Method for detecting the maximum expansion of cracks in an object, in particular a building, within a time period, - wherein a fibre optic cable is arranged in the object, in particular the building, or on the surface of the object, in particular the surface of the building, wherein the fibre optic cable is surrounded by a cladding layer structure such that when a prespecified limit force (F s) is exceeded the fibre optic cable is displaced locally in longitudinal direction relative to the cladding layer structure, - wherein at the beginning of the time period a distributed fibre-optic reference measurement is performed in the fibre optic cable and the measurement result determined thereby is kept available as a reference measurement result, - wherein at the end of the time period a distributed fibre-optic follow-up measurement is performed in the same fibre optic cable and based on the differences between the reference measurement and the distributed fibre-optic follow-up measurement, a distribution of the strain values (ε) caused by cracks in the object, in particular in the building, is determined over the cable length of the fibre optic cable, characterised in that, - a model is specified, which for a number of maximum crack widths (w max) respectively has at least one local model strain measurement value curve which specifies the local strain (ε) of the fibre optic cable over a portion of the cable length of the fibre optic cable in the region of a crack with the respective maximum crack width (w max), - in that at least one section within the distributed strain measurement value is identified in which the strain values (ε) exceed a predetermined threshold, - in that the determined distributed strain measurement value in the at least one identified section is brought into line with the local model strain measurement value curves of the model and a search is made for the maximum crack width (w max) which is assigned to the local model strain measurement value curve which coincides best with the determined distributed strain measurement value in the identified section, and - in that the crack width used with optimal alignment in the specified model is regarded as a measurement value for the maximum crack width (w max) of the respective actual crack within the time period in the object, in particular the building. - wherein a fibre optic cable is arranged in the object, in particular the building, or on the surface of the object, in particular the surface of the building, wherein the fibre optic cable is surrounded by a cladding layer structure such that when a prespecified limit force (F s) is exceeded the fibre optic cable is displaced locally in longitudinal direction relative to the cladding layer structure, - wherein at the beginning of the time period a distributed fibre-optic reference measurement is performed in the fibre optic cable and the measurement result determined thereby is kept available as a reference measurement result, - wherein at the end of the time period a distributed fibre-optic follow-up measurement is performed in the same fibre optic cable and based on the differences between the reference measurement and the distributed fibre-optic follow-up measurement, a distribution of the strain values (ε) caused by cracks in the object, in particular in the building, is determined over the cable length of the fibre optic cable, - a model is specified, which for a number of maximum crack widths (w max) respectively has at least one local model strain measurement value curve which specifies the local strain (ε) of the fibre optic cable over a portion of the cable length of the fibre optic cable in the region of a crack with the respective maximum crack width (w max), - in that at least one section within the distributed strain measurement value is identified in which the strain values (ε) exceed a predetermined threshold, - in that the determined distributed strain measurement value in the at least one identified section is brought into line with the local model strain measurement value curves of the model and a search is made for the maximum crack width (w max) which is assigned to the local model strain measurement value curve which coincides best with the determined distributed strain measurement value in the identified section, and - in that the crack width used with optimal alignment in the specified model is regarded as a measurement value for the maximum crack width (w max) of the respective actual crack within the time period in the object, in particular the building.