page_title: Crack closure
section_title: Description
hierarchical_section_title: Crack closure / Description
caption_attribution_description: English: Crack closure effect (example of R=0). Stress intensity factor, K, is calculated from the external applied force P, and the crack tip opening displacement, CTOD, varies with time during a sinusoidal cycle of K. Nominal stress intensity factor range, delta_K, is calculated from K_max and K_min. However, crack closure occurs when K &lt; K_cl even though under positive K,allowing us to define an effective stress intensity range, delta_K_eff from K_max and K_cl, which is less than the nominal applied delta_K.
context_page_description: Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. As the load is increased, a critical value will be reached at which time the crack becomes open. Crack closure occurs from the presence of material propping open the crack faces and can arise from many sources including plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in the crack, or roughness at cracked surfaces.
context_section_description: During cyclic loading, a crack will open and close causing the crack tip opening displacement (CTOD) to vary cyclically in phase with the applied force. If the loading cycle includes a period of negative force or stress ratio  (i.e. ), the CTOD will remain equal to zero as the crack faces are pressed together. However, it was discovered that the CTOD can also be zero at other times even when the applied force is positive preventing the stress intensity factor reaching its minimum. Thus, the amplitude of the stress intensity factor range, also known as the crack tip driving force, is reduced relative to the case in which no closure occurs, thereby reducing the crack growth rate. The closure level increases with stress ratio and above approximately , the crack faces do not contact and closure does not typically occur.
  The applied load will generate a stress intensity factor at the crack tip,  producing a crack tip opening displacement, CTOD. Crack growth is generally a function  of the stress intensity factor range,  for an applied loading cycle and is
  However, crack closure occurs when the fracture surfaces are in contact below the opening level stress intensity factor  even though under positive load, allowing us to define an effective stress intensity range  as
  which is less than the nominal applied .
caption: Crack closure effect (example of R=0)
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