*2.1. Experimental Setup*

The testing methodology applied generally follows the MIL-A-8867C(AS) protocol [1], and it is similar to the wind turbine blade qualification testing approach [60]. Sixteen composite spars were tested to bending failure while instrumented with AE sensors. Load was applied to a cantilevered spar in a stepwise repeated (pseudo-cyclic) loading manner while test samples were monitored by an AE sensing network. All tests were performed with flight hardware bushings installed. Slack was taken out of the loading apparatus before the load was applied to a spar. A given load was applied with a load rate of approximately 200 lbf/s. Once the target load was reached, it was held for approximately 150 s and then released before the spar was loaded to the following load level. Care was taken to apply the load slowly and steadily, to avoid imparting sudden impact loading. The applied load was distributed in the spar using the "whiffle-tree" device shown in Figure 1; a concentrated crane-load applied at the top of the loading apparatus was distributed through a series of beams and connectors to the eight ribs on the spar test bed. The load was distributed along the spar at the locations indicated by yellow boxes in Figure 1. The spar was pinned in two locations at the root, and laid along the test bed. By loading in this manner, the highest shear stress and moments occurred at the spar root, while gradually reducing toward the spar tip, to accurately represent the load distribution observed by the component during flight. Four target crane-load levels were tested for each spar (identified in the text as LL1, LL2, LL3, and LL4). After the final target crane load was achieved, loading continued at a rate of 200 lbf/s until part failure.

**Figure 1.** Schematic of the "whiffle-tree" loading apparatus to simulate distributed loading. Load cell locations are indicated by yellow boxes, while acoustic emission (AE) sensor locations are indicated as red circles.

Acoustic emission activity was recorded using eight to 10 commercially available R15I resonant piezoelectric sensors (operating frequency range of 50–400 kHz, manufactured by Physical Acoustics, Princeton Junction, NJ, USA), distributed along the top of the spar from root to tail. Sensors were placed on the top surface of the spar since the top surface showed less attenuation in pre-testing and the mounting surface was relatively flat. The approximate sensor locations along the length of the spar measured as a distance from the spar root are given in Table 1 for each of the 16 tests. It should be noted that sensor locations varied by several inches from test-to-test, while no data was recorded during the first test, which was used to validate the loading method.


**Table 1.** Sensor locations along the length of each spar.

Sensors were bonded firmly in accordance with ASTM E650 and secured with tape in order to minimize the sensor loss during ultimate structural failure. Sensor cables were also taped, and sufficient cable slack was left free to allow movement during loading, unloading, and failure. Sensors were placed with sufficient spacing such that signal attenuation would not negatively impact results and they were field-calibrated in accordance with ASTM E976/1106 using a pencil lead source.
