A nonlinear viscoelastic approach to durability predictions for polymer based composite structures

Current industry approaches for the durability assessment of metallic structures are briefly reviewed. For polymer based composite structures, it is suggested that new approaches must be adopted to include memory or viscoelastic effects which could lead to delayed failures that might not be predicted using current techniques. A durability or accelerated life assessment plan for fiber reinforced plastics (FRP) developed and documented over the last decade or so is reviewed and discussed. Limitations to the plan are outlined and suggestions to remove the limitations are given. These include the development of a finite element code to replace the previously used lamination theory code and the development of new specimen geometries to evaluate delamination failures. The new DCB model is reviewed and results are presented. Finally, it is pointed out that new procedures are needed to determine interfacial properties and current efforts underway to determine such properties are reviewed. Suggestions for additional efforts to develop a consistent and accurate durability predictive approach for FRP structures is outlined

An experimental and analytical investigation of the ductile fracture of polymers

Analytical and experimental investigation of ductile fracture of polymers using adaptation of Dugdale mode

The nonlinear viscoelastic response of resin matrix composite laminates

Possible treatments of the nonlinear viscoelastic behavior of materials are reviewed. A thermodynamic based approach, developed by Schapery, is discussed and used to interpret the nonlinear viscoelastic response of a graphite epoxy laminate, T300/934. Test data to verify the analysis for Fiberite 934 neat resin as well as transverse and shear properties of the unidirectional T300/934 composited are presented. Long time creep characteristics as a function of stress level and temperature are generated. Favorable comparisons between the traditional, graphical, and the current analytical approaches are shown. A free energy based rupture criterion is proposed as a way to estimate the life that remains in a structure at any time

Numerical solution methods for viscoelastic orthotropic materials

Numerical solution methods for viscoelastic orthotropic materials, specifically fiber reinforced composite materials, are examined. The methods include classical lamination theory using time increments, direction solution of the Volterra Integral, Zienkiewicz's linear Prony series method, and a new method called Nonlinear Differential Equation Method (NDEM) which uses a nonlinear Prony series. The criteria used for comparison of the various methods include the stability of the solution technique, time step size stability, computer solution time length, and computer memory storage. The Volterra Integral allowed the implementation of higher order solution techniques but had difficulties solving singular and weakly singular compliance function. The Zienkiewicz solution technique, which requires the viscoelastic response to be modeled by a Prony series, works well for linear viscoelastic isotropic materials and small time steps. The new method, NDEM, uses a modified Prony series which allows nonlinear stress effects to be included and can be used with orthotropic nonlinear viscoelastic materials. The NDEM technique is shown to be accurate and stable for both linear and nonlinear conditions with minimal computer time

Rate and time dependent behavior of structural adhesives

Studies on two adhesives (Metlbond 1113 and 1113-2) identified as having applications in the bonding of composite materials are presented. Constitutive equations capable of describing changes in material behavior with strain rate are derived from various theoretical approaches. It is shown that certain unique relationships exist between these approaches. It is also shown that the constitutive equation derived from mechanical models can be used for creep and relaxation loading. A creep to failure phenomenon is shown to exist and is correlated with a delayed yield equation proposed by Crochet. Loading-unloading results are presented and are shown to correlate well with the proposed form of the loading-unloading equations for the modified Bingham model. Experimental results obtained for relaxation tests above and below the glass transition temperature are presented. It is shown that the adhesives obey the time-temperature superposition principle

Connecting Youth Through Multiple Pathways

Presents findings from a field scan of efforts to help vulnerable youth graduate from high school, what is working, what should be done, and what opportunities exist for Casey's involvement. Explores risk factors and alternative programs. Lists resources

Youth at High Risk of Disconnection

Updates the 2003 report "Connected by 25: Improving the Life Chances of the Country's Most Vulnerable 14-24 Year Olds" with demographic changes in disconnected and at-risk youth -- teens who are dropouts, mothers, in foster care, and/or incarcerated

A critical phenomenology of civilization

Civilized culture is killing the planet. At present, we are facing the largest extinction event in 65 million years and the cause, according to most scholars, is patently human. My question, however, is not whether the mass destruction of the biosphere is the result of an unfortunate and misguided particularity within civilization (e.g., over consumption, driving too much, etc.), but rather: Is it the case that civilization, by its very nature, entails the destruction of the natural world and of both human and non-human communities? In the vein of a fairly recent movement in scholarship, my answer is a resounding yes. Taking a cue from one of the foremost voices of this recent movement, Derrick Jensen, I\u27ll briefly trace the genesis and justification of the following premise: Civilization is not and can never be sustainable, as well as the philosophical fallout of what this may mean for us today. Employing the thought and method of certain strands of phenomenology, I first examine how it is that civilization appears in our collective everydayness and how certain movements within this appearance give way to its replication, continuation, and (largely) unquestioned legitimacy. From there, I move to incorporate the insight of Theodor Adorno and other critical theorists, uncovering the finer ideological strands that tie us to civilization. From the arguments outlined by Jensen, John Zerzan, and others, I make a case for the active rejection and dismantling of civilization, ultimately attempting to articulate a philosophically based strategy of resistance

Extracting Polymer Nanocomposite Samples from Full-Length Documents

This paper investigates the use of large language models (LLMs) for extracting sample lists of polymer nanocomposites (PNCs) from full-length materials science research papers. The challenge lies in the complex nature of PNC samples, which have numerous attributes scattered throughout the text. The complexity of annotating detailed information on PNCs limits the availability of data, making conventional document-level relation extraction techniques impractical due to the challenge in creating comprehensive named entity span annotations. To address this, we introduce a new benchmark and an evaluation technique for this task and explore different prompting strategies in a zero-shot manner. We also incorporate self-consistency to improve the performance. Our findings show that even advanced LLMs struggle to extract all of the samples from an article. Finally, we analyze the errors encountered in this process, categorizing them into three main challenges, and discuss potential strategies for future research to overcome them

Diatoms at >5000 Meters in the Quelccaya Summit Dome Glacier, Peru

Diatoms were found in late Holocene age ice-core samples recovered from the Quelccaya Summit Dome in the tropical Andes of Peru and were imaged by environmental scanning electron microscopy and identified. Freshwater diatoms in the genera Hantzschia, Pinnularia, and Aulacoseira were the most common taxa in the samples and indicate a freshwater source for the material, which also is suggested by the presence of the freshwater alga Volvox. The overall species composition of the diatoms suggests that the majority of taxa originated from a high-elevation lake or wetland in the cordillera surrounding the ice cap. The abundant diatom valves, up to 70 µm in size, likely were transported to the ice via wind