Crush Strength vs Friability vs Durability vs Hardness: Don't Confuse These Pellet Tests
Crush strength, friability, durability, and hardness are four mechanical-property tests that procurement engineers, QC technicians, and lab managers routinely confuse. The four words are used interchangeably in catalogues, in supplier datasheets, and in casual conversation, but they map to different ASTM and pharmacopoeia methods, different equipment types, and different industry conventions. A specification written for one test cannot be measured on the equipment that runs another. This page draws clean lines between the four — what each one actually measures, which standards apply, which industries use it, and how to choose the right test for your pellet, granule, or formed-particle product.
Quick Answer
Crush strength is the peak compressive force needed to fracture a single pellet, measured by ASTM D4179, D6175, or D7084. Friability is the mass loss percentage from abrasion of granules or pellets in a vibrating cylinder, measured by Ph. Eur. 2.9.41 or USP <1216> in the pharmaceutical industry. Durability is the mass loss percentage of feed pellets after tumbling in a Holmen tester or equivalent device. Hardness is a generic term often misused to mean any of the above; in materials science it specifically means indentation resistance (Brinell, Rockwell, Vickers) and does not apply to pellets.
What is Crush Strength?
Crush strength is the maximum compressive force a single pellet can withstand before it fractures. The pellet is placed between two flat parallel platens, the upper platen is lowered at a controlled rate, and the load cell records the peak force at the moment of fracture. The number is reported in newtons (N) or pound-force (lbf), with typical catalyst values in the 0 to 220 N range and typical iron ore pellet values in the 250 kg (about 2,450 N) range. Three ASTM standards cover crush strength for different particle types. ASTM D4179 covers regular formed shapes — spheres, short cylinders (length less than diameter), and tablets — used for catalyst pellets and pharmaceutical-style formed shapes. ASTM D6175 covers extruded catalyst pellets where length-to-diameter ratio is at least 1, with the extrudate loaded radially across its cylindrical surface. ASTM D7084 is different again: it loads a small bulk volume of catalyst particles into a cylindrical sample holder, applies a piston load through a 30-second hold, and reports the pressure at which 1 percent fines form. Crush strength testing is the dominant mechanical-property test for catalyst, fertilizer, and iron ore industries. Equipment requirement: a precision compression frame with flat platens, a NIST-traceable load cell, a constant-velocity drive, and at least 1 kHz data acquisition to capture brittle fracture peaks.
What is Friability?
Friability is the percentage mass loss of granules or pellets after they have been subjected to a controlled abrasion or vibration regime. Unlike crush strength, friability does not load any individual pellet to fracture. Instead, a known mass of pellets is placed into a vibrating cylinder, a rotating drum, or a similar abrasion device, the device is run for a defined time at a defined speed or amplitude, and the sample is then sieved and reweighed. The mass loss percentage is the friability number. The pharmaceutical industry uses friability tests heavily for tablets and granules. European Pharmacopoeia method 2.9.41 covers friability of granules and spheroids using a vibrating cylinder. United States Pharmacopoeia general chapter <1216> covers tablet friability using the rotating drum method. Both methods specify the device geometry, the test duration, and the acceptance limits — typically a friability below 1 percent is acceptable for finished tablets, while granule friability limits depend on the downstream process. Friability is fundamentally different from crush strength because it measures fragmentation and surface attrition under abrasion rather than peak fracture force under static compression. Equipment for friability testing is a vibrating cylinder or rotating drum, plus a precision balance and sieves; it does not use compression platens or load cells.
What is Durability?
