Non-contact imaging for surface-opening cracks in concrete with air-coupled sensors

Rapid and accurate non-destructive evaluation (NDE) techniques are needed to assess the in-place condition of concrete structures. However the time and effort required to perform NDE tests using conventional surface-mounted contact sensors hinder rapid evaluation of large full-scale structures. The suitability of surface waves and non-contact sensing techniques to detect the presence of concrete defects is examined here. First, the ability to detect leaky surface waves in concrete with air-coupled sensors is demonstrated. Surface waves in a concrete slab specimen are generated by an electrically-controlled impact source. Next, the data and signal processing needed to improve leaky surface wave data, with respect to eventual application to velocity and attenuation images, are demonstrated. Finally velocity and wave attenuation data collected from a concrete slab specimen that exhibits surface cracking are presented. Test results show that the proposed energy ratio (attenuation) criterion is more sensitive to existence of cracks than the velocity criterion.     

Statistics of defects in one-dimensional components

Equations related to spatial statistics of defects and probability of detecting defects in one-dimensional components have been derived. The equations related to spatial statistics of defects allow to estimate the probability of existence of safe, defect-free zones between the defects in one-dimensional components. It is demonstrated that even for a moderate defect number densities, the probability of existence of clusters of two or more defects at a critically small distance is substantial and should not be neglected in calculations related to risks of failure. The formulae derived have also important application in reliability and risk assessment studies related to calculation of the probability of clustering of evens on a given time interval. It is demonstrated that while for large tested fractions from one-dimensional components, the failures are almost entirely caused by a small part of the largest defects, for small tested fractions almost all defects participate as initiators of failure. The problem of non-destructive defect inspection of one-dimensional components has also been addressed. A general equation has been derived regarding the probability of detecting at least a single defect when only a fraction of the component is examined.     

NDE in aerospace-requirements for science, sensors and sense

The complexity of modern NDE (nondestructive evaluation) arises from four main factors: quantitative measurement, science, physical models for computational analysis, realistic interfacing with engineering decisions, and direct access to management priorities. Recent advances in the four factors of NDE are addressed. Physical models of acoustic propagation are presented that have led to the development of measurement technologies advancing the ability to assure that materials and structures will perform a design. In addition, a brief discussion is given of current research for future mission needs such as smart structures that sense their own health. Such advances permit projects to integrate design for inspection into their plans, bringing NDE into engineering and management priorities. The measurement focus is on ultrasonics with generous case examples. Problem solutions highlighted include critical stress in fasteners, residual stress in steel, NDE laminography, and solid rocket motor NDE.     

Non-destructive microstructural analysis by electrical conductivity: Comparison with hardness measurements in different materials

The use of non-destructive evaluation (NDE) techniques for assessing microstructural changes in processed materials is of particular importance as it can be used to assess, qualitatively, the integrity of any material/structure. Among the several NDE techniques available, electrical conductivity measurements using eddy currents attract great attention owing to its simplicity and reliability. In this work, the electrical conductivity profiles of friction stir processed Ti6Al4V, Cu, Pb, S355 steel and gas tungsten arc welded AISI 304 stainless steel were determined through eddy currents and four-point probe. In parallel, hardness measurements were also performed. The profiles matched well with the optical macrographs of the materials: while entering in the processed region a variation in both profiles was always observed. One particular advantage of electrical conductivity profiles over hardness was evident: it provides a better resolution of the microstructural alterations in the processed materials. Moreover, when thermomechanical processing induces microstructural changes that modify the magnetic properties of a material, eddy currents testing can be used to qualitatively determine the phase fraction in a given region of the material. A qualitative relation between electrical conductivity measurements and hardness is observed.     

Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

Long‐standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high‐performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.     

