Title: Fatigue Test Laboratory Experiment
Course: Materials of Engineering > Section: MET 2200

MET 2200 FATIGUE STRENGTH EXPERIMENT

INTRODUCTION:

Materials will fail via the mechanism of fatigue when the applied stresses are cyclically changed from tensile to compressive over an extended period of time.  Higher stress levels will cause the material to fail more quickly than lower stress levels.  There is a stress level, below which, the material will appear to be immune to fatigue failure for a number of stress change cycles which greatly exceeds the expected useful life of the product.  This stress level is called the “endurance limit”.

OBJECTIVE:

It is the objective of this experiment to determine the endurance limit of the test specimens using a rotating beam fatigue tester.

EQUIPMENT & MATERIALS:

Instron R.R. Moore fatigue tester.  AISI 1018 cold finished material was used to make specimens which fit the Instron equipment.

PROCEDURE:

1) Measure the dimensions of the fatigue test samples to ensure consistency.
2) Determine the tensile strength of the AISI test samples from handbook values or from tensile testing a representative sample.
3) Mount and load a test sample in the fatigue tester at no higher than 0.50 of the nominal tensile strength of the sample. Set the fatigue test counter to zero (0) and test the sample to failure, noting the counter value at fracture.
4) Repeat step 3 at other calculated stresses to fill in the data points needed to find the fatigue strength of the alloy.  Each time reset the fatigue tester counter to zero at the start of the test and record the counter value at fracture.  Collect both load and stress data.

DATA ANALYSIS:

Discuss how all variations of experimental equipment, material, procedure, and data collection may have changed the results of the experiment and include reasons for these observations.  Plot a graph of the available data using logarithm of cycles to failure as the abscissa and tensile stress as the ordinate of the graph.  Use a spreadsheet or a French curve to form a smooth curve through the data.  The endurance stress limit is determined by finding the tensile strength at which the curve appears to become flat.  For an application where the total required cycle life and the safety factor (extra cycles needed to ensure adequate performance) are known, an “effective” endurance stress limit can be determined graphically from the curve.

DISCUSSION:

The following are required discussion points.  How does this experiment compare to expected theoretical or handbook results?   Discuss the mechanisms involved with the failures of the specimens.  Discuss how the visible features of the fractures are indicative of fatigue and the order in which they likely appeared during the test.  How can the observations of this experiment be adapted to failure analysis of other samples which failed in service?

CONCLUSION:

What conclusions can be drawn form this experimental testing of the target alloy?