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Why do Brass Tongues Break?


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Greetings,

 

I often see in these forums mention that brass reeds tend to break much more than steel reeds, with the conventional reason for it being that brass “work hardens.” This popular belief, however, confuses two different properties of metals: cold work hardenability - also called strain hardening - and fatigue strength.

 

In order to strain harden a metal, the metal must be deformed; i.e., stressed beyond its elastic limit, or yield strength. Normal vibration of a concertina reed tongue does not deform the metal, and thus, the brass tongue is not work hardened in usage. (It is work hardened before being fashioned into a reed.) In addition, carbon steel also strain hardens, yet steel reed tongues are known to be very durable. These facts suggest that the property of strain hardening is not the key property in explaining why tongues break.

 

Metal fatigue is usually the reason why most tongues break. Metal fatigue occurs sometimes in metals when they are subjected to varying, repeated cycles of stress that can be much lower than their yield strength (elastic limit). Dislocations occur in the crystal structure and grow to macroscopic fractures. With a vibrating reed tongue, metal near the base experiences stress cycling from compressive to tensile, and in use, the number of cycles experienced can go well above millions.

 

Metallurgists recognize another metallic property, called fatigue limit, which is the stress, for a given number of cycles, that will cause fatigue failure. Some metals seem to have an “endurance limit,” a stress level, below which the material can be subjected to an indefinite number of cycles without failure, though some recent research suggests that such a limit does not exist. This issue is complex and sometimes moot because of the large number of cycles required for its resolution. It’s clear, however, that metals vary greatly in their ability to withstand fatigue, and the lower the maximum stress level, the higher will be the number of survivable cycles, for any metal. Aluminum and pure Copper have relatively poor endurance, while steel and Titanium alloys perform well.

 

So why do brass reeds break? The answer lies in the peak stress they undergo during vibration and whether this stress is more than their fatigue limit for the number of cycles required. An example can show how this works, and how design can affect the results. Figure 1 in the attached file plots the peak stress experienced in steel and brass tongues during vibration, and also plotted are the reported endurance limit stresses for 1094 spring steel and 260 brass, with H10 hardness (extra spring), common reed tongue candidates. The endurance limit for steel is taken as one half its ultimate strength, and the endurance level for brass is taken as 40 % of its ultimate strength, which are generally accepted engineering values, and so, for this particular application can be considered only approximate.

 

In making this plot, I used some reasonable approximations, which I think are acceptable for the lessons learned. For the steel tongues, I measured the amplitude of tip vibration of steel accordion reed tongues at maximum playing pressures. Knowing this amplitude, measured tongue length and measured tongue thickness, we can calculate the maximum stress in the vibrating tongue. This stress is calculated from the curvature of the bending tongue at its root, which in turn is calculated from the known shape the tongue undergoes during vibration, defined now by the measured amplitude. There is some inaccuracy in these calculations, because I used a simple, constant cross area geometry, when in some cases, the cross section is not constant. However, these inaccuracies are confined primarily to the smallest two tongues I present data for, and as will be seen shortly, the important discussion involves only the longer tongue lengths.

 

For the brass plot in Figure 1, I chose a tongue geometry having the same length, the same vibration frequency, and the same amplitude as the corresponding steel tongue. In order to vibrate at the same frequency, the thickness of the brass tongue, however, must be different from that of the steel tongue, and this thickness can be found by calculation, which in this case amounts to about 41% thicker than the steel tongue. Finally, the (approximate) maximum tensile stress in the brass tongues can then be calculated and plotted, as was done for the steel tongues in Figure 1.

 

Notice that peak stresses in the steel tongues generally lie well below the endurance limit for steel, except for the longest reed, and so one might guess that these tongues have generally good design for longevity. Peak stresses in the brass tongues, however, generally lie above their endurance limit, and so these tongues, are likely to break after relatively short time in use. Notice also that the larger length (lower pitch) tongues are expected to have lower lifetimes.

 

If we make the brass reed tongues shorter than the steel tongues, yet keep the pitch the same, we can reduce the maximum stress in the brass tongues to a level below their endurance limit. Figure 2 in the attachment shows this approach, where the brass tongues are made to be 50% of the steel tongues having the same frequency. I also assume the brass tip vibrational amplitudes to be 50% of those for the corresponding steel tongues. For the brass tongues to have the same vibration frequency, calculations reveal that the brass tongues must now be only about 35% of the steel tongue thicknesses, as labeled in Figure 2. With these designs for the brass tongues, peak stress in the brass tongues never exceeds their endurance limit, as shown in Figure 2.

