PSI - Issue 84

L. Pisani et al. / Procedia Structural Integrity 84 (2026) 1144–1150 L. Pisani et al. / Structural Integrity Procedia 00 (2026) 000–000

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When an X-ray beam strikes a crystalline structure, the atoms diffract the incident radiation, and the radiation diffracted by each atom belonging to crystalline planes determines a phase difference. If the distance between the two adjacent beams is a multiple of the incident radiation wavelength, a detectable constructive interference occurs, showing a diffraction peak at a given theta angle When a sample is subjected to stress, the distance between the atomic planes parallel to the direction of the stress increases in case of traction and decreases in case of compression. This effect determines a variation of the Bragg angle which is a function of the atomic distance variation Δd and therefore of the strain ε according to Hooke's law applied at the crystalline level. ε= − 0 0 = Δ 0 (2) Knowing the Young's modulus and Poisson's ratio of the material under test, it is possible to determine the corresponding applied stress.

Fig. 1 Stress determination as relation between ε and sin 2 (Ψ) (Source GNR). Since crystalline materials are composed of crystallites oriented randomly in space, and these are subjected to different internal stresses depending on their orientation, to measure the total stress, it is necessary to collect the contributions of as many crystallites as possible. To do this, it is necessary to move the sample at different Ψ angles, which is obviously impossible for massive samples such as a bridge strand. In this case, the scanning is performed by moving the measuring head.

Fig. 2 Crystallites response at different sin 2 (Ψ) .

2.2. Magnetic Barkhausen Noise (MBN) Barkhausen effect is the phenomenon of discontinuous, stepwise changes in the magnetization of a ferromagnetic material when subjected to a slowly changing external magnetic field. This occurs because the magnetic domains within the material, which are microscopic clusters of aligned atomic magnets, jump from one position to another as their boundaries move. These jumps, or "Barkhausen jumps," produce small, abrupt changes in the magnetic flux, which can be heard as a crackling or clicking sound through an amplifier and speaker, or measured as electrical pulses.