## 1 Introduction  
Since introducing nickel-titanium (NiTi) to dentistry, the material, design, and modes of movement for endodontic NiTi instruments have been improving in an endless \"In search of excellent\" process [1]. NiTi instruments help clinicians overcome complicated issues in modern endodontic therapy, especially concerning root canal preparation [2], in addition to advancing endodontic materials [3, 4]. The most troublesome feature of this elastic rotary instrument concerns a fracture during root canal preparation with no visible or precautionary signs [5]. Many elements regarding a complicated mechanism influence the breakage of instruments in the root canal enlargement phase of endodontic therapy [6]. There are two causes for a continuously rotated instrument fracture, relating to the fatigue of the material: one is torsional, and the other is cyclic. When the tip of the file is stuck in the canal, the remaining section of the file is still freely movable, the torsional resistance of the instrument reaches the threshold, and the file breaks [7]. Concerning cyclic failure, the instrument has to bear the repeated loads in a tensile and compressive way when it is freely rotated in the curved canal, and it will fracture when the threshold of cyclic fatigue is over [7]. With the advancements of technologies in different stages from designation to production, such as cross-sectional design, metallurgy, production, before or after surface treatment, thermal process, mode of rotation, endodontic NiTi instruments have been developed continuously to improve cyclic fatigue resistance [6]. Manipulation of NiTiendodontic instruments with dynamic movement is advocated to reduce the proportion of instrument failure during root canal preparation [6]. To simulate the clinical setting, many offers have been recommended and conducted in previous studies [8; 9; 10; 11; 12; 13; 14]. Not only is the cyclic fatigue resistance of root canal instruments addressed in these static situations, but also the dynamic condition [2] in both in-and-out or pecking movements is tested [2; 11; 15]. The static situation has some positives, and outcome results are invaluable, but it does not replicate the actual clinical condition and appears to reduce the cyclic fatigue resistance of the instruments [16]. The dynamic framework, therefore, is more appropriate than the former for the fatigue experiment [16]. The stainless steel artificial canal and other devices have been commonly used in many previous studies [16; 17]. Although there are recognized standard requirements for the testing of endodontic instruments by international organizations, these standards have been applied only for standard instruments with a taper of 2%. In particular, these criteria have not included the cyclic fatigue resistance of the instrument [18]. The important shortcoming of the inside diameter of the tube that the instrument rotates in and the fitness of the instrument was mentioned in a previous review [17]. Although many valuable results of the previous studies have added considerable knowledge to the dental literature, there are many factors embedded in the designs of the previous studies that produce conflicting results related to these investigations [17]. Differential scanning calorimetry (DSC) investigation of a nickel-titanium material reveals the phase transformation temperatures and other characteristics, such as phase compositions, amount of endothermic peaks, and enthalpy changes [19]. Not only are the phase components of an instrument important factors, but also the phase transformation temperatures identify whether the austenite, martensite, or both, are dominant in working conditions, determining the instrument's characteristics in the clinical setting [18; 19].