Elongation test

The same two paint systems (Hempel’s 15500 and 45880) were tested for tensile strength. The elongation test was conducted for two different widths: 19mm and 27mm. Each of the width were tested at three different thicknesses: 100цm, 200цm and 300цm., and each of the thicknesses at three different speeds: 2mm/min, 10mm/min and 50mm/min. Each test was carried out using five replicates or more. A schematic description of the experiment plan on the figure given.

 The aimed gauge length of the samples was 100mm in the beginning, but it was changed to just 40mm due to a problem with the paper used as surface material during painting and drying. The paper had small dents/lines dividing it into four sections. These dents deformed the samples, easing thereby breakage in the line of deformation. We were not aware of this problem until we were finished making our samples. Almost all our samples had that small deformation in the middle of the length. All deformed samples were therefore manually broken at the deformation line. This breaking of samples added more replicates to the later testing.A few samples that was not deformed was used in their initial length. These were used in the Sticks-Full size test.

 

Preparing elongation samples

Due to the tight schedule, we had to make 180 samples in only one day. That is why we had to find a painting method that was efficient and fast. We considered two, in principle, different methods- the “cutting method” and our so-called “tape method”. In both methods the paint was applied on paper that already had the right dimensions. In that way, we would avoid having to cut in films that had been made already. Some earlier experiments in doing the cutting after filmmaking has been conducted at Hempel with both good and not so good results depending on the paint system (how brittle it is). Cutting method: The method was used on most of the preparations. It was carried out by cutting sticker-paper into the desired shape of the samples and placing them above a platform, which in our case was a wooden stick. The paper was attached to the stick using tape at both ends.         The wooden stick had two purposes. It had to elevate the sample while spray-painting and it had to prevent the paper from bending when painted.

Figure 1 shows how the sticks was lined up before painting

This method was very efficient because we were three people working in what looked to be a production line. It will be very slow though if one works alone. Tape method: Another way to prepare the samples was by dividing a large plate using tape into sections. Removal of the tape immediately after painting causes the surface to split into areas that has the same form as the desired samples. This depends of course of the way the tape was placed before the spray painting. It is very crucial that the tape is placed correctly if it is desired to have the same width throughout the whole sample. The method is even more time demanding than the cutting method. 

Figure 2 shows how the Tapes was lined up before painting

When making the samples by the tape method, one must make sure that the paper is held in place as explained before.To ensure that the sample sticks made by the two methods can be differentiated we use the term sticks for the ones made by the cutting method and tapes for the ones made by the tape method.    After spray painting, the tape strips was immediately removed, and the paint was left to dry for 24 hours at room temperature (20,4°C) (both the sticks and the tapes).

 

 Figure 2 shows how the sticks was removed from the paper after 24 hours of

drying

 Then removed from the paper and cured/dried 48 hours in the oven at 40°C and 24 hours at 60°C. The reason for the temperature change was to boost the curing process. The samples should have been in oven at 60°C over weekend but by mistake the oven was not switched on correctly. 

Measuring

The samples were taken out from the oven and placed in a climate room for 12 hours before measuring. The climate room was kept at a temperature of 23,5°C and with a humidity of 50%. To measure the elongation, a Zwick tensile testing machine was used. Two clamps were used to hold the sample during elongation; one at the top and one at the bottom. The clamps were squeezed together using a force equal to 2 bar.  Placing the sample correctly was difficult if the sample was narrow. It had to be placed vertical between the clamps so that the elongation will take place in the long direction only. It was a subjective judgment whether or not the sample was correctly placed. This could have caused an error in the report. The maximum force would be influenced by this error.         The force used to pull the sample apart was measured. Both the maximum force and the force at break were printed out in a report (Fmax = Fbreak in 99,5% of the cases). By dividing the maximum force by area before elongation of each sample’s cross-section one obtains the engineering tensile strength. And by dividing the force at break by area before elongation of each sample’s cross-section one obtains the engineering stress at break Also the strain in mm was measured and printed out in the report. The strain in was given in percentage as well. A graph showing the force as function of the strain in percentage was also included in this report. The sample length was used by the computer to calculate the strain. Unfortunately one of the Zwick software weaknesses was pronounced in all our reports as it did not calculate the strain based on the manually entered gauge length. Instead the length was based on a default length that had to be entered in the program’s “wizard”. A normal user would think that it was logical that the calculation should be based on data entered on the screen, but Zwick’s software demands from the user to run a wizard. This could have been acceptable if it was stated clearly in the user manual, which was not the case. Apparently, nobody was aware of this and wrong calculating were been accepted all along. The strain calculation in all our reports was based on a length of 150mm. We calculated the real values of the strain and strain percentage manually in Excel, and we added a real “scale to all our reports”. The new scale showing %-strain, is placed underneath the original one. 

Elongation testWhen treating the data from the tensile test, hundreds of plots could be made. Because of big variance of the tensile strength, Fmax, and the %-strain the focus will be on the modulus obtained at 0,1mm elongation, which turned out to be very consistent. 

