Indian Journal of Fibre & Textile Research Vol. 3 1 , June 2006, pp. 309-3 1 2

Mechanical properties of ribbon parachute fabrics for cluster submunitions

L Gnal'

Department of Textile Engineering, Erciyes University, 38039 Kayseri, Turkey

Received 7 Jalluary 2005; revised received alld accepted 13 April 2005

An attempt has been made to study the load-extension behaviour of the fabric for cluster submunitions. The effect of warp density and fibre type on the ribbon fabric performance using strip tensile and tearing test data has been studied and important points for the design of submunition parachutes highl ighted. Four different samples were manufactured using Spectra® and polyester fibres with three different weave types, two of which were derivatives of plain weave and the last one was regular plain weave. ANOVA was also performed for significance level of the test results. I t is found that as the warp density increases at core-zone of the fabric, the breaking load and Young' s modulus increase for strip tensile testing. However, the effect of weave type is found to be insignificant for tear test.

Keywords: Load-extension behaviour, Parachute fabric, Polyester fibre, Spectra® fibre, Submunitions, Warp density IPC Code: Int . n8 0030 1 3/00

1 Introduction The industrial yarns have been extensi vely used in

aerospace industry and mi l itary for the manufacturing of parachute fabric s ince World War I I . In addition to the paratroopers, parachutes have been effectively used to decelerate and stabi l ize munitions s ince Vietnam war.

Munitions can be stabi l ized in flight by parachute assemblies. These assemblies attach to the rear section of the munition to help nose-down positioning during their descent. The term 'submunition' generically describes any item of ordnance which is carried in or i s ejected by a dispenser. ' The dispenser munitions are designed to disperse submunitions over a large area, thereby increasing the radius of destructive effect over a target. Cluster bombs are the most well-known d ispenser type munitions, which could be delivered by arti l lery or planes. Each cluster bomb can carry up to 200 bomblets that are 20 cm long, 6 cm in diameter and weigh 3 .5 pound. The ribbon fabric is attached to the submunition from the top (Fig. 1 ) .

Polyamide 6 and polyamide 6.6 are commonly accepted fibres for manufacturing canopy and ribbon fabrics (particularly as webbings) for parachutes. Edberg2 investigated stress-strain curve of aged nylon parachute fabric . Starting from the late 1 970s, Kevlar® 29 was used i n parachute fabrics because of i ts superior resistant to strength degradation at elevated temperatures, l ight weight and less bulk.3.5

aE-mail : lonal @erciyes.edu.tr

Ericksen et al.6 studied the effect of warp and weft variables on the jointed webbing strength of Kevlar® 29 parachute webbi ng without increasing the weight of the structure. In 1 992, Ericksen et al. 3 experimentally and theoretically studied deflection­force curve. Some researchers 7,8 evaluated the properties of ribbon fabrics (woven and braided) for reinforcing parachutes,

Studies on parachute fabrics have particularly focused on canopy and ribbon parachute fabrics for paratroopers or mi litary cargo deployment. No speci fi c research has been reported on the fabrics used to stabi lize the submunitions. Therefore, i n the present work, the load-extension behaviour and mechanical properties of woven ribbon fabrics for the cluster submuni tions have been studied, Contribution of weave type and fibre type to the mechanical properties of ribbon fabric is also statistically analyzed using ANOV A.

-.

Fig. I - Cluster submunitions (at the bottom edges of picture) with attached ribbon parachute from the top of munition (US M42 PICM ) 1

3 1 0 INDIAN J . FIBRE TEXT. RES., JUNE 2006

2 Materials and Methods

2, 1 Materials

Polyester (PES) and Spectra® multifi lament yarns were used (Table 1 ) . Samples were manufactured in a narrow weaving machine. PES multifi lament was used as weft yarn for al l sample groups. Numbers of warp and weft ends were kept constant ( 1 5 picks/em and 49 warps/em). The detai led sample information i s given in Table 2 . Three different weaves (derivat ives of plain weave) were used. The derivations are based on the warp density along fabric width, instead of differences on interlacing points of warp and weft yarns ( l ike rib weaves). Each warp yarn consecutively interlaced with a weft yarn in a region in the middle of the fabric for certain length (core-zone). Therefore, the appearance of plain weave was conserved. For the weave type I (WT I ) , three warp yarns interlaced with a weft yarn (three warps per dent) i n a region in the middle of fabric for the length of 6 mm. For the weave type 2 (WT2), two warp yarns interlaced with a weft yarn (two warps per dent) i n a region i n the middle of fabric for the length of 6 mm. For the weave type 3 (WT3) , there is not any warp density difference along the fabric width and therefore WT3 is a regular plain weave fabric . Thi s region i s centered along the width of fabric (Fig. 2) . Fabric width of samples is around 1 5 mm and varies based on the warp yarn density along the fabric .

