DC Potential Drop Technique Selecting Probes Distances Properly for Sizing Deep Surface Cracks

D-C potential drop (DCPD) technique is a powerful tool for quantitative NDE of cracks. The technique using four probes which are in close proximity to each other has been proposed for NDE of surface cracks; that is the closely coupled probes potential drop (CCPPD) technique. It has been shown that the sensitivity of CCPPD technique to evaluate a small crack is enhanced significantly in comparison with the usual method. On the other hand, since CCPPD technique has been developed to evaluate a small crack sensitively, it is not fit to evaluate deep cracks which are sometimes found in the structural components of power plants. The objective of this study was to enhance the sensitivity of evaluating deep surface cracks. By extending the distance between current input and output probes, the change in potential drop with the change in the depth of deeper crack becomes large. But the voltage of potential drop becomes small to measure, because the current density in the material becomes low. The voltage of the potential drop can be increased by increasing the applying current, but the current would also be limited by the equipment or contacting probes. Then the way to select the appropriate distances between probes from the viewpoints of the sensitivity and the required current has been shown.

Development of Crack Depth Indicator for Deep Surface Crack

Crack depth indicator using a direct current potential drop technique has been generally applied to the nondestructive inspection in the thermal power plants such as steam pipe and turbine casing. In this paper, the authors presented practical studies concerning the development of the advanced crack depth indicator for the deep surface crack. The authors have conducted experimental measurements using the customary crack depth indicator inspecting the fatigue pre-cracked plate specimens of some materials. The results showed that the crack depth indicator was effective to inspect the surface shallow crack, however, it was hardly applied to the deep crack. And the sensor of this equipment was found to be difficult to apply some practical conditions such as contact surfaces and mechanical designs of inspected structures. To get over the restrictions, the authors have developed a new type of crack depth indicator and sensor. The authors have conducted additional experimental measurements applying this equipment using some pre-cracked specimens such as plates and structural components of some materials.

Methods to Improve the Accuracy of Registration for Multimodal Inspection of Mechanical Parts

Registration refers to the process of aligning corresponding features in images or point data sets in the same coordinate system. Multimodal inspection is a growing trend wherein an accurate measurement of the part is made by fusing data from different modalities. Registration is a key task in multimodal data fusion. The main problem with high-accuracy registration comes from noise inherent in the measurement data and the lack of the one-to-one correspondence in the data from different modalities. We present methods to deal with outliers and noise in the measurement data to improve registration accuracy. The proposed algorithms operate on point sets. Our method distinguishes between noise and accurate measurements using a new metric based on the intrinsic geometric characteristics of the point set, including distance, surface normal and curvature. Our method is unique in that it does not require a-priori knowledge of the noise in the measurement data, therefore fully automatic registration is enabled. The proposed methods can be incorporated into any point-based registration method. It was tested with the traditional ICP (Iterative Closest Point) algorithm with application to the data registration among point, image, and mesh data. The proposed method can be applied to both rigid and non-rigid registration.

Evaluation of Solid Particle Erosion Damage on E-Glass/Epoxy Composites Using Acoustic Emission Activity

The effect of solid particle erosion on the strength properties of E-glass/epoxy composite was investigated. Solid particle erosion with SiC particles 400 μm to 500 μm in diameter was simulated on 12 ply [45°/−45°/0°/45°/−45°/0°]s E-glass/epoxy composites with constant particle velocity of 42.5 m/s at impact angles of 90°, 60°, and 30° for 30, 60, 90 and 120 seconds. Damaged and undamaged specimens were subjected to tensile tests while monitoring their acoustic emission (AE) activity. An erosion damage parameter was defined as a function of the particle impact angle and erosion duration to determine the residual tensile strength of the composite. Scanning electron microscope (SEM) images of the erosion damaged specimens revealed the same damage mechanism occurred at different impact angles. The distribution of AE events by event duration, ring down counts and energy distribution were used to characterize the different damage mechanisms that occurred during tensile loading of damaged and undamaged specimens. The results showed AE activity could be used to distinguish between different damage mechanisms within the composite, such as fiber/matrix debonding, delamination and fiber fracture. The Weibull probability distribution model and the AE stress delay parameter model were developed to relate the AE activity to the erosion damage and residual strength. The results showed both the Weibull probability model and the stress delay model could be used to predict residual strength of the composites.

