Delamination is currently a largely undetectable form of damage in composite laminate materials. This thesis will develop a method to more easily detect delamination damage within composite materials. Using finite element analysis modeling and lab testing, a new method from interpreting the results obtained from existing structural health monitoring techniques is developed. Lamb waves were introduced and recorded through an actuator and sensors made of piezoelectric material. The data was then analyzed through a novel data reduction method using the Fast Fourier Transform (FFT). Using the data from FFT, the idea of covariance of energy change was developed. By comparing the covariance of energy change in beams with differing delamination size, thickness and depth, correlations were able to be developed. With these correlations, the severity and of damage was able to be detected.

Delamination at the free edge is analyzed as a mode of failure for uniaxial compression of postbuckled structural components. Analyses are performed for I-section stiffeners and for a dropped-ply laminate, all of which failed due to free edge delamination in earlier experimental studies. These specimens were made of AS4/3502 graphite-epoxy unidirectional tape. The nonlinear response of the specimens is modeled by a geometrically nonlinear finite element analysis. A variable-complexity modeling scheme is developed to economize computer resources for the nonlinear analysis. The three-dimensional stress field near the free edge is investigated by employing 20-node solid elements in that region. A dimensionless delamination index is calculated to investigate the severity of the interlaminar stress state. Numerical results correlated reasonably well with the experiments on load-end shortening plots. The nodallines and/or inflection points of the out-of-plane deflection along the free edge in the postbuckled configuration are found to be delamination critical sites. Both interlaminar shear stress tangent to the free edge and tensile interlaminar normal stress are significant at these critical locations and are likely to initiate delamination. / Master of Science

Free vibration, buckling and postbuckling analyses of laminated composite plates with multiple delaminations are presented. A fInite element method based on a layer-wise laminated composite plate theory is developed to formulate the problem. Geometric nonlinearity in the sense of von Karman and the imperfection in the plate in the form of initial global deflection and initial delamination openings are included. A simple contact algorithm which precludes the physically inadmissible overlapping between delaminated surfaces is proposed and incorporated in the analysis. A sublaminate concept is adopted in the analysis to reduce the computational efforts, and found to be efficient. Numerical results are obtained for through-the-width, circular and rectangular delaminations addressing the effects of the number of delaminations, their lengths and through-the-thickness and axial locations on the critical buckling load and buckling mode shapes as well as free vibration frequency and modes. Postbuckling responses are investigated with respect to different magnitudes and directions of initial imperfections. The effects of material anisotropy and contact condition between delaminated surfaces are also considered. It is found that the proposed approach is very efficient and powerful for solving the above mentioned problems. / Ph. D.

