Summary of system
Electronic speckle pattern interferometry (ESPI) is an optical method for measuring very small displacements and strains. It relies on interference between diffusely-reflected light from two beams, or from diffusely-reflected light from one beam and from a reference beam. In the form which our system implements, it can measure 3D displacements on a specimen (though the out-of-plane displacements can o nly be measured relative to other points on the specimen). It is extremely sensitive and can detect displacements in the order of 50 nm in-plane and 20 nm out-of-plane, but the disadvantage of this sensitivity is that the system is also extremely sensitive to vibration, so is normally used in a vibration-isolated environment. The displacements are presented as maps of displacement components and can be differentiated with respect to spatial coordinates to give maps of in-plane strain. It is suitable for measuring small strains (in the order of 50-100 microstrain upwards) making it appropriate for work in the elastic region. Large strains can be measured by applying the load in a number of small steps.
CAMERA has purchased a Dantec Q300 3D ESPI system for mounting on a dedicated Instron 5966 10kN testing machine. These are both mounted on a vibration isolation table. Our system is supplied with long arms to enhance the in-plane sensitivity of the system. It incorporates a 1.4 MP monochrome camera, and is equipped with a 6-15 mm zoom lens to cover a range of fields of view from around 40mm x 40mm (TBC) up to around 200mm x 200mm (limited by laser intensity).
Because of the vibration sensitivity, the system is recommended for use only in L4-144 which is equipped with a large vibration isolation table and a dedicated tensile testing machine. Experience indicates that reasonable results are unobtainable unless the table is “inflated”. Specimens will need to be prepared with a diffuse surface such as that obtained by spraying with matt white paint.
System includes two 70 mW borderline IR/visible class 3B lasers. In order to become an authorised user of this equipment, the researcher must have a current designated laser user and must also have either completed the ESPI training course in the last year, or have been a previous user of the system less than a year ago. The system will change with time as it adapts and new protocols might be introduced as usage develops.
To discuss use of the system contact Arthur Jones