G. P. Tandon

Durability Assessment of Styrene- and Epoxy-based Shape-memory Polymer Resins

The present study is a baseline assessment of the durability of styrene- and epoxy-based shape memory polymer resin materials being considered for morphing applications when exposed to service environment. The approach for the experimental evaluation is a measurement of the shape memory properties and elastomeric response before and after separate environmental exposure to (i) water at 49°C for 4 days, (ii) in lube oil at room temperature and at 49°C for 24 h, and (iii) after exposure to xenon arc (63°C, 18 min water and light/102 min light only) and spectral intensity of 0.3—0.4 watts/m2 for 125 cycles (250 h exposure time). Parameters being investigated include modulus in the rubbery and glassy state, stored strain, shape fixity, stress recovery ratio, and linear shape recovery. In addition, we monitor changes in specimen color, weight, and dimensions along with onset of damage due to conditioning and subsequent thermomechanical cycling.

Long-Term Durability of Polymeric Matrix Composites

Long-Term Durability of Polymeric Matrix Composites presents a comprehensive knowledge-set of matrix, fiber and interphase behavior under long-term aging conditions, theoretical modeling and experimental methods.This bookcovers long-term constituent behavior, predictive methodologies, experimental validation and design practice. Readers will also find a discussion of various applications, including aging air craft structures, aging civil infrastructure, in addition to engines and high temperature applications.

Using Optical Microscopy to Monitor Anisotropic Oxidation Growth in High-Temperature Polymer Matrix Composites

Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. In this work, unidirectional G30-500/PMR-15 composite specimens are aged at elevated temperature in air resulting in oxidation propagation parallel and perpendicular to the fibers. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage in composite specimens through the use of fluorescence imaging using dye impregnation in conjunction with optical microscopy. Optical micrographs are taken on polished internal sections and viewed in the dark-field mode to measure the degree, depth, and distribution of thermal oxidation development from external surfaces perpendicular and parallel to the fibers. It is shown that alternative pathways for transport of oxygen into the interior of the composite are fiber—matrix debonds and transverse matrix cracks that propagate with the oxidation front for which the oxidation front consistently precedes the debond crack front.

Heterogeneous Thermo-oxidative Behavior of Multidirectional Laminated Composites

This article examines the through-the-thickness heterogeneity observed in oxidation profiles of composite laminates. The effect of ply angle on oxygen diffusivity and the influence of ply stacking sequence on oxidation behavior of isothermally aged multidirectional composites are discussed by correlating experimental observations and a three-dimensional oxidation model. Experimental observations of oxidation growth are made using dark-field optical microscopy, while damage initiation and growth is detected using fluorescence imaging with dye impregnation. Oxidation growth in laminated systems is simulated using a diffusion-reaction-conversion model developed earlier for resin oxidation along with homogenization techniques. Several stacking sequences of carbon fiber-reinforced polyimide composites are studied. The effects of the orthotropy in the diffusivity tensors of each ply and the influence of neighboring ply on oxidation growth are clearly illustrated. The simulation results presented in this article are valid till the onset of damage, beyond which coupling effects between oxidation growth and damage evolution have to be addressed.

Fiber–Matrix Interfacial Failure Characterization Using a Cruciform-Shaped Specimen

In this study, a cruciform-shaped test specimen is utilized to characterize the fiber–matrix interface under transverse and combined (tensile and shear) loading. We first present an overview of past references of how the cruciform geometry is optimized to promote interfacial failure. We then discuss a modification of the cruciform specimen where face-sheets are adhesively bonded to reinforce the sample. These face-sheets serve a twofold purpose, namely, to prevent premature failure in the fillet region and to encourage debond initiation at the center of the gage length. Finally, an off-axis cruciform geometry, in which the wings of the cruciform sample are inclined at an angle with respect to the loading direction, is introduced to characterize the fiber–matrix interface under combined transverse and shear loading. Using the measured value of applied stress at debond initiation, and the evaluated stress concentration factor at the fiber–matrix interface, a mixed-mode failure envelope is then constructed in the normal-shear stress space, and a quadratic failure criterion is proposed.

