Tensile strength Rm is the maximum tensile stress a material reaches in a tensile test before it fractures. It is calculated as Rm = Fmax / A0 and given in megapascals (MPa) or newtons per square millimetre (N/mm²), both units being identical. Which standard applies depends on the material: metals are tested to ISO 6892, plastic films to ISO 527, while papers and nonwovens are tested to ISO 1924 in force per width rather than stress. This page walks through the tensile test, shows real values for metals and for the insulation materials GOBA has worked with since 1959, and explains why tensile strength differs between machine direction and transverse direction.
How is tensile strength determined in the tensile test?
In a tensile test, a standardised material sample is clamped into a testing machine and stretched with increasing force until it fails. The highest point of the resulting stress-strain curve is Rm. The maximum force Fmax always enters the value relative to the original cross-sectional area A0 of the sample, because force alone says nothing about the material without its cross section.
On the same curve, yield strength Rp comes before Rm. It marks the point from which a material deforms permanently, that is, plastically, while Rm marks fracture. Between the two values lies the usable safety margin of a design: components are dimensioned so that operating stress stays well below Rp, with Rm as the outer limit for a fault condition.
Which standard applies to tensile strength testing, depending on the material?
The same property is determined under three different test regimes for the materials relevant to GOBA, which explains why datasheets sometimes give MPa and sometimes N/cm or kN/m:
- Metals are tested to ISO 6892 (equivalent to DIN EN ISO 6892), force relative to the cross-sectional area of the sample, result in MPa.
- Plastic films such as PET film or polyimide film are tested to ISO 527, or to ASTM D882 in the US, again in MPa, because film thickness can be measured precisely enough.
- Papers and nonwovens such as aramid paper or pressboard are instead tested to ISO 1924 or TAPPI T494 in force per width, usually N/cm or kN/m, not in stress. The reason: the thickness of web material varies locally too much for a reliable cross-sectional stress value.
Anyone comparing values from different datasheets should check the unit first. MPa and N/cm cannot be converted directly without knowing the actual material thickness.
What tensile strength do metals and insulating materials have compared?
A blanket statement like "plastics stay under 100 MPa" does not hold for technical insulating films. Hostaphan PET film reaches 300 MPa in machine direction according to the manufacturer datasheet, higher than the tensile strength of aluminium or copper:
| Material | Tensile strength Rm | Standard |
|---|---|---|
| Structural steel (S235) | 360 to 510 MPa | ISO 6892 |
| Quenched and tempered steel (42CrMo4) | 1000 to 1200 MPa | ISO 6892 |
| Aluminium (AlMgSi) | 200 to 350 MPa | ISO 6892 |
| Copper (Cu-ETP) | 200 to 360 MPa | ISO 6892 |
| Hostaphan PET film, machine direction | approx. 300 MPa | ISO 527 |
| Hostaphan PET film, transverse direction | approx. 250 MPa | ISO 527 |
| Kapton HN polyimide film, 23 °C | approx. 231 MPa | ASTM D882 |
| Polyamide PA 6.6 | 70 to 85 MPa | ISO 527 |
Insulating papers and nonwovens are deliberately left out of this table. Their datasheets state tensile strength in force per width, not MPa, and would not compare cleanly without the exact thickness of the sample tested.
Why does tensile strength differ between machine direction and transverse direction?
Web material such as PET film is oriented in two directions during stretching, lengthwise (machine direction, MD) and crosswise (transverse direction, CD or TD). The two stretch ratios rarely match exactly, so tensile strength differs too: for Hostaphan, the gap between MD and CD runs around 15 to 20 percent. For pure material selection that is a footnote. For processing it matters: during slitting and rewinding, web tension must stay safely below the tensile strength in machine direction, otherwise the web tears or wrinkles form. GOBA sets slitting and winding parameters for the weaker of the two directions, not for the best value on the datasheet.
How does tensile strength relate to fracture behaviour and elongation?
Ductile materials such as steel or aluminium visibly neck down before fracture and stretch noticeably before they fail. Brittle materials, and many insulating films at low temperature, fracture abruptly without prior deformation. Elongation at break, the distance a material travels before it tears, is therefore a separate property alongside Rm. A high Rm value combined with low elongation at break means the material carries significant load but gives little warning before it fails.
GOBA Takeaway
For insulating materials, tensile strength acts mainly as a processing limit for slitting, punching and winding, and far less often as a structural design limit the way it does for a load-bearing steel part. Anyone reading a datasheet should check the test standard and the direction, MD or CD, before comparing two values. For questions on actual material thickness and tensile strength for a running project, GOBA advises based on the web material actually being processed, not just the datasheet.
