NDT Methods Research Articles

Laser line scanning thermography for surface breaking crack detection : modeling and experimental study

Crack detection in metallic samples at high surface temperature, hostile and hazardous environments, etc. is a challenging situation in any manufacturing industries. Most of the present NDE methods are suitable only for lower surface temperatures, especially at room temperature. In this situation, we need a fast and non-contact NDT method which can be applied even in high sample surface temperature. Laser thermography is one of the techniques having a high potential in non-contact inspection. As a preliminary investigation, in this article, we have studied the potentiality of laser line thermography in crack detection at room temperature. In laser line thermography, a continuous wave (CW) laser is used to generate a laser line, which in turn is used to scan the metal surface. The heat distribution over the sample surface is recorded by an infrared thermal (IR) camera. Two different approaches are reported in this work. Firstly, a stationary laser line source and its interaction with cracks; secondly, moving laser line source scanning over a surface with crack. When the distance between crack centre to laser line centre increases, crack detectability will decrease; and when laser power increases, crack detectability will increase. A dedicated image processing algorithm was developed to improve the detectability of the cracks. To understand the heat transfer phenomenon, a simplified 3D model for laser thermography was developed for the heat distribution during laser heating and was validated with experimental results. Defects were incorporated as a thermally thin resistive layer (TTRL) in numerical modeling, and the effect of TTRL in heat conduction is compared with experimental results.

Неразрушающий контроль композиционных материалов

Composite materials are widely used in a number of industrial sectors from aviation, space, to boat building, automotive, and sports goods. In recent years composite structures have seen a substantial increase of their use in the new generation of airplanes. The nondestructive testing and inspection of composite structures, both for manufacturing quality assurance and for in-service damage detection, has prompted the development and adaptation of a number of methods and techniques over the years. In this paper we review the various NDT methods for inspecting composites including water- and air-coupled ultrasound, bond testing, manual and automated tap testing, radiography, thermography, and shearography with emphasis given to techniques that demonstrated benefits to the practice of composite inspection.

Experiment Study of Ultrasonic Pulse Velocity Test of R/C Column Under Axial Load Variation

This study presents a simple NDT method with Ultrasonic Pulse Velocity (UPV) to detect repair of R/C columns by injection and grouting. In this paper, the technique applied is mainly based on the use of epoxy under pressure into the R/C column crack system for lightly damage, whereas grouting used for repair of medium and heavy damaged R/C columns. The specimen is initially charged to increase axial loads up to 0.1P0, 0.3P0 and 0.5P0. After repair, all specimens were retested with the same method. The test results show that the repair technique is in a good category with a pulse velocity value of 4.21km/s. Pulse velocity transmitter analysis provides conceptual support to increase column capacity through repair methods. Direct methods can be recommended for post-repair structural strength with high accuracy. This method is a potential candidate for future development, especially to predict the level of damage and building rehabilitation after an earthquake.

Durability Assessment of Concrete by using Electrical Resistivity & Rapid Chloride Permeability Technique

The durability of concrete is generally evaluated by many NDT technique, among the different NDT technique RCPT is one of the popular NDT method used by many researchers & practicing Engineers for evaluation of concrete durability. However the RCPT technique has some limitation on field test of concrete, considering the limitation of the RCPT technique a new technique has been intr oduced called Electrical resistivity technique is used for evaluation of concrete durability in both field & lab test. Durability of concrete is mainly depends on the properties of concrete microstructure i.e. pore size distribution, shape & connectivity of the microstructural pores in concrete. In general smaller pore size, with lesser connectivity results lower permeability & shows more durability, while concrete with porous microstructure having larger pore size distribution along with larger degree of interconnections results in higher permeability and shows poor durability. In this paper an experimental studies were conducted for evaluation of concrete durability on different types of concrete samples by using electrical resistivity & RCPT technique. From the experimental results it has been observed that concrete with lower level of Chloride permeability shows higher Electrical resistivity. Thus Electrical resistivity of concrete is directly influencing the durability of concrete.

Characteristic Curve and Its Use in Determining the Compressive Strength of Concrete by the Rebound Hammer Test.

