## 2 Materials and Methods
### Testing Procedure  
As explained in Section 2.1, the structure under test (structure-A or -B) is clamped at one end, whereas the other (free) end is attached to a stinger, i.e., a thin aluminum rod connected to the mini-exciter and shown on the left in Figure 2. Tests involve vibrating structure-A and -B at normal and at failing joint conditions. Normal joint conditions involve firmly fastened joints, obtained in the current case by applying a torque of up to 8 N/m. They are referred to as NJ-A for structure-A and NJ-B for structure-B. Failing joint conditions correspond to fastening the bolt using almost zero torque. Hence, even though the bolt bears contact with the washer, it is difficult to visually detect whether the joint is loose or not. For the same reason, the word \"failing\" is used (i.e., still operating in a substandard manner) and not \"failed\" (as for disrupted) joints. Failing J1 joint is referred to as FJ1-A or -B, for structure-A (Figure 2a) or structure-B (Figure 2b), respectively. On the other hand, only structure-B involves failing joints J2, which is a condition referred to as FJ2-B. With respect to testing at failing joint conditions, structure-A may, obviously, be tested with only one failing joint (FJ1-A). Structure-B was tested with either J1 (FJ1-B) or J2 (FJ2-B) in failing condition. Test cases with two simultaneously failing joints (FJ1-B and FJ2-B) were not considered, since, in real life, identifying the first failing joint should call for applying remedial action to revert to a condition with tightly fastened joints. Note that, irrespectively of the location of the failing joint, excitation is always provided at the same point. This has not always been the case in structural vibration testing in the past. Since the excitation position does not have to be defined according to the location of the monitored joint, the current framework could be used with vibration data recorded from normal operation/loading of the structure. As explained in Section 2.1, the starting point for producing a vibratory force from the mini-exciter is the definition of an initial suitable profile in the waveform generator. This is then used for driving the mini-exciter, which theoretically produces a force/motion profile matching the characteristics of the supplied waveform. In practice, this profile is somewhat different due to mechanical constraints (inertia of stinger and various internal parts, possibility of saturating actuators), bandwidth limitations of the exciter's armature response, parasitic loading from the stinger's off-centered motion during its operation etc. With respect to the profile of the initial waveform, a repetitive triangular form was preferred to pulses or perfect sinusoids. The reason is that loading profiles produced in several real-life cases rarely correspond to pulses or perfect sinusoids, with examples involving loading from people walking on the surface (not at a marching pace), or from operating machinery with rotating parts (internal combustion engines, for instance). On the other hand, with respect to selecting the waveform's frequency, the aim was to produce mechanical force/motion of low amplitude and frequencies corresponding to those from operation of machinery with rotating parts. Initially, tests were intended to be performed at 20-40 Hz, which is roughly the case of an internal combustion engine rotating quite slowly at 1200-2400 rpm. Following tests with the waveform generator, it soon became clear that frequency components of 50 Hz of quite significant magnitude were always present, even for triangular waveforms of 20 Hz. This meant that testing at frequencies over 50 Hz was the only dependable scenario. In practice, selecting a frequency of 80 Hz for the triangular waveform means that the mini-exciter should be ultimately producing mechanical force at somewhat lower frequencies (due to reasons stated at the beginning of this paragraph), which would roughly correspond to an engine turning at mid-range (4000-4500 rpm).