It’s that time of year again when I need to scrap the windows on my car. And the other day I was thus engaged when lo and behold — my scrapper broke. Because I was running just a tad bit late, I had to find another one in a hurry. Fortunately I did and arrived to my morning class on time.
But after the dust settled later that day, I returned home and reflected on my experience. That ice scraper lasted me about 11 years. I first got it while living in upstate New York, and it saw me through many winters there, upstate South Carolina, Seattle, and Idaho. As I examined the broken pieces of my ice scrapper, I naturally began to do what any good metallurgist would and conduct a failure analysis.
Having a key to the physics lab at work proved really helpful. Although the lab itself isn’t really set up to perform professional grade failure analysis work, I can find more equipment there than I can at home. At home I don’t have hardly any of the necessary equipment. Even with what I could find in the physics lab, my only camera is inside my phone, so it's not the best. But like any good engineer, I did what I could with what I had and made it work.
I started by laying out the broken pieces for measurement to provide both a sense of scale and the relative position of the fracture. Whole, the scraper measures about 60 cm in length. The fracture occurs at an approximate 60̊ angle from the length measurement line at about 10 cm from the scraper end.
A closer view of the fracture zone provides great detail not only of the angle of fracture but also of the chipped locations on the edge of the scraper facing the camera view. These chips result from the occasional vertical impact forces to which I subjected the end of the scraper as I tried to break through ice on my car. Note the metal scale at the bottom of the photo is in centimeters the same as the wooden meter stick towards the top of the photo.
I then placed the fractured end of the scraper into a clamp and positioned it for photographing. First I tried to get a closer view of the chips along the crossways support piece which evidence the manner in which I sometimes used my ice scrapper --- to crack the ice so as to remove more quickly. This photo doesn't provide the best documentation, but it still shows fairly well the type of impact forces to which I subjected my ice scraper over the years. Looking back, I realize I should have found a way to include the metal scale in the photo. I guess I was too excited. It has been a few years since I was this intimately involved with a failure analysis.
I then proceeded to document the fracture surface. Of the several photos I took, this one seems to provide the best resolution. (I use that word best loosely, since I realize a scanning electron microscope produces the truly best resolution of an uneven surface.) Note the darker gray discoloration of the central web piece in contrast with the lighter gray discoloration of the remainder of the fracture surface. Observing the positioning of what appear to be striations on the fracture surface (red arrows mark the edges of three of these striations), a fatigue crack initiated in the lower left corner of the central web cross-section as indicated by the yellow arrow. The vertical impact forces I mentioned earlier likely initiated the crack.
Once the crack had grown so as essentially to remove the central web from the load distribution in the cross-section, the overload in the remainder of the cross-section quickly advanced the crack to final fracture, severing the functional end from the rest of the scraper.
Admittedly I don't know much about fracture in polymers, so I was surprised to see the striation effect on the fracture surface as I would in metals, and especially at such low magnification. Doing some research into the subject, I've learned about chain scissons --- the breaking of chemical bonds in a polymer backbone due to intense localized heat. This heat can result from externally applied heat (not this case), ionizing radiation (negative on that), chemical reactions (not really likely), or mechanical stress (bingo). It makes sense then that the breaking of these bonds resembles a crack with growth fronts marked by striation-like features on the surface.
On that note, it would be helpful to know what material the manufacturer chose for the ice scraper. Mechanical properties of polymers vary wildly because they depend greatly on the arrangement and composition of the polymer chains within the material. Thus, processing plays a huge role in determining the mechanical properties of polymers. Due to the nature of the intended application (variable cycle impact loads in low temperature environments), I would have selected a thermoset polymer. But that's all conjecture. Without more information, I really can't do much more with the analysis of this specific failure.
Still, it was great to be back in the saddle again, if only for a brief moment. Despite the mini-emergency it gave me, this incident has benefited me by leading me to reflect on my future career plans and to realize what I really need --- someway, somehow --- is a mentor who can show me the ropes in the fracture of many types of materials. Looking ahead ten or more years down the road, being able to provide fractographic interpretations of fracture surfaces, regardless of material, could prove quite beneficial to my future career. I'll have to consider ways of finding such a mentor.
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