Finite-Element Model for Failure Study of Two-Dimensional Triaxially Braided Composite

A new three-dimensional finite-element model of two-dimensional, triaxially braided composites is presented in this paper. This mesoscale modeling technique is used to examine and predict the deformation and damage observed in tests of straight-sided specimens. A unit cell-based approach is used to consider the braiding architecture and the mechanical properties of the fiber tows, the matrix, and the fiber tow-matrix interface. A 0°/±60° braiding configuration has been investigated by conducting static finite-element analyses. Failure initiation and progressive degradation has been simulated in the fiber tows by using the Hashin failure criteria and a damage evolution law. The fiber tow-matrix interface was modeled by using a cohesive zone approach to capture any fiber-matrix debonding. By comparing the analytical results with those obtained experimentally, the applicability of the developed model was assessed and the failure process was investigated.     

Rate-Dependent Shell Element Composite Material Model Implementation in LS-DYNA

A previously developed constitutive model has been modified in order to incorporate the rate dependence of elastic modulus of the polymer matrix constituent into the nonlinear, strain-rate-dependent deformation analysis of polymer matrix composites. To compute the inelastic strains in the polymer matrix, state-variable-based viscoplastic equations originally developed for metals are modified in order to account for the effects of hydrostatic stresses, which are significant in polymers. The polymer constitutive equations are implemented within the strength of a material-based micromechanics method in order to predict the nonlinear, strain-rate-dependent deformation of the polymer matrix composite. The polymer and the composite models are implemented into a commercially available explicit finite-element code, LS-DYNA, as user defined materials (UMATs). The deformation behaviors of several representative polymers and two polymer matrix composites of various fiber configurations are simulated in LS-DYNA with the UMATs for a wide range of strain rates, and the numerical results agree well with the experimental data. UMAT is applied for simulations of braiding/weaving composites using the modified through-thickness integration points method.     

FE Modeling of FRP-Repaired Planar Concrete Elements Subjected to Monotonic and Cyclic Loading

In this paper, a nonlinear finite-element model is developed for the analysis of plane stress members, such as RC beams and walls, strengthened either unidirectionally or bidirectionally with fiber-reinforced polymer (FRP) composites and subjected to either monotonic or cyclic loading. The model takes into account the effects of the bonded interface between the FRP and concrete while allowing slippage in each direction. A two-dimensional membrane contact element is developed to model the effects of local bond-slip with debonding failure between the FRP and concrete capable of being captured. The model has been incorporated into a finite-element program for the analysis of RC members subject to plane stress with verification against test data of FRP-strengthened RC joints, beams, and walls. The numerical results show good agreement with the experimental data for both load-displacement responses and for the overall failure mechanisms.     

Effects of the constraint parameter triaxial stress on the failure behaviour of steels

The constraint parameter triaxial stress which results from loading types and component geometries plays an important role in the failure behaviour of materials. Especially the local failure initiation of materials depends very strongly on triaxial stress. The failure behaviour of a material under different stress states (i.e. different constraint) differs. In this paper effects of triaxial stress on the failure behaviour of steels are presented by means of theoretical analysis, experiments and finite-element-calculations. A theoretical model describing the effects of triaxial stress on failure initiation of materials has been developed based on theoretical analysis. Corresponding experiments were conducted to determine the material constants in the failure model of materials. In order to obtain local damage parameters finite-element calculations have been conducted. According to the results from theoretical analysis, experiments and finite-element calculations the damage curves of materials, in which the critical equivalent plastic strain is plotted as a function of triaxial stress, have been obtained. Furthermore, the failure behaviour of materials under different triaxial stresses has been discussed.     

Influence of Fiber Undulations on Buckling of Thin Filament-Wound Cylinders in Axial Compression

The filament-winding process introduces inherent geometric defects into thin-shell cylinders in the form of fiber-bundle, or tow, crossovers. This research identified the micromechanical geometry in the fiber crossover regions of filament-wound cylinders. A stiffness model was developed for the crossover regions based on a modified classical lamination theory. The local stiffness-coupling values predicted by this model were incorporated into a global finite-element model of thin-shell, filament-wound cylinders. Eigenvalue buckling analyses performed using this enhanced finite-element model are compared to eigenvalue analyses performed without incorporating the influence of the stiffness couplings at the fiber crossover regions. Experiments were carried out in which 16 filament-wound cylinders with four different surface patterns were fabricated and tested. The results showed that the accuracy of a finite-element analysis improved significantly when the stiffness coupling effects due to fiber undulations were properly accounted for in the analytical model.     

