A Multiscale Nonlocal Progressive Damage Model for Composite Materials

In this paper, the advantages of a nonlocal progressive damage formulation are described and demonstrated. An approximation of the nonlocal formulation was implemented coupled with the MAT162 composite damage model as a User defined material model in the LS DYNA environment. A comparison of the local model and the nonlocal model is simulated for an 8-ply laminate under tension is carried for increasing mesh densities. The results show the regularization achieved by nonlocal models by providing mesh independent results

High Strain Rate Response of Adhesively Bonded Fiber-Reinforced Composite Joints A Computational Study to Guide Experimental Design

Adhesively bonded carbon fiber-reinforced epoxy composite laminates are widely used in aerospace applications. During a high energy impact event, these laminates are often subjected to high strain rate loading. However, the influence of high strain rate loading on the response of these composite joints is not well understood. Computational finite element (FE) modeling and simulations are conducted to guide the design of high strain rate experiments. Two different experimental designs based on split Hopkinson bar were numerically modeled to simulate Mode I and Mode II types loading in the composite. In addition, the computational approach adopted in this study helps in understanding the high strain rate response of adhesively bonded composite joints subjected to nominally Mode I and Mode II loading. The modeling approach consists of a ply-level 3D FE model, a progressive damage constitutive model for the composite material behavior and a cohesive tie-break contact element for interlaminar delamination

Experimental Investigation of Transverse Loading Behavior of Ultra-High Molecular Weight Polyethylene Yarns

Ultra-high molecular weight polyethylene (UHMWPE) Dyneema® SK-76 fibers are widely used in personnel protection systems. Transverse ballistic impact onto these fibers results in complex multiaxial deformation modes such as axial tension, axial compression, transverse compression, and transverse shear. Previous experimental studies on single fibers have shown a degradation of tensile failure strain due to the presence of such multi-axial deformation modes. In this work, we study the presence and effects of such multi-axial stress-states on Dyneema® SK-76 yarns via transverse loading experiments. Quasi-static transverse loading experiments are conducted on Dyneema® SK-76 single yarn at different starting angles (5°, 10°, 15°, and 25°) and via four different indenter geometries: round (radius of curvature (ROC) = 3.8 mm), 200-micron, 20-micron, and razor blade (ROC ~ role= presentation style= box-sizing: border-box; max-height: none; display: inline; line-height: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; position: relative; \u3e~2 micron). Additionally, transverse loading experiments were also conducted for a 0.30 cal. fragment simulating projectile (FSP) and compared to other indenters. Experimental results show that for the round, 200-micron indenter, and FSP geometry the yarn fails in tension with no degradation in axial failure strain compared to the uniaxial tensile failure strain of SK-76 yarn (2.58%). Whereas for the 20-micron indenter and razor blade, fibers fail progressively in transverse shear followed by progressive strength degradation of the yarn. Strength degradation of yarn occurs at relatively low strains of 0.6–0.7% with eventual failure of the yarn at approximately ~1.8% and ~1.5% strain for the 20-micron indenter and razor blade, respectively. Breaking angles (range of 10°–30°) are observed to have little effect on the failure strain for all indenter geometries

Direct Material Property Determination: One‐Dimensional Formulation Utilising Full‐Field Deformation Measurements

A direct approach is described to determine the elastic modulus distribution in a nominally heterogeneous material subject to tensile/compression loading and primarily experiencing deformations in the axial direction. The formulation is developed for uniaxial applications using basic theoretical constructs, resulting in a computational framework that has a matrix form [A] {E} = {R}, where the [A] matrix components are known functions of measured axial strains and axial positions, {R} components are known functions of axial body forces, applied loads and reactions and {E} components are the unknown elastic moduli at discrete locations along the length of the specimen. For a series of one-dimensional (1D) material property identification procedure with known axial strains at discrete locations and various levels of random noise, results are presented to demonstrate the accuracy and noise sensitivity of the methodology. Finally, experimental measurements for a heterogeneous bone specimen are compared to our 1D model predictions, demonstrating that the predictions are in very good agreement with independent estimates at each load level of interest along the length of the bone specimen

Transverse impact of ballistic fibers and yarns: fiber length-scale finite element modeling and experiments

