Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Youqi Wang / Ballistic performance of textile fabric is affected by numerous elements, such as fabric architecture, material property, and projectile characteristics. Near fiber-level microstructures of soft body armor composed of multi-layer Kevlar KM-2 fabrics are generated for numerical simulation. The modified digital element approach (DEA) is applied to determine the ballistic limit of textile fabrics against fragment simulating projectiles (FSP). Different from other numerical models, the DEA takes a considerable amount of fiber-level detail into consideration and models the fabric at filament-level. In this approach, fabric is an assembly of yarns weaved and relaxed into pre-arranged pattern; yarn is simulated as a bundle of digital fibers. When the number of digital fibers per yarn reaches the number of actual fibers per yarn, fiber-level simulation is achieved. The DEA model successfully simulates real scale multi-layer fabric impacted by spherical projectile and accurately predicted fabric displacement and failure mechanism. It was assumed that the digital fiber is fully flexible and its bending rigidity is negligible. Shear force was thus neglected. However, for projectiles with sharp edge(s), such as FSP, due to resultant shear force, fabric failure starts where it interacts with projectile edge. As a result, the numerical results derived from the previous DEA overestimated the impact strength of fabrics against projectiles with shape edges. Therefore, shear force and fiber bending rigidity must be considered. In the modified DEA approach, numerical tests are employed to determine the effective bending rigidity of digital fiber. A combined tension-shear failure model is then incorporated into the DEA in order to calculate the shear force applied to fibers. The 3-D microscope is applied to measure the radius of FSP along the edge. The surface of the FSP is meshed into triangle elements. A unique algorithm is developed and employed to search contacts between textile fabric and projectile of arbitrary shape. In this research, first, an overview of ballistic impact analysis is discussed; the previous DEA model used in simulating ballistic impact and penetration process is presented. Second, the modified DEA approach used in simulating arbitrary shape projectile perforation process is established and verified. The method of searching and calculating contacts between textile fabric and solid body projectile is explained. The convergence and accuracy of digital element mesh are investigated statistically using tension-shear failure model. Third, fabric shear force and fiber bending rigidity are investigated using tension-shear failure model. The effective digital fiber area moment of inertia is numerically determined. Fourth, standard ballistic tests of real scale multi-layer Kevlar KM2 fabrics are simulated using FSP. Numerical results are compared to high-resolution experimental test data. The modified DEA is validated.

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / Youqi Wang / The strength of the fabric system is based on fiber strength and fabric mechanics. Modeling a fabric system accurately requires research into fiber behavior within the yarn and yarn behavior within the fabric. Limited computer resources require new approaches to yarn modeling and fabric modeling especially in regards to ballistic impact. The fabric is discontinuous. There are many factors which require modeling the physics in order to accurately simulate and design fabric systems. Weaving yarns into fabrics can introduce fiber level damages such as surface defects and crimps through sliding friction and bending and thus add variance to the tensile strength of the fibered yarn. A Weibull distribution is an often used method to develop a statistical model and is developed to calculate the strength of the yarn. It is necessary to carefully remove the fibers from the as woven fabric and use a standard ASTM single fiber tensile test to create a Weibull distribution of tensile strength. In general in Kevlar systems the edge radius for laboratory projectiles is much larger than the actual dimeter of the fiber; however, the yarn itself can be sheared, and this fibered yarn system requires modeling. There is no direct measurement of Kevlar fiber shear strength, so combined tensile-twist test data is used to develop equations to determined shear strength. DFMA is modeling software developed to create digital fabrics in a method that accurately models yarn shape with limited computer resources using a concept of a digital fiber. The digital fiber represents multiple real fibers, so it is necessary to use the digital yarn effective bending rigidity developed with numerical simulation of experimental results. Since the yarn is composed of hundreds to thousands of fibers, the physical yarn cannot be modeled in full scale fabrics. The yarn composed of digital fibers is structurally similar to real yarns and is capable of representing the real fabric mechanics. In the process of impact, within the relatively short time frame, the distribution of stress is mostly in principal yarns at a time when the event is considered complete through penetration or projectile rebound. The hybrid mesh method represents the small number of principal yarns with high density mesh and the rest of the fabric (the non-principal yarns) with coarse mesh. With hybrid mesh, the full scale simulation of actual fabrics is possible. The projectile geometry for real threats is variant depending on the types of projectiles in use (projectiles for maximum energy transfer to the target or projectiles for high shear). The laboratory projectiles are therefore variant in order to represent threats. In this research the RCC is the threat and two standard weights are modeled with local geometry. The local laboratory projectile geometry is controlled however it is bounded by a tolerance much larger than the Kevlar fibers studied here. It does act against the fibered yarn which will shear mechanically dependent on fiber to fiber interactions and possibly fiber shear strength.