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Контроль и испытания
DOI: 10.14529/engin160106
EXPERIMENTAL EVALUATION OF THE IMPACT STRENGTH OF LAMINATED COMPOSITES WITH A THERMOPLASTIC MATRIX
S.B. Sapozhnikov, ssb@susu.ac.ru,
M.V. Zhikharev, zhi-misha@yandex.ru,
O.A. Kudryavtsev, kudriavtcevoa@susu.ac.ru
South Ural State University, Chelyabinsk, Russian Federation
Tensile tests were performed to obtain the quasi-static mechanical properties of the ara-mid fabrics (Twaron®, RUSLAN®-SVM). The elastic modulus of filaments, pulled out from the fabrics was measured with compact testing machine INSTRON 5942. Filaments pull-out tests were carried out to compare the frictional forces in different aramid fabrics. Eight various types of ballistic panels with the thermoplastic matrix based on polyethylene were fabricated and two types ballistic panels based on UHMPE (Dyneema®).
Extensive ballistic tests have been carried out on various ballistic panels using 6.35 mm steel ball. Special powder gun stand for acceleration of projectiles with terminal velocity up to 900 m/s was developed The ballistic performance was assessed in terms V50 threshold as well as post V50 limit.
After the test, the comparison was produced of effectiveness between all of materials used in this work. Laminates based on UHMPE fibers are much better than others in respect to the values of on indicators of V50 (about 10 %) and of the absorbed energy (about 25 %) under high-velocity impact conditions. But their energy absorption capability can sharply drop down when projectile's velocity exceeds the ballistic limit. When selecting reinforcing aramid fabric for ballistic application, it is important to consider not only the mechanical properties of the fibers and the type of fabric construction, but also the material should have good results on all parameters of ballistic efficiency such as filaments width, etc. Best aramid fabric composite was SVM S125 with twill construction and LDPE films.
Keywords: high-velocity impact, fragment protective structure, UHMPE, aramid fabric, thermoplastic matrix.
Introduction
Protective structures on the basis of durable composite materials are widely used for protecting manpower and vehicles against fire arms bullets and explosives fragmentations [1]. Typically they have low surface density, high ballistic efficiency, and can be used as main element of protective structure, or as support material for metallic or ceramic face armor layer.
Most commonly used for manufacturing ballistic composites are aramid fibers (RUSLAN®-SVM, Kevlar®, Twaron®, Rusar®, Teksar®), ultra-high molecular polyethylene fibers (UHMPE), such as Dyneema®, Spectra®, glass and carbon fibers [2-6]. Composite materials, based on PBO (Zylon®), basalt and organic fibers, are less common, as later are relatively less strong [7], and PBO fibers have a tendency to aging, that can lead to sharp decreasing of ballistic features [5].
Composites with low resin content (less than 20 % per weight) are also very attractive for using in armor structures. Different thermoplastics with high flexibility are usually used as matrix for such composites.
Using of such materials has some advantages:
• low adhesion between matrix and fibers allows later to face maximum deformations and elongate in the impact point [8];
• additional energy dissipation mechanisms, related to stratifying and cracking [9, 10];
• fibers, not contacting directly with the projectile, are loaded [9-11];
• decreasing blunt trauma if to compare with soft armor [9, 12];
• sufficient bending stiffness for using as support.
Protective structures ballistic effectiveness is determined by several parameters. Ballistic limit (V50) - one of the main parameters, determined as speed of the striker, leading to material penetrating with 50 % possibility [13]. Fragmentations simulating devices of different shapes and weights are used during testing protective structures [13, 14]. There is a special standard in Russian Federation, regulating armor structures testing [15]. According to this, standard tests should implement steel spherical ball 6.35 mm in diameter with weight of 1.05 g, manufactured of ShKh15 steel.
Composites on the basis of aramid and UHMPE fibers assure high ballistic effectiveness [5, 16]. UHMPE fibers based composites behavior under ballistic loads was widely studied in theory and experimentally [17-21].
