Fatigue Performance of Powder Bed Fused Ti-6Al-4V Component with Integrated Chemically Etched Capillary for Structural Health Monitoring Application

Abstract

Fatigue performance of additively manufactured (AM) components is still uncertain and inconsistent. Structural health monitoring (SHM) systems offer a solution to continuously monitor the structural integrity of a structure. The effective Structural Health Monitoring (eSHM) system is the first SHM principle developed with the principal purpose to monitor AM components. The eSHM principle exploits the design freedom offered by AM to integrate a capillary inside the component. The capillary is put under low vacuum and the pressure is monitored during the operation of the component. As-built AM surfaces report elevated surface roughness and are one of the principle causes of premature fatigue initiation and fatigue failure. The current study will investigate the effect of a chemical etching (CE) post-process on the capillary surface and evaluate its effect on the fatigue performance.

On the Influence of Capillary-Based Structural Health Monitoring on Fatigue Crack Initiation and Propagation in Straight Lugs

“…Nevertheless, the diameter/aspect ratio of the capillary can be tuned, and this part of the study aims at quantifying in which proportion the capillary diameter/aspect ratio influences the crack initiation behavior. Besides, the “edge-to-edge” distance between the bottom of the capillary and the lug hole surface, referred here as distance “a” (in accordance with the terminology used in the works [26,30,27]), is also a parameter in the engineering of the crack detection system. A smaller edge-to-edge distance would imply a quicker detection of the propagating crack, and again, one aims at quantifying the influence of this design parameter on the crack initiation.…”

“…However, the influence it has on stress raise in the initiation region and at the capillary surface has to be studied. In the experimental investigations around the eSHM, the distance “a” is traditionally set to 2 mm [26,27]. Therefore, for the purpose of this parametric study, the diameter of the capillary was kept constant at 1.5 mm, and edge-to-edge distances equal to 3 mm, 2 mm and 1 mm have been considered (respectively shown in red, black and green on Figure 5b).…”

“…This has been thoroughly studied in the work by Günther et al [22]. This is also the reason why experimental testing around the eSHM has traditionally used an edge-to-edge distance of 2 mm [26,27]. From this, it can be inferred that capillary roughness is the main challenge to the practical implementation of the eSHM, as the presence of the capillary itself should not affect the crack initiation site nor significantly the crack initiation life, provided the capillaries are small with respect to the lug dimensions.…”

“…In the present research, the effect of capillary roughness on the reduction of fatigue life has not been taken into account. Indeed, technologies such as hybrid manufacturing (combination of additive and subtractive manufacturing) or chemical etching could potentially enable a drastic reduction in capillary roughness levels in the near future [23,24,25,26]. Moreover, in the context of the eSHM system, the roughness influence has already been experimentally investigated (see [27]).…”

Additive Manufactured Metallic Smart Structures to Monitor the Mechanical Behavior In Situ

“…This increased the fatigue strength values (at 500,000 cycles) to significantly above the yield value of the conventional material. A further improvement of the system was reached by reducing the internal roughness of the capillary system by a chemical etching procedure [13].…”