Bazle A. Gama

Aluminum foam integral armor: a new dimension in armor design

Closed-cell aluminum foam offers a unique combination of properties such as low density, high stiffness, strength and energy absorption that can be tailored through design of the microstructure. During ballistic impact, the foam exhibits significant non-linear deformation and stress wave attenuation. Composite structural armor panels containing closed-cell aluminum foam are impacted with 20-mm fragment-simulating projectiles (FSP). One-dimensional plane strain finite element analysis (FEA) of stress wave propagation is performed to understand the dynamic response and deformation mechanisms. The FEA results correlate well with the experimental observation that aluminum foam can delay and attenuate stress waves. It is identified that the aluminum foam transmits an insignificant amount of stress pulse before complete densification. The ballistic performance of aluminum foam-based composite integral armor (CIA) is compared with the baseline integral armor of equivalent areal-density by impacting panels with 20-mm FSP. A comparative damage study reveals that the aluminum foam armor has finer ceramic fracture and less volumetric delamination of the composite backing plate as compared to the baseline. The aluminum foam armors also showed less dynamic deflection of the backing plate than the baseline. These attributes of the aluminum foam in integral armor system add a new dimension in the design of lightweight armor for the future armored vehicles.

Dynamics of metal foam deformation during Taylor cylinder–Hopkinson bar impact experiment

Abstract Analytical solutions for dynamic deformation of foam materials during the Taylor cylinder–Hopkinson bar impact experiment were obtained. It was shown that shock wave of foam collapse appears during the fast impact. The results of this experiment can be used in estimating the average material properties of the foam under dynamic loading conditions. Results show that the un-deformed and change in length of foam specimens are in good agreement between theory and experiment, as well as numerical analysis.

High-velocity plate impact of metal foams

Abstract The ballistic impact of a massive, effectively 1-D plate on an initially stationary foam layer is considered. It is shown that four discrete velocity regimes must be considered. Two of these regimes are of major interest for ballistic impact studies. Regime 2 considers the case when the initial velocity of the plate is lower than the sound velocity of the constitutive material of the foam, but higher than the linear sound velocity of foam. Regime 3 considers the case when the initial plate velocity is lower than the linear sound velocity of the foam; but remains higher than the effective sound velocity for a perturbation in which the amplitude lies in the so-called “plateau region” of the static stress–strain diagram. Analytical solutions for dynamic deformation and energy absorption of foam materials under the plate impact condition for Regimes 2 and 3 are developed. It has been shown that in both cases, a compressive shock wave appears. The physical difference between these two regimes entails not only the creation of a shock front associated with the collapsing foam, but also an acoustic precursor in the case of Regime 3. As a result, the efficiency of energy absorption in Regime 2 depends only on the initial density of the foam, the density of the constitutive material of the foam, and the areal mass of the impacting plate, whereas the efficiency of energy absorption for Regime 3 also depends on the Mach number and the critical stress of the foam. Numerical plate impact simulations have been carried out in impact Regime 2. Explicit finite element analysis is performed using LS-DYNA 960. The time history of dynamic deformation and energy of the impact plate is presented. The numerical prediction is found to be in good agreement with the analytical results.

A Comparison of Oven-cured and Induction-cured Adhesively Bonded Composite Joints

The advantages of using adhesives for joining composite structures are now well accepted. Adhesive joints may offer, over bolted joints, advantages such as a lower assembly weight, a superior stress transfer and an improved fatigue resistance. However, in some applications the above advantages may be offset by the processingconditions required to cure the adhesive. Indeed, in the conventional oven curing process the thermal energy must diffuse through the composite layers to heat the joint interfaces, resultingin longand expensive processingtime as well as wasted energy. A novel method of achieving adhesive bonds is addressed in the present study. The method of electromagnetic heating is well suited for rapid and efficient localized heatingof adhesive bond lines, provided suitable susceptors are used at the interfaces. This paper presents the results of a study on the use of induction heating for bondingcomposite adherends. Single-lap shear tests and double-cantilever beam fracture experiments were performed on oven-cured and induction-cured adhesively bonded joints made from woven-fabric composites. The feasibility of usingthe induction-heatingtechnique for the hardeningof adhesives in composite bonded joints was demonstrated. It was found that the strength of single-lap shear joints was not significantly affected by the choice of the hardening method of the adhesive. Furthermore, the critical fracture energy of the oven-cured and induction-cured double-cantilever beams was found to be only dependent on the adhesive type. Induction-cured specimens were found to be as tough as corresponding oven-cured specimens.

Stress Wave Propagation Effects in Two- and Three-Layered Composite Materials

Multilayer materials consisting of ceramic and glass/epoxy composites have been subjected to high strain rate compression testing using the Split Hopkinson Pressure Bar. The samples were extensively strain gaged so that dynamic data were generated directly from the samples during testing. Output data from the experiments were compared with numerical simulations of the same experiments and good agreement was noted. It was found that the stress distribution within samples was quite inhomogeneous and that stresses were highest in the region of the bar-sample interface. The presence of a rubber interlayer between the ceramic and glass/epoxy decreased the stress in both components but dramatically increased the degree of stress in homogeneity.

