J. M. A. Rebello

Hydrogen damage detection by ultrasonic spectral analysis

The non-destructive testing technique known as ultrasonic spectral analysis was applied to steel samples for the detection of minute cracks produced by hydrogen in a H2S environment. The cracks were characterized and quantified by metallographic analysis and compared with the ultrasonic results. The backwall echoes and the backscattered signals were analyzed in the frequency domain and the characteristics of these spectra were evaluated by second order moments, which emphasizes the higher frequency components. The second order moments showed a greater variability for hydrogen attacked materials than for non-attacked ones, denoting good sensitivity to this type of material degradation.

Evaluation of the relevant features of welding defects in radiographic inspection

The use of X-ray as an inspection technique to ensure the integrity of industrial products dates from the beginning of the 20th century. Therefore, it is a tool of non-destructive inspection widely known. Nowadays, however, there are several researches forward on the optimization of such inspection technique, mainly for the development of an automatic system of radiographic image analysis. That is, a system that can identify and classify the defects in the radiography. An important step in the construction of this system is the classification of defects, which is usually done by using some of their features. The purpose of this work is to study the relevance of some defect features in order to classify some of the main classes of defects. The employed technique is the linear correlation between the defect features and the classes of defects. A non-linear pattern classifier is used, implemented by a neural network, to evaluate the performance in the classification. The results showed the efficiency of the method used.

Relationship between acoustic emission and CTOD testing for a structural steel

Abstract Acoustic emission testing may be employed as a nondestructive technique to detect flaws in materials and equipment. For pressure vessel construction, the test requires loading by internal pressure in order to open the flaw tips and to generate acoustic emission signals. The present work tries to investigate whether the required pressure level causes flaw propagation, risking vessel integrity. The material employed was a fine grained normalized C-Mn structural steel (ASTM A 516 gr. 60). Fracture mechanics crack tip opening displacement (CTOD) tests were conducted and the acoustic emission probes were placed on the testpieces, which enabled a correlation to be established at any moment between the crack acoustic behaviour and its CTOD value. A relevant result was that intense acoustic activity was detected before the first stable crack extension occurs. On a quantitative basis, it was found that the CTOD value corresponding to the first detectable acoustic emission from the loaded crack tip is lower than the CTOD for the initiation of stable crack propagation. As a final conclusion from this work, it can be said that for this specific steel, acoustic emission testing may be reliably performed at sufficiently low CTOD values, where the risk of fracture may be discarded.

Signal processing in advanced nondestructive materials inspection

Correspondence should be addressed to Jo˜ao Manuel R. S. Tavares, tavares@fe.up.ptReceived 31 October 2010; Accepted 31 October 2010Copyright

Model for the variation of the residual stress state during plastic deformation under uniaxial tension

Abstract A theoretical model has been developed to explain the variation of surface residual stress introduced by shot-peening with external plastic deformation, during a uniaxial tensile test. The model is based on the difference of yield stress values of the shot-peened surface layer and the remaining bulk material. It has been shown that the model fits well with experimental results obtained for the base metal and heat-affected zone of a 5.0Cr-0.5Mo steel.

DETECTION OF LACK OF FUSION WELD DEFECTS BY RADIOGRAPHY

In this work, radiography was employed as the NDT technique for detection of flaws in circumferential girth welds of steel pipelines used in offshore installations in the petroleum industry. The kind of defect specifically focused was lack of fusion. It is currently accepted in the literature that radiography is not as sensitive as ultrasonics to detect lack of fusion defects. Unfortunately, the radiographic inspection can barely detect lack of fusion and only when it is associated to inclusions and voids of considerable size. However, in a previous article (“Reliability of radiographic inspection of steel pipeline girth welds,” QNDE Conference, 2007), the authors showed that it is possible to detect lack of fusion defects if, in the radiographic tests, the angle of incidence is the same angle that the weld bevel makes with the test piece surface, which means lowering the angle of disorientation between the flaw and the radiographic beam. However, no concerns were made to sizing the defects. Computational...

