GLARE CALIBRATION BLOCKS

STUDY OVERVIEW

The need for lightweight aerospace materials with superior mechanical properties to conventional aluminium alloys has led to an interest in fibre metal laminates (FML) like Glass laminate aluminium reinforced epoxy (GLARE).

The material properties of GLARE are similar to aluminium metal sheets, but it has a better tolerance to impact damage, and it is lighter. GLARE is made from layers of aluminium with layers of Glass Fibre Reinforced Polymer (GFRP) pre-preg between them. The layers of GFRP help prevent any cracks in the aluminium layer from propagating throughout the thickness of the component. While there are significantly higher material costs associated with using GLARE, the longer lifetime until failure and lower weight can make GLARE cost-effective over the lifetime of an aircraft.

An Airbus A380 operator provided dolphitech with three GLARE blocks of different thicknesses with artificial reflectors that simulate impact damage. GLARE is prominently used in the Airbus A380, where GLARE is the structural material for the upper fuselage shell and these blocks are used to assess the performance of NDT equipment.

Photographs of just one of the blocks are shown (Figure 1) since the three blocks look very similar. All the blocks are 150 mm in both width and length, though the thickness of each block is different. This thickness is given in Table 1. Each has two artificial reflectors: a flat bottom hole and a rectangular notch. The notch runs across the entire length of the sample and is 6 mm wide. In each block, these two flaws can be seen from the back face (Figure 1), and they share the same depths as given in Table 1. The ultrasonic signature of these reflectors should be similar to impact damage as impact damage results in delamination and an air gap in the material.

Table 1 also contains the type of GLARE used for each block. There are six grades of GLARE. The grade of GLARE used, Grade 3, means the glass fibre ply orientations are 0°/90°.

Figure 1. Photographs of one GLARE block (labelled Block 3). (a) is the inspection face and (b) is the back face.

Table 1. Type and nominal dimensions of GLARE blocks. The naming convention for the type is GLARE grade – number of aluminium layers/number of glass fibre layers – aluminium layer thickness. The depths of the flaws are from the back face.

CHALLENGES

The material is highly attenuative as the multiple layers within the GLARE material make it harder for sound to penetrate the material. Here, sound energy is reflected at the interface between each layer. This not only reduces the amount of sound energy further transmitted into the part, but it also impedes signal interpretation due to the “noise” from the reflected interface echoes.

CONCLUSION

This study has demonstrated that the dolphicam2+ can be used to inspect GLARE samples of a range of thicknesses from 1.7 mm to 5.6 mm with a single TRM. It works well on this highly attenuative material to clearly resolve both artificial reflectors and the back wall of the material. These results demonstrate the potential for dolphicam2 to be used to inspect for impact damage, as the delaminations and corresponding air gaps produced by impact damage would be ultrasonically similar to these reflectors.

GLARE inspection is one of many in-service A380 applications well-suited to the dolphicam2 for both aluminium and composites.