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,
Predicting fracture and fatigue crack growth properties using tensile properties
The safe-life assessment of components requires information such as the plane stress (Kc), plane strain (KIc), part-through fracture toughness (KIe), and the fatigue crack growth rate properties. A proposed parametric/theoretical approach, based on an extended Griffith theory is used to derive fracture toughness properties and generate fatigue crack growth rate data for a range of alloys. The simplicity of the concept is based on the use of basic, and in most cases available, uniaxial stress–strain material properties data to derive material fracture toughness values. However since the methodology is in part based on an empirical relationship a wide ranging validation with actual data is required. This paper uses steel, aluminum and titanium based alloys from a pedigree database to quantify material properties sensitivity to the predictions for KIc and Kc and the subsequent estimation of ΔKth threshold and the Paris constants, C and n values. A sensitivity analysis using experimental scatter bounds show the range of da/dN predictions can be achieved. It is found KIc/ΔKth ratios designated as α has a range of 5–25 irrespective of tensile ductility, ef, and is insensitive to it. The value of ΔKth for all the alloys considered was found to be proportional to the final elongation, ef, and an empirical relationship describing ΔKth as a function of ef was established. Furthermore it is suggested that, with the knowledge of appropriate tensile properties and the estimated range of KIc/ΔKth ratios for the different alloys applying this method could be an appropriate tool that can be used to conservatively predict fracture and fatigue in similar alloy categories. Thus helping to reduce costs and optimize the number of experimental tests needed for alloy characterizations.
Evaluation of buckling load and dynamic performance of steel shear wall retrofitted with strips made of shape memory alloy
One of the most imperfections of the steel shear wall is out of plane displacements that cause severe damage in both structural and non-structural elements. In this paper, the effects of shape-memory alloys on steel shear walls are investigated. First, a numerical analysis using the finite element method in ABAQUS software has been carried out according to an experimental test. The results of the numerical analysis have been verified with experimental results. Next, shape-memory alloy fibers have been added vertically and horizontally to various parts of the steel shear wall. The results show that retrofitting with the shape-memory alloy reduces the out-of-plane displacement of the steel shear wall under both cyclic and seismic loadings. Besides, the buckling load in the steel shear wall increases when it is retrofitted with the shape-memory alloy. Also, the total out-of-plane movement (accumulated absolute displacements) of the steel shear wall and non-structural damage are controlled by the characteristic of a shape-memory alloy material called “super-elastic”.