Issue 43

F.Z. Seriari et alii, Frattura ed Integrità Strutturale, 43 (2018) 43-56; DOI: 10.3221/IGF-ESIS.43.03 45 T HEORETICAL BACKGROUND Stress intensity factor alculation of stress intensity factor at the repaired crack tip is need in order to predict the fatigue life or the fatigue life enhancement of bonded patch repaired of structures or tests specimens. The most commonly materials used in bonded patch repair are boron, graphite, and Glare. Each of these materials has an application in composite patch repair depending on the metallic material thickness, type of load spectrum, and the stress level in the spectrum [30]. It recognized that repair to a cracked structure involves an upper bound on stress intensity [31]. The stress intensity factor after repair of a cracked structure is lower, by an order of magnitude than for other repair. The thickness of the patch can be chosen based on the following equivalent stiffness criterion [32]:  . . r r p p E t S E t (1) where Ep is the cracked structure’s Young modulus, tp is the cracked structure’s thickness, E R is the equivalent Young modulus of the patch perpendicular to the crack and t R is the patch thickness. The stress intensity factor for a cracked structure with no repair can is written as:          . I a K a f w (2) where “  ” is the remote uniform tensile stress, “a” is half crack length and f(a/W) is finite geometrical correction function. In patched specimen, the stress intensity factor depends on presence of composite patch and width of the patch and numbers of plies. Evaluation of boundary correction function for evaluation of stress intensity factor is detailed in research of Ratwani [33], Boyd et al. [30] and Ricci et al. [34]. This analysis is based on a Green's function method. Currently, this developed formulation implemented in Afgrow code [35] is only valid for the following conditions: through the thickness cracks, thin structure (<3.17 mm), non-stiffened panels and crack remains under the patch. The stress intensity factor in the cracked structure with repair patch and applied stress of  is K IP . The equivalent stress  e acting on a sheet with no repair patch and giving stress intensity factor of K IP is given by:            . IP e a K a f w (3) K IP may be expressed as:    . / IP e I K K The stress transferred to the repair patch is the difference between the remotely applied stress in the repaired structure and the uniform in-plane stress that produces the same stress intensity factor in a single sheet as in the repaired structure. The stress transferred,  t, may be written as:      t e (4) then,      / t IP I K K In the case of one sided repairs, Ratwani [33] provided a bending correction factor “BC” and the stress intensity factor is expressed by:     1 IP I K BC K (5) The bending correction factor is given by: C