The application of procedures for designing and evaluating structural integrity of various construction techniques is discussed. The fatigue performance improvement of the following conditions is described: (1) conical fasteners, (2) hole preparation, (3) interference fit, and (4) interference protection. The application of fatigue tests to determine fail-safe conditions is analyzed

Chaumette, D.

Peyrony, M.

English

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PRACTICAL ASPECTS OF DESIGNING FOR AND
EVALUATING STRUCTURAL INTEGRITY
By Marcel Peyrony and Daniel Chaumette
Avions Marcel Dassault
Saint-Cloud, France
INTRODUCTION
The objective of this paper is not to show the results of very scientific studies,
but only to put forward some points which can be of practical use to the designer. The
following procedures apply generally in the tests discussed:
(1) Skins are machined, either chemically or mechanically
(2) Their surfaces are blasted with glass beads and wet sand
(3) They are given a surface protection and painted
(4) No bonding is used.
FATIGUE PERFORMANCE IMPROVEMENT
It is well known that bolts with tight fit give a definite increase in fatigue life.
However, this increase is guaranteed only if every bolt is mounted with the right fit
and if no undetectable fault may change the assumed condition.
Conical Fasteners
For a long time we have been using bolts or fasteners with tight fits of 5 to
30 microns, with very satisfactory results. But it is quite a problem to achieve a
guaranteed fatigue life of 40 000 hours while saving much weight, especially in the
joints or in the lower skin of the wing. The search for a higher admissible stress is
then a constant undertaking.
Conical bolts had at first sight appeared most interesting. There was indeed
the danger of stress corrosion when a tight fit of 90 microns was used with alloys such
as 2024-T3 and 2014-T6, but riveting on short transverse components is infrequent,
and less sensitive alloys may be used. A test program was started with two types of
test specimens, a "dog bone" type and a lap joint. (See fig. 1.)
The first test results (fig. 2) were most encouraging, until a test specimen broke
after a disappointingly low number of cycles. A more thorough inspection of the test
piece (which had been inspected prior to testing) showed a reaming fault as in figure 3.
The bolt bears only on four regions.
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It was necessaryto determine why this fault existed andwhy it hadescapedinspec-
tion. The first problem was easy to resolve. It took manyyears to learn howto drill
a perfectly circular cylindrical hole. Conical holes, becauseof their high drilling
torques, will surely require still more developmentandtooling for reliable results.
With limited tooling, we succeededin drilling correct holes up to 6 mm in diameter and
8 mm deep. But for larger diameters, very rigid jigs and expensivetooling were needed.
Both the cost of tooling andthe drilling time were found to be prohibitive for an extensive
conical fastening.
Furthermore, inspection was most difficult, with the necessity of blueing checksat
every hole, which increased the cost of the total operation. Finally, a subsequenttest
with holes purposely drilled incorrectly showeda dramatic decrease in fatigue life. (See
fig. 4.) These results, addedto the dangerof stress corrosion, led to the decision not to
useconical fasteners in the Mercure.
Hole Preparation
In anattempt to increase the fatigue life of structures, the following processes
were examined:
(1) The way in which the hole is made:
(a) Normal reaming
(b) Hell-Armor reaming
(c) Broaching
(2) The finish given after reaming:
(a) Deburring
(b) Roll over
Significant differences were foundbetweentreated anduntreatedholes without rivets
(fig. 5). On the other hand, oncethe bolt is set, thesedifferences decreaseand even
disappear (fig. 6).
Interference Fit
Ononehand, interference fit has a positive influence onfatigue life, but on the other
hand,beyonda certain level of interference (30microns), fretting under the fastener
becomestoo important unless special care is taken. Figure 7 showsthe type of failure
in each case. With the free bolt, the crack started in the cylindrical part of the hole, but
with the interference-fit bolts, the cracks started by fretting in the countersink.
406
Antifretting Protection
In all previously described tests standard sealing and surface protection treatments
were used; that is, rivets were wet mounted with PR 1422 or Blendexite and the test
specimens were painted with PR 1460 or Cellolac 78-28. Figure 8 shows the effect on
fatigue life when one of these two protections is omitted. The fatigue life was reduced by
fretting underneath the fastener collar when the specimen was not painted, and in the
countersink when the rivet was dry mounted.
Selecting Parameters for Mounting a Fastener
The results of these experiments led to the selection of the following procedures:
(1) Use of a moderate amount of interference (bearing in mind the problem of stress
corrosion)
(2) Painting and wet mounting
The influence of the way in which the hole is obtained is not so obvious. Broaching
and Hell-Armor give comparable results, but broaching is an extremely reliable method
of obtaining holes of a high standard, while Hell-Armor may be less reliable. The
"miracle" alloy for the best life has appeared to be 2024-T3. (We have not tested
7075-T73.)
FAIL-SAFE DESIGNS
General Considerations
The greatest risk of crack initiation is surely incurred in joints. Multiplying stress
concentrations by using, for example, riveted reinforcements around door openings should
be avoided as much as possible. Integral structures mechanically or chemically milled
might seem a good solution, but then the difficult problem of fail-safe design enters the
picture.
We have already made some remarks about this problem at Melbourne, having been
unfavorably impressed by some examples of so-called fail-safe design, the most classic
being a structure cut in two pieces and bolted back together. Since then the situation has
apparently worsened, judging by the design of some recent tail-unit hinges and control-
I
surface bearings.
Are the regulations responsible? It is certain that FAR 25-573 encourages a
designer who does not want to put questions to himself to demonstrate that a structure
can sustain the required static load when one of the elements has failed. This test
allows him to claim that his structure is fail-safe. Moreover, the same paragraph allows