Environmental Durability of Fabric-Reinforced Shape-Memory Polymer Composites

This study is a baseline assessment of the environmental durability of current state-of-the-art, fabric-reinforced shape-memory (SM) materials being considered for morphing applications. Tensile dog-bone-shaped specimens are cut along three different directions, namely, along (0°), perpendicular (90°), and oblique (45°) to the planar orientation of the fabric. The elastomeric response and shape memory properties before and after simulated environmental exposure to moisture, lubrication oil, and UV radiation are measured. Weight loss of the as-received and conditioned specimens is monitored and the dog-bone-shaped specimens are subjected to recovery following fixation. Parameters being investigated include modulus in the glassy and rubbery state, stored strain, shape fixity, recovery stress, and unconstrained shape recovery. There is a twofold decrease in the composite stiffness as the material is cycled between room and elevated (above the glass transition) temperature. At room temperature, the 0-degree specimen has the maximum stiffness (5.8 GPa), failure strength (94 MPa), and failure strain (5.4%), while above the Tg, the 90-degree specimen has the least stiffness (∼18 MPa) and largest strain to failure (>200%). Thus, the composite exhibits large deformation in its rubbery state. Parameters which are strongly affected by the damage developed during the first SM cycle include rubbery and glassy (or unloading) moduli values measured during the second SM cycle, while smaller changes are observed in shape fixity and recovery properties.

Characterization of Thermo-Oxidation in Laminated and Textile Composites

An understanding of the effects of thermo-oxidation in high-temperature PMCs for structural components subjected to arbitrary service environments is critical to life-performance predictions. Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. The thermo-oxidative behavior of the composite is significantly different from that of the constituents as the composite microstructure, including the fiber/matrix interphases/interfaces, architecture, and ply layup introduce anisotropy in the diffusion and oxidation behavior. In this work, light microscopy and scanning electron microscopy techniques are used to characterize the oxidative process in laminated and textile carbon-fiber-reinforced polyimide composites. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage through the use of fluorescence imaging using dye impregnation. It is shown that alternative pathways for transport of oxygen into the interior of the composite are fiber–matrix debonds and matrix cracks that propagate with the oxidation front. It has been further determined through closer examination of the oxidation front and the crack front for discrete regions of the various composite specimens, that the oxidation front consistently precedes the crack front. This mechanism for accelerated oxidation is an excellent example of the intrinsic coupling of chemical oxidative aging and damage, which needs to be properly represented in predictive models.

Fatigue Cycling of Shape Memory Polymer Resin

Shape memory polymers have attracted great interest in recent years for application in reconfigurable structures (for instance morphing aircraft, micro air vehicles, and deployable space structures). However, before such applications can be attempted, the mechanical behavior of the shape memory polymers must be thoroughly understood. The present study represents an assessment of viscoelastic and viscoplastic effects during multiple shape memory cycles of Veriflex-E, an epoxy-based, thermally-triggered shape memory polymer resin. The experimental program is designed to explore the influence of multiple thermomechanical cycles on the shape memory performance of Veriflex-E. The effects of the deformation rate and hold time at elevated temperature on the shape memory behavior are also investigated.

Interaction Between a Nanofiber and an Arbitrarily Oriented Crack

The interaction between a hollow cylindrical nanofiber and a crack in an infinite medium is investigated. The two-dimensional problem, when the crack lies in the cross-sectional plane of the fiber, is solved for a transversely isotropic fiber in an isotropic matrix phase. The reported work to date on crack—fiber interaction problems seems to be based on a conventional solid fiber cross-section, whereas in this study, a hollow fiber cross-section is considered. The crack is considered at various orientations with respect to the nanofiber. Mismatch between Youngs modulus and Poissons ratios of the fiber and matrix, fiber diameter, and fiber wall thickness, are some of the parameters in the study. Finite element method is used as the analysis tool to compute energy release rates in mixed mode conditions. Fiber—matrix interface tractions are also computed to investigate secondary failure mechanism. The behavior of a hollow nanofiber is shown to be significantly different from that of a fiber with a solid cross-section. However, with increasing nanofiber wall thickness the results of the present investigation are seen to converge to the solutions of a solid fiber. Finally, the influence of the properties of the interphase region between the hollow nanofiber and matrix on energy release rates is investigated.

Thermomechanical characterization of environmentally conditioned shape memory polymer using nanoindentation

Shape memory polymers (SMPs) are an emerging class of active polymers that have dual-shape capability, and are therefore candidate materials for multifunctional reconfigurable structures (i.e., morphing structures). However, the SMPs have not been fully tested to work in relevant environments (variable activation temperature, fuel and water swell, UV radiation, etc.) required for Air Force missions. In this study, epoxy-based SMPs were conditioned separately in simulated service environments designed to be reflective of anticipated performance requirements, namely, (1) exposure to UV radiation for 125 cycles, (2) immersion in jet-oil at ambient temperature, (3) immersion in jet-oil at 49°C, and (4) immersion in water at 49°C. The novel high-temperature indentation method was used to evaluate the mechanical properties and shape recovery ability of the conditioned SMPs. Results show that environmentally conditioned SMPs exhibit higher moduli in comparison to an unconditioned one. During free recovery, the indentation impressions of all SMPs disappeared as temperature reached above Tg, indicating that the materials ability to regain shape remains relatively unchanged with conditioning.