Strengthening of glass

Question 1

How does fracture occur in brittle solids?

Answer 1

~ imagine a solid as cross linked chains

~ if there is a crack/break in continuity, stress will re distribute to first point of continuity, the crack tip

Question 2

Why do brittle materials fail at stresses far below predicted?

Answer 2

~ because of stress intensification at crack tips

Question 3

What is Griffith-Irwin theory?

Answer 3

~stress at fracture is proportional to 1/sqrt(c)

~ c is crack length

Question 4

What is fracture strength dependent on in brittle materials?

Answer 4

~ size of the largest microscopic flaw

Question 5

How do stresses travel in ductile materials?

Answer 5

~ away from crack tip

Question 6

What’s the theoretical fracture strength of glasses? Real?

Answer 6

~ 9,800 MPa

~ 50 MPa

Question 7

What’s the fracture strength of fiberglass?

Answer 7

~ 3,300 MPa

~ pristine fiber coated w/ a polymer to protect against abrasion

Question 8

What is static fatigue?

Answer 8

~ caused by stress corrosion

~ under static load, strength of glass reduces over time

~ glass network is stretched open, giving easy flow of H2O into crack tip, leeching out alkali (replaced by H3O+) and converting SiO2 to Si(OH)4

Question 9

How does flame polishing work?

Answer 9

~ cracks smooth out with flame, less stress itensification

~ glass is a bad conductor, heat will stay at the surface

Question 10

How does etching with hydrofluouric acid work?

Answer 10

~ HF attacks the Si(4+)-O(2-) bond, long time causes part to dissolve

~ brief exposure preferentially attacks sharp corners = similar crack smoothing as flame polishing

Question 11

What are the fracture strengths after flame polishing? HF etching?

Answer 11

~ normally handled: 690 MPa

~ flame: 1400 MPa

~ acid: 1700-3400 MPa

Question 12

What happens during tempering?

Answer 12

~ glass is heated above Tg and and cooled rapidly

~ inside: cools more slowly than surface, puts regions in compression

~ applied tensile load must first overcome surface tension before crack tip experiences tension

~ safety glass, cracks into granules rather than large shrads

Question 13

What is the strength of tempered glass?

Answer 13

~ 5x that of annealed glass

Question 14

What happens with ion exhange to strengthen glass?

Answer 14

~ glass containing Na+ immersed in KNO3 soln. at a temp below Tg

~ driven by entropy of mixing, some Na+ in the glass is replaced by K+

~ K+ is bigger atom, puts glass surface in COMPRESSION

Question 15

What is laminated glass?

Answer 15

~ un-tempered glass panels PRESSURE JOINED to plastic layers

~ used in car windshields

~ bullet proof