Endochronic-Based Approach to the Failure of the Lower San Fernando Dam in 1971

The Lower San Fernando dam failure, which took place in 1971, is one of the most reported cases of seismic liquefaction damage in the geotechnical literature. For this reason, it has been analyzed by almost all of the numerical models developed since that year. In this paper, a comparison between numerical simulations, using a new endochronic model, with measured response of this dam during the earthquake of 1971, as well as numerical results previously obtained by other researchers, is presented. The main particularity of this new constitutive law is a nonassociative flow rule, related to a parameter quantifying degradation with shaking duration, in terms of stiffness reduction. It is incorporated in the model to represent soil dilation. Furthermore, contractive, dilative, and collapse trends of soil behavior are jointly embodied into the new developed constitutive law, which has been implemented in a two-dimensional coupled finite-element model. By so doing, the failure mechanism and the critical locations of the dam are identified and compared with field observations, and the approximate time for the beginning of the upstream sand fill slide is determined.     

Analysis of secondary oxide-scale failure at entry into the roll gap

Both numerical analysis based on finite-element (FE) modeling and experimental evidence concerning the secondary oxide-scale failure at entry into the roll gap are presented and reviewed for a better understanding of events at the roll-workpiece interface, in turn, leading to better definition of the boundary conditions for process models. Attention is paid to the two limit modes leading to oxide-scale failure, which were observed earlier during tensile testing under rolling conditions. These are considered in relation to the temperature, the oxide-scale thickness, and other hot-rolling parameters. The mathematical model used for the analysis is composed of macro and micro parts, which allow for simulation of metal/scale flow, heat transfer, cracking of the oxide scale, as well as sliding along the oxide/metal interface and spallation of the scale from the metal surface. The different modes of oxide-scale failure were predicted, taking into account stress-directed diffusion, fracture and adhesion of the oxide scale, strain, strain rate, and temperature. Stalled hot-rolling tests under controlled conditions have been used to verify the types of oxide-scale failure and have shown good predictive capabilities of the model. The stock temperature and the oxide-scale thickness are important parameters, which, depending on other rolling conditions, may cause either through-thickness cracking of the scale at the entry or lead to entry of a nonfractured scale when the scale/metal interface is not strong enough to transmit the metal deformation.     

Creep of Polymer Matrix Composites. II: Monkman-Grant Failure Relationship for Transverse Isotropy

This research concerns polymer matrix composite (PMC) materials having long or continuous reinforcement fibers embedded in a polymer matrix. The objective is to develop comparatively simple, designer friendly constitutive equations intended to serve as the basis of a structural design methodology for this class of PMC. Here (Part II), the focus is on extending the damage/failure model of an anisotropic deformation/damage theory presented earlier. A companion paper (Part I) by the writers deals with creep deformation of the same class of PMC. The extension of the damage model leads to a generalization of the well known Monkman/Grant relationship to transverse isotropy. The usefulness of this relationship is that it permits estimates of (long term) creep rupture life on (short term) estimates of creep deformation rate. The current extension also allows estimates of failure time for various fiber orientations. Supporting exploratory experiments are defined and conducted on thin-walled specimens fabricated from a model PMC. A primary assumption in the damage model is that the stress dependence of damage evolution is on the transverse tensile and longitudinal shear traction acting at the fiber/matrix interface. We conjecture that a supplemental mechanism of failure is the extensional strain in the fiber itself. The two postulated mechanisms used in conjunction suggest that an optimal fiber angle may exist in this class of PMC, maximizing the time to creep failure.     

Testing and Modeling of Soil-Structure Interface

An accurate modeling of soil-structure interfaces is very important in order to obtain realistic solutions of many soil-structure interaction problems. To study the mechanical characteristics of soil-structure interface, a series of direct shear tests were performed. A charged-coupled-device camera was used to observe the sand particle movements near the interface. It is shown that two different failure modes exist during interface shearing. Elastic perfect-plastic failure mode occurs along the smooth interface, while strain localization occurs in a rough interface accompanied with strong strain-softening and bulk dilatancy. To describe the behavior of the rough interface, this paper proposes a damage constitutive model with ten parameters. The parameters are identified using data from laboratory interface shear tests. The proposed model is capable of capturing most of the important characteristics of interface behavior, such as hardening, softening, and dilative response. The interface behaviors under direct and simple shear tests have been well predicted by the model. Furthermore, the present model has been implemented in a finite element procedure correctly and calculation results are satisfactory.     

