Failure Analysis - Part 3

Last week we talked about a fracture morphology that exhibited elongated dimples.

Dimples generally form in ductile materials from loads that exceed the yield stress of the material. In these materials, there are inclusions, weak grain boundaries, particles, and cleavage planes. When the material starts to yield, or plastically deform, it starts to form cracks around these particles, inclusions, or at weak grain boundaries. As the material continues to deform, these cracks form into microvoids in the material. The material around these voids continues to stretch creating a cupping effect.

These cupping microvoids are what give the fracture morphology dimples (Figure 1). When the material starts to yield these microvoids expand laterally until they coalesce with other microvoids and form a central crack in the material.

In a torsional load, the microvoids elongate when the material starts to yield (Figure 2). The matching faces of the failure will show the elongated dimples in opposite directions.

The other way they can form is from tearing as shown in Figure 3. The matching faces of the failure will show the elongated dimples in the same direction.

Figure 1 - Microvoid forming froma tension load around an inclusion and what it looked like in the failed state under a Scanning Electron Microscope

Figure 2 - Microvoid forming from a torsion load around an inclusion and what it looked like in the failed state under a Scanning Electron Microscope

Figure 3 - Microvoid forming from a tearing load around an inclusion and what it looked like in the failed state under a Scanning Electron Microscope

Want to see something we do? Have a question about something?

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Failure Analysis - Part 2

Last week I showed you a simple failure analysis of a bolt that had broken into two pieces. Even with EPI Materials Testing Group's experience and knowledge, we like to look at every part of a failure to ensure we have accurate answers and with as much detail as we can find.

So even after looking at this fracture surface with the naked eye, we took a closer look with our stereomicroscope and our Scanning Electron Microscope (SEM).

In this case, a closer look at the fracture surface with our SEM revealed the fracture morphology in detail. When we looked at the morphology, elongated dimples were evident (picture below).

The dimple morphology of a fracture surface indicates a ductile fracture mode. These dimples were elongated which are indicative of shear loading or in this case torsional load.

Next week I will show you how ductile dimples are formed during a failure.

Want to see something we do? Have a question about something?

Let us know in the comments and we will give you an answer or even write about it!

Failure Analysis - Part 1

EPI Materials Testing Group is always trying to get answers for our clients. One of our specialties is Failure Analysis.

Obviously our clients don't want failures in the first place, but we help them solve their problems to prevent future failures. To figure out how something failed we look very closely at the fracture surface.

Below is a basic failure analysis of a bolt that had broken in two.

The fractured bolt failed from rotation and reversed bending occurring at high nominal stress. Which is to say someone pulled from Top to Bottom while trying to tighten then bolt. The dark area at the bottom is where the final tear off occurred.

Next week I will show you the elongated dimples on the fracture surface near the top using our Scanning Electron Microscope (SEM).

Forensic Engineering - Interesting stuff!

Scanning Electron Microscope Sample

We do quite a few things here at EPI Materials Testing Group.

One thing we do quite a bit is taking pictures with our Scanning Electron Microscope (SEM). We are always doing something new, so you never know what you are going to see. Particularly when you are looking at a specimen at 30 times to 10,000 times magnification!

The picture below shows the surface of a metal coupon that had gone though some corrosion testing. Thankfully our client failed to clean the sample off all the way. The picture was taken at 1,500x magnification and the corrosion product on the sample is Iron Sulfide.

It turned out to be one of our favorite pictures of the year!

Happy New Year

Happy New Years from EPI Materials Testing Group!

EPI MTG is proud to announce that we had our best year ever in 2011. We were able to help over 80 different companies and law firms last year!

EPI MTG will strive to make 2012 even better.  Our goals start with assisting our existing and hopefully many new clients with their research and development, failure analysis, and materials testing.

To aid with our goals, EPI Materials Testing Group plans to build a new facility this year!  This will provide the space necessary to expand our capabilities and provide our clients with even more quality services.  The extra space will allow for better service in hosting inspections.  EPI MTG will have laboratory and conference rooms set aside specifically for those purposes. 

We look forward to having another great year and hope that our clients and readers of 'TnT' also have great year!