Our Sites

A better way to assess the tensile strength of 3D-printed parts

Researchers use interlayer shear testing instead of uniaxial tensile testing to gauge Z-direction strength of plastic parts 3D-printed on filament printers

FDM 3d printing

A new way to test the shear strength of FFF 3D-printed parts provides a better assessment of weld strength. Images: AON3D

The following article provides an overview of a novel method for testing the Z-axis strength of parts 3D-printed on FFF (fused filament fabrication)-style printers. The article, posted to the AON3D website, excerpts information from a paper published by the standards organization ASTM International. Titled “Improved Test Methods for Polymer Additive Manufacturing Interlayer Weld Strength and Filament Mechanical Properties,” the authors are Richard G. Cole, National Research Council Canada, Aerospace Research Centre; Kazem Fayazbakhsh, Aerospace Engineering Department, Toronto Metropolitan University (formerly Ryerson University); Abraham Avalos, AON3D; and Nicholas A. Nadeau, AON3D.

Those familiar with fused filament fabrication (FFF) 3D printing know that parts are often weakest in the Z direction. Understanding the intricacies of additive manufacturing’s layer-by-layer deposition process and characterizing interlayer strength has been instrumental to ensuring the reliability of 3D-printed parts. And, as additive manufacturing grows beyond rapid prototyping, there is a greater emphasis on repeatable performance for functional parts.

However, specific standards that control the printing or evaluation process have not yet been established for 3D-printed parts.

In this article, we discuss the limitations of current Z-direction tensile testing and explain why a novel method of interlayer shear testing provides greater accuracy for assessing the weld strength of printed thermoplastics.

Limits of Uniaxial Testing

Today, uniaxial tensile testing is the most popular choice for assessing the interlayer strength of 3D-printed parts. With this evaluation method, test coupons are printed vertically, then mechanically pulled apart to separate the printed layers during testing.

It’s a misconception, though, that these tests can assess the weld strength between printed layers—allowing one to draw correlations between part performance and process conditions—or generalize about the performance of polymers in the Z direction.

However, what becomes immediately clear to the 3D printer operator are the issues that arise with printing the coupon geometry and the large variability in tensile results. This is because there is no consensus on the optimal printing strategies to assess the layer-to-layer weld strength or whether the tensile results are representative of other printed geometries.

In the following, we’ll break down the specifics of why the geometry of vertical coupons is a cause for concern.

The tensile coupons consist of a grip, gauge, and transition regions, where the profile changes from wide to narrow (see Figure 1). In the transition region, each consecutive layer is narrower than the previous, which creates notches at the interlayers. These notches are sites for stress concentrations during testing and can result in premature failure of the material at the transition region.

In addition, changes in the time per layer as the coupon’s profile narrows would influence the weld strength at different sections of the coupon and provide a nonuniform thermal history. This could lead to failures in the nongauge section of the coupons and inaccurate tensile results.

additive manufacturing

Figure 1: Shown are the two main 3D printing orientations used to assess the tensile strength of FFF-printed parts.

According to ASTM Standard D638, “Standard Test Method for Tensile Properties of Plastics,” coupons should only fail at the gauge section of the geometry. Figure 2 shows an optimal failure and an unacceptable failure at the transition region.

The unacceptable tensile failure of Z-axis coupons, coupled with the large scatter of strength results, can limit the use of 3D printing because of uncertain part performance.

Interlayer Shear Testing

AON3D, in collaboration with the National Research Council of Canada and Toronto Metropolitan University, recently published a new method for evaluating Z-direction properties by using interlayer shear testing instead of uniaxial. The work is being extended with a goal of establishing new additive manufacturing-specific test standards for 3D-printed thermoplastics.

The proposed method is based on ASTM Standard D3846, “Standard Test Method for In-Plane Shear Strength of Reinforced Plastics,” which involves using notched coupons that are loaded in compression. Blocks were printed from ABS on an AON M2 printer using a set of optimized parameters. The parts were dimensionally accurate and exhibited few to no visible voids when observed under a microscope (see Figure 3).

What becomes immediately clear are the issues that arise with printing the coupon geometry and the large variability in tensile results. This is because there is no consensus on the optimal printing strategies to assess the layer-to-layer weld strength or whether the tensile results are representative of other printed geometries.

Two shear-evaluation test methods were developed to characterize the interlayer weld strength in the printed ABS (see Figure 4). The first comprised the ASTM D3846 coupons, which had notches cut and then were loaded in compression to shear the printed layers apart. The second method used a highly modified version of the ASTM standard. It featured a smaller, unnotched miniature block, which allowed for the direct measurement of strain and determination of shear modulus.

The Test 1 and Test 2 experiments both provided a coefficient of variation between tested samples of 5% or less for strength, demonstrating the repeatability of this method over Z-axis tensile testing. The notched D3846 coupons (Test 1) suffered from a moderate amount of interference from the test fixture. The minishear coupons (Test 2) did not exhibit this interference during testing and provided a more accurate assessment of shear strength, with an average coefficient of variation across three batches of 2.2%.

Why In-Plane is Better

Along with better experimental precision, the benefit of interlayer shear testing over Z-axis tensile testing is that uniform shear stress can be applied over several beads and interlayers. Coupons printed vertically provide an assessment of the bulk behavior of printed parts, which is specific to the coupon geometry and cannot be extrapolated to predict the performance of different part geometries.

Shear testing, on the other hand, can isolate the stress to a thin section of printed beads, and the weakest plane, which is often the interlayer weld region, fails as expected.

additive manufacturing

Figure 2: According to ASTM Standard D638, tensile failure should occur only at the gauge section of the coupon (left). Tensile failure in the transition area of the coupon is unacceptable (right).

Therefore, in-plane shear testing provides a more accurate assessment of the Z-axis mechanical strength. In addition, the printed blocks have a uniform cross section in the Z direction so that the temperature- and time-dependency of layer-by-layer welding can be studied more clearly.

(Click here to read an article AON3D posted about factors that can degrade layer-weld strength.)

About the Author

AON3D

AON3D is a Montreal-based additive manufacturing hardware, software, and materials science company.