Although the shorter brass tongues now appear as durable as the steel tongues, it's expected that the shorter brass tongues will not speak as loudly as the steel tongues, and this might present practical problems. A compromise is thus suggested here: one between a brass reed tongue that’s loud enough with one that lasts long enough. Since the longer tongues appear more susceptible, the higher pitched brass tongues demand less compromise.

 

It’s significant that, in this study, it was necessary to reduce the longer brass tongues to a length only half as big as the corresponding steel tongue, for the same longevity as for steel. It’s my gut feeling that makers would be reluctant to make such short tongues for the required pitches, but here, I need input from makers. Is it true, first of all, that makers construct brass tongues having lengths shorter than that of a steel tongue that has the same vibration frequency? Second, how much smaller are makers willing to go, mindful of the reduced volume the reed will produce, and just as importantly, mindful of the requirement that all reeds of the instrument be able to compete with each other in volume? The requirement to go as small as 50% for the longest reeds thus uncovers practical issues, and so, I’m doubtful that makers go that far for those longer tongues, and from this, we can understand why brass tongues break.

 

In practice, another important consideration regarding fatigue strength is surface finish. Thus, one might expect that the practice of tuning reeds by scratching could make tongues – especially those made from brass - more susceptible to fatigue failure, and such a practice can perhaps result in breakage that is more frequent than indicated here. For this reason, it’s perhaps best to perform any necessary scratching near the base of the tongue in the axial direction, with many shallow, broad scrapes, and ideally, tongue designs should be conservative enough to account for this practice.

 

One more complication with fatigue should also be mentioned, and that is the number of expected cycles. For instance, take a tongue vibrating at 250 Hz, near middle C. Using this tongue for 140 seconds every day adds up to thirteen million cycles for a year, a number that brings us into the engineering realm of extended usage. Although the concept of endurance limit should encompass tens or even hundreds of millions of cycles, we can perhaps appreciate that musical instrument reed tongues present a very demanding application.

 

In summary, the design of brass reed tongues can greatly affect their longevity. If one requires that the brass tongue put out as much volume as the steel tongue, and if the brass tongue is made as long as the steel tongue that operates at the same frequency, it’s likely that most brass tongues will fail by fatigue. If, however, lower volumes are acceptable, then generally shorter tongue lengths for brass would extend their lifetimes. For the lower pitch notes, however, it may not be practical to reduce brass tongue length far enough to render the brass tongue as durable as the steel tongue, and it’s not unreasonable to expect that longer tongues so constructed will eventually fail.

 

I welcome comments from reed makers, on what guidelines they go by in the making of brass tongues, and whether these guidelines are intended to address lifetime and volume issues.

 

Best regards,

Tom

www.bluesbox.biz

Steel and Brass Stress.doc

Edited by ttonon
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In my years of engineering I was always told (rightly or wrongly) that brass not only work hardens, but age hardens.

It is possible to increase the spring tension in soft brass wire by gently tapping the wire along it's length with a hammer.

You have made a very extensive and useful research into work hardness and other factors can cause stress breaking such as the heavy use of course files where the fracture mostly seem to occur along a file mark. The use of brass in reeds greatly reduces the manufacturing time of reeds as it's workability is easier,the resultant sound is ideal for accompaniment of singing,

You have made a very interesting thread and I too will look forward to see how it develops.

Many thanks

Al

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The use of brass in reeds greatly reduces the manufacturing time of reeds as it's workability is easier,the resultant sound is ideal for accompaniment of singing,

 

Al,

 

Probably not true. The higher volume modern makers use surface ground (both for size and profile) reeds and I don't think brass would take this well. The relative softness of brass might favour a person who makes an occasional concertina and uses hand tools only on the reeds, but the availability of brass sheet ready to use is not there, hardening would need to be done by hand.

 

The question I would like to hear the answer to is not whether brass reeds are durable, but why brass reeds create a softer sound if the sound is created by chopping air at a specific frequency. Is it because brass reeds have not typically been made to the same close tolerances as steel reeds? And therefore not creating the higher partials typical of a close reed.

 

Chris

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The question I would like to hear the answer to is not whether brass reeds are durable, but why brass reeds create a softer sound if the sound is created by chopping air at a specific frequency. Is it because brass reeds have not typically been made to the same close tolerances as steel reeds? And therefore not creating the higher partials typical of a close reed.

 

Chris

 

 

 

One way would be to build some " close tolerance" reeds from brass and test the results.

 

I recall several top quality 'tropicalised' Wheatstones that had Copper based alloy ( perhaps a spring temper Bronze) reeds that were not weak in tone... a little mellow maybe. I tend to think that the reason why these reeds are not as BRIGHT as their Steel counterparts could be that at the point of change of direction during the vibrating cycle there might be a less crisp (slower) reverse of motion...... just a thought... could be proved using a high speed video.

