Anthony Waas | University of Washington (original) (raw)

Papers by Anthony Waas

48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007

43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2002

51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 18th AIAA/ASME/AHS Adaptive Structures Conference 12th, 2010

Extending our earlier work on Mode-I crack propagation presented in earlier SDM conferences, this... more Extending our earlier work on Mode-I crack propagation presented in earlier SDM conferences, this manuscript details the research work currently underway in our group to objectively simulate mixed-mode crack propagation in laminated fiber reinforced composite materials. The analytical framework and numerical implementation using the variational multi-scale cohesive method (VMCM) is discussed. Further, mixed-mode curved crack propagation simulations and their qualitative comparison with experimental observations is presented.

$Research paper thumbnail of In-plane fracture of laminated fiber reinforced composites with varying fracture resistance: Experimental observations and numerical crack propagation simulations$

International Journal of Solids and Structures, 2010

A series of experimental results on the in-plane fracture of a laminated composite panel is analy... more A series of experimental results on the in-plane fracture of a laminated composite panel is analyzed using the variational multi-scale cohesive method (VMCM). The VMCM results demonstrate the influence of specimen geometry and load distribution on the propagation of large scale bridging cracks in laminated fiber reinforced composites. Experimentally observed variation in fracture resistance is substantiated numerically by comparing the experimental and VMCM load-displacement responses of geometrically scaled single edgenotch three point bend (SETB) specimens. The results elucidate the size dependence of the traction-separation relationship for this class of materials even in moderately large specimens, contrary to the conventional understanding of it being a material property. The existence of a "free bridging zone" (different from the conventional "full bridging zone") is recognized, and its influence on the evolving fracture resistance is discussed. The numerical simulations and ensuing bridging zone evolution analysis demonstrates the versatility of VMCM in objectively simulating progressive crack propagation, compared against conventional numerical schemes like traditional cohesive zone modeling, which require a priori knowledge of the crack path.

Journal of Composite Materials, 2006

A discrete cohesive zone model (DCZM) is implemented to simulate the mode I fracture of two dimen... more A discrete cohesive zone model (DCZM) is implemented to simulate the mode I fracture of two dimensional triaxially braided carbon (2DTBC) fiber composites. The 2DTBC is modeled as an elastic-one-parameter (a66) plastic continuum. The plastic behavior of the 2DTBC was characterized by measuring a66. Mode I fracture tests are carried out by using a modified single edge notch bend (SENB) configuration. Fracture toughness (GIC) as a function of crack extension is measured by a compliance approach. The fracture tests are then simulated by using the DCZM based interface element in conjunction with the commercial software ABAQUS® through a user subroutine UEL. The simulated results, carried out under conditions of plane stress, are compared with the experimental results and also verified for mesh sensitivity. The results presented provide guidelines and a basic understanding to model structural response of non-homogeneous materials, incorporating fracture as a damage mechanism and using co...

49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 16th AIAA/ASME/AHS Adaptive Structures Conference 10t, 2008

$Research paper thumbnail of Investigation of progressive damage and fracture in laminated composites using the smeared crack approach$

53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA, 2012

Materials, 2019

Representative volume elements (RVEs) are commonly used to compute the effective elastic properti... more Representative volume elements (RVEs) are commonly used to compute the effective elastic properties of solid media having repeating microstructure, such as fiber reinforced composites. However, for softening materials, an RVE could be problematic due to localization of deformation. Here, we address the effects of unit cell size and fiber packing on the transverse tensile response of fiber reinforced composites in the context of integrated computational materials engineering (ICME). Finite element computations for unit cells at the microscale are performed for different sizes of unit cells with random fiber packing that preserve a fixed fiber volume fraction—these unit cells are loaded in the transverse direction under tension. Salient features of the response are analyzed to understand the effects of fiber packing and unit cell size on the details of crack path, overall strength and also the shape of the stress-strain response before failure. Provision for damage accumulation/cracki...

Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, Jan 13, 2016

This paper is concerned with predicting the progressive damage and failure of multi-layered hybri... more This paper is concerned with predicting the progressive damage and failure of multi-layered hybrid textile composites subjected to uniaxial tensile loading, using a novel two-scale computational mechanics framework. These composites include three-dimensional woven textile composites (3DWTCs) with glass, carbon and Kevlar fibre tows. Progressive damage and failure of 3DWTCs at different length scales are captured in the present model by using a macroscale finite-element (FE) analysis at the representative unit cell (RUC) level, while a closed-form micromechanics analysis is implemented simultaneously at the subscale level using material properties of the constituents (fibre and matrix) as input. The N-layers concentric cylinder (NCYL) model (Zhang and Waas 2014 Acta Mech. 225, 1391-1417; Patel et al. submitted Acta Mech.) to compute local stress, srain and displacement fields in the fibre and matrix is used at the subscale. The 2-CYL fibre-matrix concentric cylinder model is extended...

Integrating Materials and Manufacturing Innovation, 2015

The effect of the curing process on the mechanical response of fiber-reinforced polymer matrix co... more The effect of the curing process on the mechanical response of fiber-reinforced polymer matrix composites is studied using a computational model. Computations are performed using the finite element (FE) method at the microscale where representative volume elements (RVEs) are analyzed with periodic boundary conditions (PBCs). The commercially available finite element (FE) package ABAQUS is used as the solver, supplemented by user-written subroutines. The transition from a continuum to damage/failure is effected by using the Bažant-Oh crack band model, which preserves mesh objectivity. Results are presented for a hexagonally packed RVE whose matrix portion is first subjected to curing and subsequently to mechanical loading. The effect of the fiber packing randomness on the microstructure is analyzed by considering multi-fiber RVEs where fiber volume fraction is held constant but with random packing of fibers. The possibility of failure is accommodated throughout the analysis—failure c...

Integrating Materials and Manufacturing Innovation, 2015

A mesh-objective two-scale finite element approach for analyzing damage and failure of fiber-rein... more A mesh-objective two-scale finite element approach for analyzing damage and failure of fiber-reinforced ceramic matrix composites is presented here. The commercial finite element software suite Abaqus is used to generate macroscopic models, e.g., structural-level components or parts of ceramic matrix composites (CMCs), coupled with a second finite element code which pertains to the sub-scale at the fiber-matrix interface level, which is integrated seamlessly using user-generated subroutines and referred to as the integrated finite element method (IFEM). IFEM calculates the reaction of a microstructural sub-scale model that consists of a representative volume element (RVE) which includes all constituents of the actual material, e.g., fiber, matrix, and fiber/matrix interfaces, details of packing, and nonuniformities in properties. The energy-based crack band theory (CBT) is implemented within IFEM’s sub-scale constitutive laws to predict micro-cracking in all constituents included in...

55th AIAA/ASMe/ASCE/AHS/SC Structures, Structural Dynamics, and Materials Conference, 2014

The flexrual response of a layer-to-layer orthogonal interlocked 3D textile composite (3DTC) has ... more The flexrual response of a layer-to-layer orthogonal interlocked 3D textile composite (3DTC) has been investigated through quasi-static three-point bending. Fiber tow kinking on the compressive side of the flexed specimens has been found to be a strength limiting mechanism for both warp and weft panels. The digital image correlation technique has been utilized to map the deformation and identify matrix micro-cracking on the tensile side prior to the peak load in the warp direction loaded panels. It is shown that the geometrical characteristics of textile reinforcement play a key role on the mechanical response of this class of material. A mechanism based multiscale computational model that considers the influence of textile architecture has been developed to capture the observed damage and failure characteristics. The fiber tows and surrounding polymer matrix are modeled at the mesoscale, while the 3DTC is homogenized at the macroscale. The pre-peak nonlinear response of the fiber tow is modeled using a novel, two-scale model, in which the subscale micromechanical analysis is carried out in close form. The influence of the matrix microdamage at the microscale manifests as the progressive degradation of the fiber tow stiffness at the mesoscale. The post-peak strain softening responses of the fiber tows and the surrounding polymer matrix are modeled through the smeared crack approach (SCA). The load-deflection response, along with the observed damage events, including matrix cracking, tow kinking, and tow tensile breakage, are successfully captured through the proposed computational model. Therefore, the proposed multiscale model is suitable for progressive damage and failure analysis of 3DTCs and specifically to study the influence of textile architecture on macroscopic response.

