*2.2. Experimental Overview*

The two full-scale composite beam specimens have been prepared for load tests, as shown in Figure 2a. The dimension of each beam is 160 mm deep × 260 mm wide × 3300 mm long, kindly provided by an industry partner. The experimental investigations are conducted in accordance with EN 13230 (Test material, specifications, support conditions, loading procedures, and other requirements needed for bending tests on railway track concrete sleepers). Note that EN 13230 has some limitations in order to detect the failure mode of flexible composites. Especially, some experimental arrangements are adapted to examine the structural damage and the failure mode of the full-scale FFU composite beams [38–41]. In this study, modal tests have been conducted using an impact excitation technique in a free-support condition (or '*free-free condition*'). The damage and failure are observed using three-point bending tests following EN 13230, in order to investigate the damage and failure of the beams. The modal parameters of FFU composite beams under different conditions are then investigated.

**Figure 2.** (**a**) FFU 17-06 specimens and (**b**) rubber cushions.

#### 2.2.1. Modal Testing

The dynamic modal parameters have been identified for both healthy and damaged conditions. It should be noted that bending tests are conducted to trigger different levels of damage. Firstly, both specimens are tested under healthy condition. This test requires laying two soft rubber cushions shown in Figure 2b. These very soft cushions have been placed underneath each sample, so that the free-free boundary conditions can be incited for the modal parameters of the sole specimens. This free-free condition is imperative if the dynamic parameters of an individual component are required in any like-for-like performance comparison.

Secondly, the experimental modal analysis has been performed to identify dynamic parameters of the specimens under different severity states of damaged conditions. The equipment used for this test is a Prosig-P800 impact hammer as given in Figure 3a. The 34 uniform locations have been marked on the surface of each sample as the excitation locations of the impact hammer. The accelerometer is fixed at one corner to record the acceleration, as shown in Figure 3b. According to the EN 13230 criterion [41], the dynamic responses up to 2100 Hz are recorded. In addition, these attributes are clearly defined using a curve fitting method. Data modal analysis is a package that can create optimisation algorithms and provide relevant frequency-dependent shapes to explain the data sets. This data can be transformed into curve images; and mode shapes can be determined by the '*animation drawing suite*'.

**Figure 3.** (**a**) PROSIG-P800 impact hammer and (**b**) excitation locations and an accelerometer position.
