This study aims to investigate the fatigue behavior and failure modes of bolted lap joints using Modified Tensile Specimens (MTS) under various cyclic load conditions. Emphasis is placed on identifying the relationship between load amplitude, fatigue life, and damage progression in low-carbon steel assemblies.

An experimental approach was adopted using MTS specimens fabricated from St 12 03 cold-rolled steel, joined with Grade 8.8 M4 bolts. Cyclic fatigue tests were conducted under zero-based loading at seven distinct force levels. Fracture surfaces were visually analyzed to identify dominant failure mechanisms.

The results revealed a strong inverse correlation between applied cyclic load and fatigue life. Three distinct failure modes were identified: bolt shear at high loads (5.4 kN), interface cracking and slippage at moderate loads (4.9-5.1 kN), and plate tearing or stable fatigue behavior at lower loads (=4.1 kN). The results highlight a progressive transition in failure mechanisms, from bolt shear at high loads to plate tearing and interface cracking at lower loads, providing essential insights for fatigue-resistant bolted joint design.

This study offers original insights into the fatigue behavior of bolted lap joints using MTS, a relatively underexplored configuration in fatigue assessment. By experimentally evaluating failure modes under varied cyclic load levels, the authors uncover critical transitions in damage mechanisms—from bolt shear to interface cracking and plate tearing—depending on the applied load. Unlike many existing studies focused on numerical modeling or bonded joints alone, this work provides empirical data rooted in real-world fastening conditions using cold-rolled low-carbon steel.