0
$\begingroup$

A common problem with ultra-high vacuum viewport windows is that often the window material deforms, into a saddle or Pringles chip shape. .

An engineer from a viewport manufacturer explained to me that this can happen when the hoop stress around the window is not fully symmetric. If it were symmetric, the window would simply bulge or cup in the middle. That seems intuitive, but why would an uneven stress cause a saddle-like strain in particular? What is a simple physical explanation for this? So far, the fact that uneven stress in x/y directions cause different curvatures seems intuitive. What is also important is that glass is a very stiff material and cannot stretch (so volume/thickness is assumed to be conserved).

Here is a rough illustration of a stressed window in a metal sleeve typically found in experiments:enter image description here

EDIT: the brazing pad (thick black) is where the metal sleeve (thin black lines) is brazed to the braze adapter (red lines representing another cylinder). The glass is hermetically sealed to the bottom of the metal sleeve.

$\endgroup$
0
$\begingroup$

Your sketch is not clear, But any collapse and wrinkling is a process of least energy configuration.

If you had very fast video camera and reviewed the recording at normal speed, you would see every step of deformation follows the path of least strain energy.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ The metal sleeve will also deform, and since the brazing etc will never be perfectly symmetrical it will tend to form an elliptical shape rather than a perfect circle. That is equivalent to pinching the glass harder at two opposite points, which will cause the saddle shape in the glass. $\endgroup$ – alephzero Feb 2 '19 at 9:20
  • $\begingroup$ @alephzero, yes, the stress lines look elliptical (confirmed by the mentioned numerical simulation), but why must the deformation look saddle-shaped? That is my main question. $\endgroup$ – wcc Feb 3 '19 at 22:40
0
$\begingroup$

The pringling is caused by the positive (compression) hoop stress causing buckling around the perimeter. The positive hoop stress is created by the radial tension trying to pull the edges towards the middle as the center is displaced axially. Since there's not enough space for all the glass to move inwards, some of it goes up and some down.

You can see the same effect take place if you lay a piece of paper on top of a drinking glass and then push down in the middle. The center will move down and then the paper will buckle / fold such that some parts go up and some parts go down allowing the perimeter to move inward. As the material get thicker, the resistance to buckling goes up and folds/ bends get less extreme. This is why you'll only get a saddle rather than folding/ shattering.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.