Page 90 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

68 INTRODUCTION AND FORMS OF CORROSION

1.8.2 Types of Tests
(i) cycles-to-failure (complete fracture) or (ii) crack propagation (crack growth) test.
In the cycles-to-failure test, samples are subjected to a number of stress cycles to
initiate and propagate cracks until complete fracture occurs. This test involves using
smooth or notched samples. This test cannot distinguish between corrosion fatigue
crack (CFC) initiation and CFC propagation.

1.8.3 Sampling in CF Tests
A sample has generally three regions; namely, a test section and two grip ends. The
design and type of sample depends on the fatigue testing machine and the objective
of the study. The test section of the sample is of reduced cross section to prevent
failure at the grip ends. Rounded samples used in axial fatigue may be threaded,
buttonhead or constant-diameter types for clamping in V-wedge pressure grips. In
rotating-beam machines, short, tapered grip ends with internal threads are used, and
the sample pulled into the grip by a draw bar. Torsional fatigue samples are generally
cylindrical. Flat samples for axial or bending fatigue tests may be generally reduced
in width and thickness in the rest of the section (4).
The fracture mechanics approach in CF provides the basics for many fatigue crack
growth studies. The relationship is

√

Δ = ( max − min
where ΔK = max − K min is the stress intensity range, and is the magnitude of the
mathematically ideal crack tip stress field in a homogeneous linear-elastic body and
is a function of applied load and crack geometry, is the stress, and is a function
of the geometry of the rupture and test sample.
The growth or extension of a fatigue crack under cyclic loading is mainly con-
trolled by the maximum load and minimum/maximum stress ratio. However, as in
crack initiation, there are other factors that have an effect, especially in the presence
of an aggressive environment.
The established standard test method for CF crack growth rate is ASTM E647
(30). In this constant load test method, the crack length is measured visually as a
function of elapsed cycles followed by numerical analysis of the data to obtain the
crack growth rate. Crack growth rates are then expressed as a function of crack tip
intensity range Δ , which is based on linear-elastic stress analysis.
The crack growth rate da/dN where “a” is the e crack length and “N” the number of
cycles as a function of Δ gives results that are independent of sample geometry, and
this permits the comparison of data obtained from a variety of sample configurations
and loading conditions (4).
Various studies on crack growth rates for many metallic structural materials have
shown that da/dN versus Δ plots have three distinct regions of behavior. In an inert
environment, the crack growth rate depends strongly on at levels approaching K 1c
(plane-strain fracture toughness) at the high end (region III) and at levels approaching