There are several works, dedicated to ballistic composites with thermoplastics matrix (polypropylene, polyvinilbutyral, and vinilester), based on aramid fiber [9, 22, 23]. There is no information in literature sources on ballistic features of the polyethylene matrix composites. Low pressure polyethylene (HDPE) is a cheap, easy melting material, binding aramid fibers with each other, which has appropriate viscosity, not allowing full filaments saturation. That is why HDPE is used as light and thin binding agent for aramid layers.
This work includes analysis of laminated composites on the basis of aramid fabrics with HDPE matrix. Pressed panels have passed ballistic tests with a striker, presented by fragmentation simulating device according to GOST R 50744-95. In order to compare ballistic efficiency parameters same tests were performed for panels, based on UHMPE fibers. In order to determine ballistic limit, experiments data were processed using Lambert-Jonas empirical-formula dependence. All manufactured laminated panels have got similar surface density (4.2 ± 0.2 kg/m2). Aramid fibers mechanical properties under quasistatic load are also presented in this work.
1. Materials and methods
Fig. 1 presents a photo of the surface of aramid fabrics, used in this work.
1.1. Fibers mechanical properties study
As composite materials properties depend, first of all, on the fibers' properties, elastic and strength filaments' characteristics were determined on compact testing machine INSTRON 5942 during filaments static elongation tests.
In order to exclude machine rigidity influence on determined elastic modulus, we used maximal possible filament length of 450 mm. Elastic modulus was measured during unloading from stress, equal to ~50 % of destructing (initial filament condition after pulling out of fabric is characterized by crimp).
In order to test filaments strength we used special clamps INSTRON SG-1, where filaments were rolled on drums 51 mm in diameter, and filaments ends were clamped in microgrips. Friction on drums allowed unloading clamping area and obtain destruction in operating range.
Table 1 includes results of testing separate filaments (series of 10 filaments along basis and woof) from all investigated woven fabrics. Where E, av, sv - elastic modulus, breaking strength, and breaking deformation. Material density was assumed equal to 1.44 g/cm3.
These data allow evaluating fibers quality and, subsequently, armor materials, manufactured of them: elastic moduli vary very slightly, in the range of 1-2 % (maximum of 5 % for Twaron® 613). Twaron® also has slight variation of strength properties - not more than 8 %. Same time CBM strength properties demonstrate variation coefficients of 13 % (along basis direction). CBM 56334 fibers have highest strength - about 3.5 GPa. Russian aramid filaments CBM are generally more durable than foreign filaments, so sound speed in them is about 20 % higher. This gives advantages under impact loading.
Fig. 1. Fabrics structure: А - Twaron® Microflex Б - Twaron® 613; В - СВМ 56334; Г - Twaron® 709 Д - СВМ S-110; Е - СВМ P-110; Ж - СВМ А-145 З - СВМ S-125
Table 1
Aramid filaments mechanical properties
Filaments from fabric Cross section area, mm2 Average E, GPa E variation coefficient, % Average av, MPa Variation coefficient av, % Average Sv, %
СВМ А 145 basis 0.021 129 0.45 2 980 2.9 2.31
СВМ А 145 woof 0.021 131 0.55 3 350 3.8 2.55
СВМ P 110 basis 0.021 132 0.76 2 700 11.0 1.96
СВМ P 110 woof 0.021 124 0.93 3 510 2.6 2.81
СВМ S 125 basis 0.021 133 0.47 3 080 10.0 2.31
СВМ S 125 woof 0.021 133 0.83 3 460 9.4 2.61
СВМ S110 basis 0.021 132 0.80 2 590 22.6 1.95
СВМ S110 woof 0.021 132 0.70 3 490 5.5 2.65
СВМ 56334 basis 0.021 136 0.91 3 420 3.9 2.52
СВМ 56334 woof 0.021 136 0.70 3 600 2.3 2.65
Twaron® Microflex basis 0.038 103 4.21 1 470 4.2 1.43
Twaron® Microflex woof 0.038 103 5.11 1 675 5.9 1.63
Twaron® 613 basis 0.039 90 0.19 2 560 7.7 2.84
Twaron® 613 woof 0.039 96 3.70 2 840 2.7 2.97
Twaron® 709 basis 0.063 100 0.19 2 570 5.1 2.57
Twaron® 709 woof 0.063 99 3.70 2 640 3.3 2.68
1.2. Pulling fibers out of the ballistic materials
It is known that friction between filaments has major effect on efficiency energy absorption by multilayer fabric protective structures under high speed impact [8]. Experiments on filaments pulling out
were performed for comparing friction forces between filaments in different aramid fabrics. Testing machine INSTRON 5942 with pneumatic rubber-covered grip INSTRON 2712-019 is used for tests performing. This grip clamps one central fiber of the 50 x 50 mm specimen. Four specimens for each ballistic material were tested (two along basis direction and two along woof direction). Typical results of testing by pulling fibers out for Twaron® 613 material are presented on Fig. 2. For this material maximum pulling out force is equal to 2.25 N for basis and 1.15 N for woof.
