PSI - Issue 66

Karolina Głowacka et al. / Procedia Structural Integrity 66 (2024) 108 – 121 Author name / Structural Integrity Procedia 00 (2025) 000–000

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the material strength in different directions, layers are arranged according to the expected way of loading the element during operation. However, in the case of theoretical studies, in many cases, samples with unidirectional reinforcement are tested, and then the strength in different directions is calculated using appropriate formulas and matrices [1]. An essential aspect in the case of the destruction of layered, composite materials is the variety of the fracture mode [2]. Similar to homogeneous materials, a fracture may occur across the sample. Then, we talk about a translaminar fracture, in which both the matrix and the fibers fracture [3]. The second case is delamination, i.e., a situation in which the fibers are not destroyed (or after a previous matrix crack). Still, the fracture occurs in the matrix or at the interface between the fibers and the matrix [4]. At the same time, fiber bridging often occurs [5], and despite the fracture of the matrix, fibers still ‘hold’ the structure for a while, slowing down the destruction process [6]. However, in both cases, the fractures occur suddenly, which can lead to a catastrophe. Therefore, it is necessary to predict the destruction in advance in order to prevent a sudden fracture of the element. It is also worth predicting what type of destruction may occur in the structure in order to increase the probability of predicting or even preventing the destruction [7]. Various measurement techniques are used to assess whether a given composite element has yet to undergo deformation exceeding the allowable deformation. They are used both at the stage of theoretical research and, in many cases, also in actual structures at the stage of operation. Both contact techniques (the measurement device is located in the material structure) and non-contact techniques (implemented at a certain distance from the tested structure) are used. Contact techniques, such as strain gauge or fiber optic [8], use the change in strain gauge resistance during deformation or the behavior of a light beam in a deformed fiber optic cable. However, there is a risk that these sensors will peel off from the structure during testing or that they will be destroyed during testing or operation of an actual, operating composite element. The use of non-contact techniques, such as Digital Image Correlation [9], Electronic Speckle Pattern Interferometry ESPI [10], or photogrammetry [11] comes down to the analysis of the element surface and the assessment of the extent to which it has been changed. This results in a conclusion about the value of the element's deformation on the entire analyzed surface. In the case of using digital image correlation, it is necessary to previously apply an appropriate random pattern (two-color, with contrasting colors, preferably black and white). Then, it is possible to analyze the entire surface after subjecting the element to deformation and refer the current image to the reference image in an undeformed state. In this paper, it was decided to conduct research to determine how the sample dimensions and, consequently, the different ratios of normal to shear stresses affect the type of destruction that should be expected. This is very important from the point of view of protection against destruction. Thanks to the appropriate design of the element, for example, by planning a thin-walled structure, the element's strength can be improved, even while maintaining the same mass [12]. Another critical aspect of the study is recording measurements and using digital image correlation for surface analysis. The research will confirm the possibility of using DIC to predict composite destruction before the first signal of a possible failure. DIC is such a flexible and universal measurement technique that cyclic photography of the element surface and then its application to the obtained images will allow obtaining information on the current state of deformation, as well as determining the location, and therefore also the type of possible cracks [13]. It is also essential to analyze the subsequent stages of material destruction after reaching the maximum force value, meaning that the first crack has already appeared. Although, as is known, composites break suddenly, and it is not possible to use the composite element after the first crack, by correlating the graph of the force dependence on the displacement of the loading pin during bending with the view of the sample surface at subsequent stages, it is possible to determine the scale of potential destruction and define factors that may cause incorrect conclusions in the case of analyzing the value of the loading force alone. Also it is worth knowing how the broken material behaves after being

subjected to further loading. 2. Materials and methods

The paper analyses the strength of polypropylene reinforced with continuous glass fibers (type E). Although samples of different dimensions were tested, in all cases, the fibers were arranged in the same way, i.e., unidirectionally, in the direction parallel to the length of the sample. The samples were prepared using the hot press technique. A detailed description of the sample production technique and their microstructure are described in [14], and much information on the materials obtained with this technique can be found in [15, 16]. Importantly, from the point of view of the analysis carried out in this paper, the technique used, although commonly used for composites