Structural Integrity Analysis

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The analyses reveal that the degree of agreement between predicted pressures and experimentally measured values depends rather markedly on the crack size for the tested pipes. Moreover, the analyses also show a possible weak dependence of the predicted pressures on the constraint-based correction scheme. Overall, the results validate the use of FAD-based methodologies for defect assessments of axially cracked pipelines.

We use cookies to help provide and enhance our service and tailor content and ads. By continuing you agree to the use of cookies. Standard practice still relies heavily on extensive testing at the subelement, element, subcomponent, component, and full-scale levels. Design details are frequently optimized through test programs.

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Scale-up effects are handled through a building-block approach that relies on testing to verify the anticipated structural performance at each scale level. Full-scale static and fatigue tests are conducted to identify "hot spots," to verify adequate structural integrity for design limit and ultimate loads, and to verify durability and damage tolerance requirements. Hence the designation "semiempirical" for current practice, which is necessarily expensive and time-consuming. In spite of over 60 years of experience by researchers and industry in designing metallic structure and 25 years of experience designing composite structure, the lack of rigorous analytical methods to predict residual strength and fatigue life can inhibit the cost-effective introduction of advanced materials with superior specific strength and stiffness relative to current conventional materials.

Such methods would also provide the means to assess the effects of service history on the durability and damage tolerance performance of the structure. This chapter examines current issues and assess the impact of advanced methods on the introduction of new materials and the design of more-structurally efficient and cost-effective primary structures on next-generation aircraft.

Metallic materials tend to fail due to the formation and growth of a dominant microcrack that eventually reaches a critical length and then more rapidly propagates to failure. However, the recently recognized phenomenon of multiple-site damage has been shown to be a critical issue in aging of commercial transport aircraft. While fracture mechanics is now a mature part of standard practice in engineering, rigorous prediction methodology only exists for brittle materials that exhibit limited plasticity. The fatigue crack growth behavior and fracture processes exhibited by ductile materials are reasonably well understood.

However, the development of rigorous elastic-plastic analytical methods has been hampered by the complicated three-dimensional effects present in most structures. In thin-sheet planar structural components, local constraint effects on the development of plastically deformed material frequently result in highly inaccurate solutions when obtained from two-dimensional plane-stress or plane-strain assumptions. Three-dimensional analyses of cracks in geometries such as lugs and fittings may be inaccurate due to uncertainties in modeling the crack-front singularity. Three-dimensional models of the geometry and crack configurations are computationally intensive and have been impractical for the practicing engineer.

Therefore, improved. In addition, methods that compute reliable, stress intensity factors for various crack geometries using stresses obtained from uncracked stress analysis models should also be developed. Most fracture mechanics analyses are performed on refined models of local structural details that use stress boundary conditions determined from a global structural analysis.

Therefore, the computed stress intensity factors will never be more accurate than the global structural analysis, regardless of the sophistication of the fracture mechanics model. The structural models must account for global details such as frames, stiffeners, tear straps, shear ties, and shear clips; must accurately capture local details such as interference-fit stresses and clamp-up stresses at riveted fasteners; and must also accurately treat combined load effects and geometric nonlinear behavior. This level of fidelity in the local stress analysis can only be achieved by the development of more-robust and computationally efficient structural analysis methods that exploit global-local and other hierarchical computational strategies.

However, it is not sufficient to only generate highly accurate local stresses.


Other features that promote crack initiation and growth need to be modeled as well. These features include metal-forming defects, machining and fabrication defects, residual stresses such as from drilling holes and forming rivets, and mechanisms such as fretting and pitting corrosion.

These features may be modeled best by an approach such as the "equivalent initial flaw size" to facilitate the fatigue and residual strength analysis. The design of longer-life and more-durable metallic structures could also be significantly enhanced by the incorporation of models that reliably account for the effects of the environment on fatigue life and residual strength. The aluminum alloys currently used in airframe structures are highly susceptible to corrosion damage.

Elaborate corrosion prevention and control programs are used by the manufacturers and airline operators. These programs are successful at delaying the development of widespread corrosion, but are not percent effective in preventing localized effects such as pitting that may cause crack nucleation.

Furthermore, fatigue crack growth rates may be significantly accelerated when newly created fracture surfaces are exposed to the environment. An improved understanding of the role of the environment on alloy behavior and better analytical methods to reliably predict spectrum loading effects, such as load sequencing, overloads, and stress reversals, on fatigue life will remove conservatism in current designs and may accelerate the use of advanced materials for corrosion-critical components.

For those alloys that exhibit a strong crack-closure effect, models should be developed that incorporate the effects of the environment into closure mechanics such as plasticity-induced closure. For other materials, superposition models need to be developed which attempt to treat time-dependent effects and cyclic mechanical load effects independently. Typically, these models are computationally simple and rely on Paris Law formulations, but may require more test data to fully characterize mean stress and stress ratio effects.

The effects of widespread fatigue damage, including multiple-site damage MSD and multiple-element damage MED , have been critical in the understanding of aging of metallic aircraft structures. Structural mechanics research is needed to develop analytical methods to predict the residual strength of a thin-sheet stiffened shell structure with fatigue cracks or damage caused by an in-flight accident.

