Ioannis P. Giannopoulos

Bio: Ioannis P. Giannopoulos is an academic researcher from National Technical University of Athens. The author has contributed to research in topics: Creep & Technora. The author has an hindex of 5, co-authored 8 publications receiving 97 citations. Previous affiliations of Ioannis P. Giannopoulos include University of Cambridge.

Prediction of the long-term behaviour of high modulus fibres using the stepped isostress method (SSM)

Abstract: A new accelerated technique, called the stepped isostress method (SSM), is presented that allows accelerated testing of materials to determine their creep response, and in particular, their creep-rupture behaviour. The approach in SSM testing is similar to the more familiar stepped isothermal method (SIM) but the acceleration is now obtained by increasing the stress in steps rather than stepping the temperature. Additional stress provides energy to the system in an analogue of the effect of heat in SIM. This method relies on the time–stress superposition concept. Various theories, assumptions and the different steps of the method are described in detail. This method is advantageous when compared with SIM because there is no need to use elevated temperatures, which may affect the chemical properties of the tested materials. The applicability of this method is investigated. The paper presents testing on Kevlar 49 yarns using SSM. The resulting creep curves and rupture times are compared with those obtained from SIM and conventional creep testing carried out in the past. The results show good correlation between the three test methods. The ability to carry out reliable creep tests in a reasonable time at low stress levels allows a designer to have much more confidence in the data for creep-rupture behaviour for fibres and allows confident prediction of structural lifetimes.

Accelerated and real‐time creep and creep‐rupture results for aramid fibers

Abstract: This article presents results from conven- tional creep tests (CCT) and two accelerated test methods (the stepped isothermal method (SIM) and the stepped isostress method (SSM)) to determine the creep and creep- rupture behavior of two different aramid fibers, Kevlar 49 and Technora. CCT are regarded as the true behavior of the yarn, but they are impractical for long-term use where fail- ures are expected only after many years. All the tests were carried out on the same batches of yarns, and using the same clamping arrangements, so the tests should be directly comparable. For both materials, SIM testing gives good agreement with CCT and gave stress-rupture lifetimes that followed the same trend. However, there was significant variation for SSM testing, especially when testing Technora fibers. The results indicate that Kevlar has a creep strain capacity that is almost independent of stress, whereas Tech- nora shows a creep strain capacity that depends on stress. Its creep strain capacity is approximately two to three times that of Kevlar 49. The accelerated test methods give indirect estimates for the activation energy and the activation volume of the fibers. The activation energy for Technora is about 20% higher than that for Kevlar, meaning that it is less sensitive to the effects of increasing temperature. The activation volume for both materials was similar, and in both cases, stress dependent. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 125: 3856-3870, 2012

Stress limits for aramid fibres

Abstract: Aramid fibres should have many structural applications: they should be used as tendons in prestressed concrete, as stay cables in bridges and as ropes in marine industry owing to their good tensile properties, low weight and lack of corrosion. However, uncertainty about their ability to carry significant loads for a long period of time has meant that engineers have been reluctant to adopt them. Two new techniques (stepped isothermal method and stepped isostress method) are now available that allow accelerated testing to be carried out at low stress levels, in such a way that the long-term creep and creep rupture properties can be determined without having to extrapolate more than one decade on a log-time scale. Such tests have now been carried out on two slightly different aramid fibres: Kevlar 49 and Technora. The paper shows how this information can be used to predict the safe operating stresses for these fibres when under sustained load, which is precisely the sort of application for which they are mos...

Seismic Assessment of a RC Building according to FEMA 356 and Eurocode 8

Abstract: Seismic engineers are increasingly turning to non-linear methods of analysis which predict directly the amount and location of plastic yielding within a structure. This is in many ways a more satisfactory procedure than relying on elastic analysis reduced by a ductility factor. Non-linear static (pushover) analysis is a commonly used technique, which is finding prominence in standards and guidance material like Eurocode 8 and FEMA 356. The purpose of this paper is to compare the methods given by these two documents. In this paper a typical existing five-story RC frame which has been designed for moderate seismicity according to the past generation of Greek seismic codes is investigated. A non-linear static (pushover) analysis is carried out for the building using SAP 2000 and the ultimate capacity of the building is established. Few critical sections are selected and the rotational ductility supply at various limit states as predicted by FEMA 356 and Annexe A of EC8 Part 3 is calculated. The two predictions are compared with each other and with results from the SAP2000 analysis. The outcome of this paper is to provide useful information for further development of Eurocode 8.

Creep and strength retention of aramid fibers

Abstract: Creep tests at ambient conditions have been carried out on Kevlar 49 and Technora yarns cover- ing a wide stress spectrum (10-70% average breaking load) for a long period of time (up to a year). The results confirm that Kevlar 49 and Technora yarns show a non- linear behavior at stresses below 40% of the breaking load and a linear behavior at stresses above 40%. The strength retention following creep for Kevlar 49 and Technora has also been examined. The results show a significant difference in the behavior of the two materials. Kevlar 49 appears to lose strength almost linearly with time, while Technora seems to lose strength much more rapidly. These results would have significant implications

High-Performance Fibers

Abstract: The article contains sections titled: 1. Introduction 1.1. Definition and Classification 1.2. Historical Overview. 2. Heat-Resistant Fibers 2.1. meta-Oriented Aromatic Polyamide Fibers 2.1.1. Solution Polymerization 2.1.2. Interfacial Polymerization 2.1.3. Fiber Formation 2.1.4. Washing, Drawing, Heat Treatment 2.1.5. Fiber Properties 2.1.6. Uses 2.2. Fibers from Heterocyclic Polymers 2.3. Fibers from Melt-Processable Polymers 2.4. Cross-linked Fibers 3. High-Strength and High-Modulus Fibers 3.1. para-Oriented Aromatic Polyamide Fibers 3.1.1. Fibers from Lytropic Solutions 3.1.2. Fibers from Isotropic Solutions 3.1.3. Fiber Structure 3.2. Polyazole Fibers 3.2.1. Fiber Structure 3.3. Fibers from Thermotropic Polymers 3.4. High Strength Polyethylene Fibers 3.4.1. Gel Spinning 3.4.2. Extrusion Process 3.4.3. Fiber Structure 3.5. Properties and Uses of High Strength Fibers 3.5.1. Fiber Properties 3.5.2. Uses