1. Rene Herrmann Compounding and Composites

2. Analyzing composite strength Testometric measurement of a composite specimen is meaningful because lamination quality varies greatly depending cleanness and procedure. The properties to be found are E-modulus, maximum permitable strain and maximum strength. This will be valid for a batch of resin and some tpes of fiber. Resin quality varies with 30%. See datasheets for this. Measurement of these properties are to be done either/or with tensile/compression tests and 3 point bending

3. Material tests When using tensile tests the specimens width has to be large as compared to thickness and the width must contain fibers as long as 10times the thickness. Specimen fiber direction will effect the choice of sample dimension. Tensile measurement is simple but the preparation of the specimen is problematic because the predefined break point is to be thinned out. Reducing cross section can be done by a) drilling a hole in the middle or b) grinding a notch in both sides. Both procedures damage the samples fibers however. The test result will be less than a prefect laminate would hold

4. 3 point bending The sample is wider than thick (2/1) or more and is at least 14 times longer than thick. Samples are cut to size and placed on 2 supports. Observe that test result is influnced greatly by if the supports are simple or clamps. The test has limitations. First the bending produces side effects while bending. This reduces effectively the width and therefore the I modulus. It can often be seen by eyes if it happens. For E-modulus measurements only this can be helped by a small rubber between specimen and stamp. This will however render strain measurement erronous. Additionally the sample is i 3 point bending also shear loaded, especially for short specimen.

5. Data from Testometric tests The machine itself can calculate material properties. However this is effected by the machines algorithm to select meaningfull sections of the overall measurement data. One particular problem is the measurement of the slope of the force versus displacement data. The test data may be distorted at the beginning because the stamp did not touch the specimen perfectly flat. The from machine measured slope (E-modulus) will be too small The test data may also be distorted be a virbration on the machine leading to an early selection of a break point. Test data sections are best extracted by human judgement.

6. Equations for data extraction (1) Tensile or compressional test behaves according to the follwoing equations. First force versus displacment. Stress and strain relatd by E – modulus, Strain is measured by,

7. Equations for data extraction (2) Forces caues displacement in the center of the beam accorind to, The slope in the linear section of the test data can be used for E – modulus measurement, use The I modulus is The stress is These equations are different for the case when the simple supports are clamps.

8. Compare analytic model to test data Use the test data and extract E – modulus, plot an analytic curve in the same graph as the test data and see when the test data departs from the measurement data by lets say 5%. Calculate how great a strain is exerted on the specimen at this point as compared to the measured maximum strain. Draw the specimen i solid edge using mm scale and import the stl file to COMSOL with scale factor 0.001. Load the specimen in COMSOL equivalently at the center and plot displacement with at least 5 different mesh sizes. Record the COMSOL data in a table. Plot the results then in the same graf as physical measurement data and analytic data. Compare the difference and measure the relative error, measurement data is true fact.