Anthony Waas - Profile on Academia.edu (original) (raw)

Papers by Anthony Waas

Integrating materials and manufacturing innovation, Apr 15, 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 can take place during the curing process prior to the application of in-service mechanical loads. The analysis shows the differences in both the cured RVE strength and stiffness, when cure-induced damage has and has not been taken into account.

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Jan 2, 2015

Progressive damage and failure in open hole composite laminate coupons under tensile and compress... more Progressive damage and failure in open hole composite laminate coupons under tensile and compressive loading conditions is modeled using Enhanced Schapery Theory (EST). The input parameters required for EST are obtained using standard coupon level test data and are interpreted in conjunction with finite element (FE) based simulations. The capability of EST to perform the open hole strength prediction accurately is demonstrated using three different layups of IM7/8552 carbon fiber composite. A homogenized approach uses a single composite shell element to represent the entire laminate in the thickness direction and this requires the fiber direction fracture toughness to be modeled as a laminate property. The results obtained using the EST method agree quite well with experimental results.

54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Apr 5, 2013

The compressive response of 3D woven textile composites (3DWTC), that consist of glass fiber tows... more The compressive response of 3D woven textile composites (3DWTC), that consist of glass fiber tows and an epoxy matrix material, is studied using a finite element (FE) based micromechanics model. A parametric Representative Unit Cell (RUC) model is developed in a fully three-dimensional setting with geometry and textile architecture for modeling the textile microstructure. The RUC model also accoutns for the nonlinear behavior of the fiber tows and matrix. The computational model is utilized to predict the compressive strength of 3DWTC and its dependence on various geometrical and material parameters. The finite element model is coupled with a probabilistic analysis tool to provide probabilistic estimates for 3DWTC compressive strength.

47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 14th AIAA/ASME/AHS Adaptive Structures Conference 7th, May 1, 2006

A mechanism-based progressive failure analyses (PFA) approach is developed for fiber reinforced c... more A mechanism-based progressive failure analyses (PFA) approach is developed for fiber reinforced composite laminates. Each ply of the laminate is modeled as a nonlinear elastic degrading lamina in a state of plane stress according to Schapery theory (ST 1 ). In ST, the lamina degradation in the transverse directions, is characterized through laboratory scale experiments. In the fiber direction, elastic behavior prevails until fiber tensile failure (in tension) or the attainment of a limit load instability in compression. The phenomenon of fiber microbuckling, which is associated with the limit load instability, is explicitly accounted for by allowing the fiber rotation at a material point to be a variable in the problem. The latter is motivated by experimental and numerical simulations that show that local fiber rotations in conjunction with a continuously degrading matrix are responsible for the onset of fiber microbuckling leading to kink banding. These features are built into a user defined material subroutine that is implemented through the commercial finite element (FE) software ABAQUS. Thus, the present model disbands the notion of a fixed compressive strength of a lamina. Instead the mechanics of the failure process is used to provide the in-situ compression strength of a material point in a lamina, the latter being dictated strongly by the current local stress state, the current state of the lamina transverse material properties and the local fiber rotation. The inputs to the present work are laboratory scale, coupon level test data that provide information on the lamina transverse property degradation (i.e. appropriate, measured, strain-stress relations of the lamina transverse properties), the elastic lamina orthotropic properties and the geometry of the structural panel. The validity of the approach advocated is demonstrated through numerical simulations of the response of a composite structural panel that is loaded to complete failure. A centrally notched 90-ply unstiffened stitched panel subjected to axial compression is selected for study. The predictions of the simulations are compared against experimental data.

A novel computational model is introduced to analyze the effect of the curing process on the subs... more A novel computational model is introduced to analyze the effect of the curing process on the subsequent mechanical response of fiber reinforced composite structures. Evaluations are made at the microscale where representative volume elements (RVEs) are analyzed with periodic boundary conditions (PBCs) using the finite element (FE) method. The commercial software ABAQUS is used as a solver, supplemented by user written subroutines. The transition from a continuum to damage and failure is effected by using the Crack Band model which preserves mesh objectivity. Results are presented for a hexagonal 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. It is noted that throughout the analysis, the possibility of failure is accommodated, i.e. failure can take place during the cure cycle and prior to application of mechanical loads under appropriate external conditions.

53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA, Apr 23, 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.

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.

