Yoseph Bar-Cohen

Analysis of leaky Lamb waves in bonded plates

Abstract The feasibility of using ultrasonic nondestructive evaluation (NDE) methods to determine the quality of bonds in structural components is considered. The severe limitations of conventional NDE method in yielding quantitative results are indicated. Some recent results of a joint theoretical and experimental program of research using leaky Lamb waves (LLW) in laboratory specimens are presented. The LLW technique is shown to have several advantages over conventional techniques. Potential applications of the technique to nondestructively determine the elastic properties of bonds in several models are discussed.

Inversion of leaky Lamb wave data to determine cohesive properties of bonds

Abstract A systematic inversion scheme is presented to determine the cohesive properties of adhesive bonds from guided wave phase velocity data. The approach is based on an iterative least-squares procedure in which the nonlinear perturbation of the unknown parameters is linearized at each step. For an accurate and adequate data set, a continuous convergence zone for the procedure is shown to exist in the neighborhood of the solution point in the parameter space. The convergence zone can be reached by an automatic search routine. The bond-sensitive parts of the data are shown to have an important effect on the convergence behavior. Thus attention must be paid to the identification and acquisition of bond-sensitive data. Inaccurate or false data is found to have a strong negative influence on the success of the inversion scheme. The improper data may be detected and removed by the present procedure.

Determination of the properties of composite interfaces by an ultrasonic method

Abstract The feasibility of using a recently developed ultrasonic technique to determine certain macroscopic properties of the interface zones of composite laminates is studied. The strong influence of the elastic properties and the thickness of the interface zone on the phase velocity of guided waves is demonstrated by means of a simple model of a single fiber embedded in a layer of the matrix material. The overall dynamic elastic moduli of a unidirectional graphite-epoxy composite laminate are determined through inversion of guided wave dispersion data obtained by the leaky Lamb wave experiment. The thickness and elastic properties of the interlaminar interface zone in a cross-ply graphite-epoxy laminate are also estimated by the same approach.

Nondestructive Evaluation of Adhesive Bonds Using Leaky Lamb Waves

Adhesive bonding is a means for transferring load between structural components of an assembly. Proper transfer can be accomplished only through a continuous adhesive medium between the adherends. Furthermore, the adhesive must have sufficiently high strength to allow the structure to meet design requirements.

Recent Advances in the Application of Leaky Lamb Waves to the Nondestructive Evaluation of Adhesive Bonds

Abstract In earlier work, the feasibility of applying the Leaky Lamb Wave (LLW) method to the nondestructive evaluation (NDE) of bonded rubber/metal structures was demonstrated. The capability of LLWs to detect and delineate flaws at the bond line was proven, even when the adherends remain in intimate contact. However, variations in adherend properties, surface orientation and thickness can adversely affect detection of bond flaws and assessment of bond quality. In this paper, parameters which degrade both LLW sensitivity and resolution to bond flaws are discussed. Examples of the effects of cold work, thickness change and specimen tilt are presented along with bond flaw detection and characterization results. Also, advances in the theory of bond flaw NDE by LLWs are presented.

Deep Drilling and Sampling via Compact Low-Mass Rotary-Hammer Auto-Gopher

Increasingly, NASA exploration missions are including in-situ sampling tasks. The difficulties of acquiring samples have been identified by the designers of the 2007 Mars Phoenix Scout and the 2011 Mars Science Laboratory missions. In particular, it has been indicated that planetary drilling faces great challenges. The associated challenges grow significantly with the depth of drilling and it is the objective of the current study to develop an autonomous wireline rotary-percussive drill (called Auto-Gopher) that can reach a depth of several meters. In developing the Auto-Gopher efforts are made to take advantage of the hammering capabilities of the Ultrasonic/Sonic piezoelectric mechanism to fracture rocks using low preload. This mechanism was demonstrated in 2005 to reach about 2-m deep in ice at Lake Vida, Antarctica. The augmentation of the hammering by rotation of the bit having flutes provides both effective cuttings removal and faster drilling. The progress in developing the Auto-Gopher will be described and discussed in this paper.

NONDESTRUCTIVE CHARACTERIZATION OF DEFECTS USING ULTRASONIC BACKSCATTERING

Ultrasonic backscattering measurements were made for discontinuities having a wavenumber-length smaller than 1. These tests were conducted for multilayered materials consisting of metals, composites, and their combination as an ARALL material. Amplitude measurements of the backscattered wave in the time domain were found to provide a parameter for detection and characterization of various types of defects. Some unexplained phenomena were observed that cannot be predicted by any of the documented theories. The experimental results and their potential application to nondestructive evaluation are considered in this paper.