a failure to be considered only partial if it is obvious.
407
As far as fatigue is concerned,the basic idea, in itself quite legitimate, of
assuring security by meansof residual strength hasbeenput in a wrong way.
Fatigue and Fail-Safe
Of course a fatigue test is not a fail-safe demonstration by itself, but it is not neg-
ligible in assessingstructural fail-safe designs. Also, fatigue testing is a goodmethod
for determining inspection schedulesfor the various parts of the airplane.
We do not believe that anelement cut in two, but in which cracks appearandgrow
rapidly, is sufficiently fail-safe. Whathappensto the half of this element carrying the
whole load whenthe other half hasbroken?
Somerecent mishapswith elementsworking in parallel shouldprove of interest.
Oneinstance was the F-14 prototype, in which two hydraulic tubesused on separatecir-
cuits, but subjectedto the same fatigue duty, gaveup in a 5-minute interval.
Another example of a more structural nature was found during a fatigue test on a
military aircraft. The main frame supporting the bending moment of the wing was made
of two rings working in parallel. (See fig. 9.) Cracks appeared and grew ahnost identi-
cally in the two rings, and a rupture occurred in each, the load being then supported by
the remaining structure outside.
Another example concerns a wing attachment (fig. 10). Cracks grew at the same
time from five holes and, what is more, on both wings. To have separated the attachment
into halves would only have given a formal fail-safe structure without increasing enough
the safety of the design.
These problems occurred because the fatigue life was short enough for all the
pieces involved to be damaged. A quite different case appeared in the test of the Mercure
main frame. An artificial crack in the flange of the frame grew only on one half of the
flange, not passing the "wall" of the web, and at a rate low enough to be found in
inspections.
Here the stress was lower, and this explains most of the difference. Thus a fatigue
test may give important indications for fail-safe designs.
There is a still worse method for obtaining fail-safe. Take a beam with an I-section,
the tensioned flange of this beam being perfectly smooth, without any hole. Then replace
the integral tension flange by riveted flanges, the tensioned area being the same. Fail-
safe is not definitely guaranteed and fatigue life is severely decreased.
If you feel unable to insure safety with such a monopiece structure, you can design
a double load path while avoiding putting rivets in the tensioned zone. You will keep good
fatigue life and get a double load path at the same time, but perhaps you will have some
trouble with fretting or corrosion.
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Fail-Safe andUnexpectedCracks
The doubleload path finds its soundestjustification in unexpectedcracks. However,
this shouldnot deter the designer from looking at the types of remaining risks and finding
a solution to them. These risks canbe definedas flaws in the material, fretting, and
stress corrosion. With correct designs, care, andinspection thesedifficulties canbe
overcome, andwe haveto work for that in anypossible way.
Important Conclusions Regarding Fail-Safe
Certainly a low stress level is an important factor for fail-safeguarantee. For
less important elements where weight loss is small, no regrets should be had in designing
with important margins for a theoreticallyinfinitelife. But this is not enough. During
the fatiguetests and after,itis necessary to monitor the crack propagation rates at every
point which may be critical. The results should be linked with the inspection schedule of
these particular points.
Double load path must not be neglected. But its reliabilitymust be assured with
regard to fatigue considerations as well as corrosion and fretting,and the structure must
not be weakened by a bad design.
OUR FAIL-SAFE APPROACH
It must be admitted that we have often used the classic fail-safe methods described
here. However, even in these cases we applied the procedures described in the following
paragraphs to investigate crack propagation.
Photoelastic Tests
For the Mercure design, we had previously developed tests on photoelastic models
and on metal parts coated with photostress material (figs. 11 to 13). Thus, we were able
to study stress concentrations on a particular component, and even on an entire element
of the fuselage. Also, we were able to determine critical areas where we could provoke
cracks.
Partial Fatigue Test
Usually the area where we want fail-safe capability of a one-piece structure is
greatly overdimensioned for fatigue. So we perform tests with normal fatigue loads,
followed by cycles at higher loads. During these tests, cracks must not originate.
409
Crack Propagation
Next we artificially provoke cracks in critical areas - that is, areas where calcu-
lations andphotostress investigations showstress concentrations. (Seefig. 14.) The
crack growth rate is plotted against time in order to define a scheduleof inspections that
will provide detection in service before the risk of dangerousfailure is encountered.
This implies that the tests are basedonparticular conditions.
It is best to have two types of cycles. The first, with only normal fatigue loads, is
applied on specimens to monitor the crack growth rate. The second, includingthe same
normal fatigueloads, also includes "fail-safe"loads, and is applied on other specimens to
evaluate the criticallength of crack beyond which a staticfailuremay occur.
Another possible applicationfor fail-safedesign is found in secondary effects (fuel
leakage, for instance).
C ONC LUSIONS
The double load path, although a good fail-safe concept in many cases, is not entirely
satisfactory. It may be insufficient when fatigue life is too short, and superfluous when
the stress is low, detection is easy, or fail-safe capability is achieved by other means.
The best procedure is to rely on crack growth-rate studies and guaranteed crack
detection by inspection in service.
The use of double load path to cope with unexpected phenomena such as stress cor-
rosion or flaws seems rather makeshift, and it is better to seek specific action (improved
forgings, inspection, protection) in each case.
The easier the inspection, the better for fail-safe.
410
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Practical aspects of designing for and evaluating structural integrity

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