This seems a reliability angle and I think it is ok for a large number of structures (samples). However, for a single structure, this approach does not tell you it is safe or not.
There are various 'damage indexes'. These are discipline-specific and quite subjective.
The web link below posts the Proceedings of the 9th Int Conf for Damage Assessment of Structures. It is open access.
Six Sigma method can be applied to anything and bring to a common value as Z score... We use Z score for Medical product to aircraft designs, hence I do not find any reason that should prevent using Six Sigma as a metric.
Six-sigma Principles can be very well applied to the defects/damages index. Your question is not clear, hence with the available information its not possible to suggest the suitable method for your Case.
Please refer to the attachments, you will understand which method you can apply to fix the limits (Upper control limit (UCL), Lower control limit (LCL)), and check whether your process is within the limits or not.
Further, based on your process performance, you can see whether it will fit into six-sigma limits or not. It is obvious that, applying the six-sigma principles to any process and achieving it, requires lot of streamlining, man power and management support, and also it takes minimum 5-6 years to achieve it.