Fracture Mechanics of Metals, Composites, Welds, and Bolted Joints
Overview of Composites In certain structural applications the use of fiber composites is advantageous over metallic structures (lighter weight, and higher stiffness). Polymer matrix composite (PMC) materials with continuous high modulus graphite fibers are very effective under high-cycle fatigue loading due to the ability of fibers to transmit and disperse the high-frequency vibration loads. A well-configured PMC structure can achieve practically infinite fatigue life, carrying relatively high loads. PMC structures subjected to high cycle fatigue usually experience damage initiation by transverse tensile failures (perpendicular to applied loading direction) in the matrix. As a result, a large number of microscopic cracks appear in the matrix, parallel to the fibers. The structural response properties and resistance to damage propagation usually remain intact at the presence of a large number of microscopic transverse tensile crack [1]. At the end of fatigue life, fiber fractures occur at stress concentration sites, such as interlaminar boundaries. The resulting local delaminations indicate onset of the damage propagation and failure stages. Composite failure is not predictable with a higher reliability compared to metallic structures due to the large number of material parameters and structural elements that contribute to the composite load redistribution and load carrying capability. The cumulative uncertainty from the large number of structural parameters that are present in PMC materials is less affected by the uncertainty associated with any one material variable. As a result, PMC structures can be designed with a higher degree of reliability. Composites are also well suited for damage tolerant design by the proper selection of fiber reinforcement patterns at damage critical regions. Composite structures are better able to redistribute stresses and redirect load paths around both discrete and distributed damage zones. PMC materials are particularly effective for damage tolerance due to flexibility of the polymer matrix that allows for small local adjustments of fiber alignment to help reduce stress concentration [2]. Fiber reinforcement configurations can be designed for the purpose of containing accidental damage. Hybrid composites that use both high-modulus and intermediate modulus fibers are effective in reducing stress concentrations as well as increasing damage energy absorption capability of PMC materials. High modulus fibers are typically made from layered anisotropic graphite, whereas intermediate modulus fibers are usually made from homogeneous materials such as fiberglass or Kevlar. Page 1 of 31 FRACTURE MECANICS OF METALS, COMPOSITES, WELDS,
Progressive Fracture of Fiber Composites
Refined models and procedures are described for determining progressive composite fracture in graphite/epoxy angleplied laminates. Unique Lewis Research Center capabilities are utilized including the Real-Time Ultrasonic C-Scan (RUSCAN) ex perimental facility and the Composite Durability Structural Analysis (CODSTRAN) computer code. CODSTRAN is used to predict the fracture progression based on com posite mechanics, finite element stress analysis, and fracture criteria modules. The RUSCAN facility, CODSTRAN computer code, and scanning electron microscope are used to determine durability and identify failure mechanisms in graphite/epoxy com posites. Results indicate that RUSCAN/CODSTRAN is an effective method of study ing progressive fracture of composites.
Electron dose-rate conversion factors for external exposure of the skin from uniformly deposited activity on the body surface.
Dose-rate conversion factors have been calculated for external exposure of the skin from electrons emitted by sources that are deposited uniformly on the body surface. The dose-rate factors are obtained from electron scaled point kernels developed by Berger (Be71; Be73; Be74). The dose-rate factors are calculated at depths of 4, 8, and 40 mg cm-2 below the body surface as recommended by Whitton (Wh73), and at a depth of 7 mg cm-2 as recommended in ICRP Publication 26 (ICRP77). The dependence of the dose-rate factors at selected depths on the energy of the emitted electrons is displayed. The dose-rate factors for selected radionuclides of potential importance in radiological assessments are tabulated.