# Delamination Assessment in Composite Laminates through Local Impulse Excitation Technique (IET)
## 2 Materials and Methods
### Materials  
The composite plates adopted in the present work cover a wide range and involve composite laminates made of glass or carbon fibers through resin infusion or pre-preg consolidation and with unidirectional or fabric textures. In total, five different laminates have been retained: (i) a unidirectional carbon fiber, (ii) a plain weave carbon fiber, (iii) a twill 2 \(\times\) 2 carbon fiber, (iv) a twill 2 \(\times\) 2 pre-preg glass fiber, (v) a twill 2 \(\times\) 2 glass fiber made by vacuum bag infusion. All of the laminated plates had dimensions of at least 300 \(\times\) 300 mm.  
The unidirectional and plain weave were carbon fiber composites and were manufactured by HP Composites (Ascoli Piceno, Italy) in autoclave from pre-preg. The unidirectional plate was made of 8 layers stacked in the same direction (stacking sequence [0]\({}_{8}\)) and had a total thickness of 0.96 mm. The plain weave was made of 4 layers, each 0.24 mm thick and all stacked with the same orientation.  
The third retained material was an 8-layer twill 2 \(\times\) 2 carbon fiber, with the commercial name XC110, stacked in the same direction and pre-impregnated with epoxy resin. The curing cycle was accomplished in an oven at 120 \({}^{\circ}\)C for 8 h. The consolidated thickness was 1.8 mm.  
The fourth and fifth materials were made of glass fibers. The fourth was made of 4 layers pre-impregnated with epoxy resin with a total plate thickness of 2.1 mm. The material was cured in an oven under a vacuum at 120 \({}^{\circ}\)C for 8 h. The glass fabric of the fifth material was twill with dimensions of 2 \(\times\) 2, made of E-glass fibers, whose concentration in the warp and weft directions is almost equivalent. The epoxy resin, commercial name EP-IN2-S-5, was vacuum-infused, and the plate was cured under a vacuum for 24 h and post-cured in an oven at 100 \({}^{\circ}\)C for 3 h. The plate had a consolidated thickness of 1.2 mm. The fifth material also presented a delaminated zone, whose geometry can be approximated as a circle with a diameter of 50 mm, as shown in Figure 1.  
Delamination was induced through a water jet machine by spreading a release wax film in the middle of the laminate, i.e., between the fourth and the fifth layers, before the resin infusion, which prevented the local adhesion of the resin to the glass fabric. When water jet cutting the plate in correspondence to the wax film, the high-pressure water encountered a discontinuity in the matrix phase, which facilitated the propagation of the water in the in-plane direction, thus inducing the delamination.  
Figure 1: Example of local delamination in 8-layer glass fiber composite.  
Table 1 reports the material properties of the laminates in accordance with the main material directions.  
The dynamic elastic properties were determined through flexural and torsional IET tests in accordance with the ASTM standard E1876 [26]. Bar specimens of 20 \(\times\) 100 mm were cut from the plates using the water jet machine. Table 1 also reports the Tsai modulus and the trace of the flexural matrix \([D]\) of each material [27], which are considered to account for the material elasticity. The first four materials were used to calibrate the FE model of the local-IET test, while the fifth material was used for validation.