Development of Materials for Naval , Fluvial and Military Applications

Resumen Desarrollo de Materiales para Aplicaciones Marítimas, Fluviales y Militares Date Received: July 7th 2017 Fecha de recepción: Julio 7 de 2017 Date Accepted: November 14th 2017 Fecha de aceptación: Noviembre 14 de 2017 Development of Materials for Naval, Fluvial and Military Applications 1 Department of Materials, Faculty of Mines, Universidad Nacional de Colombia. Medellín, Colombia. Email: fasuarez@unal.edu.co 2 Department of Materials, Faculty of Mines, Universidad Nacional de Colombia. Medellín, Colombia. Email: odbarriosr@unal.edu.co 3 DynaComp S.A.S. Calle 80 Sur # 47 D163 Bodega 1, Sabaneta, Colombia. Email: anderson.valencia@dynacomp.com.co 4 Institute for Molecular Engineering, University of Chicago, Chicago, IL, USA. Email: jphernandezo@wisc.edu DOI: https://doi.org/10.25043/19098642.164 Ship Science & Technology Vol. 11 n.° 22 (63-75) January 2018 Cartagena (Colombia) 64 From the beginnings of the human race, man has made elements for his personal protection. Leather, wood and some metals were used as materials to fabricate helmets, body-armor, shields and other items. Today, metals are used for these purposes and intensive research is done to improve their ballistic performance. Steel, aluminum and titanium alloys are widely used for these applications [1-11]. Nevertheless, metals are heavier than polymers and some ceramics, limiting their application for lightweight body and structural armors for land, air and water vehicles. In addition, metal processing requires large quantities of energy. Ceramics, polymers and composites have become the new option to avoid these restrictions [12-15]. However, each material and composite faces challenges due to the constant improvements in the development of projectiles and weapons and in the specific conditions of new applications. According to Grujicic [16], fiber-based composites were used for the first time in body-armor in the Korean war. They were constructed with nylon fabric and E-glass fibers within an ethyl cellulose matrix composite. Currently, high performance polymeric fibers such as polyaramids (Kevlar[17,18], Twaron [18,19], Technora [18, 20]), highly oriented and crystallized ultra high molecular weight polyethylene (UHMWPE) (Spectra [18, 21] and Dyneema [18, 22]), polybenzobis-oxazole PBO (Zylon [18, 23]) and polypyridobisimi-dazole PIPD (M5) [18], are widely used for ballistic composites. Under tension, these materials differ significantly from the nylon fibers, having a high absolute stiffness, extremely high specific strength, and quite low (<4%) strains-tofailure. These fibers essentially behave in tension as rate-independent linear elastic materials. When they are subjected to transversal compression tests they show large plastic deformations similar to the strains measured for the nylon and without having significant losses in their tensile load capacity. This behavior radically differs from carbon and glass fibers, which tend to flake under a tensile load condition. The ballistic performance of polymeric fibers is evaluated according to their capacity to absorb the kinetic energy from a projectile and how fast they can disperse that energy to their surroundings, avoiding local conditions for failure [16]. The last aspect is governed by the sound speed in the fiber where E is Young’s modulus and ρ the material density) [16,18]. UHMWPE and aramid fibers (like KEVLAR) had become popular reinforcement materials due to their high performance. They had replaced traditional ones like glass and nylon fibers [16]. However, in extreme applications, where a NIJ-III is required or IV protection levels, ceramic materials are preferred. Among them, alumina (Al2O3) is widely used due to its low manufacturing cost and the multiple options to process them, i.e. slip casting, pressing or injection molding. According to Medvedovski special and expensive equipment is not needed for their processing [12, 24-26]. These materials are characterized by good mechanical properties and a relative low density (3.95 g/cm3 for the alumina; aluminum density is about 2.7 g/cm3). In this paper, we are describing a platform to design composite materials of polymeric matrix that are special for military applications. Our platform integrates lightweight laminar composite materials, designed from their impact energy dissipation and storage mechanisms, with stateof-the-art manufacturing process and material characterization techniques. There are multiple mechanisms that have been identified to stop a projectile. Some of them are associated to the energy absorption during the localized and progressive damage of the target material while projectile pass through it, i.e. matrix cracking, shear delamination, compressioncutting, tensile-cutting, hydrostatic compression, melting (Karamis’ zones), adhesion and abrasion [27-32]. Other mechanisms are associated to the temporary energy storage, the elastic deformation Ballistic Materials and Design Trends Energy Dissipation and Storage Mechanisms under Impact Ship Science & Technology Vol. 11 n.° 22 (63-75) January 2018 Cartagena (Colombia) Suarez-Bustamante, Barrios-Revollo, Valencia, Hernández-Ortiz

