INDIAN INSTITUTE OF TECHNOLOGY KANPUR

Department of Materials and Metallurgical Engineering

Virtual Laboratories for Thermal Processing and Characterization of Materials

Mechanical Testing Laboratory

Experiment 3

Determination of Weibull Modulus of Brittle Materials

Objectives

  1. To examine the variability of fracture strength of brittle materials (specifically glass).

  2. To determine the Weibull Modulus and the elastic modulus of glass.

Theory

Brittle materials undergo rapid fracture and without plastic deformation. Measurements also show large variability of fracture strengths (as well in their elastic modulus). This variability arises because of the presence of cracks. This variability is quantified by fitting the data to a Weibull distribution function and determining the Weibull Modulus.

Brittle materials (e.g., glass) in the shape of plates are subjected to 3 point bending by a specially designed jig which can be attached to a standard tensile testing machine. The loading geometry is shown below.

                                                                                   

Methodology

Three point bending experiments are to be done on glass slides. Two batches of 15 glass slides are to be taken: the first batch is tested as is (i.e., without any treatment); the second batch is treated by dipping in HF for about 1 minute. The two batches are then subjected to three point bending experiments and the fracture strengths obtained for glass slides for each batch is to be tabulated separately.

The three point bend jig is mounted on a Lloyd's Tensile Testing Machine.

Now establish a remote connection to the tensile testing machine. After establishing the connection, you will see a live image of the machine. Conducted the test till the slide fractures.

The load-displacement data is saved in an excel file. Transfer the file from the remote virtual laboratory location to the local computer. From the load-displacement data the slope of the straight lines and the fracture load can be determined.

Results

  1. Note the span of the support in the three point bend test and also note the width of the glass slides.

  2. Conduct the test on all the glass slides from both, the untreated batch and the treated batch of glass slides.

  3. Tabulate the values of fracture loads and the slopes of the load-displacement curves for each test.

  4. Compute the bending strength and elastic modulus from the slope of the load-displacement curve for each test. Tabulate this data as well.

  5. Compute the Weibull modulii to show the variability of bending strength and elastic modulus.

Discussion and Conclusions

  1. Discuss the variability of the bending strengths and elastic modulii within the untreated batch.

  2. Comment on the differences in the results for the untreated and treated glass slides.

  3. List the important conclusions obtained in the experiment.

Questions

  1. A material with a high value of Weibull modulus would show large variability in mechanical properties?

  2. As the sample size is reduced the variability of mechanical properties reduces. Justify.



Global View of glass and and also variability
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