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Thesis (Ph.D.)--Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Realff, Mary L.; Committee Member: Graham, Samuel; Committee Member: Jacob, Karl I.; Committee Member: May, Gary; Committee Member: Shofner, Meisha.

An investigation of the effect of R-ratio on the mode II fatigue delamination of AS4/3501-6 carbon/epoxy composites has been undertaken. Experiments have been performed on end notched cantilever beam specimens over a wide range of R-ratios (-l ≤R ≤0.50). The measured delamination growth rate data have been correlated with the mode II values of strain energy release rate range ∆G[formula omitted]), maximum strain energy release rate (G[formula omitted]) and stress intensity factor range (∆K[formula omitted]). The growth rate is dependent on the R-ratio over the range tested. For a constant level of ∆G[formula omitted], the crack growth rate decreases with increasing R-ratio. A similar trend is observed when the data is plotted as a function of G[formula omitted]. The effect of plotting the growth rate as a function of ∆K[formula omitted] is to produce an R-ratio dependence opposite to that obtained by either the ∆G[formula omitted] or G[formula omitted] approach. For a constant level of ∆K[formula omitted], the crack growth rate increases with increasing R-ratio. Master equations which completely characterize the fatigue behaviour as a function of ∆G[formula omitted] and ∆K[formula omitted] have been derived, based on the observation that the growth rate law exponent, n and constant, A are unique functions of R-ratio. Values for n are surprisingly large and increase with increasing R-ratio whereas values for A decrease with increasing R-ratio. The effect of time-at-load has been considered in an attempt to explain the existence of the R-ratio dependence of the growth rate. The correct trend can be established for the exponent, n but not for the constant, A. Friction between the crack faces, particularly at higher R-ratios, is proposed as a possible explanation for the observed anomaly. Further evidence of a frictional mechanism operating at higher R-ratios has been discovered through a postmortem fracture surface examination. Additional fractographic observations are presented over the entire range of R-ratios tested. In regions subjected to negative R-ratio cycling, there is no evidence of the characteristic mode II hackle features. Instead, loose rounded particles of matrix material are found. An extensive amount of hackling is observed in regions subjected to low positive R-ratio cycles. The extent of hackle damage visibly decreases in areas where higher levels of R-ratio are imposed. A correlation between the general fracture surface morphology and the fatigue data provides support for the hypothesis that energy for delamination is always available in sufficient quantity, and that growth is dependent on the stresses ahead of the crack tip being sufficiently high. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Submitted in fulfilment of the requirements for the degree of Master of Engineering: Mechanical Engineering, Department of Mechanical Engineering, Faculty of Engineering and the Built Environment, Durban University of Technology, Durban, South Africa. 2017. / In this study, the fatigue life of fiber reinforced composite (FRC) materials system was investigated. A nano-filler was used to increase the service life of the composite structures under cyclical loading since such structures require improved structural integrity and longer service life. Behaviour of glass fiber reinforced composite (GFRC) enhanced with various weight percentages (1 to 5 wt. %) of Cloisite 30B montmorillonite (MMT) clay was studied under static and fatigue loading. Epoxy clay nanocomposite (ECN) and hybrid nanoclay/GFRC laminates were characterised using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The mechanical properties of neat GFRC and hybrid nanoclay/GFRC laminates were evaluated. Fatigue study of the composite laminates was conducted and presented using the following parameter; matrix crack initiation and propagation, interfacial debonding, delamination and S–N relationship. Residual strength of the materials was evaluated using DMA to determine the reliability of the hybrid nanoclay/GFRC laminates. The results showed that ECN and hybrid nanoclay/GFRC laminates exhibited substantial improvement in most tests when compared to composite without nanoclay. The toughening mechanism of the nanoclay in the GFRC up to 3 wt. % gave 17%, 24% and 56% improvement in tensile, flexural and impact properties respectively. In the fatigue performance, less crack propagations was found in the hybrid nanoclay/GFRC laminates. Fatigue life of hybrid nanoclay/GFRC laminate was increased by 625% at the nanoclay addition up to 3 wt. % when compared to neat GFRC laminate. The residual strength of the composite materials revealed that hybrid nanoclay/GFRC showed less storage modulus reduction after fatigue. Likewise, a positive shift toward the right was found in the tan delta glass transition temperature (Tg) of 3 wt. % nanoclay/GFRC laminate after fatigue. It was concluded that the application of nanoclay in the GFRC improved the performance of the material. The hybrid nanoclay/GFRC material can therefore be recommended mechanically and thermally for longer usage in structural application. / M

The response of a flake mat to the mechanical stress applied during consolidation is a function of mat structure, raw material properties and the environmental conditions created during pressing. This research project was aimed at improving the understanding of raw material behavior during the hot-pressing of wood based composites by examining the response of a wood flake mat to the compression encountered during press closure in the flakeboard manufacturing process. The structure of a flakeboard mat can be considered to be that of a cellular material, the properties of which are governed by the cellular geometry, or arrangement of cells, and the properties of the solid cell wall material. A method for quantifying the cellular structure of a wood flake mat was developed and implemented. The structure of thin mat sections and small flake mats was quantified using image analysis techniques. The applicability of theories developed for modelling the compressive behavior of cellular materials to the consolidation of wood flake mats was investigated using thin mat sections. Narrow mat sections and small laboratory mats has similar void sizes but significantly different void shapes. Void size was not significantly affected by flake orientation, but void shape was. / Master of Science

The objective of this investigation is to predict the residual strength of notched composite Iaminates with various layups, subjected to low frequency fatigue loading with constant amplitude at room temperature, by using a material modeling approach, fracture and fatigue mechanics and the finite element method (FEM). For simplicity, after thousands of cycles, the geometry of a circular hole of the deformed laminate was categorized as (1) uniformly expanded hole into elliptic shape, (2) crack propagation around the hole transversely. Both types were studied for 12 cases of layups with various proportions of 0, 45, -45 and 90 degree plies. The effect of geometry change during fatigue on residual strength was attributed to the elliptical hole, longitudinal splitting, matrix cracking (reduction moduli of plies), crack propagation and local delamination. Due to the thin through-the-thickness notched laminate, two-dimensional FEM was used and interlaminar stresses were not considered. Reduction of stress concentration is a reason for the increase of the residual strength of the notched laminate. The stress concentration factor decreases while the elliptic hole becomes more slender; that was examined by the FEM. The residual strength and stiffness were determined by the material modeling with moduli reduction and damaged zone, and the numerical result was obtained by FEM. Laminate theory, point stress criterion, polynomial failure criterion, ply discount method, and fatigue and fracture mechanics (Paris' Power Law) were also included in this research. Geometry change and moduli reduction are two major effects that are considered to predict the notched strength. The WN point stress fracture model is adopted for simplicity, instead of the average stress criterion. K_tg that corresponds to the unnotched strength in the normalized stress base curve is used to obtain the characteristic length (d_o). We find that K_tg decreases when the elliptic hole becomes more slender and more moduli are reduced (more plies crack). At the time d_o that is determined from K_tg in the base curve is not necessarily a fixed material constant. The correlation between the fatigue life and the residual strength as predicted by the model and those determined numerically is found within acceptable errors in comparison with the experimental data. / Ph. D.