Averaged maximum friction forces values for different materials are indicated in Table 2. Measurements demonstrate that in all materials, except CBM 56334, friction force along woof is higher than along basis. Twaron® Micro-flex has highest friction forces in both directions. Lowest friction forces were obtained for materials CBM 56334 and CBM A145. Both materials have sateen construction. 1.3. Manufacturing pressed ballistic panels When performing this work we've used ballistic panels 85 x 85 mm with surface density of about 4 kg/m2, manufactured of aramid fabrics and ballistic polyethylene. Intermediate layers are presented by thermoplastic films - low density polyethylene (HDPE) - with initial thickness of 40 ^m, that were placed between aramid fabric layers. Panels of ballistic polyethylene HB2 and HB80 were pressed without additional intermediate layers. Table 3 presents data on used materials.
1
VM клп/г ^^^warp
weft wM 'Шш
0 10 20 30 40 50
Displacement, mm
Fig. 2. Filament pulling out diagram for Twaron® 613 fabric
Table 2
Results of fibers pulling out tests
Fabric type Woof/basis F, Н
СВМ А145 0.4/0.3
СВМ P110 4.5/1.7
СВМ S125 1.3/1.1
СВМ S110 0.85/0.65
СВМ 56334 0.29/0.45
Twaron® Microflex 13.5/5.05
Twaron® 613 2.25/1.15
Twaron® 709 1.8/1.6
Table 3
Data on used materials
Fabric type Surface density, g/cm2 Layer thickness, mm Number of layers Panel thickness, mm Construction type
Twaron® Microflex 218 0.275 17 3.63 Linen
Twaron® 709 195 0.255 17 3.50 Linen
Twaron® 613 137 0.175 23 3.49 Linen
СВМ 56334 145 0.190 23 4.36 Satin
СВМ А-145 145 0.220 22 4.21 Satin
СВМ Р-110 110 0.170 27 3.72 Linen
СВМ S-110 110 0.160 27 4.03 Twill
СВМ S-125 125 0.170 25 4.12 Twill
Dyneema® HB2 257 0.320 16 4.43 Four unidirectional UHMPE fibers layers with laying them 0/90/0/90 and with thermoplastic matrix
Dyneema® HB80 145 0.235 30 4.70 Four unidirectional UHMPE fibers layers with laying them 0/90/0/90 and with thermoplastic matrix
Set of aramid filaments was heated in an oven up to 145 ± 5 °C during 2 hours up to reaching uniform temperature distribution over the set height. Temperature monitoring was performed with a thermocouple, installed in the middle part of the set. Sets from ballistic polyethylene were heated up to temperature of 120 ± 5 °C during 2 hours. When pressing pressure has reached 100 ± 10 bar, exposure time was equal to 10 minutes, set was cooled down to 60 °C, after which the set was disassembled and cooled down under air.
This has resulted in manufacturing 10 different variants of ballistic panels, 6 specimens for each
type.
2. Ballistic tests
Ballistic tests were performed according to GOST R 50744-95 by spherical striker 6.35 mm in diameter (1.05 g) of tempered ball bearing steel. We've used SUSU ballistic test bench, Fig. 3 [24].
Fig. 3. General view of the ballistic test bench
Fig. 4 demonstrates photos of composite panels after ballistic tests. Panels deflection and delami-nating area increase as ball speed decreases. Left part of Figure 4 shows panel deflection in the impact spot (1.85 mm). Right part of the Figure shows deflection of 6.44 mm. Initial speeds and ball speed after penetration are presented in Table 4.