Computational Constitutive Model for Predicting Nonlinear Viscoelastic Damage and Fracture Failure of Asphalt Concrete Mixtures

A computational constitutive model was developed to predict damage and fracture failure of asphalt concrete mixtures. Complex heterogeneity and inelastic mechanical behavior are addressed by the model by using finite-element methods and elastic–viscoelastic constitutive relations. Damage evolution due to progressive cracking is represented by randomly oriented interface fracture, which is governed by a newly developed nonlinear viscoelastic cohesive zone model. Computational simulations demonstrate that damage evolution and failure of asphalt concrete mixtures is dependent on the mechanical properties of the mixture. This approach is suitable for the relative evaluation of asphalt concrete mixtures by simply employing material properties and fracture properties of mixture components rather than by performing expensive laboratory tests recursively, which are typically required for continuum damage mechanics modeling.     

Poro-Damage Approach Applied to Hydro-Fracture Analysis of Concrete

An approach using mechanics of saturated porous media is presented to model strongly coupled hydromechanical effects in concrete. Fracture mechanisms of the matrix are taken into account by introducing a tensorial damage variable, which makes it possible to describe orthotropic damage states as well as their effects on hydromechanical parameters (permeability and Biot tensor). An experimental procedure, allowing simultaneous control of pore pressure and applied stresses in a concrete specimen, leads to the identification of material parameters introduced in the constitute model. This model is implemented in the finite-element code CASTEM 2000; numerical simulations of a hydraulic fracture test are then performed and show that the damage-dependence of hydraulic parameters has significant influence on the global response of the structure.     

Methodology for Impact Modeling of Triaxial Braided Composites Using Shell Elements

In this paper, a two-dimensional triaxial braided composite model has been studied using the nonlinear explicit finite-element code LSDYNA. The unit cell consists of six subcells and material properties associated with shell element integration point simulate braiding architecture. The local material properties were selected by correlation of the global behavior of a coupon model with static specimen tests. By changing subcell size and orientation angle at integration points, different braids architectures were obtained. Panel ballistic models were performed with benefits of computation efficiency of shell elements. Mechanical properties, panel impact threshold velocities, and failure initiations for braids with bias angles of 75, 60, 45, and 30° were studied. Boundary effects were also investigated.     

Micromechanical modeling of unidirectional continuous sigma fiber-reinforced Ti-6Al-4V subjected to transverse tensile loading

A micromodeling analysis of unidirectionally reinforced Ti-6-4/SM1140+ composites subjected to transverse tensile loading has been performed using the finite-element method (FEM). The composite is assumed to the infinite and regular, with either hexagonal or rectangular arrays of fibers in an elastic-plastic matrix. Unit cells of these arrays are applied in this modeling analysis. Factors affecting transverse properties of the composites, such as thermal residual stresses caused by cooling from the composite processing temperature, fiber-matrix interface conditions, fiber volume fraction, fiber spacing, fiber packing, and test temperature are discussed. Predictions of stress-strain curves are compared with experimental results. A hexagonal fiber-packing model with a weak fiber-matrix interfacial strength predicts the transverse tensile behavior of the composite Ti-6-4/SM1140+ most accurately.     

Shakedown Approaches to Rut Depth Prediction in Low-Volume Roads

Rutting, due to permanent deformations of unbound materials, is one of the principal damage modes of low traffic pavements. Flexible pavement design methods remain empirical; they do not take into account the inelastic behavior of pavement materials and do not predict the rutting under cyclic loading. A finite-element program, based on the concept of the shakedown theory developed by Zarka for metallic structures under cyclic loadings, has been used to estimate the permanent deformations of unbound granular materials subjected to traffic loading. Based on repeated load triaxial tests, a general procedure has been developed for the determination of the material parameters of the constitutive model. Finally, the results of a finite-element modeling of the long-term behavior of a flexible pavement with the simplified method are presented and compared to the results of a full-scale flexible pavement experiment performed by Laboratoire Central des Ponts et Chaussées. Finally, the calculation of the rut depth evolution with time is carried out.     

Damage Localization and Finite-Element Model Updating Using Multiresponse NDT Data

New techniques for both finite-element model updating and damage localization are presented using multiresponse nondestructive test (NDT) data. A new protocol for combining multiple parameter estimation algorithms for model updating is presented along with an illustrative example. This approach allows for the simultaneous use of both static and modal NDT data to perform model updating at the element level. A new damage index based on multiresponse NDT data is presented for damage localization of structures. This index is based on static and modal strain energy changes in a structure as a result of damage. This method depicts changes in physical properties of each structural element compared to its initial state using NDT data. Deficient or potentially damaged structural elements are then selected as the unknown parameters to be updated by parameter estimation. Error function normalization, error function stacking, and multiresponse parameter estimation methods are proposed for using multiple data types for simultaneous stiffness and mass parameter estimation. Also, multiple sets of measurements with various sizes and missing data points can be utilized. This paper uses a laboratory grid model of a bridge deck built at the University of Cincinnati Infrastructure Institute and the corresponding NDT data for validation of the above damage localization and model updating methods. Multiresponse parameter estimation has been utilized to update the stiffness of bearing pads, and both the stiffness and mass of the connections, using static and dynamic NDT data. The static and modal responses of the updated grid model presented a closer match with the NDT data than the responses from the initial model.     

