By Robert Baker-Turley

#10299713

University of Technology, Sydney

Hypothesis :

Upon seeing the failed material, I was able to make several deductions about possible causes of failure. Because of the nature of the object, being a grip in a tensile testing machine, it is highly probable that the piece failed from one of 2 methods. Either :
A) It failed from a stress load so high it caused a fairly fast fracture of the piece.

B) The piece failed from fatigue (possibly at a much lower stress than the calculated yield/fracture stress)

Aim :

The aim of the project was the determine the possible origin of failure, and use this origin to try and determine the possible cause of the failure of the piece. Additionally, the aim was to determine certain key factors about the composition and strength of the piece.

Method and Background :

Failure analysis is a systematic examination of failed devices to determine the root cause of failure and to use such information to eventually improve product reliability. Failure analysis is designed to: identify the failure modes (the way the product failed); identify the failure site (where in the product failure occurred); identify the failure mechanism (the physical phenomena involved in the failure); determine the root cause (the design, defect, or loads which led to failure); and recommend failure prevention methods.

The process begins with the most non-destructive techniques and then proceeds to the more destructive techniques, allowing the gathering of unique data from each technique throughout the process. This data when properly analysed leads to a viable mechanism for the failure. The use of destructive techniques early in the process is discouraged as it can result in the loss of valuable information that might be required later.

Failure Analysis

- Non-Destructive Evaluation -

Non-Destructive Evaluation (NDE) is designed to provide as much information on the failure site, failure mechanism, and root cause of failure without causing any damage to the product or obscuring or removing valuable information. A significant amount of failure information is available through visual inspection and the more traditional NDE methods, such as Scanning electron Microscopy, or simply taking macro photos using a common camera, or microscope.

Method for Non-Destructive Evaluation.

1. Take macro photos of the product using a camera, and also a microscope.

a. Focus on areas such as possible failure initiation site, possible sites that contributed to the failure, and other important features of the piece.

i. It is also important to take these pictures with a scale IN the picture. Use this method so when the photos are re-sized, the scale is not affected. This is more important when using a digital camera than using a microscope, where the magnification is known.

2. Use the scanning electron microscope, as described below.

Scanning Electron Microscopy (SEM)

In failure analysis, electron microscopy is a natural extension of optical microscopy. The use of electrons instead of a light source provides much higher magnification (>10,000x), unique imaging, and the opportunity to perform elemental analysis and phase identification. SEM photos appear like the one displayed to the right, allowing people to determine the structure of the product given to them, and also allows them to help decide on the method of fracture.

Using the SEM, take photos of your product. Try to include all important features, and make sure to take a picture of the possible site of initial failure (primary failure). Take photos of the surface of the material, and later on, after some destructive testing is done, take pictures of the inside of the material, so the 2 can be compared. This will allow us to determine if the material has been treated in any way.

Failure Analysis

- Destructive Evaluation -

Having completed the non-destructive analysis, the next step is to use destructive sample preparation techniques to reveal the internal structure of the sample. As much information as non-destructive evaluation (NDE) provides, destructive evaluation is often necessary to verify the failure mechanism and root cause.

Microsectioning, also known as cross-sectioning, is performed to reach a surface which reveals an important feature of the sample. The cross-sectioned surface is often examined using optical microscopy and electron microscopy after being treated by a metallurgist.

Method for Destructive Evaluation

1. Microsection your piece of material.

a. This involves cutting out a small piece of the material. This is usually done on a machine, and the product is continuously hit with water to prevent heating.

2. Take the piece of material, and mount it using the white powder and goo.

a. Make sure that when you mount the material, one of the ?cut? faces is facing the bottom of the mount.

3. After the sample has dried, grind the sample using a relatively course grain of paper. Do this on a machine, so that the bottom surface stays flat, and so the machine can pump water on the sample to avoid the sample heating up.

a. Proceed grinding the sample with smaller grains, each time rotating the sample 90 degrees. Move to the next grain when you cannot see the scratches from the previous grain.

b. It is wise to continuously clean the sample, to remove gunk picked up while grinding. Clean the sample using an alcohol.

4. After you have proceeded to grind the mounted sample with a sufficiently small grain, polish the sample with a couple of grades of polishing cloth. This is also done on a machine.

5. Next, clean the sample using an alcohol, dry the sample, then etch the sample in a liquid. Etching should only be done for a few seconds, before the sample should be dunked under running water.

6. The sample is now ready for the SEM and optical photos.

7. After both of these steps are completed, the sample can undergo hardness tests (such as the Vicors hardness test)

8. Additionally, you may wish to use a mass spectrometer on your sample to determine its exact composition. This test should be performed on the surface of the product, on the surface of the primary failure site, and on a point inside the product.

Results :

Photos

These photos reveal the overall structure of the product, and show all cracks and failure.

Macro Photos

These 3 pictures show the cracks on a macro level. In the 3^rd picture, it is possible to see the slight curve formed on the edges of the cracks. This is typical of ductile failure as this is plastic deformation. This leads me to believe this was not the result of a fracture caused by fatigue (fatigue failure typically has brittle failure signs, even in ductile materials).

SEM Photos

These 3 photos reveal the microstructure in more detail. The dimples on the material suggest that this material underwent ductile failure, and not brittle failure. This is more evidence that the material did not in fact fail due to fatigue.

Mass Spectrometer

This is a mass spectrometer output graph. This graph can be analysed and the exact elemental composition of the product can be determined. I didn?t get around to it.

Hardness Tests

Hardesness Test (1) ? 162 Micron (using 50 grams of force)

Vicor Hardness ? 751.8 Vicors

Conclusions :

Due to the short period of time that we had, the only conclusion I can draw is that the product underwent ductile failure. Because of this, failure by the means of fatigue can be discounted. The site of primary failure was most likely towards the top left hand side of the piece (see photo, note that it is turned 90^o to the right). This can be deduced as it forms a fork from the left to the right, and other cracks originate from the fork.