Ultra-high molecular weight polyethylene (UHMWPE) composites are used in a wide array of protective armor systems. Design of these systems is largely performed through empirical studies which can be costly and time consuming. Modeling tools that enable enhanced performance through exploitation of the design of laminated composite architectures are desired. In this effort, we present a multi-scale, finite element-based representative volume element (RVE) approach that uses laminate mechanics to capture the ply-level material nonlinearity and strain-induced fiber reorientation of UHMWPE composite laminates subjected to low-velocity impact (LVI) loading. The effects of strain rate on the ply-level material response and predicted LVI response of UHMWPE composites are explored. An LVI methodology is developed to characterize the impact performance of thick-section UHMWPE composite materials and applied with the RVE finite element model to calibrate material and delamination properties for Honeywell (R) SpectraShield (R) II SR-3136 and DSM Dyneema (R) HB210. The multi-scale RVE approach accurately captures the peak back-face deformation and extent of delamination for laminates of these materials across three impact energies. The LVI methodology provides a means to evaluate and rank the impact performance of various UHMWPE composite materials, laminate architectures, and processing condi-tions. The LVI methodology coupled with the multi-scale RVE approach produces ply-level behavior and delamination properties linked to materials and processing conditions, ultimately creating a modeling tool that is capable of exploring the design of laminate architectures for UHMWPE composite structures.

Insertion of composite materials into next generation vehicles requires an understanding of their response over the range of relevant loading conditions encountered in the field. This paper focuses on modelling the low velocity, high energy impact on structural composite materials. An experimental procedure, using the four quadrant impact methodology, is presented to characterize composite material response and damage during these events. A model is developed and verified (within 9% for peak force and peak deflection) to predict the composite response to low velocity, high energy events. The model is then exercised to gain a deeper understanding about how mechanical and damage properties influence the structural response of the composite. The model was most sensitive to delamination failure stress, which changed the damage area by a factor of 8 over the range of values examined. Through investigating the effect of boundary conditions, a 67% increase in peak force was found by switching from simply supported to clamped. This work demonstrates a first level analysis and model to predict the composite response to low velocity, high energy loading conditions.

In this effort, the interfibrillar behavior in UHMWPE fibers subjected to tension is investigated using a unique experimental setup that allows for the in-situ monitoring of the deformations of sub-fiber level components within the fiber. Digital image correlation (DIC) is used to measure fiber surface strains, quantify the stress-strain fiber response and interfibrillar sliding between adjacent macrofibrils within the fiber’s microstructure, and estimate the length of macrofibrils within the fiber. Finite element (FE) based models of the UHMWPE fibers are developed that account for the complex fibrillar microstructure and macrofibrillar interactions governing the macro-scale fiber response under uniaxial tension. The FE-based models are compared to both experimental results and previously developed analytical fiber models to study the effects of underlying model assumptions on the fiber’s stiffness and deformation response. The shortcomings of a continuum material model for the fiber are revealed. A unique beam connector model of the fiber is presented which mimics the inherent interfibrillar deformation mechanics observed in the experiments. A Design of Experiments (DoE) approach is used to facilitate development of the model. Predictions are shown to compare favorably with experimental results for both the fiber stress-strain response and the degree of interfibrillar sliding. The findings presented in this paper contribute towards the fundamental understanding of the complex microstructure-property relations in UHMWPE fibers and can ultimately be used to help guide the development of high performance fibers widely used in armor ballistic protection systems.

