Section 7.3. Quality Control Testing
The quality control tests
provide data that (a) support the initial quality design assessments and
non-destructive inspection (NDI) requirements, or (b) ensure the uniformity of
the production product. Because many of
these tests will be conducted during the production run they are fairly simple
tests. Requirements for these tests are
defined after the preliminary sizing and the identification of fracture
critical parts. Quality control data
covered by this category of tests include equivalent initial quality (EIQ)
data, continuing assessment of the non-destructive evaluation (NDE) capability,
and component prolongation tests for fracture toughness and crack growth
resistance.
One sure method for minimizing damage tolerant problems due to the
presence of the manufacturing induced
rogue flaw is to take ample precautions on the production line to minimize the
probability that such defects could
be present in safety-of-flight structures.
The manufacturer, during design, will typically suggest methods
for ensuring strict production line control of material preparation,
fabrication and joining techniques. The
manufacturer’s control can be periodically checked using the same type of
testing and analysis approaches that were utilized in design to justify the
choices of materials and defect sizes for the airframe’s damage tolerant
analysis.
Throughout the procurement
cycle of several recent weapon systems, fracture toughness was controlled to
specified design minimum levels for airframe safety-of-flight type
structure. The particular fracture
toughness property used for quality control was the plane-strain fracture
toughness – KIc, (see
Section 7.2). Since some manufacturing
processes are such that they alter the microstructure of some materials (and
thus the fracture resistance), it was believed necessary to monitor the behavior of material subjected to
the gamut of processes that precede final assembly. In fact, the B-1A material quality control
program was designed so that the fracture toughness was sampled for each fracture critical part after each major
manufacturing process; such a sampling program
provided an immediate indication if any process was detrimental to the fracture
toughness.
In almost all the past cases
where fracture toughness was controlled, ASTM E399 was employed to obtain a
valid plane-strain fracture toughness (KIc)
value. As a result of the difference in
cost between a KIc test
and other much simpler mechanical tests such as the tensile test, engineers
have been giving attention to the development of tests that are both simple and
representative of the fracture toughness property of interest. The double-edge notched specimen and the
round edge-notched specimen are two notched geometries that have been examined. Both notched geometries are prepared with
sharp root radii, i.e. radius < 0.002 inch, but do not contain fatigue
precracks. A Chevron Notch Test for KIV (ASTM E1304)
can also be used as a KIc
indicator.
For quality control
purposes, the manufacturer might prepare a series of round-edge notched
specimens and KIc
specimens with the same microstructure (from the same lot of material) and
determine the relationship between, for example, notched tensile strength and
fracture toughness. The series of tests
would be repeated for different microstructure (different lots of material)
until every possible combination of
microstructure was covered. The
manufacturer would then formulate a global relationship between notched
tensile strength and fracture toughness of this material. Using standard
statistical techniques, the manufacturer could then establish the required
level of notch tensile strength that should be measured during production in
order to achieve the minimum allowable level of fracture toughness.
While the crack growth property is actually of greater concern than the
fracture toughness property, controlling the level of subcritical crack
growth resistance via a quality control test has not been attempted for any large weapon system due to the
expense and complexity of the crack growth rate test. The Air Force funded one study to explore the possible
development of an inexpensive crack growth test but the results of this
program were mixed [Creager & Sommers, 1977]. With the advent of automated
fatigue crack growth rate test methods, future quality control programs could
incorporate a test for controlling the subcritical crack growth resistance of
fracture critical parts.