Bruce K. Fink

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.

High Strain-Rate Behavior of Plain-Weave S-2 Glass/Vinyl Ester Composites

Thick-section composites made from plain-weave S-2 glass fabric (24 oz./sq. yard) and vinyl ester (411-C50) resin have been tested over a wide range of strain-rates (200-1600 s -1) using a compression split Hopkinson pressure bar (SHPB) with a momentum trapping device. Experiments were performed in two material directions: thickness and fill. Three different types of specimens having rectangular cross sections were tested with thickness ranging from 3.8 mm to 12.7 mm. The strain-rate effects on maximum stresses and maximum non-linear strains have been characterized. The dominant failure modes of the material have been determined through optical and scanning electron microscopy (OM and SEM). It has been identified that the dynamic ultimate stress and failure strain is higher than the corresponding quasi-static values. The ultimate stress is found rate insensitive for both thickness and fill direction loading. The non-linear failure strain is also found to be rate insensitive in the case of thickness direction loading; however, the failure strain increases with strain-rate in the case of fill direction loading. The dominant dynamic failure modes in thickness direction loading are compressive matrix cracking, fiber breakage, and lateral flow of fiber bundles. In the fill direction loading, the dynamic failure modes are kink band formation, delamination, transverse matrix cracking, and longitudinal splitting.

Performance Metrics for Composite Integral Armor

Future combat systems necessarily focus on lightweight, highly mobile and transportable armored vehicles. Lightweight composite integral armor systems are being developed to meet these needs. The goal of this paper is to centrally document the myriad design requirements for composite integral armors that serve multifunctional roles including ballistic, structural, shock, electromagnetic, and fire protection. Structural and ballistic performance requirements as well as manufacturing and life-cycle performance issues of integral armor are presented. Specific areas addressed include high-strain-rate testing and modeling, ballistic testing and modeling, low-cycle fatigue, damage tolerance, repair, reduced-step processing, through-thickness reinforcement, energy dissipation and rate-dependent failure mechanisms, and non-linear mechanics.

A study on the induction heating of conductive fiber reinforced composites

A unified approach, considering three possible heating mechanisms: fiber (Joule losses) and fiber crossover junction (dielectric hysteresis and contact resistance), to identify dominant heating mechanisms during induction processing of conductive fiber reinforced composites is presented. Non-dimensional parameters were proposed to identify the relationships between heating mechanisms and process and material parameters. Parametric studies showed that junction heating mechanisms dominate fiber heating for the material systems considered, with the exception of relatively low contact resistance (< 10 3). Results for dielectric hysteresis and low contact resistance were consistent with individual models in the literature. A design map relating the three mechanisms is presented that can help identify the dominant heating mechanism, given the properties of the composite.

Resistive Susceptor Design for Uniform Heating during Induction Bonding of Composites

A novel susceptor concept for metal mesh susceptors, designed to achieve uniform in-plane temperatures during induction heating, is documented. The process involves redirecting eddy current flow patterns in the resistive mesh susceptor by specifically designed cut patterns in the mesh. A theoretical model was developed to predict heat generation in metal mesh susceptors with any prescribed network pattern. Initial results for meshes with cut patterns show significant changes in heating compared to an uncut mesh. Cut patterns can be optimized to reduce temperature gradients in the susceptor to within the processing window of the composite. Experimental results are presented for qualitative comparisons.

Assessment of flow and cure monitoring using direct current and alternating current sensing in vacuum-assisted resin transfer molding

