Methods for Measuring Toughness

3D printing materials often sacrifice mechanical properties for printability compared to their extruded and injection-molded counterparts. 

In fused deposition modeling (FDM), the standard material is PLA, which has poor toughness. PETG, ABS, ASA, and PC offer superior toughness to PLA, but in order for these materials to print well, the material grades need to have lower molecular weights, co-polymers, or other components that reduce the performance of the final parts when compared to polymers that have been molded or extruded. 

Stereolithography (SLA) is also known for using brittle plastics, as most standard acrylate resins have relatively poor toughness. Over time, tougher materials have been introduced, like Formlabs’ Tough 1500 Resin and Tough 2000 Resin. While these materials have significantly better toughness than most standard acrylate materials, they still lag behind most thermoplastic polymers in terms of toughness. 

The printing process with the toughest materials is selective laser sintering (SLS). The default Nylon 12 Powder is much tougher than PLA or most acrylic resins, and even tougher options like Nylon 11 Powder create incredibly tough parts. These materials do not need to be modified for printability in the way that FDM materials need to be. Additionally, SLS has much better isotropy for toughness in all axes.

When considering such a broad array of available formulations, measuring the toughness of 3D printed materials is incredibly important to be able to quantitatively assess their fitness for more demanding applications outside of prototyping.

Toughness is the ability of a material to absorb energy and plastically deform without fracturing, but there are several ways to quantify toughness.

One way is to perform a tensile test like ASTM D638, where a specimen is pulled apart by a pair of grips. The force on the cross-sectional area of the part, or stress, can be plotted relative to the elongation, or strain. Work is the dot product of force and distance. By taking the integral to calculate the area underneath the stress-strain curve, the energy absorbed before fracture can be determined.

This particular way of measuring toughness is often called “Tensile Toughness,” and while it’s useful for helping understand the meaning of toughness, it is not commonly measured or reported, making it more difficult to quantitatively compare toughness values and is rarely reported in a technical data sheet (TDS).

Elongation at break is sometimes used as a stand-in for toughness. When materials have a similar ultimate tensile strength, it is a reasonable approximation of the area underneath the stress-strain curve. This approximation is less useful when comparing materials with significantly different ultimate tensile strengths. Elongation at break on its own is a measure of ductility and not toughness.

*Densetec extrusion grade homopolymer samples were obtained from McMaster-Carr (Item # 8742K129) and tested internally by Formlabs under identical conditions as the printed resin samples.

The most common way to distill toughness down to a single number in a simple, easy-to-repeat test, is with Notched Izod impact testing. In this test, a hammer on a pendulum swings through a part, and the height of the hammer after the impact is measured. The difference between the hammer’s starting position and ending position represents a difference in gravitational potential energy, and this difference corresponds with the energy absorbed during the impact with the test specimen. This energy value is typically divided by the length or area of the test specimen, which gives us a value in J/m or J/m². 

Charpy impact strength is a similar test, but with a slightly different setup, and is used more commonly with metals. Either test can be “notched” or “unnotched.” With the notched variation a small notch is cut into the part to act as an initial crack source. Formlabs uses ASTM D256-10 for evaluating our materials.

While Izod impact strength is more commonly reported since it is quick and easy to get a quantitative value, it is tested on a thick (~12 mm) specimen, which is often not representative of the geometries used in plastic parts.

Gardner impact strength (ASTM D4226) is a better test for measuring the impact toughness of a thinner sheet of material. The test uses a dropped weight to impact a thin specimen, which is then examined to see if the plunger penetrated the specimen. The result is expressed either in terms of the maximum height achieved before failure, or the maximum energy absorbed. 

Because the specimen geometry for a Gardner impact strength test is more representative of most plastic parts, this is a more meaningful metric for impact performance in the real world. Formlabs material engineers therefore optimized the formulation of Tough 1500 Resin V2 to perform better in this metric.

Plastics behave differently based on the rate that loads are applied to them. With impact testing, a load is applied very quickly, but real-world materials also need to absorb energy when forces are applied more gradually.

One way to measure toughness at lower strain rates is by measuring fracture toughness.

This can either be expressed as a value called K_C, which is the critical stress intensity factor for a crack to propagate and cause brittle fracture; or W_f, which is the work (or energy) required to propagate a crack through the material.

Fracture toughness is one of the most important toughness measurements because it represents resistance to brittle fracture under most non-impact loads.

Cracks and fractures can also occur over time, rather than in loading like in fracture toughness. The gradual propagation of crack repeated loads is called fatigue.

To measure fatigue properties in plastics, we typically use a test called Ross Flex (ASTM D1052) which repeatedly bends bar-shaped specimens thousands of times.