Assessment of creep life fraction for in-service high-temperature components

The grain boundary etching method for the assessment of material degradation has received much attention because it is simple, inexpensive and easy to apply to in-service high-temperature components. The effectiveness of the method is verified by successfully applying it to in-service components of aged fossil power plants such as superheater and reheater headers. A degradation parameter is employed using intersecting point ratio. The material degradation for in-service high-temperature components is estimated by a Larson–Miller parameter which is calculated from aging temperature and aging time. Creep tests are performed to characterize the correlation between intersecting point ratio and life fraction of service material. The possibility is confirmed to apply the grain boundary etching method to the creep life assessment of in-service high-temperature components.     

Diffuse ultrasonic backscatter in a two-dimensional domain

The scattering of elastic waves in polycrystalline materials is relevant for ultrasonic materials characterization and nondestructive evaluation (NDE). Diffuse ultrasonic backscatter measurements are used widely to extract the microstructural parameters such as grain size and also to detect flaws in materials. Accurate interpretation of experimental data requires robust scattering models. Line transducers are often used for ultrasonic experiments such that an appropriate model for these two-dimensional problems is needed. Here, a theoretical expression for the temporal diffuse backscatter is derived for such domains under a single-scattering assumption. The result is given in terms of transducer and microstructural parameters. In addition, the problem is examined in terms of numerical simulations using Voronoi polycrystals that are discretized using finite elements in a plane-strain formulation. The material properties of the individual Voronoi cells are chosen according to appropriate material distributions. Such numerical models also allow scattering theories, including the one discussed here, to be examined for well-controlled microstructures. Example numerical results for materials with varying degrees of scattering that are of common interest are presented. The numerical results are compared with the theory developed with good agreement. These results are anticipated to impact ultrasonic NDE of polycrystalline media.     

Nondisruptive material sampling and mechanical testing

The aging and inservice degradation of industrial equipment has underscored a need for efficient and reliable evaluation of the suitability of such equipment for continued service. The structural components of traditional energy production facilities, such as fossil- and nuclear-fueled electric power plants, are prime examples of aging equipment for which integrity during extended service is of major concern. The paper describes a recently developed nondisruptive miniature material sample removal and test approach that is being applied to a range of operating electric power plant components from turbine generators to pressure vessels, and to petrochemical plant reactor vessels for inservice integrity assessment. Thein situ removal of a thin wafer-like sample (less than 25 mm in diameter and 2.5 mm in thickness) from the component surface generally has no effect on component integrity. The miniature specimen small punch (disk bend) test has been developed to mechanically test the as-removed material, and is being used to estimate the material tensile behavior and fracture properties (ductile-to-brittle transition temperature and fracture toughness) required for a reliable component integrity assessment.     

Assessment of wood load condition by Near Infrared (NIR) spectroscopy

The assessment of the mechanical properties of wood using non-destructive evaluation (NDE) tools has been widely developed and refined. These NDE tools mainly rely on vibrational, ultrasonic or stress-wave approaches. Vibrational techniques generally show higher correlations between the estimated modulus of elasticity (MOE), or modulus of rupture (MOR), and the measured MOE, or MOR, than stress-wave techniques. They are, however, relatively difficult to apply in the field due to boundary conditions common in many timber structures. Thus, improved tools for assessing timber structures are still needed. Recently, near infrared (NIR) spectroscopy (500–2400 nm) has shown promise for predicting the MOE and MOR of wood. This work focuses on extending the use of NIR for measuring the load applied to small wood beams. The reflectance NIR spectrum was measured as the applied load was increased. Good correlations (r > 0.96) between the measured load and the predicted load were obtained using spectra taken from both the tension and compression surfaces of the small wood beams.     

Nondestructive Evaluation of Ceramic Candle Filter with Various Boundary Conditions

Nondestructive evaluation (NDE) using a dynamic characterization technique was conducted to study ceramic candle filters. Ceramic candle filters are hollow cylindrical structures made of porous ceramic materials used to protect gas turbine in coal-fired power plants. Deterioration and failure of ceramic filters occurs after being exposed to high-temperature and high-pressure operational environment over a period of time. This paper focuses on the development of an NDE method that can predict the in-situ structural stiffness of the candle filters while still being attached to the plenum. A combination of laboratory testing, theoretical analysis, and finite element method (FEM) simulations are presented. The candle filters were tested using a laser vibrometer/accelerometer setup with variable boundary restraints. A variable end-restraint Timoshenko beam equation was derived to determine the dynamic response of the candle filters with simulated in-situ boundary conditions. Results from the FEM simulation were verified with the analysis to determine the stiffness degradation of the candle filters as well as the boundary conditions. Results from this study show that the vibration characteristics can be used effectively to evaluate both the structural stiffness and the in-situ boundary restraints of the ceramic candle filters during field inspections.     

