The Mechanical Properties Of Glass

Introduction to Glass TechnologyThe Mechanical Properties of GlassTheoretical strength, practical strength, fatigue, flaws, toughness, chemical processesGlass Engineering150:312Professor Richard LehmanDepartment of Ceramics and Materials EngineeringRutgers University, New Brunswick, New Jersey, USASpecific Strength and Stiffnessof Selected Bulk Materials27.5Glass, ChemTemperedGlass, Phys.Tempered27.0GlassE/p [specific 25.5SteelSteel, hotrolled, 1% 23.50.050.0100.0150.0200.0250.0Yield/p [specific strength]1

Introduction to Glass TechnologyTheoretical Strength of Glass Most glasses have strong covalent bondsÆ Si-O bonds, 435 kJ/moleÆ Yields theoretical strength of 17 GPa Practical strength is much lessÆ Depends on flaws, usually on onsany mechanically distinguishable phaseÆ Manufactured glass has nominal strength of 70 MPa [10 KPsi]Æ Design stresses often 7 MPa or less for high reliability structuresÆ Low surface area structures [e.g. fibers] have higher strengthsUnits PropertyUnitsPressure, Stress Pascal, PaToughnessMPa-m0.5Pressure,StressPounds persquare inchFundamental2ConversionN/mkg/m-sec2As above1000 psi 7MPaN/Albs (force)/in21 MPa 143 psi2

Introduction to Glass TechnologyPractical Strength of Glass σ failure stress, i.e. strength of thematerial c flaw size in meters KIC Critical stress intensity factor formode I crack propagation. KIC has low values for brittle materials,high values for tough materials Value 0.75 - 1.0 MPa-m0.5 for glassσ K ICπcFAcFσ AFlaw Sensitivity of Glass Calculatedstrengths of glasswith various flawsizes and KICvalues. Typical flaws sizeranges and KICvalues arehighlighted0.5c, 1630844603154223015771115788558394279197139993

Introduction to Glass TechnologyMeasuring KIC Double Cantilever beam test apparatus formeasuring KIC Allows controlled propagation rate of flawChemical Aspects of Glass Strength Fracture process is one of breaking bonds Si-O bonds are the principal bonds in silicate glassesOOSiOSiOSiO4

Introduction to Glass TechnologyChemical Aspects of Glass Strength Si-O bonds are susceptible to scission by nucleophilic attack. Negatively charges attacking the positively charged nucleus Result: Si O Si H2O 2 [ Si OH]The presence of water or other polar substances greatlyaccelerates the crack propagation processExperimental Demonstration of the Effects ofFlaws and Chemical Attack on Glass Strength Ordinary bulk glass as a nominal strength of 70MPa Bulk synthetic glass [SiCl4 - derived] has astrength of 500 MPa [7X increase] due to fewerimpurity defects Drawing fibers from synthetic glass reduces thesurface area under test and increases strength to4 GPa for 10 cm gage length Bending fibers to test strength reduces gagelength to a few microns. Strength increases to 8GPa. Perform bending test in liquid nitrogen [77 K] tozero chemical effects. Strength is statistically thesame as theoretical strength! MPa7050040008000170005

Introduction to Glass TechnologyDoes Water Aid Fracture or Increase Strength?[It depends on form of the water!] Water vapor promotes failure:Æ SiO2 2H2O Î Si(OH)4Æ Bonds broken, glass fractured Liquid water promotes crack blunting/healingOOSiOSiOSiOMechanical Properties of Materials -- minum, Alloy 1100-H142.76911012040.725.6Steel, High Strength, low range7.8520034055043.325.5Magnesium, low1.8438014044.423.9Steel, hot rolled, 1% C7.8520058096073.925.5Glass, Phys Tempered2.67021021080.826.9Steel, High Strength, high range7.8520010001200127.425.5Aluminum, Alloy 2014-T62.772410500151.926.7Magnesium, high1.843280340155.623.9Titanium, low4.5110760900168.924.4Glass, Chem. Tempered2.670500500192.326.9Titanium, high4.511010001200222.224.4Glass, Fibers2.670400040001538.526.96