Although yarn counts are very close for both fibre types, their fabric thicknesses are different because of the number of fi lament per tow and bulkier characteristics of PES fibre as compared to Spectra® fibre.

2.2 Methods

Fabric testing was performed according to the relevant ASTM standards. I nstron 44 1 1 universal tester was used for tear and tens i le tests of fabric using strip method. S ince the fabric surface is smooth and prone to s l ipping, the special attachments were produced to prevent sl ippage.

Strip method (ASTM D 5035) was used for the evaluation of elongation and breaking force of narrow fabrics. Five samples, which were 20 cm i n length and the same fabric width as given i n Table 2, were tested. Only warp-wise testing was performed. Testing machine was set for a loading rate of 300 mm/min.

The same number of samples, sample length and loading rate was used for the tear test (ASTM D 226 1 ). Samples were cut from the middle to the length of 75 mm.

Table I-Yarn properties

Yarn Spectra® 1 000 Polyester

Yarn count, dtex

Number of filament/tow

Ultimate tensi le strength, g/den

Modulus, g/den

Elongation, %

239 60 38

1 320 2.9

Table 2 - Sample specifications

Sample code

A B

C D

Warp Weft Fabric Fabric lhickness width

mm mm

PES PES 0.4 1 5 .0 PES PES 0.4 1 5 .0 PES PES 0.4 1 5 .4

Spectra® PES 0.3 1 4.8

Regions having regular

c o n .� D Q) (/) 3 l:. a, c Q) ....J

warp density

(regular plain weave) /

/

/ /

• FabriC Width

•

Core-zone having

d ifferent warp

. density for the

length of 6 m m

245 220 7.9 1 35 8

Weave type

WT I

WT2

WT3

WT I

Fig. 2 - Fabric scheme for spec ify ing warp distribution and fabric layout

Fabric thickness was measured based on the ASTM D 1 777 using the James H.Heal c loth thickness tester. Five measurements were performed and the average was taken.

3 Results and Discussion Several points need to be considered while

designing parachutes for stab i li zation purpose. Fabric width determines air drag and contributes to t ime interval for turning the submunition nose-down. The effect of these factors requires rel atively wider fabric to allow quick and safe nose-down positioning. However, the opti mization of fabric width is necessary in order to pack submuni tions effectively. To avoid reduction in the strength of ribbon parachute, the selection of weave type (warp

ONAL: MECHANICAL PROPERTIES OF RIBBON PARACHUTE FABRICS 3 1 1

distribution) becomes important. By i ncreasing warp density per tooth of reed, it i s possible to adjust the fabric w idth during fabric manufacturing.

Strip test i s used for determin ing the breaking force and elongation of fabric. Fi ve samples were tested for each group. The average values along with the standard deviations (in brackets) are shown I n Table 3 .

The breaking load and the Young' s modulus of Sample A are sl ightly higher than those of Sample B for very close breaking displacement, although both of them are made from the same fibre type. It i s observed that breaking load and Young' s modulus increase, as the warp density i ncreases. Sample D has higher breaking load and modulus as compared to Sample A, although they were manufactured for the same weave type and number of warp ends. Thi s i s attributed to the superior fibre modulus and tensi le strength of Spectra@ fibre compared to PES fibre.

Fibre type determines the load-extension behaviour of the fabric. The strain-stress graph (Fig. 3) is steeper for Spectra@ fabric (Sample D) than for PES fabrics (Samples A, B and C). The slope and breaking behaviour of fabrics could also be observed from Fig. 3, where Sample D suddenly broken off at lower strain and therefore having lower toughness. This i s attributed t o the higher stiffness o f Spectra@ fibre as

Table 3 - Strip test results

Sample Breaking Breaking Young's Energy to code load displacement modulus breaking point

N mm N/mm J

A 465 ( 1 4) 1 8 .89 (0.0 I ) 24.49 (0.52) 3 .32 (0.98) B 46 1 (22) 1 8 .97 (0.07) 23.05 ( 1 .23) 3 .64 (0.44) C 454 (64) 1 8 .99 (0.06) 20.28 ( 1 .08) 3.76 (0.03) D 478 ( 1 9) 1 2.56 (0.02) 37.7 1 (2. 1 2) 2 .84 (0.66)

"Yalues in parentheses are the standard deviations

IiOO

500

400 � a; D1 .9

200

1 00

0

I .. · · · · SampIe A - Sample B - - - ' S ample C - - - - Sample DI ' 1 / I

I 1 / '

" , I 1 I 1

/ : ....

" I

"'\"'.

