Objective Composite materials have the advantages of high specific strength, specific modulus, and fatigue resistance and have been widely used in aerospace, ships, vehicles, and other fields. However, the properties of composites are easily influenced by their internal defects. Shearography has the advantages of full field, high sensitivity, anti-environmental-interference, and no special requirements of material types. Compared with temporal-phase-shift-based shearography, spatial-phase-shift-based shearography has a fast detection speed and is suitable for real-time detection. Spatial carrier frequency introduction is the most commonly used spatial-phase-shift method. However, in the Michelson-based spatial-phase-shift system, the shear amount and spatial phase shift are both controlled by a rotating mirror. To obtain a separated spectral diagram, a large amount of shear is required, but the effective measurement area is reduced, which leads to excessive sensitivity. In the Mach-Zehnder-based spatial-phase-shift double-imaging system proposed by Gao et al., the shear amount and spatial phase shift can be controlled independently. The imaging lens is placed after the Mach-Zehnder shear part, and the detection area of the system is limited by the size of the first beam-splitter prism in the shear part. When the distance between the shearography system and the detected material is fixed, the field of view of the system is usually small and fixed. Because of the small field of view, the detection efficiency is low. To solve this problem, this paper introduces an improved Mach-Zehnder-based spatial-phase-shift double-imaging system. The advantage of the independent adjustment of shear amount and spatial carrier frequency is retained, the field of view is enlarged, and the detection efficiency is improved.
Methods The solid state laser with a wavelength of 532 nm is expanded by the beam expander and irradiates on the surface of a rough material. The speckle produced by the rough surface of the material is reflected and imaged by an imaging lens on its focal plane. The focal point of lens 1 coincides with that of the imaging lens, so after passing through lens 1, the light reflected by the rough surface becomes parallel light. After passing beam-splitter 1, the two light beams are reflected by mirror 1 and mirror 2, respectively, where mirror 2 is used to introduce shear. The spatial carrier frequency is introduced in the two beams and is generated after passing through apertures 1 and 2. The two beams converge via lens 2' and lens 2″, and after passing through beam-splitter 2, the two light beams interfere with each other on the CCD target; thus, the speckle pattern is obtained. The spatial carrier frequency is introduced by the dislocation of apertures 1 and 2 in the spatial position. When the beam is passing through the aperture, the spatial carrier frequency is introduced into the beam. In a certain system in which the wavelength of laser and the distance between the aperture and CCD camera are fixed, the spatial carrier frequency is influenced only by the spatial positions of the two apertures. After the Fourier transform of the image obtained by the CCD camera, the spectrum with phase information can be separated. The inverse Fourier transform is applied to the spectrum containing the phase information, and the deformation distribution can be obtained by subtracting the phase information before and after the deformation.
Results and Discussions Two specimens were analyzed in this work: a defect-free aluminum plate and a composite plate with internal defects. The Mach-Zehnder-based spatial-phase-shift double-imaging system with a large field of view uses an imaging lens with a focal length of 35 mm to analyze the two specimens. The first derivative of the out-of-plane deformation distribution is shown in Fig. 6(a) and the internal defect distribution can be seen in Fig. 6(b). The result shown in Fig. 6 proves the suitability of the system for surface deformation and defect detection. The same specimen was analyzed using the traditional and Mach-Zehnder-based spatial-phase-shift double-imaging system with a large field of view including two imaging lens with different focal lengths, and Fig. 7 shows the contrast experimental results. From Fig. 7, the double-imaging system has a large field of view compared with the traditional double-imaging system. The use of imaging lenses with different focal lengths can change the field of view. When the camera is unchanged, the enlarged field of view causes the image resolution to decrease. In the actual detection process, according to the field of view and the quality requirements, a camera with higher resolution and a larger target surface and matching short-focal-length imaging lens can be used.
Conclusions This paper introduces a Mach-Zehnder-based spatial-phase-shift double-imaging system with a large field of view that can be used to detect deformation and internal defects. The spatial carrier frequency can be adjusted by changing the relative position of the two apertures placed in front of the lens. The advantage of adjusting spatial carrier frequency and shear amount independently is retained. The experimental results reveal that the field of view in double-imaging shearography can be enlarged by changing the focal length of the imaging lens. According to the actual situation, the field of view is adjustable by changing the focal length of the imaging lens, which leads to an improvement in efficiency.