From the beginnings of the human race, man has made elements for his personal protection.Leather, wood and some metals were used as materials to fabricate helmets, body-armor, shields and other items.Today, metals are used for these purposes and intensive research is done to improve their ballistic performance.Steel, aluminum and titanium alloys are widely used for these applications [1][2][3][4][5][6][7][8][9][10][11].Nevertheless, metals are heavier than polymers and some ceramics, limiting their application for lightweight body and structural armors for land, air and water vehicles.In addition, metal processing requires large quantities of energy.Ceramics, polymers and composites have become the new option to avoid these restrictions [12][13][14][15].However, each material and composite faces challenges due to the constant improvements in the development of projectiles and weapons and in the specific conditions of new applications.
According to Grujicic [16], fiber-based composites were used for the first time in body-armor in the Korean war.They were constructed with nylon fabric and E-glass fibers within an ethyl cellulose matrix composite.Currently, high performance polymeric fibers such as polyaramids (Kevlar [17,18], Twaron [18,19], Technora [18,20]), highly oriented and crystallized ultra high molecular weight polyethylene (UHMWPE) (Spectra [18,21] and Dyneema [18,22]), polybenzobis-oxazole PBO (Zylon [18,23]) and polypyridobisimi-dazole PIPD (M5) [18], are widely used for ballistic composites.Under tension, these materials differ significantly from the nylon fibers, having a high absolute stiffness, extremely high specific strength, and quite low (<4%) strains-tofailure.These fibers essentially behave in tension as rate-independent linear elastic materials.When they are subjected to transversal compression tests they show large plastic deformations similar to the strains measured for the nylon and without having significant losses in their tensile load capacity.This behavior radically differs from carbon and glass fibers, which tend to flake under a tensile load condition.
The ballistic performance of polymeric fibers is evaluated according to their capacity to absorb the kinetic energy from a projectile and how fast they can disperse that energy to their surroundings, avoiding local conditions for failure [16].The last aspect is governed by the sound speed in the fiber where E is Young's modulus and ρ the material density) [16,18].UHMWPE and aramid fibers (like KEVLAR) had become popular reinforcement materials due to their high performance.They had replaced traditional ones like glass and nylon fibers [16].However, in extreme applications, where a NIJ-III is required or IV protection levels, ceramic materials are preferred.Among them, alumina (Al2O3) is widely used due to its low manufacturing cost and the multiple options to process them, i.e. slip casting, pressing or injection molding.According to Medvedovski special and expensive equipment is not needed for their processing [12,[24][25][26].These materials are characterized by good mechanical properties and a relative low density (3.95 g/cm 3 for the alumina; aluminum density is about 2.7 g/cm 3 ).
In this paper, we are describing a platform to design composite materials of polymeric matrix that are special for military applications.Our platform integrates lightweight laminar composite materials, designed from their impact energy dissipation and storage mechanisms, with stateof-the-art manufacturing process and material characterization techniques.
Polymer matrix composite materials with laminar structures are another kind of ballistic materials that combine a set of interesting mechanisms to stop a bullet.Gama and Gillespie [30][31][32] worked with a S-2 glass/SC15 composite and proposed a model based on a Quasi-Static Punch Shear Tests (QS-PST) to quantify and classify the energy dissipation into elastic and absorbed energies as a function of the penetration displacement and support span.The energy partition is based on the identification of the mechanisms that take place during the five phases of the bullet-penetration: (i) impact-contact and stress wave propagation; (ii) hydrostatic compression and local punch shear; (iii) shear plug formation under compressionshear; (iv) large deformation under tension-shear; and (v) end of penetration and structural vibration.Contrary to a dynamic ballistic impact, where typical bullets are engineered projectiles, flat nose cylinder projectiles were used, with the assumption that they are considered rigid bodies.Classical Ballistic Limit Analysis (CBLA) applies for rigid projectiles and establishes that the limiting velocity V 50 , i.e. the velocity for which the probability that a projectile penetrates a target, is related with the impact velocity V I , the residual velocity V R and the projectile mass m p : Equation (1) establishes that the transferred energy to the target by the projectile in case of whole penetration (V I > V 50 ) equals the sum of the kinetic energy associated to V 50 and the kinetic energy of an equivalent mass m E that moves with residual velocity V R .Consequently, V R is defined as follows This velocity can be estimated from the Lambert and Jonas equation [1] as where β and p are fitting parameters.Equations ( 1) and ( 2) with experimental data for impact tests using different V I provide an estimation of the limiting velocity of a projectile-target system (see Fig. 1).
