Question

1. The fracture stress of your sample (in units of N m⁻²) is given by dividing the load (in N) at the fracture point by the cross-sectional area of the original sample (in m²). The typical thicknesses of plastic bags are 8 × 10⁻⁶  m for flimsy bags and 7 × 10⁻⁵  m for thicker bags. Select one of these values and use it to obtain an estimate for the fracture stress of your bag. Ensure you state the thickness value you chose.

2.Typical values of the fracture stress of high-density polyethylene (HDPE) range from 20 MPa to 30 MPa, and the fracture stress of low-density polyethylene (LDPE) ranges from 8 MPa to 12 MPa. Compare the value you obtained with these values, and use the comparison to decide which material was used to make your bag.

3. In around 25 words, comment on how the masses of the individual weights that you used to stretch your sample will affect the accuracy of your result. By comparing the weight that caused fracture with the heaviest weight that did not break the sample, refine your estimate for the fracture stress and state the range of values that your fracture stress falls between.

Test the properties of plastic film This experiment required the following equipment: an empty ice cream container some sticky tape a convenient piece of scrap wood kitchen scales an assortment of food tins. This equipment is shown in Figure 4.3a. Figure 4.3a Equipment used for tensile testing experiment Two strips of plastic were cut from a plastic bag. One end of each strip was taped to the wood, and the other ends were taped to the ice cream container (Figure 4.3b). Note that this needs a long strip of wide tape pressed firmly onto the surfaces, otherwise failure will occur when the tape comes apart rather than when the plastic breaks. Using narrow strips of plastic (at most 20 mm) also helped t<o ensure that the plastic, rather than the join, failed.

Answer 1

Ans) Given,

Width of each strip = 20 mm or 0.020 m

Thickness of flimsy bag = 8 x 10^-6 m

Thickness of thicker bag = 7 x 10^-5 m

Cross sectional area of flimsy bag plastic = 0.020 x 8 x 10^-6 = 1.6 x 10^-7 m²

  Cross sectional area of thicker plastic = 0.020 x 7.5 x 10^-5 = 1.5 x 10^-6 m²

Let the fracture point be average of two values = (13.70 + 16.3)/ 2 = 15 N

We know, stress = Load / Area

Number of strip = 2

Load on each strip = 15/2 = 7.5 N

Stress for flimsy plastic = 7.5 / 1.6 x 10^-7

= 4.6875 x 10⁷ N/m² or 46.875 MPa

Stress for thicker plastic = 7.5 / 1.5 x 10^-6 = 5 MPa

According to question , stress for HDPE plastic range from 20 - 30 MPa but our calculated value is far better than this value.

On the other hand, stress for LDPE plastic ranges from 8 - 12 MPa but our calculated value is less than this range .

Therefore, it is recommended to use HDPE plastic for the bag with thickness of 8 x 10^-6 m

It is recommended to put lower weight first and then slowly increase them instead of putting heavy weight directly to get more accurate results. For example, suppose we have two type of weight 500 gm and 100 gm then instead of putting 500 gm place 100 gm weight first then increase it slowly till failure. If failure point is 1600 gm then increase 100 gm weight successively till 1600 gm instead of putting 500 gm weight after some value say 1200 gm, so failure occur at 1200+ 500 = 1700 gm .Since , original failure point is 1600 gm so 1700 gm will not give accurate result.

As stated above Fracture point is 15 N or 1530 gm , so add 1530 - 1402 = 128 gm weight instead of putting some heavy mass more then 128 gm. Therefore , Fracture point occur between 13.7 N to 15 N