The probability of a large defect occurring is higher in a large volume.
All materials contain some defects such as pores, inclusions, and micro-cracks. These defects lead to reduced material strength. In the case of a ductile material (copper, mild steels, etc.), defect frequency and mean size are important factors; whereas in the case of a brittle material (e.g., hardened steels and carbide), the frequency above a certain size limits the strength. Consequently, this latter phenomenon makes the mechanical strength volume dependent as the probability of finding a large defect increases with increased material volume.
Transverse rupture strength
The transverse rupture strength (Rbm) is determined as the fracture stress in the surface zone Rbm=3FLk/2bh2 where F=maximum fracture load and k=chamfer correction factor (normally 1.00-1.02)
Transverse rupture strength (TRS) or bending strength testing is the simplest and most common way of determining the mechanical strength of cemented carbide used in cemented components and tools, like carbide rolls. 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. TRS reaches a maximum at cobalt content of about 15% (by weight) and a medium to coarse WC grain size.
The carbide rotary toolmaking sector (i.e., carbide rolls or carbide rotary cutters) in industry has adopted a modified TRS testing method more applicable to the geometry of solid carbide tooling and allowing a rapid testing procedure. In this test, a modification of the standard test specimen according to EN 23 327 (ISO 3327) is used. This comprises a cylindrical 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 (i.e., carbide rolls or carbide rotary Cutters), the edge effect of the rectangular standard specimen is avoided.
As a result, the data gained from this test shows TRS values higher than those of the rectangular test piece. Typically, the TRS values obtained from the cylindrical specimens exceed the level of the square specimens by about 20%. Thus, caution must be used when data are compared.
Transverse rupture strength decreases with increasing temperature. At prolonged load times and high temperatures, cemented carbides will exhibit creep behavior.
Mechanical strength for carbide roll.
One of the most important properties of cemented carbides is their extremely high compressive strength under uniaxial loads.
A suitable method of compressive strength determination is defined by EN 24 506 (ISO 4506). To obtain accurate values with cemented carbide, a modified specimen geometry must be used in order to overcome the edge and contact effects associated with a simple cylindrical test piece.
When the load is applied, there is first an elastic deformation, but prior to fracture, there will also be a certain amount of plastic deformation.
The compressive strength increases with decreasing binder content and decreasing grain size. A cemented carbide grade with a small WC grain size and a low binder content has a typical compressive strength approaching 7000 N/mm2.
The compressive strength decreases with increasing temperature. The proportion of plastic deformation increases dramatically with temperature, leading to a barrel-shaped specimen before fracture, thereby making results uncertain.
Wöhler curves from compressive fatigue testing of different carbides. Lower load limit is 250 N/mm2
The fatigue strength of cemented carbide under pulsating compression loading is normally 65 to 85% of the static compressive strength at 2×106 cycles. No definite fatigue strength limit, which corresponds to an infinite life, has been found as in the case of steel and other metals. The fatigue strength increases with decreasing WC grain size and decreasing binder content.