Unfortunately for engineers, the spec sheet isn’t going to tell you everything you need to know about how a material will perform in the real world. Standardized tests from organizations such as ASTM and ISO are designed to be easy to perform and relevant to a broad range of applications and materials. In this regard, they represent a “lowest common denominator” approach. There purpose is the be “standard”, not to be the best representation of reality.
In this article, I’m going to pick a fight with three standardized tests that are ubiquitous in polymer 3D printing.
1. UTS doesn’t matter
Seriously - when was the last time that you broke something by pulling as hard as you can on both ends? The ultimate tensile strength (UTS) is commonly mistaken for real-world strength. Sorry, it simply doesn’t matter. There are 3D printing polymers with a UTS of 80 MPa that shatter instantly with any impact, flex, or torsion. That’s not strong.
UTS is widely reported because its easy to measure. When you do a tensile test, you can get the Young’s modulus (aka elastic modulus aka tensile modulus) at the same time as the UTS. The modulus is a geometry-independent, standardized way of assessing the stiffness of a material, which depends on both the material properties and the object’s geometry. The UTS is the point at which the stress is highest - for brittle materials, this is when the material fails. For ductile materials, such as polySpectra’s Cyclic Olefin Resins, this is when the material begins to yield, undergoing plastic deformation.
Because UTS is easy to measure, easy to modulate (with fillers, for example), and easy to understand because every engineer learns about it in school - most 3D printing companies like to boast about their UTS. It’s too bad it doesn’t bear any resemblance to the real-world strength of 3D-printed parts. From the same tensile test - a more meaningful metric is the toughness, which is the area under the stress-strain curve. But unless you are playing tug-of-war, the tensile toughness isn’t actually a true measurement of the materials toughness. For example, some of the 3D printing resins with the highest tensile toughness can be torn with your bear hands, because they have no tear strength.
The bottom line is that tensile testing is great for learning about the physics of elastic and viscoelastic materials, and it contains useful information that engineers might need to know while designing components and devices, but for 99% of applications, the UTS has nothing to do with the real-world failure modes of materials. As an engineer - you need to decide what “strength” is most relevant to your application: compressive strength, impact strength, tear strength, flexural strength? The spec sheet won’t do that for you.
2. HDT is misleading
The Heat Deflection Temperature, or HDT, is even worse. This is truly a lowest common denominator test that someone designed to be super easy, and for that reason, it’s super irrelevant to the real world. The HDT test goes like this: a specific shaped rectangular bar is subjected to a certain force, and when the bar deflects by a certain amount at a certain temperature, that’s called the HDT.
This has absolutely nothing to do with the true working temperature of a material. The hint that this is irrelevant should come from the fact that there are two different HDTs at two different forces (Method A and Method B), which gives 3D printing companies the opportunity to pick the higher one and report that as their working temperature.
The HDT measurement doesn’t say anything about what else is happening to that material at that temperature. Without pointing any specific fingers, we’ve found that many 3D-printed polymers completely degrade during the HDT test. In our work to benchmark other photopolymers, it isn’t uncommon for the test specimen to come out of the test looking like a charcoal briquette. So sure, the HDT on the spec sheet is technically true, but it is not representative of the real world working temperature.
What might matter more to your application is:
- Thermal cycling (going between the high and low temperature ranges of the real-world environment)
- Impact strength at low temperature (giving you a better sense of the full temperature failure range)
- Flexural modulus as a function of temperature (providing a better sense of high-temperature failure modes)
If you really want to see what is happening to the mechanical properties as a function of temperature, then Dynamic Mechanical Analysis (DMA) is much more appropriate than HDT. But because a DMA trace contains a lot of information, it’s also not simple enough to just report a single number and call it a day.
Unfortunately, HDT is what everybody puts on the spec sheet, and for the undiscerning engineer, it is commonly passed off as a proxy for working temperature.
3. Notched Izod is not real-world impact
Last but not least, let’s talk about the notched Izod impact test. In Izod testing, you hit a bar of a certain size with a pendulum of a known weight and measure the energy that’s transferred to the bar by seeing how high the pendulum goes on the other side.
Here’s the thing about the notching: it’s an attempt to do two things:
- First, it tries to “say more” about the bulk property of the material by pre-creating a defect in the surface for the pendulum to hit.
- Second, it attempts to standardize that defect.
When you buy an impact tester, you also get sold the notching machine. If you don’t use it correctly, you can get wildly different results. The notching machine needs to be calibrated as much as the impact tester itself.
I don’t have a problem with the reasoning behind these choices. It makes sense why all of those steps are taken and why that’s the necessary way to standardize it. But just like the other two examples, this has absolutely no analog in a real scenario.
Notched Izod is the real world equivalent of first slashing something with a knife and then taking an axe and perfectly hitting the exact groove on the surface that you slashed, perfectly putting all of the axe’s force inside the notch. If you’re actually thinking about impact strength and how something might survive being hit with a blade, nicked by some other component, or shot with a bullet, the unnotched impact strength is much more meaningful.
For example, a COR part hanging out in a blender experiences forces that are much more like the unnotched impact test than they are like the notched impact test. This again is something that can be really confusing because we’re naturally inclined to just compare things at face value. We might look at some material that we know has a certain notched Izod value, then look at another material with a lower value, and think all of a sudden that it can’t possibly be an acceptable replacement. In reality, it certainly could be.
As a simple example of this, we were communicating with a team that makes “battle bot” robots, who were completely obsessing over the notched Izod while comparing COR to the materials they’re used to. Unfortunately, the notched Izod has very little to do with the real-world resilience to being whacked, slashed, and hacked by another robot’s weapons. The blender test does.
For impact strength, we’ve found that our industrial and defense customers often use much less quantitative tests simply because they’re a much more reliable indicator of real-world performance:
- Dropping a bowling ball on a part of a specific geometry from a specific height
- Trying to see if anyone in the room can break the actual part with their bare hands
- Taking a panel to the firing range and shooting it with a gun
Conclusion
In summary, there’s no substitute for real-world testing. Engineers need to be wary not to take these standardized test proxies at face value. At polySpectra, we specialize in making the world’s most rugged photopolymer resins that actually survive demanding real-world environments. Our mission is to help engineers think their ideas into reality.
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