Karen S. Whitley

Analysis of the finite deformation response of shape memory polymers: I. Thermomechanical characterization

This study presents the analysis of the finite deformation response of a shape memory polymer (SMP). This two-part paper addresses the thermomechanical characterization of SMPs, the derivation of material parameters for a finite deformation phenomenological model, the numerical implementation of such a model, and the predictions from the model with comparisons to experimental data. Part I of this work presents the thermomechanical characterization of the material behavior of a shape memory polymer. In this experimental investigation, the vision image correlation system, a visual–photographic apparatus, was used to measure displacements in the gauge area. A series of tensile tests, which included nominal values of the extension of 10%, 25%, 50%, and 100%, were performed on SMP specimens. The effects on the free recovery behavior of increasing the value of the applied deformation and temperature rate were considered. The stress–extension relationship was observed to be nonlinear for increasing values of the extension, and the shape recovery was observed to occur at higher temperatures upon increasing the temperature rate. The experimental results, aided by the advanced experimental apparatus, present components of the material behavior which are critical for the development and calibration of models to describe the response of SMPs.

Thermal/Mechanical Durability of Polymer-Matrix Composites in Cryogenic Environments

In order to increase the reliability of the next generation of space transportation systems, the mechanical behavior of polymeric-matrix composite (PMC) materials at cryogenic temperatures must be investigated. This paper presents experimental data on the residual mechanical properties of a carbon fiber polymeric composite, IM7/PETI-5 as a function of temperature and aging. Tension modulus and strength were measured at room temperature, -196 C, and -269 C on five different specimens ply lay-ups. Specimens were preconditioned with one set of coupons being isothermally aged for 576 hours at -184 C, in an unloaded state. Another set of corresponding coupons were mounted in constant strain fixtures such that a constant uniaxial strain was applied to the specimens for 576 hours at -184 C. A third set was mechanically cycled in tension at -184 C. The measured properties indicated that temperature, aging, and loading mode can all have significant influence on performance. Moreover, this influence is a strong function of laminate stacking sequence. Thermal-stress calculations based on lamination theory predicted that the transverse tensile ply stresses could be quite high for cryogenic test temperatures. Microscopic examination of the surface morphology showed evidence of degradation along the exposed edges of the material because of aging at cryogenic temperatures. ________________

Thermal/Mechanical Response of a Polymer Matrix Composite at Cryogenic Temperatures

The mechanical behavior of a polymeric-matrix composite at cryogenic temperatures was investigated. Experimental data are presented on the residual mechanical properties of a carbon-fiber polymeric composite, IM7/PETI-5, both before and after aging. Both tension and compression modulus and strength were measured at room temperature, -196°C, and -269°C on five different laminate configurations consisting of [0] 12 and [90] 12 unidirectional laminates, [±25] 3s and [±45] 3S angle-ply laminates, and a 13-ply laminate [45/90/90/90/-45/0/0/0/-45/90/90/90/45]. One set of specimens was aged isothermally for 576 h at -184°C in an unconstrained state. Another set of corresponding specimens was aged under constant uniaxial strain for 576 hours at -184°C. Based on the experimental data presented, it is shown that trends in stiffness and strength that result from changes in temperature are not always smooth and consistent. Moreover, it is shown that loading mode and direction are significant for both stiffness and strength and that aging at cryogenic temperature while under load can alter the mechanical properties of pristine, unaged laminates made of IM7/PETI-5 material.

The Combined Influence of Molecular Weight and Temperature on the Physical Aging and Creep Compliance of a Glassy Thermoplastic Polyimide

AbstractThe effect of molecular weight on the viscoelastic performance of anadvanced polymer (LaRC™-SI) was investigated through the use of creepcompliance tests. Testing consisted of short-term isothermal creep andrecovery with the creep segments performed under constant load. Thetests were conducted at three temperatures below the glass transitiontemperature of five materials of different molecular weight. Through theuse of time-aging-time superposition procedures, the material constants,material master curves and aging-related parameters were evaluated ateach temperature for a given molecular weight. The time-temperaturesuperposition technique helped to describe the effect of temperature onthe timescale of the viscoelastic response of each molecular weight. Itwas shown that the low molecular weight materials have higher creepcompliance and creep rate, and are more sensitive to temperature thanthe high molecular weight materials. Furthermore, a critical molecularweight transition was observed to occur at a weight-average molecularweight of

Innovative Materials for Aircraft Morphing

{\bar M}

MEASUREMENT OF ELASTIC CONSTANTS FROM AXIAL-TORSIONAL LOADING OF RECTANGULAR POLYMER COMPOSITE MATERIALS1

w∼ 25,000 g/mol below which, the temperature sensitivity of thetime-temperature superposition shift factor increases significantly. Theshort-term creep compliance data were used in association with Struikseffective time theory to predict the long-term creep compliance behaviorfor the different molecular weights. At long timescales, physical agingserves to significantly decrease the creep compliance and creep rate ofall the materials tested. Long-term test data verified the predictivecreep behavior. Materials with higher temperature and lower molecularweights had greater creep compliance and higher creep rates.

The use of doubler reinforcement in delamination toughness testing

An experimental and analytical study was performed on the use of tension stress relaxation to characterize the effects of elevated temperature and physical aging on the linear viscoelastic behavior of IM7/K3B. Isothermal stress relaxation tests on a [′45] 2 s laminate were run over a range of sub-glass transition (T g ) temperatures. The sequenced test method most commonly employed for creep was successfully adapted to the stress relaxation test and from those sequenced tests, material parameters such as aging shift rates and momentary master curve coefficients were developed for use in the analytical model. The analytical viscoelastic model was based on classical lamination theory, the hereditary integral formulation type constitutive law, and effective time theory. Time-aging time superposition, effective time theory, and viscoelasticity were used to determine the physical aging related material parameters from the relaxation tests. Results were compared to previously measured isothermal creep compliance results via known relationships for the convolution of compliance to modulus. Time-temperature superposition was also used to evaluate master curves and related shift factors. All of the results illustrated that the relative influence of temperature and aging must be considered when assessing long-term performance and that the loading mode may have to be considered when accurate predictions of viscoelastic behavior are required.

Permeability and Life-time Durability of Polymer Matrix Composites for Cryogenic Fuel Tanks

Reported herein is an overview of the research being conducted within the materials division at NASA Langley Research Center on the development of smart material technologies for advanced airframe systems. The research is a part of the Aircraft Morphing Program which is a new six- year research program to develop smart components for self- adaptive airframe systems. The fundamental areas of materials research within the program are computational materials; advanced piezoelectric materials; advanced fiber optic sensing techniques; and fabrication of integrated composite structures. This paper presents a portion of the ongoing research in each of these areas of materials research.