Flexural vs Compressive Strength in Graphite
It is an oddity of the graphite business, and in many other endeavors in life, that the more you know about something, the more you realize you still don’t know. At Semco, we do know there are many elements in graphite component design. We’ll go ahead and claim to know most of them (hey, decades in the same business can teach you a lot). But what we recognize is that when a client brings to us a problem with a graphite component that we machined, we may not immediately know what the problem is nor how to fix it. In fact, we’ve learned that rushing a solution (say, just machining a new part and hoping for the best) causes more problems that it solves. With diligence and a willingness to overlook nothing, we are able to solve our customers’ issues with graphite components. After all, graphite is a simple material. There aren’t many elements in graphite (just one, really -- carbon) and it’s not a new material (humans have used graphite for centuries). Problems with graphite applications can be figured out. A recent experience with a long-time client demonstrates Semco’s prowess at doing just that: taking the time to figure it out. This experience also highlighted the distinction between compressive and flexural strength, a distinction the layman may find uninteresting, but the Semco graphite engineer finds to be crucial. The following story will explain why.
First, some definitions. Compressive strength is the capacity of a material to withstand loads tending to reduce its size. Measured, it is the value obtained by plotting applied force against deformation. At its compressive strength limit, the material will fracture or deform irreversibly.
Flexural strength, on the other hand, is defined as the ability of a material to resist deformation under load. It is measured with the help of the Transverse Bending Test. In this test, a sample of a material is bent until it fractures, using a 3-point flex test technique. Flexural strength is measured at the highest stress experienced within the material at the moment of rupture. These specifications apply to graphite materials and are commonly found on material specification sheets that we provide our customers to help them choose the ideal material for their graphite components.
Flexural strength (pay attention now -- this is an important fact to our story) of most graphite material is on average only 50% to 60% of its compressive strength.
If these definitions are not correctly taken into account when designing graphite components, confusion (and breakage) can occur. Both parameters refer to the ability of a material to resist deformation (i.e., to withstand loads). The difference between the two parameters is due to the methodology applied to calculate the loads. Compressive strength is measured when force is applied uniformly on one surface while the opposite face is fully supported. Flexural strength is measured by what is essentially a bend test. The material is supported on two points placed at the edges of the material while force is applied to its center. Intuitively, the compressive strength of a material will be greater than its flexural strength.
Now (finally!) for the story. This was an experience that demonstrates how the wrong interpretation of these materials specifications (compressive vs. flexural strength) can result in component failure. We received a customer call concerning an application using graphite plates in a sintering process. The customer informed us that some of these graphite components had fractured during use. We had worked closely with the customer to produce these plates based on their process requirements. Additionally, the customer used these plates for years with no failures. You can see how tempting it might be to just assume the last batch were lemons for whatever reason and go ahead and machine some of the same plates we had always made. But at Semco, we know enough to know that assuming the last batch of plates was an anomaly could be a very costly mistake.
The client’s system consisted of one graphite plate used as a base, fully supported by the bottom of the furnace, a stack of parts to be sintered, and finally a top graphite plate. The graphite plates allowed for a threaded metal rod secured to the bottom of the furnace to penetrate the center of the assembly. A large round nut was used to secure everything in place and apply the necessary pressure. This design resulted in an assembly that was subjected to mainly compressive forces throughout. The graphite grade was more than capable of withstanding the compressive forces applied within the system as originally designed.
When investigating the fracturing issue with the customer, we looked into any possible changes within the process. We discovered that the originally designed part, which was virtually a solid disk, has been re-designed by the customer, for weight savings, into a ring with a large inner diameter. Our team realized that the top lid was no longer supported. Once force was applied to the center of the lid, the material fractured.
In this instance, the lid was no longer experiencing compressive strengths, but was in fact experiencing flexural strengths. This resulted in reducing the maximum force the lid could withstand by approximately 50% (told you that fact would come in handy). With this design change, the original material was no longer suitable in this application. The solution was relatively simple: higher strength graphite material was used to replace the original graphite grade.
While the customer’s process did not change, the different geometry of the parts caused a switch from compressive force to flexural force. The updated lids with adequate flexural strength ran flawlessly when tested, and the lids are currently in full production and performing as expected.
Experiences like this one with problematic graphite components remind us to be even more diligent, patient, and open-minded the next time a customer rings us up with a problem. This is why our customers stick with us — they can count on us to collaborate with them and not be dismissive of their concerns. Of all the elements in graphite manufacturing, this kind of customer service may be the most important.