Parts are frequently weakest in the z-direction, which is well known to those with experience in fused filament fabrication (FFF). The reliability of 3D printed items has been greatly increased by quantifying the interlayer strength and comprehending the complexities of the layer-by-layer deposition process. There is more focus on repeatable performance for functional items as additive printing progresses beyond fast prototyping. However, there are currently no set standards that regulate the printing or evaluation process for 3D printed products. The limits of the present z-direction tensile testing are discussed in this paper, along with the benefits of the unique interlayer shear testing technique for determining the weld strength of printed thermoplastics.
Limitations of z-direction tensile testing
Today, the method of choice for determining the interlayer strength of 3D printed objects is uniaxial tensile testing. Test coupons are printed vertically for this examination, and the printed layers are mechanically torn apart during testing. It is erroneous to assume that these tests can evaluate the weld strength between printed layers in order to correlate part performance with process parameters or to generalize polymer z-direction performance. An AM operator, however, quickly notices the problems with printing the coupon geometry and the wide range of tensile results. This is due to the lack of agreement on the best printing techniques to evaluate the strength of the welds between layers or whether the tensile results are typical of other printed geometries. Let’s break down the specifics of why the geometry of vertical coupons are a cause for concern.
The grip, gauge, and transition zone where the profile shifts from wide to narrow make up the tensile coupons. Each layer that follows the previous one in the transition region is thinner, resulting in notches at the interlayers. Stress concentrations can occur at these notches during testing, which may cause the material at the transition area to fail before it should. The weld strength at various points on the coupon would be impacted by variations in the time-per-layer as the coupon's profile narrows, resulting in a non-uniform thermal history. As a result, the coupons' non-gauge part fails and the tensile readings are incorrect. Only the gauge part of the geometry is permitted for the coupons to fail in accordance with ASTM D638 for reliable strength.
Since there is a significant amount of uncertainty in component performance, the application of 3D printing for businesses is limited by the poor tensile failure of z-axis coupons in combination with the broad scatter of strength values. Using interlayer shear testing rather than uniaxial tensile tests, AON3D has recently published a new technique for assessing z-direction characteristics in partnership with the National Research Council of Canada and Toronto Metropolitan University. The purpose of this ongoing endeavor is to create new AM-specific test standards for thermoplastics used in 3D printing. The advantages of interlayer shear testing over z-direction strength characterization are outlined in the following section.
How does interlayer shear testing work?
The standard test method for in-plane shear strength of reinforced plastics, ASTM D3846, which employs notched coupons loaded in compression, is the foundation for the proposed shear testing procedure. Using a set of optimal parameters, ABS blocks were produced on the AON M2. When examined under a microscope, the M2 hardware could create pieces that were dimensionally correct and had few to no visible voids.
To describe the interlayer weld strength in the printed ABS, two sets of shear evaluation methodologies were created. The first was made up of ASTM D3846 coupons with notches cut out of them that were loaded tightly to tear the printed layers apart. The second approach, which allowed for the direct measurement of strain and determination of shear modulus, used a smaller un-notched miniature block and a significantly modified version of the ASTM standard. The corresponding test setups are shown in the photographs below.
The repeatability of this method over z-axis tensile testing was shown in both studies, which had a coefficient of variation for strength between tested samples of 5% or less. The test fixture caused some little interference with the notched D3846 coupons (Test 1). The mini-shear coupons (Test 2), which had an average coefficient of variation of 2.2% over three batches, did not display this interference during testing and gave a more precise evaluation of shear strength.
Why in-plane shear is better for interlayer strength testing
The advantage of interlayer shear testing over z-axis tensile testing is that consistent shear stress can be delivered over a number of beads and interlayers in addition to higher experimental precision. The bulk behavior of printed parts can be evaluated using coupons that have been printed vertically, but this assessment is exclusive to the coupon geometry and cannot be generalized to predict the performance of other component geometries. The weakest plane, which is frequently the interlayer weld zone, fails as expected in shear testing, which isolates the stress to a thin portion of printed beads. As a result, in-plane shear testing offers a more precise evaluation of the mechanical strength of the interlayer. The printed blocks also have a uniform cross section in the z-direction, allowing the temperature and time to be controlled. Our previous article on common issues in 3D printing goes into further detail on what causes poor layer weld strength. This in turn enables better traceability to process conditions and better predictability on part performance.
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