# Fatigue Crack Growth in Managing Steel Obtained by Selective Laser Melting
## 1 Introduction  
Selective Laser Melting (SLM) is a technology for additive manufacturing consisting of the fusion of a fine metal power layer by layer. It is an iterative procedure consisting of (1) deposition of a thin layer of powder; (2) selective melting with a laser; and (3) descent of the manufacturing platform. The laser moves in the build area with controlled speed and scan pattern. The unmelted powder is collected and therefore there is no waste of material. The geometry is defined in a CAD model which is read by the SLM equipment. This procedure is very interesting to generate components with complex geometry and eventually composed of different materials. Therefore, SLM is becoming prominent in the automotive, aerospace, medical and injection molding industries. Various materials, such as titanium alloys [1], nickel-based alloys, iron, aluminum, bronze, copper, stainless steels [2] and high-speed steels of metal matrix, can be considered in this process.  
Several studies, primarily focused on the influence of the selection of metal powder and sintering parameters on the microstructure of the sintered components, state that SLM components can provide static mechanical properties competitive with those of conventional bulk materials. Nevertheless, in service the components are typically dynamically loaded; therefore, the design must include the analyses of fatigue performance. Studies focused on the fatigue behavior of sintered materials are relatively scarce. Most previous studies developed tested smooth specimens under stress control [1; 3; 4] or strain control [2]. Notched specimens have also been tested under stress control in order to obtain S-N curves [5]. The presence of porosity, high surface roughness and thermal residual stresses,typical of these materials, reduce the fatigue initiation life, increasing the importance of the crack propagation regime. Pores result from powder contaminations, evaporation or local voids after powder-layer deposition. However, few studies on fatigue crack growth can be seen in the literature. [PERSON] et al. [6] obtained da/dN-AK curves in compact tension (CT) specimens made of TiAl6V4 (width, W = 40 mm and thickness, t = 10 mm). [PERSON] et al. [7] presented da/dN-AK curves obtained in four-point bending specimens and using replica technique to measure the crack length.  
The main objective of this work is to study the fatigue crack propagation in AISI 18 Ni300 managing steel obtained by SLM. The mechanisms behind the phenomenon were analyzed using two different approaches. First, the fatigue crack growth rate, da/dN, was obtained using compact tension specimens, in agreement with ASTM E647 standard, and da/dN-\(\Delta\)K curves are plotted. Second, the crack tip plastic deformation was analyzed, and da/dN is presented versus plastic Crack Tip Opening Displacement (CTOD), predicted numerically for the C(T) specimen. The material plastic behavior was studied using results obtained from low-cycle fatigue tests on smooth specimens under constant amplitude strain range. The stress-strain hysteresis loops were used for fitting the hardening models.