Residual Strength Evaluation after Impact Tests in Extreme Conditions on CFRP Laminates Academic research paper on "Materials engineering"

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Procedía Engineering 167 (2016) 138 - 142

Procedía Engineering

www.elsevier.com/locate/procedia

Comitato Organizzatore del Convegno Internazionale DRaF 2016, c/o Dipartimento di Ing. Chimica, dei Materiali e della Prod.ne Ind.le

Residual Strength evaluation after impact tests in extreme conditions on CFRP laminates

V.Lopresto*, A. Langella, I. Papa

Department of Chemical, Materials Engineering and Industrial Production, University of Naples Federico II, Piazzale Vincenzo Tecchio 80, _80125 Naples_

Abstract

Compression after impact tests were performed using a new apparatus, designed and realized by University of Naples "Federico II", within the Solid Mechanics Program financed by ONR, that allow a more simple functioning and more reliable data respect to the one suggested by the ASTM D7137 Standard.

Impact tests were carried out on carbon fibre in vinylester resin laminates of Naval interest, loaded when supported by the devise suggested by the EN6038 European Standard and also when supported by a water pillow, to investigate the influence of the water- material interaction.

The results about the residual compression strength evaluation, obtained in different test conditions like increasing impact energy levels, different loading temperatures, different specimens thickness, were compared: they showed a negligible influence of the temperature, even if T = -25°C represents a crucial condition, evidencing a different trend. On the other side, a sensible influence of the load application was noted: the residual strength of the laminates tested on the water is sensibly higher than the one obtained by the traditional pure impact tests.

© 2016 The Authors.PublishedbyElsevierLtd. Thisis an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Organizing Committee of DRaF2016

Keywords: Low velocity impact tests, low temperature, residual strength, CAI.

1. Introduction

An environmental condition, critical for structures made of composite material, is related to low temperatures, especially in presence of dynamic loads. This problem is really important in the Artic Ocean navigation. The most crucial, but important information, is the residual compression strength of a laminate after an impact load [1-3]. It is due to the importance to know if the panel in service, mounted on a structure, can continue to work or if it is necessary to replace the part. The problem related to composite materials is due to the not simple damage mechanisms and their complex interaction, and to the fact that the damage is not always a visible damage: it could be present between the internal plies but it is not visible [4]. One of the most potentially dangerous aspect in the

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Organizing Committee of DRaF2016

doi: 10.1016/j.proeng.2016.11.680

impact response of composite structures, is the difficulty to detect damage by visual inspection, even when considerable strength and rigidity losses have occurred.

In the present research, supported by the ONR Solid Mechanics Program, Compression After Impact tests were carried out on carbon fibre laminates made by Ashland Derakane 510A vinyl ester matrix, used in the shipbuilding industry and supplied by the Naval Surface Warfare Center (NSWC). The impact tests were conducted in the traditional way, by a falling weight machine equipped with an instrumented steel impactor with hemispherical nose 19,8 mm in diameter.

The specimen was placed, first, on the support suggested by the EN6038 and then, using a test equipment designed and realized, within the mentioned Program, to reproduce the real material behaviour in the presence of water and to obtain a load distribution on the entire surface of the specimen. Condition this which reproduce the impact on a hull, with a solid body suspended in the water.

Thanks to a thermal chamber, that allows to carry out tests in the temperature range between +70 and -50°C, the impact tests were carried out at room and the low temperatures of -25°C and -50°C.

The effect of the impact load on the laminates has been related with the delaminations, evaluated by ultrasonic scanning, and with the indentation, the plastic deformation left by the indenter on the surface of the material. Compression after Impact tests were performed using a new apparatus, designed and realized within the Solid Mechanics Program, that allow a more simple functioning and more reliable data.

The results about the residual compression strength evaluation showed a negligible influence of the temperature, even if T = -25°C represents, one more time, a crucial point evidencing a different trend. On the other side, a sensible influence of the load application was noted with a residual strength of the laminates tested on the water sensibly higher than the one obtained by the traditional impact tests.

2. Materials and experimental set-up

The carbon laminates supplied by Naval Surface Warfare Center (NSWC) and fabricated by the vacuum infusion process by overlapping 7, 10 and 13 T700 carbon fabric plies (0°/90°) 300g/sqm, resulting in 2, 3 and 4 mm nominal thickness, were impacted at low velocity. For the square panels, 600x600mm, Ashland Derakane 510A vinyl ester resin was used since the US Navy applications. The final volumetric fibre percentage, Vf, was 48%. Rectangular specimens, 100x150mm, suggested by the EN6038 or ASTM7137 Standard to allow the Compression After Impact tests, were cut from the panels.

The experimental tests were carried out by a falling weight machine Ceast Fractovis and the specimens were impacted by an instrumented impactor cylindrical in shape with an hemispherical nose 19.8 mm in diameter. The impact velocity of about 4.0 m/s was measured.

The specimens were loaded up to penetration, to obtain the complete load displacement curve. In correspondence of some characteristics points on the curve, evidencing a change in material behaviour after a damage, increasing impact energy levels, 5J, 10J and 20J, were measured and used to investigate the damage start and propagation. The updating of the machine by a thermal chamber allowed tests at low temperatures. Impact tests, were, first, carried out at room and at the temperature of -25°C and -50°C on laminated specimens using the clamping device suggested by the above recalled Stardard.

