Viorel Anghel

Extraction of dispersion curves for waves propagating in free complex waveguides by standard finite element codes.

This paper presents a fast and reliable method, for obtaining all the range of dispersion curves for wave propagation usually used in practice, by numerical simulation only, via common commercial finite element codes. Essentially, the method is based on a simple and robust approach, consisting in a few series of modal analyses for a representative part of the inspected structure. In this way, for different wave lengths, one can find the mode shapes and corresponding natural frequencies by solving some real, symmetric and well numerically conditioned eigenvalue problems. The method allows the extraction of propagating modes only and, in spite of not producing continuous dispersion curves, it is not susceptible to aliasing effects, as some similar methods are. Additionally, complete graphical representations of guided waves are possible with some minor calculus effort.

Results of Nondestructive Inspection of Layered Composites Using IR Thermography and Ultrasonics

Polymeric layered composites exhibit a variety of damages following in service loading conditions, like delamination, matrix cracking or even fibre breaking. Detection of such damages and assessing their extension and severity is vital during maintenance cycle, in view of keeping the normal operational reliability. For local inspections, IR thermography and ultrasonic scanning are among the best valued NDT methods. The paper describes the inspections performed by IR active thermography, in different variants, and pulse-echo ultrasonic scanning on GFRP. A variety of layered composites and defects/damages were inspected and the results are evaluated independently, in some cases being compared each other, with valuable conclusions for the users of the mentioned NDI techniques.

Linking the Impact Force History with Residual Mechanical Characteristics and NDI - A Smart Way to Perform SHM in Composites

Structural integrity monitoring (SHM) and evaluation of residual mechanical performance are highly needed in assessing the post-impact behaviour of composite materials and structures. The link between impact force history and the damage level was not followed enough in research studies upon the SHM of composites. The authors put in evidence a clear link in this matter in a variety of layered composite materials. The link was assessed by evaluating the residual mechanical performance and by nondestructive inspection (NDI) – ultrasonics and infrared thermography (IRT) - on the impacted samples. Such a link may prove a very useful and reliable shortcut for backing the online SHM and condition based maintenance.

Aspects of Lamb-Wave Generation and Transmission

This paper presents numerical and experimental results concerning Lamb-wave actuation and reception in metallic or layered composite panels and pipes. The numerical simulations were carried out with LS-Dyna, run under ANSYS platform. The main goals of this approach were to find better models for actuating Lamb waves and tracking the wave transmission in order to get efficient in situ structural health monitoring. Prior adequate choice of the signal frequency using the dispersion curves proved to be beneficial for better transmission, in terms of signal detection, and it extended inspection range. The effects of the transducer-inspected item interface and transducer shape on the wave transmission were also evaluated. Signal generation with rectangular transducers, batteries of round piezoceramic transducer (PZT) patches or the use of prisms, which value Snell’s law, were alternatively considered for getting guided waves in the inspected item. The experiments showed the performance of various transducers and devices used on the experimental chain. The Lamb waves were generated by PZT, in various shapes and configurations, aiming to lower the inspection cost. Experiments always aligned with theoretical results so that robust models could be selected for further investigations.

Evaluation of the structural behaviour of a port chamber following damages produced in the refractory lining

The port chamber used in oil refineries is an exhaust chamber mounted after the catalytic reactor for retaining and unifying the exhaust hot process gases. The thick steel shell has a refractory lining made of refractory cement called gunite or shotcrete. Damages produced in the refractory lining have serious effects on the thermal generated stresses in the steel shell. The research work aimed to find the level of these stresses in a numerical/experimental approach and to evaluate its consequences on the service life of this important component of the oil refinery. A complex sequentially coupled physics thermal-structural analysis was performed using the ANSYS code in order to get the stresses generated by a deep crack in the shotcrete layer. The results concerning the thermal mapping were based on previous inspections made with infrared thermography (IRT) and made possible an estimation of the remained service life of the port chamber.

Particular Aspects Concerning Low Velocity Impact on Composite Tubes

The study had in view various aspects which can arise during the low velocity impact tests made on composite pipes/tubes. It implied numerical simulations, made by ANSYS and LS-DYNA codes, on glass fiber/epoxy composite pipes. The geometry and material characteristics were taken from real pipes, which have been experimentally tested in parallel, using a drop weight impact tower. The main parameter in view was the impact force history, which gives most information upon the impact event and, accordingly, is used by most of researchers for characterizing the damages produced on the impacted body and for assessing the impact installation.

Contributions in the Study of Hybrid Layered Composites under Static and Variable Loads

The research work behind this paper focused a rather extensive assessment of hybrid composites made of pure aluminium sheets, alternating with GFRP and CFRP layers. Static, fatigue and low velocity impact tests were performed, combined with NDI on damaged samples, using Lockin thermography, on specimens obtained from the two hybrid laminates and from genuine GFRP and CFRP laminates, all having five layers. The static and fatigue tests were made on parallel specimens, un-notched and having a central 5 mm drilled hole, with various failure modes. The low velocity impact tests were followed by CAI tests, meant to evaluate residual mechanical performance and damage tolerance. Lockin thermography was used for prior assessment of damage.

Damage Assessment through Impact Force History Recording

Low velocity impact is a frequent and inevitable in-service event, with higher occurrence in transportation structures. The damages following such an event are more diverse, extended and with more severe consequences in the case of composite materials and structures. The research work presented here concerns fibre reinforced polymeric composites in the forms of plates and pipes. It is continuing an effort meant to allow customers exploiting such structures to have a short cut in monitoring the integrity of this kind of structures. To this end, it is proposed a careful following of the impact force history recording, which can offer valuable and more direct information about the damage level produced under this insidious loading.

Structural Integrity Monitoring of Composite Pipes

Composite pipes enjoy increasing interest in the sector of petroleum and gas transportation, due to a number of qualities, concerning especially the corrosion resistance and light weight, face to the traditional steel pipes. As composite materials are prone to a various range of defects and damages which can seriously affect their service ability, reliable inspection methods have to be tested in order to assure the required in service reliability. The paper presents progress made in applying complementary global/local non-destructive inspection (NDI) methods, such as Lamb wave method and infrared thermography (IRT) method, to effective structural health (SHM) monitoring of composite pipes.