The use of additive manufacturing (AM) to make metallic parts is becoming increasingly popular in the modern industrial sector for its ability to create complex-shaped and one-off parts quickly and efficiently. However, there is one key step in the AM production process which has traditionally been neither fast nor cost-effective: AM part qualification.
This step, which involves testing the material properties of 3D-printed parts, has traditionally been a lengthy and costly affair due to the need for machining tensile coupons from each part and then performing tests on each of them.
Fortunately, recent advances in testing technology have made it possible to significantly reduce the time and cost associated with qualifying metal parts produced by additive manufacturing. One such method is PIP testing, developed by the former University of Cambridge Scientists at Plastometrex, which can produce stress-strain curves for whole parts in under five minutes. This means that testing times can be reduced 80-fold and cost savings can amount to 99%, without compromising on data accuracy.
The current “gold standard” for testing the material properties of AM parts, and anything metallic, is tensile testing, which involves applying tension to a sample (also known as a coupon) to produce a nominal stress-strain curve, which is then converted into a true stress-strain curve. Let’s break down the process:
Step 1: Produce a “dog bone” coupon of material (Figure 1)
This may be done by printing a row of coupons alongside your part and/or machining your part into coupons. The coupons usually measure around 100-220 mm in length and 10s of mm in width so a significant amount of valuable AM material and time is required to produce them. If machining is required, this will be done using a CNC lathe or mill – a step that is often outsourced to dedicated machine shops, often resulting in lengthy lead-times before the coupons have even been loaded into the testing machine - another step which may also be outsourced.
!(assets/fig-1.-astm-e8-coupon.png "Figure 1: ASTM E8 \\"dog bone\\" coupon, for conducting a tensile test")
Step 2. Load coupons in test machine
The coupons will then be loaded into a tensile testing machine fitted with an extensometer (this can be an extensive outlay, with the price of tensile machines ranging from $50k - $620k). A set of grips will hold the sample at either end, which then pull apart to apply tension and the load and displacement is monitored.
Step 3. Plot nominal stress-strain curve
Once data has been gathered at the end of the test, the yield strength, ultimate tensile strength, elongation, and reduction of area, are used to create a stress-strain curve (Figure 2). For most people, this will be the final step in the process. However, for those looking at using their data for more advanced modelling, some work needs to be done on converting this nominal stress-strain curve.
Step 4. Converting nominal to true stress-strain
Using standard analytical equations, the nominal stress-strain curve can be converted to a true stress-strain curve up to the ultimate tensile strength (Figure 3). After the UTS, stress and strain fields are not uniform, meaning that these equations will not apply beyond this point. The true stress-strain data is often required for use in finite element analysis of the part to ensure it is fit for purpose in service.
In summary, this process is hardware, material, energy, and man-power intensive, which all adds up in terms of cost, carbon footprint, and time associated with each test, even when the whole process is conducted in-house. Where parts of the process, such as machining, are outsourced, this can bump up the cost further and cause significant, sometimes months of delays.
For AM users looking to drive efficiencies in their qualification processes, there are options. One such option is the novel PIP testing method based on 15 years of research by former University of Cambridge materials scientists and developed by Plastometrex. PIP testing is already in use by major AM OEMs, and top-tier academic and industrial institutions across the world.
Having looked at the steps involved with conducting a tensile test, let’s now compare that with the process of conducting a PIP test, using the Plastometrex Benchtop Indentation Plastometer (Figure 4):
Step 1: Producing the sample
PIP can test any sample with a flat surface and minimal dimensions of 3 x 3 x 1.5 mm. This means that there is no need to create samples specifically for PIP testing as it can be performed on small blocks such as density cubes. Alternatively, PIP can test real parts, mapping property variation on a millimetre scale.
Step 2: Sample preparation
The only sample preparation required is a quick grind on a basic grinding wheel. It is advised to go up to a P2500 (1200-grit) grind but P1200 (600-grit) can work as well. This process usually takes no more than 15 minutes depending on the material and can take as little as 30 seconds.
Step 3: Testing the sample
All that's left is to place the sample on the sample tray of the Plastometer and press start on the software. The testing procedure is fully automated, making it very easy to use, and eliminating much of the risk of human error.
Step 4: Looking at the results
After 5 minutes, both the full nominal and true stress-strain curves will be plotted on the software as well as the numerical values of the UTS, Yield Stress, and Uniform Elongation.
With the click of two simple buttons this information, and the profile data, can easily be exported to either pdf for easy overview or csv for further in-depth data analysis.
All of this is done by one machine – no lathes or mills, extensometers, or tensile machines, no calculations or conversions, and best of all – no outsourcing. An Indentation Plastometer is also a fraction of the size of a tensile machine and weighs in at just 35kg, meaning it can comfortably sit on a desk.
When the time and cost of each step is taken into consideration, we see that PIP testing can save weeks (often months) of waiting and operational time, as well as delivering a 99% cost-saving, when compared to conventional AM part qualification. Here’s how we worked it out:
Time (Figure 5):
Cost (Figure 6):
With the added benefits of providing the ability to measure hardness numbers and property mapping, the PIP test doesn’t just deliver faster and cheaper, but also more insightful AM part qualification.
Over 45% of the 450 organisations surveyed by Plastometrex indicated that being able to map property variations across parts would add value to their organisation. AM offers unparalleled design freedom, empowering engineers to design complex parts and systems. These complex geometries often lead to heterogeneous mechanical properties across a printed component. Probing these differences is critical when trying to fully understand a component’s mechanical performance and how this is related to component geometry, processing parameters, and material composition.
The Indentation Plastometer has a much greater spatial resolution than the traditional tensile test. It allows users to carry out an array of millimetre-scale indents across the surface of a component in order to map spatial variations in yield stress, hardening behaviour, and ultimate tensile strength. This functionality provides data that would otherwise be impossible or extremely expensive to achieve via traditional tensile testing (which would require multiple coupons to be machined from a single part).
Conventional testing methods have been holding back the industrial adoption of additive manufacturing by hampering the main selling point of AM: speed. With new methods, such as the PIP test, AM part qualification times and costs can be slashed, enabling engineers to quickly identify any structural defects or weaknesses in AM-produced parts without having to cut off sections for manual inspection or conducting destructive tests on multiple components. By monitoring the stress-strain curves produced by PIP testing during each phase of production and assembly, any problems can quickly be detected before they become serious issues that could lead to downtime or product failure.
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