Transverse rupture strength (TRS) also known as "modulus of rupture", "bend strength", or "flexural strength". It's a material property, defined as the stress in a material just before it yields in a TRS test. Simply put, It's the point just before it breaks and shatters. If you have every had an end mill break in half, you have exceeded the TRS value.
When an carbide end mill is "bent" (Fig. 1), it experiences a range of stresses across its depth (Fig. 2). At the edge of the object on the inside of the bend, B, (concave face) the stress will be at its maximum compressive stress value.
At the outside of the bend, A, (convex face) the stress will be at its maximum tensile value. These inner and outer edges of the carbide rod are known as the 'extreme fibers'.
Most materials fail under tensile stress before they fail under compressive stress. If you break a pencil in half it doesn't crack close to you but awn from you. The maximum tensile stress value that can be sustained before the rod fails is the transverse rupture strength (TRS) of a given grade of carbide rod.
The Standard method of measuring TRS is with a square material sample, as shown in Fig 1, of a given length and supported at both ends with a force, the yellow arrow above, exerted on the bar stock. It's done according to the standardized method EN 23 327 (ISO 3327):
"a specimen of a specified length with a chamfered, rectangular cross section is placed on two supports and loaded centrally until fracture occurs. TRS is taken as the median of several observed values."
The carbide rotary toolmaking sector has adopted a modified TRS testing method that is more applicable to the geometry of solid carbide rods. In this test a modification of the standard test specimen according to EN 23 327 (ISO 3327) is used.
This test comprises a cylindrical carbide specimen, Ø 3.25 x 38 mm. This modified test has been adopted as an industry standard and is now proposed to be included in the ISO standard.
By using this cylindrical test specimen, as used to make carbide rotary end mills, the edge effect of the rectangular standard specimen is avoided.
NOTE: Higher TRS values increase the TOUGHNESS of the tool: it will "bend" a bit more before catastrophic failure, but you sacrifice WEAR RESISTANCE when you increase toughness. More on that below...
Why TRS is important
Take a look at the picture of the two end mills in the above section. If you are using a longer overall length (OAL) end mill, the longer tool will be more prone deflection when it's in-the-cut and under load.
It's essentially the same effect if you try to push too hard on the head of a pencil point. If you apply too much pressure, it will eventually break and lead to catastrophic failure of the tool. Ideally, the longer the reach of the tool the higher your would prefer the TRS value.
It's important to keep in mind that it's really about a carbide rod length to diameter ratio, The longer the overall length of the tool, when compared to cutting diameter, the more important the TRS becomes.
Because of this, the TRS value is also very important to understand when using miniature end mills. Higher TRS values enable you to take a heavier cut (chip load per tooth) without catastrophic failure of the tool.
How you increase the Transverse Rupture Strength
The TRS reaches a maximum at cobalt content of about 15% (by weight) and a medium to coarse Tungsten Carbide WC grain size. Typically, the cobalt content of an end mill ranges between 8-12% (by weight) of the carbide in most end mills.
It's important to know that the cobalt content of a carbide end mill is measure by weight and NOT volume.
Think about mixing up a cake. You pour your milk into a measuring cup based upon the VOLUME of milk you need. In contrast, when mixing carbide rod, you MEASURE THE WEIGHT of the carbide and the WEIGHT of the cobalt on scale for the proper mix.
Carbide weighs A LOT more than Cobalt! To see this for yourself hold a cobalt end mill in one hand and a carbide end mill in the other. Because Cobalt weighs much less than Carbide it takes up MORE VOLUME: It's a bigger pile as you increase the percentage of cobalt.
It bears repeating, Carbide substrate is measured by weight.
If you where to measure the VOLUME of the cobalt in a 12% Cobalt (by weight) carbide end mill, that volume may be as high as 24-28% (depending on the grain size of the carbide). That's the reason for the reduced wear resistance of the higher cobalt content but also the reason that those end mills have a higher TRS value and greater "toughness"
For a much more detailed breakdown of carbide substrates and how carbide is made please take a look at our technical section at this link: CARBIDE SUBSTRATE.