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,
Virtual Testing and Its Application in Aerospace Structural Parts
In many occasions, metallic parts will undergo plastic deformation either during their service usage or prior to their actual operation in the manufacturing and assembly phases. Under these circumstances, material properties may change considerably and must be accounted for when estimating the residual strength capability of parts. The change in material properties will occur when load has been removed and the work-hardening phenomenon has caused increase in material yield value, reduction in percent elongation, and degradation in fracture allowables. For these reasons, new static and fracture data must be generated if safe-life assessment must be performed on fracture critical parts. Fracture data are currently obtained through the ASTM testing standards, which are costly and labor intense. Difficulties associated with preparing the specimen, precracking the notch, recording and monitoring the data, obtaining the variation of fracture toughness versus part thickness, capturing data in the threshold region, and repeating the test in many cases due to invalid results make the virtual testing technique extremely helpful to overcome the time and cost related to the above-mentioned testing difficulties. Farahmand (Fatigue & Fracture Mechanics of High Risk Parts, Chapman & Hall, 1997, Chapter 5; Fracture Mechanics of Metals, Composites, Welds, and Bolted Joints, Kluwer Academic Publisher 2000, Chapter 6) utilized the extended Griffith theory to obtain a relationship between the applied stress and half a crack length by using the full stress–strain curve for the material under consideration. The extension of this work led to estimation of material fracture toughness and construction of full fatigue crack growth rate curve (Farahmand, J. Fract., 2007, Vol. 75, pp 2144–2155). This technique will be very useful to implement when materials undergo plastic deformation (the work-hardening phenomenon) and fracture allowables are needed to conduct a meaningful life analysis assessment of parts. First, Farahmand’s virtual testing theory is described briefly and, subsequently, the application of this methodology to two cases of aerospace pressurized tanks, exposed to plastic deformation, presented in this chapter.
Probabilistic analysis of bladed turbine disks and the effect of mistuning
Probabilistic assessment of the maximum blade response on a mistuned rotor disk is performed using the computer code NESSUS. The uncertainties in natural frequency, excitation frequency, amplitude of excitation and damping have been included to obtain the cumulative distribution function (CDF) of blade responses. Advanced mean value first order analysis is used to compute CDF. The sensitivities of different random variables are identified. Effect of the number of blades on a rotor on mistuning is evaluated. It is shown that the uncertainties associated with the forcing function parameters have significant effect on the response distribution of the bladed rotor.