Mike DeMicco wrote:
> "Phil, Squid-in-Training" <[email protected]> wrote
> in news:FUY_f.715$fG3.27@dukeread09:
>
> > [email protected] wrote:
> >> amakyonin wrote:
> >>> The cyclingnews article on the bike quotes a Trek represe ntative in
> >>> discussing the steerer: "No, it's aluminium; it's been blasted then
> >>> anodized black". Presumably by "blasted" he means bead blasted.
> >> bead blasting causes the outer aluminum to deform plastically,
> >> compressing in the radial direction. (That's why it has that nifty
> >> flat, textured finish). This plastic deformation causes residual
> >> stress, keeping the outer aluminum in a "compressed" state.
> >>
> >> The claim is that this resitual stress improves fatigue life. Since
> >> cracks don't typically grow in regions under compressive stress
> >> (tension is generally what makes cracks propogate) then keeping the
> >> outer "fibers" of aluminum in compression keeps microcracks from
> >> propagating and that's where the improved fatigue life comes from. As
> >> I said, that's the claim. It makes sense to me from a mechanics of
> >> materials perspective, but I have no idea whe ther the theory actually
> >> translates into real-world fatigue-life benefits.
> >
> > It doesn't, according to my instructor from yesterday's lecture. With
> > a mean stress below 0, fatiguing cycles still produce fatigue
> > failures, even if there i s a compressive stress. I'm trying to
> > remember exactly what it was he said, but I'll ask again and find out.
> >
>
> Well, your prof. is wrong. Shot peening has been proven to increase
> fatigue life.
Why do you assume it's the prof's fault? Remember, grasshopper doesn't
remember exactly what was said.
BUT:
(a) bead-blasting is not shot-peening, shot-peening improves fatigue
life, bead-blasting does not. Shot peening is particularly important
before an aluminum part is either anodized or heavily machined.
(b) we don't know whether the steerer actually was bead-blasted, or in
fact shot-peened. I surely hope Trek had the sense to shot-peen such a
crucial part before anodizing it. Just because the mechanic says it was
"blasted" doesn't mean anything.
One of the objections I have to threadless headsets is they clamp the
steerer too tight. With a regular stem, in a crash the bars turn and
can be straightened out with no ill effect. Not necessarily the same
with threadless, depending.
FYI: below first is a manufacturer's blurb about shot peening Ti
frames. It's useful because it distinguishes bead blasting from shot
peening. After is a more general discussion from elsewhere about shot
peening and its uses re machining and anodizing.
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Shot Peening- Why Do We Do It?
Shot peening has 4 major benefits in the crafting of titanium bicycle
frames:
Fatigue life is enhanced significantly increasing the useful life of
the frame The frame is stress relieved so that all of the component
parts work together Surface hardness is enhanced increasing resistance
to scratches Provides an esthetically pleasing finish
Shot peening is a cold working process in which the frame is bombarded
with small spherical metal balls called shot at a precise angle. Shot
act like tiny ball peen hammers and create a uniform dimpled texture on
the surface of the frame. This compacts the outer layer of the
material.
The processes of butting and welding titanium tubes to make a bike
frame are known to create tensile stresses in the frame material.
Tensile stresses make the area in question want to pull itself apart.
This is a bad property to impart to a bicycle frame as any minor notch
or micro crack in the frame will want to propagate and further
compromise the material. The induced tensile stresses are most
concentrated in the heat affected zone - the area of the welds. Thus,
strength is compromised precisely where you would like it to be
greatest.
Shot peening of a welded titanium joint substantially increases both
fatigue strength and fatigue life as compared to the same joint which
is not shot peened. Shot peening imparts what is called residual
compressive stress which counteracts the residual tensile stress, which
is created in the process of cutting, grinding and welding. Typically,
fatigue strength of a welded titanium joint after shot peening is
double that without shot peening. Fatigue life is enhanced by shot
peening to an even greater degree.
By shot peening the frame after it is welded together, we are able to
relieve the stresses in the material providing compressive qualities,
which are known to reduce micro cracking and enhance fatigue life.
Without stress relieving, each of the tubes will retain tensile
stresses which tend to conflict with one another. Stress relieving
allows the component tubes of the frame to work together as designed,
acting as a unified structure rather than a collection of competing
parts.
The shot peening process work hardens the surface of the tube, while
giving it a finely textured surface. These two properties together
create an attractive finish that is highly resistant to scratches. If
scratched, the scratch is harder to see because the surface is
textured. The textured surface glitters in the sun in a manner similar
to that of a pearl paint job.
Some people may confuse shot peening with sand blasting or bead
blasting. At IF, we use sand blasting on our steel frames to remove
contaminants. We also use it impart a microscopic tooth to the surface
of the metal to provide a mechanical bond with the paint. Bead blasting
is used for cosmetic purposes to provide a uniform finish to the
surface of the metal. Neither sand blasting nor bead blasting improve
the mechanical properties of the metal.
