Cruciform specimens' experimental analysis in ultrasonic fatigue testing

Abstract

In this paper, two special aluminium cruciform specimens are designed and tested in an ultrasonic fatigue machine. They were designed based on Single‐Input‐Multiple‐Output (SIMO) modal analysis to induce in‐plane biaxial stress combinations (in‐phase tension‐tension [T‐T] and out‐of‐phase compression‐tension [C‐T]) when at resonance at 20 kHz. The geometries were subjected to both numerical analysis and experimental testing to understand if they can indeed create the intended biaxial state of stresses. Both numerical and experimental results showed an impact of nearby resonant modes of noninterest on the correct functioning of the specimens, especially regarding the T‐T specimen where a large deviation from the mode of interest was measured. This means that future work includes re‐designing T‐T specimens taking into account these mode shapes. Only out‐of‐phase specimens demonstrated to work properly and tests until failure were conducted. The first failure results showed to be consistent with literature when out‐of‐phase biaxial stress is applied cyclically.

Review of Multiaxial Testing for Very High Cycle Fatigue: From ‘Conventional’ to Ultrasonic Machines

“…An original approach to biaxial UFT was proposed in order to achieve VHCF regime using cruciform specimens [25]. Having as starting point the same principles used in the design of the ultrasonic fatigue testing machines and design rules for cruciform specimen design as in [16,24,55], it was shown that at least when using cruciform specimens for in-plane tension-tension (biaxial) testing, only the specimen needs to be redesigned and no changes are required to be made to the machine. Figure 16 shows the UFT machine setup described by Montalvão and Wren [25].…”

“…The result was a cruciform specimen with the same central section geometry and thickness as the original specimen, but only slightly shorter (or even narrower) arms (Figure 18). To achieve a working specimen, several dimension combinations were numerically tested and experimentally tested until they have the resonance mode of interest within the working frequency [24]. The same principles as the ones described in the previous Section 4.2.2 were used to design specimens that are able to deliver biaxiality ratios ∈ −1, 1 [26]:…”

“…The problem with cruciform specimens, as reported by Costa et al [24]-in a study that combined FEA analysis, experimental UFT results, and thermographic analysis-is that these specimens may present 'undesired' mode shapes in the vicinity of 20 kHz. In such a case, the specimen's deformation shape will be the result from the combination of the two mode shapes being excited, and not just predominantly from one.…”

“…Machines 2020, 8, x FOR PEER REVIEW 23 of 30 Figure 26. Cruciform C-T specimen failed after tension/tension (out-of-phase) biaxial UFT at VHCF [24].…”

“…As already mentioned, most UFT in the literature is based on uniaxial tension-compression machines [17][18][19], although pure torsion UFT machines have also been developed [20][21][22]. More recently, the concepts of biaxial and VHCF fatigue were combined with the development of cylindrical specimens for tension/torsion UFT [15,23]; cruciform specimens for tension/tension in-plane UFT [24][25][26]; and disc-shaped specimens for multiaxial bending UFT [27].…”