Ball Indentation Hardness

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Ball indentation hardness

Fundamentals

The ball indentation method was introduced to determine the hardness of plastics. The method is based on measuring the penetration depth of a steel ball into the surface of a test specimen under the influence of a test load. This test load is applied for a defined period of time after a preload has been applied. The fundamental difficulty with this method is that the indentation depth is not a linear function of the load. This is compensated for by limiting the indentation depth to a value that is small compared to the ball diameter. A uniform indentation depth range of 0.15 mm to 0.35 mm is achieved by applying four test load levels. The following test loads are specified for this purpose: 49 N, 132 N, 358 N, and 961 N. Discontinuities occur at the load transition points from one load level to the next, resulting in different hardness values for each test load.

Evaluation and calculation of characteristic values

When evaluating the test, a nearly continuous transition at the junction of two test load levels is achieved by mathematically taking into account the non-strictly linear indentation depth–load function. To determine the hardness, the test load is selected which, 30 seconds after application, produces a indentation depth that lies within the aforementioned range of 0.15 to 0.35 mm (gray area in Fig. 1).

Fig. 1:

Area of validity of ball indentation hardness

Fig. 2:

Test procedure for measuring ball indentation hardness

Figure 2 shows a schematic representation of the test procedure for determining ball indentation hardness. Figure 3 shows different hardness testers from ZwickRoell and Instron-Wolpert (see: manufacturer of material testing machines).

Fig. 3:

Examples of hardness testing devices for measuring ball indentation hardness

The following conditions apply to the evaluation:

• If a indentation depth of < 0.15 mm is determined after the test period has elapsed, the test load must be increased!

• If a indentation depth > 0.35 mm is determined after the test time has elapsed, the test load must be reduced!

• If an indentation depth in the range of 0.15–0.35 mm (gray) is determined after the test time has elapsed, the hardness value is determined manually or by the software!

If this condition is met, the hardness is generally calculated using the following equation:

$HB\,=\,{\frac {F}{\pi \,dh}}$            (N/mm2)

Since the load frame bends during the test, which can be described by the compliance (see: tensile test compliance), the hardness value must be corrected:

$HB\,=\,{\frac {F_{r}}{\pi \,dh_{r}}}$            (N/mm2)

with	$F_{r}\,=\,{\frac {0{,}21}{\left(h-h_{r}\right)+0{,}21}}\cdot F$            (N)

with

F0		preload (N)
FL		test load (N)
F		total load (N)
Fr		reduced test load (N)
d		ball diameter d = 5 mm
h		indentation depth (mm)
hr		reduced indentation depth

Characteristic values for moulding compounds and thermoplastics

The following Table lists some material values of ball indentation hardness for various moulding compounds and plastics.

Table 1: Material values of ball indentation hardness for thermoplastics and moulding compounds

Product group	HB (N/mm2)	Product group	HB (N/mm2)
Curing compounds		Thermoplastics, unreinforced
Phenol resin	... 200	Fluoropolymers	30 ... 70
Urea resin	... 150	Polyacetals	... 140
Melamine resin	... 200	Polyamides	... 100
Polyester resin	... 200	Polycarbonates	... 100
Epoxy resin	... 200	Polymethyl methacrylates	... 200
		Polyethylenes	10 ... 65
		Polypropylenes	60 ... 75
		Polystyrenes	... 120
		Polyvinyl chlorides	... 120
		PVC, impact-resistant	30 ... 100

Influence of orientation

Figure 4 shows the dependence of ball indentation hardness on the measurement location on a multipurpose test specimen, i.e., the orientation (see: tensile test residual stresses orientations), for a polypropylene copolymer.

Fig. 4:

Dependence of ball indentation hardness on orientation (measurement location) and storage condition in washing lye at 95 °C for a PP copolymer PPC2)

100 h-storage::	If the PP copolymer is stored in washing lye at 95 °C for 100 hours, the blue curve is obtained. It can be seen that lower hardness is also recorded in the shoulder areas due to the lower orientation. Compared to the initial values, a reduction in the average hardness level can also be observed as a result of the influence of tempering (reduction of residual stresses).

1,000 h-storage::	If this material is stored for up to 1,000 hours under identical conditions, the red curve for the measurement points 10 mm apart is obtained. It is clear to see that the hardness level has evened out in the shoulder area and the middle plane-parallel section. This effect is caused by the greater mobility of the polymer chains and the resulting deformation and orientation compensation.

See also

• IRHD hardness
• Conventional hardness testing
• Ball or pin impression method

References

• ISO 2039-1 (2001-12): Plastics – Determination of Hardness – Part 1: Ball Indentation Method
• Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 182/183 (ISBN 978-1-56990-806-8; e-Book ISBN 978-1-56990-807-5; see AMK-Library under A 22)
• Koch, T., Bierögel, C., Seidler, S.: Conventional hardness values. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume VIII/6A3, Springer, Berlin (2014) 357–379, (ISBN 978-3-642-55165-9; see AMK-Library under A 16)