Papers by Author: Ştefan Sorohan

Abstract: 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.

Abstract: 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.

Abstract: Static loading and impact tests of ceramic alumina tiles have shown a very interesting behaviour of a tough, brittle, and high energy absorbing material. Strain gauges techniques and data acquisition were used throughout this research. A static-dynamic equivalence is done through a calibration procedure of the measuring system. The experimental strain history is completely acquired till the failure of the ceramic tile. Numerical simulation of impact tests gives additional insight on the complicated phenomena. Low velocity impact testing revealed different patters of failure, depending on the conditions of impact, and the capacity of this material to be used in particular purpose designed applications.

Abstract: Adhesive bonding is a particularly effective method of assembling complex structures, especially those made from dissimilar materials. If the joint is well designed and correctly executed, the adhesive bond ought to be one of the strongest components of the structure and most certainly should not be the reason for reducing the load capacity or fatigue life. The major factors determining the integrity of an adhesive bond are selection of the most appropriate adhesive, joint design, preparation of the bonding surfaces, strict quality control in production and monitoring in service. This work focuses on the evaluation of the load capacity of some configurations of adhesively bonded single-strapped joints based on finite element analyses. The adhesive layer thickness, the overlap length, the adherent and strap thicknesses were varied as well as the materials properties.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.