## 8 Impact and Shock Measurements on Critical Structures
This threshold was used because at a pressure level lower than this, the obtained measurement signals would be mixed with large scattering due to the various noise sources in the experimental environment. Related to explosion loading, structural development is also accounted for in terms of sensors and measurement. Naval vessels, which are subject to explosion risk during the war state, demand more structural safety. One study improved the naval hull with a multi-layer projective structure to withstand blast and fragment/debris loadings [139]. To measure its crashworthiness, dynamic strain gauges were placed on the outer and inner plates of the liquid cabin, while accelerometers were set on the longitudinal defensive bulkhead to record impulse acceleration during the blast ignition. The experiment results concluded that the perforations due to the fragments from the ends were mainly concentrated on a square plate (side length \(l\) = 600 mm), although the travel distance of these fragments in the air reached almost 2.5 m. Additionally, the function of the liquid cabin was noted, which was installed in the second layer of the multi-layer projective structure. This part was designed to halt high-speed fragments from causing critical damage to the inner structure. Impact loadings that possibly occur are not always in the form of high-speed contact. In the case of ship collision and hard grounding, the contact is mostly classified as low-velocity impact. A series of structural trials were conducted by [PERSON] et al. [140] by considering the development of ship structure by arranging sandwich beams. Two cores were deployed, namely, a corrugated type and a y-frame type, as specimens that are subjects of projectile impact in the forms of quasi-static and drop-weight loading as representatives of collision and grounding. The measurements displayed different tendencies in terms of the high initial load, which was not shown by the quasi-static test. To investigate this phenomenon, an additional test was conducted, in which the initial hypothesis stating that the measured initial peak loads during the drop-weight test are highly affected by the mass of the roller and load cell was confirmed, which is the loading applied to the target beam. The development of the structural type subjected to low-velocity impact continued in 2016 with a study in which, instead of modifying the beam geometry, a steel-concrete-polymer (SCP) composite was used to arrange a barrier to defend the internal tank construction [141]. Considering that anisotropic material was used, material characterization was performed, including the corrosion resistance sensitivity, as the application was aimed at maritime-based vessel operation. In this stage, the test was conducted according to ASTME2661/E2661M-10, which was followed by an idealized collision resistance test using a sphere with a diameter of \(\mathcal{O}\) 300 mm as the indenter. In this experiment, the deformation of the construction was recorded by displacement gauges installed at the representative location points. In 2017, [PERSON] et al. [142] re-performed the explosion test presented by the previous scholars but in a new way, i.e., underwater conditions, which was later known as an underwater explosion (UNDEX) experiment. In this work, three underwater pressure sensors (at radius \(r\) 2 m, 4 m, and 6 m from the explosive source (TNT)) and three data acquisition (DAQ) systems were deployed to measure the generated reflected wave pressure from the wall of the water tank and pressure from bubbles. In this case, an underwater shock wave was superimposed on the hydrostatic pressure. Based on the assessment of the recorded pressure history, the shock wave at a fixed location begins with an instantaneous pressure level (it reaches the peak point), which is followed by a decline. Generally, simplified estimation is represented by an exponential function. Similar to the explosion concept, shock is considered necessary in the structural performance of ship construction.