Structural analysis methods are needed to evaluate the damage tolerance of aircraft for MSD and MED and to provide a basis for assessing the integrity of repairs on existing aircraft structures. Stiffened shell analysis methods should include automated, adaptive remeshing capability, as well as a capability to analyze structures with nonlinear materials properties and at large deformations.

Structural integrity

These improved methods should be integrated into the structural analysis methodology to predict fatigue crack growth and residual strength of aircraft structures. In cooperation with industry, a structural test program, through the subcomponent and subscale test article level, will be necessary to verify the integrated fracture mechanics and structural analysis methods.

In contrast to metallic alloys, composite materials exhibit complex failure modes that may involve the interaction of several different damage mechanisms. The complexity of the failure modes becomes a major design consideration when addressing durability and damage tolerance requirements. The prediction of composite structural behavior is complicated by the fact that there are virtually no material properties of the constituents that can be measured independently and used to predict the response of the composite.

Most design allowables depend on the as-cured properties of the constituents and the fiber architecture. The process of inferring local micro behavior from global macro response tends to promote empirically based design practices. The current design practices rely heavily on empirical approaches to conservatively estimate the effects of damage on durability and damage tolerance.

Structural integrity and failure - Wikipedia

For example, simple tests such as open-hole compression and compression-after-impact tests are often used to establish "knock-down" factors for reducing the working strain levels to avoid damage growth. In the absence of mechanical methods to predict damage initiation, growth, and failure modes, all test data must be obtained on the specific laminate stacking sequence or fiber preform architecture to be used in the design.

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As described in the introduction to part 2, structural concepts are tested at the. This approach would be significantly enhanced by the introduction of rigorous, mechanics-based analytical methodology that would enable the design of composite structures that are more structurally efficient. There are several key technical barriers to developing reliable methods to predict failure of composite structure.

The first barrier is related to the complex three-dimensional stress states produced by the fiber architecture. The bridge had been built at a time when the Model T Ford was the most popular automobile on the roads, but by the s cars were significantly larger and heavier, further increasing the load on the bridge. Corrosion of the steel structure of the Silver Bridge was likely, especially as the acidity of the rain was enhanced locally by industry nearby.

An example of how corrosion can lead to failure of a bridge structure is described in Box 5: Fall of the Kinzua Viaduct. Many historic bridges and viaducts show the effects of long-term corrosion, which can often lead to catastrophic failure. Such a failure happened to the Kinzua Viaduct during the passage of a large storm through western Pennsylvania in July It was reconstructed in steel in Figure 32 to allow for the greater loads on the structure. The viaduct was retired from active service in , but the structure remained as the centrepiece of a national park, taking occasional traffic from sightseeing tours.

However, the bridge required extensive renovation owing to rusting and was closed again to traffic. Renovation was started in February , but on 21 July an unusually severe storm hit the region. During the storm, 11 of the viaduct's 20 towers fell, destroying the landmark Figure Forensic investigation of the fall showed that the centre of the structure had oscillated from side to side as two tornados with wind speeds of about mph struck the site.

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It was estimated that about four complete cycles occurred before its collapse. Final failure had occurred at the rusted base bolts holding the structure to the foundations, which had fatigue cracked over a long period of time Figure 34 , assisted by internal rusting. The lattice superstructure fell in several parts, the railway line ending up hundreds of feet away.

Failure assessment diagram

The failure bears some similarity to the classic fall of the Tay Bridge in a severe storm in , the main difference lying in the way the joints in the structure behaved. Most of the cast-iron joints of the Tay Bridge failed either before or during the fall, so the railway line ended up close to the foundations. The steel joints at Kinzua remained mostly intact. The year-old Silver Bridge collapsed suddenly at about 5 p. The first signs of collapse were later recounted by the survivors. After this, the bridge started disintegrating fast; girders and hangers fell, followed by collapse of the roadway itself near the centre of the bridge.

The towers then fell, bringing the rest of the chains with them. The entire structure collapsed within about a minute, disgorging 31 of the 37 vehicles into the river below. Those who were trapped inside their sinking vehicles had little chance of escape, however, given that the river reached a depth of 20 m near the centre.

Some broke their vehicle windows and managed to escape their sinking or sunken cars, swimming to the surface. The fall from the road deck and impact with the water rendered many of the victims unconscious and they drowned, trapped in their sunken vehicles. Despite heroic rescue attempts from both sides of the river, the disaster claimed 46 victims, although remarkably, three people from the centre section survived.

Recovery of the bodies took some time, and they were the first priority after all the swimming or stranded victims were rescued.

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  • However, it was vital to determine the cause of the accident, so the river bed was trawled thoroughly for all the metalwork that had fallen. It was difficult work as well, because the river was deep and fast-flowing at that point, as well as being very cold. Although there had been many other bridge collapses in the USA before , only one had been worse: the collapse of a railway truss bridge in at Ashtabula, in which over died.