Caglar Oskay

A Multiscale Failure Model for Analysis of Thin Heterogeneous Plates

This manuscript presents a new multiscale framework for the analysis of failure of thin heterogeneous structures. The new framework is based on the asymptotic homogenization method with multiple spatial scales, which provides a rigorous mathematical basis for bridging the microscopic scales associated with the periodic microstructure and thickness, and the macroscopic scale associated with the in-plane dimensions of the macrostructure. The proposed approach generalizes the Caillerie—Kohn—Vogelius elastostatic heterogeneous plate theory for failure analysis when subjected to static and dynamic loads. Inelastic fields are represented using the eigendeformation concept. A computationally efficient n-partition computational homogenization model is developed for simulation of large scale structural systems without significantly compromising on the solution accuracy. The proposed model is verified against direct 3D finite element simulations and experimental observations under static and dynamic loads.

A Nonlocal Multiscale Fatigue Model

A nonlocal multiscale model in time domain is developed for fatigue life predictions. The method is based on the mathematical homogenization theory with almost periodic fields. The almost periodicity reflects the effects of irreversible deformations in time domain in the form of accumulation of damage. Multiple temporal scales are introduced to decompose the original boundary value problem into micro-chronological (temporal unit cell) and macro-chronological (homogenized) problems. A nonlocal Gurson type constitutive law is revisited for cyclic loading, calibrated and validated against fatigue crack propagation experiments on 316L austenitic stainless steel specimens.

Computational modeling of polyurea-coated composites subjected to blast loads

This manuscript presents computational modeling and simulation of woven E-glass fiber-reinforced vinyl-ester (EVE) composites and polyurea-coated EVE composites subjected to blast loading. The response of polyurea is idealized based on a temperature- and pressure-dependent visco-elastic constitutive model. The response of the EVE layers is modeled based on a multiscale computational damage model that includes adiabatic heating and rate-dependence in the constituent (i.e. matrix and fiber) behavior. Experimentally validated numerical simulations of EVE composite and polyurea-coated EVE composite specimens subjected to blast loading indicate that the proposed models are capable of accurately capturing the inelastic and failure characteristics of the specimens. The significant shock mitigation effect of polyurea coating is numerically demonstrated. Predictive simulations suggest better blast mitigation characteristics with increasing polyurea thickness and confining the perimeter of the polyurea layers.

Experimentally-validated mesoscale modeling of the coupled mechanical–thermal response of AP–HTPB energetic material under dynamic loading

This manuscript presents a combined computational–experimental study of the mesoscale thermo-mechanical behavior of the Hydroxyl-terminated polybutadiene (HTPB) bonded ammonium perchlorate (AP) composite energetic material subjected to dynamic loading conditions. The computational model considers the AP–HTPB interface debonding, post-debonding interface friction and temperature rise due to viscoelastic dissipation as well as dissipative interfacial processes. The interface is modeled using a cohesive zone model combined with a contact algorithm to account for the interface separation, particle/binder contact and heat generation. The HTPB binder is modeled as viscoelastic with adiabatic temperature rise. Three experiments are conducted to calibrate and validate the model. Raman spectroscopy and indentation experiment are employed to determine the interface properties, whereas Kolsky bar tension test along with in-situ synchrotron X-ray diffraction measurements are used to validate the model and understand the interface separation characteristics under dynamic loading.

Polycrystal plasticity modeling of nickel-based superalloy IN 617 subjected to cyclic loading at high temperature

A crystal plasticity finite element (CPFE) model considering isothermal, large deformation and cyclic loading conditions has been formulated and employed to investigate the mechanical response of a nickel-based alloy at high temperature. The investigations focus on fatigue and creep-fatigue hysteresis response of IN 617 subjected to fatigue and creep-fatigue cycles. A new slip resistance evolution equation is proposed to account for cyclic transient features induced by solute drag creep that occur in IN 617 at 950 °C. The crystal plasticity model parameters are calibrated against the experimental fatigue and creep-fatigue data based on an optimization procedure that relies on a surrogate modeling (i.e. Gaussian process) technique to accelerate multi-parameter optimizations. The model predictions are validated against experimental data, which demonstrates the capability of the proposed model in capturing the hysteresis behavior for various hold times and strain ranges in the context of fatigue and creep-fatigue loading.

