Empirical modeling of in-cure volume changes and elasticity development of thermoset polymers

In the interest of evaluating the cure-induced residual stresses that develop during manufacturing of a thermoset polymer composite, a course of research has been carried out to determine the nature of volume changes that occur during a cure cycle These volume changes are known to be from two different sources thermal expansion and cure shrinkage. A well-characterized aerospace material, Hercules 3501-6 Epoxy, has been used to develop a model for the cure shrinkage and thermal expansion during cure. The developed methods are also applied to RS-3 and EX-1515 resins. The primary equipment used includes a volumetric dilatometer, which monitors total sample volume change dung cure, a differential scanning calorimeter (DSC), which is used to evaluate sample degree of cure, and a cure induced stress test (CIST) which monitors the cure induced stresses that are developed in embedded fibers. Cure shrinkage was isolated by monitoring isothermal cure processes, in which case there are no thermal volume changes to account for. Cure shrinkage was shown to be linear with respect to degree of cure. Thermal volumetric effects were determined by investigating cure cycles of constant heat rate. The thermal expansion coefficient was shown to be constant over a wide range of temperature and degree of cure. Based upon experimental data, a model has been developed to predict total volume changes for an arbitrary cure cycle. As a follow-on study, the volume change data has been combined with analysis of the polymer stress state to develop an investigation of m-cure development of elastic modulus. The development of elasticity was shown to mimic the progression of degree of cure

Development of Lightweight CubeSat with Multi-Functional Structural Battery Systems

This collaborative multi-disciplinary effort aims to develop a lightweight, 1-unit (1U) CubeSat (10x10x10 cm) which utilizes improved and fully integrated structural battery materials for mission life extension, larger payload capability, and significantly reduced mass.The electrolytic carbon fiber material serves the multifunctional capacitive energy system as both a lightweight, load bearing structure and an electrochemical battery system. This implementation will improve traditional multifunctional energy storage concepts with a highly effective energy storage capability

Tbx1 regulates Vegfr3 and is required for lymphatic vessel development

Defects in lymphangiogenesis are added to the broad clinical manifestations of DiGeorge syndrome, caused by deletion of the T box transcription factor Tbx1

Advanced Power Technology Development Activities for Small Satellite Applications

NASA Glenn Research Center (GRC) has a long history related to the development of advanced power technology for space applications. This expertise covers the breadth of energy generation (photovoltaics, thermal energy conversion, etc.), energy storage (batteries, fuel cell technology, etc.), power management and distribution, and power systems architecture and analysis. Such advanced technology is now being developed for small satellite and cubesat applications and could have a significant impact on the longevity and capabilities of these missions. A presentation during the Pre-Conference Workshop will focus on various advanced power technologies being developed and demonstrated by NASA, and their possible application within the small satellite community

Dynamic micromechanical modeling of textile composite strength under impact and multi-axial loading

Micromechanical finite element modeling has been employed to define the failure behavior of S2 glass/BMI textile composite materials under impact loading. Dynamic explicit analysis of a representative volume element (RVE) has been performed to explore dynamic behavior and failure modes including strain rate effects, damage localization, and impedance mismatch effects. For accurate reflection of strain rate effects, differences between an applied nominal strain rate across a representative volume element (RVE) and the true realized local strain rates in regions of failure are investigated. To this end, contour plots of strain rate, as well as classical stress contours, are developed during progressive failure. Using a previously developed cohesive element failure model, interfacial failure between tow and matrix phases is considered, as well as classical failure modes such as fiber breakage and matrix microcracking. In-plane compressive and tensile loading have been investigated, including multi-axial loading cases. Highly refined meshes have been employed to ensure convergence and accuracy in such load cases which exhibit large stress gradients across the textile RVE. The effect of strain rate and phase interfacial strength have been included to develop macro-level material failure envelopes for a 2D plain weave and 3D orthogonal microgeometry

Failure investigation of in-flight separation of afterburner nozzle flap in a turbojet engine

A turbojet fighter experienced a separation of the afterburner nozzle flap during flight. Three mounting bolts were intended to fasten the nozzle flap assembly, but complete fracture of these bolts was the primary reason of failure. One of them was lost during the incident and two shank parts of the mounting bolts remained in the engine component. Laboratory investigation of the failed components of the nozzle flap assembly indicated that the mounting bolts were improperly installed during the last maintenance. Detailed fractographic study and metallurgical analysis focused on the fractured bolts and revealed fatigue striations with an overload dimple morphology in all fractured surfaces of the bolts. The investigation established sufficient evidence of improper installation of the primary flap assembly due to deformation of the alignment hole which inevitably resulted in higher operational loads on the bolt. It is concluded that the missing bolt had failed first, and other bolts fractured in succession in the manner of fatigue fracture. In response, preventive measures regarding inspection strategy have been improved to avoid similar failures. •Turbojet fighter experienced in-flight separation of the exhaust nozzle assembly.•Improper installation of the parts resulted in higher operating load acting on the mounting bolt.•Mounting bolts were fractured in succession in the manner of fatigue failure.•Inspection strategy has been improved to prevent similar failure

Failure investigation of hydrogen induced cracking of stabilator skin in jet aircraft

In this study, a 1 m long crack found in the stabilator skin of a jet aircraft has been investigated. SEM observation revealed that the crack initiated from the free edge of the inboard side where fatigue striations and ratchet marks were observed, then propagated transversely through the skin in the manner of a brittle crack. Predominantly intergranular fracture with quasi-cleavage morphologies was observed on the entire fractured surface. Fractography with hydrogen measurement as well as hardness testing indicated that hydrogen induced cracking (HIC) was the dominant cracking mechanism. Visual observation identified undesirable loads induced by several improper maintenance activities. Finite element analysis was used to correlate the experimental observations with these identified abnormal load cases. The location of the stress concentration determined by finite element analysis showed good correlation with the actual cracking location. It was concluded that an undesired assembly force induced a high residual stress, and operational aerodynamic loads combined with these maintenance loads contributed to exceeding the HIC threshold stress in the degraded metal due to hydrogen embrittlement. •Top skin of stabilator including honeycomb core was cracked longer than 1 m.•Intergranular fracture was identified as the dominant cracking mode.•Evidence of undesired loads caused by improper maintenance activities were identified.•Stress concentration location based on FE analysis is coincident with experimental observation.•Hydrogen induced cracking associated with service loads resulted in catastrophic brittle failure

Dynamic micromechanical modeling of textile composites with cohesive interface failure

Micromechanical finite element modeling has been employed to investigate the failure of several compositionally varied textile composite materials under dynamic loading. A previously developed cohesive element failure model for interface strength is employed at the phase boundary between the fiber tows and the interstitial matrix to determine the effects of interface properties on the failure behavior of a 2D plain weave and 3D orthogonal weave S2 glass/BMI composite. Thus, tow pullout and separation have been included in addition to more classical micro-level failure modes such as fiber breakage and matrix microcracking. The dynamic response of a representative volume element (RVE) is determined at strain rates of 1000 and 10,000 strain/s in an explicit finite element formulation. A parametric study has investigated compositional effects on impact strengths of two weave geometries with a relatively ‘strong’ versus ‘weak’ interface property at 10,000 and 1000 strain/s in tension and compression. The successful implementation of the cohesive failure scheme into the textile RVE framework is shown, and fundamental macro-level failure cases are investigated to relate micromechanical parametric variation to consequent strength effects. </jats:p