Understanding the Mechanical Properties of 3D Prints
The Stress-Strain Curve
Selecting the right material for your application is a critical component of good design, but the material data sheets (spec sheets) can be confusing to the untrained eye. Luckily, just a handful of mechanical properties can go a long way to making your printed prototype a success and understanding them is easy using the stress-strain curve. While math is the basis of all of the concepts explained here, it isn’t necessary to get an intuitive understanding of each property.
The stress-strain curve is created by means of a tensile test (shown above). In a tensile test, a standardized sample of material, the test specimen, is clamped at either end in a machine that pulls the specimen apart with a steadily increasing force. The specimen is stretched until it fails and the relationship between stress and strain is recorded in the form of a graph (shown below). The exact shape of the plotted line depends on the material being tested.
Strain is a measure of the stretch of the material with respect to its original length and is recorded as a percentage. The strain at the point when the specimen breaks is recorded on the spec sheet as “elongation at break” or “percent elongation” and is a good measure of how ductile the material is. The lower the percent elongation, the less ductile (or more brittle) the material is.
Stress is a measure of the internal forces felt by the material. The larger the force applied, the higher the material stress must be to counteract that force. From this measurement, comes our second common material property, tensile strength. Tensile strength is a measure of how much constant force a material can withstand.
The relationship between the stress and the strain (the stress-strain curve itself) follows a predictable pattern. Firstly, there’s a linear region that represents the portion of stretching which is non-permanent. Like an elastic band, if the force is released from the material within the linear region, the specimen will return to its original shape. For this reason, the linear region is sometimes referred to as the elastic region and the stretching that occurs here is called elastic deformation. Then, at the end of the elastic region, we have the yield point. Beyond the yield point, the graph becomes nonlinear, and the stretching that occurs there will remain even if the force is released. The portion of the curve after the yield point is called the plastic region and the stretching that occurs there is called plastic deformation. Within the plastic region, the stress increases all the way up to the ultimate tensile strength (UTS). Beyond UTS, the stress decreases as the internal structure of the material begins to fail and the specimen continues to stretch until it reaches the point of failure.
The pattern in the structure of the stress-strain curve gives rise to a few more properties that can be found on your material spec sheet. Firstly, the steepness or slope of the elastic region is known as “Young’s Modulus” or the “modulus of elasticity” and it represents the stiffness of the material. A larger Young’s Modulus makes for a stiffer material since stress builds much more quickly for a given amount of strain. Second, yield strength is the amount of stress a material can withstand without plastic deformation and ultimate tensile strength is the maximum possible amount of stress that the material can withstand. The tensile strength that was mentioned earlier refers to one of these two properties. Finally, there is an implicit bonus property; toughness. Toughness is the ability of a material to absorb energy by deforming (think helmets or tupperware) and is represented by the area under the stress-strain curve. The toughness is generally shown on the spec sheet as “Impact strength”, “izod impact”, or “charpy impact”.
Now that the stress-strain curve is understood, comparisons between materials on the spec sheets as well as between two different stress-strain curves becomes much easier. The graph below shows four curves which each represent a different plastic. It can be seen that stiffness and strength increase from right to left while ductility increases from left to right. Toughness is highest at neither extreme, the red line represents the plastic of highest toughness, the orange or blue plastic would be the second most tough, and the green plastic the least tough. The same conclusions can easily be drawn from the material spec sheets by referencing the values for tensile strength, elongation at break, young’s modulus, and impact toughness.
For all those who have found the mechanical properties on spec sheets confusing or overwhelming, we hope this post will help you nurture a better intuition about which material will help you achieve your prototyping goals.
And, if the decision is still too hard, don’t hesitate to reach out to firstname.lastname@example.org/, and our team of experts will be happy to discuss your needs and make a suitable material recommendation.