PSI - Issue 65

D.G. Solomonov et al. / Procedia Structural Integrity 65 (2024) 275–281 D.G. Solomonov and M.Sh. Nikhamkin / Structural Integrity Procedia 00 (2024) 000–000

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fatigue limits as texted by ASTM Standard D 3479/D 3479M–96 (2007) and GOST 57143-2016. Such tests are expensive and require the use of special samples and a long time. In some cases, this hinders the use of polymer composite materials (PCM) in highly loaded structural elements. Unfortunately, it is not always possible to use fatigue resistance characteristics obtained in this way in design practice. The fact is that these characteristics can change significantly due to the influence of reinforcement schemes, part shape, technology features as texted by Alam et al. (2019), Sevenois and Van Paepegem (2015), Strizhius (2020), Reis et al. (2009) and Kulkarni et al. (2018). This forces to test full-scale components or typical structural elements as texted by Vallons et al. (2013), Guseinov et al. (2021), Wang and Soutis (2018). Such tests are more complex, they require special techniques and expensive samples. Accelerated methods for obtaining experimental data on the fracture resistance of materials from multicycle fatigue are of undoubted practical interest as texted by Collins (1981). One of the accelerated methods is based on the use of infrared thermography (the IRT method). It was originally developed for metals and described by La Rosa G. and Risitano A. (1981) and Luong (1995). The method is based on the effect of heat release during cyclic deformation of a material. Block cyclic loading of the sample is carried out. The amplitude of the load in each subsequent block increases. When the amplitude of alternating stresses exceeds the fatigue limit, intensification of self-heating of the samples is observed. This effect is due to the activation of heat release when fatigue damage occurs. In works of Collins (1981) La Rosa G. and Risitano A. (1981), a good agreement of the obtained estimates of fatigue limits with the results of standard fatigue tests was demonstrated for metals. The IRT method was successfully applied by various authors to polymer composite materials by Luong (1995), Huang et al. (2019), Montesano et al. (2013), Colombo et al. (2019) and Muller et al. (2021). The main mechanisms of fatigue damage accumulation that cause energy dissipation and self-heating of reinforced composites under cyclic loading are viscoelastic deformation of the matrix material, matrix cracking, friction at the fiber–matrix interface, fiber reorientation, and fiber destruction as texted by Montesano et al. (2013). Using the IRT method to study the fatigue of full-scale structures or typical structural elements requires the development and verification of a specific technique. This study deals with the fatigue strength of flange joints, which are typical elements of composite structures. They are widely used in various structures made of composite materials. The purpose of the study is to develop a technique for rapid assessment of resistance to high-cycle fatigue of a typical carbon fiber shell with a flange by the IRT method.

2. Experimental technique

The object of research is the critical zone of a typical shell with a Γ-shaped flange. The sample for research is a rectangular shell fragment cut from a full-scale structure (Fig. 1). The shell is made of laminated carbon fiber based on unidirectional carbon fabric and an epoxy matrix, and the flange is made of laminated carbon fiber based on an equally strong fabric; they are glued together. The sample dimensions are 150×40 mm, the shell thickness is 2 mm.

Fig. 1. The test sample: 1 – shell fragment, 2 – flange, 3 – adhesive layer

Fig. 2. The scheme of sample securing and loading: 1 – sample, 2 – weight, 3 – vibration stand table, 4 – strain gauge