Fig. 4. Photos of samples from CBM S110 material after penetration
Table 4
Initial/final striker speeds
Specimens numbers 1 2 3 4 5 6
Twaron® 613 444/0 490/206 589/446 636/504 725/602 785/702
Twaron® Microflex 455/0 489/223 591/456 700/632 725/645 865/806
Twaron® 709 401/0 497/265 578/396 713/522 768/694 776/698
CBM 56334 442/0 498/113 559/321 600/376 770/660 865/756
CBM A145 525/0 603/338 671/462 785/670 850/763 855/773
CBM S110 430/0 566/362 587/372 673/563 760/692 852/803
CBM S125 508/0 570/309 608/438 649/500 758/655 837/746
CBM P110 503/0 543/306 648/524 720/639 774/688 874/822
Dyneema® HB2 457/0 526/0 571/0 619/391 811/680 865/732
Dyneema® HB80 595/0 608/0 690/418 771/558 800/639 888/715
3. Ballistic tests results
During damage of composite panels we've noticed fibers rupture, filaments major separation and pulling out. Separation was observed in all panels without exclusions (on the basis of aramid filaments and UHMPE). Low binding between matrix and fibers allowed pulling out filaments, directly contacting the striker (Fig. 5).
Experiment data on impact with a fragmentation simulating device were processed using classical Lambert-Jonas dependence [25]
f0ifV < V50
У„ =•
A ■ (Vm - V5mJ'm if Уг > У5
50
where A, V50, and m - are parameters, determined from condition of calculated residual penetration speeds best corresponding with experimental data (least squares technique). V50 - speed, at each 50 % of strikers penetrate through the material. This parameter, as well, as surface density, is used when designing protection system, as well, as when comparing different armor structures. Vr and Vi - residual and initial striker speeds accordingly. This dependence should be used with care, as parameters, determined from it, will depend from material and geometry. Nevertheless, it helps analyzing behavior of different materials under ballistic load from the energy balance point of view. Lambert dependency parameters values and composite panels surface density are presented in Table 5.
Table 5
Lambert dependency parameters values and composite panels surface density
Fabric type V50, m/s A m Surface density, kg/cm2
Dyneema® HB80 656 0.87 4.401 4.35
Dyneema® HB2 604 0.861 6.322 4.11
СВМ S125 555 0.933 4.168 4.09
СВМ A145 525 1.162 1.941 4.04
СВМ P110 511 1.003 3.089 4.01
СВМ S110 505 1.082 2.471 4.01
Twaron® 613 476 0.954 3.004 4.04
СВМ 56334 490 1.023 2.25 4.23
Twaron® 709 450 1.066 2.185 3.98
Twaron® Microflex 473 0.994 3.027 4.37
3.1. Ballistic effectiveness
Table 6 includes summary on ballistic properties of homogeneous and hybrid composite panels, faced high speed impact load. Two values: A = V50/p and ¥ = (mp-(V50)2) / 2p (where mp = 1.05 g -weight of the striker, p - panel surface density) were used for comparing ballistic effectiveness of different composites.
Value ¥ indicates maximum panel absorbed energy.
Table 6
Composite panels ballistic parameters comparison
Fabric type Average ctv, MPa F, Н Construction type Д (AMmax)X X100 % Y (Y/Ymax)x X100 % Rating
Dyneema® HB80 - - Unidirectionally oriented fibers 151 100 51.9 100 1
Dyneema® HB2 - - Unidirectionally oriented fibers 147 97 46.6 90 2
СВМ S125 3270 1.2 Twill 137 91 39.5 76 3
СВМ A145 3160 0.35 Satin 130 86 35.8 69 4
СВМ P110 3100 3.1 Linen 128 85 34.2 66 4
СВМ S110 3040 0.75 Twill 126 83 33.4 64 5
Twaron® 613 2700 1.7 Linen 118 78 29.4 57 6
СВМ 56334 3510 0.37 Satin 116 77 29.8 57 7
Twaron® 709 2600 1.7 Linen 113 75 26.7 51 8
Twaron® Microflex 1570 9.25 Linen 108 72 26.9 52 9
3.2. Mechanisms, effecting the ballistic effectiveness
3.2.1. Materials
Above indicated results demonstrate that Dyneema® HB80 has the highest ballistic parameters among all tested composites. This is because this material has high quantity of unidirectionally oriented layers (120 in our case) of high strength UHMPE fibers. Fibers strength and elastic modulus can reach 2.8 GPa and 200 GPa accordingly [21]. This composite material absorbs 10 % more energy than Dyneema® HB2 and 50 % more energy than Twaron® 709 based composite. Best aramid fabric CBM S125 based composite absorbs approximately 25 % less energy than Dyneema® HB80. It should be noted that energy absorbing by UHMPE dramatically decreases when projectile speed exceeds the ballistic limit, see Fig. 6.
