Seite - 58 - in 3D Printing of Metals

Metals 2017,7, 91 struts.Atameasurement lengthof4mm, theλCusedwas0.25andthenumberofmeasurement lines was9. Thedistancebetweenthe lineswas40μm. Figure6.Positionforsurfaceroughnessmeasurementsonall fourstruts. Representative samples were prepared to obtain information on the microstructure. Polishedsampleswereetchedbyimmersion inamixtureof100mLH2O,3mLHNO3and3mLHF. Themicrostructureswereexaminedunder thedigitalmicroscope—KeyenceVHX2000. In order to study themechanical properties of the produced samples, twomechanical tests wereperformed: (1)All scaffoldsweremechanically testedviauniaxial compressionaccordingtoDINstandard (DIN EN 50106). A universal testing machine (INSTRON E 10,000; Instron GmbH, Darmstadt, Germany)was used to conduct themechanical testing. Compression testing of the sampleswas performedwithaconstant traversevelocityof2mm/min. Thestrain rate for thepresent results is approximately1.6×10−2 s−1.All testswereperformedunderstandardatmosphericconditions(24◦C, 50%rel. humidity). Thepartswere testedwith therodsorientedperpendicular to the loaddirection. Thetopandbottomareasof thespecimenwerealignedparallel to the loadbearingcapacity.Values for loadanddisplacementwerecontinuouslyrecordedduringtesting.All fabricatedscaffoldswere tested withfivespecimensuntilmechanical failureoccurred. EvaluationwasalwaysbasedonthecrosssectionareaAdeterminedwith thedigitalmicroscope andthemaximumloadFbefore thefirst fractureordecrease in loadoccurred. Basedonthesedata, thecompressionstrengthwasdeterminedas σ= F A (2) Alldeterminedstrengthswereconsideredwithrespect to theslendernessratioλ. Theslenderness ratioλwasdeterminedas thequotientof theproductofcolumneffective length factorβ timesLength of the strutLandgyration radius i. The columneffective length factor for all calculationswas0.5, equivalent to theEuler loadingcase4. λ= β×L i (3) Thegyrationradius iwascalculatedasrootof thequotientof thegeometricalmomentof inertia I andthecrosssectionareaA. i= √ I A (4) Basedonthestrutcrosssectionareasdetermined, theequivalentdiameter (Figure7)wasused to calculate the gyration radius. The equivalent diametermatched thediameter of a comparable circularstrutandwascalculatedfromtheresults for thecrosssectionareasdeterminedwiththedigital microscope—KeyenceVHX2000. 58