Laboratory Report: Effect of Creep on Lead

This lab demonstrates the effect of creep on lead. Three similar lead specimens were subjected to different applied loads and the effects of the load where measured by data logging software on a computer. The purpose of the lab was to explore the topic of creep, as creep is an important design factor in engineering.

Introduction

Creep is the changes in shape weather gradual or constant that occurs when a specimen is under a constant applied load or applied stress, combined with temperatures that normally are or exceed one third of the melting point of the material.

The stresses involved in creep are stresses below the yield strength and normally applied for long periods of time before a high level of creep occurs.

Some types of creep are dislocation creep where dislocations and movements of atoms lead to elongation of the object, Nabarro-Herring creep atoms diffuse through the lattice causing grains to elongate along the stress axis and Coble creep the atoms diffuse along grain boundaries to elongate the grains along the stress axis

Temperature is a major factor on creep the higher the temperature i.

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e. the closer to the melting point the faster creep takes place. It is diffusion within the material that causes dislocations and the movement of atoms, causing the length of a specimen to increase in gauge length while under a constant applied load.

In this lab lead will be used as not many other materials have Tm/3 (melting point temperature) at room temperature, which will remove the need to heat the specimen to the necessary temperature.

The tests will consist of taking lead specimens and applying a constant load by means of hanging weights. The total weight applied to each specimen will be less than the failure strength of the material. The specimen will then gradually increase in length until its cross sectional area will not be sufficient to support the applied load and fracture will occur.

Theory

Creep behavior in materials typically progresses through three stages:

1. Primary Stage: This stage exhibits an instantaneous strain when a stress is applied. However, the rate of creep decreases due to work hardening, where the material becomes more resistant to deformation.
2. Secondary Stage: The secondary stage is characterized by a constant rate of creep. During this stage, there is a balance between work hardening and softening due to annealing, resulting in a steady creep rate.
3. Tertiary Stage: In the tertiary stage, damage becomes evident as voids and cracks begin to form within the material. This stage eventually leads to failure, and the material's strength deteriorates rapidly.

The strain of the lead specimen can be calculated using the formula:

Strain (ε) = (Change in Length / Original Length)

In this experiment, the focus is primarily on the rate of change of strain with respect to time, calculated as (dε/dt).

Procedure & Equipment

The following lead specimens were used in the experiment, with their respective dimensions and applied loads:

1. Place specimen 1 in a fixed position on the tecquipment machine.
2. Start the data logger program.
3. Gradually apply the weight until the desired load is reached (as specified in the table above) to avoid initial impulse damage.
4. If the specimen does not fracture within a set time, stop the test.
5. Record the data measured by the data logger and save it to a file.
6. Repeat the above process for specimens 2 and 3.

The experiment was conducted using a tecquipment machine, as shown in Figure 1, which utilizes an 8:1 mechanical advantage lever system. An LVDT transducer continuously measured the strain on the specimen, and the data was recorded using data logging software on a computer.

Results

Test 1

The table below illustrates the strain behavior of specimen 1 during the test:

In this test, the specimen did not fracture within the test duration, but it was estimated that failure might have occurred after 4-5 hours.

Test 2

The table below illustrates the strain behavior of specimen 2 during the test:

In test 2, the specimen did not fracture within the test duration.

Test 3

The table below illustrates the strain behavior of specimen 3 during the test:

Test 3 exhibited the primary, secondary, and tertiary stages of creep, with the specimen ultimately failing. The primary stage was abrupt, transitioning to the secondary stage after 25 seconds. The tertiary stage began around 140 seconds, and the specimen failed just after 170 seconds.

A U shape graph was what one would expect from a creep test, though as failure did not occur, the specimen did not reach the tertiary stage, so the final slope is not on the table.

This table is more like what one would expect to see, showing all three slopes for each of the three stages. The first as it's subjected to the applied force. While in the secondary stage, it remains linear with a low strain rate before the cross-sectional area finally reaches a level where it cannot cope with the applied stress, and the specimen begins to fail.

Steady State Creep Rate

To determine the values for 'A' and 'n' in the equation for steady-state creep rate, we plotted a graph of log ss against log ?, where ? represents the applied load (Mass x gravity x 8) divided by the cross-sectional area. The linear regression equation for the graph was:

y = 0.103x + 3.22

Using the method described in the theory section, we calculated the values for 'A' and 'n' as follows:

Log A = 3.22 (implies A ≈ 25.03)

n log ? = 0.1013x (implies n ≈ 0.1013)

For this material, the steady-state creep rate was found to be less than 1.06 x 10^-9 per pascal (Pa).

Additionally, to achieve a total time to failure of more than 10 years, the applied stress (?), when the specimen failed, had to be less than or equal to 19.6 x 10^-9 Pa.

Discussion

The experiment may be prone to various errors, including:

1. Errors in measuring the dimensions of the specimens, particularly the gauge length.
2. External factors like temperature fluctuations in the room during testing.
3. Possible bending of the specimens during handling, which could affect measurements.
4. Electrical noise that could impact data recording.

Real-world creep tests are often performed at higher stresses and shorter durations to simulate the conditions a material will experience over its lifespan. For example, a 10-week test at higher stress levels may be conducted to assess the behavior of a material expected to last 10 years in service.

It's worth noting that creep can have both positive and negative effects. In some cases, as seen in concrete, creep can prevent cracks and enhance the material's performance.

Conclusion

In this laboratory experiment, dislocation creep was observed in lead specimens subjected to different applied loads. The tests revealed the stages of creep, including primary, secondary, and tertiary stages. Specimen 3 exhibited all three stages and ultimately failed. Creep is a critical consideration in engineering design, and an incorrect assessment of creep behavior can lead to catastrophic failures. The results of this experiment underscore the importance of understanding creep in materials and its impact on structural integrity.

Recommendations

For future experiments, it is advisable to minimize potential errors by ensuring accurate measurements, controlling environmental conditions, and carefully handling specimens. Additionally, further research into the effects of creep on different materials and the development of strategies to mitigate its negative consequences in engineering applications is recommended.