# Validation of Acoustic Emission Waveform Entropy as a Damage Identification Feature
## 1 Introduction  
Liquefied natural gas (LNG) carriers are ships used for long distance transportation of LNG. With the increase in demand of natural gas, supersized LNG carriers are emerging as an economical way for long distance transportation of LNG. According to the figures presented in reference [1], 473 LNG carriers were in operation by 2017. The cargo containments in the LNG carriers are double-walled and are often integrated with the carriers. According to the World LNG report [1], the primary wall of 74% of LNG carriers is constructed from austenitic stainless steel because its construction is cheaper and requires less engineering maintenance work [2]. Despite their advantage, primary walls constructed from austenitic stainless steel are subjected to a tremendous amount of sloshing impact during voyages [3; 4; 5]. Yielding as a result of sloshing impact can cause crack initiation in the primary wall of the cargo containment [6; 7]. If there is a leak of more than 5 mm in the secondary wall, the cryogenic fluid could potentially reach the hull of the ship, causing brittle failure [8; 9]. Both leakage of LNG and failure of the hull can increase the risk to the safety of life and property. To avoid catastrophic failure (i.e., leakage above 5 mm and failure of the hull) of the containment, a set of regulations for inspection have been set by the International Maritime Organization (IMO). According to the regulations, the entire containment has to be inspected thoroughly every five years by a surveyor(e.g., Lloyd's Register) [10]. Despite the regulations, there have been several damages in the primary wall during a voyage, as reported by a French LNG carrier manufacturer [11]. Therefore, to reduce the risk associated with failure of the LNG carriers, there is a need to replace traditional maintenance with a condition-based maintenance (CBM) strategy [12]. CBM is a strategy that continuously monitors the structural health condition to predict whether maintenance is required. In cargo containment, if the initial stages of damage (i.e., yielding of primary wall/austenitic stainless steel) can be monitored, appropriate maintenance can be carried out to avoid a catastrophic failure. In order to gather real time information regarding yielding, structural assessment of the wall of LNG containment with a suitable structural health monitoring (SHM) technique is a primary solution. Determining the severity of damage such as yielding in a material is a challenging task (because unlike the other damage mechanism, such as crack initiation, there is no separation of the material). To achieve this goal, the SHM technique must be sensitive to the damage mechanism over a long time (e.g., the entire operational life of the LNG carrier). Considering most of the widely used SHM techniques (e.g., modal analysis, long range guided waves, vibration, and acoustic emission), only acoustic emission (AE) is capable of identifying the onset of yielding because of its sensitivity to the microscopic changes in the material [13]. AE is commonly used in the industry as a passive SHM technique [13]. The studies presented in references [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24] showed that it is sensitive in identifying the onset of yielding in austenitic stainless steel. For instance, it has been shown by [PERSON] et al. [14] that the peak AE activity occurs before the onset of macro yielding. This phenomenon is attributed to the dislocation generated from the [PERSON] source. AE activity during and after yielding is well understood for both nuclear and commercial grade austenitic stainless steel [15; 16; 17]. These studies suggest that the significant increase in AE activity during yielding of nuclear grade austenitic stainless steel is due to dislocation activity. However, in commercial grade austenitic stainless steel, the major source of high AE activity during yielding is inclusion de-cohesion (i.e., damage initiation as a result of the separation of the matrix-inclusion interface).