Spring 2019 MSE 245L Brittle Fracture Lab Module (Student)
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Louisiana Technical College *
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189
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Material Science
Date
Apr 3, 2024
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MSE245L – Introduction to Materials Science and Engineering Lab Brittle Fracture Week of 2/4/19 Group A, Week of 2/11/19 Group B Print this out and bring to lab – it contains the procedure for the lab. Required Reading Before the Lab Date: 1. Callister & Rethwisch: “Materials Science & Engineering – An Introduction” Chapter 8: Failure, Sections 8.1- 8.6 (Fracture) Chapter 12, Section 12.9 Stress- Strain Behavior (Flexural Strength) https://en.wikipedia.org/wiki/Flexural_strength 2. Lab write-up – read before lab to see what you’ll be expected to include in your results/reports. It is recommended to print and bring with you to lab. Summary of Experiment: Students will learn how brittle materials fail by fracture. In this lab, we will stress glass samples to fracture using the four- point bend test. Deliverables (documents to be turned in during the lab): Electronic copy of the group data collected to be emailed to TA. No hardcopy assignment is due during lab. A lab write-up is due the following week for each group and is posted to BB. Theory: The Four-Point Bend Test may be used to determine the flexural strength of a sample. It uses a rod sample having either a circular or rectangular cross section that is stressed until fracture using a four-point loading technique. In our lab, the cross section for our samples is rectangular. 4-point loading technique: The sample is placed on 2 supports a set distance apart and 2 loading pins are placed at an equal distance from the center. These 2 loading pins are then gradually lowered at a set rate until the sample fails. The advantage of the 4-point bend test over the 3-point bend test is the homogeneous stress state between the two loading points. For a 3-point bending test, the stress peaks at the central load (the point of contact). For a 4-point bending test with the same sample, a larger volume exhibits a constant load. This is important to note because the flexural strength is controlled by the largest flaw. The largest flaw may be found anywhere throughout the sample. By probing a large volume, the 4-point bend test will give more accurate results. At the point of loading, the top surface of the specimen is placed in a state of compression, whereas the bottom surface is in tension. The maximum tensile stress will then occur at the bottom surface of the sample. The stress at fracture using the bend test is known as the flexural strength or flexural stress. Nomenclature (in SI units): 𝜎𝜎
𝑓𝑓
Flexural Strength (
𝑀𝑀𝑀𝑀𝑀𝑀
)
𝐹𝐹
Load at fracture (
𝑁𝑁
)
L Support Span (
𝑚𝑚𝑚𝑚
)
𝑏𝑏
Width of test beam (
𝑚𝑚𝑚𝑚
)
𝑑𝑑
Thickness of test beam (
𝑚𝑚𝑚𝑚
)
Flexural stress,
𝜎𝜎
𝑓𝑓
for a rectangular cross- section: 𝜎𝜎
𝑓𝑓
=
3
4
𝐹𝐹𝐹𝐹
𝑏𝑏𝑑𝑑
2
Page 2
of 3
[Ref: https://en.wikipedia.org/wiki/Flexural_strength
] Lab Summary:
1)
The flexural strength of glass will be examined in this lab module 2)
You will utilize the data collected from your Section # Group # only, that is 60 tests. (For example, Section 001 Group A will not use Group B data, and will not use Group A data from other sections). Data used is for your lab session only. 3)
All data collected must be sent to the TA of that lab group by email to be posted by COB next day 4)
Data will be posted to Blackboard for each lab group, by the TA, by Close of Business the following day. Experimental Procedure: A demonstration of the steps will be given by the lab assistant (LA). Glass slides will be provided for this experiment. Safety: Wear safety goggles & ensure cover is in place before load is applied to samples Safety: Dispose of glass slides in breakables box ONLY. Split into pairs (or 3 if odd #). 4 Test frames are available and 1 PC. Each pair should test samples, then rotate to the lab PC to add their data to the running spreadsheet. 1)
Data Collection: a)
Create a table to keep track of specimen number, dimensions (width, thickness), surface treatment (unscratched or scratched), and failure load. b)
Measure the span length using calipers or ruler (dimension of the bottom loading points, L) c)
Label the slide # using a sharpie for tracking purposes i)
Measure dimensions using a caliper, 3 measurements for each dimension, at the center of the slide ii)
Place scotch tape across the entire length of all slides - The purpose of the tape is to keep the sample together when it breaks. It also allows us to examine the crack patterns and helps to identify where the fracture originated. Make notes as needed within your spreadsheet for each sample. The tape also helps with cleanup. d)
Scratch 30 glass slides using the provided sand paper e)
Each pair of students should test 12 slides (6 scratched, 6 unscratched) f)
Each student should clean up each broken slide before proceeding with the next test. To Test a Slide: 1)
Retract the top plate manually
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