Class Note for MSE 257A at UA
Popular in Course
Popular in Department
This 4 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Arizona taught by a professor in Fall. Since its upload, it has received 15 views.
Reviews for Class Note for MSE 257A at UA
Report this Material
What is Karma?
Karma is the currency of StudySoup.
Date Created: 02/06/15
Glass failure Structure Isotropic independent of direction Fracture is dependent on the distribution of applied stress 0 Load 0 Thermal Conchoidal fracture pattern 0 Smooth from initiation point 0 Roughens at about 1 cm where crack velocity approaches 1400 ms Obsidian tools through prehistory Brittle High strength Low toughness Cracks are hard to initiate but propagate easily once started Fracture strength is related to whether a flaw capable of initiating fracture is present in the material Fused SiOz rodthread Handled glass 1000000 psi 10000 psi Griffith Crack Theory Griffith flaw Griffith proposed in 1920 that tiny flaws act as stress concentrators and that cracks initiated at flaws can propagate under certain conditions What happens when a crack propagates 1 Stress applied to the object is initially stored as elastic energy When a crack propagates the stress is relieved ie there is a decrease in stored elastic energy 2 When a crack propagates new surfaces are generated ie there is an increase in surface energy For a crack to be able to propagate the first has to exceed the second 1 Release of stored elastic energy G n52cE G Energy release rate this is the amount of stored elastic energy that would get released upon the propagation of the crack 5 applied stress c half the crack length E modulus of elasticity stiffness ie how much the material is able to deform temporarily as we apply stress 2 Increase in surface energy that has to be overcome Crack resistance R is equal to the increase in surface energy of the crack It is a material constant R 2y 7 surface energy of material energy per unit area A crack will propagate when the stored elastic energy in the material G is greater than or equal to the increased energy associated with new surfaces G 2 R n52cE 2 2y As c half crack length increases G increases meaning that there is a greater energy release rate for a constant crack resistance y hence R does not change Given constant applied stress 5 rapid failure occurs Thermal Stresses Trim i Tfoot Taddition i Tbody Tsurface i Tinterior Tfurnace i Tair High Temperature inside a Low Temperature 5 surface 5 d o E 8 l Shorter Longer Length Coefficient of expansion how much does a material shrink or stretch given a certain amount of temperature change L2 L1T2T1 CONSTANT FOR A MATERIAL To remain coherent Glass at the interior stretches or goes into tension Glass at the surface shrinks or goes into compression Prince Rupert s Drops Nephew of Charles of England 1661 Small drops chill so quickly produce a hollow with a vacuum in it in the center Surface in compression so great that can hit bulb with hammer but if scratch surface of tail or drop sand on it the object disintegrates into fine particles Transition zone SO nearto surface in tail Averting failure from Griffith cracks Reduce surface flaws abrasion surface treatments avoided Put surface into a lower stress state ie compression 0 Exchange of positive ions Na glass dipped in K salt Replacement results in compressive stress at surface 0 Tempered glass cool rapidly through transition 0 Corelle glass low expansion glass coating makes 3 layers Compression on cooling Reduce internal stresses anneal and slowly cool Change path of crack propagation eg toughen with inclusions Role of annealing and slow cool Equalize temperatures below softening point Stresses dissipate Use of thermal stress for aesthetic ends Ice Glass or Verre Craquel XVI Century Venice Plunge partially blown glass into cold water Immediately reheat slightly don t want cracks to close Blow to enlarge and create larger fissures