Shot peening is used precisely because it improves the performance
characteristics of the finished parts. It is used in the aerospace
industry, in high performance cars and motorcycles, and in light weight
bicycle stems and bars where light weight and high strength are
performance imperatives.
What's the big deal? Don't a lot of frame builders shot peen their
frames?
To the best of our knowledge we are the only titanium frame builder
using the shot peening process. This process should not be confused
with bead blasting, which is used to provide a cosmetic finish to ti
bikes. Any company that claims to bead blast over a shot peened finish
does not understand shot peening. Any type of finish polishing brushing
or bead blasting applied after shot peening negates the benefits of
shot peening.
1. INTRODUCTION
1.1 Shot Peening Described
Shot peening is a cold working process in which the surface of a part
is bombarded with small spherical media called shot. Each piece of shot
striking the material acts as a tiny peening hammer, imparting to the
surface a small indentation or dimple. In order for the dimple to be
created, the surface fibers of the material must be yielded in tension.
Below the surface, the fibers try to restore the surface to its
original shape, thereby producing below the dimple, a hemisphere of
cold-worked material highly stressed in compression. Overlapping
dimples develop an even layer of metal in residual compressive stress.
It is well known that cracks will not initiate or propagate in a
compressively stressed zone. Since nearly all fatigue and stress
corrosion failures originate at the surface of a part, compressive
stresses induced by shot peening provide considerable increases in part
life. The maximum compressive residual stress produced at or under the
surface of a part by shot peening is at least as great as half the
yield strength of the material being peened. Many materials will also
increase in surface hardness due to the cold-working effect of shot
peening.
Benefits obtained by shot peening are the result of the effect of the
compressive stress and the cold working induced. Compressive stresses
are beneficial in increasing resistance to fatigue failures, corrosion
fatigue, stress by corrosion cracking, hydrogen assisted cracking,
fretting, galling and erosion caused by cavitation. Benefits obtained
due to cold working include work hardening, intergranular corrosion
resistance, surface texturing, closing of porosity and testing the bond
of coatings. Both compressive stresses and cold-worked effects are used
in the application of shot peening in forming metal parts.
1.2 Residual Stresses
Residual stresses are those stresses remaining in a part after all
manufacturing operations are completed, and with no external load
applied. These residual stresses can be either tensile or compressive.
For example, a welded joint will contain high magnitude residual
tensile stresses in the heat-affected zone (HAZ) adjacent to the weld.
Conversely, the surface of induction hardened components may contain
residual compressive stresses. In most applications for shot peening,
the benefit obtained is the direct result of the residual compressive
stress produced.
2. MANUFACTURING PROCESSES - EFFECT ON FATIGUE LIFE
2.1 Beneficial Manufacturing Processes
Surface hardening which typically leaves the surface of the part with a
residual compressive stress is a beneficial manufacturing process.
Honing, polishing and burnishing are surface enhancement processes
which remove many of the defects and stress risers which can occur
during other manufacturing operations. Surface rolling will leave the
surfaced in residual compressive stress but is limited to regular
geometries while shot peening, properly controlled, produces the best
results.
2.2 Detrimental Manufacturing Processes
Manufacturing processes are known to have significant effects on
fatigue properties of parts. The effects are either detrimental to
fatigue properties or beneficial. Detrimental processes can include
through-hardening, grinding, abusive machining, plating and welding.
These processes tend to induce residual tensile stresses into the part,
thereby lowering fatigue characteristics. Nontraditional machining,
(ECM, EDM) also results in fatigue debits. In EDM (Electro-Discharge
Machining) a recast layer is produced which can be brittle and notch
sensitive and under residual tensile stress. Fatigue failures can
originate in the recast layer and propagate into the base metal. In ECM
(Electro-chemical machining), the chemical used can attack the grain
boundaries leaving them in a weak, notch sensitive condition.
2.2.1 Grinding
Residual tensile stresses and surface brittleness can be caused by the
generation of high surface temperatures during severe grinding
operations. It has been found that residual tensile stresses created by
grinding can approach the ultimate tensile strength of the material
itself. Residual tensile stress will dramatically reduce the fatigue or
stress corrosion resistance of ground parts. Shot peening after
grinding can overcome the detrimental effect of tensile stresses
induced by severe grinding, as shown in Figure 7.
2.2.5 Anodizing
Anodizing is another application in which shot peening improves fatigue
resistance of coated materials. Benefits similar to those shown for
plating are illustrated in Figure 13 where the high-strength aluminum
base metal was shot peened prior to the hard anodize process.
Fig. 13 The Influence of Hard Anodizing and Shot Peening on the Failure
Strength of Duralumin (LI)
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