A method of assessing a structural defect presence in a structure, the method comprising the steps of: determining at least one critical damage strain value of the structure, capturing a first image of a surface of the structure under a first loading condition; capturing a second image of the surface of the structure under a second loading condition; assigning a position matrix on the captured first image; obtaining a deformation matrix comprising a deformation value at each position of the position matrix by using an image correlation technique on the first and second images, and by comparing deformation of corresponding parts of the surface captured therein; calculating a strain matrix using the obtained deformation matrix; and determining a micro-crack to be present at a position if an element of the strain matrix representing the strain at the position is greater than or equal to a predetermined critical damage strain value, wherein the critical damage strain value is a strain value at which a micro-crack is detected or predicted to appear and/or propagate, characterised by a step of determining and/or detecting a crack propagation path comprising the sub-steps of: determining a potential damage zone which is a set of all the neighbouring positions determined to have a micro-crack present; obtaining a potential damage strain matrix by selecting the elements relating to the potential damage zone from the strain matrix; determining at least one local maximum strain value for each row and/or column of the potential damage strain matrix if an element therein represents a strain with greater value than both the previous and subsequent elements in the potential damage strain matrix; and determining the crack propagation path to be a collection of positions related to the at least one local maximum strain values. determining at least one critical damage strain value of the structure, capturing a first image of a surface of the structure under a first loading condition; capturing a second image of the surface of the structure under a second loading condition; assigning a position matrix on the captured first image; obtaining a deformation matrix comprising a deformation value at each position of the position matrix by using an image correlation technique on the first and second images, and by comparing deformation of corresponding parts of the surface captured therein; calculating a strain matrix using the obtained deformation matrix; and determining a micro-crack to be present at a position if an element of the strain matrix representing the strain at the position is greater than or equal to a predetermined critical damage strain value, wherein the critical damage strain value is a strain value at which a micro-crack is detected or predicted to appear and/or propagate, characterised by a step of determining and/or detecting a crack propagation path comprising the sub-steps of: determining a potential damage zone which is a set of all the neighbouring positions determined to have a micro-crack present; obtaining a potential damage strain matrix by selecting the elements relating to the potential damage zone from the strain matrix; determining at least one local maximum strain value for each row and/or column of the potential damage strain matrix if an element therein represents a strain with greater value than both the previous and subsequent elements in the potential damage strain matrix; and determining the crack propagation path to be a collection of positions related to the at least one local maximum strain values. determining a potential damage zone which is a set of all the neighbouring positions determined to have a micro-crack present; obtaining a potential damage strain matrix by selecting the elements relating to the potential damage zone from the strain matrix; determining at least one local maximum strain value for each row and/or column of the potential damage strain matrix if an element therein represents a strain with greater value than both the previous and subsequent elements in the potential damage strain matrix; and determining the crack propagation path to be a collection of positions related to the at least one local maximum strain values.