CLPEX Chat Board

I have seen what I interpreted as stretching artifacts, that resembled friction ridge detail in materials capable of stretching and contracting, such as vinyl tape or plastic wrap. I have seen not only exceptionally small instances, but also greatly enlarged instances. If you have the tape available, is there a three dimensional aspect to the apparent ridge detail beyond the adhesive layer?

Could you post the photos, hopefully with a scale or at size reference?
Regards, RLT

Ruler is cm.

That looks a lot like the artifacting mentioned above. I've seen it on tape and also a couple of different hard plastic surfaces.
One of the hard surfaces was a thin plastic top to some sort of jewelry box (I didn't take the lift). It was plastic on the sides and top and was likely pinched when opened. One side resembled a whorl in pattern, but had little to no detail - it was mostly concentric circles.
The other, also not lifted by me, was noted to be from a vending machine. The patterning was very long. My guess was that too hot of a bulb was placed behind it and there was some melting that led to the appearance of 'ridges'.
Get some black electrical tape and see if you can duplicate what you see.

Alphabrit, the second photo I do not believe is ridge detail but some type of artifact. I have seen something similar with gasoline but not sure how it is formed. Some type of light pressure in a confined space.

The first one I do not think is ridge detail but not real sure. I downloaded the image into CS3 but when I tried to enlarge it pixelated badly. Although I do not think it is friction ridge detail I am not absolutely sure unless I can get a closer look.

A few months ago there was an e-mail floating around about 'sprue marks' or what I was taught to call them as extrusion marks. The first one could be some type of artifact like that but would take a closer look.

Like RL and Steve it is probably some type of artifacting. On the second one you can see a line then it will branch out, travel, branch out, travel branch again. It is branching over very short distances

You might consider Schallamach waves.

Phenomenal thank you Ernie- I did a brief search on Schallamach waves- had never heard of such a thing. Going to do some more research.So how does one duplicate Schallamach waves?
Charlie, thank you, I am sorry the photos didn't enlarge well, I had to re size them for the website to get them to upload, they were too big. Gentlemen do you think both are artifacts? Arson 3 appears to have level 3 detail where Arson 2 does not.
Kate.

Not sure how, or if, Schallamach waves (also called "waves of detachment') can be purposedly duplicated. It seems as if the occurence of hair-line markings in certain materials is a result of a chance event involving stress, friction or slippage in certain areas. They just happen.

Thank you so much, I am amazed that in 16 yrs I never ran into it before, totally fascinating.
Steve, thank you for you input, we did play with some electrical tape but of course we were unable to reproduce these effects!

Was anyone able to enlarge/view Arson 3? this one really does look as if it has pores ( I know its ugly)
Kate

Awareness of Schallamach waves is a spinoff of another forensic discipline involving comparative examinations.

The other possibility is Hysteresis depending on the physical properties of the electrical tape.
I.e. The tape is stretched as it is broken off from the roll and subsequently shrinks over time on the application.

Shane

The patterns bear some resemblance to some effects of solvents on some plastic surfaces. Might be worth considering, since it is an arson case with volatiles and perhaps some environmental heat. Perhaps even the effect of a solvent film under the tape when it was applied. And maybe a combination of effect, such as stretching and contraction of tape that has been exposed in part to a solvent. While the patterns exhibit "bifurcations" and other elements common to friction ridges, there appears to be far too much consistent directional branching. That suggests a "growing" network of branches developing over time from an area most effected.

Illustrations/images of Schallamach waves are difficult to locate. There are some images of interest at:

http://courses.washington.edu/overney/C ... slides.pdf

There is an area of arson_2.jpg that is fun to play with the 'ridge' formations in comparison to "wave' formations in one of the images.

I WAS PROCESSING A 'P' LAB AND CAME ACROSS THE SAME PATTERNING ON A PLASTIC BOTTLE, LIKE A 2LTR MILK BOTTLE. MY IMMEDIATE REACTION WAS 'EUREKA' , THEN REALISED THAT IT WASN'T A BIG PALM PRINT BUT THE PLASTIC OF BOTTLE. IT WAS SO CLEAR I WAS AMAZED, NO BODY ELSE THOUGHT SO AND DIDN'T UNDERSTAND WHAT I WAS ON ABOUT, MORONS!