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A New Possibility in Design of Composite Materials for

Improved Impact Properties

Seminar byRANJITH K

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1.Introduction• Composites have been widely used in applications where there is a

risk of impact, due to the excellent properties these materials display for absorbing impact energy.

• However, composites during impact situations typically generate an enormous number of small pieces, due to the energy absorption mechanism of these materials.

• This can prove dangerous, as the small fragments of the original structure may cause damage to other vital components near to it and may even result in harm to human life.

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• This study was designed to explore the possibility of incorporating a material which, while maintaining a high level of energy absorption without any plastic deformation, was able to maintain its original form, or at least significantly reduce the number of pieces generated after impact.

• The addition of a polyamide layer, NOMEX®, to a monolithic carbon fibre fabric laminate was investigated.

• NOMEX® has been investigated for use in the aerospace industry as a core for sandwich composite structures in the form of a honeycomb structure.

• In the current study NOMEX® is placed in the laminate, like any other composite layer, but with the particularity that it is not pre-impregnated.

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2. Description and Manufacturing of the material

• Woven fabric carbon/epoxy pre-impregnated ply: Hexply W3T-282-42- F593-14, with a density of 1442 kg/m3

• Polyamide: NOMEX® Comfort E502 220i, with a density of 608 kg/m3 and a thickness of 0.34 mm

• To obtain the test samples, several panels were manufactured, some containing just woven fabric and others using both woven fabric and NOMEX®.

• In all cases, the stacking sequence was symmetrical and the NOMEX® layer was placed on the plane of symmetry.

• After curing, the panels were cut and inspected with ultrasounds to check that there was no presence of porosity in them.

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Fig.1. Micrograph of the composite with NOMEX® (the different layers are detailed).

NOMEX® (Honey comb) Woven carbon fibre fabric

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3. Testing and Results

• The following tests have been carried out on samples of carbon fibre woven fabric pre-preg, both with NOMEX® and without NOMEX®.

Test Number of Samples UsedTensile test 5

Interlaminar fracture toughness energy test 3

Impact test 5

Compression after impact test 5

73.1. Tensile Test

• The tensile test was carried out to calculate the ultimate strength of the material.

• The tests were carried out using an Instron 4482 universal testing machine, at a speed of 1 mm/min.

• They were completed following the standard UNE-EN 2561, which specifies that the samples must have the following size: 250 mm (length), 15 mm (width) and 1 mm (thickness).

• The mean, the maximum and the minimum values of the strength reached in the 5 samples are shown, plus the standard deviation (STD) and the covariance (CV).

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Tensile strength with respect to fabric area (tensile strength with respect to total area)

Material Fabric Fabric + NOMEX®

Stacking sequence

[0]4 [(0)2/NOMEX®/(0)2]

Mean [MPa] 328.90 315.53 (264.2)

Maximum [MPa] 337.10 342.09 (286.5)

Minimum [MPa] 325.60 292.27 (244.7)

STD [MPa] 13.40 23.54 (19.7)

CV [%] 4.11 7.46

Table 1: Results of the tensile strength test.

Fig.2. Tensile test specimen results without

(a) and with (b) NOMEX®.

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3.2. Interlaminar Fracture Toughness Energy Test

• In this test, the adherence of the NOMEX® to the laminate was checked, measuring the interlaminar fracture toughness energy (GIC).

• The objective of the test is to measure the energy needed to propagate a crack throughout the laminate.

• These tests were carried out following the standard AITM 1.0005, which specifies that the samples must have the following size: 250 mm (length), 25 mm (width) and 3 mm (thickness).

• The crack length to calculate the energy, as stated in the specification, is 100 mm.

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Interlaminar fracture toughness energy

Material Fabric Fabric + NOMEX®

Stacking sequence

[0]6s [(0)6/NOMEX®/(0)6]

Mean [J/m2] 322.40 630.00

Maximum [J/m2] 348.96 638.04

Minimum [J/m2] 283.48 623.77

STD [J/m2] 34.49 7.30

CV [%] 10.69 1.16

Table 2: Results of the interlaminar fracture toughness energy test.

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3.3. Impact Test

• In the test, a mass with a semi spherical tip falls from a certain height onto the laminate.

• The parameters measured are the perforation depth (i.e., indentation) and the delaminated area.

• These tests were carried out in a drop tower following the standard ASTM D7136, which specifies that the samples must have the following size: 150 mm (length), 100 mm (width).

• The weight of the impactor was 4.47 kg. ASTM D7136 specifies a value of 5.50 kg for the impactor, but allows the use of other values, the test in this case being designated as a non-standard test.

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• The height chosen was 76.50 cm.

• Before the impact, four points are marked at a distance of 20 mm from the centre of the zone that is to be impacted.

• After the impact, four points are measured with a depth gauge (D1, D2, D3 and D4).

• The indentation is the result of the following equation: id = Dmax – (D1 + D2 + D3 + D4) / 4

• After the test, the samples were inspected manually using ultrasonic techniques to measure the area of the laminate where the damage appears.