Simplified Braiding through Integration Points Model for Triaxially Braided Composites

A simplified methodology has been developed for modeling two-dimensional triaxially braided composite plates impacted by a soft projectile using an explicit nonlinear finite-element analysis code LS-DYNA. The fiber preform architecture is modeled using shell elements by incorporating the fiber preform architecture at the level of integration points. The soft projectile was modeled by an equation of state. An arbitrary Lagrangian–Eulerian formulation is used to resolve numerical problems caused by large deformation of the projectile. The computed results indicate that this numerical model is able to simulate a triaxially braided composite undergoing a ballistic impact effectively and accurately, including the deformation and failure with a reasonable level of computational efficiency.     

Fiber damage during the consolidation of PVD Ti6Al4V coated nextel 610TM alumina fibers

Titanium matrix composites reinforced with sol-gel synthesized α-alumina fiber tows have attracted interest as a potentially low cost continuous fiber reinforced metal matrix composite system. We have conducted a detailed investigation of fiber damage during high temperature consolidation of PVD Ti6Al4V metallized sol-gel alumina fiber tows. Using both hot isostatic pressing and interrupted vacuum hot press consolidation cycles, the two principal mechanisms of fiber damage have been experimentally identified to be microbending/fracture and fiber matrix reaction. A time dependent micromechanics model incorporating the evolving geometry and mechanical properties of both the fibers and matrix has been formulated to simulate the fiber bending/failure mechanism in a representative unit cell and explore the effect of fiber strength loss due to reaction with the matrix. This model has been used to design a process cycle that minimizes damage by exploiting the enhanced superplastic deformation of the initially nanocrystalline PVD Ti6Al4V matrix.     

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.     

A theoretical and experimental study of continuous-casting tundishes focusing on slag-steel interaction

A model of a tundish has been developed that takes into account the steel, slag, and refractory phases. Predicted temperatures and velocities in the steel and refractory from the model were earlier found to agree well with measured velocities and temperatures. The model was also used to determine the optimal location of flow devices, making the temperature distribution in the steel more even and enhancing the removal of inclusions to the slag. In this study, the focus was on using the model to study the slag/steel interface in the tundish. Predictions showed that slag is dispersed into the steel close to the interface as well as close to the ladle shroud. In order to confirm these predictions, the momentary interfacial solidification sampling (MISS) method was developed. Using this method, a sample of the steel/slag interface could be taken that represented almost an instantaneous picture of the interface. The MISS sampler was used for sampling low-carbon steel in the tundish. Samples were analyzed using ultrasonic testing, optical microscopy, and scanning electron microscopy (SEM). Analysis results confirmed the presence of nonmetallic particles close to the slag/steel interface and close to the ladle shroud, as suggested by the modeling results. The analyses also showed that the slag/steel interface is very irregular, despite the low velocities.     

Characterization of Damage in Triaxial Braided Composites under Tensile Loading

Carbon fiber composites that utilize flattened, large tow yarns in woven or braided forms are being used in many aerospace applications. The complex fiber architecture and large unit cell size in these materials present challenges for both understanding the deformation process and measuring reliable material properties. In this paper composites made using flattened 12k and 24k (referring to the number of fibers in the fiber tow) standard modulus carbon fiber yarns in a 0°/+60°/?60° triaxial braided architecture are examined. Standard straight-sided tensile coupons were tested with the 0° axial braid fibers either parallel to (axial tensile test) or perpendicular to (transverse tensile test) the applied tensile load. The nonuniform surface strain resulting from the triaxial braided architecture was examined using photogrammetry. Local regions of high strain concentration were examined to identify where failure initiates and to determine the local strain at the time of failure initiation. Splitting within fiber bundles was the first failure mode observed at low to intermediate strains. For axial tensile tests the splitting was primarily in the ±60° bias fibers, which were oriented 60° to the applied load. At higher strains in the axial tensile test, out-of-plane deformation associated with localized delamination between fiber bundles or damage within fiber bundles was observed. For transverse tensile tests, the splitting was primarily in the 0° axial fibers, which were oriented transverse to the applied load. The initiation and accumulation of local damage caused the global transverse stress-strain curves to become nonlinear and caused failure to occur at a reduced ultimate strain for both the axial and transverse tensile tests. Extensive delamination at the specimen edges was also observed. Modifications to the standard straight-sided coupon geometry are needed to minimize these edge effects when testing the large unit cell type of material examined in this work.