Guided Wave C.U.I. Inspection

Using guided wave, tens of meters of pipe can be quickly and completely screened from a single location. The ability of this system to send waves along the length of the pipe means that very difficult to inspect areas, such as road crossings, can be interrogated from a remote and easily accessible location. This paper discusses the practical application of a guided wave system to real world inspection and highlights the benefits and limitations.

Dynamic Crack Propagation and Arrest in PWR Pressure Vessel Steel: Interpretation of Experiment With the X-FEM Method

In the frame of analysis of the pressure thermal shock in a PWR RVP and the associated R&D activities, some developments are performed at CEA on the dynamic brittle propagation and crack arrest. This paper presents a PhD work on the modeling of the dynamic brittle crack growth. For the analyses, an important experimental work is performed on different geometries using a French RPV ferritic steel: Compact Tension specimens with different thickness, isothermal rings under compression with different positions of the initial defect to study a mixed mode configuration, and a ring submitted to thermal shock. The first part of this paper details the test conditions and main results. To propose an accurate interpretation of the crack growth, a viscous-elastic-plastic dynamic model is used. The strain rate influence is taken into account based on Cowper-Symond’s law (characterization was made from Split Hopkinson Pressure Bar tests). To model the crack propagation in the Finite Element calculation, eXtended Finite Element Method (X-FEM) is used. The implementation of these specific elements in the CEA F.E. software CAST3M is described in the second part of this paper. This numerical technique avoids re-meshing, because the crack progress is directly incorporated in the degrees of freedom of the elements crossed by the crack. The last part of this paper compares the F.E. predictions to the experimental measurements using different criteria. In particular, we focused on a RKR (Ritchie-Knott-Rice) like criterion using a critical principal stress in the front of the crack tip during the dynamic crack extension. Critical stress is found to depend on crack speed, or equivalently on strain rate. Good results are reported concerning predictive simulations.

Improved Design for Hypercompressor Packing Cups

Second stage cylinders for polyethylene hyper compressors are among the most stressed pressure vessels in the industry. Due to the high operating pressure and pressure fluctuations between suction and discharge, these cylinders must be designed to withstand high fatigue loads. Packing cups are the most critical cylinder component and, in particular, the lube oil ducts are the bottleneck for reliability. This work deals with a structural optimization of packing cups focused on the most critical packing cup area: the lube oil hole. Various pre-compression techniques (i.e., shrink-fit and autofrettage) are used to improve the fatigue behavior. Longer life and reliability are investigated with advanced design techniques such as finite element modeling and design of experiments. New design cups with a 50% higher fatigue safety factor have been obtained.

Remote Measurement of the Pipe Thickness Reduction by Microwaves

A method which can inspect a pipe in a large scale and measure the thickness reduction remotely was demonstrated. A copper pipe having 17 mm inner diameter, 1 mm wall thickness, and 900 mm length was measured. The thickness reduction having the value from 10% to 80% of the wall thickness was detected significantly. By building up a resonance for the microwave signal propagated in the pipe, it is possible to determine the thickness reduction using the measured resonance frequency.

Autofrettaged Spherical Pressure Vessels Design

Autofrettage process, adopted by the pressure vessel industry, enhances the static limit pressure of components. In addition, a significant increase in the fatigue life autofrettage components is also observed due to the inhibition of crack initiation and propagation. The application of autofrettage treated vessels can be extended to the power generation industry (fossil and nuclear), the petrochemical industry, the food industry (bacterial eradication container), and automotive applications (injection pump), among many others. In particular, spherical pressure vessels, due to their inherent stress and strain distributions require thinner walls compared to cylindrical vessels; therefore, they are extensively used in gas-cooled nuclear reactors, gas or liquid containers rather than heads of close-ended cylindrical vessels. In this paper analytical expressions have been derived for stress and strain during autofrettage process of spherical vessels with different material models. These formulas have been applied to evaluate the residual stresses, and optimized design in monotonic and cyclic loading conditions.

A New Ultrasonic Inspection Technique for Flaw Depth Sizing Based on Short Path of Diffraction

This paper describes a new tip diffraction technique, short path of diffraction (SPOD), for accurate depth sizing of flaws on the surface opposite to the inspection surface in which a set of an angle beam and a longitudinal 0-degree transducers is used as a transmitter and a receiver. In this technique, flaw depth is just calculated from the time difference between two echoes of a longitudinal diffracted wave at the tip of a flaw, which arises after an incident wave from the angle beam transducer impinges on the flaw. One echo is related to a wave traveling directly from the tip to the receiver, and the other is a wave which is reflected at the back wall and subsequently travels to the receiver. The two echoes are detected by the longitudinal 0-degree transducer located right above the flaw. Incident angles and incident points have no effect on determining flaw depth in this technique, therefore, more accurate and efficient flaw sizing becomes possible for the proposed technique. The new technique will be demonstrated by applying to depth sizing of slits and stress corrosion cracks which are machined into welded specimens made from austenitic stainless steel.

Autofrettage of a Spherical Pressure Vessel

There is a numerical procedure for modeling autofrettage of thick-walled cylinders that incorporates Bauschinger effect as a function of prior plastic strain and Von Mises’ yield criterion. In this paper the numerical procedure is extended to solve the analogous problem of a spherical, thick walled steel vessel. An equivalent new analytical solution for the case of a spherical vessel is also formulated. The analytical and numerical solutions are shown to be in close agreement. It is demonstrated numerically that a re-autofrettage procedure, previously proposed for cylindrical vessels, may be extremely beneficial for spherical vessels. Additional commentary is provided on the limitations of certain analytic solutions.

