Modal impact testing for estimating bolted joint tightness

The tension level in a bolt is difficult to determine and control. Based on simple hammer impacts, a novel technique is proposed to quantify the level of bolt tightness by analyzing natural frequencies and damping ratios of the bolt. The technique is investigated experimentally by testing two different bolts (short and long) and measuring accelerations in the transverse directions.

At low tension the squared natural frequency of the first bending mode increases strongly with tension. As the bolt is gradually tightened, the squared frequency starts changing more weakly and approximately linearly with tension. By signal processing the transient response from the hammer impact, and treating problems of beat-frequencies due to the near cross sectional symmetry in the bolt, the corresponding effective linear damping ratios can also be obtained.

Further, a scanning laser Doppler vibrometer (SLDV) is used to measure the mode shapes of the long bolt and it is studied how these change with tension.