Theoretical and experimental evidence for using impact modulation to assess bolted joints

This paper investigates the feasibility of extending the application of impact modulation from crack detection to the assessment of bolted joints. To support this effort, both theoretical and experimental evidence of the effectiveness of impact modulation to detect changes in bolt torque are presented. First, a nonlinear, single degree of freedom model is used to verify the correlation between changes in the nonlinear coefficient that represents the nonlinear contact forces present at a bolted interface and the amplitude of the sidebands generated in impact modulation testing. Next, a finite element representation of a two-beam, one-bolt assembly is presented to study effects not captured by the single degree of freedom model. Results of impact modulation simulations using the finite element model revealed that the amplitude of the sidebands in the response spectra depends not only on the nonlinearity present at the bolted interface, but also on other test parameters including impact amplitude and location, probing force amplitude and frequency, and sensor location. These findings are corroborated with experiments in which impact modulation was performed on a two-beam, one-bolt assembly similar to the assembly used in the simulations. Finally, impact modulation testing was conducted on the two-beam, one-bolt assembly using different bolt torque levels to demonstrate the method’s sensitivity to changes in bolt torque. A modulation index based on the amplitudes of the sidebands in the response spectrum was developed to quantify the results of the impact modulation testing. Results showed that the magnitude of the modulation index increased as the bolt torque decreased. The rate of change in the modulation index was largest when the bolt torque was $$<$$ 20 % of its maximum value.