ISO 10545-4 Modulus of Rupture Testing of Ceramic Tiles

What the test does

A tile is supported on two parallel rods and loaded through a third along its centreline, bending it in three-point flexure until it breaks. Force is applied at a rate that raises the stress in the tile by 1 ± 0,2 N/mm² every second. Two quantities come out: breaking strength, a force, and modulus of rupture, a stress derived from that force and the tile geometry.

What it measures, and why it matters

A floor tile spans the high points of an imperfect bed of adhesive and is walked on, loaded by furniture and occasionally struck. What resists that is its ability to carry a bending load, and this method quantifies it.

The two reported quantities answer different questions, and that is the distinction which matters most here. Breaking strength is the force the tile carried and it scales with thickness: it is the practical number for whether a tile will survive a duty, and what the ISO 13006 product classifications use. Modulus of rupture is a stress, normalised for the tile's dimensions, describing the ceramic body rather than the article. A thin porcelain tile can have a very high modulus of rupture and a modest breaking strength; a thick, coarser tile the reverse.

A specification quoting only modulus of rupture has said something about the body and nothing directly about whether the tile will break underfoot.

Whole tiles wherever possible

This is a product test: the article that is sold is, wherever it will fit the apparatus, the article that is broken. That is a stated preference rather than a prohibition on cutting, and the standard prescribes exactly when a tile is cut down.

Specimen

A whole tile wherever possible, glazed face upThe standard says whole tiles shall be tested whenever possible, and that in case of doubt whole-tile results are preferred to results from cut tiles. Cut tiles are noted in the report.

When tiles are cut

Work size thickness 7,5 mm or more: tiles longer than 600 mm are cut to fit the apparatus, no larger than the rod lengthWhere the long-to-short side ratio is 3 or less the original aspect ratio is kept until the long side reaches 600 mm — 1 200 mm × 600 mm is cut to 600 mm × 300 mm. Cuts are held within 10 mm.

Thin tiles

Work size thickness under 7,5 mm: whole only from 324 mm² to 40 000 mm² maximum area; above that, 200 mm × 200 mm specimens are cutFor a tile whose short side is under 200 mm the specimen is that short side by 200 mm, centred on the tile.

Condition

No visible damage or cracks, and not previously tested

Scope

All ceramic tiles

Number of specimens

Tabulated against maximum area and work size thicknessThe tables are the technical revision the 2019 fourth edition introduced. Read that edition rather than the 2014 one on specimen selection.

Drying

To constant mass before testA ceramic body that has absorbed water behaves differently, and untried tiles read low and variably.

Rod covering

Rubber of (50 ± 5) IRHD, measured to ISO 48-2, on both supports and the loading rodThickness and rod diameter are tabulated against the tile's long side. The covering lets a tile that is not perfectly flat bear evenly rather than sitting on one high point.

Replace the rod coverings when they harden

DakA worn or set covering stops distributing the load and the tile breaks at a contact line instead of at its weakest section.

Test speed

Loading

Three-point bending through a central rod

Rate

A stress increase of 1 ± 0,2 N/mm² per secondNot a crosshead speed and not a fixed force rate. The machine has to raise the bending stress in the tile at that rate, which means the force rate is different for every tile size and thickness.

As a force rate

dF/dt = (dR/dt) × 2bh² / 3LDakThe modulus of rupture expression rearranged. b is the tile width, h the thickness and L the span between the support rods.

Worked: 300 mm square floor tile, 9 mm thick

About 58 N/s, or 3,5 kN/minDakSpan 280 mm, after the 10 mm overhang beyond each edge support that Table 1 sets for a tile whose long side is 95 mm or more. The ± 0,2 tolerance gives 46 to 69 N/s. Recompute it for the tile actually being tested.

Span

Set by the tile size, as the method definesSpan appears in the modulus of rupture formula, so a span set for a different tile size changes the reported stress directly.

Reported

Breaking strength in newtons, and modulus of rupture in N/mm²

What the machine must be capable of

What the method asks of the apparatus is a recording pressure gauge accurate to 2,0 % and rubber of (50 ± 5) IRHD on all three rods. It does not cite ISO 7500-1; a frame verified to ISO 7500-1 Class 1 is our recommendation, not a requirement of the standard. Breaking forces run from a few hundred newtons on a thin wall tile to several kilonewtons on thick porcelain, so 10 to 50 kN with a cell chosen for the product range covers it.

The fixture is a three-point bend arrangement, the span set by the tile size. No specimen is larger than the rod length and the longest side the method produces is 600 mm, so a rig spanning 600 mm covers all of it. Rod diameter, rubber thickness and the overhang beyond the supports are tabulated against the long side and are measured rather than assumed.

The rate is a stress rate — 1 ± 0,2 N/mm² per second — so the force rate is different for every tile. Rearranging the modulus of rupture expression gives it: dF/dt = (dR/dt) × 2bh² / 3L. On a 300 mm square floor tile 9 mm thick, spanning 280 mm once the 10 mm overhang the table sets is taken off each end, that is about 58 N/s — 3,5 kN/min — with the tolerance giving 46 to 69 N/s. Recompute it from b, h and L for the tile in hand.

Deflection is not part of the result; the reported quantities come from the breaking force and the geometry.

Grips and fixtures for this method

Adjustable spanTJ-124

Three Point Bend Fixture

Two support rods and a central loading rod, all covered in (50 ± 5) IRHD rubber so an imperfectly flat tile bears evenly. Rod diameter, rubber thickness and the overhang are tabulated against the tile's long side. No specimen exceeds the rod length, and the longest side the method produces is 600 mm, so a rig that spans a 600 mm specimen covers the whole of it.

Specifications

Self-identifying

Load Cells

Breaking forces run from a few hundred newtons on a thin wall tile to several kilonewtons on thick porcelain. A cell chosen for the product range keeps the low end resolved rather than sizing everything for the heaviest tile.

Specifications

How the test runs

1. 01Select tiles at random from the lot, and cut them down only where the method requires it for that work size and thickness.
2. 02Dry to constant mass.
3. 03Check the resilient coverings on all three rods.
4. 04Set the span for the tile size.
5. 05Place the tile glazed face up, centred on the supports with the specified overhang.
6. 06Load through the central rod so the stress rises at 1 ± 0,2 N/mm² per second, until the tile breaks.
7. 07Record the breaking load.
8. 08Measure the minimum thickness along the fracture.
9. 09Compute breaking strength and modulus of rupture.
10. 10Report both, with the number of tiles.

Calculations

What the report has to contain

• Reference to ISO 10545-4 and the edition
• Tile type, work size and nominal thickness
• Number of tiles tested
• Drying condition before test
• Span used
• Rod diameters and the resilient covering
• Loading rate
• Breaking load for each tile
• Minimum thickness measured along each fracture
• Breaking strength and modulus of rupture, both reported

What goes wrong in practice

Rod coverings that have hardened or worn stop distributing the load and the tile breaks at a contact line. A span set for a different tile size changes the modulus of rupture directly, because span is in the formula. Tiles tested without drying read low and variably. And reporting modulus of rupture alone, when the specification wanted breaking strength, is an error that reaches purchase decisions.