PSI - Issue 28

Behzad V. Farahani et al. / Procedia Structural Integrity 28 (2020) 226–233 Behzad V. Farahani et al./ Structural Integrity Procedia 00 (2020) 000–000

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processing techniques and image-based sensing, being very definite and sharp non-contact field techniques for experimental mechanics, brought about a paradigm change in phenomena sensing (Farahani, Sousa, et al. 2018). Therefore, numerous widely applicable optical approaches are playing a significant role in support of experiments. The present review manuscript marks out advanced image based approaches for structural integrity monitoring to assess the fracture and failure of different material behaviors, and focuses on approaches such as Digital Image Correlation (DIC) (McCormick and Lord 2010) and Thermoelastic Stress Analysis (TSA) (Thomson 1852; Rocca and Bever 1950). These full-field techniques depend on image processing to evaluate mechanical behaviors, and to evolve even as reviews such as this are being written, which justifies a special effort to keep abreast of this progress. Damage tolerance analyses can be performed through the linear elastic fracture mechanics (LEFM) theory in which some fracture parameters would be involved including T-stress, higher-order terms and the stress intensity factor (SIF) where the SIF plays a considerable role. The LEFM theory together with the crack growth laws, i.e. Paris’ law, is generally adopted to examine and predict the crack growth and accordingly the fracture behavior of structural components. Structural components’ lifetime under fatigue loading might be enhanced within a proper study on the cracked regions. Some research works can be found in the literature focused on constructions and essential factors governing crack paths, dealing with distinct theories such as Maximum Tangential Stress (MTS) (Aliha and Ayatollahi 2012), Strain Energy Density (SED) (Huffman et al. 2017), and Strain Energy Release Rate (SERR) (Carpinteri and Paggi 2007). Thus, SIF determination is commonly considered in fracture mechanics to describe the stress state close to a fatigue crack tip caused by a remote load or residual stresses. Amongst mentioned criteria, the SIFs can be obtained consistently through the SERR in the LEFM. It is categorized as direct and indirect approaches. Computationally, direct procedures are the more straightforward on the SIF determination, which fundamentally rely on the correlation of crack opening displacements, obtained directly from the finite element (FE) solution (Farahani, Tavares, Moreira, et al. 2017). In the literature, several studies carried out to characterize the SIF for different material behaviors and various loading conditions. From the experimental point of view, optical measuring tools were used to document the stress/strain close to the cracked region. Amongst all, DIC and TSA proved the validity of the obtained data to deal with the accurate SIF determination. Numerically, advanced discretization techniques resorting to the FEM and its extended version (XFEM) in addition to meshless methods attracted a significant attention amongst the researchers. Moreover, there are some analytical solutions suggested by the literature in order to obtain the exact/reference solution to SIF. In this regard, the compliance function together with the stress dead-zone concept is of interest in this review study [(Farahani, Eslami, et al. 2019; Farahani, de Melo, da Silva Tavares, et al. 2020; Farahani, de Melo, Tavares, et al. 2020)]. 2. Digital Image Correlation Digital image correlation (DIC) (McCormick and Lord 2010) denotes to the digital acquisition and image processing of an investigated surface to acquire full-field measurements of its deformation. This is attained by dividing the image into small regions, known as subsets (facet field), and tracking the position of each subset in images captured under different loading conditions. In common, a high contrast speckle pattern is hired over the areas associated with the surface to be measured with DIC. Typically, the DIC strategy begins with acquisition of a reference image of the specimen prior to the load application, and a series of images is therefore taken over the deforming process. These captured images evolve the displacement field associated with the random dot pattern related to the initial non deformed reference image. Afterwards, the full-field deformation is evaluated by correlating subsets of the deformation images with the original reference and an evolving displacement map is achieved thereof. If the measured surface and its displacement field remain completely in two dimensions, it is sufficient to use one camera to capture suitable images for DIC. However, if the surface is not planar or if there are out-of-plane displacements, a stereovision system, using two or more cameras, must be used (Sutton, Orteu, and Schreier 2009). As an illustration, in the scope of the LEFM, Farahani et al. (Farahani, Tavares, et al. 2018; Farahani, Tavares, Belinha, et al. 2017) conducted a study on a standard AA6082-T6 compact tension (CT) specimen (with � � 40 , thickness of � � ��� , and � � ���4 , see Fig. 1-a). It aimed to evaluate the mode I SIF by means of DIC. The experimental SIFs were measured for different crack lengths � � upon a fatigue loading condition