eRocks

3.2 Types of Test Methods

3.2.1 Loading
A number of rock strength tests have been developed which apply a known load to a specimen of fixed geometry. There are a number of ways in which a force can be applied to a body depending on the geometry of a test piece, which can range from irregular lumps to highly machined complex geometric shapes. There are several different designs for the method by which a load is applied to the test piece, from points to flat surfaces. In a typical test the load at which breakage occurs is correlated to the material strength. Loading tests that give a measure of tensile strength have been shown to correlate well with comminution energy requirements [Bearman 1989, 1997]. A typical example of a test that produces a tensile strength parameter is the Brazilian test in which, although the test piece is loaded in compression, causes the sample to break in tension due to the biaxial loading applied in the procedure.

A significant number of tests to predict comminution characteristics have been introduced since the early work of Kick [1885] and Rittinger [1867] and fall into one of three main categories. Grinding methods were covered in Chapter 1 with the remaining types being loading and impact based.

3.1 Factors Affecting Results
A number of variables in both the specimen and the actual test method itself can alter the results obtained. A full review of such variables can be seen in Vutukuri et al [1974].

3.1.1 Specimen Geometry
Depending on the actual details of the test itself a sample can fall into one of a number of geometric shapes. Typical shapes include cylinders, bars, cubes and irregular lumps. The method of force application can lead to different values of strength for a similar shape. Therefore differently shaped particles require the use of modified equations. These variations have lead to the need for a standardisation. Core samples have proved to be the most popular [Brook, 1977]. The effect of shape cannot be overlooked as Lundborg [1976] found that for tests using cylinders the strength decreased with increasing size.

3.1.2 Length to Diameter Ratio (L/D)
It has been recommended [Vutukuri et al, 1974] that to obtain reliable results L/D ratios should be in the region of 2.5-3.0 when tests are compressive in nature. However this is not always possible in practice, and it has been found by Bearman [1998] that values of as low as1:1 can be used effectively. Using values of greater than 3 can lead to additional stresses, for example the effects of twisting. Direct tensile tests differ, as there is a need to grip the specimen, therefore requiring a larger L/D.

3.1.3 Rate and speed of Loading
The rate of loading is where the applied force is increased linearly by adjusting the speed of the loading body. In comparison with a constant speed of loading force where the load observed can increase non-linearly. Willard and McWilliams [1968] found that point-loaded discs loaded at a higher rate were able to absorb more energy than those loaded at slower rates. This is likely to be due to the fact that the samples were not homogenous in nature. In contradiction to this Vutukuri et al [1974] reviewed work that concluded that the rate of loading has no significant effect on compressive strength. It was concluded the rate of loading has little effect on rocks that are homogenous, low in porosity and are elastic in nature.

3.1.4 Rock Fabric
The fabric of a rock is usually multi-phase and as a result cannot easily be defined. For example, some inclusions may be very hard while the main matrix itself is very soft. It is highly likely that the interfaces between any phases within the rock are the weakest points provided that the forces are applied in the correct orientation. Figure 3.1.4 shows the effect of layering and how it could affect experimental results.

Figure 3.1.4 Effect of Rock Fabric

As can be seen, the force applied to sample A has the ability to run along the particle interface, as opposed to sample B, where the force has to cross the interface. This is likely to increase the energy required to break sample B when compared to that of sample A.

Another factor that can be linked with the fabric of the rock is the possibility of flaws arising from porosity. These flaws can take the form of ‘holes’ or microcracks. Microcracks can reduce the observed strength of rock due to the fact that pre-propagated cracks are present. Griffith [1920] showed that cracks could reduce the actual strength of a material when compared to a theoretical strength by as much as two orders of magnitude.

3.1.5 Surface Finish
The surface finish can be an important factor that requires consideration with both the sample and the loading platten. If the loading surfaces are rough in nature, this can give rise to increased stress concentrations, which can subsequently alter the experimental value achieved.

The surface finish of the test specimen can be critical in some cases. For example the chevron bend test has been used extensively in the metallurgical applications and has been subsequently adapted to rock mechanics. It is widely accepted that the quality of the surface affects the final values for KIC, and therefore the surface finish of the test piece must be the same as the bulk sample to ensure that a scientific comparison can be made.

This is a picture or a reclaimer that scoops up stored iron ore that was left by the stacker. At the bottom right side of the pictutre you can make out the large rotating buckets that scoop up the iron ore for loading onto the massive ships.

The iron ore is channeled along long conveyorbelts to it final destination.

These too, are big buggers!!

