DISP - Impact Resistance - Week 7

Question 1

Is protecting the eye a first step where industrial eye safety is concerned?

Answer 1

No, it is actually a last resort
Identifying the hazard, eliminating it or controlling it is the first step.

Question 2

What 9 considerations must be given where industrial eye safety is concerned?

Answer 2

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

Question 3

What is the most common mechanism of injury to the eye (industrially)?

Answer 3

Working with metal

Question 4

Define impact resistance.

Answer 4

A material’s ability to resist the force of a sudden impact

Question 5

List 7 patients who should definitely be fitted with impact resistance lenses.

Answer 5

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

Question 6

List 4 lens materials that have very poor impact resistance. Are these more dangerous than no lenses or do they still confer some protection?

Answer 6

Untreated crown glass
LHI
THI
Zeiss 1.9
Frequently more dangerous than no lenses

Question 7

Do impact resistant materials typically have general resistance to impact or might they be more resistant to certain projectiles over others?

Answer 7

Some are resistant to certain types of impacts but not others

Question 8

Define hardening.

Answer 8

Increasing impact resistance
-also scratch resistance

Question 9

What lens material is hardening only used on?

Answer 9

Crown glass lenses

Question 10

Can photochromic lenses be hardened?

Answer 10

Yes, but the results are generally poor

Question 11

Can LHI be hardened?

Answer 11

Yes, but the results will be very poor

Question 12

Can CR-39 and/or plastics be hardened?

Answer 12

Noh

Question 13

Describe briefly what happens when a lens is hardened.

Answer 13

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.

Question 14

Describe how impact forces are dispersed in a hardened lens vs unhardened.

Answer 14

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.

Question 15

Briefly describe the procedure to thermally harden crown glass lenses.

Answer 15

Lens is heated to 620-730C for 50-300 seconds
Both surfaces is then rapidly cooled using compressed air or submerged in oil

Question 16

What happens if the lens is too thin when hardening?

Answer 16

No effective difference between the surface and core when cooling, meaning it will have poor impact resistance

Question 17

What happens if the lens is too thick when hardening?

Answer 17

Increased lens bulk results in longer heating times with possible lens warping

Question 18

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?

Answer 18

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

Question 19

Briefly describe the procedure to chemically harden crown glass lenses.

Answer 19

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

Question 20

List 6 advantages chemical hardening has over thermal hardening.

Answer 20

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

Question 21

List 3 disadvantages chemical hardening has over thermal hardening.

Answer 21

Takes 16 hours
Thin compression layer - scratches can significantly reduce the effect of hardening
Poorer fracture pattern - long sharp splinters

Question 22

What happens to fracture velocity of crown glass when it is hardened?

Answer 22

It increases compared to unhardened glass.

Question 23

What is the fracture pattern of crown glass in large and small missiles?

Answer 23

Large - small harmless glass cubes (sharp splinters consistently found however)
Small - more splinters

Question 24

Which lens material has very high fracture velocities?

Answer 24

Polycarbonate

Question 25

What is better for large missiles, CR-39 or crown glass?

Answer 25

Similar performance

Question 26

What is better for smol missiles, CR-39 or crown glass?

Answer 26

CR-39

Question 27

Does chemical hardening tend to produce higher or lower fracture velocities compared to thermal hardening?

Answer 27

Similar fracture velocities

Question 28

List two advantages of hardened crown glass.

Answer 28

Most scratch resistant material
Has reasonable fracture velocities for large missiles

Question 29

List three disadvantages for hardened crown glass.

Answer 29

Fracture pattern is not always benign, especially small high velocity missiles
Has reasonably poor fracture velocities for small missiles
Relatively heavy

Question 30

List three advantages of CR-39 (in terms of impact resistance).

Answer 30

Generally benign fracture pattern
High fracture velocity
Relatively lightweight

Question 31

List three advantages of CR-39 (in terms of impact resistance).

Answer 31

Scratches easily in an industrial environment
Addition of a hard-coat or anti-reflection coating reduces impact resistance

Question 32

What lens material are high index plastics similar to?

Answer 32

CR-39

Question 33

List three advantages of high index plastics (in terms of impact resistance).

Answer 33

Benign fracture pattern
Fracture velocity can be similar or worse than CR-39
ightweight

Question 34

List a disadvantage of high index plastics (in terms of impact resistance).

Answer 34

Scratches reasonably easily in an industrial environment

Question 35

List three advantages of polycarbonate (in terms of impact resistance).

Answer 35

Outstanding impact resistance
Lightweight
Absorbs UV below 380nm

Question 36

List four disadvantages of polycarbonate (in terms of impact resistance).

Answer 36

Very poor scratch resistance
Optically not ideal (Abbe value is 30)
Addition of a hard coat or anti-reflection coating reduces impact resistance
Impact resistance is severely reduced by solvents like acetone

Question 37

What is the lens material of choice if safety is a prime concern?

Answer 37

Polycarbonate

Question 38

Why is polycarbonate not optically ideal?

Answer 38

Abbe value is low (~30)
Correctly centred lenses should not cause any visual problems however

Question 39

List an advantage of plano clip-ons/goggles (in terms of impact resistance).

Answer 39

High fracture velocity

Question 40

List four disadvantages of plano clip-ons/goggles (in terms of impact resistance).

Answer 40

Optically inferior
Poor aesthetically
Annoying if poorly fitted
Scratches reasonably easily in an industrial setting

Question 41

What aspect of eyewear for protection should not be underestimated?

Answer 41

Despite wearing protective gear, some things can fly around/under/over the eyewear