Fatigue behavior dynamic

Fignre9.4 Schematic of expected dynamic fatigue behavior for cracks subjected to Region 1 sub-critical crack growth. 

M.P. Hanson, Static and Dynamic Fatigue Behavior of Glass Filament-Wound Pressure Vessels at Ambient and Cryogenic Temperatures, NASA TN D-5807, National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio (1970). 

Various criteria are utilized to describe fatigue behavior under dynamic load that are generally derived from the intended application ... 

Kayabasl O et al. (2006) Static, dynamic and fatigue behaviors of dental implant using finite element method. Advances in Engineering Software 37 649-658... 

As one can see, there are two families of constraints with which the bonding will have to deal successively the constraints due to the launch, mainly static and dynamic, over a short period of time and the constraints due to the space environment over a long period of time, mainly thermo-mechanical. If one does not have to deal with fatigue behavior, as on aircrafts, one does have to deal with ageing due to the thermal-mechanical stresses, mismatches between adhesive and adherend, metallic or composite, regarding the extreme temperature variations. 

An adequate description of material behavior is basic to all designing applications. Fortunately, many problems may be treated entirely within the framework of plastic s elastic material response. While even these problems may become quite complex because of geometrical and loading conditions, the linearity, reversibility, and rate independence generally applicable to elastic material description certainly eases the task of the analyst for static and dynamic loads that include conditions such as creep, fatigue, and impact. 

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

The behavior of materials under dynamic load is of considerable importance and interest in most mechanical analyses of design problems where these loads exist. The complex workings of the dynamic behavior problem can best be appreciated by summarizing the range of interactions of dynamic loads that exist for all the different types of materials. Dynamic loads involve the interactions of creep and relaxation loads, vibratory and transient fatigue loads, low-velocity impacts measurable sometimes in milliseconds, high-velocity impacts measurable in microseconds, and hypervelocity impacts as summarized in Fig. 2-4. 

Long time dynamic load involves behaviors such as creep, fatigue, and impact. T vo of the most important types of long-term material behavior are more specifically viscoelastic creep and stress relaxation. Whereas stress-strain behavior usually occurs in less than one or two hours, creep and stress relaxation may continue over the entire life of the structure such as 100,000 hours or more. 

Dynamic mechanical measurements are performed at very small strains in order to ensure that linear viscoelasticity relations can be applied to the data. Stress-strain data involve large strain behavior and are accumulated in the nonlinear region. In other words, the tensile test itself alters the structure of the test specimen, which usually cannot be cycled back to its initial state. (Similarly, dynamic deformations at large strains test the fatigue resistance of the material.)... 

Composite adhesives are being used for a growing number of applications outside the automotive industry as well. For example, Dow Formulated Systems recently developed a foam core system for wind blades that is bonded by epoxy adhesives. The new system offers long-term dynamic behavior and shear strength properties that deliver the excellent mechanical strength and fatigue resistance necessary to achieve blade durability. 

Loads on a fabricated product can produce different t3q>es of stresses within the material. There are basically static loads (tensile, modulus, flexural, compression, shear, etc.) and dynamic loads (creep, fatigue torsion, rapid loading, etc.). The magnitude of these stresses depends on many factors such as applied forces/loads, angle of loads, rate and point of application of each load, geometry of the structure, manner in which the structure is supported, and time at temperature. The behavior of the material in response to these induced stresses determines the performance of the structure. 

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