General observationAfter plotting the thickness as function of modulus, one very consistent information is obtained. Even though this was not expected, the thickness has an influence on the modulus. This influence is observed for both paints and on all samples .It seems to have the same degree of impact on the two paints making the slopes about the same for the two paints. Where a modulus in the area of 3000 MPa is found for a thickness of 300m the modulus increases significant to about 5000 MPa when the thickness changes to 100 m (15500 paint).This phenomenon will be discussed below where several possible explanations are listed. 

The apparatus used for elongation test Length of samples Curing time of paint

 

ApparatusWhen performing a mechanical measurement like the tensile test, it is always related to an uncertainty of some degree. Because of many moving parts inside the machine, some errors will occur. The measurement of the force used to elongate the sample is measured using a so-called strain gage. A strain gage is designed to convert mechanical motion into an electronic signal. A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor. Even though the elongation of the strain gage is very small, it will lead to an error in the measurement. When the sample, as in this situation, is very short, the error coming from the strain gage becomes larger.Unfortunately, it has not been possible to find any numbers about these uncertainties in the instruction manual for the tensile testing apparatus.

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Length of samples

As mentioned earlier, the length of the samples turned out to be smaller than wanted because of problems with the paper used for the filmmaking. Due to these small samples, the error within the apparatus will be pronounced. When placing the film between the clamps in the tensile tester, it is now of even bigger importance that the film is in total vertical position, since unwanted forces from the edge would break the film more easily. However, this type of error has not been seen. Actually, the opposite is observed. The short films had surprisingly a longer %-strain than the long samples and hence do not seem more brittle as expected if placed wrong. The short films also breaks at higher forces, Fmax, than the long ones. An explanation to this can be, that the longer the sample, the bigger the chance to have defects within the sample; especially along the edges as in the figure below. 

Figure 1 shows a sample defects along the edges. The longer the samples the more defects there is. (3 x zoom)

When stretching a sample, some degree of biaxial stress will be transferred to the sample. The smaller the ratio between length and width, the larger the biaxial stress. However, the results do not indicate this phenomenon since the short samples elongates more than the longer ones. Unfortunately there is no minimum or standard length given for the tensile tester. After looking at dimensions of some metals for tensile testing, a recommended ratio of 6 between length and width was given. The ratio used for these experiments were:(40 x 27) mm = 1,5; (40 x 19) mm = 2; (120 x 27) mm = 4,5. Curing time of paintIf the paint has not cured enough, different phases will be present in the film. If the surface layer is stronger than the inner layer, then plots of thickness vs. modulus will indeed have a negative slope as found in the results. The smaller the thickness, the more dominating the surface layer will be. The hardness test indicated a difference in the curing of the two paints. As expected the 15500 was the most brittle one. The hardness test conducted on the 45880 showed a correlation with the film thickness. The thicker the film, the more soft the material. This was observed on all the samples – also on the plates which had been exposed to higher temperatures for several days. If the curing time for the 45880 was not long enough, this could explain the lower modulus at higher thickness. However, the correlation between thickness and modulus is also seen on the 15500 system without having varying hardness when thickness changes.  

Change of parameters during elongationIn the following discussion, the focus will be on the various changes of parameters.

 Comparison of paint systemsBy looking at the different plots of the thickness as function of modulus, a difference through all the plots is observed. As expected, the one 15500 has a higher modulus than the 45880. Comparison of widthBefore measuring, you could perhaps expect that the modulus from the paint with dimension 40 x 27 mm would be more brittle than the same paint with dimensions 40 x 19 mm because of larger biaxial tensions within the film caused by the low ratio between length and width. This was not observed.Instead, the width seems to have a slight effect on modulus for the 15500 paint system only - at all speeds. A theoretically explanation for this behaviour cannot be found. Instead, a physical problem during preparation of the film might give a hint of this surprisingly unexpected result. You observe a larger modulus (larger stiffness) on the 19 mm films compared to the similar 27 mm ones. This can be due to the used sticks of wood when making the free films. On the 19 mm wood sticks the paint from the paper adhered in some degree to the stick itself making it a little difficult to remove. The 27 mm sticks had a slight different design with bends on the edges making the paper with film easier to remove. Because of this observation, it is possible that the 19 mm films has an extra thin layer of paint along the edges making it more stiff (brittle) which can explain the higher modulus on the 19 mm samples. This can be confirmed by looking at this microscope photo in figure 2.

Figure 2. shows a 19 mm 15500 stick with an extra thin layer of paint along the edge. (10 x zoom)

Comparing the results above with the other paint (45880), only an effect as above mentioned is seen on the modulus at low speeds. At higher speeds no difference between the two widths is observed. That the speed should have a larger influence on this particularly paint is difficult to explain. It is interesting though, that in most of the cases (at speeds higher than 2 mm/min) you do not observe any difference between the different widths. This could be explained by the more soft paint. Because of longer curing time, when pulling off the film from the sticks, it seems easier to remove than the other paint. Even though it is attached in some degree to the 19 mm stick of wood it does not attach in the same degree as the 15500 paint mentioned above. The result of this could very well be that the thickness along the edges is not dominating as much in these films. 

