DISP - Impact Resistance - Week 7 Flashcards
Is protecting the eye a first step where industrial eye safety is concerned?
No, it is actually a last resort
Identifying the hazard, eliminating it or controlling it is the first step.
What 9 considerations must be given where industrial eye safety is concerned?
Regular replacement of the safety specs Who is responsible for ensuring proper use Prescription safety specs - cost and procedure Penalties for not using them Rewards for using them Provision of safety specs Provision for site visitors Education program Protocol for eye injury events Employer/employee/union involvement
What is the most common mechanism of injury to the eye (industrially)?
Working with metal
Define impact resistance.
A material’s ability to resist the force of a sudden impact
List 7 patients who should definitely be fitted with impact resistance lenses.
Good vision in one eye only
Chilren
Elderly - especially if unsteady on their feet
Patients working in dangerous industrial situations
Patients involved in handyman activities
Patients with dangerous hobbies or sports
Motorcyclists
List 4 lens materials that have very poor impact resistance. Are these more dangerous than no lenses or do they still confer some protection?
Untreated crown glass LHI THI Zeiss 1.9 Frequently more dangerous than no lenses
Do impact resistant materials typically have general resistance to impact or might they be more resistant to certain projectiles over others?
Some are resistant to certain types of impacts but not others
Define hardening.
Increasing impact resistance
-also scratch resistance
What lens material is hardening only used on?
Crown glass lenses
Can photochromic lenses be hardened?
Yes, but the results are generally poor
Can LHI be hardened?
Yes, but the results will be very poor
Can CR-39 and/or plastics be hardened?
Noh
Describe briefly what happens when a lens is hardened.
Rapidly cool a heated lens, resulting in the outer region cooling.
When the inner region cools and contracts, it exerts tension on the rigid outer surface.
Describe how impact forces are dispersed in a hardened lens vs unhardened.
When a hardened lens receives a blow that frwould fracture ordinary glass, compressive force on the opposite side to the site of impact offsets the tension generated.
Briefly describe the procedure to thermally harden crown glass lenses.
Lens is heated to 620-730C for 50-300 seconds
Both surfaces is then rapidly cooled using compressed air or submerged in oil
What happens if the lens is too thin when hardening?
No effective difference between the surface and core when cooling, meaning it will have poor impact resistance
What happens if the lens is too thick when hardening?
Increased lens bulk results in longer heating times with possible lens warping
Whar pattern is the result of the cooling process in lens hardening? How can it be viewed? What does the pattern depend on (2)? What happens if the lens is chemically toughened?
Strain/quench pattern
Can be viewed with a polariscope or strain tester
Pattern depends on the power and shape of the lens
Patterns are not detectable if chemically toughened
Briefly describe the procedure to chemically harden crown glass lenses.
Lens is placed into moleten potassium nitrate at 440C for 16 hours, producing ion exchange
More potassium enters microfractures in the lens surface in place of sodium
List 6 advantages chemical hardening has over thermal hardening.
Uses a lower temperature, lower chance of warping Works with thinner lenses Reduces chance of mistakes Can treat a variety of lenses at once Stronger than thermal hardened lenses Can be used with photochromic lenses
List 3 disadvantages chemical hardening has over thermal hardening.
Takes 16 hours
Thin compression layer - scratches can significantly reduce the effect of hardening
Poorer fracture pattern - long sharp splinters
What happens to fracture velocity of crown glass when it is hardened?
It increases compared to unhardened glass.
What is the fracture pattern of crown glass in large and small missiles?
Large - small harmless glass cubes (sharp splinters consistently found however)
Small - more splinters
Which lens material has very high fracture velocities?
Polycarbonate
