Lifetime prediction of laser-precracked fused silica subjected to subsequent cyclic laser pulses
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Ansgar W. Schmid Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623-1299
John C. Lambropoulos Laboratory for Laser Energetics and Department of Mechanical Engineering, University of Rochester, 250 East River Road, Rochester, New York 14623-1299
Stephen J. Burns Department of Mechanical Engineering, University of Rochester, 250 East River Road, Rochester, New York 14623-1299
Semyon Papernov Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623-1299 (Received 13 May 1999; accepted 23 February 2000)
Measurements of fatigue failure strength of laser-cracked fused silica in air at room temperature for different numbers of laser shots and laser fluences are presented. The failure-strength variability is found to be due mainly to the spectrum of crack depths. Agreement with theory suggests the incorporation of a residual term into the failure–strength equation. Due to its sign, the residual stress is of mouth-opening displacement nature at the crack. Analysis of the residual stress data shows a linear proportionality with crack depth, whereas the failure–strength is inversely proportional to the square root of the crack depth.
I. INTRODUCTION
Fused silica has good transparency in the ultraviolet (UV) spectral range; it is widely used as optical-window material in high-fluence, 351-nm positions on glass and 248-nm KrF lasers.1–5 The useful output of these highpower lasers is limited, however, by laser-induced damage to optical components, including fused silica. Often these optical components (large lenses and windows) separate atmospheric pressure from vacuum areas (10−3 to 10−4 torr),1 such as on spatial filters and target tanks, experiencing not only high fluence irradiation conditions but also pressure differential-induced tensile stresses. Under fatigue conditions, i.e., when the strength diminishes for increasing loading duration (here, the number of laser pulses), the damage is initiated by slow crack growth and culminates in catastrophic crack growth and implosive lens or window failure when the stress intensity approaches the critical value.6,7 Analysis of the strength characteristics then proceeds in accor1182
J. Mater. Res., Vol. 15, No. 5, May 2000
dance with the basic crack propagation equation, i.e., the stress-intensity-factor equation of the form K ⬀ a1/2, representing the driving force on the crack of characteristic dimension a subjected to an applied stress . Since laser-induced cracks cannot be eliminated entirely, the behavior of cracked structures under service conditions must be quantified to be predicted. Systematic scientific rules must be devised to characterize laserinduced cracks and their effects and to predict if and when it may become necessary to replace the cracked components. This paper makes a contribution toward this end. Consider a structure in which a laser-induced crack develops. Upon application of repeated laser pulses,8 or due to a combination of laser fluences and numb
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