# Failure Mechanism and Structural Analysis of Chain Slings with Non-Standard Connections
## 1 Introduction  
Chain hoists and chain slings are essential devices used for the stable and efficient lifting and handling of heavy loads, and they are widely employed across various industries such as manufacturing, construction, logistics, and shipbuilding. Their ability to safely move loads even in spatially constrained environments makes them highly versatile, and their importance lies in enhancing both operational efficiency and safety at worksites. However, in industrial settings, safety regulations related to chain use are frequently neglected or poorly managed, leading to structural deficiencies in chain systems and subsequently increasing the risk of accidents. The safety of chain systems depends not only on material strength or manufacturing quality but also on a complex interplay of factors, including the geometric design of components, connection methods, loading conditions, usage frequency, and maintenance quality [1]. In particular, improper adherence to specifications such as sling angles, working methods, load limits, and safety factors can result in localized stress concentrations and permanent deformation, significantly increasing the risk of chain failure [2, 3]. Nevertheless, non-standard connection practices--such as directly wrapping chains around eyebolds or hooks without using master links--are frequently observed in the field. Such practices adversely affect the structural integrity of the chain, alter stress transfer paths, and generate localized stress concentrations that may lead to unexpected fractures. Uneven stress distributions on the surface of chain links can also initiate fatigue cracks during repeated use, potentially leading to catastrophic system failure [4]. In addition, repeated use and aging of chain systems are major contributing factors to degradation. Chains that have been in service for extended periods often exhibit reduced durability due to accumulated damage such as wear, corrosion, and microcracking. Internal defects that are not visible to the naked eye can serve as critical weaknesses, causing sudden failure under overload conditions. Therefore, regular inspection and accurate life prediction are essential. Structural analysis, beyond simple visual inspections, is required to assess the chain's true condition. Prior studies have also confirmed that long-term use and repeated repairs significantly contribute to failure [5]. Numerous researchers worldwide have conducted studies to better understand the failure mechanisms of chain systems, particularly focusing on fatigue life and related analyses. [PERSON] et al. [6] investigated residual stresses in mooring chains using neutron diffraction, hole-drilling experiments, and finite element analysis, revealing that residual stresses significantly affect fatigue crack initiation and fatigue life. [PERSON] et al. [7] developed a probabilistic S-N based fatigue model incorporating mean load and corrosion effects, proposing a reliability assessment method for the design and life extension of mooring chains. [PERSON] et al. [8] evaluated the fatigue performance of large-diameter, high-strength offshore mooring chains under seawater conditions through full-scale testing. [PERSON] et al. [9] and [PERSON] et al. [10] experimentally examined the effects of mean load on the fatigue life of chains with varying diameters and grades, finding that increased mean loads result in reduced fatigue life. [PERSON] et al. [11] validated fatigue crack propagation in offshore mooring chains by combining experiments, finite element analysis, and analytical SIF-Paris law, including residual stresses induced by proof loading during manufacturing, clearly demonstrating the significant influence of residual stresses on crack growth behavior. [PERSON] et al. [12] analyzed the impact of mean load and corrosion on the fatigue life of mooring chains and proposed an extended S-N curve model incorporating these effects. [PERSON] et al.