APPLICABILITY OF COMPOSITE CHARPY IMPACT METHOD FOR STRAIN ... · Since most test methods, used for...

2nd International RILEM Conference on Strain Hardening Cementitious Composites 12-14 December 2011, Rio de Janeiro, Brazil

399

APPLICABILITY OF COMPOSITE CHARPY IMPACT METHOD FOR STRAIN HARDENING TEXTILE REINFORCED CEMENTITIOUS COMPOSITES

J. Van Ackeren P

(1)P, J. Blom P

(1)P, D. Kakogiannis P

(1)P, J. Wastiels P

(1)P, D. Van Hemelrijck P

(1)P, S.

Palanivelu P

(2)P, W. Van Paepegem P

(2)P, J. Vantomme P

(3)

(1) Vrije Universiteit Brussel, Mechanics of Materials and Constructions, Brussels, Belgium (2) Universiteit Gent, Dept. of Materials Science and Engineering, Gent, Belgium (3) Royal Military Academy, Civil and Materials Engineering Department, Brussels, Belgium Abstract

Since most test methods, used for strain hardening cementitious composites, are adopted

from polymer composites test standards, this paper investigates the applicability of a standard impact test (ISO 179-1:2000), which is used in the polymer composites world, on a specific textile reinforced cementitious composite using an inorganic phosphate cement (IPC) as a matrix material. The aim is to provide a test method to investigate the local impact behaviour of textile reinforced cementitious composites. It was found within this work that this method is applicable to cementitious composites provided some adaptations to the specimens’ dimensions. The Charpy impact strength of an IPC matrix reinforced with different kinds of fibre types is successfully investigated. This test method can be used to qualitatively rank different cementitious composite materials.

1. INTRODUCTION In the last few decades, many new cementitious materials were developed in order to reach

thinner and stronger construction elements. The evolution started with the introduction of loose steel or polymeric fibres into the fresh concrete mixture, leading to fibre reinforced concrete (FRC): the addition of fibres into concrete provides toughness or energy absorption capacity to the inherently brittle concrete. A new generation of FRC’s are the so called textile reinforced concretes (TRC) in which mainly fibre textiles (glass, polymers …) are applied as reinforcement: they can provide also a strain hardening capacity when a sufficiently high quantity of fibres is used. In a recently held conference on these kinds of materials [1], the potential of these new composites was again demonstrated. In an attempt to better define high performance fibre reinforced cementitious composites (HPFRCC), Naaman and Reinhardt [2] proposed a clear classification of these materials according to their possibility to exhibit strain hardening under bending or in tension. Within this classification a distinction is made between the four following categories: (1) crack control, (2) deflection hardening, (3) strain hardening and (4) high energy absorption.


400

Strain hardening cementitious composites (SHCC) not only show strain hardening under tensile loading, but also can absorb a large amount of energy. The cementitious material studied in this paper can most certainly be classified in the fourth category. It exists of an inorganic phosphate cement (IPC) matrix with different kinds of textile reinforcements. IPC was developed at the Vrije Universiteit Brussel and consists of a liquid component based on phosphoric acid solution containing inorganic metal oxides and a calcium silicate powder component. Next to the advantages of being incombustible (EN13501-1), this material also possesses a neutral pH after hardening, which allows cheap E-glass fibres to be used as reinforcement. It is however also possible to process other types of fibres, such as polymeric fibres or carbon fibres, into this cementitious matrix. Figure 1 shows typical tensile stress-strain behaviour of IPC composites with different kinds of fibre reinforcements. The fibre volume fraction is kept constant for all materials at about 20 %. It is clear from this figure that indeed all tested composite specimens can absorb a significant amount of energy due to their pronounced strain hardening behaviour after multiple matrix cracking. Note that depending on the used fibre reinforcement one can obtain a completely different composite material. For instance the curve of PE-reinforced IPC shows stiff and strong (up to 400 MPa) behaviour, while on the right side of the graph the PVA reinforced IPC shows ductile (maximum strain more than 5 %) but less strong behaviour.

This composite material has been investigated in a project that studied the possibility of producing sandwich structures that are able to absorb a significant amount of energy under blast or impact loading. The textile reinforced IPC was meant to protect the energy absorbing core against fire and to distribute the load from the explosion to the sandwich core [3]. Apparently, the material itself is able to absorb energy under blast and impact loading without failing. Under impact loading only local damage can cause failure of the material. Globally, there will be matrix cracking reducing the stiffness, but the material keeps its load bearing capacity. In this paper, the local impact damage and the corresponding energy absorption are investigated by means of impact tests.