Effect of the manufacturing process on the interfacial properties and structural performance of multi-functional composite structures

A composite integral armor (CIA) structure consists of various layers such as ceramics, rubber and polymer composites assembled in a precise sequence to provide superior ballistic and structural performance at low areal density. CIA structures were originally manufactured in a labor-intensive multi-step process. In recent years, vacuum-assisted resin transfer molding (VARTM) has emerged as an affordable manufacturing method for CIA structures. In this paper, the relationship between the manufacturing processes (i.e. VARTM and multi-step) and the mechanical performance of CIA beams is investigated by four-point bend tests. The behavior of the CIA is found to be highly dependent on the mechanism of stress transfer between the layers and the structures are found to fail progressively and provide significant ductility and capacity. The VARTM process is found to produce structures with superior mechanical performance. Moreover, the level of interface adhesion achieved during processing is shown to control the structural behavior of the CIA. Consequently, the Mode I fracture testing of VARTM and multi-step manufactured double-cantilever beams, representative of one interface of the CIA, is characterized. The resistance to crack growth of the specimens is also related to the manufacturing process, with the VARTM specimens achieving the highest fracture toughness.

Plastic limit analysis of cylindrically orthotropic circular plates

The plastic limit analysis of cylindrically orthotropic circular plates is developed using a piecewise linear orthotropic yield criterion. The yield criterion is a modification of an isotropic formulation that consists of a series of weighted piecewise linear components. The piecewise linear yield criterion enables an analytical solution for the plastic limit load of cylindrically orthotropic circular plates. Plastic limit analysis for both simply supported and clamped circular plates under uniformly distributed load are carried out. Parametric studies are conducted to investigate the sensitivity of the plastic limit loads to material orthotropy and influences of orthotropic ratio and chosen yield criteria on the plastic limit loads of the circular plates are discussed. It is found that the plastic limit loads of the orthotropic circular plates are affected significantly by the orthotropic ratio. Enhancement of the circumferential yield moment will increase dramatically the plastic limit load of the plates. Moment and velocity fields of the plates in plastic limit state are also derived and discussed. The results obtained from the present study are helpful in understanding the failure mechanism of orthotropic circular plates and is useful for design.

Progressive Damage and Delamination in Plain Weave S-2 Glass/SC-15 Composites Under Quasi-Static Punch Shear Loading

Quasi-static punch-shear tests are carried out on plain weave (PW) S-2 glass/SC-15 epoxy composite laminates with a right circular cylinder punch to identify the sequence and extent of damage and the corresponding displacements at which they occur for a wide range of laminate thicknesses. Two different support spans of 25.4 mm (1 in) and 101.6 mm (4 in) diameter with different layers (0.6 mm ply thickness) of composite laminates are tested under quasi-static loading to identify compression-shear and tension-shear dominated modes of damage. Numerical punch shear experiments are conducted using LS-DYNA 970. The numerical modeling is carried out using a newly developed composite damage model, namely MAT 162, which has been incorporated into LS-DYNA. MAT 162 uses damage mechanics principle for progressive damage and material degradation. Input data required in MAT 162 have been calibrated to match the experimental results of 22-layer composite plate of both spans (25.4 mm and 101.6 mm). The calibrated material properties have been used to simulate other thicknesses, and the simulated results show good agreement with experiment results. It has been found that the dominant damage mechanisms are delamination and fiber breakage due to shear and tension.Copyright

Structural Repair of Composite Structural Armor

Composite structural armor (CSA) is a multifunctional material that provides ballistic protection, stiffness, and strength at minimum weight. It consists of a multilayered architecture of polymer composites, rubber and ceramic tiles, stacked in a precise manner to obtain optimal performance. During its lifetime, CSA will experience manufacturing defects and be subjected from low to high velocity impact threats that will reduce the performance of the structure. The ability to repair a CSA is consequently an important factor in the future deployment of the CSA. The present study addresses the problem of the repair of CSA in a minimum number of steps. Several potential repair methods are introduced, and an adhesively bonded plug repair scheme is then further analyzed. Virgin (control) and repaired CSA beams are manufactured and their static behaviors are compared in a four-point bend test. The performance of a scarf patch repair is assessed. The effect of three scarf angles and two repair adhesive systems, one being low-temperature cure and the other room-temperature cure, are compared. The room-temperature cure repair system offers only a limited ability to bond to the different material layers, thereby limiting the performance of the repaired CSA. On the other hand, it is found that the low-temperature cure repair system provides sufficient interfacial strength to allow the repair to recover an appreciable amount of the original structural performance of the beams. Furthermore, it is found that the repair efficiency increases as the scarf angle decreases. This study clearly demonstrates that it is possible to repair the through-thickness section of CSA, in a single-step operation, and renew structural performance.

Static and Dynamic Strength of Scarf-Repaired Thick-Section Composite Plates

Abstract : One of the many challenges facing weapon developers is the requirement for a highly lethal, lightweight, and compact large caliber gun system. One concept recently investigated by the U.S. Army is that of a swing-chamber gun, necessitating the use of telescoped ammunition. Such ammunition not only reduces the volume available for the propellant change, but also places severe geometric constraints on both the distribution of the propellant and the location and functionality of the ignition system. Lumped parameter codes cannot capture the influence of these configurational complexities on the processes of flame spreading and the formation of ensuing pressure waves. One-dimensional, two-phase flow interior ballistic simulations reveal the likelihood of such waves and raise the concern for possible damage to the ammunition (projectile). Multidimensional, two-phase interior ballistic simulations provide quantitative predictions of the flow in the annular region between the sidewall of the telescoped projectile and the cartridge case, also show the formation of pressure waves, and further the concern over projectile damage. Initial results are shown from ongoing work to couple an interior ballistics code with a gun/projectile structural dynamics code. Pressure waves in the charge produce a very demanding environment for the projectile which necessitates the use of a more substantial structure, with the attending sharp reduction in cargo capacity, or the use of exotic materials in order to insure a successful launch.