The processing conditions used in the production of advanced polymer fibers facilitate the formation of an oriented fibrillar network that consists of structures spanning multiple length scales. The irregular nature of fiber tensile fracture surfaces suggests that their structural integrity is defined by the degree of lateral (interfacial) interactions that exist within the fiber microstructure. To date, experimental studies have quantified interfacial adhesion between nanoscale fibrils measuring 10-50 nm in width, and the global fracture energy through applying peel loads to fiber halves. However, a more in-depth evaluation of tensile fracture indicates that fiber failure typically occurs at an intermediate length scale, involving fibrillation along interfaces between fibril bundles of a few 100s of nanometers in width. Interaction mechanisms at this length scale have not yet been studied, due in part to a lack of established experimental techniques. Here, a new focused ion beam-based sample preparation protocol is combined with nanoindentation to probe interfaces at the intermediate length scale in two high-performance fibers, a rigid-rod poly(p-phenylene terephthalamide) and a flexible chain ultrahigh molecular weight polyethylene fiber. Higher interfacial separation energy recorded in the rigid-rod fiber correlated with less intensive fibrillation during failure and is discussed in the context of fiber chemistry and processing. Power law scaling of the total absorbed interfacial separation energy at three different scales in the polyethylene fiber is observed and analyzed, and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. The contribution of these mechanisms to the overall integrity of the fiber is discussed, and the importance of the intermediate scale is elucidated. Results from this study provide new insights into the mechanical implications of hierarchical lateral interactions and will aid in the development of novel fibers with further improved mechanical performance.

In this work, fibrillation is introduced as an energy absorbing mechanism in the modeling of Kevlar® KM2 single fibers subjected to quasi-static transverse compression. Fibrillation is simulated using a finite element model of the fiber cross-section containing discrete fibrils connected by interfibrillar cohesive zones. Model predictions of nominal stress-strain response for an assumed bilinear cohesive traction-separation interfibrillar behavior are compared to experimental data. Analysis shows that modeling of the microstructural fibril network, represented by a distribution of strong cohesive interactions, is necessary to capture the experimental response. The model provides valuable insight into the unique deformation mechanisms governing fiber fibrillation under transverse compression.

In this work, we present a novel approach for improving the delamination resistance and durability of structural composite laminates using compliant thermoplastic polyurethane (TPU) interlayers that allow decoupling of the structural plies within the laminate thus preventing catastrophic delamination. Laminates with different compliant interlayer thicknesses are fabricated and characterized under multiple low velocity impacts (LVI) to demonstrate the significant improvement in delamination resistance that can be achieved. Finite element analysis and experimental data on samples of the interlayer laminates subjected to three-point bending is presented to provide additional insight into the unique decoupling phenomena enabling significant improvements in delamination resistance of structural composite laminates.

Ultra-high molecular weight polyethylene (UHMWPE) fiber-reinforced composites have received widespread attention in the literature due to their attractive ballistic protection attributes. Recently, investigators are recognizing and demonstrating the significant role that interlaminar shear has on their ballistic performance. In this paper, we present a characterization methodology to quantify the quasi-static interlaminar shear strength and nonlinear interlaminar shear stress-strain response of UHMWPE composite laminates. The methodology uses a tension loaded double-lap coupon design to introduce interlaminar shear loading. Coupon displacement measurements using Digital Image Correlation (DIC) coupled with Finite Element Analysis (FEA) incorporating nonlinear material behavior and traction-separation behavior is an integral part of the data reduction scheme. This research provides a unique methodology for developing interlaminar shear constitutive models for UHMWPE composite laminates, which are critically needed to improve the accuracy of ballistic impact simulations for the development of more efficient armor designs.

In this paper, an approach for transferring the complex interactions of polyethylene fibrils from molecular dynamics (MD) simulations to finite element (FE) modeling is developed. Simulations of intermolecular interactions using an all-atom MD model with the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential are used to calibrate the force descriptions of surface-based interactions in an equivalently configured finite element simulation. The anisotropic material description of the finite element fibril is derived from MD simulations of crystalline polyethylene subjected to mechanical tension, compression, and shear in all directions. Fibril pull-out simulations are conducted in both MD and FE domains to determine the inter-fibril, surface-based Lennard-Jones interaction parameters for the continuum fibril model. This approach will help elucidate the complex interaction forces that exist at the atomistic level and ultimately enable scale-bridging towards the micro-and macro-fibril length scales.

This article represents our contribution to Part B of the 2nd Worldwide Failure Exercise (WWFE-II), where comparisons between epoxy/lamina/laminate response and failure predictions, based on maximum strain failure criterion, and experimental results are made. Correlations between actual test data and our nonlinear stress-strain response and failure envelope predictions under multi-axial loading for 12 different case studies are presented and discussed. Although our approach to modeling composite failure ranked high when applied to biaxial in-plane loading analysis, the theory does not consistently capture the inherent strengthening mechanisms that can be attributed to triaxial loading and hydrostatic pressure. It is expected that the incorporation of a strain-based ply-level failure criterion that introduces some form of strengthening mechanism under a three-dimensional stress state would significantly improve the accuracy of our predictions.