Gillespie, John, Jr.Keefe, MichaelBallistic impact onto flexible textile fabrics is a complicated multi-scale problem owing to the structural hierarchy of the materials, anisotropic material behavior, projectile geometry, impact velocity and boundary conditions. While this subject has been an active area of research for decades, the fundamental mechanisms such as material failure, dynamic response and multi-axial loading occurring at lower length scales during impact are not well understood. This work provides new insights into the fundamental deformation and failure mechanisms during ballistic impact onto textile fabrics at the micron length scale. In this research, a hybrid computational-experimental systematic approach is adopted to understand the mechanisms and deformation modes of high performance polymer fibers, specifically Kevlar KM2, that is widely used in ballistic impact applications. Fiber length-scale 3D finite element (FE) models are developed to better understand and complement the complicated transverse impact experiments. The fiber length-scale study suggests that fibers are subjected to multiaxial stress states including transverse compression, axial tension, axial compression and transverse shear significant enough to cause fibrillation in the fiber during impact. A dispersive flexural wave mode is predicted by the model due to the finite longitudinal shear modulus of the fiber. The flexural wave induces curvature in the fiber significant enough to cause compressive kinking and, in turn, local fibrillation in the fiber. A fiber length-scale yarn model is developed by explicitly modeling all the 400 fibers in a KM2 600 denier yarn. The yarn transverse compression results show that fiber-fiber contact plays a significant role in the spreading and deformation of individual fibers that is consistent with experimental results. When subjected to transverse impact, the model indicates significant transverse compressive strains in the fiber that increase with impact velocity and a flexural wave that induces curvatures in the fibers significant enough to induce compressive kinking and fibrillation. In addition to the transverse wave, a spreading wave develops due to fiber-fiber contact interaction that spreads the fibers to a large extent resulting in non-uniform loading and progressive failure of fibers within the yarn. Guided by the computational models, single-fiber micromechanical experiments for axial compressive kinking and transverse compression deformation modes are developed. The average tensile strength of the kinked fibers is found to be reduced by 7% compared to the virgin fibers. An experimental methodology is developed to determine the single fiber constitutive behavior in quasi-static transverse compression by removing the geometric nonlinearity due to the growing contact area. The fibers exhibit nonlinear inelastic behavior under large compressive strains. The fibers subjected to 60% nominal strains (80% true strains) showed a 20% reduction in average tensile strength compared to the virgin fibers. A nonlinear inelastic constitutive model is implemented as a user defined material (UMAT) suitable for the commercial FE code LS-DYNA explicit analysis. During impact, the inelastic behavior results in a significant reduction in the fiber bounce velocity and a reduction in the projectile-fiber contact forces by 40% compared to an elastic constitutive behavior. The inelastic dissipation and reduced bounce leads to an inelastic collision rather than an elastic collision. The longitudinal shear modulus and the inelastic behavior are found to govern the failure response of the fibers during impact. Modeling the single fiber quasi-static multiaxial loading experiments indicate fiber failure may be initiated based on a gage length dependent maximum axial tensile strain in the fiber. Regardless of the material behavior (elastic or inelastic), fiber length-scale impact models show a gradient in the axial tensile strain (stress) in the fiber cross section at the location of failure consistent with multiaxial loading experimental observations. Fiber-level yarn breaking speed predictions based on a maximum axial tensile strain (stress) criterion are much lower than the breaking speed based on classical theory and they are consistent with experimental measurements. Therefore, the reduction in experimental yarn breaking speed compared to theoretical Smith solution is attributed to the stress concentration and property degradation mechanisms due to multiaxial stress states at the location of failure.University of Delaware, Department of Mechanical EngineeringPh.D

Role of Inelastic Transverse Compressive Behavior and Multiaxial Loading on the Transverse Impact of Kevlar KM2 Single Fiber

High-velocity transverse impact of ballistic fabrics and yarns by projectiles subject individual fibers to multi-axial dynamic loading. Single-fiber transverse impact experiments with the current state-of-the-art experimental capabilities are challenging due to the associated micron length-scale. Kevlar® KM2 fibers exhibit a nonlinear inelastic behavior in transverse compression with an elastic limit less than 1.5% strain. The effect of this transverse behavior on a single KM2 fiber subjected to a cylindrical and a fragment-simulating projectile (FSP) transverse impact is studied with a 3D finite element model. The inelastic behavior results in a significant reduction of fiber bounce velocity and projectile-fiber contact forces up to 38% compared to an elastic impact response. The multiaxial stress states during impact including transverse compression, axial tension, axial compression and interlaminar shear are presented at the location of failure. In addition, the models show a strain concentration over a small length in the fiber under the projectile-fiber contact. A failure criterion, based on maximum axial tensile strain accounting for the gage length, strain rate and multiaxial loading degradation effects are applied to predict the single-fiber breaking speed. Results are compared to the elastic response to assess the importance of inelastic material behavior on failure during a transverse impact

Molecular Dynamics Modeling of the Effect of Axial and Transverse Compression on the Residual Tensile Properties of Ballistic Fiber