\ ' . . ,

0 5 10 1 5 20 EXlension (mm)

Fig_ 3 - Load-extension graph of each sample under unidirectional tensile loading

25

compared to PES fibre. Samples A, B and C have h igher toughness with lower Young 's modulus (Table 3). Relatively longer extension zone is observed for Samples A, B and C.

Strip test results suggest that the fibre type for warp yarn needs to be selected depending on the available conditions, where environmental factors as well as the releasing attitude for munition should be considered. Fabrics made from Spectra@ fibre offer incomparable stiffness and higher breaking load, while having lower toughness as compared to PES fibre. Cost i ssue is also i n favour of PES fibre as well .

Free-ends of the ribbon fabric are folded and connected using stitching technique to put into parachute form. Ribbon parachute i s attached to submunition with a hole i n the middle of this stitching zone. Hence, tear strength of the fabric i s an i mportant i ssue for parachute manufacturing. Table 4 shows the tear strength test results; the average of five tests and standard deviations ( in brackets) are reported.

Breaking load of Samples A, B and C is bound to be very close to each other for tearing test. This tendency was attributed to the usage of same fibre as fi l l ing yarn. Even breaking load of Sample D is very close to other samples (Fig. 4) . The graphs for Samples B and C are not shown i n Fig. 4 i n order to avoid confusion i n reading i ndividual data. However, the toughness of Sample D is lower than that of others. This could be explained with bias effect. Although tearing test is a uniax ial testing process, bias

Sample code

A

B

C

D

100 90 Ell 70

6 60 " 50 i3 40 ..J

30 20 10 0

0

Table 4 - Tear test results

Breaking Breaking Energy to load displacement breaking point N m m J

85 ( 1 8) 70.58 (7.67) 5 . 17 (0.53) 84 ( I I ) 73.56 (7.82) 5.2 (0.42) 82 (7) 73.88 (5.69) 5 .2 1 (0.8 1 )

87 ( I I ) 68.88 (5.77) 4.52 (0.48)

J-sampleA · · · · . . · Sampe D I

20 40 60 60 100 120 140 1 60 160 200 220 Eltension (mm)

Fig. 4 - Load-extension graph for tear test

3 1 2 INDIAN J . FIBRE TEXT. RES. , JUNE 2006

Table 5 - ANOV A chart for Young's modulus of strip test resu lts

Source of SS dJ MS F-valuc P-value F-critical variation

Between 28.9 1 007 2 1 4 .45503 5 .38679 0.04577 5 . 1 43249 groups

Within 1 6 . 1 0053 6 2.683422 groups

Total 45.0 1 06 8

SS - Sums of squares. dJ - Degrees of freedom, MS - Mean square, P-value - Probabi l ity of F value, and F-critical - Critical value of F

Table 6 - ANOV A chart for encrgy to breaking point of tear test results

Source of SS dJ MS F-value P-valuc F-critical variation

Bctween 0.459267 0.459267 1 5 .66572 0.0 1 6706 7.70865 groups

Within 0. 1 1 7267 4 0.0293 1 7 groups

Total 0.576533 5

effect might become effective where Spectra@ fibre i s less tough than PES . Another possibi l i ty can be very minor sl ippages despite all cautions taken during tearing test. The latter might also i ncrease bias effect.

Replicated one-way ANOY A was performed i n order to determine whether d ifferences at the core­zone lead to significance in breaking load and Young' s modulus of plain weave fabrics for strip test. Contributions of breaking load and energy to breaking point are analyzed for tearing test. The statistical software package JMP@ was employed to interpret the experimental data for 95% confidence level. The results were evaluated based on F-ratio and probabil ity of F-ratio (prob>F) . The lower the probability of F-ratio, the more significant is the variable. Weave type is found to be significant only for Young's modulus taken from strip test. However, weave type is i nsignificant for tear test results . The ANOY A chart for strip test is given in Table 5 .

ANOY A was also performed for determining the significance of fibre type using the samples of same weave type. Fibre type is found to be s ignificant for

the Young' s modulus of strip test. Although the discrepancy between the results of PES and Spectra fibres is high, the analysis concludes that the fibre type is not significant for energy to breaking point results from tear test. This is attributed to the higher standard deviation of the results given in Table 4. The ANOY A chart for strip test i s given in Table 6. It should be noted that when the significance level of the results, given i n Tables 5 and 6, are compared based on probabil i ty of F-ratio (prob>F), it i s realized that prob>F for the result i n Table 5 i s h igher than that i n Table 6.

4 Conclusions The studies show that the warp density contributes

higher breaking load and Young' s modulus at strip testing. Weave type has no contribution for the mechanical properties of fabric in tear test. ANOY A shows that the weave type and fibre type are only significant for the Young' s modulus results taken from strip tensile test.

Acknowledgement The author would l ike to appreciate the support

from 2nd Air Supply and Maintenance Command, Turkey.

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