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You usually want to play a 0.01 strain rate per minute or 0.10 strain rate per minute so that you can see in full detailed curve of how the material slowly reacts as stress increases over time.
considering you have a 125 mm length, you are looking at a load rate of about 0.01 mm/min and you are aiming to allow the material to reach 0.05 strain (about 6 mm) or failure. That is a total of 600 minutes to reach 0.05 strain.
to be honest I don’t clearly remember how to calculate maximum depth of testing. Worth revisiting. -Tommy
Maximum 3 mm deflection should be enough to see the carbon fibre sample eventually reach failure. You will see in-person the stress curve and if it does not fail, then feel free to continue the experiment.
From experience, and for practical purposes, you can start at 0.5 mm/min. The testing lab technician will also like you better for this.
If you are doing multiple tests, and you think the maximum stress will be easily captured, you can speed up to 1 mm/min.
Data Processing
Source: https://en.wikipedia.org/wiki/Three-point_flexural_test#Standards
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Results
Table #1: List of variables recorded for experiment
Variable | Quantity | Note |
|---|---|---|
L = Support Span (mm) | 31.6 | Approximate measurement |
b = Width of test beam (mm) | Varied by Specimen | Averaged measurement |
d = depth of beam (mm) | Varied by Specimen | Averaged measurement |
Load rate (mm/min) | 1 mm/min | Greater than standard, but was used for sake of time and practicality |
Maximum deflection (mm) | 3 mm | Greater than standard, but allowed to show the material’s properties consistently up to failure |
Table #2: Dimensions of test specimens
Specimen | L = Support Span (mm) | b = Width of test beam (mm) | d = depth of beam (mm) | m (N/mm) |
|---|---|---|---|---|
1 | 31.6 | 16.51, 15.92, 15.53 avg = 15.987 | 1.15, 1.33, 1.09 avg = 1.19 | 120 |
2 | 31.6 | 14.63, 15.61, 14.39 avg = 14.877 | 1.38, 1.59, 1.42 avg = 1.463 | 94 |
3 | 31.6 | 15.24, 15.35, 15.10 avg = 15.23 | 0.98, 0.96, 0.93 avg = 0.957 | 35.7 |
4 | 31.6 | 13.70, 14.42, 14.62 avg = 14.247 | 0.98, 0.92, 0.92 avg = 0.94 | 10.4 |
Tensile Testing
Specimen Preparation
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The minimum length of the specimen is equal to the total gripping length + 2 x width + gauge length, but greater lengths are recommended in order to minimize bending stresses.
Need to ask Mark about gripping length and tabbing options
tabbing is required to reduce the chance of material failure at the grips
Can ask Mark about a wedge grip that helps to reduce stress concentrations
Can bond steel tabs at the ends of specimen
Load Rate
The Definitive Guide to ASTM D3039 - Instron
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