Application of the small punch test to determine the fatigue properties of additive manufactured aerospace alloys

Abstract

Abstract. Additive layer manufacturing (ALM) processes are becoming increasingly prevalent in the aerospace industry as design engineers look to profit from the numerous advantages that these advanced techniques can offer. However, given the safety critical nature and arduous operating conditions to which these components will be exposed to whilst in service, it is essential that the mechanical properties of such structures are fully understood. Transient microstructures are a typical characteristic of ALM components and resulting from the thermal cycles that occur during the build operation. Those microstructures make any mechanical assessment an involved procedure when assessing the process variables for any given parameter set. A useful mechanical test technique is small-scale testing, in particular, the small punch (SP) test. SP testing is capable of localised sampling of a larger scale component and presents an attractive option to mechanically assess complex parts with representative geometries, that would not be possible using more conventional uniaxial test approaches. This paper will present the recent development of a small-scale testing methodology capable of inducing fatigue damage and a series of novel tests performed on different variants of Ti6Al-4V.

Small punch fatigue testing of a nickel superalloy

“…Figure 2 shows the SP fatigue test setup which for testing positive R ratios, as in the case in this research, is consistent with that of a traditional SP test under a constant displacement rate, as a result the dimensions are as those prescribed by the European Small Punch Code of Practise [1]. Previous work in the authors' research group has gone into more comprehensive detail regarding this unique test arrangement [9,10]. Tests were performed with an R=0.1 load ratio at a 1Hz frequency using a sinusoidal waveform.…”

“…The advancement and worldwide implementation of the SP technique has highlighted the necessity to understand the behaviour of the arrangement under cyclic deformation, particularly in aerospace alloys where fatigue performance is a life limiting parameter. Thus far, efforts have seen a SP test with a cyclic loading capability [7,8] implemented using compressive loading ratios (R>0), whilst a second arrangement has been developed to accommodate positive loading regimes (R=0.1) and fully reversed cycles (R=-1) by means of a twin indenter layout [9,10].…”

Modelling the small punch tensile behaviour of an aerospace alloy

“…Since the early 1990s, Swansea University has invested significant effort into exploiting the promise of the SP test, where previous research has adopted the technique to characterise single crystal turbine blade alloys, 7,8 next-generation titanium aluminide materials 9 and additive layer manufactured (ALM) components. 10 One such form of ALM that is readily receiving significant investment is electron beam melting (EBM). 11 In this process, an electron beam provides the power source to melt a metal powder that lies on a powder bed to then build up a component layer by layer, typically within a near-vacuum environment.…”

Mechanics of the small punch test: a review and qualification of additive manufacturing materials

“…Present works are directed at starting to fill these gaps, as shown in [31,179], including test control variations for cyclic loading, as well as introduction of a dual-punch test setup to introduce fully reversed cyclic loading in an effort to replicate traditional fatigue testing damage. Concurrent work completed by Lancaster et al, utilized a similar dual punch setup to fatigue test variants of Ti-6Al-4 V manufactured by both electron beam melting and conventional forging [180]. Regardless of its scarcity, however, the literature on this topic shows the potential for fatigue testing with SPT, especially since sample size does not seem to have a significant effect.…”