Experimental and Numerical Investigation of the Effect of Short Bolt Length on the Tensile Strength of Bolt-Nut Assemblies

Fracture in the reduced threaded zone and thread stripping at the bolt-nut interface are two of the most common and critical failure mechanisms observed in high-strength steel bolts used in structural applications. These failure modes can significantly compromise the integrity of bolted connections in steel structures, particularly when bolts are subjected to high levels of tensile loading. Understanding these mechanisms is essential for ensuring the safety and durability of such connections in civil and mechanical engineering systems. Among the factors that influence thread stripping, several key parameters stand out. These include the number of threads engaged within the nut, the mechanical properties of both the bolt and nut materials, the geometry of the threads, and the magnitude of the pretension force applied during assembly. The number of engaged threads directly affects the load transfer capability between the bolt and nut. Inadequate engagement may lead to excessive stress concentrations, eventually causing localized failure in the threads, even if the bolt material itself is sufficiently strong. According to standard structural design codes and industry guidelines, bolts should be installed in such a way that at least three full threads remain exposed beyond the nut after the tightening process is complete. This requirement helps ensure that a sufficient number of threads are fully engaged within the nut, thereby minimizing the risk of thread stripping under service loads. However, in practice, this guideline is not always adhered to, especially in scenarios involving preassembled steel elements where bolts may be incorrectly sized or improperly installed. Errors during fabrication, misalignment during assembly, or the use of shorter bolts for convenience can all lead to insufficient thread engagement. Despite the practical importance of this issue, the negative effects associated with insufficient thread engagement have not been thoroughly explored in the existing body of research. Many previous studies have focused more broadly on bolt tensile strength or fatigue performance without isolating the specific influence of thread engagement length. The study presented here aims to fill that gap by combining experimental testing with numerical simulations to investigate how reduced bolt lengths affect the tensile behavior of bolt-nut assemblies. In this research, high-strength bolts classified as grade 10.9, with a nominal diameter of 20 mm, were used. Two sets of specimens were prepared for tensile testing. One group had bolts with three full threads protruding beyond the nut, consistent with standard guidelines, while the other group had bolts with no exposed threads, simulating a case of insufficient engagement. Each group consisted of five specimens. The experimental tests were supplemented by finite element simulations performed using SolidWorks and ABAQUS software, which allowed for detailed visualization and analysis of stress distribution and failure progression. The results of both the physical tests and simulations consistently showed that thread stripping was the dominant failure mode. Specimens with proper thread engagement not only withstood higher loads but also exceeded the minimum tensile strength requirements defined in design codes. In contrast, specimens without protruding threads failed at significantly lower loads, underperforming relative to design expectations. These findings highlight the importance of adhering to proper thread engagement practices during bolt installation and underscore the potential structural risks associated with deviating from standard assembly procedures.