Introduction to Glass TechnologyTheoretical Strength of Glass Theoretical strength of glass can be calculated from the energy needed toseparate a plane of atoms, see diagram Si - O covalent bonds are strong, circa 104 kcal Calculated value gives: 18 GPa or 2.6 million Psi Most practical and design strengths are 1000 times less!Strength of Glass Decreaseswith Increasing Flaw SizeMPa PSImm mils7

Introduction to Glass TechnologyStrength of Glass from Various ProcessesStrength Depends on Surface Condition A pristine, protected surface is important for high strengthFlaws concentrate stress and reduce strengthEtching or fire-polishing can remove flawsLacquer protects surfaces, mechanically and chemicallyFlexural Strength,1 h loaddurationSurface ConditionMPaKPsiAs Received142.0Severely Sandblasted456.51725250Acid Etched and coated with lacquer toprotect surfaces8

Introduction to Glass TechnologyEffect of Glass Composition and FiberDiameter on Tensile StrengthA Fused silica in vacuum afterbakingB Fused silica in dry atmosphereC Fused silica in moist atmosphereD Borosilicate glass in dry airE Borosilicate glass in airF Soda lime silicate glass in airFatigue Behavior of Glass Breaking Stress versus durationMPa KPsiA Static loadB Cyclic @ 14 HzC Cyclic @ 10,000 Hz9

Introduction to Glass TechnologyFatigue Behavior of GlassBreaking strength versus timeMPaKPsi A Annealed glass in air B Tempered glass in air C Annealed glass invacuum.Glass Failure and Weibull Statistics Glass failure does not follow normalstatistics Weibull generated a new approach forbrittle materials. σµ threshold failure stress σo characteristic stress m Weibull modulus σ σ µf (σ ) σo m10

Introduction to Glass TechnologyWeibull Statistics -- Example Sort measured strength data in ascending order Devise a way to calculate frequency, simple one shown Graph data on linear axesFatigue Behavior of Glass Graph data on log - probability axes Data should give a straight line Blue line indicates one Weibull distribution athigh strengths -- intrinsic behavior of process Red line indicates a second flaw distribution atlower strengths Approach to increase glass strengthÆ Determine cause of red line failures9 Begin at weakest failure and work up9 Can estimate flaw size from Griffin relationshipÆ Eliminate or fix process step that generates failuresÆ Move on to next distribution11

Introduction to Glass TechnologyFracture Behavior of GlassFeatures form as crackaccelerates from origin Origin Mirror Mist HackleCrack Propagation in Glass Bullet velocity 800 m/s Fracture velocity 1500 m/s Shock wave velocity 4500 m/s Exposure time 1 µs12

Introduction to Glass TechnologyFracture Behavior of Glass Low Energy Impact Distinct point of origin Low to moderateradial cracking Few fragmentsFracture Behavior of Glass High Energy Impact Distinct point oforigin Extensive radial andcircumferentialcracking pattern Many fragments13

Introduction to Glass TechnologyFracture Behavior of Glass Impact failure oftempered glass Enormous number ofsmall fragments Glass not penetrated inthis instance Glass continues to supportsome load.Methods for Strengthening Glass Fire Polishing improves strength the most Quench-hardening is most common commercially14

Introduction to Glass TechnologyASTM Test Specifications and Methods for GlassASTM Test Specifications and Methods for Glass[Continued]15

The Mechanical Properties of Glass Theoretical strength, practical strength, fatigue, flaws, toughness, chemical processes Glass Engineering 150:312 Professor Richard Lehman Department of Ceramics and Materials Engineering Rutgers University, New Brunswick, New Jersey, USA Specific Strength and Stiffness of Selected Bulk Materials 23.5 24.0 24.5