Ultrasonic Stress Measurement: Application to Preload Assessment on Already Tightened Bolts

When a material is under mechanical load, the stresses modify the velocity of acoustic waves. This consists in acoustoelastic effect. This property can be exploited for stress measurement in the material itself when the stress concerns the surface of the material, or in the bulk material, as in bolts. The ultrasonic bolt tightening control allows knowledge of preload stress in screws. The accuracy is much better than in the case of torque wrench. In fact, the use of this mechanical-based measurement equipment leads to uncertainties due to random resistant torques, induced by mechanical manufacturing tolerances. The ultrasonic measurement of the pre-load gives the information about the tension inside of the bolt, independently of mechanical uncertainties. This consists in an in-situ measurement. More, in some cases, the ultrasonic method allows knowledge of the preload on bolts already tightened, without untightening. This method is called bi-wave method, and has many connections with birefringent method. Examples of application of the method are presented about the differential mode case, which is largely known and industrialized, and in the case of bi-wave method, which lead to several on site applications, performed by CETIM with a prototype equipment.