Fatigue Performance of Ti-6Al-4V Additively Manufactured Specimens with Integrated Capillaries of an Embedded Structural Health Monitoring System

Abstract

Additive manufacturing (AM) of metals offers new possibilities for the production of complex structures. Up to now, investigations on the mechanical response of AM metallic parts show a significant spread and unexpected failures cannot be excluded. In this work, we focus on the detection of fatigue cracks through the integration of a Structural Health Monitoring (SHM) system in Ti-6Al-4V specimens. The working principle of the presented system is based on the integration of small capillaries that are capable of detecting fatigue cracks. Four-point bending fatigue tests have been performed on Ti-6Al-4V specimens with integrated capillaries and compared to the reference specimenswithout capillaries. Specimens were produced by conventional subtractive manufacturing of wrought material and AM, using the laser based Directed Energy Deposition (DED) process. In this study, we investigated the effect of the presence of the capillary on the fatigue strength and fatigue initiation location. Finite element (FEM) simulations were performed to validate the experimental test results. The presence of a drilled capillary in the specimens did not alter the fatigue initiation location. However, the laser based DED production process introduced roughness on the capillary surface that altered the fatigue initiation location to the capillary surface. The fatigue performance was greatly reduced when considering a printed capillary. It is concluded that the surface quality of the integrated capillary is of primary importance in order not to influence the structural integrity of the component to be monitored.

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

“…Fatigue failure occurred at a load level of 818 MPa in the HIPped specimen without chemical etching (Specimen 1). As compared to previous tests, this result equals the benchmark of wrought Ti-6Al-4V specimens [5,6] as well as the stress relieved specimen produced using laser based PBF [6] and clearly outperforms all specimens tested under as-built conditions [6]. It is clear that HIP has certainly improved the fatigue performance, but fatigue initiation eventually occurred at the capillary surface.…”

“…The step method was used to accelerate testing, with a run-out 500,000 cycles in each step. The initial stress level was 114 MPa, well below the expected fatigue strength [5,6], and increased with 78.1 MPa in each step. The test frequency was 15 Hz and a stress ratio (R ratio) of 0.1 was used.…”

“…Strantza et al [3] presented that the eSHM system has reached TRL level 3. The capillaries of the eSHM system were produced either using laser based Directed Energy Deposition (DED) [4,5] or laser based Powder Bed Fusion (PBF) [6].…”

Fatigue failure monitoring of 316L stainless steel coupons using optical fibre based distributed strain sensing

“…It has been shown that the capillary does not act as an initiation site for fatigue damage and that the fatigue properties of the specimen remain unaltered when including such capillaries [29,30]. A summary of fatigue tests of specimen with and without capillary is shown in table 1 in [31].…”

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

“…The fatigue performance was greatly reduced when considering a printed capillary. It is concluded that the surface quality of the integrated capillary is of primary importance in order not to influence the structural integrity of the component to be monitored [14,15].…”