Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:9) has been observed to be a site for fatigue crack nucleation at low applied stress level ( max= 13 Y), where the combination of cyclic and residual stress is not enough to cause stress relaxation (Peters et al., 2002).
Category: