## 4 Discussion
### Relationship between UTT and Combined MSS and SPT  
The MSS protocols have been found to produce reliable measurements of the stress-strain response of the material in the initial elastic regime and up to small plastic strains (about 0.5% plastic strain). On the other hand, the SPT protocol was found to reliably estimate the stress-plastic strain response of the material (characterized using the Swift equation). These observations open the opportunity to combine these protocols to recover a stress-strain curve for the material that could be compared directly with the UTT measurement. As already discussed, the main advantages of MSS and SPT protocols are that they have tremendous potential for dramatic savings in cost and effort while requiring only small material volumes.  
The combined stress-strain curve estimated from the MSS-SPT protocols is compared with the direct measurement from UTT Sample number 7 in Figure 10.  
Figure 10: A graph showing both MSS and SPT stress-strain curves from Sample number 7 overlaid on the corresponding UTT stress-strain curve.  
Figure 9: Yield strength and ultimate tensile strength (UTS) results from all SPTs showing average and standard deviations, along with the corresponding UTT measured yield strengths and UTS results.  
The MSS and SPT stress-strain curves were combined graphically after the MSS curves were scaled (see Equations (10)-(13)) and both MSS and SPT stress-strain curves were converted from true stress-strain curves to engineering stress-strain curves for comparison with UTT measurements. The scaled MSS data is shown in greater detail in Figure 5.  
The compact nature of the indentation machine used for MSS and SPT is well suited for materials and processes development on-board an orbiting laboratory such as the International Space Station (ISS). MSFC-SPEC-3717 and MSFC-SPEC-3716 [46; 48] requires AM metallography samples to qualify AM parameters and processes, and these samples are of sufficient size to both provide metallographic imaging and the small amounts of material which are required for MSS and SPT. SPT samples are essentially slices of material no greater than 500 \(\upmu\)m so 3-5 SPT samples may easily be extracted from a metallography sample before it is mounted. The mounted and polished metallography samples may then be used for imaging and are highly conducive to MSS. The reduced material required for both SPT and MSS, as compared to UTT, allows for reduced costs to extract the combined MSS-SPT stress-strain response which can be used to determine modulus, yield strength, and ultimate strength. [PERSON] et al. [49] performed fatigue testing using SPT on Ti-6 Al-4V and ascertained that the small sample volumes required for SPT can drastically reduce cost without compromising the accuracy of a fatigue test. [PERSON] et al. [43] performed a trade study on the cost comparison between MSS and UTT and evidenced that both reduced material and testing costs associated with MSS drastically reduce its cost compared to UTT. This overall reduced cost for MSS and SPT is particularly advantageous when high-cost materials, such as high entropy alloys, bulk metallic glasses, or refractory metals, need alloy or processing development. The cost reduction and potential for automated high-throughput testing using MSS and SPT make it possible to produce a larger dataset of yield, modulus, and ultimate strength values for a lower cost and shorter time period than UTT.