Prediction of progressive damage and strength of IM7/977-3 composites using the Eigendeformation-based homogenization approach: Static loading

This paper presents the results from the authors’ participation in the Air Force Research Laboratory’s Damage Tolerance Design Principles Program. The Eigendeformation-based reduced order homogenization method was employed to predict the mechanical response of a suite of open hole and unnotched IM7/977-3 composite laminates under static tension and compression. Damage accumulation, effective stiffness, and ultimate strength blind predictions are included in addition to the results of the recalibration study. In blind predictions, the proposed multiscale model produced predictions with an average error of 13.1% compared to the experiments for static ultimate strength and 13.6% for stiffness. After recalibration, the average prediction error was improved to 8.7% for static ultimate strength and 4.4% for stiffness. Details of the blind predictions and the recalibration are discussed.

Nonlocal Homogenization Model for Wave Dispersion and Attenuation in Elastic and Viscoelastic Periodic Layered Media

This manuscript presents a new nonlocal homogenization model for wave dispersion and attenuation in elastic and viscoelastic periodic layered media. Homogenization with multiple spatial scales based on asymptotic expansions of up to eighth order is employed to formulate the proposed nonlocal homogenization model. A momentum balance equation, nonlocal in both space and time, is formulated consistent with the gradient elasticity theory. A key contribution in this regard is that all model coefficients including high order length-scale parameters are derived directly from microstructural material properties and geometry. The capability of the proposed model in capturing the characteristics of wave propagation in heterogeneous media is demonstrated in multiphase elastic and viscoelastic materials. The nonlocal homogenization model is shown to accurately predict wave dispersion and attenuation within the acoustic regime for both elastic and viscoelastic layered composites.

Modeling compression-after-impact response of polymer matrix composites subjected to seawater aging

This manuscript presents a computational investigation of the compression-after-impact (CAI) response of polymer matrix composites subjected to seawater-induced environmental degradation. A multiscale computational model that accounts for seawater-induced property degradation in composite constituents is proposed to predict the CAI response of E-glass reinforced vinyl-ester composites. Predictions of the CAI response as a function of specimen saturation are validated against experimental observations. The investigations revealed that partially-saturated vinyl-ester matrix composites undergo a significant reduction in the CAI strength, part of which is recovered upon full matrix saturation. The proposed computational model captures the response characteristics of partially- and fully-saturated specimens.

Three-Dimensional Modeling of Short Fiber-Reinforced Composites with Extended Finite-Element Method

AbstractThis manuscript presents a modeling approach based on the extended finite-element method (XFEM) for modeling the mechanical behavior of three-dimensional short fiber composites including interface debonding. Short fibers are incorporated into the XFEM framework as deformable elastic two-dimensional rectangular planar inclusions. Enrichment functions account for both the presence of axial deformable fibers within the composite domain and the progressive debonding along the fiber matrix interfaces. A modeling strategy is provided that is particularly suitable for failure analysis of composites with high-aspect-ratio inclusions, in which direct numerical analysis is computationally intractable. The performance of the proposed XFEM model is numerically assessed by comparing model predictions to the direct finite-element method.

Modeling Random Short Nanofiber- and Microfiber-Reinforced Composites Using the Extended Finite-Element Method

AbstractThis manuscript presents the formulation and implementation of an extended finite-element method (XFEM) for random short fiber-reinforced composite materials. A new enrichment function is proposed to incorporate the effect of random fiber inclusions within the XFEM framework to eliminate the need of using finite-element meshes compliant with fiber inclusions. The motion of the fiber inclusions are modeled by constraining the deformation field along the domain of the fiber inclusions. Coupling the XFEM along with the new enrichment function and constraint equations formulate the elastic response of short fiber-reinforced composites. Numerical integration procedures are provided for accurate evaluation of the system response for fiber tips that lie on arbitrary positions within the problem domain. The performance of the proposed model is verified against the direct finite-element method.