Impact Tester Model 304

Description

Surface coatings are often exposed to impacts and impact stress in real-world practice. These can lead to deformation of the substrate and affect the adhesion and cohesion of the coating. Ball impact testers are used to test the resistance, deformability and ductility of coatings and substrates. They can also test the adhesion of coatings. The ball impact test provides you with a test that can simulate this type of stress under standardized conditions. In this case, a defined weight, the sample-side end of which is designed as a ball head (for ASTM and ISO-1) or as a male tappet (for ISO-2) with a fixed diameter, is dropped onto the sample through a guide tube.

General Description

The Impact Tester, Model 304, in all its variations, consists of a sturdy base plate with attached holding arm into which the slotted fall tube is clamped (transverse thread screw). In the ISO versions the screw is tightened with a clamping lever enabling the apparatus to be set quickly for different specimen thicknesses. The lever can also be pulled out a short way against a spring force. It is thus disengaged and can be turned freely. Below the falling tube and carried in the base plate is mounted the die in accordance with the relevant standard. The die iseasily exchangeable but at the same time is accurately fitted so that the centre lines of guide tube and die coincide.

Located at the bottom end of the falling weight is the ball punch or male die tappet appropriate to the die being used on its side, it has a protruding pin which is guided in the slot along the length of the tube, used to lift the weight to the desired height manually. On the ISO versions the mass of the falling weight can be doubled by bolting on an additional weight (a total weight up to 4 kg is possible).

Scales are mounted along the slot which in the case of instruments for ISO standards are considered in „cm“, or for ASTM instruments in „inch lbs“.

Procedure for the Impact Test

After preparing the specimen as laid down in the standards (in respect of surface treatment of base material, application of coating, hardening procedure, storage, coating thickness measurement, possible cross cut test etc.) two fundamental decisions have to be taken:

• The ball impact is directed onto the coating for a concave deformation (intrusion) or on the other side for a convex deformation (extrusion). The standards listed give the user the choice between these two, to select or make an agreement for one or the other method
• In respect of the energy for the deformation the first possibility is to employ an agreed value for the potential energy at the beginning of the fall. The impact test in this case gives the go/no-go test or pass/fail answer in respect of the resistance of the coating from the point of view of the formation of cracks under rapid deformation. This method gives only a qualitative result but enables a batch of specimens to be tested in rapid succession.

A quantitative result is obtained, if repeated impact tests are performed to establish the minimum energy to damage the material. In this case, the distance of drop and hence the energy of impact is varied until the formation of cracks and/or loss of adhesion is observed. The value of energy which resulted in such damage has to be confirmed by repeated tests using also further specimen panels. If different results are obtained, it is recommended to establish a mean value.

A fundamental point – and this applies also to the go/no-go test - is to ensure that the test is conducted at an adequate distance from the edge (at least 35 mm) and also from the previous tests on the specimen (minimum 70 mm centre to centre).

Evaluation and Interpretation

The specimens deformed by the ball impact are normally examined for cracks and peeling visually perhaps with the aid of a magnifying glass. To ensure that less obvious cracks will still be identified, the ASTM D 2794 standard suggests two more sensitive methods of examination:

• Application to the specimen of copper sulphate solution to enable the smallest faults in the coating to be shown up in clear contrast. This procedure is only effective, if the base material is steel and if any anti-corrosion coating such as phosphating has also broken through as a result of the impact.
• In the case of electrically insulating coatings applied on a metallic base, the test area can also be examined with a porosity test instrument. For this, simple conductivity testers using a 9 VDC supply and a dampened sponge as test probe are all that is required.

The impact energy is expressed in different terms in the inpidual standards. In ISO, DIN, NF and SNV the dropping height (in mm) in conjunction with the weight of the impact body is used for a relative energy scale. The remaining impact test standards lay down the use of absolute units of energy: kg m (ISO 6272, ASTM D 2794), inch lbs (ASTM D2794).

These units of energy relate to one another as follows:

0.1 kg m = 8.8 inch lbs

The conversion factors can be used to compare the energy values that can be set on the various versions of the instrument. Because of the differing ball and die dimensions, it is, however, not possible to convert results obtained with the different ball impact test methods by any method of accurate calculation.

Scope of delivery

• Impact Tester, Model 304 ISO-1
  • Basic plate and side plate with set-up and guide tube
  • ISO-1 falling weight incl. ISO hemispherical head (Ø 20 mm/0,79') – 1 kg
  • Screw-on weight 1 kg (additional weight)
  • ISO-1 die (Ø 27 mm/1,1')
  • Additional arm with mounted specimen clamping device
  • Impact depth limitation