Opti 523 Wenrui Cai

Tensile stress will occur just outside the contact area and will form cracks into subsurface of the glass. If damage does occur, will the component survive subsequent applied stresses? What effect does contact damage have on strength? The project is to analysis this phenomena using finite element method and predict its effect on the glass strength with experimental data.

 To determine a critical load P c (lb/in) and contact radius R, with which a deep enough crack can be made to degrade the strength of the glass.  Or the relationship between load, contact radius and the probability to fail of the glass.

Contact pressure  c = (PE*/  R*)^.5 Max tensile stress  t = (1-2 g )  c /3 For example, a 25 lb/in load with contact radius 0.01 in, we got 16 ksi tensile stress at the surface. Far more than 1ksi but still survive. Why? Because the stress region is shallow, crack fail to propagate. The strength of glass may be already degraded, just we don’t know.

governed by the random distribution of the size, orientation, and location of the surface flaws in relation to regions under stress. stress intensity factor   0 the nominal stress perpendicular to the stress plane  a depth of the flaw.  A flaw will result in a fracture if KI > fracture toughness Weibull distribution  F(  ) Probability of failure at bending stress    0 Characteristic strength (F(  0 ) = 63,21 %)  Weibull factor (scatter of the distribution.)

Work with Brian Cuerden to use ANSYS  Mesh control  Nonlinear analysis  Flaw will change the contour of tensile stress field Need to find out: 1, Will the indentation make a crack deeper than critical depth?(for opti-polish >.003in (75um)) 2, If there is a exsiting flaw in the tensile region, will it grow?

 B270 Flat windows 2’’ diameter 1.15mm thick  Three indentors with different contact radii: Sharp edge (about.0002in) R=0.01 in; R=0.1 in (harden steel)  Support metal rings OD 0.75’’ID 0.38’’3/8’’ height

Aluminum tube (for double ring strength test)  OD 1.25’’ID 1.12’’  OD 1.75’’ID 1.62’’  Rubber rings between metal and glass  COSMOSworks  Agree with calculation from Roark’s 30 lb force gives about 4000psi tensile stress on the surface

 Need lots of samples to fit the Weibull distribution load vs. probability to fail (Without indentation)

 Strength vs indentation loads  need lots of samples to determine strength  Once the strength is determined, the plot will show a critical load to degrade the strength

 Theoretical solution for relations R vs a, F vs a  FEA singularity at surface.  Large enough yield strength of steel(keep the radius) to make indention.