Контроль и испытания EXPERIMENTAL EVALUATION OF THE IMPACT STRENGTH OF LAMINATED COMPOSITES WITH A THERMOPLASTIC MATRIX

Tensile tests were performed to obtain the quasi-static mechanical properties of the aramid 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 ﬁ bers 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.


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.
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][10][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 (V 50 ) -onе 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.
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/m 2 ). Aramid fibers mechanical properties under quasistatic load are also presented in this work. Fig. 1 presents a photo of the surface of aramid fabrics, used in this work.

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 Е,  v ,  v -elastic modulus, breaking strength, and breaking deformation. Material density was assumed equal to 1.44 g/cm 3 .
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.  Average

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.  Table 2. Measurements demonstrate that in all materials, except CBM 56334, friction force along woof is higher than along basis. Twaron ® Microflex has highest friction forces in both directions. Lowest friction forces were obtained for materials CBM 56334 and CBM A145. Both materials have sateen construction.

Manufacturing pressed ballistic panels
When performing this work we've used ballistic panels 85 × 85 mm with surface density of about 4 kg/m 2 , manufactured of aramid fabrics and ballistic polyethylene. Intermediate layers are presented by thermoplastic films -low density polyethylene (HDPE) -with initial thickness of 40 µm, 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.      Table 6 includes summary on ballistic properties of homogeneous and hybrid composite panels, faced high speed impact load. Two values:  = V 50 / and  = (m p ·(V 50 ) 2 ) / 2 where m p = 1.05 gweight of the striker,  -panel surface density) were used for comparing ballistic effectiveness of different composites.

Ballistic effectiveness
Value  indicates maximum panel absorbed energy. . This composite material absorbs 10 % more energy than Dyneema ® HB2 and 50 % more energy than Twaron ® 709 based composite. Best aramid fabric СВМ 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.

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 СВМ 56334 and СВМ 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.

Conclusion
This work deals with researching of ballistic effectiveness of thermoplastics on the basis of aramid fabrics (СВМ 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.