No nonsense
with NDT
Nondestructive testing methods in infrastructure
Photo courtesy of Peddinghaus Corp., Bradley, Ill.
By Jesse A. Grantham and
Andrew J. McGoldrick
Infrastructure is defined as the basic
physical systems that serve a community’s population, such as roads, bridges,
utilities, water, and sewage. These systems
are essential for enabling productivity in the
economy, so it is crucial to ensure they are
fit for use. Nondestructive testing (NDT)
techniques determine whether a component
has reached its useful service life and needs
repair or replacement.
In-service load conditions produce incremental movement in materials and structural
members. Whenever a force is applied to a
member, it becomes stressed. The stresses
cause strains, or movements, which are explained by the material properties. In-service
conditions cause stress, strain, distortion,
fatigue, and corrosion, which can manifest as
discontinuities or defects.
A discontinuity is an interruption of the
typical structure of a material, such as a lack of
homogeneity in its mechanical, metallurgical,
or physical characteristics. A discontinuity is
not necessarily a defect. A defect is discontinuity that either by nature or by accumulated
effect renders a part unable to meet minimum
applicable acceptance standards or specifications. In other words, the part is rejected.
NDT Methods
NDT locates discontinuities or defects in critical areas without causing structural damage.
Two fundamental categories of discontinuities that are located by NDT are surface
and subsurface discontinuities.
Surface discontinuities, including cracks,
porosity, slag inclusions, excess reinforcement,
inconsistent weld bead profile, undercut, and
blisters, usually can be identified by visual testing. To enhance surface VT, magnetic particle
testing or liquid penetrant testing is employed.
Visual Testing (VT). VT is the most
basic, cost-effective NDT method. It should
take place prior to, during, and after welding.
Many standards require it before other methods because there is no point in submitting
an obviously bad weld to sophisticated
inspection techniques. Welding codes always
state that welds subject to nondestructive
examination shall have been found acceptable
by visual examination. V T requires good eyesight in the technician and sufficient light, a
weld size gauge, a magnifying glass, and a 6-in.
metal ruler.
Magnetic Particle Testing (MT).
Magnetic particle testing detects surface and near-surface defects in ferromagnetic materials
only. Defects in magnetized materials will
distort the magnetic field, causing a leakage
field. When fine ferromagnetic particles are
applied to the surface, they concentrate at the
defect by getting caught in the leakage field.
Liquid Penetrant Testing (PT). PT
detects surface-breaking defects in any nonporous material. A liquid penetrant is applied
to the surface and is drawn into defects
by capillary action. Once a preset dwell time
has passed and excess penetrant removed, a
developer is applied to draw out the penetrant
from the defect. Visual inspection is then
performed.
PT also reveals surface cracks and pinholes
that are not visible to the naked eye. It is used
to locate leaks in welds and can be applied
with austenitic steels and nonferrous materials
W12 2009 WELDING UPDATE FOR INFRASTRUCTURE