Strength of welded joints of Cr–Mn steels with elevated content of nitrogen in hydrogen-containing media

We analyze the operating characteristics of welded joints of austenitic high-nitrogen steels. The influence of hydrogen- and chloride-containing media on the structurally sensitive properties of heterogeneous welded joints is investigated. It is shown that the heat-affected zone, where the residual welding stresses are superimposed on the stresses caused by the influence of the ambient media and in-service defects, proves to be especially sensitive to the influence of hydrogen degradation.     

Integrated niobium DC SQUID gradiometers for non-destructive evaluation and biomagnetism

We describe the design and fabrication of thin-film Nb gradiometers with integrated DC SQUIDs for use in non-destructive evaluation (NDE) and biomagnetism. Issues of sensitivity, imbalance and field response are considered. Results are presented from eddy-current NDE in an unshielded environment of aluminium plates with sub-surface flaws, and from biomagnetic measurements of spinal and peripheral nerve evoked fields.     

Compensation of the Skin Effect in Low-Frequency Potential Drop Measurements

Potential drop measurements are routinely used in the non-destructive evaluation of component integrity. Potential drop measurements use either direct current (DC) or alternating current (AC), the latter will have superior noise performance due to the ability to perform phase sensitive detection and the reduction of flicker noise. AC measurements are however subject to the skin effect where the current is electromagnetically constricted to the surface of the component. Unfortunately, the skin effect is a function of magnetic permeability, which in ferromagnetic materials is sensitive to a number of parameters including stress and temperature, and consequently in-situ impedance measurements are likely to be unstable. It has been proposed that quasi-DC measurements, which benefit from superior noise performance, but also tend to the skin-effect independent DC measurement, be adopted for in-situ creep measurements for power station components. Unfortunately, the quasi-DC measurement will only tend to the DC distribution and therefore some remnant sensitivity to the skin effect will remain. This paper will present a correction for situations where the remnant sensitivity to the skin effect is not adequately suppressed by using sufficiently low frequency; the application of particular interest being the in-situ monitoring of the creep strain of power station components. The correction uses the measured phase angle to approximate the influence of the skin effect and allow recovery of the DC-asymptotic value of the resistance. The basis of the correction, that potential drop measurements are minimum phase is presented and illustrated on two cases; the creep strain sensor of practical interest and a conducting rod as another common case to illustrate generality. The correction is demonstrated experimentally on a component where the skin effect is manipulated by application of a range of elastic stresses.     

Modelling and fatigue life assessment of complex structures

This paper presents some of the motivations and main conclusions from a series of joint Nordic research initiatives in which an integrated research approach to the development of future generations of advanced fabricated structures have been employed. The integrated research approach includes coordinated efforts in several key technologies: high‐speed welding processes, high strength materials, cost‐effective NDE, post‐weld treatments and FE‐based design assessment tools. Traditionally, fatigue assessment methods for welded structures have been developed based on small‐scale test specimens and verification studies for large structures are rarely published. Applications on complex structures have led to several new assessment concepts and areas for future work. A modified structural stress method that proposes a multi‐linear stress distribution through the plate thickness is introduced. Also, a crack growth assessment method in which the constraint equations of a sub‐model are linked to the global model is presented. Both these new methods are promising for complex structures. The crucial role of boundary conditions for complex structures is highlighted as is the future challenge of understanding and making use of the residual stress state for welded structures.     

