Radiographic Determination of the Lay-up Influence and Loading Configuration on Fatigue Damage Development under Bearing/Bypass Loading Conditions

The goal of this academic project was to study the effects of several variables on the damage progression within a mechanically fastened joint. The variables tracked included the composite layup, loading configuration (single shear vs. double shear), R value, stress level, and damage mechanisms observed in each specimen. In-situ x-ray of the individual laminates recorded the extent of damage, mostly longitudinal splitting and bearing, as a function of the cycle count. The following lay-ups were included: [04/45/03/90/0]s, [45/90/45/02/45/02/-45/0]s, [±5/65/(±5)2/-65/±5]s, and [±5/65/(±5)2/-65/5/65]s. The stress levels at which detectable damage develops was determined. The researchers chose to apply 50,000 cycles at each stress level. Once damage was detected, the stress level was typically raised 2.5 ksi. Another 50,000 cycles was then applied until the bolt hole diameter elongated by 10% of it’s original length. The damage length versus stress level is plotted as a way to compare damage onset stresses and growth as a function of lay-up, stress ratio.