Auto-Gopher-II: a wireline rotary-hammer ultrasonic drill that operates autonomously

An important challenge of exploring the solar system is the ability to penetrate at great depths the subsurface of planetary bodies for sample collection. The requirements of the drilling system are minimal mass, volume and energy consumption. To address this challenge, a deep drill, called the Auto-Gopher II, is currently being developed as a joint effort between JPL’s NDEAA laboratory and Honeybee Robotics Corp. The Auto-Gopher II is a wireline rotaryhammer drill that combines breaking formations by hammering using a piezoelectric actuator and removing the cuttings by rotating a fluted bit. The hammering is produced by the Ultrasonic/Sonic Drill/Corer (USDC) mechanism that has been developed by the JPL team as an adaptable tool for many drilling and coring applications. The USDC uses an intermediate free-flying mass to convert high frequency vibrations of a piezoelectric transducer horn tip into sonic hammering of the drill bit. The USDC concept was used in a previous task to develop an Ultrasonic/Sonic Ice Gopher and then integrated into a rotary hammer device to develop the Auto-Gopher-I. The lessons learned from these developments are being integrated into the development of the Auto-Gopher-II, an autonomous deep wireline drill with integrated cuttings and sample management and drive electronics. In this paper the latest development will be reviewed including the piezoelectric actuator, cuttings removal and retention flutes and drive electronics.

Synchronous separation, seaming, sealing and sterilization (S4) using brazing for sample containerization and planetary protection

The return of samples back to Earth in future missions would require protection of our planet from the risk of bringing uncontrolled biological materials back with the samples. This protection would require “breaking the chain of contact (BTC)”, where any returned material reaching Earth for further analysis would have to be sealed inside a container with extremely high confidence. Therefore, the acquired samples would need to be contained while destroying any potential biological materials that may contaminate the external surface of the container. A novel process that could be used to contain returning samples has been developed and demonstrated in a quarter scale size. The process consists of brazing using non-contact induction heating that synchronously separates, seams, seals and sterilizes (S4) the container. The use of brazing involves melting at temperatures higher than 500°C and this level of heating assures sterilization of the exposed areas since all carbon bonds (namely, organic materials) are broken at this temperature. The mechanism consists of a double wall container with inner and outer shells having Earth-clean interior surfaces. The process consists of two-steps, Step-1: the double wall container halves are fabricated and brazed (equivalent to production on Earth); and Step-2 is the S4 process and it is the equivalent to the execution on-orbit around Mars. In a potential future mission, the double wall container would be split into two halves and prepared on Earth. The potential on-orbit execution would consist of inserting the orbiting sample (OS) container into one of the halves and then mated to the other half and brazed. The latest results of this effort will be described and discussed in this manuscript.

Drilling, Coring and Sampling Using Piezoelectric Actuated Mechanisms: From the USDC to a Piezo-Rotary-Hammer Drill

NASA exploration missions are increasingly including sampling tasks but with the growth in engineering experience (particularly, Phoenix Scout and MSL) it is now very much recognized that planetary drilling poses many challenges. The difficulties grow significantly with the hardness of sampled material, the depth of drilling and the harshness of the environmental conditions. To address the requirements for samplers that could be operated at the conditions of the various bodies in the solar system, a number of piezoelectric actuated drills and corers were developed by the Advanced Technologies Group of JPL. The basic configuration that was conceived in 1998 is known as the Ultrasonic/Sonic Driller/Corer (USDC), and it operates as a percussive mechanism. This drill requires as low preload as 10N (important for operation at low gravity) allowing to operate with as low-mass device as 400g, use an average power as low as 2-3W and drill rocks as hard as basalt. A key feature of this drilling mechanism is the use of a free-mass to convert the ultrasonic vibrations generated by piezoelectric stack to sonic impacts on the bit. Using the versatile capabilities of the USDC led to the development of many configurations and device sizes. Significant improvement of the penetration rate was achieved by augmenting the hammering action by rotation and use of a fluted bit to remove cuttings. To reach meters deep in ice a wireline drill was developed called the Ultrasonic/Sonic Gopher and it was demonstrated in 2005 to penetrate about 2-m deep at Antarctica. Jointly with Honeybee Robotics, this mechanism is currently being modified to incorporate rotation and inchworm operation forming Auto-Gopher to reach meters deep in rocks. To take advantage of the ability of piezoelectric actuators to operate over a wide temperatures range, piezoelectric actuated drills were developed and demonstrated to operate at as cold as -200 o C and as hot as 500 o C. In this paper, the developed mechanisms will be reviewed and discussed including the configurations, capabilities, and challenges.