Hardness Testing
Hardness is not an easy property to define. In general, hardness is considered to be the resistance demonstrated by a material to being scratched or indented.
It is a measurement taken prior to fracture and is a function of the rigidity of the material and its modulus of elasticity.
It is important to stress that the results obtained using one method of measuring hardness cannot be compared with those obtained using any other method. However, they can be empirically correlated.
In fact, hardness correlation tables are available to correlate different types of hardness with tensile strength measurements from different materials.
Methods of Determining Hardness
Three methods of determining hardness are most commonly used on plastic materials.
Shore Hardness: A and D
Shore durometer A consists of a 1 kg weight and Shore durometer D consists of a 5 kg weight. The general standard used is UNE-EN ISO 868.
Using durometer D is recommended when measurements above 90 are obtained with test A, and using durometer A is recommended when measurements below 20 are obtained with test D.
The thickness of the test piece should be at least 4 mm. Hardness must be measured at least 12 mm from any edges on the test piece. At least five repetitions should be carried out.
- Support base (flat, horizontal surface)
- Point of indentation: steel bar with a diameter of 1.1-1.4 mm.
- Hardness indicator scale
The measurement is taken after 15 seconds of application. The result is expressed directly as “Shore hardness”.
Hardness of the Ball
There are different scales for measuring the hardness of the ball: 132 N, 358 N and 961 N. This method involves measuring hardness by applying a constant preload (9.8 N) on a steel ball, followed by application of the test load.
After holding the force for a specific time and then removing it, the total depth of indentation in the ball should be measured, from which an amount can be deducted for elastic recovery. Each scale division represents a vertical displacement of 0.002 mm of the punch.
At least 10 repetitions should be carried out. The measurement should be taken 30 seconds after application. The general standard used is UNE-EN USO 2039-1.
The ball hardness method is very common for materials used in vehicle interiors (dashboards, doors, etc.) in accordance with the specific regulations of each vehicle brand.
Barcol Hardness
Barcol hardness is applied to reinforced and non-reinforced plastic materials. Greater deviations occur in reinforced plastic due to hardness differences between the resin and the load that comes in contact with the indenter. The general standard used is UNE 53270.
The durometer consists of a cone-shaped steel indenter with an angle of 26° and a base diameter of 0.157 mm. The quadrant should be divided into 100 sections, each of which represents a depth of penetration of 0.0076 mm.
Test pieces should have a thickness of at least 1.5 mm and measurements must be carried out at least 3 mm from any edges. The number of repetitions will depend on the initial hardness measurement obtained. The measurement should be taken immediately.