A novel, two-scale computational model has been developed to predict the progressive damage and f... more A novel, two-scale computational model has been developed to predict the progressive damage and failure responses of fiber-reinforced composite laminates using the material properties at the constituent (fiber and matrix) level. These properties were measured from coupon level tests on a unidirectional lamina of the same material system. In the proposed computational scheme, the macroscale finite element analysis (FEA) was carried out at the lamina level of a 3D laminate model, while micromechanical analysis was implemented concurrently at the subscale to compute the local fields at the fiber and matrix scale. Thus, the influence of matrix microdamage at the microscale manifests as the progressive degradation of the lamina stiffness, resulting in the nonlinear evolution of the stress versus strain response, while the lamina stiffness matrix remains positive-definite. The lamina post-peak strain softening response resulting from catastrophic failure modes including fiber tensile breakage, fiber kinking and matrix cracking, were modeled using the smeared crack approach (SCA). The interlaminar failure due to delamination was accounted for through cohesive elements inserted in-between the layers. The predictive capability of the proposed method is illustrated by comparing the computational results with experiment for three different lay-ups of IM-7/977-3 carbon fiber composite laminates subjected to various loading conditions, including both un-notched and open-hole specimens subjected to remote tensile and compressive loading, respectively.

54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Apr 5, 2013

The open hole tensile and compressive strengths are important design parameters in qualifying fib... more The open hole tensile and compressive strengths are important design parameters in qualifying fiber reinforced laminates for a wide variety of structural applications in the aerospace industry. In this paper, we present a unified model that can be used for predicting both these strengths (tensile and compressive) using the same set of coupon level, material property data. As a prelude to the unified computational model that follows, simplified approaches, referred to as "zeroth order", "first order", etc. with increasing levels of fidelity are first presented. The results and methods presented are practical and validated against experimental data. They serve as an introductory step in establishing a virtual building block, bottom-up approach to designing future airframe structures with composite materials. The results are useful for aerospace design engineers, particularly those that deal with airframe design.

55th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Jan 10, 2014

A systematic yet simple way to measure the fracture toughness value for a Mode I crack that occur... more A systematic yet simple way to measure the fracture toughness value for a Mode I crack that occurs perpendicular to the fiber direction in a unidirectional composite is presented. Cross-ply single edge notch tension (SENT) tests combined with finite element analysis areutilized to obtain the in-plane fracture toughness value under tension in the direction of fibers. The notch length of the SENT specimen is used as a parameter to back out the fracture toughness value in a consistent manner. Interlaminar fracture in polymer matrix composite (PMC) laminates, often called delamination, is defined as an out-of-plane discontinuity between two adjacent plies of a laminate. Delamination behavior has been studied by many researchers and now can be characterized in a standardized manner. Fracture properties of Mode I, Mode II, and mixed-mode (between Mode I and Mode II) delamination can be obtained from standard tests in conjunction with finite element analysis (FEA). However, a systematic approach to quantify and measure intralaminar fracture properties is not available in the literature. This presentation pertains to the development of a suitable test-analysis protocol to obtain such toughness values that can be used in virtual tests. Intralaminar cracks are defined as in-plane discontinuities that advance through the entire laminate thickness in the direction parallel to the fiber direction. Intralaminar fracture modes are typically characterized as three major intralaminar failure mechanisms, which are distinct from the microdamage mode. They are transverse (Mode I) matrix cracking, shear (Mode II) matrix cracking, and axial (Mode I) fiber fracture as shown in Figure . These fracture modes are consistent with the in-plane failure typically observed in PMC laminates. The interlaminar cracks, once initiated, can trigger other failure mechanisms such as delamination and/or act as delamination migration pathways between adjacent interfaces, leading to the further growth of the delamination. Presence of such damage can cause significant reduction of the overall performance of structural components and thus, an accurate measurement and quantification of the fracture modes are fundamental to the understanding of the onset, growth, and interaction of interlaminar and intralaminar cracks. However, this activity cannot occur in isolation of the subsequent modeling effort and use of finite-element based structural analysis. This presentation will describe a combined experimental and numerical study to provide a systematic and reliable way for measuring the intralaminar fracture properties. Especially, the fracture toughness value for the Mode I fiber failure will be obtained from single edge notch tension (SENT) tests combined with FEA simulation. It is assumed that the fracture toughness value for Mode I trasverse cracking is the same as the value for the interlaminar Mode I fracture and the vaue for shear Mode II matrix cracking is the same as the vaue for the interlaminar Mode II fracture. SENT specimens are designed to have different notch lengths with the same dimensions elsewhere in order to control the main in-plane fracture mode. The measured experimental force-displacement data, converted to a suitable stressstrain response will be compared to corresponding FEA simulations to back out the individual intralaminar fracture properties.

55th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Jan 10, 2014

Materials, Aug 12, 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/cracking in the matrix is made possible via the Bazant-Oh crack band model. The results suggest that the choice of unit cell size is more sensitive to strength and less sensitive to stiffness, when these properties are used as homogenized inputs to macro-scale models. Unit cells of smaller size exhibit higher strength and this strength converges to a plateau as the size of the unit cell increases. In this sense, since stiffness has also converged to a plateau with an increase in unit cell size, the converged unit cell size may be thought of as an RVE. Results in support of these insights are presented in this paper.

