Moisture distributions and the effects of hygrothermal cycling on moisture gradients can be characterized. Extensive work has been accomplished to understand the diffusion of moisture in composites and the resulting composite properties Fried, 1967; McKague et al. Typically, these conditions have been identified by cyclic thermal or hygrothermal exposure testing Brunner, 1994; Sensmeier, 1994 , with test acceleration gained by rapid cycling, increased temperature range, or increased ply thickness multiple adjacent plies of one orientation. Early experiments used a flat paddle-shaped gage that was inserted into the crack; as the crack opened, the paddle gage rotated, and an electronic signal was sent to an x-y plotter. Note 4: Fatigue crack growth can be significantly influenced by load history.
The constitutive parameters in these strength models are studied as a function of the service inputs and environments using kinetic or rate theory allowing fatigue, creep, creep rupture, aging, oxidation, and other time-dependent and cycle-dependent effects to be introduced. This implies that the material can plastically deform, and, therefore, is tough. Accelerating Creep Tests There are several empirical and analytical methods for accelerating a test, most focusing on accelerating creep tests Manson and Ensign, 1979. Figure 6 shows the data collected and shows the signals from the notch corner and tip — compare to Figure 2 to understand the corresponding ultrasonic image area. In this method, strain versus time data are fit to an equation involving four constants: Θ1, Θ2, Θ3, and Θ4. Results The video of the test showing the growth and failure due to a fatigue crack can be found. We typically perform pre-cracking using the compliance technique.
Accelerated exposures are used to produce end-of-life microstructure or damage states for subsequent characterization tests. Fatigue crack growth rate data are derived from the crack length data discrete pairs of crack length and cycle count data and test load data. Approach The verification test setup was simple. The effects of design mean stress, the stress range for ground-air-ground cycles, the simulated thermal stress, the number of stress cycles per flight, and salt corrosion were included in their studies. It was clear that given the likely vibrations during the fatigue testing the ability of the probe to remain attached to the specimen would be an issue.
The substantiation test methodology has been very similar to the process described above for lower-temperature applications except that a different criterion is used to determine the upper-use temperature for a material system. The jet engine and the electric power generation industries have experienced such concerns in the past in the design of their components and therefore have experience in designing against creep-fatigue failures. For the airframe of a Mach 2. Moreover, preexisting flaws or sharp design features may significantly reduce or nearly eliminate the crack initiation portion of the fatigue life of a component. The SiC-based fibers typically show primary creep over their whole life, and phenomenological models can be accurately fitted to the data to allow interpolation and extrapolation Jia et al. Several theories have been proposed to explain crack kinking and crack propagation in mixed-mode loading, and two are highlighted below.
Claeys and Jones 1984 attempted to accelerate a creep test of 6061-T6 by accelerating an elevated-temperature exposure. The results of this study show that the primary source of variability associated with fatigue crack growth rate testing is the experimental procedure used to obtain the raw test data crack length versus elapsed cycles. In this section, let's consider cracks that grow straight-ahead from the application of a resulting in a single. This complicating factor needs to be considered in using constant-amplitude growth rate data to analyze variable amplitude fatigue problems 11. Oxidation measurements at high pressure must be carried out to extend the model into the range of realistic operating conditions under fuel-rich conditions.
The oxides generally consisted of a dual-layered structure with an inner layer of chromium enriched oxide relative to the metal substrate and an outer layer of iron rich oxide. No standard test procedures are available for conducting creep-fatigue crack growth testing, however, it is a subject of active research. While this method provided curves that satisfactorily reproduced real long-time test data, it has several drawbacks. The strain energy release rate can physically be understood as: the rate at which energy is absorbed by growth of the crack. Specifically, creep stresses for different levels of creep strain were of interest.
During the development effort, questions regarding thickness, buckling, excessive plasticity, accuracy of crack length measurements, length-to-width ratio and proximity of welds to the notch for welded inserts were systematically addressed. It is important to test aging response at the size scale where the degradation takes place. If a crack is present in a specimen that undergoes cyclic loading, the specimen will plastically deform at the crack tip and delay the crack growth. Mark, Burak Erman and Mike Roland, Editor s , , The Science and Technology of Rubber, Fourth edition, Academic Press, Boston, 2013, pp. The linear elastic and elastic-plastic fracture mechanics approaches for predicting brittle and ductile fracture, respectively, are well established and are not discussed in this report.
Analysis The data presented can be plotted on a graph of crack height a from the original position of the notch tip see Figures 2 and 6 against the time. An electrical potential method of measuring crack growth rates is employed on miniature center-cracked-tension specimens 1. Today we utilize the finest systems available to perform these testing features supported by state-of-the-art data acquisition and machine automation. However, the R-curve was not widely used in applications until the early 1970s. The structure is impacted prior to the fatigue test, substantiating both barely visible impact damage requirement for the life of the structure and the inspection interval required for visible impact damage. The effects of scale, geometry, surface quality, coatings, and diverse individual service conditions must be considered together with their possible synergistic interactions. The preliminary test program should determine the magnitude of the transient and the crack growth increment required to establish steady-state behavior after a new condition is introduced.