PSI - Issue 43

Raghu V. Prakash / Procedia Structural Integrity 43 (2023) 190–196 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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defect volume (such as blind drill hole of certain depth). Table 1 presents the comparison of stiffness degradation and damage volume fraction for an impacted CFRP specimen. The damage volume fraction was normalized with specimen volume for a typical dimension of 50 mm length x 45 mm width x 4.5 mm thickness. The actual damage volume estimated was approx. 410 mm 3 while the impact only specimen had a damage volume of 170 mm 3 . Further the void volume of each void present in the laminate due to hand lay-up manufacture of composite was found to be approx. 1 mm 3 and not too many voids were found in the finally prepared laminate. At the outset, it may appear that the damage volume over the area of interest is much smaller compared to the gross property change – viz., the stiffness, but the CT imaging provides accurate information about the nature of damage that exists in an impacted laminate. Table 1. Stiffness and damage volume fraction comparison of impacted CFRP laminate at failure due to fatigue loading. First cycle stiffness (kN/mm) Failure cycles Stiffness at failure cycles (kN/mm) Loss in Stiffness (% of initial stiffness) Estimated Damage Volume fraction (% of typical section considered – 50 mm x 45 mm x 4.5 mm)

19.58

1057940

12.81

34.58

4.05

a)

b)

c)

Fig. 10 – Three dimensional CT image of an un-impacted quasi isotropic CFRP laminate.

Fig. 11 – Image analysis of CFRP laminate – a) Ct Image of damage in front view (F), b) binary mage from ImageJ software and c) damage area identification for a specific depth of specimen.

The damage volume estimates were plotted as a function of stiffness drop based on test results from multiple specimens tested at different stress levels and loading conditions and the same is shown in Fig. 12. The figure presents the information in the form of stiffness drop as well as terminal stiffness. Thus, if instantaneous stiffness is known at any instance of operation, the same can be used to estimate the damage volume fraction.

a) b) Fig. 12 – Graph of damage volume fraction as a function of terminal stiffness as well as % loss in stiffness for CFRP laminate.

The results presented here though may suggest as specific to a particular class of polymer composite material, the concept can be extended to other polymer composite materials as well and we found its applicability to glass fiber