Implementation of a Smeared Crack Band Model in a Micromechanics Framework NASA/TM-2012-217603 Ju... more Implementation of a Smeared Crack Band Model in a Micromechanics Framework NASA/TM-2012-217603 June 2012 NASA STI Program . . . in Profi le Since its founding, NASA has been dedicated to the advancement of aeronautics and space science. The NASA Scientifi c and Technical Information (STI) program plays a key part in helping NASA maintain this important role. The NASA STI Program operates under the auspices of the Agency Chief Information Offi cer. It collects, organizes, provides for archiving, and disseminates NASA's STI. The NASA STI program provides access to the NASA Aeronautics and Space Database and its public interface, the NASA Technical Reports Server, thus providing one of the largest collections of aeronautical and space science STI in the world. Results are published in both non-NASA channels and by NASA in the NASA STI Report Series, which includes the following report types:

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Jan 9, 2012

A micromechanical model for 2-D triaxial braided composites is presented. The tow arrangement in ... more A micromechanical model for 2-D triaxial braided composites is presented. The tow arrangement in the representative unit cell is modeled by solving the packing problem using analytical optimization, avoiding the need to measure the geometric properties experimentally. These results are used in the development of a closed-form micromechanical model predicting the full 3-D stiffness matrix of the homogenized representative unit cell. The results show good agreement with experimental data; however, the true significance of this work is that it enables design optimization using 2-D triaxial braided composites because the need for experimental calibration for each set of design variables is eliminated.

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.

47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 14th AIAA/ASME/AHS Adaptive Structures Conference 7th, 2006

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

57th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2016

Progressive damage and failure in open hole composite laminate coupons subjected to flexural load... more Progressive damage and failure in open hole composite laminate coupons subjected to flexural loading is modeled using Enhanced Schapery Theory (EST). Previous studies have demonstrated that EST can accurately predict the strength of open hole coupons under remote tensile and compressive loading states. This homogenized modeling approach uses single composite shell elements to represent the entire laminate in the thickness direction and significantly reduces computational cost. Therefore, when delaminations are not of concern or are active in the pre-peak regime, the version of EST presented here is a good engineering tool for predicting deformation response. Standard coupon level tests provides all the input data needed for the model and they are interpreted in conjunction with finite element (FE) based simulations. Open hole bending test results of three different IM7/8552 carbon fiber composite layups agree well with EST predictions. The model is able to accurately capture the curvature change and deformation localization in the specimen at and during the post catastrophic load drop event.

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2015

Room-temperature tensile responses of oxide-oxide woven ceramic matrix composites (CMCs) have bee... more Room-temperature tensile responses of oxide-oxide woven ceramic matrix composites (CMCs) have been studied for two different lay-ups, including [0/90]s and [+45/0/-45/0]s. The CMC is made of aluminosilicate matrix reinforced by 3M Nextel TM 610 alumina fibers (AS/N610). The dry fiber preform consists of multiple layers of eight-harness satin weave (8HSW) fabric. The digital image correlation (DIC) technique was utilized to map the deformation histories and identify modes of failure for both un-notched and single-edgenotched tension tests. A lamina-level constitutive model was developed for a single 8HSW CMC ply. It is assumed that the pre-peak nonlinear response is only attributed to the in-plane shear. The post-peak strain softening responses were related to the tractionseparation laws through the smeared crack approach. Utilizing the ply properties measured from the un-notched [0/90]s and [+45/0/-45/0]s tension tests, the proposed computational model shows accurate predictions for the single-edge-notched tension tests.

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

The growth of transverse cracks in a composite layer is investigated using the semi-analytical hi... more The growth of transverse cracks in a composite layer is investigated using the semi-analytical high-fidelity generalized method of cells to represent a repeating unit cell of the composite. This method discretizes the domain into a number of subcells, each of which is occupied by a constituent of the composite. A non-linear displacement field approximation is used within each subcell, and displacement continuity and traction continuity are enforced, on average, at each subcell interface. Discontinuities are introduced at the subcell interfaces through the evolving compliance interface model. In this model traction and separation are related via a time dependent parameter. The predicted response for two simple examples of composite micro-architectures is presented and discussed.

Integrating materials and manufacturing innovation, Apr 15, 2015

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Jan 2, 2015

54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Apr 5, 2013

47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 14th AIAA/ASME/AHS Adaptive Structures Conference 7th, May 1, 2006

53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference 20th AIAA/ASME/AHS Adaptive Structures Conference 14th AIAA, Apr 23, 2012

54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Apr 5, 2013

55th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Jan 10, 2014

Materials, Aug 12, 2019

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Jan 9, 2012

47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference 14th AIAA/ASME/AHS Adaptive Structures Conference 7th, 2006

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

57th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2016

56th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2015

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