Modeling loss of fatigue life in aircraft landing gear due to residual stresses from hard landings

当飞机着陆时承受的载荷超过设计极限载荷的10%时，操作员假定起落架部件将发生局部塑性变形。这导致齿轮中留下持久的残余应力 **，预计会对部件的疲劳寿命产生负面影响。在过载被认为是严重的情况下，起落架必须停止使用，进行热处理和长时间隔离-这对飞机运营商来说显然是一个昂贵的程序。用于确定是否需要从服务中移除的程序和检查取决于应力和疲劳分析，如果可以对其进行改进和验证，则可以采用不太保守的方法。这可以大大降低运营成本，而不会影响安全性。在与该公司的初步接触中，我们建议研究起落架疲劳分析的一个基本方面。具体来说，我们将研究如何准确地将残余应力的影响纳入疲劳寿命模型。通常的假设是，残余应力可被视为等效于恒定的平均载荷，该载荷叠加在使用中预期的循环载荷上。我们可以通过有限元分析和低周疲劳下简化几何形状的试件试验相结合来检验这一假设。在6个月的项目结束时，我们希望能够提供对 ** 平均应力假设的初步评估。预计这将指导进一步的研究细化建模 ** 的残余应力的影响和改进的本构模型用于预测疲劳寿命。

When loads imparted to aircraft upon landing exceed 10% of designed limit loads, operators assume that**localized plastic deformation will occur in landing gear components. This results in persistent residual stresses**left in the gear, which are expected to negatively impact the fatigue life of the components. In cases when the**overloading is considered to be significant, landing gear must be taken out of service, heat-treated and**quarantined for long periods of time - an obviously expensive procedure for aircraft operators. Procedures and**inspections used to determine the need for removal from service depend on stress and fatigue analyses that**may, if they could be refined and validated, permit a less conservative approach. This could substantially**reduce operating costs without any compromise in safety.**In this initial engagement with the company, we propose to study a fundamental aspect of landing gear fatigue**analysis. Specifically, we will examine how to accurately incorporate the effect of residual stresses within the**fatigue life model. The common assumption is that a residual stress can be treated as equivalent to a constant**mean load superimposed on the cyclic loading expected in service. We can test this assumption through a**combination of FE analysis and experiments on test coupons with simplified geometries under low-cycle**fatigue. By the end of the 6-month project, we hope to be able to offer a preliminary assessment of the**mean-stress assumption. It is anticipated that this will guide further research into refinements of the modeling**of residual stress effects and improvements to the constitutive models used to predict fatigue life.