Crack fatigue

The frequency-dependent rate of disentanglement of the molecular coils in the fatigued fibrils seems to account for part of the frequency effect on growth rate. 

In addition hysteresis heating occurs in the strained craze material. Both effects combine to give rise to a distinct frequency sensitivity of A for a variety of materials such as PC and PMMA [219, 220], and PPO, PVC, PA 66, PC, PVDF, and PSU [220]. As has been noted by Skibo et al. [220] the frequency sensitivity varies with temperature. It reaches a maximum at that temperature where the external (fatigue) frequency corresponds to the internal segmental jump frequency (of the jS-relaxation process). 

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Lafarie-Frenot, M.C. and Henaff-Gardin, C., Formation and Growth of 90° Ply Fatigue Cracks in Carbon/Epoxy Laminates , Composites Science and Technology Vol. 40, p. 307-324, 1991... 

YOSHIOKA,T. Detection of Rolling Contact Sub-Surftice Fatigue Cracks Using Acoustic Emission Technique. Lubrication Engineering, Vol.49, No.4, 1993, p.303-308. 

In the near future the technique will be further evaluated using ultrasonic signals from natural defects, e.g., fatigue cracks. The performance measure and the parameter optimization procedure wilt also be refined in order to obtain a computationally efficient implementation, easy to use for a trained operator. 

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

Fig. 2. Typical rf signal from a 28 mm deep fatigue crack in 56 mm thick carbon steel sample...

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

H.Toda Quantiative evaluation of fretting fatigue cracks on the surface of the railroad axles by grazing SH Wave ultrasonic method. Journal of JSNDI, Vol. 40, March, pl58-164, (1991)... 

There are no classification requirements for routine NDE beyond thickness testing and visual inspection except for repairs, modifications or where service history has identified a specific problem in which case the Surveyors will request NDE at the same locations in similar ships. Under circumstances where visual inspection has found evidence of fatigue cracking the Surveyor can also call for NDE to assess the full extent... 

The cyclic loading applied to hull structure will eventually lead to fatigue cracking. 

For existing ships the only NDE method nominated by classification for the detection of fatigue cracks is close-up visual inspection - although all Surveyors have the option of requesting additional NDE when warranted. The sensitivity of visual inspection is influenced by the degree of surface preparation and the level of lighting at the inspected surface - which may not always meet the level of 500 lux nominated by some NDE specifications. 

Doyle, J. L., Wood, G. R., and Bondurant, P. D. Using Laser-Based Profilometry to Locate and Measure Corrosion Fatigue Cracking in Boiler Tubes, Materials Evaluation, D The American Society of Nondestructive Testing, Inc., Vol. 51, No. 5, pp. 556-560 (1993). 

Corrosion fatigue is a type of failure (cracking) which occurs when a metal component is subjected to cyclic stress in a corrosive medium. In many cases, relatively mild environments (e.g., atmospheric moisture) can greatly enhance fatigue cracking without producing visible corrosion. 

Eig. 9. Schematic fatigue crack growth data showing the regions of growth rate. 

Eracture mechanics concepts can also be appHed to fatigue crack growth under a constant static load, but in this case the material behavior is nonlinear and time-dependent (29,30). Slow, stable crack growth data can be presented in terms of the crack growth rate per unit of time against the appHed R or J, if the nonlinearity is not too great. Eor extensive nonlinearity a viscoelastic analysis can become very complex (11) and a number of schemes based on the time rate of change of/have been proposed (31,32). 

ASTM E647-93, "Measurement of Fatigue Crack Growth Rates," Annual Book of ASTM Standards, ASTM Puhhcations, Philadelphia, 1993. 

This optimum condition is designed to ensure that the botes of all components yield at the same time. If the cylinder is subjected to fatigue conditions, it has been suggested (39) that a better design criterion would arrange for the maximum normal stress, which controls fatigue crack propagation, to be the same in each component. 

The fracture surfaces, revealed when the tube is broken open, are found to be smooth with a rippled appearance characteristic of fatigue. This type of behavior is sometimes known as leak before break. On the other hand, if the material lacks toughness, the propagation of the fatigue crack may be intermpted part way through the wall by the intervention of fast fracture, resulting in what is sometimes known as the break before leak mode of failure. 

Another important appHcation of LEFM is the rate of growth of a fatigue crack under cycHc loading. This is also controlled by the stress intensity factor through an equation of the following form (110) ... 

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

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