Portable and Hybrid Laser Generation / Air-coupled Detection System for Non Destructive Inspection

This review presents a portable head instrument, using a hybrid laser generation/aircoupled detection system for ultrasonic non-destructive inspection. The equipment is made of a low-cost Nd:YAG pulsed laser linked to a multimode optic fiber to generate Surface Acoustic Waves (SAW). Instead of a long-pulse detection laser coupled to an interferometer, a small aircoupled ultrasonic transducer is used to perform the detection. A compact probe holder supporting the optical assembly and the air-coupled transducer has been designed, and integrated with an XY scanner system to move the probe above the sample. Inspection tests are carried-out on representative samples, and the performances of the portable head are evaluated for surface and subsurface defect detection in aeronautic composite materials. Introduction: Conventional ultrasonic techniques for Non-Destructive Testing (NDT) of materials manufactured by the aeronautic industry have traditionally relied on systems using contact piezoelectric transducers for both generation and detection. These standard inspection systems, including pulse-echo, through transmission and tandem mode require an immersion medium (usually water), or a coupling agent (gel, mineral oil...) between the sample to be inspected and the piezoelectric transducer. In the aeronautic and aerospace industry, a variety of composite materials (such as water-sensitive, porous or skin-honeycomb structures) do not allow the contact with this coupling medium. To overcome this obstacle and to improve speed inspection, there has been a steady effort to develop alternative NDT methods that are coupling free. These techniques can include thermography [1,2], shearography [3,4] or laser ultrasonics [5,6]. However, each one of these methods present attractive features and drawbacks. For example, laser-based ultrasonic techniques provide a number of advantages such as non contact generation and detection, broad bandwidth, ability to operate on curved and rough surfaces, but remain a complex and very expensive technology. Furthermore, a certain reflectivity of the detection surface is required. During the last decades, many improvements have been made in the design of air-coupled ultrasonic transducers. These type of transducers has already been investigated in the through transmission mode for NDT of aeronautic composite materials [7,8]. The major drawback is the low signal amplitude, due to the high acoustic impedance mismatch between solid and air (leading to low transmission of ultrasound across the interface). Despite of this, the flexibility of this new technique offers a potential for new compact and low-cost NDT systems. This paper describes the development of a hybrid portable prototype equipment suitable for surface and subsurface defect detection in metallic and composite materials. This device is based on a low cost pulsed Nd:YAG laser to generate Surface Acoustic Waves (Rayleigh waves) in the thermoelastic regime. Rayleigh wave motion is confined to a layer with thickness equal to about one wavelength [6,9]. This wave mode is always present, and its magnitude is the largest of all the other waves modes: this is because it only propagates over a two-dimensional surface rather than throughout three-dimensional space. To provide flexibility of operation under safety conditions, a high power optical fiber is used to deliver the laser energy onto the testing area. Instead of a long pulse detection laser coupled to an interferometer, a small air-coupled transducer performs the ultrasound detection. Although the bandwidth of air-coupled transducers is narrow compared to the bandwidth of interferometers, the variety of available commercial aircoupled transducers enable to operate in a frequency range between 0.1 MHz and 4-5 MHz. But with high frequencies, the ultrasound attenuation increases significantly in air, so the distance between the transducer and the sample must be reduced, involving difficult alignment. A probe holder supporting the optical assembly and the air-coupled transducer has been designed, and integrated with an XY scanner system to move the probe above the sample. This hybrid system is packaged as a compact NDT equipment, and could be easily transposed for in-service aircraft inspection. The performances of the portable laser generation/air-coupled detection system are evaluated on representative composite samples, and the results are compared with those obtained by other NDT methods. Equipment: The compact pulsed laser used for the generation of ultrasounds is a Q-swiched QUANTEL BRILLIANT Ultra laser. The maximum energy is 45 mJ @ 1064 nm, the repetition rate can be adjusted between 0 and 20 Hz, and the pulsewidth is 8 ns. The Nd:YAG oscillator is coupled to a 1 mm diameter multimode optical fiber (ended with an SMA connector), and a variable optical attenuator is used to adjust the maximum power level that can be delivered onto the sample without damage. In order to concentrate as much as possible the ultrasound energy in the direction of the air coupled transducer, a focusing probe was developed to focus the laser beam into a line. In this way sufficient energy can be deposited without raising the power density so high as to cause ablation, and the wave propagation direction is restricted to be perpendicular to the line. The focusing arrangement is made of a plano-convex lens to collimate the laser beam emerging from the fiber, and a cylindrical lens to focus the beam into the line (about 10 mm x 0.5 mm) at the top surface of the sample. The optical assembly is placed into a stackable lens tube.