Laser-ultrasonic measurement of elastic properties of a thin sheet and of tension applied thereon

A method for non-contact characterization of a rapidly moving sheet, comprising the steps of:

• illuminating the sheet at a first location with a first pulsed laser beam for generating acoustic waves propagating through the sheet thickness and over its surface,
• illuminating the sheet at a second location with a second laser beam, on the same side or the opposite side of the sheet, this second location being spaced from the first location by a predetermined distance, said sheet scattering the second beam to produce a scattered beam with a plurality of speckles,
• collecting the scattered beam, such collected scattered beam having a phase or frequency modulation representative of the acoustic motion at the second location, and
• demodulating the phase or frequency modulation to provide an electrical signal representative of the acoustic motion at the second location, and
• analyzing the electrical signal to determine a characteristic or a set of characteristics of the sheet,
• characterized in that said first laser has a wavelength selected to ensure progressive absorption below the surface of the sheet so as to produce a distributed acoustic source below the surface and an intensity below a threshold value to avoid damage to the sheet, and said demodulating of the phase or frequency modulation is carried out with a speckle-insensitive interferometer.