Fatigue Strength Verification of Additively Manufactured Structures Considering the Local Loading Conditions and Microstructure (LBM-Fatigue)

增材制造（AM），特别是基于粉末的激光束熔化（LBM）工艺，在轻量化设计中制造结构部件具有很高的潜力。因此，一个有效的疲劳强度验证概念是必不可少的。虽然已建立的设计概念允许考虑增材制造的特定影响因素（例如，各向异性，“建成”（AB）表面等），但这通常会导致零件的实际寿命存在很大偏差。然而，目前的研究表明，在一个增材制造的部件中，微观结构、缺陷特征（包括工艺缺陷的大小、数量、位置和类型）以及力学性能都是不同的。因此，相关研究工作进一步证明了增材制造材料的缺陷容差对其疲劳寿命有很大影响。这些方面必须加以考虑，但还不能轻易地纳入现有的核查概念。此外，加载条件的影响及其与上述影响因素的相互关系必须包含在扩展设计概念中。因此，本研究项目的主要目标是阐述一个有效的疲劳强度验证概念，该概念基于局部概念和/或断裂力学，并考虑了上述增材制造的特定影响因素。这一概念将分3个阶段进行详细阐述，由3个工作包（AP）代表，使用AISI 316L制成的样品，并通过LBM制造。在AP1中，将基于单轴推拉疲劳试验，分析和量化AB面、建筑方向及其相互关系的影响。此外，还将通过使用面积法和仪器循环压痕试验（CIT）来确定不同建筑方向的缺陷容限，并对局部缺陷特征和微观结构进行分析。随后，这些关系将在第一个疲劳强度验证概念中被量化和整合。在此基础上，AP2将阐述旋转弯曲和扭转对加载梯度的影响，以及它们与AP1中分析的影响因素的相互关系。在这种情况下，一个特别的焦点将放在缺陷容忍度上。这些发现将被整合到验证概念中，并在双轴疲劳试验中进行预验证。最后，开发的概念将通过演示器的疲劳测试在AP3中进行验证，该演示器必须开发并显示由复杂几何形状引起的多轴渐变载荷。为了使AP1和AP2中阐述的方法具有可移植性，将分析缺陷特征、微观结构、表面形貌以及关键区域的力学性能。此外，将根据验证概念绘制增材制造部件的耐用设计建议。

Additive Manufacturing (AM), especially the powder-based Laser Beam Melting (LBM) process, enables a high potential to manufacture structural components in a lightweight design. Therefore, a valid fatigue strength verification concept is indispensable. Although established design concepts allow to consider AM specific influencing factors (e.g. anisotropy, “as-built” (AB) surface, etc.), this often results in a high deviation to the actual lifetime of parts. However, the current state of research shows that microstructure, defect characteristic, which comprises the size, number, position and type of process-induced defects, as well as mechanical properties can vary within an additively manufactured component. Thereby, relevant research works further demonstrate that the defect tolerance of an AM material highly influences the fatigue lifetime. These aspects must be considered, but cannot be integrated readily in existing verification concepts yet. Moreover, the influence of the loading condition and its interrelation with the influencing factors described above, must be included into an extended design concept. Consequently, the main goal of this research project is the elaboration of a valid fatigue strength verification concept, which is based on local concepts and/or fracture mechanics, and which considers the AM specific influencing factors described above. This concept will be elaborated over 3 stages, which are represented by 3 working packages (AP), by using specimens made of AISI 316L and manufactured via LBM.In AP1, the influence of the AB surface, building direction and their interrelation will be analyzed and quantified based on uniaxial push-pull fatigue tests. Additionally, the defect tolerance will be determined for the different building directions by using the √area-approach and instrumented cyclic indentation tests (CIT), complemented by an analysis of the local defect characteristic and microstructure. Subsequently, these relations will be quantified and integrated in a first fatigue strength verification concept.Based on this, the effect of loading gradients, achieved by rotational bending and torsion, as well as their interrelations with the influencing factors analyzed in AP1 will be elaborated in AP2. In this context, a special focus will be on the defect tolerance. These findings will be integrated in the verification concept, which will be prevalidated in biaxial fatigue tests.Finally, the concept developed will be validated in AP3 by means of fatigue tests at a demonstrator, which has to be developed and exhibits multiaxial graduated loadings caused by a complex geometry. To enable the transferability of the approaches elaborated in AP1 and AP2, the defect characteristics, the microstructure, the surface topography as well as the mechanical properties of the critical areas will be analyzed. Moreover, advises for a durable design of additively manufactured components will be drawn based on the verification concept.