Ballistic impact induces multiaxial loading on Kevlar® and polyethylene fibers used in protective armor systems. The influence of multiaxial loading on fiber failure is not well understood. Experiments show reduction in the tensile strength of these fibers after axial and transverse compression. In this paper, we use molecular dynamics (MD) simulations to explain and develop a fundamental understanding of this experimental observation since the property reduction mechanism evolves from the atomistic level. An all-atom MD method is used where bonded and non-bonded atomic interactions are described through a state-of-the-art reactive force field. Monotonic tension simulations in three principal directions of the models are conducted to determine the anisotropic elastic and strength properties. Then the models are subjected to multi-axial loads—axial compression, followed by axial tension and transverse compression, followed by axial tension. MD simulation results indicate that pre-compression distorts the crystal structure, inducing preloading of the covalent bonds and resulting in lower tensile properties

Validation of Large Area Capacitive Sensors for Impact Damage Assessment

Impacts in fiber-reinforced polymer matrix composites can severely inhibit their functionality and prematurely lead to the composite’s failure. This research focuses on determining the efficacy of a novel capacitive sensor, termed as the Soft Elastomeric Capacitor or SEC, to monitor the magnitude of out-of-plane deformations in composites. This work forwards the development of a sensing skin that can be used as an in situ monitoring tool for composites. The capacitive sensor can be made to arbitrary sizes and geometries. The sensor is composed of an elastomer composite that measures strains experienced by the material it is bonded to. The structure of the sensor, fabricated to function as a parallel plate capacitor, responds to impacts by transducing strains into a measurable change in capacitance. In this work, the SECs are deployed on randomly oriented fiberglass-reinforced plates with a polyester resin matrix. The material is impacted at various energy levels until the monitored composite material reaches its yielding point. The behavior of the sensor in impact detection applications below the proof resilience shows little to no change in the capacitance of the sensor. As the impacts surpass this yielding point, the sensor responds linearly with induced change in the area. The sensor performed within the expectations of the proposed model and demonstrated the efficacy of the proposed large-area sensor as a damage quantification tool in the structural health monitoring of composites.This is an accepted manuscript of an article published as Vereen, Alexander Brennan, Austin Downey, Subramani Sockalingam, and Simon Laflamme. "Validation of Large Area Capacitive Sensors for Impact Damage Assessment." Measurement Science and Technology (2023). doi:https://doi.org/10.1088/1361-6501/ad0954. This Accepted Manuscript is © 2023 The Author(s). This Accepted Manuscript is available for reuse under a CC BY 4.0

Large area capacitive sensors for impact damage measurement

Impacts in fiber reinforced polymer matrix composites can severely inhibit their functionality and lead to failure of the composite prematurely. This research focuses on determining the efficacy of a novel capacitive sensor, termed as the soft elastomeric capacitor or SEC, for the purpose of monitoring the magnitude of out-of-plane deformations in composites. This work aims to forward the development of a sensing skin that can be used as an in situ monitoring tool for composites. The capacitive sensor can be made to arbitrary sizes and geometries. The sensor is composed of an elastomer composite that inherits the strains of the material it is bonded to. The structure of the sensor, manufactured to function as a parallel plate capacitor, responds to impacts by transducing strains into a measurable change in capacitance. In this work, the large area capacitive sensors are deployed on randomly oriented fiberglass-reinforced plate with a polyester resin matrix. The material is impacted at various energy levels until the material reached its yielding point. The behavior of the sensor in impacts below the proof resilience shows little to no change in capacitance of the sensor. As the impacts surpassed this yielding point, the sensor responds linearly with induced change in area. The results performed within expectations of the proposed model and demonstrated the efficacy of the proposed large area sensor as a damage quantification tool in the structural health monitoring of composites.This proceeding is published as Vereen, Alexander B., Austin Downey, Subramani Sockalingam, and Simon Laflamme. "Large area capacitive sensors for impact damage measurement." In Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2022, vol. 12046, pp. 115-120. SPIE, 2022. DOI: 10.1117/12.2629492. Copyright 2022 SPIE. Posted with permission

Mode-I behavior of adhesively bonded composite joints at high loading rates

This study investigates the high loading rate behavior of adhesively bonded carbon/epoxy composite joints under mode I loading. A computationally guided experimental setup is developed to study the mode-I behavior of composite joints in the range of quasi-static to high loading rates. A double cantilevered beam specimen with wedge insert type loading setup is used to conduct quasi-static and dynamic experiments. For the dynamic loading, a modified split Hopkinson bar is used to load the sample at high rates. The local deformation field is measured using high Spatio-temporal resolution digital image correlation (DIC). From the experiments, the mode-I energy release rate is calculated from the load, crack extension and crack root rotation data measured using load cell and DIC. A decrease in the initiation fracture toughness with increase in loading rate was observed which is attributed to the strain rate dependent behavior of the epoxy-based film adhesive. For both quasi-static and high loading rates, a mixed adhesive-cohesive failure is observed from the fracture surface analysis