48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2007

52nd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, 2011

Integrated Computational Engineering (ICE) is a valuable and cost effective resource for ensuring... more Integrated Computational Engineering (ICE) is a valuable and cost effective resource for ensuring structural integrity and damage tolerance of future aerospace vehicles that are made with laminated fiber reinforced composite laminates. Towards that end, the variational multiscale cohesive method (VMCM) reported by the authors in previous AIAA SDM conferences, 23-26 is extended further to address problems of mixed mode in-plane crack propagation in fiber reinforced laminates. A set of experimental results obtained using a single edge notch eccentric three point bend test is used for validating the VMCM predictions. Further the applicability of VMCM is demonstrated through simulation of mixed mode in-plane crack propagation for different specimen geometries and different loading conditions.

The compressive response of polymer matrix fiber reinforced unidirectional composites (PMC's) is ... more The compressive response of polymer matrix fiber reinforced unidirectional composites (PMC's) is investigated via a combination of experiment and analysis. The study accounts for the nonlinear constitutive response of the polymer matrix material and examines the effect of fiber geometric imperfections, fiber mechanical properties and fiber volume fraction on the measured compressive strength and compressive failure mechanism. Glass and carbon fiber reinforced unidirectional composite specimens are manufactured in-house with fiber volume fractions ranging over 10 ∼ 60 percent. Compression test results with these specimens show that carbon fiber composites have lower compressive strengths than glass fiber composites. Glass fiber composites demonstrate a splitting failure mode for a range of low fiber volume fractions and a simultaneous splitting/kink banding failure mode for high fiber volume fractions. Carbon fiber composites show kink banding throughout the range of fiber volume fractions examined. Nonlinear material properties of the matrix, orthotropic material properties of the carbon fiber, initial geometric fiber imperfections and nonuniform fiber volume fraction are all included in an appropriate finite element analysis to explain some of the observed experimental results. A new analytical model prediction of the splitting failure mode shows that this failure mode is favorable for glass fiber composites, which is in agreement with test results. Furthermore, this model is able to show the influence of fiber mechanical properties, fiber volume fraction and fiber geometry on the splitting failure mode.

50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2009

Initial development of a multiscale progressive damage and failure analysis tool for laminated co... more Initial development of a multiscale progressive damage and failure analysis tool for laminated composite structures is presented. The method models microdamage at the lamina level with the thermodynamically based Schapery Theory. Transverse cracking and ber breakage, considered failure mechanisms in this work, are modeled with failure criteria evaluate at the micro-constituent level using the Generalized Method of Cells. This model is implemented using ABAQUS/Explicit nite element software and MAC/GMC Suite of Micromechanics Codes. Load versus displacement and local strain gage results for two center-notched laminates are compared against results using ABAQUS/Standard and experimental data. Furthermore, damage and failure paths are compared to C-scans and photographs of failed specimens.