In the case where a complete projectile penetration occurs V I → V 50 -and V R = 0. Therefore, it is possible to separate the kinetic energy transmitted to the target as the sum of three components: (i) absorbed energy by the target by several failure mechanisms, E absorbed ; (ii) accumulated elastic energy, E E-Acum.; and (iii) kinetic energy associated to cone movement, E K-Come .The energy transmitted to the target, E 50 is defined by, Elastic and kinetic energies are dissipated after the impact by mechanisms of structural vibration and viscous damping (mechanisms (v)).Some authors [30][31][32][33] consider these energy components as part of the energy absorbed by the system; however, this approximation is not satisfactory because a great amount of energy that is initially transmitted to the target is then transmitted to the environment and is not fixed or permanently stored by the material [30].
To estimate each contribution in E 50 , an identification of the active mechanisms is performed using ballistic punch shear tests (Ballistic-PST) and a relationship is established with those mechanisms observed in QS-PST for the same ballistic system (target-projectile) [30].From a systematic study of set of curves obtained from the QS-PST, Gama and Gillespie's team developed a quasi-static ballistic penetration model and a method to determine the ballistic limit V 50 for laminar composite materials [30][31][32].We currently are working to enhance the applicability of this model to laminar composite materials reinforced with hard ceramic particles.
In the case of ceramic materials of high alumina content (alumina-mullite AM, or alumina containing mullite, zirconia and zircon ZAS) and heterogeneous reaction-bonded ceramics, failure mechanisms are linked with brittle fracture [24][25][26].On the rear face, the affected impact region is characterized by coaxial and radial cracks generated by tensile stress (because of bending associate to the bulging) that give this area a cone shape (Fig. 2).Spalling is also observed.The size of the generated fragments varies in an extremely wide range from extra-fine powders (on the nanometer scale) up to big parts of the order of several centimeters (mesoscale).Sarva et al. [35] have discovered that this fine powder is generated by an erosive mechanism that erodes the projectile reducing its kinetic energy.This resulting powder is the combination of the projectile and target material.This erosive mechanism that is accompanied by an ejected flow is very efficient to reduce the kinetic energy of the projectile and its effect could be enhanced to vastly improve the ballistic efficiency of ceramic tiles by judiciously restraining them with membranes of polymeric composites or metal sheets.
An application that has a close relation with our composite materials is the case of lightweight laminate metallic matrix composite reinforced with ceramic particles.The work of Karamis at al. is one of the best in the field from the descriptive point of view of the ballistic impact phenomenon [27][28][29].They have designed some composites materials of this kind using an Almatrix and alumina as a reinforcement element to defeat engineered projectiles (7,62 mm x 51 mm Armour Piercing, AP) shot by a G3 assault rifle with an average impact velocity of 710 m/s.Targets are 15 mm thick and can stop this kind of projectiles.The ballistic material was manufactured by the combination of a process of hot compression and squeeze casting.Fig. 3 shows details of the impact zones of ballistic tests carried out for two different configurations of their laminate, where multiple energy dissipation mechanisms are identified.
Our polymeric matrix composite is designed to generate synergy between the matrix, the Some of the individual contributions for these mechanisms have been estimated.We are currently working on the abrasive contribution of the particles.We are, however, able to provide a crude assessment of the energy involved in this complex phenomenon.We fabricated polymeric matrix composite materials reinforced with hard particles and high performance fi bers using a state-of-the-art vacuum assisted infusion molding.Epoxy resins and a polyester based-resin are used as the polymeric matrices.Th ree kinds of high performance fi bers were used as reinforcement: glass, Kevlar and carbon fi bers.Th ese fi bers were provided as woven fabrics in four diff erent confi gurations as shown in Fig. 4. Two kinds of hard particles are used as the additional reinforcement that provides abrasive and hardness characteristics to the composite.Th e laminates are thin tiles of laminar composites, are light and stiff and made with a thickness of approximately 1 mm.Th e infusion resulted in high reproducible laminates with a standard deviation of 0.3 mm in the thickness.
Fig. 5 shows details of some setups used to manufacture samples by vacuum assisted infusion techniques.Diff erent confi gurations were used to explore how the vacuum line and the in/out points can aff ect the integrity of the manufactured samples.Th ese types of experimental exercises have permitted the identifi cation of the more relevant elements to be considered during the manufacture of the composites and how the assemblies can be simplifi ed without aff ecting the quality of the products and reducing costs and the negative impact in the environment.Fig. 6 shows some samples manufactured by these techniques.by Honeywell (E.U.A).Table 1 shows some of the mechanical and physical properties of high performance fi bers.
Notice that HMW-PE has the lowest density and an excellent combination between strength properties and toughness.Th e minimum diameters measured were around 13 µm (Table 2).
It is important to consider the possibly negative eff ect of the "knot" found in the Spectra Shield II -Th ick as a manufacture defect (see Fig. 7).Fibers of the Spectra SR121 and Gold Shield have a homogenous diameter.Th e Diameter of the fi bers is an important variable considered in the models developed to assess the elastic energy storage and to consider the eff ect of weight and cost in a fi nal product.

