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

Molybdenum-carbide-graphene composites for paper-based strain and acoustic pressure sensors

Optical measurement techniques – A push for digitization

Multiple-aperture one-shot shearography for simultaneous measurements in three shearing directions

The selection of the direction of sensitivity for digital shearography is determined by its shearing direction. As a result, directionally shaped defects could be missed in non-destructive testing using a digital shearography system with only one shearing direction. This paper reports a polarized digital shearography system based on two Mach–Zehnder interferometers, which can create two orthogonal shearing directions and record shearograms in the two orthogonal directions simultaneously. The two shearograms are separated from each other by proper polarization design so that no cross interference occurs. The phase maps of the shearograms are generated by spatial phase shift methods through the introduction of different carrier frequencies in the two orthogonal shearograms and use of the Fourier transform method. This enabled simultaneous dual directional non-destructive testing during continuous loading. Theory derivation, spectrum analysis, and non-destructive testing application results are shown in detail.

Polarized digital shearography for simultaneous dual shearing directions measurements.

Simultaneous dual directional strain measurement using spatial phase-shift digital shearography

The invention discloses a single-slit spatial carrier shearing speckle interferometry measuring system and a measuring method. The measuring system and the measuring method are characterized in that laser shot by a laser device passes through a beam expander and then irradiate a measured object in the form of diffusion light, diffuse reflection light on the surface of the measured object sequentially passes through an imaging lens, a slit diaphragm, a 4f system and a Michelson type device to be projected onto a target surface of an CCD camera. The measuring system and the measuring method can carry out nondestructive, full-field, rapid and dynamic measurement for the defects and stress deformation of the surface of the measured object and is convenient for field measurement.

Single-slit spatial carrier shearing speckle interferometry measuring system and measuring method

The invention discloses a bi-directional shearing speckle interference system based on spatial carrier and a measurement method. The system is characterized in that different-wavelength emergent light of a first light source and a second light source irradiates a measuring surface at the same time after beam expanding; diffuse reflection light of the measuring surface passes through an imaging lens, a diaphragm, a 4f system and a beam splitter prism and is divided into two beams of light; the two beams of light pass through filters and become two light beams with single wavelength, and the two light beams respectively pass through two Michelson shearing apparatuses so as to realize introduction of shearing displacement at different directions; and light with two different shearing directions passes through the beam splitter prism and irradiates on a CCD target surface. The measurement method can carry out bi-directional synchronous detection on defect of a measured object and is a nondestructive, whole-field and high-efficiency measurement method.

Liu Yingxue

Papers

Simultaneous dual directional strain measurement using spatial phase-shift digital shearography

Single-slit spatial carrier shearing speckle interferometry measuring system and measuring method

Bi-directional shearing speckle interference system based on spatial carrier and measurement method