Strengthening of glass Flashcards
How does fracture occur in brittle solids?
~ 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
Why do brittle materials fail at stresses far below predicted?
~ because of stress intensification at crack tips
What is Griffith-Irwin theory?
~stress at fracture is proportional to 1/sqrt(c)
~ c is crack length
What is fracture strength dependent on in brittle materials?
~ size of the largest microscopic flaw
How do stresses travel in ductile materials?
~ away from crack tip
What’s the theoretical fracture strength of glasses? Real?
~ 9,800 MPa
~ 50 MPa
What’s the fracture strength of fiberglass?
~ 3,300 MPa
~ pristine fiber coated w/ a polymer to protect against abrasion
What is static fatigue?
~ 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
How does flame polishing work?
~ cracks smooth out with flame, less stress itensification
~ glass is a bad conductor, heat will stay at the surface
How does etching with hydrofluouric acid work?
~ 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
What are the fracture strengths after flame polishing? HF etching?
~ normally handled: 690 MPa
~ flame: 1400 MPa
~ acid: 1700-3400 MPa
What happens during tempering?
~ 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
What is the strength of tempered glass?
~ 5x that of annealed glass
What happens with ion exhange to strengthen glass?
~ 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
What is laminated glass?
~ un-tempered glass panels PRESSURE JOINED to plastic layers
~ used in car windshields
~ bullet proof