A New kHz Rate Laser-Ultrasound Scanner for Ply-by-Ply Imaging of Defects, Pores and Inconsistencies in Composite Structures

Among NDT methods, only US and X-ray tomography can precisely locate three- dimensional defects regardless of composite type. X-ray systems, however, are very expensive, time-consuming and cumbersome. In addition, the chamber limits the size of the sample to several centimeters and makes it completely unsuitable for field applications. Conventional US pulse-echo techniques usually require couplants or full sample immersion for optimal energy transfer, which can affect overall scan speed and limit the applications where they can be used. Laser-ultrasound (LU) has many advantages over conventional US. First, laser-generated US transients are ultra-wideband, providing at least 3 times better resolution compared to conventional US transducers with the same characteristic frequency [1]. Second, the system is fundamentally non-contact and removes all issues related to US coupling. Its typical disadvantages are low sensitivity, instability, low pulse repetition frequency and high cost. We have recently overcome these limitations with a new kHz rate fiber-optic LU scanner [1-4]. Here we show that the scanner can provide nearly X-ray quality 3D images and locate almost all imperfections in composites: visualize pores [5] and wrinkles, evaluate heat damage [6] and locate impact damage with a sub-ply spatial resolution

Non-Contact Evaluation of Residual Stress in Metals with Laser-Generated Surface Acoustic Waves and a Point-Like Fiber-Optic Sagnac Detector

Stress can remain in a material after the original load is removed. It can be purposely introduced to improve the properties of structural components, but can also be undesired, shortening a component’s lifetime, changing its original geometry or even leading to failure. There is a large spectrum of problems where residual stress must be evaluated. An acousto-elastic approach is non-destructive and uses stress-dependent alterations in ultrasound (US) speed of bulk or surface modes [1]. However, these alterations are incredibly small (10-3 ÷ 10-5) and, thus, accurate measurement of both the US wave speed and propagation distance is required. Thickness measurement is not required for surface acoustic waves (SW) as compared to bulk acoustic modes. However, it requires a well determined distance between source and detector and very accurate time-of-flight measurement. Here we show that an approach based on laser-generated SW can solve this problem when a highly sensitive, point-like optical detector [2, 3] is used on receive. Using a laser beam focused to a narrow strip about 100 μm wide as a wave source and a modified Sagnac interferometer [2, 3] with an 8 μm diameter beam on receive, it is possible to use a short propagation path (5-10 mm) to obtain the required accuracy of time-of-flight measurements. For instance, the relative wave speed was estimated with an error of approx. 0.025% (i.e. 2.5*10-4) when only 20 signal averages were applied. The in-plane distribution of relative deviations of SW speed (proportional to stress) can be obtained with 2D scanning over a sample [4]. An example of relative SW speed deviations in one cross-section is shown in Fig.1 for a stainless steel sample

A New kHz Rate Laser-Ultrasound Scanner for Ply-by-Ply Imaging of Defects, Pores and Inconsistencies in Composite Structures

Among NDT methods, only US and X-ray tomography can precisely locate three- dimensional defects regardless of composite type. X-ray systems, however, are very expensive, time-consuming and cumbersome. In addition, the chamber limits the size of the sample to several centimeters and makes it completely unsuitable for field applications. Conventional US pulse-echo techniques usually require couplants or full sample immersion for optimal energy transfer, which can affect overall scan speed and limit the applications where they can be used. Laser-ultrasound (LU) has many advantages over conventional US. First, laser-generated US transients are ultra-wideband, providing at least 3 times better resolution compared to conventional US transducers with the same characteristic frequency [1]. Second, the system is fundamentally non-contact and removes all issues related to US coupling. Its typical disadvantages are low sensitivity, instability, low pulse repetition frequency and high cost. We have recently overcome these limitations with a new kHz rate fiber-optic LU scanner [1-4]. Here we show that the scanner can provide nearly X-ray quality 3D images and locate almost all imperfections in composites: visualize pores [5] and wrinkles, evaluate heat damage [6] and locate impact damage with a sub-ply spatial resolution.</p

Non-Contact Evaluation of Residual Stress in Metals with Laser-Generated Surface Acoustic Waves and a Point-Like Fiber-Optic Sagnac Detector

Stress can remain in a material after the original load is removed. It can be purposely introduced to improve the properties of structural components, but can also be undesired, shortening a component’s lifetime, changing its original geometry or even leading to failure. There is a large spectrum of problems where residual stress must be evaluated. An acousto-elastic approach is non-destructive and uses stress-dependent alterations in ultrasound (US) speed of bulk or surface modes [1]. However, these alterations are incredibly small (10-3 ÷ 10-5) and, thus, accurate measurement of both the US wave speed and propagation distance is required. Thickness measurement is not required for surface acoustic waves (SW) as compared to bulk acoustic modes. However, it requires a well determined distance between source and detector and very accurate time-of-flight measurement. Here we show that an approach based on laser-generated SW can solve this problem when a highly sensitive, point-like optical detector [2, 3] is used on receive. Using a laser beam focused to a narrow strip about 100 μm wide as a wave source and a modified Sagnac interferometer [2, 3] with an 8 μm diameter beam on receive, it is possible to use a short propagation path (5-10 mm) to obtain the required accuracy of time-of-flight measurements. For instance, the relative wave speed was estimated with an error of approx. 0.025% (i.e. 2.5*10-4) when only 20 signal averages were applied. The in-plane distribution of relative deviations of SW speed (proportional to stress) can be obtained with 2D scanning over a sample [4]. An example of relative SW speed deviations in one cross-section is shown in Fig.1 for a stainless steel sample.</p