Determination of the nontraditional lay-up influence and loading configuration on fatigue damage development under bearing/bypass loading conditions using radiography

Abstract

This article presents the effects of several variables on the damage progression within a mechanically fastened graphite/epoxy composite joint. The variables included the composite lay-up, 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 delamination, as a function of the cycle count. The following lay-ups were investigated: [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. All of these lay-ups are considered to be ‘hard’ lay-ups, much stiffer in the 0° direction than in the 90° direction. 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 to 17.25 MPa (2.5 ksi). Another 50,000 cycles was then applied until the bolt hole diameter elongated by 10% of its original length. The damage length vs. stress level is plotted as a way to compare damage onset stresses and growth as a function of lay-up and stress ratio. The new ‘non-traditional’ lay-ups are shown to offer some unique advantages over traditional lay-ups.