Kuang C. Liu

Efficient Multiscale Modeling Framework for Triaxially Braided Composites using Generalized Method of Cells

In this paper, a framework for a three-scale analysis, beginning at the constituent response and propagating to the braid repeating unit cell (RUC) level, is presented. At each scale in the analysis, the response of the appropriate RUC is represented by homogenized effective properties determined from the generalized method of cells micromechanics theory. Two different macroscale RUC architectures are considered, one for eventual finite-element implementation and the other for material design, and their differences are compared. Model validation is presented through comparison to both experimental data and detailed finite-element simulations. Results show good correlation within range of experimental scatter and the finite-element simulation. Results are also presented for parametric studies varying both the overall fiber volume fraction and braid angle. These studies are compared to predictions from classical lamination theory for reference. Finally, the multiscale analysis framework is used to predict t...

Fatigue Life Prediction Using Hybrid Prognosis for Structural Health Monitoring

Because metallic aircraft components are subject to a variety of in-service loading conditions, predicting their fatigue life has become a critical challenge. To address the failure mode mitigation of aircraft components and at the same time reduce the life-cycle costs of aerospace systems, a reliable prognostics framework is essential. In this paper, a hybrid prognosis model that accurately predicts the crack growth regime and the residual-useful-life estimate of aluminum components is developed. The methodology integrates physics-based modeling with a data-driven approach. Different types of loading conditions such as constant amplitude, random, and overload are investigated. The developed methodology is validated on an Al 2024-T351 lug joint under fatigue loading conditions. The results indicate that fusing the measured data and physics-based models improves the accuracy of prediction compared to a purely data-driven or physics-based approach.

Characterization of impact damage in woven fiber composites using fiber Bragg grating sensing and NDE

Woven fiber composites are currently being investigated due to their advantages over other materials, making them suitable for low weight, high stiffness, and high interlaminar fracture toughness applications such as missiles, body armor, satellites, and many other aerospace applications. Damage characterization of woven fabrics is a complex task due to their tendency to exhibit different failure modes based on the weave configuration, orientation, ply stacking and other variables. A multiscale model is necessary to accurately predict progressive damage. The present research is an experimental study on damage characterization of three different woven fiber laminates under low energy impact using Fiber Bragg Grating (FBG) sensors and flash thermography. A correlation between the measured strain from FBG sensors and the damaged area obtained from flash thermography imaging has been developed. It was observed that the peak strain in the fabrics were strongly dependent on the weave geometry and decreased at different rates as damage area increased due to dissimilar failure modes. Experimental observations were validated with the development of a multiscale model. A FBG sensor placement model was developed which showed that FBG sensor location and orientation plays a key role in the sensing capabilities of strain on the samples.

Guided Wave Based Fatigue Crack Detection and Localization in Aluminum Aerospace Structures

The work presented in this paper provides an insight into the current challenges to detect incipient damage in complex metallic structural components. The goal of this research is to improve the confidence level in diagnosis and damage localization technologies by developing a robust structural health management (SHM) framework. Improved methodologies are developed for reference-free localization of fatigue induced cracks in complex metallic structures. The methodologies for damage interrogation involve damage feature extraction using advanced signal processing tools and a probabilistic approach for damage detection and localization. Specifically, piezoelectric transducers are used in pitch-catch mode to interrogate the structure with guided Lamb waves. A novel time-frequency (TF) based signal processing technique based on the matching pursuit decomposition (MPD) algorithm is developed to extract time-of-flight damage features from dispersive guided wave sensor signals, followed by a Bayesian probabilistic approach used to optimally fuse multi-sensor information and localize the crack tip. The MPD algorithm decomposes a signal using localized TF atoms and can provide a highly concentrated TF representation. The Bayesian probabilistic framework enables the effective quantification and management of uncertainty. Experiments are conducted to validate the proposed detection and localization methods. Results presented will illustrate the usefulness of the developed approaches in detection and localization of damage in aluminum lug joints.Copyright

Low Speed Projectile Impact Damage Prediction and Propagation in Woven Composites

This paper presents a novel multiscale framework to analyze low speed impact damage in woven fiber composites. The three length scales considered were micro, meso and macro. At the microscale, the individual constituents of the yarn were modeled using the Generalized Method of Cells to predict the elastic properties and strengths. At the mesoscale, a double homogenization technique was employed to determine the strengths and elastic properties as a function of the microscale results. At the macroscale, an explicit finite element model developed to investigate the response of various weaves under impact loading. The developed procedure has been used to obtain the strength and elastic properties of various woven fiber composites.