During the construction of concrete structures, it is often useful to know compressive strength at an early age. This is an amount of strength required for the safe removal of formwork, also known as stripping strength. It is certainly helpful to determine this strength non-destructively, i.e., without any invasive steps that would damage the structure. Second only to the ultrasonic pulse velocity test, the rebound hammer test is the most common NDT method currently used for this purpose. However, estimating compressive strength using general regression models can often yield inaccurate results. The experiment results show that the compressive strength of any concrete can be estimated using one’s own newly created regression model. A traditionally constructed regression model can predict the strength value with 50% reliability, or when two-sided confidence bands are used, with 95% reliability. However, civil engineers usually work with the so-called characteristic value defined as a 5% quantile. Therefore, it appears suitable to adjust conventional methods in order to achieve a regression model with 95% one-sided reliability. This paper describes a simple construction of such a characteristic curve. The results show that the characteristic curve created for the concrete in question could be a useful tool even outside of practical applications.

Effect of Thermal Loading on Selected Parameters of Reinforced Concrete Obtained by Acoustic NDT Method

The paper presents the results of experiments carried out on test samples made from reinforced concrete. Within this experiment, concrete beams with steel reinforcement in the middle were made. The concrete samples were tested by a non-destructive acoustic Impact-echo method before and after high temperature loading (after cooling to room temperature). We focused on the dominant frequencies shift in the frequency spectra obtained by this method. The aim was to assess the ability of aforementioned acoustic method to detect the thermal damage of steel reinforced concrete.

Comparison of infrared thermography, ground-penetrating radar and ultrasonic pulse echo for detecting delaminations in concrete bridges

Buildings, bridges, roads, and other structures need to be regularly maintained if they are to last. In general, if a defect is detected early, the cost of its remediation is typically much lower than if the structure is allowed to degrade for months or years. A key means of locating hidden defects are non-destructive testing methods. This paper describes measurements that compare three non-destructive methods applied to a concrete bridge and a laboratory specimen with artificial defects to simulate cavities. Three methods (infrared thermography, ground-penetrating radar and ultrasonic pulse echo) were used to measure the depth and size of cavities in the concrete panel. Furthermore, the measurements were made using two different brands of radar and two different thermal imaging cameras from the same manufacturer. Lastly, the NDT methods were compared across general performance criteria in terms of accuracy, testing practicality, and costs.

Experimental Testing of Aramid Composite Applied in Ballistic Armour by Ultrasonic IR Thermography

Reference samples, in which artificial discontinuities (defects) are fabricated, are used to verify non-destructive testing procedures. Artificial discontinuities are known defects of reference samples and enable verification the feasibility of the verified NDT method for the identification of location and depth of discontinuities. Thermographic inspection of reference samples also helps to determine the required thermal input source parameters to test samples. This paper presents the results of experimental testing for ultrasonic IR thermography of defects in aramid composite laminate intended for light ballistic armour. Specimens of the aramid composite material included artificial defects and were inspected at various ultrasonic frequencies. Optical thermography and X-ray inspection were carried out on the same specimens, and their results were compared to those determined with ultrasonic IR thermography.

Development of NDT Methods for Control of Building and Bridge Structures in Bulgaria

Development of NDT Methods for Control of Building and Bridge Structures in Bulgaria

Shear failure characterization in masonry components made with different mortars based on combined NDT methods

In this paper, an experimental study conducted on Red Clay brick masonry “triplets” built with Cement and Lime-based mortars is presented. Monitoring of fracture is executed by Acoustic Emission (AE) and Digital Image Correlation (DIC). First, for a profound understanding of individual properties, small scale experiments were conducted for characterization of AE signatures of mortar and brick separately. Representative masonry triplet elements are tested in shear for investigation of the fracture mechanisms in relation to the properties of different mortars. Localization of the cracks as well as source identification is performed by means of AE parameter-based analysis. Their correlation with DIC, accurately showed the location of macro cracking and the developing of strain concentrations even before fracture. This is one of the first investigations where full field strain pattern from DIC are successfully correlated with AE indices in masonry. Numerical simulations were also performed for validation and prediction purposes.

Monitoring of Thermal Damage Evolution in Concrete Parts by Acoustic NDT Methods

The paper presents the results of the experimental work which was focused on the use of the Impact-echo method for testing of concrete specimens damaged by high-temperature. The test specimens were heated to 600 and 800 °C for sixty, one hundred twenty and three hundred minutes. After heating, the samples were continuously cooled to room temperature and tested by Impact-echo method.