Multilayered materials and structures are of special interest to both academic researchers and industrial engineers as they have been used in an increasing number of applications, such as micro- electromechanical system (MEMS) on polymer substrate, protective coating on metal structure for anti-corrosion, ceramic coating on metal substrate for abrasion resistance, thermal barrier coatings, metal or metal oxide coating on metal-coated polymer for reflectance tuning and protection, as well as laminated composites and structures, etc. However, the surface layer and joint interface are commonly prone to premature failures because of cracking and delamination due to their different thermal and mechanical properties. In return, the premature failures will affect the performance and structural integrity of the device and eventually cause the failure of the multilayered structures to perform their functions. Therefore, it is essential to understand the mechanical behavior and corresponding failure mechanisms of the multilayered systems. This Ph.D. dissertation focuses on the study of opening-mode fracture (OMF) behavior and interfacial delamination in different types of multilayered structures, such as the aluminum plate or wire with alumina protective coating, multilayered advanced polymeric reflectors, and asphalt pavements. In addition, another failure mechanism, i.e. the material degradation and aging, is also studied. Firstly, a two-dimensional (2D) elasoplastic fracture model in Cartesian coordinates is developed to study the OMF in the thin alumina film fully bonded to an aluminum plate undergoing large- scale yielding. The stress field in the coating layer is described by one section between two adjacent cracks. The 2D plane strain formulations are employed to analyze the elastic field in the thin film, while a one-dimensional (1D) linear hardening plastic model is applied to account for the large plastic deformations in the substrate under substantial yielding. An elastoplastic shear lag model is established to transfer the tensile stress in the substrate to the thin film. General formulations and explicit expressions of the elastic/elastoplastic solutions of the thin film/substrate system under different loading stages have been presented. The elastic field in the thin film is then verified with the finite element (FE) results. The fracture energy release rate (ERR) is calculated and corresponding elastoplastic fracture analysis is conducted. Experimental characterization is further conducted to validate the present model; the results show that this fracture modle can capture the fracture initiation, infilling, and saturation in the thin film successfully. This model has been extended to cylindrical coordinates, where the alumina coating is fully bonded to an aluminum wire/rod. Due to the axial symmetry, the proposed 2D elasoplastic fracture model has been reproduced in polar coordinates and be used to study the OMF behavior of alumina coating fully bonded to an aluminum wire. For the case where thermal loading is applied on coating/substrate structures, neither the plane strain nor plane stress assumption can be applied because the thermal loading would introduce multi axial normal stress, therefore, a three-dimensional (3D) elastic fracture model is then developed to study the OMF in the coating fully bonded to the elastic substrate. When the temperature change reaches a certain level, block cracking will initiate in the surface layer to release the accumulated thermal stress. For simplicity, we assume the top surface of the coating would keep flat and in a rectangular shape after deformation. Then the elastic field in both coating and substrate is analyzed and verified with the FE results. The fracture ERR is then obtained based on the solved elastic field and used to analyze the fracture initiation, infilling, and saturation. In order to verify the fracture analysis, the theoretical fracture analysis results are compared with FE simulation results based on the cohesive zone model (CZM) and experimental data from the literature. The good agreement demonstrates the accuracy of the proposed 3D fracture model. In addition, this model for coating/substrate system is extended to study multilayered structures with arbitrary number of layers. In order to verify this extended model, the predicted elastic field in an advanced polymeric solar reflector is compared with FE simulation results and parametric studies are conducted to investigate the effect of geometry on the accuracy of this model. Furthermore, the fracture behavior of the surface layer in the advanced polymeric solar reflector is studied using the calculated fracture ERR. Additionally, the delamination behavior, as another common failure mode of the coating/substrate structures, is then studied. The delamination fracture energy of a multilayered glass solar reflec- tor is tested by employing the width-tapered beam method. The testing results indicate that the weakest interface of the multilayered solar reflector would be the glass-copper interface with a de- lamination fracture energy 4.4 J/m2. Using the tested fracture energy as an input, an FE model is built based on the CZM and the returned peeling force from simulation is then compared with the test results to verify the accuracy of the test method. The good agreement between the simulation and test results demonstrates that the width-tapered beam method is accurate enough to measure the delamination fracture energy of this multilayered solar reflector. Additionally, the effect of aging on the delamination fracture energy is investigated by measuring the delamination fracture energy after 50 hrs’ accelerated aging test. The results show that the aging has minor effect of delamination fracture energy for samples with alumina (Al2O3) protective coating, while it reduces the delamination fracture energy for samples with titania (TiO2) protective coating. As another failure mechanism, the material degradation or aging behavior is studied in this dissertation. The weight percentage of oxygen (WPO) in three types of asphalt binders extracted from reclaimed asphalt pavements (RAPs) and one extracted from fresh Hot Mix Asphalt (HMA), that have been aged under continuous ultraviolet (UV) or UV/moisture/condensation exposure for different period, is measured using a energy-dispersive X-ray spectroscopy (EDX). Then the tested data are fitted based on two classic aging models, namely the fast-rate constant-rate (FRCR) and nonlinear differential dynamic (NDD) models. The good fitted results show that both FRCR and NDD models can capture the aging behavior of asphalt binders extracted from both RAPs and fresh HMA under continuous UV or UV/moisture/condensation exposure. Meanwhile, although exposed under UV for the same time, the WPO in samples after UV/moisture/condensation aging are lower than those in samples after continuous UV aging, which indicates that condensation and moisture reduce the UV-induced photo-oxidative aging rate.

