investigation of the mechanical properties of hybr

Research ArticleInvestigation of the Mechanical Properties of HybridCarbon-Hemp Laminated Composites Used as ThermalInsulation for Different Industrial Applications

M. L. Scutaru andM. Baba

Department of Mechanical Engineering, Transilvania University of Brasov, 29 Eroilor Boulevard, 500036 Brasov, Romania

Correspondence should be addressed to M. L. Scutaru; [email protected]

Received 11 February 2014; Accepted 26 March 2014; Published 22 April 2014

Academic Editor: Bogdan Mitrica

Copyright 2014 M. L. Scutaru and M. Baba. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Carbon-hemp composite laminate provides good thermal properties. For this reason this type of material is presently being usedfor various applications like insulator for airplanes, spaceships, nuclear reactors, and so forth. Unfortunately their mechanicalproperties are less studied.These characteristics are very important since they should be guaranteed also for important mechanicalstress in addition to the thermal one. The present paper presents a study regarding the impact testing of some hybrid compositelaminate panels based on polyester resin reinforced with both carbon and hemp fabric. The effects of different impact speeds onthe mechanical behavior of these panels have been analyzed. The paper lays stress on the characterization of this hybrid compositelaminate regarding the impact behavior of these panels by dropping a weight with low velocity.

1. Introduction

Composite materials for thermal insulation [13] are usedin a wide range of applications; however, they are usedwith prudence in applications where transverse loadingsappear, for instance, loadings given by transverse impactwith low velocity. Since this type of materials is used alsofor applications involving new technologies like aeronauticindustry, space missions, or nuclear reactors, the mechanicalproperties must be more carefully investigated. In general,failures and imperfections are inevitable in composite struc-tures. In this context, design concepts of composite structuresare used to take into account these failures, such as damagetolerance and damage resistance. Damage resistance is con-nected to the materials capability to minimize the failureseffects given by impact, while damage tolerance is given bythe materials capability to maintain its properties even afterfailures appearance in material. Usually, these properties arecalled residual properties. One of the difficulties regardingthe properties and evaluation of composites is, ironically,an advantage, namely, the capability to allow users to tailortheir properties to suit the design needs [4, 5]. There are a

huge number of fibers ranges and combination ways, resins,additives, stacking and orientation ways, manufacturing pos-sibilities (thermal treatments) and therefore is very difficultto extrapolate the composite behavior depending on theseparameters for a certain combination of them [6, 7]. Inapplications, the use of composites based on natural fibers isyet limited at the so-called nonstructural components suchas inner components of cars. One of the main reasons forthis limitation consists in the sensitivity of these compositesat impact and the difficulty in critical evaluation regardingdamages caused at impact.

2. Material and Method

The research has been carried out on ten composite panelspresenting a rectangular shape and being underpinned on alledges. All panels have the same materials structure:

(i) a layer of thermosetting resin reinforced with carbonfabric;

(ii) a layer of thermosetting resin reinforced with hempfabric;

Hindawi Publishing CorporationAdvances in Mechanical EngineeringVolume 2014, Article ID 829426, 4 pageshttp://dx.doi.org/10.1155/2014/829426

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2 Advances in Mechanical Engineering

(iii) a layer of thermosetting resin reinforced with carbonfabric;

(iv) a layer of thermosetting resin reinforced with hempfabric.

The plies sequence has been carried out in the hand lay-up process using a roll for resin impregnation of carbon andhemp fibers. Finally, the structures thickness has been 4mm.The laminate panel has beenmaintained at room temperaturefor twoweeks fromwhich ten specimens of rectangular shape(150 100mm) have been cut. The specimens have beensubjected to impact by dropping a weight according to thestandard ASTM-D5420-98a (Figure 1). The impact testingby dropping a weight is used to characterize the dynamicbehavior of a material. The experimental setup consistsin a two-column frame and a weight which can be liftedand released in free fall with minimum friction by slidingalong columns under own weight (Figure 2). The indenterpresents a hemispherical head with a 16mm diameter andits mass is equal to 1.9 kg. This indenter hits the middle ofthe rectangular specimen. The accelerometer is fixed on theupper part of the indenter and the signal (acceleration) istaken over in computer by help of an acquisition device typeNI USB 6521 BNC. Using this kind of testing, some dataregarding the mechanical properties of a material can beobtained, namely:

(i) the energy, , absorbed during impact;(ii) the variation of impact force, F, at the impact

moment;(iii) the variation of inventors displacement, , versus

time, and so forth.

In case of impact testing by weight falling, the onlymeasured feature versus time is the contact force, (),exerted by the weight which falls on specimen while thespecimens deflection is determined as a function of timeby numerical integration of the indentors motion equation.The acquisition of experimental data (acceleration) as well ascomputing the response parameters described above has beencarried out using a block diagram conceived by the LabViewprogram.

The height from which the weight is released has beencomputed with the well-known relation:

=

V2

[m] , (1)

where represents the gravitational acceleration. The inden-tors motion equation can be written by help of the followingrelation:

() =

()

(m/s2) , (2)

where represents the impactors mass.The absorbed energy can be computed according to the

following relation:

() =

0

() V () (J) . (3)

ImpactorAccelerometer

1006.25

150

35

Rubber-tipped clamps

Coupon specimen

Wood plate

30

6

Steel plate

12.5

75

12.5

125

Hemispherical indentor 16

Figure 1: The specimens geometry and fixing mode.

