Functionally Graded Adhesive Joints with Improved Strength and Stability

随着对纤维增强复合材料作为金属轻质替代品的依赖程度越来越高，结构胶粘剂在飞机、航天器、风力涡轮机叶片、体育用品和机动车辆等高性能应用中的连接材料中变得越来越重要。在典型的粘接中，粘合剂的外缘承担了大部分剥离载荷，是最有可能发生故障的地方。通过创建具有柔软、灵活的边缘和更坚硬的核心的功能梯度粘接接头，可以更均匀地分布应力，显著提高接头强度；事实上，在迄今为止进行的有限的测试中，通过这种方法已经报告了高达60%的增强。虽然这样的结果非常有希望，但还没有人找到一种方法来创造性能随时间稳定的功能梯度粘接接头。这项工作旨在调查功能梯度胶粘剂背后的基础科学，这种胶粘剂的性能通过调节辐射暴露来控制。将为这项工作配制环氧树脂；将测量其热和机械性能，并通过暴露于不同剂量的伽马射线对基于这些材料的粘接接头进行分级，并测试其强度。同时，计算机模拟将使用测量的特性来预测关节性能，并使用实际的关节性能数据来验证模拟。由此产生的计算设计工具将提供关于如何最好地设计功能梯度节点的信息。这些努力将通过消除目前阻碍此类研究的关键障碍，开辟与功能梯度胶粘剂的制备和研究相关的几乎完全未知的领域。这项工作的成功将使工程师能够创造出更坚固、更轻、更可靠的粘合剂，并更有效地利用高性能复合材料。最直接的影响可能会出现在大量使用复合材料和轻质金属的行业--航空航天、造船、地面运输和风能--随着这些努力的影响和好处变得明显，许多其他领域也将随之应用。此外，成果将被整合到本科和研究生课程中，将开展工作，通过本科工程顶峰项目和课外活动(如F1 SAE竞赛团队)展示应用，研究生将与REU学生一起接受培训并融入研究工作，结构胶接接头边缘存在应力集中问题，而对制造缺陷的敏感性使许多人不愿使用它们。通过功能梯度粘合剂(FGA)逐渐引入载荷--例如，边缘柔软，中间僵硬--解决了这两个问题。最近的理论工作预测了使用FGA后关节强度的显著提高，这一结论得到了文献中报道的几个实验努力的证实。这一领域进展缓慢的原因是难以制定一致、稳定和有力的森林生态系统。再加上在该领域缺乏经过验证的知识，这阻碍了它们的应用。总体目标是了解如何设计、创建、表征和预测FGA接头的特性，长期目标是提高接头性能、稳定性和可靠性。这项工作的中心假设是，使用预测模型设计的稳定性能梯度将使粘接接头更牢固、更稳定。这项工作将对FGA的设计、创建和行为有一个基本的理解。将确定FGA接头的设计规则，实现创建和表征FGA和FGA接头的一般方法，并验证FGA接头性能的预测。这一努力将扩大我们对复杂接头进行建模的能力，并总体上制造和表征功能梯度材料。这些努力将通过消除目前阻碍这类调查的主要障碍，开辟一个几乎完全未被探索的领域，涉及到一般的食品添加剂的编制和研究。它们还将为行业提供新的选项，承诺在粘合方面的能力大幅提升，导致更安全、更坚固、更轻的结构，更有效地利用有限的资源。

With greater reliance on fiber-reinforced composites as lightweight alternatives to metals, structural adhesives are increasingly important for joining materials in high performance applications such as airplanes, space vehicles, wind turbine blades, sporting goods, and motor vehicles. In a typical adhesive bond, the outside edges of the adhesive carry most of the peeling load and are the most likely point of failure. By creating a functionally graded adhesive joint with soft, flexible edges and a stiffer core, the stress may be distributed more evenly, increasing joint strength substantially; indeed, enhancements of up to 60 percent have been reported via this approach in the limited testing conducted to date. While such results are highly promising, no one has identified a means of creating functionally graded adhesive joints whose properties are stable over time. This work seeks to investigate the basic science behind functionally graded adhesives whose properties are controlled by adjusting exposure to radiation. Epoxy resins will be formulated for this effort; their thermal and mechanical properties will be measured, and adhesive joints based on these materials will be graded via exposure to varying doses of gamma rays and tested for strength. In tandem, computer simulations will use the measured properties to predict joint performance, with actual joint performance data used to validate the simulations. The resultant computational design tools will provide information on how best to design functionally graded joints. The efforts will open up an almost entirely unexplored field related to the preparation and study of functionally graded adhesives by removing key barriers that currently prevent such investigations. The success of this work will enable engineers to create stronger, lighter, more reliable adhesive bonds, and make more efficient use of high performance composites. The most immediate impact is likely to be felt in sectors where composites and lightweight metals are heavily used - aerospace, ship-building, ground transportation, and wind energy - with many other areas of application to follow as the implications and benefits of these efforts become apparent. Furthermore, results will be integrated into both undergraduate and graduate courses, work will be carried out to demonstrate applications through undergraduate engineering capstone projects and extracurricular activities (such as Formula SAE Competition team), and graduate students along with REU students will be trained and integrated into the research effort, Structural adhesive joints suffer from stress concentrations at their edges, while sensitivity to manufacturing flaws has made many reluctant to use them. The gradual introduction of load through a functionally graded adhesive (FGA) - soft at the edges, stiff in the middle, for instance - addresses both issues. Recent theoretical work predicts dramatic gains in joint strength using FGAs, a conclusion confirmed by the few experimental efforts reported in literature. The slow progress in this area stems from the difficulties in making consistent, stable, robust FGAs. Coupled with a lack of validated knowledge in the area, this has precluded their application. The overall objective is to understand how to design, create, characterize and predict the properties of FGA joints, with the long-term goal of increased joint performance, stability, and reliability. The central hypothesis of this work is that stable property gradients designed using predictive models will enable stronger, more stable bonded joints. This work will provide a fundamental understanding of the design, creation and behavior of FGAs. Design rules for FGA joints will be identified, general methods to create and characterize FGAs and FGA joints will be realized, and predictions of FGA joint properties will be validated. This effort will expand our ability to model complex joints and make and characterize functionally graded materials in general. The efforts will open up an almost entirely unexplored field related to the generalized preparation and study of FGAs by removing key barriers that currently prevent such investigations. They will also provide industry with new options that promise a significant advance in capabilities as far as bonding is concerned, leading to safer, stronger, lighter structures that make more efficient use of limited resources.