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Testing Artifacts For AM

Testing artifacts can be used for different objectives.  They had to be versatile for different types of non-destructive testing while being practical for printing and numerical simulations.  Furthermore, their geometrical characteristics needed to be quickly and accurately measurable using standard tools.

 At the same time, the structure of the testing artifact had to allow users to alter process parameters.  It needed to have a reasonable size, thickness, and length to enable the effect of the different process parameters to be observed and measured.

 

Simple structures like cantilever and constrained plates and beams also have been employed for analyses of the mechanical properties and have proven to be practical for estimating isotropic, orthotropic, and anisotropic elastic properties.  The applied approaches also measured the natural frequencies of the vibrating object to evaluate the elastic moduli in complex multi-layer structures, such as composite materials, which lent itself to additively manufactured plates and beams.

Cantilever_beam_NDT.png

Fig. 1. Digital holographic interferometric setup sensitive to out-of-plane displacement: (a) DHI setup: BS represents beam splitter; M1, mirror; PZT, piezo for phase stepping; DL1 and DL2, beam expanders; SF1, spatial filter; OB and RF, object beam and reference beam; FI, fiber; CP, fiber coupler; and CCD, video camera; and (b) The setup in operation with a close view of the optical head. Source: K.P. Dissertation

Besides, the American Society for Testing and Materials (ASTM) established standard number E-1875 and E-1876 to explain the procedure of evaluation of the dynamic mechanical properties through modal analyses.   

Some of the critical and recommended criteria of the testing artifacts being utilized for similar non-invasive characterization gleaned from the various reference are listed here:

  • artifact has to be large enough to examine the performance of the machine near the extremes of the platform as well as near the center;

  • artifact should not take too long to build and require no post-treatment or manual intervention;

  • artifact should be easy to measure the properties of interest,

  • artifact should allow measuring the repeatability;

  • the plate and beam have to be rectangular;

  • plate and beam should have homogeneous or near-homogeneous in-plane mass distribution; 

  • a slender ratio of length to cross-section should be preferably as large as 25 but must not be less than 5;

  • width-to-thickness ratio must be 5 for shear modulus measurements of rectangular bars (for the central thin section of the testing artifacts);

  • the testing artifact for the fundamental flexural frequencies must range between 100 to 10,000 Hz, and a fundamental torsional resonant frequency must be in the range from 200 to 30,000 Hz;

  • the mass of the specimen must be a minimum of 5g to avoid coupling effects;

  • all surfaces on the rectangular specimen must be flat.  Opposite surfaces across the length, thickness, and width must be parallel to within 0.1 %;

  • for the plate section, the length-to-thickness and width-to-thickness ratios should be higher than 20 to satisfy the thin plate theory;

  • the ratio between Young’s modulus and shear modulus should be lower than 50;

  • the length-to-width ratio must not equal (E1/E2)1/4.  

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There are also some criteria gathered from experimental perspectives and the capability of the tools in the lab:

  • shaker frequency should range between 20 Hz to 15KHz;

  • achievable resolution for out-of-plane distortion is 50µm;

  • thickness is one of the most critical and influential parameters and has to be measured precisely when it is less than 3 mm;

  • the thickness deviation measured for three different locations must be within 0.1% for the calculation error to be less than 0.3%;

  • the length and width of the specimens must be measured with 0.1% resolution to warranted the measurements with an error of less than 0.3%.

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