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

添加制造(AM)，特别是基于粉末的激光熔化(LBM)工艺，使得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校核概念是不可或缺的。虽然已建立的设计概念允许考虑AM特定的影响因素(例如各向异性、竣工(AB)表面等)，但这往往会导致零件的实际寿命出现较大偏差。然而，目前的研究状况表明，在添加制造的部件中，微观结构、缺陷特征(包括工艺引起的缺陷的大小、数量、位置和类型)以及机械性能都会发生变化。因此，相关研究工作进一步证明，AM材料的缺陷容限对疲劳寿命有很大影响。必须考虑这些方面，但还不能轻易地将它们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括在扩展的设计概念中。因此，本研究项目的主要目标是阐述一种有效的疲劳强度验证概念，它基于局部概念和/或断裂力学，并考虑上述AM特定的影响因素。这一概念将由3个工作包(AP)代表，使用由AISI316L制成的由LBM制造的试件，在AP1中，将基于单轴推拉疲劳试验来分析和量化AB表面、建筑方向及其相互关系的影响。此外，缺陷容限将通过√面积法和仪器化循环压痕测试来确定不同建筑方向的缺陷容限，并辅之以局部缺陷特征和微观结构的分析。随后，这些关系将被量化并整合到第一个疲劳强度验证概念中。在此基础上，通过旋转弯曲和扭转获得的载荷梯度的效果，以及它们与AP1中分析的影响因素的相互关系将在AP2中详细阐述。在这种情况下，将特别关注缺陷容忍度。这些发现将被整合到验证概念中，并将在双向疲劳试验中进行预先验证。最后，所开发的概念将在AP3中通过在演示器上的疲劳试验进行验证，该演示器必须开发并展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转换性，将分析关键区域的缺陷特征、微观结构、表面形貌以及机械性能。此外，还将根据验证概念为附加制造部件的耐用设计提供建议。

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.