51st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 18th AIAA/ASME/AHS Adaptive Structures Conference 12th, 2010

A continuum-level, dual internal state variable, thermodynamically based, work potential model, S... more A continuum-level, dual internal state variable, thermodynamically based, work potential model, Schapery Theory, is used capture the effects of two matrix damage mechanisms in a fiber-reinforced laminated composite: microdamage and transverse cracking. Matrix microdamage accrues primarily in the form of shear microcracks between the fibers of the composite. Whereas, larger transverse matrix cracks typically span the thickness of a lamina and run parallel to the fibers. Schapery Theory uses the energy potential required to advance structural changes, associated with the damage mechanisms, to govern damage growth through a set of internal state variables. These state variables are used to quantify the stiffness degradation resulting from damage growth. The transverse and shear stiffness' of the lamina are related to the internal state variables through a set of measurable damage functions. Additionally, the damage variables for a given strain state can be calculated from a set of evolution equations. These evolution equations and damage functions are implemented into the finite element method and used to govern the constitutive response of the material points in the model. Additionally, an axial failure criterion is included in the model. The response of a center-notched, buffer strip-stiffened panel subjected to uniaxial tension is investigated and results are compared to experiment.

53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA, 2012

A thermodynamically-based work potential theory for modeling progressive damage and failure in fi... more A thermodynamically-based work potential theory for modeling progressive damage and failure in fiber-reinforced laminates is presented. The current, multiple-internal state variable (ISV) formulation, enhanced Schapery theory (EST), utilizes separate ISVs for modeling the effects of damage and failure. Damage is considered to be the effect of any structural changes in a material that manifest as pre-peak non-linearity in the stress versus strain response. Conversely, failure is taken to be the effect of the evolution of any mechanisms that results in post-peak strain softening. It is assumed that matrix microdamage is the dominant damage mechanism in continuous fiber-reinforced polymer matrix laminates, and its evolution is controlled with a single ISV. Three additional ISVs are introduced to account for failure due to mode I transverse cracking, mode II transverse cracking, and mode I axial failure. Typically, failure evolution (i.e., post-peak strain softening) results in pathologically mesh dependent solutions within a finite element method (FEM) setting. Therefore, consistent character element lengths are introduced into the formulation of the evolution of the three failure ISVs. Using the stationarity of the total work potential with respect to each ISV, a set of thermodynamically consistent evolution equations for the ISVs is derived. The theory is implemented into commercial FEM software. Objectivity of total energy dissipated during the failure process, with regards to refinements in the FEM mesh, is demonstrated. The model is also verified against experimental results from two laminated, T800/3900-2 panels containing a central notch and different fiber-orientation stacking sequences. Global load versus displacement, global load versus local strain gage data, and macroscopic failure paths obtained from the models are compared to the experiments.

54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2013

A mesh objective crack band model was implemented within the generalized method of cells micromec... more A mesh objective crack band model was implemented within the generalized method of cells micromechanics theory. This model was linked to a macroscale finite element model to predict post-peak strain softening in composite materials. Although a mesh objective theory was implemented at the microscale, it does not preclude pathological mesh dependence at the macroscale. To ensure mesh objectivity at both scales, the energy density and the energy release rate must be preserved identically across the two scales. This requires a consistent characteristic length or localization limiter. The effects of scaling (or not scaling) the dimensions of the microscale repeating unit cell (RUC), according to the macroscale element size, in a multiscale analysis was investigated using two examples. Additionally, the ramifications of the macroscale element shape, compared to the RUC, was studied.

53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA, 2012

The smeared crack band theory is implemented within the generalized method of cells and high-fide... more The smeared crack band theory is implemented within the generalized method of cells and high-fidelity generalized method of cells micromechanics models to capture progressive failure within the constituents of a composite material while retaining objectivity with respect to the size of the discretization elements used in the model. An repeating unit cell containing 13 randomly arranged fibers is modeled and subjected to a combination of transverse tension/compression and transverse shear loading. The implementation is verified against experimental data (where available), and an equivalent finite element model utilizing the same implementation of the crack band theory. To evaluate the performance of the crack band theory within a repeating unit cell that is more amenable to a multiscale implementation, a single fiber is modeled with generalized method of cells and high-fidelity generalized method of cells using a relatively coarse subcell mesh which is subjected to the same loading scenarios as the multiple fiber repeating unit cell. The generalized method of cells and high-fidelity generalized method of cells models are validated against a very refined finite element model.