A second source of low cycle vibration is perhaps repeated tuning and detuning of the instrument.
This is again difficult to quantify but it is not impossible that some tens of cycles of 10% of the axial tension are involved.
Many musicians initially initially apply higher axial tensions to stabilize the attachment of the string at the machine (tuning) head prior to reducing to the required value (relaxation tuning).
This may slightly increase the effective cyclic stress amplitude and reduce the maximum tensile stress at the bridge.
In any case, it is clear that the vibratory stresses are likely to be small in comparison with the steady mean stress.