Vacuum-assisted resin transfer molding (VARTM) is an emerging manufacturing technique that holds promise as an affordable alternative to traditional autoclave molding and automated fiber placement for producing large-scale structural parts. In VARTM, the fibrous preform is laid on a single-sided tool, which is then bagged along with the infusion and vacuum lines. The resin is then infused through the preform, which causes simultaneous wetting in its in-plane and transverse directions. An effective sensing technique is essential so that comprehensive information pertaining to the wetting of the preform, arrival of resin at various locations, cure gradients associated with thickness and presence of dry spots may be monitored. In the current work, direct current (dc) and alternating current sensing/monitoring techniques were adopted for developing a systematic understanding of the resin position and cure on plain weave S2-glass preforms with Dow Derakane vinyl ester VE 411-350, Shell EPON RSL 2704/2705 and Si-AN epoxy as the matrix systems. A SMARTweave dc sensing system was utilized to conduct parametric studies: (a) to compare the flow and cure of resin through the stitched and non-stitched preforms; (b) to investigate the influence of sensor positioning, i.e. top, middle and bottom layers; and (c) to investigate the influence of positioning of the process accessories, i.e. resin infusion point and vacuum point on the composite panel. The SMARTweave system was found to be sensitive to all the parametric variations introduced in the study. Furthermore, the results obtained from the SMARTweave system were compared to the cure monitoring studies conducted by using embedded interdigitated (IDEX) dielectric sensors. The results indicate that SMARTweave sensing was a viable alternative to obtaining resin position and cure, and was more superior in terms of obtaining global information, in contrast to the localized dielectric sensing approach.

Accelerated Curing of Adhesives in Bonded Joints by Induction Heating

This paper presents the results of a study on the use of induction heating for rapid curing of a commercially available room-temperature curing paste adhesive, with a view to application in the repair of composite structures. The repair of damaged composite structures using adhesively bonded patches is often a very time-consuming process. This is partly due to the long times required for complete and satisfactory cure of the adhesive systems used. While the curing process can be accelerated by application of heat through devices such as heating blankets and lamps, these methods are inefficient since considerable heat is lost to the surrounding material and environment. The method of electromagnetic heating, however, is well suited for rapid and localized heating of the adhesive bondline provided suitable susceptors are used. In this paper, it is shown that induction heating can be successfully used to cure a room-temperature curing paste adhesive. Furthermore, results of single lap shear and double notched shear specimen tests show no substantial reduction in the strength of bonded joints with the use of embedded susceptors.

A study on the induction heating of carbon fiber reinforced thermoplastic composites

Recent work in the literature has identified a new heating mechanism during induction processing of carbon thermoplastic prepreg stacks: contact resistance between fibers of adjacent plies. An experimental methodology has been developed to estimate the contact resistance through heating tests based on the properties of the composite and geometry of the specimen. Measured values indicate comparable resistance values at the contact region, compared to resistance in the fiber direction, for AS-4/PEI prepreg stacks under vacuum pressure. The measured values can serve as inputs for induction heating models and process models of carbon thermoplastic prepreg stacks.

Feedback control of the vacuum-assisted resin transfer molding (VARTM) process

The Vacuum Assisted Resin Transfer Molding (VARTM) technique is a liquid-molding process that offers the potential to significantly reduce fabrication costs for large-scale composite structures. The VARTM workcell is used to evaluate control strategies and sensors such as SMARTweave to provide feedback for an intelligent control system. Current VARTM systems lack automated control systems resulting in part to part variability. This research presents a continuously controlled vacuum actuator system and the influence of vacuum gradients on resin flow front control.

Development of a numerical model to predict in-plane heat generation patterns during induction processing of carbon fiber-reinforced prepreg stacks

A numerical model is proposed to describe in-plane heat generation spatial response during induction processing of carbon fiber-reinforced thermoplastics. The model is based on a unified approach that considers three possible heating mechanisms: fiber heating (Joule losses in fiber), noncontact junction heating (dielectric hysteresis), and contact junction heating (Joule losses at junctions). A lumped meshing scheme is used to construct a numerical representation for cross-ply and angle-ply orientations of 2-ply prepreg stacks. Heat generation patterns are calculated based on voltage and current conservation laws and verified with induction heating of AS4 carbon fiber-reinforced polyetherimide (AS4/PEI) prepreg stacks. Excellent agreement is found except at very low angle-ply orientations where the predicted heating patterns show significant deviations from the experiment results. A sensitivity analysis is also performed to assess the relationship between heating patterns and material and process parameters. The results show that the stack angle between plies and intrinsic prepreg microstructure can significantly affect the heating patterns.