Creep failure of a steam pipe girth weld and NDT strategy on creep damage

12Cr1MoV is a commonly used heat-resistant steel for the steam pipelines of subcritical power boilers in China. Usually, the girth weld joints are the weakest areas of the entire steam pipelines. In this work, a leakage failure of girth weld joint in a 12Cr1MoV steam pipe with about 10 years' service was analyzed. The crack is along the intercritical heat affected zone (ICHAZ) in the hoop direction, and the fracture completely duplicating the geometry of the fusion line. The microstructure had degraded seriously by carbides spheroidization. In the region of ICHAZ and FGHAZ (type IV zone), creep cavities were observed along the grain boundaries. These phenomena indicate the weld joint had suffered type IV creep cracking. The formation and accumulation of the cavities in type IV zone of the weld joint before cracking is a unique phenomenon of creep. Therefore, the proper NDT method and signal extraction with strong ability on cavities recognition should be the key points for detecting the creep damage in early stage. The sensitivity and selectivity of the signal are both important for the practical application on the creep inspection.

Current possibilities of the diagnosis of prestressing steel in structures

The field of diagnosis of structures has been experiencing a dynamic development in the last decades caused by the development and expansion of new NDT methods and instrumentation. At the same time, it is possible to observe changes in the focus of interest in this field, or more precisely, alternating waves of interest in specific types of diagnosis targeted at special types of structures. These waves of interest are always caused by a certain trend in the development of the society or building industry, and most often, by some major even in the field. In the last few years, particular attention has been paid to prestressed structures, especially to bridges, in connection with the recent accidents of bridges made from prestressed concrete in the Czech Republic and abroad. Considering the relatively large number of these bridges in the Czech Republic built in the second half of the 20th century, attention is paid to basic changes in the pre-construction condition surveys aimed at determining the condition and the remaining life of these structures. The paper focuses on the analysis of possibilities of assessing the condition of prestressing steel at the level of the present state of diagnosis of structures.

Shearography Technique on Inspection of Advanced Aircraft Composite Material

The A320 aft fixed fairing is an advanced composite structure fabricated from laminate carbon woven and combines with honeycomb core which is externally mounted on the flap track fairing as a wing part of Airbus aircraft. These flaps are movable control surfaces to widen up the surface of the wing in order to increase the lifting. This paper discussed on the inspection of A320 aft fixed fairing by using shearography NDT technique. Shearography technique reveals the stress-affected zone due to additional loading that can be realized by the laser speckle correlation on the inspected composite panel. This A320 aft fixed fairing is identified as a scrap composite panel which having different sizes and orientations of defect. The results showed that the changes of shearing direction in laser shearography technique are an effective means to reveal the defects propagation in the composite component. The shearing at Y-axes direction is able to detect the cracks at horizontal orientation while shearing at X-axis has successfully detects the cracks at vertical orientation. Therefore, shearography could be utilized as an alternative inspection for other NDT method for inspection of composite material.

Inspection of casting defects and grain boundary strengthening on stressed Al6061 specimen by NDT method and SEM micrographs

Non-destructive testing is one part of the function of quality control and is complementary to other long-established methods. By definition, non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service. The methods covered are Radiography, Magnetic Particle Crack Detection, Dye Penetrate Testing, Ultrasonic Flaw Detection, Eddy Current and Electro-Magnetic Testing. Here in this project work by using the radiography method of NDT testing is followed to identify the internal defects of the Al6061 cast specimen that was made by stir casting method. The aluminum cast structure involved to mono and multi-heat treated at about 450°C and 600°C in a muffle furnace and is compressed mechanically by using hydraulic press arrangements before the brief inspection through NDT testing. This technique is suitable for the detection of internal defects in ferrous and nonferrous metals and other materials. Furthermore, the denser the material greater will be the absorption. After the primary stage of inspection, most of the defects were identified by the NDT methods. In the second part of this project, SEM images can be taken for the compressed casting structure. The structural changes were successfully made on the cast specimen and the bond strength, as well as hardness were also increased. The comparison was made between the NDT inspected values and the SEM microstructure. It shows the clear representation of the grain structure and the bond strength.

