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,
Effect of ARB Processing on Fatigue Crack Closure in Commercially Pure Titanium
Fatigue crack closure in commercially pure titanium sheets severely deformed using the accumulative roll-bonding (ARB) process were investigated. Sheet processed through 6 cycles of ARB consists of fine equiaxed grains and elongated lamellar grains and shows a high 0.2% proof stress of 790MPa. Fatigue crack closure was not observed in the ARB processed sheet. The effective threshold stress intensity factor ranges, "Kth.eff, of the ARB processed sheets and starting sheet are nearly the same. On the other hand, the resistance of the fatigue crack growth rate of the ARB processed sheet is higher than that of the starting sheet. It was found that ARB increases the resistance for the fatigue crack propagation but has no effect on the onset of propagation. [doi:10.2320/matertrans.M2012352]
Micromechanics of intraply hybrid composites: Elastic and thermal properties
Composite micromechanics are used to derive equations for predicting the elastic and thermal properties of unidirectional intraply hybrid composites. The results predicted using these equations are compared with those predicted using approximate equations based on the rule of mixtures, linear laminate theory, finite element analysis and limited experimental data. The comparisons for three different intraply hybrids indicate that all four methods predict approximately the same elastic properties and are in good agreement with measured data. The micromechanics equations and linear laminate theory predict about the same values for thermal expansion coefficients. The micromechanics equations predict through-the-thickness properties which are in good agreement with the finite element results.