Testing standard
Tensile properties of paper and paperboard (using constant rate of elongation apparatus)
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
TAPPI T494 stretches a strip of paper or paperboard to break on constant-rate-of-elongation equipment and reports four properties from one curve: tensile strength, stretch, tensile energy absorption and tensile stiffness. It does not apply to combined corrugated board. The current revision is TAPPI/ANSI T 494 om-22.
A strip of paper or paperboard of specified width is clamped between two jaws a set distance apart and stretched at a constant rate of elongation until it breaks. Force and extension are recorded throughout, so the whole load–elongation curve is available and not only the breaking point.
Four properties come out of that curve: tensile strength, stretch, tensile energy absorption and tensile stiffness. The method applies to all types of paper and paperboard within the limits of the instrument, whether the machine tests horizontally or vertically and whether it is manually operated or computer controlled.
Tensile strength is the property a converting line feels most directly. A web that breaks on a press, a bag that splits at the gusset, a label that tears off its liner — all of them are tensile events, and this is the number quoted against them.
Stretch and tensile energy absorption matter more than they look. Stretch is the elongation at break, and it separates a paper that will absorb a shock from one that will simply snap. Tensile energy absorption is the area under the curve up to break, which is the work the sheet can absorb — the property that governs whether a sack survives being dropped rather than merely being lifted.
Direction is fundamental. Paper is anisotropic because the fibres align with the machine direction during forming, so machine-direction and cross-direction results differ substantially on the same sheet, and the ratio between them describes the forming. A tensile figure quoted without its direction is not a result.
The method does not apply to combined corrugated board, and it is applicable to handsheets only with the modifications given in TAPPI T 220.
Two things have to be recorded before the strip is cut: which direction it came from, and what atmosphere it was conditioned in. Neither can be recovered afterwards.
Maximum force divided by specimen width
Reported per unit width. Divided again by grammage it becomes tensile index.
Elongation at break as a percentage of the test span
Separates a sheet that absorbs a shock from one that snaps.
The area under the load–elongation curve up to break, per unit area
The work the sheet can absorb. This is the property that governs whether a sack survives being dropped rather than merely lifted.
The slope of the initial straight part of the curve, per unit width
The in-plane elastic response, used in converting and runnability work.
Air-actuated vice jaws at a capacity matched to paper and board. Foot-pedal closure lets the strip be positioned with both hands and clamped without being disturbed, which is what keeps a specimen square in the jaws.
Specifications
For tissue and light grades, where a face that grips a heavy liner would cut the sheet. Breaking loads of a few newtons need a grip that closes gently and a cell sized to match.
SpecificationsA cell chosen for the grade rather than the frame. The range from tissue to heavy board spans two orders of magnitude, and one cell will not resolve both ends of it.
SpecificationsBreaking loads run from a few newtons on tissue to a few hundred newtons on heavy board, so the load cell has to be chosen for the grade rather than for the frame. A low-capacity frame with clean resolution at the bottom of its range is right for this work, and force accuracy to ASTM E4 is the normal requirement.
Constant rate of elongation is the defining requirement. The method is named for it, and results from a constant-rate-of-loading instrument are not equivalent — the two families of machine were historically different pieces of equipment and the distinction survives in the title for that reason.
Grips must hold paper without cutting it and without letting it slip. Pneumatic vice-action jaws with a face suited to the grade are the usual choice: serrations that grip a heavy liner will cut a tissue. Elongation is normally taken from grip separation over the defined span rather than from a clip-on extensometer, which would load the sheet.
Slippage that looks like stretch is the classic error and it inflates both stretch and tensile energy absorption while leaving the strength almost untouched. Breaks at the jaw line are discarded, not recorded. Edges cut with worn blades give a spread of low results that looks like variable paper. And conditioning skipped for speed produces figures that are internally consistent, entirely plausible and simply describing the wrong humidity.
Three documents cover the same physical test and give numbers that are not interchangeable.
| TAPPI T494 | ISO 1924-2 | ISO 1924-3 | |
|---|---|---|---|
| Loading | Constant rate of elongation | Constant rate of elongation | Constant rate of elongation |
| Rate basis | Specified in the method | A defined elongation per minute | 100 mm/min, short span |
| Region | North America | International | International |
| Reports TEA and stiffness | Yes | Yes | Yes |
| Interchangeable | No | No | No |
Quoting a T494 figure against an ISO 1924-2 specification, or the reverse, is the commonest error in this area. The tests are the same in principle and the rates are not, and paper is rate-sensitive.
TAPPI T494 is the North American method for the tensile properties of paper and paperboard using constant-rate-of-elongation equipment. A strip is stretched to break and four properties are taken from the one curve: tensile strength, stretch, tensile energy absorption and tensile stiffness. The current revision is TAPPI/ANSI T 494 om-22.
Because the alternative exists and gives different answers. Constant-rate-of-loading instruments increase force at a fixed rate; constant-rate-of-elongation instruments increase displacement at a fixed rate. Paper is rate-sensitive, so the two produce different strengths and very different stretch figures. Historically they were different machines, and the distinction is carried in the method title so that nobody assumes equivalence.
It is the area under the load–elongation curve up to break, expressed per unit area — the work the sheet absorbs before it fails. Strength alone describes a peak; TEA describes toughness. It is the property that separates a sack paper that survives a drop from one that has the same breaking load and splits, which is why it is specified on packaging grades and largely ignored on printing grades.
Because paper is strongly anisotropic. Fibres align with the machine direction as the sheet forms, so machine-direction strength is typically much higher than cross-direction strength on the same paper, and stretch usually runs the other way. A tensile figure without its direction cannot be compared with anything. The ratio between the two directions is itself useful, because it describes how the sheet was formed and dried.
No. Combined corrugated board is explicitly outside the scope — the structure is not a sheet in tension and the result would describe the geometry rather than the material. The component papers are tested by this method before they are corrugated, and the board itself is characterised by crush tests instead: edgewise, flat and ring crush.
A low-capacity frame with clean resolution at the bottom of its range and a load cell chosen for the grade. Breaking loads run from a few newtons on tissue to a few hundred newtons on heavy board, which no single cell resolves well. Grips have to hold paper without cutting it: pneumatic vice-action jaws with a face suited to the grade are the usual answer, and elongation is normally taken from grip separation rather than from a clip-on gauge that would load the sheet.
Conditioning, more often than anything else. Paper is hygroscopic and loses strength as moisture content rises, so a sheet tested straight off a warm machine and the same sheet tested after conditioning in a damp plant will differ by more than most people expect. The figures look plausible in both cases. Where two laboratories disagree, the atmosphere each conditioned in is the first thing to compare, before the paper is questioned.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | A few newtons on tissue to a few hundred newtons on heavy board — no single load cell resolves both ends | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Pneumatic vice-action grips with a face suited to the grade | Our vice-action grips, built to the specimen |
| Environment | Standard atmosphere for testing paper; conditioned and tested in it | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
