## 1 Introduction  
In recent years, short fiber-reinforced thermoplastics (SFRTs) have gained importance, especially as they have begun to replace metal elements in certain applications, primarily due to their ease of fabrication and superior mechanical properties [1]. Polyamide 6 (PA-6) is one of the most widely used materials in this category due to the following characteristics: its ease of manufacture by injection moulding, high strength, stiffness, toughness, translucency, fatigue resistance, and abrasion resistance [2]. Enhancing PA-6 with short glass fibers improves its strength, stiffness, heat distortion temperature, and abrasion resistance, though it may introduce anisotropic properties [3]. Given the common use of this material in engineering components, understanding its fatigue behavior is crucial. Traditionally, the fatigue limit or endurance--the stress level below which a material can withstand an infinite number of loading cycles without fatigue failure--has been determined from Wohler curves or S-N curves [4, 5]. This approach, however, requires between 16 and 20 specimens to yield reliable results. An alternative, the Staircase method, can achieve similar outcomes with only 11 to 15 specimens [6, 7]. This paper proposes using the Locati methodology [8, 9, 10] to achieve results that are both rapid and cost-effective. The Locati method, an accelerated fatigue test, aims to determine the fatigue limit from a single specimen by applying incrementally increasing stress loads until failure. Various researchers have employed the Locati method to ascertain the fatigue limits of different materials. Nonetheless, a unified criterion for defining the fatigue limit based on Locati test results has yet to be established. The fatigue effect of laser quenching on the boronized layer of Cr12 MoV steel was investigated by [PERSON] et al. [11] through the analysis of fatigue accumulation damage. [PERSON] et al. [12] studied the high-cycle fatigue of 2024-T3 high-strength aluminum alloy, estimating endurance based on the Palmgren-Miner linear damage theory. [PERSON] et al. [13] utilized \(\upmu\)CT analysis alongside the Locati method to determine the fatigue limit of recycled aggregate mortar, and in another study, the variation in resonance frequency to define the fatigue limit of recycled aggregate concrete [14]. [PERSON] et al. [15] analyzed the fatigue failure behavior of short glass fiber-reinforced PA 6.6, using strain evolution to determine the critical step. The Locati method was also applied by [PERSON] et al. [16] to identify the fatigue limit of recycled aggregate concrete, establishing a correlation between the maximum load a specimen can withstand and its fatigue limit. Traditionally [17; 18], damage formulation is based on the concepts of stress and actual strain. Effective stress reflects the material's response to net section reduction caused by various damage mechanisms, allowing damage (\(D\)) to be defined from Equation (1). \[D=1-\frac{\widetilde{E}}{E} \tag{1}\]  
Within this framework, \(E\) represents the material's undamaged elastic modulus, while \(\tilde{E}\) denotes the apparent elastic modulus of the material once damage has occurred. The damage parameter (\(D\)) quantifies the extent of material deterioration, ranging from 0 (indicating no damage) to a critical damage level (D\({}_{\text{c}}\)) corresponding to failure. Fatigue-induced damage in polymeric composites manifests through the formation and propagation of discontinuities, including micro-cracks, voids, and cracks, which progressively undermine the material's strength leading to failure [19]. This damage can be conceptualized as the variation in energy relative to the initial state at each moment. In this study, a model to predict fatigue behavior was developed, aimed at characterizing materials swiftly, economically, and with high precision. This model is anchored in the Locati method and accommodates pauses during fatigue tests and the application of cycles with varying load amplitudes.