The mechanical reliability of electronic components has become harder and harder to predict due to the use of composite materials. One of the key issues is creating an accurate model of the delamination mechanism, which consists in the separation of two different bounded materials. This phenomenon is a very challenging issue that is investigated in the Nano Interface Project (NIP), in which this thesis is involved. The macroscopic adhesion force is governed by several parameters described at different length scales. Among these parameters, the roughness profile of the interface has a pronounced influence. The main difficulty for an accurate delamination characterization is then investigating the effects of this roughness profile and the modifications it implies for the overall cohesion. The objective of the NIP is to develop an interface model for the numerical testing of electronic components in a finite element software. The problem is that a direct modeling of all the mechanisms described previously is really expensive in term of computation time, if possible at all. This difficulty is increased by the huge mismatch of the mechanical properties of the materials in contact. A scale transition method is therefore required, which is provided by homogenization. The idea is to consider the delamination at a wider scale. Rather than modeling the whole roughness profile, the adhesion at the interface will be described by homogenized, or macroscopic, parameters extracted from a representative model at the micro-scale, the RVE. This thesis will deal with the determination of these homogenized parameters.

No description available.

Joints provide a path for transfer of load and are important components in an assembly of structures, particularly in translating joint strength improvements directly to significant cost savings. This cost savings is more evident in composite joints since manufacturing of more complex single piece components results in a reduction of both part count and labor. An improvement in joint strength for co-cured composite joints through minimized free-edge delamination was investigated for quasi-isotropic [0/±45/90]s lay-up based on the quantitative assessments of the quasi-static and fatigue strength and qualitative understanding of the fatigue damage initiation and propagation for the [0/±θ/90]s family of co-cured composite joints. A previously proposed co-cured joint concept, the Single Nested Overlap (SNO) joint, was compared against a Straight Laminate (SL) and a single lap joint. The SL represents a "perfect" joint and serves as an upper bound whereas the single lap joint represents the simplest generic joint and is the base design for the SNO joint concept. Three categorized failure types, which represented predominant failure modes in the SL, single lap and SNO joints, along with two different fatigue strength indicators were used for quasi-static and fatigue strength comparison. With fatigue run-out defined at 1 x 106 cycles, the fatigue damage initiation and propagation at high loadings was monitored with an Infrared Thermoelastic Stress Analysis (IR-TSA) technique, while a damage type comparison was used at low loadings. Quasi-static Acoustic Emission (AE) counts were observed to be Fatigue Limit (FL) predictors for [0/±θ/90]s SL and SNO joints. The validity of these FL predictors were also assessed in the damage type comparison.

