Determination of strength criteria on conditions of visual testability of impact damage in composite aircraft constructions

One of the principal criteria of aircraft composite design strength on the conditions of damage tolerance is the minimum defect size of accidental impact damage that is sure to be located during regular maintenance checks. Construction strength with similar (BVID – Barely Visible Impact Damage) and smaller damages must be ensured under design load. The analysis of operational factors affecting the criterion value is conducted in the paper. The effects of personnel qualification, surface color, level of contamination, distance to the object as well as defect size variations during the time due to relaxation of inner strains in the damaged construction were studied.

Research was carried out using 90 carbon fiber three stinger panels of different color and contamination levels which were subject to impact damages by a 25 mm spherical striker. Totally 80 blows were struck when applying energy within the range of 3–107 J. In the visual damage detectability test under different conditions 42 experts were participated including 25 airlines inspectors. For the statistical analysis of empirical data and determination of dependence for the damage detection probability on its size, the Weibull function was utilized. Determination of BVID size using "90/95" criterion was performed by means of the bootstrap method (95% of inspectors must detect not less than 90% of defects with the size not smaller than BVID). The results obtained in this study enabled to access the degree of various operational factors impact on reliability of damage detectability during the visual inspection and determine BVID values depending on the combination of the given factors. In particular, it was demonstrated that from all the viewing distances the defects in polymer composite materials are more detectable on the blue surface than on the gray and red ones. Conducted research demonstrated a significant effect of relaxation (the reduction effect of the surface defect sizes in the composite panel affected by different factors during time span) on the probability of defect detectability during operation. Also water saturation of the construction under excessive temperatures has the critical compared with other factors impact on relaxation of damages in polymer composite materials. The obtained results accord with the data in foreign publications making them more supplement and specific. On the basis of conducted research the entire range of recommendations for aircraft designers and operators utilizing composite constructions is formulated. 

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