The Polytec 3D scanning laser vibrometer, is capable of evaluating test specimen surface areas ranging from mm2 to m2 at a stand-off distance between 0。35 m and 5 m。 The laser light of the class II HeNe laser has a wavelength of 633 nm which corresponds to red light。 The velocity of a structure at one point is measured simultaneously from three different angles as illustrated in Fig。 1。 An orthogonal decomposition leads to the three-dimensional velocity field in a Cartesian coordi- nate system [16, 17]。

The PSV-400-3D consists of three inpidual sen- sor heads (PSV-I-400), each containing a vibrometer sensor (OFV-505), that are connected to a controller (OFV-5000), a digital camera (A-CAZ-1000) and     a

data management system (PSV-W-402)。 A junction box (PSV-E-401-3D) serves as an interface between the sensor heads, the controllers and the data management system。

The velocity decoder (VD-07) resolves the change in frequency due to the Doppler effect to a voltage proportional to the measured velocity with a maximal decoder frequency of 350 kHz。 Six different ranges are available that span from 1 mm s−1/V to 50 mm s−1/V。 For a peak voltage range of 10 V, this corresponds to a peak velocity range of 0。01 m s−1 to 0。5 m s−1。 The res- olution is dependent on the selected range and on the frequency。 It reaches values between 0。02 μm s−1/Hz1/2 and 0。2 μm s−1/Hz1/2。

The high precision scan unit has a scanning range of ±20◦ in x and y-direction。 The angular precision equals <0。002◦。 At a stand-off distance of 500 mm, this corresponds to a resolution of 17 μm on the the measurement object with the spot size of the laser of 40 μm。 The angular stability for the laser beams equals

<0。01◦/hr。

The standard PSV-400-3D-SLV is not feasible for strain measurements, as the errors in the laser beam po- sitioning on the object are too large。 Therefore, the sys- tem was upgraded with a triangulation software (PSV- S-TRIA Optimizing 3-D Geometry), that improves the beam positioning on the measurement object, and an external high resolution camera (A-CAZ-1000), that enhances the resolution for the measurement points defined on the camera image。 The higher  resolution of the camera leads to more precise matching of the scan points from the camera image to the measurement object。

The monochrome A-CAZ-1000 high resolution dig- ital video camera has a resolution of 1034 × 779 pixels。 The macro zoom lens has an optical zoom from 8。5 mm to 90 mm。 The smallest field of view is 6 mm × 8 mm。 To obtain optimal results the axis of the camera must

Fig。 1 Illustration of experimental arrangement for measurement of the displacement field on a flat plate, from which the strain field can be estimated for the whole surface of the structure

be aligned to the measurement axis。 This is ensured by the PSV-A-T34 tripod used for the measurement, which is depicted in Fig。 2 together with the mounted sensor heads。

The PSV software (PSV 8。7) enables recording, dis- playing and processing of the measured data on grids with up to 512 × 512 points。

Experimental Apparatus and Specimens

The specimens used to evaluate the in-plane strain measurements were positioned between the two clamps of the testing apparatus depicted in Fig。 3。 The upper clamp is connected to the 15 kN load cell (Novatech F204) and a piezo stack actuator (P-235。40 PICA)。 The flexible tip (P-176。60) was utilized to isolate the piezo stack from bending load and torsional load。 The distance between the clamps is variable and was set to 143 mm in the scope of this study。 The testing apparatus is positioned at a distance of 670 mm from the sensor heads。

The pre-loaded piezo actuator has a travel range  of

60  μm。  The  pushing  force  can  reach  a  maximum of