This can also be seen for composites on the aramid fabrics basis with linen and twill construction, see Fig. 7.
Dyneema® composite is twice more expensive than aramid fabrics based composites, therefore using aramid fabrics is more effective when there are no raised demands on protective structures weight.
Fig. 6. Ballistic curve and absorbed energy - impact energy for composite sets Dyneema® and CBM 56334
Fig. 7. Ballistic curve and absorbed energy - impact energy for composite sets CBM A145 and CBM S125
3.2.2. Fibers parameters and fabric construction
Aramid fabrics based composites ballistic parameters depend on several factors, related to material features, on fabric construction, fiber thickness, interfibers friction force, etc. It is not possible to choose one main factor. It can be clearly seen when analyzing penetration results for aramid fabrics bases composite panels.
As we've indicated above, aramid fabric CBM S125 based multilayer material has the best ballistic parameters among all tested aramid composites. This fabric is characterized with not the best fibers strength, not the highest friction force in fibers pulling out tests, has middle fibers crimping structure (twill construction). But this material is one of the best per each of these parameters, which determines its ballistic effectiveness. Other fabrics have low values of one or several criteria.
For example, CBM A145 has high fibers strength and minimal fiber bending (satin construction), but has "loose" structure. If the projectile is relatively small, than it breaks only a few central fibers, moving the rest apart not breaking them. Loose fibers construction also leads to low fibers pulling out resistance, so, energy, absorbed by interfiber friction, is lower if to compare with twill or linen construction.
Aramid fabrics P110 and S110 based composites ballistic effectiveness is approximately the same. First material has construction with high crimping degree (linen construction), increasing stress from filaments bending. Second aramid fabric S110 resistance to fibers pulling out is lower if to compare with S125. Beside this, these fabrics have lower fibers strength than CBM S125 and A145.
CBM 56336 fabric filaments have maximum strength and elastic modulus among all. It should be noted, that CBM 56334 and CBM A145 constructions are the same (eight-harness satin), so it was expected, that ballistic limit for panels from CBM 56334 would be higher than for A145. But tests have demonstrated that it is not the case. We have interpreted these results as due to loose construction -CBM 56334 filaments are located approximately 25 % wider than filaments of A145. As the result, lower fibers quantity directly contacted with the striker.
All Twaron® fabrics have linen construction. Due to high filaments strength and their small diameter Twaron® 613 based composites have higher ballistic effectiveness than other Twaron® based panels. It should be noted that not depending from low fibers strength (1.5 times lower than for Twaron® 709 filaments), due to extremely dense fabric structure and due to maximum fibers pulling out resistance, Twaron® Microflex has ballistic effectiveness nearly equal to Twaron® 709 and only 10 % lower than Twaron® 613 has. Nevertheless, in case of panel penetration and fibers destruction, panel absorbed energy dramatically decreases for Twaron® Microflex, see Fig. 8.
V„ (m/s) E0 (J)
Fig. 8. Ballistic curve and absorbed energy — impact energy for composite sets from Twaron® 613 and Twaron® Microflex
Conclusion
This work deals with researching of ballistic effectiveness of thermoplastics on the basis of aramid fabrics (CBM and Twaron®) and ultra-high molecular polyethylene (UHMPE).
UHMPE based composites have demonstrated best values of ballistics limit and absorbed energy if to compare with other materials. But their energy absorption capability dramatically drops down when projectile's velocity exceeds the ballistic limit. When selecting reinforcing aramid fabric for ballistic application, it is important to consider not only the mechanical properties of the fibers and the type of fabric construction, but also all parameters of ballistic efficiency such as filaments width, etc.