Now these beasties are quite intimidating from this distance. Now imagine being behind them when they have a belly full of iron ore in them. I was in a big 4X4 and these were 10 feet in front of me going up a steep hill (a muddy one) and just the wheels were much taller than what I was in.

I think those steps in the background are 15M each!!!

The weirdest thing was the 4X4 had to have a really long bendy pole stuck on top of them so that these muthas didn’t squash them. When they spotted you behind them they pulled out so that you could undertake them, which was again a tad weird.

Yet more red earth and blue sky!

This thing is a settling tank, which is used to get really fine particles of iron ore out of suspension. Imagine a muddy river and all the stuff that is making the mud look, well, muddy. Well that stuff is still worth money, so it needs separating from the solution, and that what this device would do. Over time the particles sink to the bottom and the particles are then scooped off the bottom of the tank.

To put the scale of the settling tank into perspective, bottom left is a big-ish 4X4. I have no idea what I was standing on at the time

Where is this located I hear you ask……not far from the big hole at Mt Whaleback:
http://www.erocks.co.uk/10/mount-whaleback-western-australia/

I think it is not far from an ore body called Marra Mamba. But if someone calls me on that and suggests I am talking rubbish, I will hold my hand up….I am doing this from memory.

This is a picture of a stacker at Port Hedland in Western Australia at the ship loading facility. I lived about 3 minutes drive from this beast of an operation. I will be putting images of those up soon which are in stark contrast to the one above.

So what does the above stacker do? Well it moves left to right on rails pouring out 1000’s of tonnes of iron ore for storage and later “reclaiming”. This reclaiming happens when it is needed and a ship needs loading.

It is difficult to get the size of one of these mutha’s, but that pile of iron ore that the beast is adding to, is maybe a 3 storey house. I may be wrong on that. In fact looking at the building , far left, that was a build multi storey building with many layers of crushing equipment.

So, yes it falls into this category of Big Crazy S###

Ever seen Crocodile Dundee or Bush Tucker man and if you are a little like me images like this leave you in an awe struck state. Now imagine this…having this image behind you as well and all around is scrub land with the odd set of rock outcrops. Oh yeah, it is as hot as hell, and your sandwiches which you put in a cool box are crispy on one side due to the heat!

Yep, you is in the outback sport!

So where is this crazy road, well it is about 3 hours drive (depending on how fast you drive) from Port Heldland, in Western Australia. I went looking for mad things, and this scene captured my imagination. More pictures to come but this is not far from a fresh water lake, that I took a swim in.

This is a picture of Mt Whaleback in Western Australia. This is about 0.5KM deep, 0.5KM across and 1Km in length. Now what I think is a bit crazy is the fact that this hole in the earth used to be a mountain!!!

This is is from the sky, http://www.hyvista.com/iom/iom_jan05.html

The picture I took is from 1995.

If you want to read a bit more about it:
http://www.portergeo.com.au/tours/iron2002/iron2002depm1.asp

Cheers

Dr Rock

This tunnel is about 1400M underground and as with the the big hole, I think you will have to concur that that is a big tunnel.

What is more amazing is the fact that it was dug out using picks and shovels and the guy on the left is 6 foot 9!!!!

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Answers on a postcard as to which two bits of information are not true:

Send them to

Bullshit Question Competition
BS House
BS County
England
(Sorry Mr PostMan Dont know the code)

2. Fracture Mechanics Theory

The study of fracture mechanics has been applied to a wide range of fields, mainly metals and rock but it can be applied to any substance that is subjected to some form of stress. In most cases the information gained is used to predict a products life span and its working limits. Within the field of rock breakage behaviour, the values obtained are used to quantify the comminution characteristics of an ore sample and hence predict energy requirements for crushing and grinding.

Giffith [1920] conducted the original work in this field and most of today’s models have been based on the conclusions of his work. A summary of what the term ‘Fracture Mechanics’ means is given by Irwin and de Wit [1983]:

“…the fracture of materials in terms of the laws of applied mechanics and the macroscopic properties of materials. It provides a quantitative treatment, based on stress analysis, which relates fracture strength to the applied load and structural geometry of a component containing defects”.

2.1 Modes of Fracture

Three basic modes of fracture exist and are illustrated in Figure 2.1 where A, B and C are modes I, II and III respectively. In Mode I the body is in a tensile or opening regime where the crack and the crack surface are perpendicular to one another. Mode II has the characteristics of being sheared or slid and Mode III is in a state of tearing or anti-plane. These modes of fracture can exist independently or combined with one or two of the others, with the most complex being a I, II and III combination.

Figure 2.1 Three basic modes of fracture