The simple sandwich structure consists of three parts: the panel, the core material and the glue joint, which transfers the load between the first two components by gluing. The effect of the sandwich structure is to allow the lightweight, thick core material to withstand shear stress while separating the two relatively tough, thin load-bearing panels.

For the panel, the main consideration is the strength and stiffness of the material, but for the core material, the main purpose is to reduce the weight to the greatest extent. In aircraft construction, the core material typically uses aluminum honeycomb, foam or chshem® honeycomb. Aluminum honeycomb or chshem® honeycomb, which has the advantages of high compression modulus and light weight, is widely used in the aviation industry, and is usually used together with carbon/glass fiber prepreg. The main discussion here is the non-destructive testing of the foam core sandwich structure. In the field of advanced composite materials, foam core sandwich structures are commonly used in wing leading edges and rudders, landing gear doors, wing/wingtip fairings, and the like. Similar to the honeycomb sandwich structure, common defects in foam composites are:

    1. Defects in composite panels, such as scratches, cracks, pores, inclusions, etc.

    2. Bonding defects of composite panels and foam cores, such as debonding

    3. Damage to the PMI foam core

In response to these defects, various non-destructive testing methods have been developed accordingly. However, the foam sandwich structure generally has a large detection area and a thin thickness, and exhibits low thermal conductivity and electrical conductivity, and the sound attenuation of the foam material is large, so that it has a significant difference from the non-destructive detection of the general composite material. At present, non-destructive testing methods for PMI foam sandwich structures mainly include ultrasonic nondestructive testing and laser misalignment speckle interference nondestructive testing.