Figure 1: tensile stress-strain behaviour of IPC reinforced with different UD-fibres

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6strain [%]

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402

The Charpy impact strength can be compared to typical values for toughness that is defined

in fracture mechanics as being the energy that is necessary for the growth of a crack. Toughness (GRcR) is a material parameter, typical values for different materials can be found in Ashby and Jones [6]. For metals, values between 100 and 1000 kJ/m² are given while for most plastics the toughness is less than 10 kJ/m². Typical values for composites start at 5 kJ/m² and can reach more than 100 kJ/m².

Three specimen types are defined in ISO 179-1 [5] as illustrated in Table 1. Note that the given tolerances are rather small for cementitious materials. In the experimental section the average values of the dimensions together with their standard deviation will be given. Specimen type 1 is used when the material does not show interlaminar shear. Specimen type 2 is designed to induce typical bending failure modes for materials that undergo ILS. These specimens fail in the middle section of the span either by tensile failure on the back of the specimen or compressive failure or buckling of fibres at the impacted side. The span to thickness ratio is fixed to 20 for this specimen type. To test the energy absorption capacity of interlaminar shear phenomena, specimen type 3 is designed with much smaller span to thickness ratio of 6 or 8. The value of 8 can be chosen if it is not possible for practical reasons to test at such small spans.

Table 1: Specimen types, dimensions and spans proposed by ISO-179 [5]

Specimen Length l Width b Thickness h Span L 1 80 ± 2 10.0 ± 0.2 4.0 ± 0.2

2

3 25h

11h or 13h 10 or 15 3 20h 6h or 8h

3. EXPERIMENTAL STUDY All tests in this work were performed on a Ceast Impactor II 7611 pendulum testing

machine. The pendulum has a capacity of 15 J and is equipped with a processing unit that immediately calculates the Charpy impact strength. The speed at impact is 3.8 m/s. Before every series of measurements the machine is calibrated by determination of the friction in the pendulum.

3.1 2D RANDOM GLASS FIBRE MAT REINFORCEMENT

The first series of specimens, which contains 2D random chopped strand glass fibre mats as reinforcement, were manufactured by hand lay-up technique [7]. The volume fraction is about 20 %. Note that only part of this fraction is contributing to the load carrying capacity of the specimen. Five different series of 20 specimens were manufactured (see table 2): the three different types of specimens according to the ISO 179-1 standard. For type 2 and 3 the influence of the width was tested (10 or 15 mm) by adding two extra series.

The results of these tests are made visible in figure 3 in form of boxplots. The the bold line in the middle of the grey box represents the median. Next to the boxplots, a figure shows the tested specimens and their failure modes.

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405

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406

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407

REFERENCES [1] W. Brameshuber, Proceedings of the International RILEM Conference on Materials Science

(MatSci), Volume 1: 2P

ndP ICTRC, 2010, Aachen, Germany ISBN: 978-2-35158-106-3

[2] A.E. Naaman, H.W. Reinhardt, High performance fiber reinforced cement composites HPFRCC-4: International workshop Ann Arbor, Michigan, June 16-18, 2003. Cement and Concrete Composites, Volume 26, Issue 6, 757-759 (2004)

[3] S. Palanivelu, W. Van Paepegem, J. Degrieck, B. Reymen, J. Nadambi, J. Vantomme, D. Kakogiannis, J. Van Ackeren, D. Van Hemelrijck, J. Wastiels, Experimental and numerical blast energy absorption study of composite tubes for sacrificial cladding structures, ECCM-14, 7-10 June, 2010, Budapest, Hungary

[4] V. Gopalaratnam, S. Shah, R. John, A modified instrumented charpy test for cement-based composites, Experimental Mechanics, volume 24, issue 2, p. 102-111 (1984)

[5] ISO 179-1: 2000, Plastics – Determination of Charpy impact properties – Part 1: Non-instrumented test

[6] M.F. Ashby, D.R.H. Jones, “Engineering materials 1: An introduction to properties, applications and design”, Elsevier Butterworth-Heinemann, Burlington, 2005, ISBN 0-7506-63804

[7] J. Blom, P. Van Itterbeeck, J. Van Ackeren, J. Wastiels, Textile reinforced inorganic phosphate cement composite moulds, NCTAM-2009, 2009, Brussels, Belgium

APPLICABILITY OF COMPOSITE CHARPY IMPACT METHOD FOR STRAIN ... · Since most test methods, used for...

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