ASTM D785 & ISO 2039-1/-2 Plastics hardness test
The ASTM D785 and ISO 2039-1/-2 standards describe methods for determining the hardness of plastics, hard rubber and related electrical insulating materials made of plastics using a loaded ball indenter. In the ISO 2039-1 standard, the hardness value is determined by means of a ball indentation test. The surface of the indentation and then the hardness are calculated from the measured indentation depth. ASTM D785 and ISO 2039-2 describe methods for determining the indentation hardness of plastics using a Rockwell hardness tester.
The method is used to test hardness in the macro range with a test force of between 49-980.7 N. This method is a depth measurement method, which means that to determine the hardness value of a test piece, the permanent indentation depth caused by the indenter is measured. The indenter is a steel ball, with different ball diameters depending on the Rockwell hardness method.
ISO 2039-1 Ball indentationASTM D785/ISO 2039-2 RockwellTesting instrumentsDownloads Request a consultation
Performing a ball indentation hardness test according to ISO 2039-1
For ball indentation hardness in accordance with ISO 2039-1, the total test force is applied in two stages. This is intended to eliminate the influence of the roughness of the specimen surface (e.g. grooves on the specimen) and the measurement error caused by the play in the indentation depth measurement.
First, the indenter is pressed into the specimen to be tested with a test preload of 9.8 N up to an indentation depth h0, where h0 defines the reference plane (base) for the subsequent measurement of the indentation depth (h).
The test force is then applied for an exposure time defined by the standard and the penetration depth is measured. The test force is to be selected from the values 49 N, 132 N, 358 N and 961 N so that the indentation depth is in the range of 0.15 to 0.35 mm after 30 s of exposure. If an indentation depth of less than 0.15 mm or greater than 0.35 mm is detected at the end of the test force exposure time, the test force must be increased or decreased.
The surface area of the indentation can then be calculated using the measured indentation depth h. The ball indentation hardness (HB) is subsequently calculated using a formula defined in the standard. The test indentations must be positioned so that there is sufficient distance from the specimen edge (border) and between the individual test indentations (each no less than 10 mm).
The high test loads and the deep indention allow for measurement across a large test surface and thus, a strong average of the measured value. This method is suitable for measuring both filled and unfilled plastics.
ISO 2039-1 Representation of the hardness value
The hardness value of the plastic test according to ISO 2039-1 is made up of two components:
- A numerical hardness value
- The two capital letters HB, which stand for ball indentation hardness (H = hardness; B = ball indentation)
Example of how to represent and read a hardness value:
- 45 HB: 45 ... hardness value; HB ... ball indentation hardness in Newtons per square millimeter (N/mm²)
ASTM D785 & ISO 2039-2 Rockwell hardness test
- ISO 2039-2 and ASTM D785 specify various Rockwell hardness scales for plastic hardness tests.
- In the Rockwell method, the total test force is applied in two steps. This is intended to eliminate the influence of the roughness of the specimen surface (e.g. grooves on the specimen) and the measurement error caused by the play in the indentation depth measurement.
- In contrast to ball indentation hardness according to ISO 2039-1, where the indentation depth is measured under load, the Rockwell method measures the indentation depth at a defined preload. Different indenters are available according to the respective Rockwell scale.
Running a test according to ASTM D785 & ISO 2039-2
When determining the plastic hardness in accordance with ISO 2039-2 and ASTM D785, the indenter is first pressed into the specimen to be tested with the corresponding test preload to the indentation depth h0. where h0 defines the reference plane (base) for the subsequent measurement of the permanent indentation depth (h).
The test force is then applied for a duration defined in the standard (several seconds), whereby the indenter penetrates the specimen to the maximum penetration depth h1.
At the end of the application time, the test force is reduced again, the indenter moves back upwards by the elastic part of the indentation depth in the total test force and remains at the level of the permanent indentation depth h. This is also referred to as the depth difference (difference between the indentation depth before and after application of the test force). Using the permanent indentation depth h, the Rockwell hardness (HR) can then be calculated according to a formula defined in the standard, taking into account the Rockwell scale used.
ASTM D785 & ISO 2039-2: differences between the test methods
The plastic hardness according to ASTM D785 and ISO 2039-2 differs in the diameter of the indentation ball and the level of the test force used (also test load).
ASTM D785 Rockwell hardness scale:
| ASTM D785 Rockwell hardness scale | Preload (kg) | Test load (kg) | Diameter of the indenter |
|---|---|---|---|
| R | 10 | 60 | 0.5000 ± 0.0001 in / 12.700 ± 0.0025 mm |
| L | 10 | 60 | 0.2500 ± 0.0001 in / 6.350 ± 0.0025 mm |
| M | 10 | 100 | 0.2500 ± 0.0001 in / 6.350 ± 0.0025 mm |
| E | 10 | 100 | 0.1250 ± 0.0001 in / 3.175 ± 0.0025 mm |
| K | 10 | 150 | 0.1250 ± 0.0001 in / 3.175 ± 0.0025 mm |
ISO 2039-2 Rockwell hardness scale:
| Rockwell hardness scale ISO 2039-2 | Preload (N) | Test load (N) | Diameter of the indenter (mm) |
|---|---|---|---|
| R | 98.07 | 588.4 | 12.7 |
| L | 98.07 | 588.4 | 6.35 |
| M | 98.07 | 980.7 | 6.35 |
| E | 98.07 | 980.7 | 3.175 |
ASTM D785 & ISO 2039-2 Representation of the hardness value
The hardness value according to ISO 2039-2 and ASTM D785 is made up of two components:
- A numerical hardness value
- The two capital letters HR, for hardness according to Rockwell
Example of how to represent and read a hardness value:
- 65 HR
- 65 … hardness value
- HR … according to Rockwell
Rockwell hardness testers from ZwickRoell feature an indenter-holder for hardness testing at difficult-to-access locations. In addition to ASTM D785 and ISO 2039-2, they also cover the classic and Super Rockwell hardness test in accordance with ISO 6508 | ASTM E18.