Neutron and Synchrotron Diffraction Measurements of Residual Stress in Steel Weldments

This paper explores the use of state-of-the-art instruments such as neutron and synchrotron diffraction for evaluation of residual stress in carbon steel welded components. The study shows significant variation in transverse residual strain/stress distribution across the length of the weld. Measurements before and after post weld heat treatment are reported together with traditional mechanical tests. The changes in residual stress distribution as weld beads are added in multi-bead welds are also presented. Important practical results are: i) The start and end of the weld are the most critical parts of the weldment. Implementations of run-in and run-out (which are subsequently ground off) can minimize the residual stress at the start and end of the weld. ii) Manipulation of the sequence especially around the weld toes can minimize the value of the residual stress in that region. iii) Post weld heat treatment is very effective at reducing residual stresses.

Coke Drum Weld Inspection

Conventional Ultrasonic Inspection of Coke Drums may require the use of Automated Pulse Echo or Time of Flight Diffraction Techniques (TOFD). The more recent application of Phased Array ultrasonic technology enables a faster and more accurate location and depth discrimination of the cracks detected in the welds. Pulse Echo ultrasonic inspection requires the use of three transducers from each side of the weld. A zero degree compression transducer and two angle transducers, most likely 60° or 70°. The advantage of this techniques is that it provides positional information as to the location of the crack in the weld and accurate length measurement. The problem is that additional techniques have to be used to determine the depth of any cracks detected. An alternative to Pulse Echo inspection is the Time of Flight Diffraction technique. The TOFD technique uses multimode transducers to insonify the weld region with Lateral, Compression and Shear Wave ultrasound. The technique accurately detects and determines the length and depth of reflectors in the weld region. The technique was initially developed for the Nuclear Industry as a sizing technique. More recently it has become used for detection and sizing of flaws. The TOFD technique does not place the flaw in the cross section of the weld in order to achieve this another technique such as Pulse Echo Ultrasound is required. The TOFD technique is not sensitive to small flaws which are open to either surface. In order to detect small flaws such as “Toe Cracks” a supplementary technique such as ACFM or Eddy Current inspection may be required. The illustration shows the format of the sound generated from a TOFD transducer arrangement. The advantage for welds < 1.50" in thickness is that careful selection of the transducers and appropriate spacing may allow the weld to be inspected in a single pass. The illustration below shows two displays, an unrectified “RF” display which corresponds to which ever cursor is active and a grey scale display adjacent. The Grey Scale Display is a stacked “RF” display where each vertical line correspond to a single location along the line of the weld non-conforming perturbations in the display indicate areas of concern which can be identified by length and depth as shown in the boxes at the lower left of the illustration. The first significant amplitude group on the grey scale display corresponds to the Lateral Wave, the second the Compression and the third the Shear Wave. Flaws detected between the Lateral and the Compression Wave are often repeated between the Compression and the Shear Waves. Phased Array technology has been available for some time, however only recently has the software been able to display the data in a format which provides clear data which can be used to locate and size of the flaws in a variety of weld configurations. Coke Drums have several significant areas of concern, Weld Seams which may be Shell to Shell, Shell to Head or Shell to Skirt format. We will consider the Circumferential Shell weld and the Skirt weld at this time. The photographs show a shell seam which reduces in section for this example the weld was inspected from one side only. The signals were corrected for Beam Path Length and the amplitudes of the signals were equalized for angle. The following data were collected: Two Notches were machined in the plate one either side of the weld on the underside. The plate was then scanned from one, the thicker, side using the Phased Array probe. The reflectors which were the same depth are depicted with a similar amplitude at their correct relative positions, one on the near and the other on the far side of the weld root. With the signals equalized all the reflector were detected from a single scan location and with similar amplitudes. The Skirt to Shell weld was simulated in a solid piece of carbon steel. EDM notch reflectors were machined in the samples at critical locations. The critical angles were calculated which would produce reflections from each of the potential crack areas and the Phased Array inspection was performed to verify the calculations. A single plot is shown as an example, containing the reflector on the Shell side near the crotch on the inside of the weld. The illustration shows the sound path of the Phased Array which detects a reflector close to the crotch on the inside between the Skirt and the Shell. Discriminating this flaw with conventional ultrasonic inspection would be extremely difficult. It is the ability of the Phased Array Sector Scan to use multiple angles on a single pass which enables flaws at multiple locations and angles to be detected by a line scan and imaged at their relative location.

AWJ Performance Characteristics at 600 MPa

Increasing the pressure of abrasive waterjet, while fixing the jet power, increases the jet’s power density and thus the cutting speed may increase. This was observed for steel, aluminum, and stone cutting. It was also observed that the kerf taper is less for higher pressure jets. Increasing the pressure while keeping the jet diameter fixed will increase both the power and the power density. This will result in increase cutting speed and less taper. The operating cost of the AWJ process consists mainly of the costs of abrasives, nozzle wear, utility, and maintenance of equipment. The cost per unit length of material (specific cost) is determined based on the cutting speed. It was found that the main advantage of increasing pressure is increasing the cutting speed or reducing the abrasive consumption per unit time or unit length of cut. The highest savings are obtained when the speed is maximized by increasing the pressure and the abrasive flow rate. Several study cases were addressed in this paper using assumptions and simple models to generalize the analysis. The analysis indicates that increasing the pressure from 400 MPa to 600 MPa may result in cost per unit length saving of over 30%.