Sunday, December 14, 2008

Green Compression Strength and Permeability

 

OBJECTIVE

This experiment measures the following properties of mould sands;

  1. Green compression strength
  2. Permeability

 

MATERIALS & EQUIPMENTS

Green sand, Steel specimen tube, Permeability machine, Universal Sand Strength testing machine, Sand rammer, Funnel, Stopper, Ruler and Weight meter.

 

THEORY

 

  1. Green Compression Strength

Green compression strength refers to the stress required to rupture the sand specimen under compressive loading. The sand specimen required to rupture the sand specimen under compressive loading. The sand specimen is taken out of the specimen tube and is immediately (any delay causes the drying of the sample which increases the strength) put on the strength testing machine and the force required to cause the compression failure is determined. The green strength of sand is generally in the range of 0.03 to 0.16 MPa.

  1. Permeability

The rate of flow of air passing through a standard specimen under a standard pressure us termed as permeability number. The standard permeability test is to measure the time taken by a 2000cmof air at a pressure typically of 980 Pa (10/sq cm), to pass through a standard sand specimen confined in a specimen tube. The standard specimen size is 50.8 mm in diameter and a length of 50.8 mm. Then, the permeability number, P is obtained by,

            P = (V.H) /

 

Where, V = volume of air = 2000cm

            H = height of the sand specimen = 50.8 mm

            = air pressure, g/cm

            A = cross-sectional area of sand specimen = 20.268 cm

            T = time in minutes for the complete air to pass through

Substituting the values:

            P = 501.28/.T

The properties of moulding sand are dependent to a great extent on a number of variables such as:

i)                    Sand grain shape and size

ii)                   Clay type and amount

iii)                 Moisture content

iv)                 Method of preparing the sand mould

 

PROCEDURE

Preparation of specimen:

Since the properties of sand are dependent to a great extent, on the degree of ramming, it is necessary that the specimen be prepared under standard conditions. To get reproducible ramming condition, a laboratory sand rammer is used along with a specimen tube. The measured amount sand is filled in the specimen tube, and a fixed weight of 6.375 to 7.25 kg is allowed to fall on the sand three times from a height of 50.8 0.125 mm. The specimen thus produced should have a height of 50.8 0.8 mm. To produce this size of specimen usually sand of 145 g to 175 g would be required.

Figure 1.0 The Funnel (blue) and Rammer (green).

i)                    Collect the sand to be tested

ii)                   Fill it in a steel tube with the help of the funnel. Place stopper at the other end of the tube.

iii)                 Bring the tube, under ramming machine to give three impacts by turning the handle three times. Adjust the amount of sand in the tube such that after three impacts, the specimen height is 50.8 mm.

iv)                 Remove the specimen from the tube.

 

  1. Green Compression Strength Testing

Figure 1.1 The Green Compression Strength Test Machine.

 

i)                    Bring the specimen to a Universal Sand Strength Testing Machine

ii)                   Set the adapter on machine and hold the specimen in the adapter.

iii)                 Take the initial reading by setting the magnet piece. Make sure the initial reading is zero.

iv)                 Start compressing the specimen by turning the handle until the specimen fails.

v)                  Note down the reading. The final reading gives the value of the specimen’s green compression strength.

 

 

 

 

  1. Permeability Testing

 

Figure 1.2 The Permeability Test Machine.

 

i)          The machine is set up.

ii)         The specimen is hold in the steel tube.

iii)                 The tube is placed on the machine. The tube is surrounded by mercury to avoid any leakage. The nozzle is inside the tube.

iv)                 The air is passed through the tube and as a standard mark, the stop watch is started.

v)                  The stop watch is stopped as soon as the 2000 mark on the floating cylinder reaches the standard mark.

vi)                 The cross sectional area and the length of the specimen is measured after removing it from the tube. The 200o mark indicates that 2000 cm of air has passed through.

 

  

Figure 1.3 Sample of specimens.

 

RESULTS

 

1. Specimen’s cross section.

            The given dimensions of standard specimen are;

            D, diameter = 50.8 mm

            H, height = 50.8 mm

            Cross section    =

                                    =

                                    =   2026.82 mm

                                    =  2.02682   m   

2. Sample of calculation.

            P = (V.H) /

                = 501.28 /

                = 501.28 / (10)(0.5)

                = 100.256

From the readings obtained according to the three samples of weight, 145 g, 165 g and 175 g, none of these samples managed to get a height of 50.8 mm. Since the value of 50.8mm falls between the weight of 145 g and 165 g, the exact theoretical value is calculated as below.