 

Geoff.

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...but the availability of brass sheet ready to use is not there, hardening would need to be done by hand.

 

Hi Chris. Have you made efforts to find hard brass? As you probably know, the copper/brass industry has set definite standards on the various conditions brass can be sold under, with hardness ranging from 1/8 hard (H00) to super spring hard (H14). From the calculations for this thread, it’s evident to me that the most common brasses should be hardened to at least Spring, or Extra Spring Temper, in order to withstand a reasonable amount of cycles (lifetime) before breakage. I could understand that such a product might not be too easy to find, but I’d be surprised if you couldn’t find it at all. (Pray tell that American Industry has sunk to such rock bottom levels!) So I guess I’m asking, how hard did you try (with no pun intended)?

 

The question I would like to hear the answer to is not whether brass reeds are durable, but why brass reeds create a softer sound if the sound is created by chopping air at a specific frequency. Is it because brass reeds have not typically been made to the same close tolerances as steel reeds? And therefore not creating the higher partials typical of a close reed.

 

As I mentioned in other posts, the only way material properties enter into the way the tongue vibrates is through the ratio Y/rho, where Y is Young’s Modulus, or Modulus of Elasticity, and rho is material density. If I might coin a phrase, let’s call this ratio the “specific stiffness” of the material. This ratio for steel is about twice that for brass, and herein lies an important clue, which to me, means that the higher specific stiffness for steel results in higher aerodynamic forces on the steel tongue, when compared to those for the brass tongue that operates at the same frequency. In other words, the steel tongue operates at a higher (air) force level than the brass tongue, and thus causes higher pressure pulses than does the brass tongue. Air pressure pulses with higher magnitudes occurring at the same frequency require larger gradients in time (you have to go a greater distance in the same time, meaning that you must accelerate more), and from Fourier analysis, this means that there must be higher harmonics in the resulting waveform. Although this explanation sounds rational, I haven’t proven it, though I wouldn’t be surprised at all if it turns out to be true.

 

As an addendum to my original post in this thread, I attach an additional figure, giving the brass tongue length fraction to steel tongue length in order that the maximum stress in the brass tongue remain at or below the material’s endurance limit, as explained in the original post. Such a plot may be interesting to makers, because it shows that the smaller (higher pitch) reed tongues need not be reduced in length as compared with the corresponding steel tongue as much as the longer tongues, from the point of view of endurance. It's also interesting to note that the smallest brass reed tongues can be made significantly larger than the corresponding steel tongues, even around twice as much, without undue risk to their endurance. Of course, many issues are not included in this discussion, including the relative volume and scaling factors among the brass reed tongues.

Best regards,

Tom

Tongue Length Ratio.doc

Edited by ttonon
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I recall several top quality 'tropicalised' Wheatstones that had Copper based alloy ( perhaps a spring temper Bronze) reeds that were not weak in tone... a little mellow maybe. I tend to think that the reason why these reeds are not as BRIGHT as their Steel counterparts could be that at the point of change of direction during the vibrating cycle there might be a less crisp (slower) reverse of motion

The 'tina in my avatar has brass?/copper-based alloy reeds. They hold their own in a session (in fact I've used this instrument to play for dance), but yet are mellow enough for song accompaniment - all-in-all a great compromise. Not sure about the behaviour at reversal, but they certainly have the punchiness of steel, without the strident sound.

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Tom,

 

I have not made any effort at all to find such a hard brass other than knowing that it is not listed among the 20 or so possibilities at the place I get my supplies of brass used for other parts in the concertina. I have no great wish to use brass, mainly because none of the brass reeded onstruments I have heard have inspired me to do so. If any modern maker has done so I imagine it would be Colin and Rosalie Dipper.

 

The possibility that greater amplitude of displacement in a steel reed and the subsequent higher travelling speed (not frequency) at the tip of a steel reed might account for the added higher partials had occurred to me after writing yesterday, and to others, I had one PM to that effect overnight.

 

Your graph of comparative lengths for brass/steel reeds based on durability is interesting, greater lengths in higher reeds could supply more volume, but lengths in higher reeds are driven by pitch rather than material capacity. Making high reeds twice as long is my idea of a really bad day!

 

Do you have a reason for this push into brass reed science Tom? It has certainly left me with a lot to think about...

 

Chris

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The possibility that greater amplitude of displacement in a steel reed and the subsequent higher travelling speed (not frequency) at the tip of a steel reed might account for the added higher partials

Hi Chris, my wild guess about why brass tongues sound different from steel tongues goes further than simply amplitude of displacement, although such a factor may be involved. My hunch also includes a possibility that air-related, aerodynamic effects can result in higher pressure pulses through other mechanisms related to how the air pushes and gets pushed by the vibrating tongue. For instance, mechanisms such as those suggested by Geoff might contribute to the understanding.