This article can form good basis for working out detail optimal model of protective structure (indicated in this work), taking into account all ballistic parameters.
Acknowledgments
This research was performed in South Ural State University (National Research University) out of Russian Scientific Fund grant (project No. 14-19-00327).
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Received 23 November 2015
УДК 620.178.76 йО!: 10.14529/впд1п160106
ЭКСПЕРИМЕНТАЛЬНАЯ ОЦЕНКА УДАРНОЙ ПРОЧНОСТИ СЛОИСТЫХ КОМПОЗИТОВ С ТЕРМОПЛАСТИЧНОЙ МАТРИЦЕЙ
С.Б. Сапожников, М.В. Жихарев, О.А. Кудрявцев
Южно-Уральский государственный университет, г. Челябинск
Проведены статические испытания нитей арамидных тканей (Twaron®, РУСЛАН®-СВМ) на растяжение для определения их упругих и прочностных характеристик на малогабаритной испытательной машине ГМБТИОМ 5942. Эксперименты по вытягиванию нитей были проведены для сравнения сил трения между нитями в различных арамидных тканях. Были изготовлены восемь различных вариантов баллистических панелей с термопластичной матрицей на основе полиэтилена и два вида баллистических панелей на основе сверхвысокомолекулярного полиэтилена (СВМПЭ марки Бупееша®).
Обширные баллистические испытания были проведены на изготовленных баллистических панелях, использовался стальной шарик диаметром 6,35 мм. Для разгона шарика до скоростей 900 м/с был использован баллистический стенд ЮУрГУ. Баллистические характеристики были оценены с точки зрения предельной характеристики материала - баллистического предела У50.
После испытаний было произведено сравнение эффективности всех материалов, исследованных в данной работе. Композиты, основанные на СВМПЭ волокнах, оказались лучшими из всех рассмотренных материалов по значению баллистического предела (превышение на 10 % по сравнению с ближайшим конкурентом) и по значению поглощенной энергии (около 25 %). Но при превышении баллистического предела способность к поглощению энергии у СВМПЭ резко снижается. При выборе арамид-ной ткани для баллистических приложений важно учитывать не только механические свойства волокон и тип переплетения, но и все параметры баллистической эффективности такие, как ширина нитей и др. Лучшим баллистическим материалом на основе арамидных тканей стал СВМ 8125 с саржевым переплетением и пленками из полиэтилена низкого давления.
Ключевые слова: высокоскоростной удар, защитная структура, СВМПЭ, арамид-ная ткань, термопластичная матрица.
Сапожников Сергей Борисович, доктор технических наук, профессор, профессор кафедры «Прикладная механика, динамика и прочность машин», Южно-Уральский государственный университет, г. Челябинск, ssb@susu.ac.ru.
Жихарев Михаил Владиленович, аспирант кафедры «Прикладная механика, динамика и прочность машин», Южно-Уральский государственный университет, г. Челябинск, zhi-misha@ yandex.ru.
Кудрявцев Олег Александрович, аспирант кафедры «Прикладная механика, динамика и прочность машин», Южно-Уральский государственный университет, г. Челябинск, kudriavtcevoa@ susu.ac.ru.
Поступила в редакцию 23 ноября 2015 г.
ОБРАЗЕЦ ЦИТИРОВАНИЯ
Sapozhnikov, S.B. Experimental Evaluation of the Impact Strength of Laminated Composites with a Thermoplastic Matrix / S.B. Sapozhnikov, M.V. Zhikharev, O.A. Kudryavtsev // Вестник ЮУрГУ. Серия «Машиностроение». - 2016. - Т. 16, № 1. - С. 72-81. DOI: 10.14529/engin160106
FOR CITATION
Sapozhnikov S.B., Zhikharev M.V., Kudryavtsev O.A. Experimental Evaluation of the Impact Strength of Laminated Composites with a Thermoplastic Matrix. Bulletin of the South Ural State University. Ser. Mechanical Engineering Industry, 2016, vol. 16, no. 1, pp. 72-81. DOI: 10.14529/engin160106