 

            The desired height is 50.8 mm and x is the desired weight.

            Using interpolation, the value of x is determined as below,

           

           

 

For the 147.05 g it undergoes Permeability Test and after three readings, the average value obtained was 98.678 while for Green Compression Test, this specimen started to crack at 58 kN/m and the maximum strength reached the value of 70 kN/m.

Theoretical value of Permeability for the mass 147.05 g is 100.256 while the experimental value determined from the experiment is 98.678.

Percentage of error,

 

  1. Table and data.

 

Mass, g

Permeability (kg/N)

Height of specimen, cm

1st reading

2nd reading

3rd reading

Average value

145

88

89

96

91

49

165

89

90

88

89

57

175

76

74

75

75

60

Table 1.0 The results obtained from the experiment.

 

 

 

 

 

 

 

 

 

 

 

 

 

DISCUSSION

 

            From this experiment, we determined the values of permeability for green sand for different quantity of mass in order to get the desired green compression strength and permeability. Firstly, according to the experiment in order to obtain a sand specimen with a height of 50.8 mm, the range of sand weight would be between 145 g to 175 g. So, the range is divided into three values of 145 g, 165 g and 175 g. For these values of weight, none of them manage to acquire the desired height. Hence, a calculation is done and resulted that a specimen with a mass of 147.05 g would have a height of 50.8 mm. This value is proven to be true when it is being experimented according to the prepared procedure.

            As for the permeability section, the readings were done three times to observe any changes that might present while the experiment is performed. However, a small number of differences seemed to appear in the readings where significantly all of them resulted an increase in values. This means that, the permeability did increase and more air can pass through the sand. For this matter, the reason that might contribute to this changes is that the volume of air that passes through the sand slowly dries the amount of water in the green sand composition. It is said that the water in the green sand composition dries more and more after each permeability test. That is why, the permeability value keeps increase from the first reading to the third reading.

            As for the third value of green sand specimen, which refers to 175 g, the permeability is much lower as the composition of water in it is much higher making harder for air to pass through it. As observed, for the first two sample of sand taken for the specimen, the sand is taken from the surface of the container and the sand itself is exposed to dry air that able to dry up some of the moisture in the green sand composition. As for the third sample of sand specimen, the sand is taken deeper from the container and believed to have more moisture compared to the sand at the surface of the container. So, as this can be seen, the moisture of sand differs from the surface to the inside. The inconsistency of moisture presence in the green sand composition do related with condensation factor. This do affected the permeability and strength values throughout the experiment. Higher moisture will cause the specimen to have low permeability.

 CONCLUSION

 

            As observed in the discussion above, it is determined that there are several factors present causing errors in readings of the data. It starts from the sand pored into the funnel directly to the test tube. While doing this, some of the sand will stick at the funnel and the accuracy of mass is totally reduced. Next, when it is being rammed in the rammer machine, the same thing happens where a small number of sand will also stick at the head of the rammer. Again, the mass of the specimen will be reduced. Even the loss of sand due to these processes is very little but it still affected the whole process especially in terms of sand volume. Since the volume drops, thus the dimension of the specimen required will not be accurate. So, the desired height of 50.8 may not be achieved.

            For the next process which is the permeability, the value of permeability did increase as the volume of air flows through the sand specimen dries it up slowly and causing the sand specimen to be drier and more air can pass by it.

            For the green sand compression strength, a machine is being used to test the specimen’s strength. The only specimen that undergoes this particular test was the 147.05g specimen which states an accurate reading of 50.8 mm in height. The specimen is tested until it started to break apart. According to the lab assistant, the permissible load that this specimen could hold is up until 60 MPa. However, this specimen started to crack when it reached the value of 58 MPa. Even so, this specimen managed to undergo loads up to 70MPa.

            In order to enhance the result for this particular experiment, the green sand need to be more equally mixed with adequate water. Excessive amount of water would make the sand harder in uniformly condensed even in a short while. One more important thing is that, when placing the sample of specimen sand in a container, it must be oftenly mixed equally to make sure that they are well prepared and the moisture would be uniform.

 

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