As a part of the Second World-Wide Failure Exercise, a three-dimensional nonlinear maximum progressive strain model, based on laminate analysis, is employed to make blind predictions for 12 test cases representing failure envelopes and stress strain curves for isotropic, unidirectional, and multidirectional composite laminates. This approach allows for redistribution of ply stresses and differentiation of the various potential modes of failure. These cases include initial and final ply failure envelopes under tri-axial loading, as well as 3 cases, requiring nonlinear stress–strain analysis. Comparison of predictions with actual experimental data will be made in Part B of the Second World-Wide Failure Exercise.

An atomistic based finite bond element model has been developed to study the effects of multiple Stone–Wales (5-7-7-5) defects on mechanical properties of graphene sheets and carbon nanotubes. The element formulation includes 8 degrees of freedom reducing computational cost compared to the 12 degrees of freedom used in other FE type models. The coefficients of the elements are determined based on the analytical molecular structural mechanics model developed by the authors. The model uses the modified Morse potential to predict the Young's modulus and stress–strain relationship of perfect and defective nanotubes and graphene sheets. The variation of ultimate stress, strain at failure, and Young's modulus values of carbon nanotubes and graphene sheets have been examined as a function of the distance between two defects aligned in the axial and hoop directions. The mechanical properties as a function of the number of defects in the hoop direction are also studied. It is found that the moduli are sensitive to the tube lengths when the total tube length is used to compute the strain. If one uses a local defective length to define the strain, a size independent modulus can be obtained for the defective region. The diameter of the affected region (2nm) from a single defect is defined as the defective length and is used for all different tube lengths examined in the present study. The effects of defect density on mechanical properties of tubes of any lengths are also discussed.

An atomistic based finite bond element model for the prediction of fracture and progressive failure of graphene sheets and carbon nanotubes is developed by incorporating the modified Morse potential. The element formulation includes eight degrees of freedom reducing computational cost compared to the 12 degrees of freedom used in other FE type models. The coefficients of the elements are determined based on the analytical molecular structural mechanics model developed by the authors. The model is capable of predicting the mechanical properties (Young’s moduli, Poisson’s ratios and force–strain relationships) of both defect-free and defective carbon nanotubes under different loading conditions. In particular our approach is shown to more accurately predict Poisson’s ratio. The numerical prediction of nonlinear stress–strain relationships for defect-free nanotubes including ultimate strength and strain to failure of nanotubes is identical to our analytical molecular structural mechanics solution. An interaction based mechanics approach is introduced to model the formation of Stone–Wales (5-7-7-5) topological defect. The predicted formation energy is compared with ab initio calculations. The progressive failure of defective graphene sheets and nanotubes containing a 5-7-7-5 defect is studied, and the degradation of Young’s moduli, ultimate strength and failure strains of defective nanotubes is predicted.

An atomistic based finite bond element model for the prediction of fracture and progressive failure of graphene sheets and carbon nanotubes is developed by incorporating the modified Morse potential. The element formulation includes eight degrees of freedom reducing computational cost compared to the 12 degrees of freedom used in other FE type models. The model is capable of predicting the mechanical properties (Young's moduli, Poisson's ratios and force-strain relationships) of both defect-free and defective carbon nanotubes under different loading conditions. In particular our approach is shown to more accurately predict Poisson's ratio. The numerical prediction of nonlinear stress-strain relationships for defect-free nanotubes including ultimate strength and strain to failure of nanotubes is identical to our analytical molecular structural mechanics solution. An interaction based mechanics approach is introduced to model the formation of Stone-Wales (5-7-7-5) topological defect. The predicted formation energy is compared with ab initio calculations. The progressive failure of defective graphene sheets and nanotubes containing a 5-7-7-5 defect is studied, and the degradation of Young's moduli, ultimate strength and failure strains of defective nanotubes is predicted.