A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Composite materials have seen widespread use in the aerospace industry and are becoming increasingly popular in the automotive industry due to their high strength and low weight characteristics. The increasing usage of composite materials has resulted in the need for more effective techniques for nondestructive testing (NDT) of composite structures. Of these techniques, digital shearography is one the most sensitive and accurate methods for NDT. Digital shearography can directly measure strain with high sensitivity when combined with different optical setups, phase-shift techniques, and algorithms. Its simple setup and less sensitivity to environmental disturbances make it particularly well suited for practical NDT applications. This paper provides a review of the phase measurement technique and recent developments in digital shearographic NDT. The introduction of new techniques has expanded the range of digital shearography applications and made it possible to measure larger fields and detect more directional or deeper defects. At the same time, shearography for different materials is also under research, including specular surface materials, metallic materials, etc. Through the discussion of recent developments, the future development trend of digital shearography is analyzed, and the potentials and limitations are demonstrated.

Digital Shearography for NDT: Phase Measurement Technique and Recent Developments

The application of fiber-reinforced polymer (FRP) composites is continuously increasing due to their superior mechanical properties and the associated weight advantage. However, they are susceptible to more complex types of damage, and advanced damage characterization systems are required to prevent catastrophic failures. Various non-destructive testing and evaluation (NDT&E) and in-situ structural health monitoring (SHM) techniques have been applied for damage detection in FRP composites. These techniques have been continuously developed to achieve reliable inspections, especially for safety-critical applications such as the aerospace industry. This review presents recent advances in NDT&E techniques and SHM techniques, particularly for damage diagnosis in FRP composites. For selecting the most suitable NDT technique based on specific criteria, the analytical hierarchy process is applied as a decision-making tool to evaluate and rank the NDT techniques. The size of the specimen is found to be the most important criterion that significantly affects technique selection. Finally, the importance of developing in-situ SHM systems is outlined, and different in-situ SHM systems are then reviewed and discussed. This review provides progress of the recent damage characterization techniques and enables researchers to devise selection criteria to select the most appropriate technique for their own work.

Advancements in fiber-reinforced polymer composite materials damage detection methods: Towards achieving energy-efficient SHM systems

Selective laser melting (SLM) at different laser fluences was used to fabricate a TiC/Inconel 625 (TiC/IN625) nanocomposites. The influence of laser fluence on surface morphology, microstructure evolution, and mechanical properties of the TiC/IN625 nanocomposites were studied. The surface morphology of the TiC/IN625 nanocomposites was affected by the applied laser fluence, and the surface morphology was optimal when the laser fluence was 208 J/mm3. The laser fluence influenced growth of the TiC/IN625 nanocomposites microstructure, ranging from insufficient growth (139 J/mm3) to sufficient growth (167 J/mm3) and refined columnar/cellular dendrites (208 J/mm3). A continuous and refined ring-like network structure with homogenously distributed reinforced TiC phases was obtained. Obvious grain refinement occurred in the TiC/IN625 nanocomposites because TiC nanoparticles acted as nuclei for heterogeneous nucleation. The TiC/IN625 nanocomposites printed at 208 J/mm3 exhibited 1219 MPa ultimate tensile strength and 31% fracture strain. The superior strength of the TiC/IN625 nanocomposites was attributed to dislocation strengthening, Orowan strengthening, grain refinement strengthening, and load-bearing strengthening.

Microstructure evolution, mechanical properties, and strengthening mechanism of TiC reinforced inconel 625 nanocomposites fabricated by selective laser melting

Evaluation of large plastic deformation for metals by a non-contacting technique using digital image correlation with laser speckles

Digital speckle pattern interferometry and digital shearography are widely used in nondestructive testing due to their advantages of high speed, full field, and high sensitivity. However, traditional speckle pattern interferometry and shearography can only measure a single variable of deformation or strain. This study presents a modified common measurement setup that can simultaneously measure deformation and its first derivative. In the optical setup, a reference beam is introduced behind the shearing device to interfere with the object beams, thereby simplifying the spectrum and improving the quality of the phase maps. Then the spatial carrier technology is used to extract phase and achieve dynamic measurements. The proposed system also expands the measurement range using a ${4}f$4f system. This study also presents a 3D optical setup based on the 1D system, which is more suitable for practical measurement applications in industrial areas. Theoretical derivation and experimental results are described and presented.

Simultaneous 3D measurement of deformation and its first derivative with speckle pattern interferometry and shearography

Because the sensitivity of shearography is determined by its shearing direction, aeolotropic defects could be difficult to detect in nondestructive testing (NDT) using a digital shearography system with a single shearing direction. We report an adjustable aperture multiplexing spatial phase-shift digital shearography system for simultaneous measurement of displacement derivatives in three directions. By setting the aperture parameters properly, three spatial carrier frequencies are produced within the speckle pattern, and the information of three interferograms can be separated in the frequency domain. Phase maps of the three shearograms can be obtained by applying a windowed inverse Fourier transform, which enables simultaneous measurement of the displacement derivative. The system is suitable for NDT with a dynamic load. The capability of the triple-directional spatial phase-shift digital shearography system is described by theoretical discussions as well as experiments.

Spatial phase-shift digital shearography for simultaneous measurements in three shearing directions based on adjustable aperture multiplexing

Shearography has been widely used in non-destructive testing due to its advantages of fast, full-field, and high sensitivity for nondestructive test and strain measurement. Shearography has been applied successfully to various industry applications. However, traditional shearography requires the surface under test to be sufficiently rough. This study presents a modified shearography setup that can be applied to workpieces with specular surface which is illuminated by light scattered from a rough plane to generate speckle patten. In the optical setup, the specular surface is conjugated to the phase map to simplify the positional correspondence between the interferogram and the internal defects. The sensitivity factor is analyzed through ray tracing method based on geometric optics which is influenced by the surface normal. The experimental results are described and presented as well.

Peizheng Yan

Papers

Simultaneous 3D measurement of deformation and its first derivative with speckle pattern interferometry and shearography

Spatial phase-shift digital shearography for simultaneous measurements in three shearing directions based on adjustable aperture multiplexing

Shearography for non-destructive testing of specular reflecting objects using scattered light illumination

An error elimination method for high-temperature digital image correlation using color speckle and camera

The application of SLM in shearography detecting system