Nano-indentation Tests
Th is kind of test was carried out to assess the magnitude of the elastic module (E) of the fi bers  (upon their cross section) and its radial variation at the interface between fi bers and matrix.Measurements were carried out using an IBIS Nanoindentation System with a Berkovich indenter.
Results of these measurements were plotted in the graphs of Fig. 9 for composites with the three basic types of fi bers used in the manufacture of laminar composites shown in Fig. 7. Results of the values measured for E are in agreement with those reported in specialized literature.Th e horizontal axis is the penetration depth (h t ).

Microhardness
Th e eff ect of the curing time and temperature time on the mechanical properties of a resin was explored using a polyester based resin prepared varying the weight percentage of its catalyst; see Fig. 10.Four days after the manufacturing of the samples microhardness was measured on the polished surfaces (red points); then, 24 days later the microhardness of the samples was measured (blue points).During this period the environmental temperature was between 21 to 28°C.Finally, 11 days later, the curing of the samples was accelerated by using a furnace at 80°C for 5 hours to guaranty a complete curing.Microhardness was then measured again (purple points).We are currently obtaining extra-hard layers using alumina particles (Al 2 O 3 ) and others.Th e goal is to place some hard layers on or near the impact face of the armors to promote the activation of some additional dissipative mechanisms as the projectile abrasion (projectile peeling), plastic deformation of the projectiles, fracture of hard particles and the fl ow of these hard particles into the polymeric matrix.
Th e hardness of Al 2 O 3 is about 1170 (94% purity) to 1440 HV (99.5% purity).In our composites, microhardness values up to 940 HV have been measured in our samples using loads of 0.49 N (50 grams).Th ese values correspond to the hardness of a mixture of alumina particles and polymeric resin; that is why the lower values were recorded because of the lesser hardness of the matrix.
Using a comprehensive characterization method, developed in our group, we measured reaction kinetics using diff erential scanning calorimetry.
Th e dynamic and dynamic-isothermal DSC data is used in combination with a Kamal-Sourour model for the reaction kinetics and cure.Th e models are integrated to fi nd the Time-Temperature-Transformation diagram of the resin (see Fig. 11).
Th e TTT diagram provides the processability window for each resin and aids the processing design towards optimization and control.

Fig. 1 .
Fig.1.Residual velocity V R as function of the impact velocity V I , for a S-2Glass/SC15 layered composite with a target thickness H C = 13,2 mm and a flat nose cylinder projectile made of steel with mass m p = 13,8 g and diameter D p = 12,7 mm[30]

Fig. 7 Fig. 4 .Fig. 5 .Fig. 6 .
Fig.7shows images obtained by a stereoscopic and optical microscopy of the cross section of four samples of laminar composites.Th ese pictures reveal important details of their specifi c confi gurations: size and shape of the branches of fi bers, fi bers distribution, defects (as pores for instance), areal percentage of components, cross sections of the composite, spaces occupied by the matrix and composite homogeneity.A primary characterization of some ballistic products was carried out at the Laboratorio

Fibre 3 Fig. 7 .
Fig. 7. Cross-sections of samples of several laminates for inspection and mechanical characterization

From Fig. 10 ,Fig. 9 .
Fig. 9. Elastic Module (E) of the transverse section of fi bers and its surroundings of anti-depth penetration matrix (h t ).

2y 2 yFig. 10 .Fig. 11 .
Fig. 10.Effect of curing time and temperature on hardness of the matrix.For each tested condition, each plotted point represents the average value of five measurements