Mechanical properties and damage characterization of triaxial braided composites in environmental conditions

Under environmental conditions, triaxial braided composites exhibit complex behavior and damage mechanisms. This paper investigates the damage mechanisms of these complex composites under varying environmental conditions. Tensile, compressive, and shear specimens of triaxial braided composite material were tested at room, hot (100℃), and hot/wet conditions (60℃/90% relative humidity). The strain field was studied using a digital image correlation system and the effect that the specimens’ edges have on the strain field was quantified. For the tension specimens, the environmental conditions caused reductions in the elastic and failure properties, whereas the compression specimens exhibited degradation exclusively in the failure properties. An increase in temperature rather than humidity was found to be a driving factor for the degradation of the mechanical properties. A non-destructive, flash thermography technique was used to characterize surface/subsurface damage in the specimens. Scanning electron microscopy was conducted to determine the microstructural modes of failure.

A Comparison of Triaxially Braided Composites Using the Generalized Method of Cells

Two distinct triaxial braid architectures are compared in this numerical study: (1) the “traditional” triaxial braid in which the axial tows are merely laid in between the woven biased (±θ°) tows and (2) the “true” triaxial braid in which the axial tows are interleaved through the biased tows. The microstructure of the triaxial braids is constrained as a function of volume fraction, braid angle, and tow geometries. A multiscale modeling methodology is developed to use the Generalized Method of Cells (GMC) micromechanics model recursively over multiple length scales in a three step homogenization process to compare the effective elastic properties of the two different types of triaxial braids. This methodology has previously been used to accurately predict experimental results for the “traditional” triaxial braid architecture; however, this paper simply compares the two types of braid architectures numerically. Preliminary results show that the “true” triaxial braid follows most of the trends of the “traditional” triaxial braid in effective stiffness properties as a function of braid angle, except for the axial modulus at higher braid angles. At braid angles in excess of 45°, the “true” triaxial braid shows an increase in axial stiffness. The axial modulus of the “true” triaxial braid exceeds that of the “traditional” triaxial braid for braid angles greater than 50°. Therefore, the “true” triaxial braid does present a viable alternate braid that may offer advantages over the “traditional” triaxial braid for certain applications. An experimental study is being prepared to compare the two triaxial braids experimentally and to validate the numerical model used in the present study.

Micromechanics model to link microstructural variability to fiber reinforced composite behavior

The microstructural variation in fiber-reinforced composites has a direct relationship with its local and global mechanical performance. When micromechanical modeling techniques for unidirectional composites assume a uniform and periodic arrangement of fibers, the bounds and validity of this assumption must be quantified. The goal of this research is to quantify the influence of microstructural randomness on effective homogeneous response and local inelastic behavior. The results indicate that microstructural progression from ordered to disordered decreases the tensile modulus by 5%, increases the shear modulus by 10%, and substantially increases the magnitude of local inelastic fields. The analyses presented in this paper show the importance of microstructural variability when small length scale phenomena drive global response.

Strain Rate Dependent Multiscale Modeling of Woven Composites

This paper presents the constitutive modeling of the viscoplastic response of woven fabric composites. The constitutive model is determined through use of the Multiscale Generalized Method of Cells and considers three length scales in the analysis. The micro, meso, and macroscales are analyzed and the constituent constitutive model, fiber/tow repeating unit cell, and woven repeating unit cell are modeled at each length scale respectively. Results are presented for both plain and five harness satin weaves the in-plane tensile, compressive, and shear responses at various strain rates. Results show good correlation with available experimental data. I. Introduction EXTILE and braided composites are both architecturally and mechanically complex composite materials. In the aerospace industry, there are several mainstream weaves and braids. Textile and braided composites differ from traditional laminated composites, in that each lamina contains fibers in more than one direction, achieved through weaving or braiding. This produces desirable effects, such as reduce propensity for delamination, thicker lamina, quasi-isotropic fabrics. However, often maximum volume fraction and subsequently strength are sacrificed. In contrast to traditional unidirectional laminated composites, textile and braided composites often have varying orientations due to undulation, warp, weft, and braid tows. Damage mechanisms in these composites contain all those of traditional laminated composites plus additional modes that arise due to their geometric features. Typically the relevant physical mechanisms that need to be considered, in no particular order, are matrix nonlinearity, matrix and fiber failure, tow splitting/first matrix cracking, fiber/matrix debonding, tow/matrix debonding, and fiber buckling/kinking. From this list, it is apparent that these mechanisms occur at various length scales occurring on the order of 1cm. Development of a high fidelity model should contain the most relevant damage mechanisms and formulation of a multiscale modeling is the most practical method to implement those although various techniques have been developed.

Strain rate dependent inelastic response of triaxially braided fabric composites via multiscale generalized method of cells

This paper presents the constitutive modeling of the viscoplastic response of triaxially braided composites. The constitutive model is determined through use of the Multiscale Generalized Method of Cells and considers three length scales in the analysis. The micro, meso, and macroscales are analyzed and the constituent constitutive model, fiber/tow repeating unit cell, and triaxial braid repeating unit cell are modeling at each length scale respectively. Results are presented for the in-plane tensile, compressive, and shear responses at various strain rates. Results show good correlation with available experimental data.