Variation of the residual stress state in a welded joint during plastic deformation in a 5.0%Cr and 0.5%Mo steel

Abstract The aim of the present work is to verify the variation of the initial residual compressive stress induced by shot-peening, during plastic deformation under uniaxial tension, in a steel with 5.0%Mo and 0.5%Cr, employing test pieces taken from a welded joint where, due to the welding process, there are regions with different mechanical properties (yield strength and tensile strength). The regions of the joint, the base metal, the heat-affected zone and the weld bead exhibited different behaviour. In the heat-affected zone the residual compressive stresses in the longitudinal direction changed to tensile stresses with 0.5-1 per cent plastic deformation. At 3 per cent the stresses attain a maximum tensile value, decreasing in an oscillating manner up to 6 per cent plastic deformation. Beyond this point the test pieces reached a constant tensile value until the fracture. The transverse residual stress values were also modified during the tensile test but no full stress relaxation was observed. Indeed, the initial compressive stress values changed under small applied plastic deformation and after 4 per cent plastic deformation they changed weakly until the test piece fracture. The weld bead was weakly deformed and no effective stress relaxation was observed. In both the longitudinal and the transverse directions, the residual compressive stresses are brought sharply from an initial value of — 470MPa to a minimum of — 250MPa for only 0.5 per cent plastic deformation. No further changes were observed until the fracture of the test piece.

Ultrasonic inspection of adhesive joints of composite pipelines

Composite pipelines are an attractive solution when traditional materials are not suitable for this purpose, which happens frequently at aggressive environments and also where the structural weight is a limiting factor. This work studies the application of the ultrasonic technique at the detection of defects as lack of adhesive and lack of adhesion, commonly found in adhesive joints of glass fiber reinforced plastic (GFRP) pipelines applied at onshore and offshore facilities. Computational simulations were conducted in CIVA 11© software (beta version) in order to obtain the best possible configuration for the inspections, applying the pulse-echo technique. Experimental results were compared to these simulations and several transducers were tested. An inspection methodology and reference blocks were developed for the calibration of the inspections. Some samples were selected for cutting in order to compare the ultrasonic results and the real condition of the joints. Results show that smaller frequencies ar...

Finite element model for the characterization of deleterious phases by eddy current technique

Avoiding accidents caused by equipment failure and ensuring structure quality are some of the challenges of the petroleum industry. Petroleum exploitation in salt-water has required the use of special materials, such as duplex stainless steels (DSS), that support highly aggressive corrosion conditions and, at the same time, are ductile facilitating equipment manufacture. But DSS can be embrittled when exposed to some heat treatments, such as welding (T > 800 deg). Deleterious phases might precipitated in this material, especially sigma phase (σ), which appears in higher volumetric fraction. This paper presents a proposal of a characterization procedure of deleterious phases in DSS using Eddy Current Technique (EC). A calculus model was elaborated using finite elements. The model was able to reproduce the behavior of this type of steel, and it will help to correctly recognize the signals generated by the material’s internal defects.

Residual stress state behaviour under fatigue loading in duplex stainless steel

The lifetime of duplex stainless steel parts experiencing cyclic fatigue is directly influenced by the residual stress levels present in the ferrite and austenite phases. This paper analyses the behaviour of the residual stress fields in both phases introduced by a shot peen-ing treatment. The X-ray diffraction technique is used to investigate the stress levels both on the sample surface and in subsurface layers, after low fatigue cycles. The results show that the compressive residual stress levels introduced by the shot peening treatment in both phases improves the fatigue life of the material. However, the cyclical loads produce partial or total relief in these residual stresses fields. A scanning electron microscopy study of the samples shows that the shot peening process induces the formation of micro cracks only in the ferrite phase. The largest variations in the total compressive residual stress level also occur in this phase.