The Point Load Test is a fast and convenient way to determine the strength and fracture toughness of an ore.  The Point Load Index can be used to simultaneously characterise rock for blastability and comminution processes.

In a Point Load Test, a sample of rock is mounted between two pointed platens and pressure is applied until failure of the sample occurs.  The peak applied load is recorded and used to calculate the Point Load Index.

The point load tester consists of a hydraulically powered ram and two pointed platens. One of these platens is stationary while the other is free to move through the application of pressure, delivered via the hydraulically powered ram. The rock specimen to be tested is placed between the two platens and force is applied to the specimen by activation of the hydraulic ram. The force applied to the rock is increased and eventually causes the rock to fail. The peak pressure applied is indicated on a pressure gauge. The force recorded by the instrument to just break the rock (P) is converted to a strength value, equivalent to a 50 mm rock. This produces the so-called Is(50) value or Size-Corrected Point Load Index.

According to Brook (1985), the formula to convert the force reading to I_s(50) value is as follows:

I_s(50) = FP / (D_e)²

Where
F = size correction factor = (D_e/50)^0.45
P = applied load (MN)
D_e = (4A/p)^0.5
A= minimum cross sectional area of the specimen (mm²)

The units of the point load index are MPa and whereas the test is considered to cause tensile failure it can be converted to compressive strength (C) by:

C = 24 I_s(50)

The shape and size of each particle tested in a Point Load Test can influence the results of the test, as will natural variations in rock quality and competence.  The best possible shape for greatest accuracy is cylindrical, such as core. If core samples are not available, square or rectangular particles are preferred.  Shapes to avoid are concave or convex particles and triangular shaped particles.  To avoid partial shearing, the surfaces where the platens contact the particle should be flat and parallel.  This will also minimise the possibility of the particle falling out during the test.  The Point Load Index is normalised to a thickness of 50 mm, however examining larger particles reduces the affect of shape and rock texture.  Samples should also be as competent as possible, without any obvious pre-existing flaws.

For every Point Load Test carried out at JKTech, a standard report is issued.  The report details the Point Load Test procedure method and presents the results in tabular form, as well as in a histogram format for each ore type.  Further interpretation of the results is available if required.


Sample Requirements

1. For a Point Load Test: sufficient core to cut 50 pieces with a length to diameter ratio of at least 2.5:1 and a core diameter ideally about 50mm, although 20 to 60 mm is acceptable. Rock pieces in -75+50mm range can also be used - again 50 pieces with 2.5:1 aspect ratio and reasonably parallel end surfaces.  Rocks or core larger than 75 mm cannot be tested due to machine constraints.  Less than 50 particles may be tested if availability is an issue, but results are more accurate if the full amount is provided.

2.  Download the Sample Transmission Forms, which include Sample Hazard Advice Form and Quarantine Declaration Forms.  A completed Sample Hazard Advice Form must accompany every sample shipped to JKTech. In addition, a Quarantine Declaration must be completed for every sample originating from outside Australia.

Contact our Laboratory Services Manager for more information about the Point Load Tests, or to arrange for a test to be carried out.