Issue 78

V. Pisarev et alii, Fracture and Structural Integrity, 78 (2026) 96-117; DOI: 10.3221/IGF-ESIS.78.06

I NTRODUCTION

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he relevance and importance of experimental investigations concerning the assessment of the impact damage influence on the residual strength of composite structures, which are made from carbon fibre reinforced polymers (CFRP), is described in introductions included into our previous papers [1, 2]. These two works also contain detailed description of novel approach to the determination of residual stress component arising near and inside of contact interaction area. It is quite evident that these components can be involved as design parameter, which is essential for reliable estimation of residual strength of damaged structure [3]. However, to date, there have been no studies in which residual stress values are used to predict compression-after-impact performance (CAI) of CFRP subjected to a low-velocity impact. The same is true with respect to reliable evaluation of the residual fatigue life of the composite structures with low-velocity impact damages. All available approaches are based on various non-destructive multi-modal monitoring techniques including ultrasonic C scan, computed tomography, infrared thermography, three-dimensional digital image correlation, and laser confocal microscopic analysis [4–6]. There is review paper, which provides a thorough overview of current knowledge regarding how natural fiber composite materials react to Low-Velocity Impacts. It highlights the key things that influence their performance and potential uses [7]. Existing methods of evaluating impact influence on post-damage behavior of composite materials can be conventionally categorized into two broad classes namely. The first of them implies creating various numerical models [8–13]. The second group considers so-called experimental-numerical techniques [14–19]. What is common in all cases is the fact that to achieve the most of declared goals indirect parameters, which are almost impossible to quantify, are used. It should be taken into account that experimental analysis of residual stress in CFRP composites is rare owing to the difficulties of determining residual stress accurately in these materials [20]. In this recent paper the contour method of residual stress measurement was applied using wire electric-discharge machining. The technique involved is quite complex even though residual stresses in regular area of cross-ply composite plate is of interest. In the case of determining residual stresses in the area of impact damage, such problems are magnified many times. Involving residual stress component values inherent in contact dimple area both for accurately predicting CAI behaviors and for damage tolerance assessment of thin-walled composite aircraft structures looks like very promising methodology. The first step in this way, which resides in recording high-quality interference fringe patterns caused by local material removal inside and outside contact interaction area, has been already made in work [2]. It was demonstrated that required quantitative data can be reliably derived for impact energy equals to 55 J. The second essential step in this direction resides in obtaining quantitative relations between the impact energy and residual stress values arising at the vicinity of contact dimple. Solving this problem is the main subject of present paper. Residual stress components are determined for three coupons impacted by different energy levels, namely 55, 65 and 75 J. Quantitative data have been obtained at eight points for each specific specimen by means of through hole drilling. We have demonstrated for the first time that reliable quantitative results can be obtained for 65 and 75 J energy impact. This is the main essential feature of performed investigation. This fact means that the upper limit of applicability of the developed approach to residual stress quantifying has been expanded to the impact energy of 75 J. The estimation of the distributions of the residual stress components over the thickness of the sample in the impact zone is the second important issue of the present paper. The necessary data was obtained by testing two similar coupons. In one of them, the initial experimental information relates to the impact side of the specimens with a contact dimple caused by 75 J impact. In the second coupon, impacted by the same energy, interference patterns are obtained by drilling probe holes from the opposite side. This information is obtained for the first time. The coordinates of the corresponding drilling points with respect to the contour of the contact dimple coincide on both surfaces.

O BJECTS OF EXPERIMENTAL STUDIES

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resent paper concerns the determination of residual stresses in four rectangular coupons of dimension 180×30×6.4 mm. Specimens are manufactured from laminated CFRP plate (320×320×6.4 mm) with cross-ply lay-up configuration

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