I think the “Aluminum Time Bomb Theory” demonstrates the lack of materials knowledge in the bike
industry. It should be common knowledge that most modern aircraft use aluminum exclusively for
their primary structures (internal frames and bulkheads) and 95% or better of their exterior surfaces,
including load bearing skins. The aircraft industry has been using these alloys for several decades. I
have recently been a passenger on some planes that I estimate were made no later than the 60’s. So
aluminum alloys have certainly proved their long term durability and high performance in the aircraft
industry. The occasional failure that has occurred has typically been due to a design or manufacturing
defect or improper maintenance.
The aircraft companies have picked aluminum because it offers the best combination of material prop-
erties and processing capability in order to create high performance, lightweight, robust aircraft. Prior
to the widespread use of aluminum alloys in airframes, I believe that Chrome Moly Steel was used in
many cases for structural members and coated fabric was used for skins.
Aircraft see high levels of shock and vibration. They are subjected to high G loads, both positive and
negative in severe turbulence and positive when landing. The most severe repetitive G loads occur on
aircraft carriers. The planes are accelerated and decelerated at high G’s by mechanical linkage with the
The example given of repeatedly bending a small piece of metal is not relevant to the durability or relia-
bility of a bicycle frame. When you permanently deform the material as in the example you are yielding
it. This is not what fatigue strength or fatigue life refers to or is about. It has no relation to fatigue
strength. Some of the highest fatigue strength materials I have used are carbon fiber and boron fiber.
They will not take a significant permanent set, breaking instead at a high force level. So these extremely
high fatigue strength fibers would rate near zero by the repeated bending test.
The example is comparing the material property to withstand reversing yield conditions. On a bike,
this would be like crashing the bike, smashing the head tube back and buckling the top and down
tubes. Then straightening the tubes back (to the degree possible) and repeating the cycle. While this
might be useful in a few circumstances, I would rather my frame did not yield so easily in the first
place. The optimum material for this reversing yield property might be a low carbon (low yield
strength) or mild steel alloy which has not proven to be a good choice for high performance bike
The statement “Alu has a shorter fatigue life than steel.” also demonstrates shortage of material
knowledge and understanding. Sure, a high strength steel alloy will exhibit a longer fatigue life at a
high fully reversing load level than a high strength aluminum alloy. These numbers always reflect per-
formance for a unit volume. But it also weighs 3 times as much for the same volume. If the mass of
the bike is unimportant, then I guess steel is the better material. If density is factored in, aluminum is
Failure of a structure due to repeated stress cycles has two main components. They are crack initiation
and crack propagation. The obvious thing to design for is to prevent crack initiation. In theory, if no
cracks can start, then we don’t need to worry about fracture toughness, or crack propagation. But this
does not work in real life. I think that all metal bike frames have millions of small cracks. It is inherent
in their metal structure. Most metals are made up of very small metal crystals or grains stuck together.
There are inherently a lot of flaws in the micro structure. The concentration of cracks is higher where
the metal has been welded or brazed, such as at the joints. I believe this is true for steel, aluminum
and titanium alloys. A tough material will allow the bike to perform adequately for a long time with a
crack in it that is below a certain crack size. The tougher the material, the larger the allowable crack.
Below this critical size, the crack will grow so slowly that it will not become a problem.
Crack initiation behavior is measured by fatigue tests.
Fatigue behavior of a given material is not at all well defined by any single number. Fatigue behavior for
a material is more accurately portrayed by a series of curves. The behavior (and number of cycles it can
withstand) will vary considerably depending on whether the load is only applied in one direction, both
directions, or is applied in addition to a static or constant load. For each type of loading condition
described above, the material will exhibit a range of fatigue cycles to failure depending on the load
level applied. The most commonly used test is the fully reversed load without static load. It is a simple
test to perform.
The fatigue life increases as the stress level is reduced. Common steel alloys and common aluminum
alloys have differently shaped curves.
The curve for steel under fully reversed loading is approximately a constant downward slope (plotted
on a logarithmic cycle’s scale) until about one million cycles, where the curve abruptly becomes hori-
zontal. It has a well-defined corner in it. This is called the endurance limit for steel.
The curve for aluminum does not have this sharp corner. The curve continues to decrease very slowly
well past one million cycles and becomes horizontal at five hundred million cycles. So the fatigue limit
for aluminum alloys is typically measured at 500 x 10^6 cycles, where the curve is no longer decreas-
ing. This is way more high stress cycles than a bicycle will ever see.
I should also add that there is typically a lot of scatter in fatigue data. Often a thick band showing the
range of cycles that the material withstood may represent the curves.
The shape of the curves sort of gives aluminum an advantage in the fatigue mode. I think the real high
stress cycles that a bike sees are more likely to be around 10,000 cycles during its expected lifetime
(about 20 years). As aluminum’s published data is typically measured at 500 million cycles, it is con-
siderably stronger at lower cycles. Steel is also stronger at lower cycles, but since it was measured at
one million cycles, the strength improvement at 10,000 is probably not as great as in the aluminum.