Grease-like coupling agent based on metal ionization method for low-frequency ultrasonic NDT

Abstract In long-distance NDT based on ultrasonic guided waves with low-frequency excitation, the most commonly used industrial coupling agents are grease-like coupling agents. However, these agents show the poor performance in NDT, because they can cause more acoustic energy loss and lower acoustic transmissivity. In the study, to improve the acoustic transmission property of grease-like coupling agents, a new method was proposed to combine different metal ions like Li+, Na+ and Ca2+ with base grease, thus forming new grease-like coupling agents. By comparing with the ultrasonic transmitting property of the regular coupling agents, selects a premium ultrasonic grease-like coupling agent. In the experimental results, when low-frequency ultrasonic guided waves transmitted through the lithium-based coupling agent, except the slight delay of peak response time, pulse signals showed the higher energy transmission efficiency with the better pulse vibration waveforms in the iron medium than other oil-based coupling agents. The newly developed grease-like coupling agent can be easily prepared at a lower cost. Importantly, the new coupling agent can improve the energy transmission performance and greatly enhance the sensitivity of ultrasonic flaw detection.

Advanced NDT Methods and Data Processing on Industrial CFRP Components

In this work, enhanced thermal data processing is developed with experimental procedures, improving visualization algorithm for sub-surface defect detection on industrial composites. These materials are prone to successful infrared nondestructive investigation analyses, since defects are easily characterized by temperature response under thermal pulses with reliable results. Better defect characterization is achieved analyzing data with refined processing and experimental procedures, providing detailed contrasts maps where defects are better distinguished. Thermal data are analyzed for different CFRP specimens with artificial defects and experimental procedures are verified on real structural aeronautical component with internal anomalies due to impact simulation. A better computation method is found to be useful for simultaneous defect detection by means of automatic mapping of absolute contrast, optimized to identify defect boundaries.

Practical Aspects of Nondestructive Induction Field Testing in Determining the Depth of Steel and Reinforced Concrete Foundations

Characterizing unknown bridge foundations has been a great concern for the Federal Highway Administration (FHWA) and Department of Transportation (DOT) in the past decades. Among conventional NDT methods, the Induction Field is a favorable nondestructive method to determine the depth of bridge foundations made of steel or reinforced concrete. Although many studies have been reported on the applicability of the method, the practical aspects of the method including the effect of major factors influencing the success of the method has not been completely discussed. In the current study, the effect of distance, soil type and soil moisture is investigated using two testbeds. The findings were valuable and provided practical guidelines to perform Induction Field tests on steel and reinforced concrete foundations.

Automated NDT inspection based on high precision 3-D Thermo-Tomography model combined with engineering and manufacturing data

Besides final inspection of components, NDT can be used during manufacturing in terms of monitoring and even controlling process quality. In addition, components are tested during their life cycle using various NDT methods. Evaluation and defect localization in 3-D is an enormous improvement towards automation. For this purpose, multiple different NDT images should be combined in three-dimensional space to improve the efficiency of the inspector in order to determine defect size and position accurately. The paper recommends an NDT inspection on large and complex components to ensure a closed-loop manufacturing process from CAD designs to real components. The main goal is to manage knowledge from the automated manufacturing processes and to reuse them with NDT inspection data in an analysis environment, i.e CATIATM. The DLR Augsburg is researching on robot-based automated thermoplastic CFRP manufacturing processes with inline-NDT inspection. Active thermography is the NDT method of choice because it is capable of visualizing defects on dry carbon fiber preforms as well as on consolidated parts. The proposed system aims firstly to collect the manufacturing process data automatically, and secondly generates a 3-D thermo-tomography model out of several 2-D thermal images. A smart manufacturing pick-and-place system with digital workflow and an automated spot welding process was used to preform 104 thermoplastic cut pieces. For the thermography investigation and traceability of defect detection depth on consolidated components, artificial defect anomalies were placed in predefined positions and depths. The results of the automated 3-D thermo-tomography model combined with engineering and manufacturing data were then confirmed with computed tomography results.

Localization and identification of gas infrastructure defects by acoustic emission

Durability, reliability and usability are three foundations of reliable construction, crucial in economic and social context. Because localization and monitor of potential defect and assessment of its impact on condition of the construction are part of the durability assessment, we should develop appropriate methods, which would detect the beginning of technical condition process getting worse and could allow monitoring its progress during exploitation of the construction, not only at areas chosen subjectively or based on unverifiable computer models. One of this NDT methods complied with user expectations is acoustic emission method based on analysis of active destruction processes. The article presents results of AE applications to locate, monitor and identify destruction processes caused by exploitation burdens made to assessment the technical condition of gas infrastructure made from cast iron, steel and plastic.