At the same time, tests to investigate the influence of the water on the load distribution, labelled as WB (standing for Water Backed), were carried out on similar carbon fibre specimens simply supported by a thick water layer. A deformable bag containing water was located under the specimen. These loading conditions are in the middle between the pure impact and the hull slamming, and they were never before studied in literature. A cylindrical rigid steel box, 200mm in diameter, covered by a black silicon thin bag, was used as support; a circular steel plate 8 mm in thickness, having a window 100x150mm to locate the specimens, was fixed on the box (Fig. 1). The box has been filled with a quantity of water necessary to create a pillow of water and avoid any resistance during the impact. The coupons were placed on the pillow to create the contact between the surface and the bag. They were, then, impacted on the other side by the device described above.

Fig. 1: Steel box design for the water backed impact.

During each test, the load curves, as well as the energy and the velocity, were recorded by the DAS16000 acquisition program. The delaminated areas were obtained by the US Multi2000 Pocket 16x64 by M2M. The impacted specimens were tested in compression to investigate the residual compression strength (CAI tests) by a new equipment designed at University of Naples "Federico II", to faster avoid the problem of the buckling and the misalignment of the specimen and the load.

Respect to the method suggested by the European and American Standards, this equipment involve a lower number of bolts and it is more rigid, avoiding the mentioned misalignments that cause load concentrations and wrong failures. The two lateral blocks are parts of the main body and the sample is placed in a single sliding guide (Fig. 2a) blocked to the rigid main body by only one bolt (Fig. 2b).

However, despite all, the tests are very complicated and the results strongly depend on the quality of the laminate. The above described problems lead to incorrect failures and so, to a not correct residual strength evaluation.

Fig. 2: CAI tests equipment: new design. Unfortunately, a large number of tests gave non acceptable results.

Four strain gauges, two on the front and two on the back side, were glued on each specimen following the suggestions of the ASTM D7137 Standard (Fig.2). This was a very long and complicated work.

3. Results

Plotting the residual strength as a function of the impact energy, the general decreasing trend found in literature [2] was confirmed for all the temperatures tested (Fig.3, laminates 3mm in thickness).

Fig.3: Residual strength: influence of impact energy, U; t = 3 mm

The same was done for the 4 mm thickness specimens: a more constant trend of the residual strength was found as a function of the impact energy (Fig. 4).

Fig.4: Residual strength: influence of impact energy, U; t = 4 mm

It denotes a less sensitive residual strength decreasing. Also a lower influence of the temperature on higher thicknesses, was noted. The contrary was observed at room temperature where the increasing thickness led to a decreasing in the residual properties, justified by the fact that most of the absorbed energy is spent to create internal damage since the increasing rigidity, whereas for thinner laminates, the bending effect is predominant [5].

The rectangular specimens were, then, impacted at the same impact energies also when they were simply supported on a water pillow, to investigate the water backed influence. The shape of the load curves was found to be different, as well as the maximum load and displacement values.

By comparing the residual strength in pure impact and water backed tests (Fig. 5), higher residual properties were showed by water backing tested, in particular for the thinnest laminates.

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Fig. 5: Comparison about residual strength after pure impact and water backed tests.

However, by comparing the delamination extensions measured in water and air backed tests (Fig. 13), a more limited area was always found in presence of the water. It is due, without any doubt, to the different interaction between the impactor, the material and the constrain conditions. To understand the influence of the latter on the dynamic response, the theoretical study is necessary and it is what University of Naples is trying to do in collaboration with NY University and it will be shown in a future paper.

Conclusions

CAI tests were carried out on more than 50 coupons and from the results obtained it was possible to conclude that:

• the general trend from literature, for the prediction of the residual strength, was confirmed;

• an interesting decreasing in residual strength at the decrease of T (larger delamination) was observed;

• a lower decrease in residual strength at the increasing thickness, was found;

• the absorbed energy was revealed lower at low temperatures; since the larger delaminated areas at low temperature, a less amount of energy is necessary to absorb and to spend to create damage;

• a different behaviour was noted between air backing and water backing load conditions: the load-displacement curves are completely different and the maximum load is sensibly lower during the impact on the water;

• higher residual properties were exhibited by specimens water backing tested;

• a more limited area was always found in presence of the water.

Acknowledgement

The authors gratefully acknowledge the ONR Solid Mechanics Program, in the person of Dr. Yapa D.S. Rajapakse, Program Manager, for the financial support provided to this research; and Dr. Loup Douglas for providing the laminates used in the research.

References

[1] S. Abrate, "Impact on laminated composites: recent advances", Appl. Mech. Rev. 1994, 47 (11): 517-544.

[2] G. Caprino, "Residual strength prediction of impacted CFRP laminates", J. Compos. Mater., Vol. 18, 1984, pp. 508-518.

[3] W. J. Cantwell and J. Morton, "The impact resistance of composite materials - A review", Composites, Vol. 22, No. 5, 1991, pp. 347-362.

[4] G. Caprino, V. Lopresto 'The significance of indentation in the inspection of carbon fibre-reinforced plastic panels damaged by low-velocity impact", Composites Science and Technology, Vol. 60 (2000), pp.1003-1012.

[5] G. Caprino, V. Lopresto, C. Scarponi, G. Briotti, Influence of material thickness on the response of carbon-fabric/epoxy panels to low-velocity impact, Comp. Science and Tech. 59: 2279-86, 1999.