Durability is the term most commonly used in the animal feed pellet industry to describe the resistance of feed pellets to handling abrasion and breakage. The standard test is the Holmen tester (or NHP100 unit), which uses pneumatic pressure to circulate a measured mass of feed pellets through a tube system for a defined time (typically 60 or 120 seconds), then sieves and reweighs the sample. The percentage of pellets remaining on the sieve is the pellet durability index (PDI). Older tumbling-box durability tests use a rotating box rather than pneumatic circulation. The feed industry has its own conventions because feed pellets behave differently from catalyst or fertilizer pellets — they are softer, more elastic, and break down through abrasion of fines rather than through brittle fracture. There is no ASTM or ISO standard for feed pellet durability; the Holmen method is industry-standard but not formally codified by ASTM International or ISO. Acceptance levels vary by feed mill and feed type — pelleted feed for ruminants and aquaculture typically targets PDI above 95 percent. Durability is conceptually similar to friability — both measure mass loss under handling stress — but the test apparatus and convention differ. The KHT Pellet Hardness Tester focuses on crush strength testing (single-pellet compression); feed durability testing uses separate Holmen-class equipment.
What is Hardness (vs Crush)?
Hardness is the most overloaded word in pellet testing. In casual industry usage, hardness often means crush strength — when a procurement engineer says a fertilizer granule is hard, they usually mean it has a high crush strength. In materials science, however, hardness has a specific technical meaning: the resistance of a material surface to indentation by a harder object. Brinell hardness uses a hardened steel or tungsten carbide ball pressed into the surface; Rockwell hardness uses a conical or ball indenter; Vickers and Knoop hardness use diamond pyramid indenters. These are governed by ASTM E10 (Brinell), ASTM E18 (Rockwell), and ASTM E92 / E384 (Vickers / Knoop microhardness). They are designed for metals, ceramics, and polymers — not for porous granular pellets. A catalyst pellet or fertilizer granule is too friable to give a meaningful indentation reading; the indenter would simply crush the surface. So when someone asks for the hardness of a pellet, the answer almost always involves a crush strength test (ASTM D4179, D6175, D7084, ISO 4700, or ASTM D5230 for carbon black) — not a Brinell or Rockwell hardness number. The KHT Pellet Hardness Tester is named hardness tester because that is the colloquial industry term, but the actual test it runs is single-pellet crush strength.
When to Use Each Test
Choosing the right test depends on the particle type, the failure mode of concern, and the standard your customer or regulator demands. For catalyst pellets — spheres, tablets, extrudates, granular bulk — use crush strength per ASTM D4179, D6175, or D7084 as appropriate to the shape. Catalyst customers care about fracture under static and dynamic loads in the reactor bed. For iron ore pellets used in steelmaking, use ISO 4700 cold compression strength plus ISO 3271 tumble strength. The two together describe both static crushing and rolling abrasion. For pharmaceutical tablets and granules, use friability per Ph. Eur. 2.9.41 or USP <1216>. Tablets are tested in the rotating drum; granules are tested in the vibrating cylinder. Crush strength of pharmaceutical tablets is also sometimes tested using a compression hardness tester per USP <1217>. For animal feed pellets, use durability per the Holmen tester. PDI is the industry-standard metric. For fertilizer granules, use crush strength per ASTM D4179 or supplier-specific test methods derived from D4179. For carbon black pellets, use individual pellet hardness per ASTM D5230 (carbon black industry standard) and the related D3313 / D1937 mass strength tests. For metals, ceramic monoliths, and engineered materials where indentation hardness is the relevant property, use Brinell, Rockwell, or Vickers per ASTM E10, E18, or E92 — but only if the material is dense enough to give a meaningful indentation reading. The first decision is the particle type; the second is whether you care about fracture under static load or about mass loss under handling abrasion.
Side-by-Side Comparison
The four tests measure four different mechanical properties, run on four different equipment types, and follow four different reporting conventions. The table below shows them at a glance. Procurement engineers, QC managers, and laboratory technicians should refer to the relevant standard's full text before specifying test equipment or interpreting supplier certificates. The KHT Pellet Hardness Tester covers crush strength testing in the single-pellet compression configuration (ASTM D4179, D6175, ISO 4700, ASTM D5230). Friability and durability testing requires separate vibrating-cylinder, rotating-drum, or pneumatic-circulation equipment that is sold by other instrument manufacturers.
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