Figure 3 shows a 10 x zoom of 45880- 19 mm sample

Comparison of different preparation methodsBefore measuring it was expected that the small cracks observed along the edges would influence the modulus; especially on the 19mm samples. This was however not observed. Actually the opposite was seen.As already mentioned above, there is a slight difference in the modulus of the two widths. It is most pronounced in the brittle paint (15500) because this had a larger degree of attachment to the 19mm sticks of wood than the other paint had. The 27mm sticks had a different design making the films easier to remove. When comparing the different preparation methods it is found that, in case of the 45880 paint the 27mm stick and the 19mm made using tape are similar. This only contributes to the theory about the error in the preparation method of the 19mm samples when using sticks. In order to avoid this in the future, the stick of wood must be approximately 2 mm shorter in width than the paper placed on top of it. By doing this, paint attached to the stick can be avoided. Comparison of different sample lengthWhen comparing the short and long samples, a considerable higher modulus is found for the long samples (120mm), which also leads to less strain. This is not expected. The tensile strength is considerably larger for the short samples. This is also not expected.An explanation for the tensile strength being smaller for the long samples can be, as mentioned earlier, that the longer the sample, the bigger the chance to have defects within the sample; especially along the edges. Comparison of different tensile speedsThe effect on the modulus from the speed of pulling is believed to have no relations on the modulus. By neglecting the only result indicating a slight influence of the speed, there is no relations between the two parameters (see the discussion in "comparison of width", 2nd section above).Of some interest, it is found that the paints have breaks at a higher Fmax the higher the pulling speed. In many cases it is seen that a speed of 50 mm/min results in a larger tensile strength than the one you get with a speed of 2 mm/min. The strain is about the same.

Example:

Comparison of Fmax between the same samples pulled at speed 2 mm/min (up) and 50 mm/min (down).

Penetration test

STANDARD PENETRATION TEST

The standard penetration test is the most commonly used in-situ test, especially for cohesionless

soils which can not be easily sampled. The test is extremely useful for determining the relative

density and the angle of shearing resistance of cohesionless soils. It can also be used to determine

the unconfined compressive strength of cohesive soils.

The standard penetration test is conducted in a bore hole using a standard split spoon sampler.

When the bore hole has been drilled to the desired depth, the drilling tools are removed and the

sampler is lowered to the bottom of the hole. The sampler is driven into the soil by a drop hammer

of 63.5kg mass falling through a height of 750mm at the rate of 30 blows per minute (IS – 2131:

1963). The number of hammer blows required to drive 150mm of the sample is counted. The

sampler is further driven by 150 mm and the number of blows recorded. Likewise, the sampler is

once again further driven by 150mm and the number of blows recorded. The number of blows

recorded for the first 150mm is disregarded. The number of blows recorded for last two 150mm

intervals are added to give the standard penetration number (N). In other words, the standard

penetration number is equal to the number of blows required for 150mm penetration beyond

seating drive of 150mm.

Figure: Standard penetration Test

If the number of blows for 150mm drive exceeds 50, it is taken as refusal and the test is

discontinued. The standard penetration number is corrected for dilatancy correction and

overburden correction.

(a) Dilatancy correction: silty fine sands and fine sands below the water table develop pore

water pressure which is not easily dissipated. The pore pressure increases the resistance of the soil

and hence the penetration number (N).

Terzaghi and Peck (1967) recommend the following correction in the case of silty fine sands when

the observed value is N exceeds 15.

The corrected penetration number  ————- (1)

Where   is the recorded value and  is the corrected value.

If  , 

(b) Overburden Pressure Correction:

In granular soils, the overburden pressure affects the penetration resistance. If the two soils having

same relative density but different confining pressures are tested, the one with a higher confining

pressure gives a higher penetration number. As the confining pressure in cohesionless soils

increases with the depth, the penetration umber for soils at a shallow depth is underestimated and

that at greater depth is overestimated. For uniformity, the N – values obtained from the filed tests

under different effective overburden pressures are corrected to a standard effective overburden

pressure.

Gibbs and Holtz (1957) recommend the use of the following equation for dry or moist clean sand.

—————(2)

Where   is the recorded value and  is the corrected value,  =effective overburden

pressure  .

The above equation is applicable for  ? 280  .

The ratio   should lie between 0.45 and 2.0. If   ratio is greater than 2.0,  should be

divided by 2.0 to obtain the design value used in finding the bearing capacity of the soil.

The correction may be extended to saturated silty sand and fine sand after modifying the 

according to equation (2), i.e.   obtained from equation (2) would be taken as   in equation

(1).

Thus, the overburden correction is applied first and then the dilatancy correction is applied.