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Indentation after impact with respect to the total thickness

Material Fabric Fabric + NOMEX®

Stacking sequence [(45/0)5/(0/45)5] [(45/0)5/NOMEX®/(0/45)5]

Mean [%] 86.56 36.69

Maximum [%] 100.00 39.91

Minimum [%] 58.88 30.91

STD [%] 20.35 3.96

CV [%] 23.51 9.65

Table 3: Results of the impact test.

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Delaminated area after impact

Material Fabric Fabric + NOMEX®

Stacking sequence

[(45/0)5/(0/45)5] [(45/0)5/NOMEX®/(0/45)5]

Mean [mm2] 4776.20 8159.10

Maximum [mm2] 5760.08 10681.10

Minimum [mm2] 3616.22 6125.92

STD [mm2] 953.52 1918.40

CV [%] 19.96 23.51

Table 4: Results of the delamination after impact test.

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3.4. Compression After Impact Test• This test is used to calculate the residual strength of the laminate

under compression after having applied the impact test to it.

• The load is applied perpendicular to the thickness of the sample, producing a uni-axial compression.

• Standard used is ASTM D7136.

Fig. 3: Front and rear view of a specimen with NOMEX® after the CAI test.

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Compression strength after impact with respect to fabric area (compression strength after impact with respect to total area)

Material Fabric Fabric + NOMEX®

Stacking sequence [(45/0)5/(0/45)5] [(45/0)5/NOMEX®/(0/45)5]

Mean [MPa] 135.60 180.92(173.8)

Maximum [MPa] 146.20 187.06(179.7)

Minimum [MPa] 125.40 175.67(170.1)

STD [MPa] 8.00 5.41(5.2)

CV [%] 5.92 2.99

Table 5: Results of the compression strength after impact test.

174. Analysis of Results

• 4.1. Tensile Test• From the tensile test it can be clearly seen that NOMEX® does not

play any role from a mechanical resistance point of view.

• NOMEX®'s lack of contribution to the strength of the total laminate, in spite of having one third of the strength of the fabric, is due to the different stress strain relation of fabric and NOMEX®.

Fig. 4: Schematic comparison between the ultimate

strength/strain values for thecarbon fabric and the

NOMEX®.

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4.2. Interlaminar Fracture Toughness Energy Test

• It can be observed that the energy associated with the samples that contain NOMEX® is higher than those samples without NOMEX®.

• These results confirm the satisfactory adherence of the NOMEX® with the prepreg, the value of GIC for the laminate with NOMEX® being almost twice as high as in the case of the monolithic fabric.

Fig. 5: load/displacement curves for the GIC test in

specimens without NOMEX® (a) and with

NOMEX® (b).

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4.3. Impact Test

• For the samples without NOMEX®, the delaminated area is small (4776.23 mm2) and located close to the impactor, but the indentation is very big (86.56%).

• In three samples the impactor penetrated the laminate completely spreading the material fragments.

• In the case of the samples with NOMEX®, the delaminated area is bigger (8159.17 mm2), but the damage is only inside the samples (the indentation is 36.69%).

• The damage has been absorbed and distributed by the NOMEX® which prevents the spread of material fragments.

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Fig. 6: Front and rear (impacted face) view of two impacted samples, one without NOMEX® (a) and the other with NOMEX® (b).

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4.4. Compression After Impact Test

• The ultimate compression strength in the case of the samples with NOMEX® (180.92 MPa) is more than 30% higher than in the case of the samples without NOMEX® (135.60 MPa).

• The dispersion of the test results is very low in both cases (2.99% and 5.92% respectively).

• Thus, including NOMEX® in the laminate reduces the effect of the impact on its strength, and as a consequence improves the behaviour of the part under subsequent loads.

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5. Conclusions• The results of the tests have shown that, by adding a layer of

NOMEX® to a composite laminate:

• The ultimate tensile strength is not affected.

• The interlaminar fracture toughness energy is enormously increased, achieving a stable propagation of the cracks as well.

• When subjected to an impact there is almost no fragmentation of the material and the damage produced by the impactor is reduced.

• The strength of the material under compression after an impact is increased, as NOMEX® layer redistributes the impact energy inside the laminate.

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• The damage produced by the impact on the carbon fibre layers was lower than in the case without NOMEX®

• Thus, the laminate can provide more capacity to resist a higher load during the compression after impact test.

• In summary, the results show that after low energy impacts the addition of NOMEX® to a composite laminate benefits the overall behaviour of the material.

• These results open up the possibility of extending these tests to materials and parts used commonly in the aeronautical, aerospace and automobile industries.

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6. References

1. Foo CC, Chai GB, Seah LK, “Mechanical properties of Nomex material and Nomex honeycomb structure”- 2007.

2. Castanie B, Bouveta C, Aminandab Y, Barrauc JJ, Thevenetd P, “Modelling of Low-energy/low-velocity impact on Nomex honeycomb sandwich structures with metallic skins”- 2008.

3. Product data HexPly F593. Resin system for advanced composites. http://www.hexcel.com/Resources/DataSheets/Prepreg-Data Sheets/F593.pdf

4.Jesús Justo, Sergio Osuna , Federico París, “Design of composite materials with improved impact properties”- 2015.

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Thank You