Using Phased Array Technology and Embedded Ultrasonic Structural Radar for Active Structural Health Monitoring and Nondestructive Evaluation

The embedded ultrasonic structural radar (EUSR) was developed based on phased array technology. It can interrogate large structural areas from a single location using ultrasonic guided Lamb and Rayleigh waves generated by tuned piezoelectric wafer active sensors (PWAS) that are permanently attached to the structure. This paper brings together several aspects of the implementation and application of EUSR to structural damage detection: (a) improving the near field damage detection; (b) designing optimized phased-array patterns; (c) designing a mini phased array for compact structures with complicated geometries and multiple boundaries. Firstly, we deduced a generic formulation for phased array directional beamforming using the exact traveling waves formulation without the limiting parallel-rays assumption used by other investigators. This algorithm has been implemented in the EUSR LabVIEW program and its performance has been verified through simulation and experimental tests. Secondly, we studied the beamforming and lobe steering characteristics of a 1-D linear array design. The influence of several geometry parameters was discussed in order to achieve the optimal directionality, including the number of sensors in the phased array, the spacing between adjacent sensors, and the steering direction angles. Extensive simulation studies have shown that the well-behaved directional beamforming can be achieved with judicious array design. Proof-of-concept experiments for testing these results have also been set up and the preliminary results are confirming the effectiveness of our approach. Thirdly, we investigated the possibility of applying the EUSR phased array method to compact specimens and proposed the design of a mini phased array. Laboratory experiments have been carried out to prove the successful implementation of this concept. Finally, the paper ends up with discussions and conclusions regarding the beamforming, optimization and implementation of the PWAS phased arrays, as well as suggestions for further work.

MMM Stress Evaluation and Fracture Analysis for Steam Turbine Failure Blade

Metal magnetic memory (MMM) technology, a new NDE method, has been applied to stress evaluation and fracture analysis for the 50MW turbine failure blade. The static and dynamic stress of failure blade is calculated and analyzed. The rupture blade of steam turbine is detected at workshop. Given are the MMM signal characteristics of stress distribution for the failure blade and fracture face. It has been found: for the blade profile, the MMM air-out side curve occurs more zero passage signals than air-in side one; for the fracture surface, the crack transition zone’s MMM variation amplitude is minimal, the crack initiation zone’s is in the middle, and the tear fracture zone’s is maximal. The result from small hole stress measuring method is consistent with MMM result. This shows MMM technology is a new tool of fracture analysis and stress testing in engineering practice.

Model Based Damage Detection in an Automotive Exhaust Hanger Component

The field of health monitoring of structures may have possible benefits for many industries. In this work, we analyze damage induced in an automotive exhaust hanger component. Damage in this system is defined as a crack development and propagation at a specific location. It is proposed and demonstrated in this work that a selected group of modal frequencies and their collective behavior is a suitable measure of tracking the damage. It is also shown in this work, via experimental and computational observations, that the second moment of inertia is very sensitive to the damage induced. Future work will include developing a predictive model for damage in this automotive exhaust system component.

Early Damage Stress State MMM Testing

The stress state testing and evaluation is the major concern in early damage testing. The metal magnetic memory (MMM) testing, based on ferromagnetic theory, is a new NDE technology that can find the most serious stress concentration zones, microscopic defect, etc. However, there is no quantitative analysis standard in engineering practice. This paper is to find out MMM critical signal characteristics of early damage to offer a foundation data for MMM quantitative testing. The relation between stress distribution and magnetic memory signal has been studied by experiments. Two different states of stress, including one-direction tensile stress state and two-direction bend stress state, were created. The critical magnetic characteristics are given in zones of stress concentration, which can serve as a rule for testing early damage. The result of metallographic examination proves the validity of MMM early damage testing.

Thermoelectric Nondestructive Characterization of Subsurface Inclusions

A comparison between published analytical results with experimental ones of the magnetic field produced by thermoelectric currents of subsurface semi-spherical pure tin inclusions embedded in a copper bar under the influence of an external heating (+45°C) and cooling (+10°C) into the specimen is presented. The measurements were taken from a 12.7 mm-thick copper bar with several subsurface semi-spherical pure tin inclusion diameters at different lift-off distances. The experimental magnetic flux density measurements show a good agreement with analytical ones as a function of the lift-off distances with the different inclusion diameters with the exception of the intrinsic material background magnetic signal that affected deeply the detectability of subtle imperfections in noncontacting thermoelectric measurements. The main sources of such adverse background signals in thermoelectric NDE are the intrinsic variations of the material properties of the specimen to be inspected.

PVDF and PZT Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM) applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive and inexpensive. PWAS achieve direct transduction between electric and elastic wave energies. PWAS are essential elements in the Lamb-wave SHM with pitch-catch, pulse-echo, phased array system and electromechanical impedance methods. PWAS are charge mode sensors and they can be used as both transmitters and receivers. A model of PWAS is shown in this paper. In vibration, impact detections applications, the PWAS response is strong due to the large dynamic change of strain. In pitch-catch, pulse-echo and phased array applications, PWAS are used to generate and receive Lamb waves and the PWAS response is small. A charge amplifier for PWAS applications is introduced in this paper. PWAS are normally made of piezoceramic Lead Zirconate Titanate (PZT). The structural integrity tests require attachment of PWAS to the material surface and there are critical applications where the rigid piezoceramic wafers cannot conform to curved surfaces. As alternative one can use flexible piezopolymer such as polyvinylidene fluoride (PVDF); such PVDF-PWAS have been studied in this paper. PVDF-PWAS were mounted on a cantilever beam for the free vibration test and on a long rod for the longitudinal impact test. The experimental results of the PZT-PWAS and PVDF-PWAS have been compared with the conventional strain gauge. The theoretical and experimental results in this study gave the basic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.