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Abstract: Featured ApplicationThe developed new method will enable us to perform contactless air-coupled NDT of composite structures possessing a complex geometry. AbstractUltrasonic-guided waves are widely used for the non-destructive testing and material characterization of plates and thin films. In the case of thin plastic polyvinyl chloride (PVC), films up to 3.2 MHz with only two Lamb wave modes, antisymmetrical A0 and symmetrical S0, may propagate. At frequencies lower that 240 kHz, the velocity of the A0 mode becomes slower than the ultrasonic velocity in air which makes excitation and reception of such mode complicated. For excitation of both modes, we propose instead a single air-coupled ultrasonic transducer to use linear air-coupled arrays, which can be electronically readjusted to optimally excite and receive the A0 and S0 guided wave modes. The objective of this article was the numerical investigation of feasibility to excite different types of ultrasonic-guided waves, such as S0 and A0 modes in thin plastic films with the same electronically readjusted linear phased array. Three-dimensional and two-dimensional simulations of A0 and S0 Lamb wave modes using a single ultrasonic transducer and a linear phased array were performed. The obtained results clearly demonstrate feasibility to excite efficiently different guided wave modes in thin plastic films with readjusted phased array. Keywords: air-coupled ultrasonic; Lamb waves; finite element modeling; plastic films
In this paper, Charpy impact tests were conducted on cracked aluminum plates repaired with FML composite patches. The effects of the crack characteristics and patch lay-up sequence on the energy absorption of the specimens were investigated experimentally. In order to reduce the test numbers, the design of experiments method was used, and the results were predicted by response surface method. The effect of repairing on the fracture parameters [stress intensity factor (SIF), J-integral, and crack propagation direction (CPD)] at the crack front was calculated using three-dimensional (3D) finite element analysis. The results show that the value of the energy absorption increases when the crack angle increases and that the patch lay-up sequence has a significant role on the efficiency of the repair. When the location of the metal layer of the patch is near the repaired surface of the specimen, the value of the energy absorption increases.
Many researchers calculated the stress intensity factor (SIF) at the crack tip of repaired cracks. Among them, Callinan et al. , Jones and Chiu , Chung and Yang , Bachir Bouiadjra et al. , and Ayatollahi and Hashemi  used the finite element (FE) method to investigate the effect of composite patching on the SIF of a crack as an important measure for analyzing the performance of the composite reinforcement technique.
Tsai and Shen  obtained the SIF for thick aluminum plates having a central crack in both repaired and unrepaired cases. They showed that the SIF decreased in the presence of composite patch and that SIF variation was very significant in a doubly asymmetric plate.
Ouinas et al.  performed a numerical investigation on cracked aluminum plate repaired with octagonal composite patch in mode I and mixed mode conditions. They examined the effect of mechanical and geometrical properties of the patch on the SIF in the crack tip. They concluded that the SIF at the crack tip is inversely proportional to the increase in the patch rigidity. They also observed that in mixed mode condition, the reduction of the SIF value in opening mode (mode I) is more important than that in shear mode (mode II).
Nabousli et al.  performed nonlinear analysis of the adhesively bonded composite patch to investigate its effects on the damage tolerance of the repaired structure. They showed that the crack-opening displacement and the SIF value of the repaired plate in geometrically nonlinear analysis are smaller than the ones in the linear analysis. Chung and Yang  conducted experimental tests on thick Al6061-T6 panels repaired by a single-sided fiber-reinforced composite patch. They showed that the fatigue life of the patched plates increases about four to six times compared to that of the unpatched plates.
Okafor et al.  used adhesively bonded composite patches for repairing cracked aircraft aluminum panels. They found that the maximum skin stress decreases significantly after the patch is bonded. The maximum value of skin stress occurs at the crack tip when the panel is unpatched, whereas for the patched panel it shifts to the patch edges.
Sabelkin et al.  performed experimental and analytical investigations on fatigue behaviors of cracked 7075-T6 aluminum panels repaired by one-sided adhesively bonded composite patch. They observed that the residual strength and the fatigue life of the patched panels increase considerably.
Khalili et al.  conducted the Charpy impact test on edge-cracked aluminum plates repaired with one-sided composite patch. They noticed that the carbon patches have better characteristics than glass patches in reinforcing the cracked plates. In another similar research , they repaired notched aluminum specimens with metallic, composite, and FML patches. They observed that FML patches were more effective than the other patches in reinforcing the notched specimens.
The aim of the present study is to assess the effect of crack characteristics (crack length and crack angle) and patch lay-up sequence on single-sided cracked aluminum plates repaired with the FML composite patches on one side. For this purpose, Charpy impact tests were conducted on the patched and unpatched cracked specimens. The effect of repairing on the fracture parameters such as SIF, J-integral, and CPD at the crack front was also calculated using 3D FE analysis (FEA).
In the present study, the cracked specimens are made of aluminum alloy plate AA1035 (Vatco Co., Tabriz, Iran) . The thickness of the specimens is 3 mm. The mechanical properties of this material were measured by tensile test (Alborz koosha Co., Tehran, Iran) according to ASTM E8M 09-2010 standard  and are given in Table 1. The geometry of the patched specimens is shown in Figure 2. The uncracked specimens were prepared with a water jet machine (HYDRAjet, GA, USA). Then, an initial crack was generated in the specimens by a wire cutting machine (Charmilles, Russia) (electrical discharge machining). This paper takes into account the effect of two main crack characteristic parameters: the crack length and the crack angle. Three different crack lengths and crack angles were considered. The ratio of the initial crack lengths to the specimen width (a/w) was considered to be equal to 0.1, 0.3, and 0.5. The crack angle θ was also considered to be θ=0°, 30°, and 45° (Figure 2). Figure 3 shows nine different configurations of the specimens.
Three different patch lay-up sequences were performed on the cracked specimens. The code C1 represents the patch lay-up sequence of fiber-fiber-aluminum (F-F-A). This means that the fiber layer is bonded to the cracked plate and the metal layer is on the furthest point from the repair surface. Similarly, the code C2 demonstrates that the lay-up sequence is A-F-F, and finally, the code C3 shows that the lay-up sequence is F-A-F. In the case of C2, the metal layer is exactly bonded to the cracked specimen, whereas in C3 the metal layer is in the middle of the patches. The digits after the letter C correspond to patch lay-up sequence, crack length, and crack angle, respectively, which are described in Table 2. The digits after the letter B correspond to crack length and crack angle, respectively. 2b1af7f3a8