A method of monitoring stress in a composite, comprising: scanning the composite using an NQR spectrometer, the composite comprising a polymer matrix and a microcrystalline material disposed in the matrix, wherein the microcrystalline material comprises molecules having nuclei with respective nuclear quadrupole moments; correlating one or more locations of the composite with NQR spectra collected at the one or more locations to produce localized NQR spectra; characterised by: generating a microscopic strain distribution index map of the composite from the localized NQR spectra, comprising determining the respective microscopic strain distribution index values for each spectral line of interest as a ratio between a full width at half maximum (FWHM) and a central NQR line frequency of the spectral line of interest; comparing the generated microscopic strain distribution index map to an original microscopic strain distribution index map of the composite; determining differences in the microscopic strain distribution index values between said maps to quantify fatigue in the composite and identifying locations that are stressed based on said ratios to identify the presence of structural fatigue, structural precursors to failure, or a combination thereof. scanning the composite using an NQR spectrometer, the composite comprising a polymer matrix and a microcrystalline material disposed in the matrix, wherein the microcrystalline material comprises molecules having nuclei with respective nuclear quadrupole moments; correlating one or more locations of the composite with NQR spectra collected at the one or more locations to produce localized NQR spectra; characterised by: generating a microscopic strain distribution index map of the composite from the localized NQR spectra, comprising determining the respective microscopic strain distribution index values for each spectral line of interest as a ratio between a full width at half maximum (FWHM) and a central NQR line frequency of the spectral line of interest; comparing the generated microscopic strain distribution index map to an original microscopic strain distribution index map of the composite; determining differences in the microscopic strain distribution index values between said maps to quantify fatigue in the composite and identifying locations that are stressed based on said ratios to identify the presence of structural fatigue, structural precursors to failure, or a combination thereof. generating a microscopic strain distribution index map of the composite from the localized NQR spectra, comprising determining the respective microscopic strain distribution index values for each spectral line of interest as a ratio between a full width at half maximum (FWHM) and a central NQR line frequency of the spectral line of interest; comparing the generated microscopic strain distribution index map to an original microscopic strain distribution index map of the composite; determining differences in the microscopic strain distribution index values between said maps to quantify fatigue in the composite and identifying locations that are stressed based on said ratios to identify the presence of structural fatigue, structural precursors to failure, or a combination thereof.