Computing the durability of WAAM 18Ni 250 maraging steel specimens

Abstract

MIL-STD 1530D requires that the certification of an aircraft part employ analytical tools that are capable of modeling crack growth. It is further stated that the durability and damage tolerance (DADT) analyses should be based on linear elastic fracture mechanics (LEFM) and follow a building block approach. This paper illustrates the durability analysis required to certify an additively manufactured part by using the examples of durability tests performed on two wire arc additively manufactured (WAAM) 18Ni 250 Maraging steel specimens.

Further thoughts on EIDS and the durability analysis of WAAM 18Ni 250 steel with rough surfaces

“…The author's recent paper 1 illustrated how to perform a durability analysis for an Additively Manufactured (AM) built part using as examples the test results of two WAAM 18Ni 250 Maraging steel specimens with rough "as built" surfaces where failure was due to the nucleation, and subsequent growth, of small surface-breaking cracks. The specific form of the crack growth equation used in the analysis reported by Peng et al 1 was…”

“…In contrast, the NASA Fracture Control Handbook NASA-HDBK-5010 5 suggests using an initial aspect ratio of 1.5. In this context, it should be noted that in the durability predictions performed by Peng et al, 1 the EIDS used was a surface-breaking semi-elliptical crack with a depth of 0.254 mm and an aspect ratio c/a of 1.5. As such, the aspect ratio used was consistent with that suggested in NASA-HDBK-5010.…”

“…To answer that question, the analysis of Specimen 1 performed in Peng et al 1 was repeated with c/a = 1 and (initial) crack depths of 0.228, 0.254, and 0.314 mm (Details of how the analysis was performed are given in Peng et al 1 ). The resulting crack growth histories are shown in Figure 2 along with the measured crack depth history and the computed crack growth history given by Peng et al 1 Here we see that if we assume an EIDS with c/a = 1, then the fatigue crack growth curves associated with each of these initial crack depths are nonconservative.…”

Analysis of the Effect of Machining of the Surfaces of WAAM 18Ni 250 Maraging Steel Specimens on Their Durability

“…Practitioners in the field of additively manufactured structures have long been aware [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] that the interaction of surface-roughness with surface-breaking material discontinuities and surface-breaking porosity/lack of fusion can significantly degrade the durability and damage tolerance (DADT) of an additively manufactured part. Indeed, ref.…”

“…[7] concluded that the fatigue life of AM Ti-6Al-4V parts built using either electron beam melt or direct metal laser sintering was determined by surface roughness effects (a picture illustrating how surface-breaking cracks can develop as a result of surface-breaking material discontinuities/defects is shown in Figure 1). The author's recent paper [1] on the durability of WAAM 18NI 250 Maraging steel specimens tested with their surfaces left in the "as built" condition aptly illustrated this phenomenon in that the specimens failed due to the interaction between the rough-surface and surface-breaking material discontinuities. Although not reported in [1], this test program also found that the presence of large, wholly contained, internal voids/porosity and large near-surface voids did not result in failure.…”