0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0 20 40 60 80 100 120 Time (Days) Crack growth rate (cm/days) Figure 6. Measured crack growth rates at different external stresses. 0 1 2 3 4 5 0 20 40 60 80 100 120 140 Time (days) a (cm) Figure 7. Measured crack lengths at different external stresses. 4. CONCLUSIONS The effect of a external stress on the hydrogen induced cracking was monitored successfully using an ultrasonic flaw detector. The presence of an external stress affected the number of initial cracks in the HIC tests, having more initial cracks as the stress increases. This effect is due to the weakening of the interface between the matrix and the nonmetallic inclusions by the superposed stress fields of the molecular hydrogen pressure and the applied external stress. It was found that the total crack length and the number crack interconnections, increased proportionally to the applied uniaxial stress, however little or no effect of the stress was seen on the crack growth rate. ACKNOWLEDGMENT Authors tank to the Instituto Politécnico Nacional and PEMEX Exploración Producción Región Sur for their support for the development of this work. This work was financed by PEP Región Sur, grant SRS GCTO 002/00.
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