Is 1.67 hi index considered tone as impact resistant as polycarbonate and would you "upgrade " to high index for a patient who requires a balance lens
Is 1.67 hi index considered tone as impact resistant as polycarbonate and would you "upgrade " to high index for a patient who requires a balance lens
no and no.
I should know better. Dude I work with insists that it is. So is there a difference between impact resistance an tensile strength? If so what is it?
Impact resistance is the ability to withstand a projectile impacting a lens without breaking, tensile strength in a nutshell is the ability to withstand being pulled apart.
Thank you Randle!
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
As Randle pointed out, they are not the same, although they are often related.So is there a difference between impact resistance an tensile strength? If so what is it?
Tensile strength refers to the ability of a material to resist being pulled apart. When a lens is struck by an object, the front surface is placed under a compressive force and the back surface is placed under a tensile force. With enough tensile force, the back surface tears, resulting in a crack that can propagate through the entire lens. So a higher tensile strength will generally increase impact resistance.
However, there are different ways to define tensile strength, since many materials do not exhibit "brittle" fracture, but instead exhibit some elasticity. There is a "yield" strength at which the material no longer exhibits elastic behavior but instead beings to deform. There is also an "ultimate" strength at which the material finally begins to tear apart, leading to fracture. Another related quality is ductility, which refers to the ability of a material to deform or stretch without tearing (beyond the "yield strength").
Polycarbonate is a very ductile lens material. Because polycarbonate is a thermoplastic without high cross-linking, the molecule chains can move past each other more, which makes the material able to "stretch" or "give" more on impact. This is good for impact resistance, because it resists fracture. But it is bad for rimless suitability, because the stretching or tearing of the lens material in the vicinity of the drill holes will essentially make the lens fail as a frame support structure.
Best regards,
Darryl
Darryl J. Meister, ABOM
one more reason to re-seal the drill holes before mountig the lenses into the frame..
Originally Posted by Darryl Meister
.......................one more reason to re-seal the drill holes before mountig the lenses into the frame.
How come they didn't compare crown glass?
Or transparent aluminum!
http://phys.org/news167925273.html
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
So we can finally transport those whales safely through time on the Enterprise.
Best regards,
Darryl
Darryl J. Meister, ABOM
Darryl, are you recommending against using poly for rimless? I would think the frame would typically bend or break before the poly lens completely fractures at the drill holes, but I am just speculating and obviously every frame is differnt. Have there been specific test of lens materials to assess which is "best" for rimless that incorporates the compromising caused by the drill holes? I know "the best lens for rimless" has been discussed in many other threads, but is there real test data?
I was mainly just illustrating that there are really a few different, albeit intimately related, factors involved when discussing tensile strength, impact resistance, and rimless suitability. In terms of tensile strength for rimless suitability, polycarbonate is certainly "strong enough." However, I do feel that there are better options for rimless frames.
In particular, certain high-index materials, like MR-8, MR-7, and MR-10, and high-impact materials, like Trivex and NXT, exhibit comparable tensile strength, but offer improved flexural strength (important when the lens serves as a frame component) combined with lower internal stress and greater chemical resistance (important to avoid failures at the drill points).
Of course, when impact resistance is a concern, polycarbonate or high-impact materials, like Trivex and NXT, should be used.
Best regards,
Darryl
Darryl J. Meister, ABOM
In our office we have been coached to fit patients, who have a high Rx and concerns of impact resistance in poly. Is there a better material to use when trying to thin down a high Rx lens and provide impact resistance? I am pretty new to the world of "opticianry" and I could literally ask a thousand questions, but this one in particular is a concern of mine, because it seems as if, in our office, we have begun defaulting to a poly lens (not even when impact resistance is not a concern for the patient) for thinning down high Rx, I am just wondering if this is the correct way to go?
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Reply With Quote
Bookmarks