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Surface strain measurement using pulsed laser shearography with fibre-optic imaging bundles

机译:使用脉冲激光剪切成像和光纤成像束进行表面应变测量

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摘要

This thesis describes the development of a shearography instrument for the quantitative measurement of surface strain on dynamic test objects. Shearography is a non-contact, full-field interferometric speckle technique used for the measurement of displacement gradient. It is often used in industry for qualitative inspection of industrial components. To fully characterize the surface strain, a total of six components of displacement gradient are required. These can be measured using shearography instrumentation with at least three measurement channels. Phase measurements from each measurement channel are combined using a matrix transformation to produce the orthogonal displacement gradient measurements. The instrument presented in this thesis possesses four measurement channels consisting of four views of the object under investigation. Images from the four views are transported to the shearing interferometer using coherent fibre-optic imaging bundles. The signals from the four views are then spatially multiplexed onto the four quadrants of a CCD camera. The optical source is a frequency doubled, pulsed Nd:YAG laser which is used to effectively ‘freeze’ the motion of the dynamic object for the duration of the laser pulse. The optical phase difference between images recorded from two laser pulses is determined using the spatial carrier technique. This method involves introducing a carrier frequency into the recorded speckle pattern using a Mach-Zehnder interferometer. A Fourier transform is used to access the phase dependent spectral features, from which the phase distribution is calculated. The instrument is first validated through the measurement of two static test objects. The results of these measurements are compared with modelled data and with results from a multiple-illumination-direction shearography system using a continuous-wave laser. The instrument is then used to investigate two dynamic objects; a plate rotating at 610 rpm and a speaker cone vibrating at frequencies in the range of 1 – 5 kHz.
机译:本文介绍了一种用于动态测试物体表面应变定量测量的剪切仪的开发。剪切成像是一种非接触式全场干涉散斑技术,用于测量位移梯度。它通常用于工业中对工业组件的定性检查。为了充分表征表面应变,总共需要六个位移梯度分量。可以使用具有至少三个测量通道的剪切成像仪进行测量。使用矩阵变换将来自每个测量通道的相位测量结果进行组合,以产生正交位移梯度测量结果。本文介绍的仪器具有四个测量通道,其中包括被调查对象的四个视图。使用相干光纤成像束将来自四个视图的图像传输到剪切干涉仪。然后将来自四个视图的信号在空间上复用到CCD摄像机的四个象限中。光源是倍频脉冲Nd:YAG激光,用于在激光脉冲持续时间内有效地“冻结”动态物体的运动。使用空间载波技术确定从两个激光脉冲记录的图像之间的光学相位差。该方法包括使用Mach-Zehnder干涉仪将载波频率引入记录的散斑图样中。使用傅立叶变换来访问与相位有关的频谱特征,从中计算出相位分布。首先通过测量两个静态测试对象对仪器进行验证。将这些测量的结果与建模数据以及使用连续波激光的多照明方向剪切成像系统的结果进行比较。然后,该仪器用于研究两个动态对象。一块板以610 rpm的转速旋转,并且扬声器锥体以1-5 kHz的频率振动。

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