Reliability Based Analysis with the Boundary Element Method

本研究的主要目的是开发技术，分析复杂的工程结构的可靠性，使用边界元法（BEM）。边界元法是一种用于结构分析的数值计算方法，可用于模拟承受载荷的结构的行为。有限元法（FEM）是结构分析的另一种数值计算方法。与有限元法相比，边界元法具有速度快、精度高等优点，这使得它在可靠性分析中非常有用。本研究将着重于边界元法的应用。结构的可靠度是指结构成功地满足一定标准的概率。例如，该标准可以是结构不断裂，或者当施加载荷时它不弯曲太多。因此，结构的可靠性是确定安全性的一个非常重要的因素。对于飞机来说尤其如此，因为结构故障的后果可能非常严重。结构的可靠性受到许多不确定性因素的影响。例如，我们可能不知道将应用于结构的确切载荷量，或者我们可能不知道结构的确切尺寸。在进行可靠性分析时，重要的是要考虑这些不确定性的来源。边界元法也将用于模拟结构中由于疲劳而产生的裂纹的增长。疲劳是由于反复施加载荷而导致的材料弱化。它是飞机结构失效的主要原因，也是其他工程结构失效的最常见原因之一。通过更好地理解疲劳裂纹扩展背后的力学原理，可以更准确地估计结构的疲劳寿命（在疲劳严重削弱结构之前，结构预计保持使用多长时间）。因此，可以更准确地确定例行检查的时间，从而提高安全性和效率。边界元法已被证明是非常有效的模拟裂纹扩展。它能够以非常有效的自动方式对裂纹扩展进行建模，既快速又准确。确定结构疲劳寿命的一个重要步骤是估计结构中裂纹的初始尺寸。这些初始裂纹称为等效初始裂纹尺寸（EIFS），边界元法可用于估算这些裂纹尺寸。还将开展工作，通过应用替代模型来提高边界元法的效率。替代建模涉及用廉价的“替代模型”替换计算昂贵的模型（如FEM或BEM模型）。这些代理模型是从FEM或BEM模型创建的。这些代理模型的计算成本非常低，但只能用于狭窄的目的。它们对于可靠性分析或估计EIFS是非常有用的。多保真度模型是代理模型的一种先进形式，并将在这项工作中使用。新的方面，这项研究：-结构可靠性分析，使用边界元法（BEM）还没有被认为是由研究界。复杂工程结构的可靠性分析技术将首次使用边界元法得到发展。由于边界元法的速度和精度，它可以被证明是更有效的其他方法的可靠性分析。因此，可以以更少的计算成本研究更复杂的工程结构。用边界元法对EIFS进行了估算。考虑到它在模拟裂纹扩展方面的有效性，它可能比其他估计EIFS的方法更有效。因此，对于更复杂的工程结构，EIFS可以确定较少的计算成本。- 多保真度模型将首次用于边界元法，它们也将首次用于估计EIFS。

The main aim of this research is to develop techniques for analysing the reliability of complex engineering structures using the boundary element method (BEM). The BEM is a numerical computational method for structural analysis that can be used to model the behaviour of structures subjected to loads. The Finite Element Method (FEM) is another numerical computational method for structural analysis. The BEM has several advantages over the FEM that make it very useful for reliability analysis, namely its speed and accuracy. This research will focus on the use of the BEM.The reliability of a structure is the probability that the structure successfully meets certain criteria. For example, the criteria could be that the structure doesn't break, or that it doesn't bend too much when a load is applied. The reliability of a structure is therefore a very important factor in determining safety. This is especially true for aircraft, since the consequences of structural failure can be very significant. The reliability of a structure is influenced by many sources of uncertainty. For example, we might not know the exact amount of load that will be applied to the structure, or we might not know the exact dimensions of the structure. When performing reliability analyses, it is important to take into account these sources of uncertainty. The BEM will also be used to model the growth of cracks in structures due to fatigue. Fatigue is the weakening of a material due to repeatedly applied loads. It is the leading cause of structural failure in aircraft, and among the most common causes of failure in other engineering structures. By better understanding the mechanics behind fatigue crack growth, the fatigue life (how long the structure is expected to remain in service before fatigue severely weakens it) of a structure can be more accurately estimated. The timing of routine inspections can therefore be more accurately determined, providing improvements in both safety and efficiency. The BEM has proven to be very effective at modelling crack growth. It is able to model crack growth in a very effective automatic manner that is both quick and accurate. An important step in determining the fatigue life of a structure is to estimate the initial sizes of the cracks in the structure. These initial cracks are called Equivalent Initial Flaw Sizes (EIFS), and the BEM can be used to estimate them.Work will also be undertaken to improve the efficiency of the BEM through the application of surrogate modelling. Surrogate modelling involves replacing computationally-expensive models (such as a FEM or BEM model) with a cheap 'surrogate model'. These surrogate models are created from the FEM or BEM model. These surrogate models are very computationally-cheap but can only be used for narrow purposes. They are very useful for reliability analyses or for estimating EIFS. Multi-fidelity models are an advanced form of surrogate models and will be used in this work.Novel aspects of this research:- Structural reliability analysis using the boundary element method (BEM) has not been considered by the research community. Techniques for the reliability analysis of complex engineering structures will be developed for the first time using the BEM. Due to the speed and accuracy of the BEM, it could prove to be more effective that other methods for reliability analysis. Therefore, more complex engineering structures can be investigated for less computational cost.- The estimation of EIFS has not been carried out before with the BEM. Given its effectiveness in modelling crack growth, it could be more effective than other methods for estimating EIFS. Therefore, the EIFS for more complex engineering structures can be determined for less computational cost. - Multi-fidelity models will be used for the first time with the BEM, and they will also be used for the first time for estimating EIFS.