of this CFRP laminate. These specimens were edge coated with a standard resin to prevent direct exposure of cut surfaces to moisture. Hot wet conditions for experiments were decided based on Aircraft operating conditions depending on different places/country. Following hot-wet conditions were selected to subject these ILSS specimens: i. No conditioning – Samples kept at Ambient condition Specimens were kept in the different conditioning chambers with aboveconditions.Weight gain was measured in the regular intervals till the saturation. Moisture absorption rate increases gradually and attains saturation at about 1 wt. % under both conditions. Once the specimens were reached saturation, three-point bend tests were conducted as per ASTM test standard specifications using a 25KNservo-hydraulic test machine todetermine ILSS properties for aged andunagedmaterial. Respective hot-wet conditions were maintained evenduring theILSS tests. Tested specimens were subjected to Scanning Electron Microscope (SEM) analyses to study the failure behavior. The specimens were subjected to ILSS test were coated with a conducting (gold) layer using a sputtering unit and examined in a Leica Scanning ElectronMicroscope (SEM). 3. Results andDiscussions Inter Laminar Shear Strength (ILSS) results are presented in the following table 1. Total five specimens were tested at each condition and averageof test results reported in this table. ii. 70 0 C/85% RH iii. 55 0 C/100% RH

Table 1. ILSS Test Results for CFRP specimens. Aging Condition

ILSS Strength (MPa)

Condition A: No conditioning – Samples kept at Ambient condition and tested at Ambient condition 93 Condition B: Aged at 70 0 C/85%RH and tested at 70 0 C/85%RH 61 Condition C: Aged at 55 0 C/100%RH and tested at 55 0 C/100%RH 62

From the results we can see that Inter laminar shear strength reduced significantly by about 33% due to presence ofmoisture. Important point to observe here is for both the Conditions Band C, the ILSS strength is almost same. The degradation includes