A Study of Machining Effects on the Surface Integrity, Strength Properties, Damage Evolution and Fatigue Strength of Composite Laminates
oberoi, harinder singh
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A Study of Machining Effects on the Surface Integrity, Strength Properties, Damage Evolution and Fatigue Strength of Composite Laminates Harinder Singh Oberoi Chair of the Supervisory Committee: Dr. Ramulu Mamidala Department of Mechanical Engineering With the increased usage of Carbon Fibre Reinforced Plastics (CFRP) Composite Laminate Materials in various industries, an understanding of changes in strength properties and fatigue performance due to manufacturing processes is becoming critical to the study of their performance characteristics. As cured laminates almost always require machining of edges and/or drilling of holes, the resultant surface integrity by such post-cure processes influences the residual strength and fatigue performance during the service life. Unfortunately post-cure manufacturing processes also result in surface and sub-surface damage which evolves during service life conditions and result in the deterioration of strength and fatigue performance. The surface conditions of any machined surface are classified as surface texture and usually represent the exterior microscale geometry of the machined surface. Surface Integrity commonly refers to the features that are sub-surface or immediately beneath the surface. In general the surface integrity consists of the structure and the stress conditions within the interior layers and subsequently dictates the surface mechanical properties. This body of work studies the influence of resultant surface integrity from trimming and drilling of composite laminates on their strength properties, damage evolution and fatigue strength. A two phased approach was utilized in this research study. In phase 1, a 10 ply thick balanced symmetric [0/-45/90/45/0]s composite laminate of unidirectional Carbon fibre prepreg tape with an Epoxy resin was used in the study. For Phase 2, a 22 ply thick balanced symmetric [90/-45/0/-45/90/45/0/-45/0/90/0]s composite laminate of unidirectional Carbon fibre prepreg tape with an Epoxy resin was used. Machining processes used in this study included Abrasive Water Jet (AWJ) and Carbide Router Endmilling for the trimming of the laminate material. For drilling of holes in the laminate material, Polycrystalline Diamond (PCD) drills and Chemical Vapour Deposition (CVD) diamond coated carbide drills were used. Test material was machined with these processes and resultant surface integrity was recorded using a Surface Profilometer, Edge Replication using acetate tape and Scanning Electron Microscopy (SEM). Test samples were generated with varying surface integrity along differing machining processes and standard ASTM Tests conducted to study the residual strength properties. Testing was conducted on edge trimmed specimens included Monotonic Strength (Tension & Compression) as well as Cyclic Strength (Tension-Tension Fatigue). For Drilled Hole Specimens testing included Open-Hole Monotonic Strength (Tension & Compression) and Cyclic Strength (Tension-Tension Fatigue). Similar tests were conducted for Pinned-Hole strength testing for static conditions. During Tension-Tension Fatigue Testing a percentage change in stiffness was used to determine resultant fatigue life and it correlation to surface integrity. During the fatigue testing process, damage evolution was studied using Optical Microscopy, Photography and Scanning Electron Microscopy. The analytical modeling of Fatigue Damage composite laminates was based on damage progression. A damage model based on a change in compliance (stiffness) was used as the analytical model in this study based on the utilization of change in stiffness and resultant fatigue life to record the damage progression during the Tension-Tension Fatigue Testing.
- Mechanical engineering