Any composite material is made up from two or more materials and therefore contains interfaces, which usually represent planes of weakness. Interfacial fractures are effectively constrained to propagate along these interfaces as mixed-mode fractures with all three opening, shearing and tearing actions (i.e. mode I, mode II and mode III), instead of kinking to maintain pure-mode-I conditions at the advancing crack front, as would typically happen in an isotropic material. This is significant because mixed-mode fracture toughness is load-dependent and not a purely intrinsic material property (although clearly the pure mode fracture toughnesses are indeed intrinsic material properties that can be determined experimentally). Therefore, in order to know the fracture toughness under general loading conditions, it is necessary to know both the interface failure criterion (that describes the fracture toughness as a function of the mode mixity), and the mode mixity of the crack under the specified loading conditions. This is a complex problem that has occupied researchers in the fracture mechanics community for decades. Consequently, the literature contains a large number of different mixed-mode partition theories. This work appears to show that, of all the partition theories assessed, Wang and Harvey s (2012a) Euler beam partition theory is able to most accurately predict the fracture toughness of a mixed-mode delamination in a fibre-reinforced polymer composite laminate. This statement is based on the outcomes of three separate studies: The first study uses data reported in the literature from a thorough programme of mixed-mode fracture testing of unidirectional and multi-directional laminates. The Euler beam partition theory is able to accurately predict the fracture toughness in all cases. Furthermore, the Euler beam partition theory, which is completely analytical, closely agrees over a large domain with Davidson et al. s (2000) independently-derived non-singular field partition theory, which was derived with the aid of experimental test results. In general, the singular-field approach based on 2D elasticity and the finite element method give poor predictions. In the second study, an original programme of mixed-mode fracture testing is carried out, which incorporates several novel aspects including new test apparatus and a methodology for testing with a wide range of applied pure bending moments. Eighty five fracture tests are performed on unidirectional glass/epoxy laminates to determine the initiation and propagation fracture toughnesses. Although the second study was inconclusive with respect to the correctness of any particular partition theory, the development of the test apparatus and test methodology are considered to be major contributions that will be useful for both design engineers and academic researchers, not only working with fibre-reinforced polymer composite laminates, but also working with other composite materials containing interfacial cracks. The third study uses digital image correlation to investigate the near-crack tip strain fields of mixed-mode delaminations to try to discover the underlying mechanics that govern the selection of a mixed-mode partition theory. The new testing apparatus is used again, and another novel testing methodology is developed. The work appears to confirm (with some caveats) that two sets of pure modes exist, that is, two pure mode I modes, and two pure mode II modes, with their numerical values roughly corresponding to those from Wang and Harvey s (2012a) Euler beam partition theory. It should be noted that, as far as the author s knowledge is concerned, Euler beam partition theory is the only one in the literature to predict the existence of two sets of pure modes. Although this work set out to conclusively determine which mixed-mode partition theory is able to most accurately predict the fracture toughness of a mixed-mode delamination in a fibre-reinforced polymer composite laminate, and also, to discover why, the outcomes cannot truly be called conclusions . Rather, they only offer strong support for Wang and Harvey s (2012a) Euler beam partition theory for predicting the fracture toughness fibre-reinforced polymer composite laminates against delamination. Despite this, the work makes major contributions that will be useful for both design engineers and academic researchers in the field, as described in the above.

No description available.

The compression strength of dropped-ply, graphite-epoxy laminated plates for the delamination mode of failure is studied by analysis and corroborated with experiments. The nonlinear response of the test specimens is modeled by a geometrically nonlinear finite element analysis. The methodology for predicting delamination is based on a quadratic interlaminar stress criterion evaluated at a characteristic distance from the ply drop-off. The details of the complex state of stress in the region of the thickness discontinuity are studied using three-dimensional solid elements, while the uniform sections of the plate are modeled with quadrilateral shell elements. A geometrically nonlinear transition element was developed to couple the shell elements to the solid elements. The analysis was performed using the COmputational MEchanics Testbed (COMET), an advanced structural analysis software environment developed at the NASA Langley Research Center to provide a framework for research in structural analysis methods. Uniaxial compression testing of dropped-ply, graphite-epoxy laminated plates has confirmed that delamination along the interfaces above and/or below the dropped plies is a common mode of failure initiation. The compression strength of specimens exhibiting a linear response is greater than the compression strength of specimens with the same layup exhibiting geometrically nonlinear response. Experimental and analytical results also show a decrease in laminate strength with increasing number of dropped plies. For linear response there is a large decrease in compression strength with increasing number of dropped plies. For nonlinear response there is less of a reduction in compression strength with increasing number of dropped plies because the nonlinear response causes a redistribution and concentration of interlaminar stresses toward the unloaded edges of the laminate. / Ph. D.