Flight service environmental effects on composite materials and structures

NASA Langley and the U.S. Army have jointly sponsored programs to assess the effects of realistic flight environments and ground-based exposure on advanced composite materials and structures. Composite secondary structural components were initially installed on commercial transport aircraft in 1973; secondary and primary structural components were installed on commercial helicopters in 1979; and primary structural components were installed on commercial aircraft in the mid-to-late 1980's. Over 5.3 million total component flight hours have been accumulated on 350 composite components since 1973. Service performance, maintenance characteristics, and residual strength of numerous composite components are reported. In addition to data on flight components; 10-year ground-based exposure test results on material coupons are reported. Comparisons between flight and ground-based environmental effects for several composite material systems are also presented. Test results indicate excellent in-service performance with the composite components during the 15 year evaluation period. Good correlation between ground-based material performance and operational structural performance has been achieved.     

Thermal Wave Imaging and Ultrasonic Characterization of Defects in Plasma Sprayed Coatings

One of the primary concerns in thermal spray coatings is bond integrity, as disbonds at the coating/ substrate interface could cause premature failure of a component. Current quality control test practices use empirical destructive mechanical test methods that do not provide direct correlation between test results and coating performance. Nondestructive evaluation (NDE) techniques used for bulk materials are not readily adaptable for the evaluation and characterization of coatings. This paper reports on the use of thermal wave imaging and ultrasonic immersion C-scan to characterize artificial disbonds in plasma spray coatings on steel substrates. This work demonstrates the capability of these techniques in characterizing total disbonds and presence of ' kissing bonds ” (where the interface surfaces are in such close contact that they give a appearance of a good bond). The effects of ultrasonic frequency on the C-scan images were also studied. A critical evaluation of both thermal wave imaging and ultrasonic techniques is presented along with their relative advantages and disadvantages.     

复合材料层压板分层缺陷超声相控阵检测与评估

针对碳纤维增强树脂基复合材料分层缺陷的无损检测与评估问题,通过制备预埋分层缺陷的标准试样,利用超声相控阵技术对缺陷进行无损检测与定量评估,并对测量误差进行分析。首先,在层压板铺层中间埋入聚酰亚胺薄膜制备分层缺陷试样;然后,对试样进行超声相控阵检测,通过超声S扫和C扫图像对缺陷进行定性分析与定量测量,并结合声场仿真对检测误差进行分析。结果表明:所制备试样内分层缺陷形状规则、埋深及大小与预设一致;超声相控阵步进方向检测尺寸比较准确,而扫查方向尺寸误差较大;超声相控阵技术能够准确识别分层缺陷的形状、尺寸及位置,具有很高的检测精度,对较小缺陷具有很好的检测效果。     

Anwendung der holografischen Interferometrie in der zerstörungsfreien Werkstoffprüfung

Usage of Holographic Interferometry for Non-Destructive Materials Testing . Holographic interferometry has meanwhile developed into a usable method for non-destructive materials testing, which permits even large-area components to be tested by using high-power lasers. Recording the fringe structures makes a subsequent detailed resolution evaluation possible. Appraisal of the video recordings makes the origin and change of the secondary interference fringes clearly recognizable and interpretable and thus substantiates very informatively the introduction of holographic interferometry for non-destructive materials testing.     

Miniature Mobile Sensor Platforms for Condition Monitoring of Structures

In this paper, a wireless, multisensor inspection system for nondestructive evaluation (NDE) of materials is described. The sensor configuration enables two inspection modes-magnetic (flux leakage and eddy current) and noncontact ultrasound. Each is designed to function in a complementary manner, maximizing the potential for detection of both surface and internal defects. Particular emphasis is placed on the generic architecture of a novel, intelligent sensor platform, and its positioning on the structure under test. The sensor units are capable of wireless communication with a remote host computer, which controls manipulation and data interpretation. Results are presented in the form of automatic scans with different NDE sensors in a series of experiments on thin plate structures. To highlight the advantage of utilizing multiple inspection modalities, data fusion approaches are employed to combine data collected by complementary sensor systems. Fusion of data is shown to demonstrate the potential for improved inspection reliability.