Page 363 - Failure Analysis Case Studies II

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Fig. 3. The fracture surface of one of the four fractures of cylinder B. The photograph has been marked to
highlight beachmarks caused either by damage during hydrotests of the cylinder or indicating changes to the
crack growth mechanism. Some growth stages might leave no clear beachmark.
a Area of different morphology indicating initial defect.
a-b Area of growth which is mainly intergranular which is consistent with SMIE. Note that in this
region some grains are very large because of recrystallisation during the heat treatment cycle
c-c'
k, Faster area of growth, plastic areas mixed with SMIE.
After c, c' Plastic rupture, surface approaches 45" to other fracture plane.

bottle broke into two. Figure 3 shows one of the cracks in the neck region. It appears that there is
a region of quite different crack growth prior to the final failure which is indicated by regions of 45"
shear. There are at least two visible beach marks, indicating crack growth stopped for a time, on
this specimen.
The characteristics of the growth observed by examining beachmarks in both the specimens
indicate fairly stable growth for a time moving mainly up the threaded portion but then there is a
tendency to move more quickly without penetrating the outer surface with some growth totally
interior to the cylinder.

3. MODELS OF CRACK GROWTH
3.1. Existing means of calculation of crack growth rate
Crack propagation due to SMIE in small specimens under different imposed K, values has been
studied previously by Ibrahim [l]. One set of specimens was cut from aluminium ingots with an
assay of lOOppm Pb. The lowest value of KI used for this series of tests was 13MPaJm. The
following equation was determined for crack growth rate in mm/h:
log,,(da/dt) = 0.234KI-7.719.
Ibrahim and Stark [lo] present additional studies on some specimens cut from Australian pressure
vessels and tested at room temperature. The alloys used in these gas cylinders had very low levels
of lead present, probably less than 10ppm. The growth was much slower and the equation derived
for crack growth rate in mm/h was
log,,(da/dt) = 0.20164K1- 8.889. (2)