Figure 2: The impact device.

3. Results

Using the LabView program, the variation of force hasbeen computed at the impacts moment after the signal hasbeen recordedwith the accelerometer through the acquisitiondevice (Figure 3). In the sameway, the following distributionshave been represented:

(i) the variation of impact force F versus the displace-ment at the impacts moment (see Figure 4);

(ii) the variation of indenters displacement at theimpacts moment (Figure 5);

Advances in Mechanical Engineering 3

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018

0

200

400

600

800

1000

1200

1400

1600

Time (s)

Forc

e (N

)

200

C1; 4m/sC5; 3m/s

C7; 2m/sC10; 1m/s

Figure 3: Force-time (F-t) distribution at the impacts moment.

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

0

200

400

600

800

1000

1200

1400

1600

Displacement (m)

Forc

e (N

)

200

C1; 4m/sC5; 3m/s

C7; 2m/sC10; 1m/s

Figure 4: Force-displacement (-) curves at the impacts moment.

(iii) the variation of energy at the impacts moment(Figure 6).

The results obtained after the impact testing of hybridcarbon-hemp composite laminate are presented in Table 1 aswell.

The indenters falling height has been computed as fol-lows:

V2 = 2 =V2

2

. (4)

4. Discussion and Conclusions

Analyzing especially the graphics from Figure 6, we cansay that these curves present leaps and inflexions due to

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.0180

0.002

0.004

0.006

0.008

0.01

0.012

0.014

Time (s)

Disp

lace

men

t (m

)

C1; 4m/sC5; 3m/s

C7; 2m/sC10; 1m/s

Figure 5: The variation of indenters displacement at the impactsmoment.

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.0180

2

4

6

8

10

12

14

16

Time (s)

Ener

gy (J

)

C1; 4m/sC5; 3m/s

C7; 2m/sC10; 1m/s

Figure 6: The variation of energy at the impacts moment.

the presence of delamination. Integrating the area underthe loading curve (force-displacement, Figure 3) until themaximum value of the force (according to the first failure),the energy required to initiate the failure, can be obtained.At the impacts moment, the energy accumulates in time andis directly proportional with the force and increases untilit reaches a constant landing. After reaching a maximumvalue of the force, this decreases in time, the energy U beingabsorbed in material and the forces decrease took placeafter reaching the landing of U. In general, at the compositelaminates the energy is frequently absorbed by creating somedelamination surfaces called delamination breaks that lead tothe strength and stiffness decrease. Analyzing the specimensafter impact testing (Figure 7) it can be noticed that the failure

4 Advances in Mechanical Engineering

(a) (b)

Figure 7: Specimen after impact testing.

Table 1: Results after impact testing.

Specimen Results ObservationsSpecimen thickness [mm] Falling height [m] Impact speed [m/s]

1

4

0.815 4 Total break2345 0.458 3 Break at first ply67 0.203 2 Trace on specimen is slightly marked89 0.05 1 Trace on specimen is not visible10

areas localized on the specimens surface (where the intenderhits) are smaller than those localized on the back front. Weconclude that the cracks have been propagated from the placewhere the indenter hits to the back front of the panel. Thus,we can conclude that the panelmanufactured from the hybridcompositematerial involved in this study is stiff. On the otherhand, as expected, the grater the impact speed is, the graterthe failure area has been noticed.

This type of materials is feasible for various applications,for example, in the automotive industry and airplanes. Moresystematic studies regarding the mechanical properties of thecomposite materials must be performed. The influence ofionising radiation over the insulating composite materialsmust be also investigated in order to increase the safety andsecurity for various types of missions.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

References

[1] P. Keerthan and M. Mahendran, TI thermal performance ofcomposite panels under fire conditions using numerical studies:plasterboards, rockwool, glass fiber and cellulose insulations,Fire Technology, vol. 49, no. 2, pp. 329356, 2013.

[2] J. G. Yang, G. Wang, and K. W. Ding, Thermal performancetest of the new sandwich insulation composite wall panels,Advanced Materials Research, vol. 676, pp. 2226, 2013.

[3] W. Yin, J. Cui, G. Xia, R. Jin, S. Xia, and S. Liu, Thedevelopment of new sandwich thermal insulation compositewall based on the housing industrialization,Applied Mechanicsand Materials, vol. 291294, pp. 9971000, 2013.

[4] N. D. Cristescu, E. M. Craciun, and E. Soos, Mechanics ofElastic Composites, Chapman & Hall/CRC, Boca Raton, Fla,USA, 2003.

[5] B. Mitrica, Design study of an underground detector formeasurements of the differential muon flux, Advances in HighEnergy Physics, vol. 2013, Article ID 641584, 9 pages, 2013.

[6] S. Vlase, H. Teodorescu-Draghicescu, M. R. Calin, and L.Serbina, Simulation of the elastic properties of some fibre-reinforced composite laminates under off-axis loading system,Optoelectronics and Advanced Materials, Rapid Communica-tions, vol. 5, no. 4, pp. 424429, 2011.

[7] H. Teodorescu-Draghicescu and S. Vlase, Homogenizationand averaging methods to predict elastic properties of pre-impregnated composite materials, Computational MaterialsScience, vol. 50, no. 4, pp. 13101314, 2011.

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