 

To Geoff’s suggestion, I would add that a very critical part of the swing cycle is when the tongue enters the slot, both from the top and from the bottom of the swing. Such details could be studied experimentally by making Variable Impedance and Laser Vibrometer measurements, as done by Cottingham at Coe College, Cedar Rapids, IA. Slight departures from pure sinusoidal motion in the tongue vibration can possibly shed much light on the issue. Comparing magnitudes and time locations of such departures made by brass tongues with those made by steel tongues I think would provide much insight.

 

Do you have a reason for this push into brass reed science Tom? It has certainly left me with a lot to think about...

Although I think the sound of brass has advantages in some musical settings, and I wonder how much of brass breakage is due to design issues, I’m primarily interested in trying to understand the things we observe in its use.

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Just as a point of interest in relation to all this:

 

The return springs that I make for the keywork on my Uilleann Pipes are of a Copper based alloy, either Brass or Nickel silver,which I 'hammer' to work harden. A competitor uses bought-in blue tempered Steel springs for this job which break with regularity. These Steel springs are much thinner than my Brass ones which suggests that the spring strength is much greater. Perhaps my competitors Steel springs are just too hard for the job, however of the thousands of hand beaten Brass springs I have made I can recall only one or two that failed.

 

Note, these are leaf type springs.

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It is interesting that to work harden brass by hammering, is just lightly tapping without even denting the surface.

Is this what you find Geoff?

If you go too far a full heat and slow cool brings it back to soft again.

Al

 

 

Pretty much the case Alan, although I tend to use the Peine end of a small hammer and tap lightly to more quickly word harden the brass with out the sheet getting too thin. I guess I find this more controlable and I can see where I have been working by the dents. I use a 'Half Hard' sheet for this so I do not have to beat so much and there are no real soft spots after the work.

 

Geoff.

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Hi Alan and Geoff,

 

The attached file gives a table with information linking the temper achieved in a Copper alloy with the amount of cold working necessary for the desired temper. Also in the file is a chart describing the ASTM temper designations. Thus, if a strip of annealed brass 0.010 inch thick were rolled to a thickness of 0.003 inches, it would undergo a 70% reduction, achieving a temper of H10, "extra spring hard."

 

I do understand that you have worked out your own trusted methods to cold work brass, and that you may be entirely satisfied with it, but for those so inclined, I don't think it would be very difficult to make a small rolling mill that would reliably and uniformly reduce strip material to the convenient thicknesses necessary for making reed tongues.

 

Best regards,

Tom

Cu Alloy Tempering Charts.doc

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Tom

 

I know this is a little OT as its nothing to do with the durability of brass, but I was moved to perform a couple of very crude, ie. unscientific, tests. A friend (Chris Vonderborch, a concertina maker from Tasmania) has a good Wheatstone with brass reeds. He measured the profiles of a range (3) of reeds, and also their maximum altitude in operation. I did the same with steel ones. I think I was hoping the brass ones would reverse at a lower height, and also be much different in amplitude. From a very non-representative test I can say the results were remarkably similar in all departments. The profiles were the same to within a thou or so and the maximum lift above the frame pretty much the same.

 

If one was to write off these two variables, (on the basis of such a casual test you probably wouldn't), then is anything left other than speed of reversal and the difference that makes to tip speed through the frame. I say that because even if the speed of reversal is the issue then it will not in itself create higher partials. They can only be created by the tip of the reed passing through the frame, and presumably the only variable in this is speed.

 

A question, is the only difference in terms of sound between a brass and a steel reed the addition of higher partials in the steel reed? So that rather than a different and mellow tone the brass reed is a fully included subset of the steel reed tone. ( I received a PM recently also suggesting this.)

 

I know a solid session of testing could nail some of this stuff. Trouble is I am so busy at the moment.

 

Cheers

 

Chris

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Tom,

 

I take it the pure sinusoidal swing you mentioned earlier would be a theoretical movement with absolutely no decel and accel, just an instant change of direction?

 

Thinking a little more on this, if the extra higher partials in a steel reed are caused by higher tip speed through the frame then the steel would have to be the one with the slower rate of deceleration and acceleration at the reversal point.Ie. taking longer to turn around. While the turnaround could be slower, the acceleration could be to a greater maximum speed. In comparison the brass, if it had a higher rate of decel and acceleration and therefore a faster turnaround could afford to travel slower through the frame because it would have more time to do so and would therefore generate fewer higher partials.

 

Having written all of this and then returned to what you said earlier I realise you have in fact said this already when you were talking about "specific stiffness" and its ramifications. It just took me a while to understand it. I will leave it here in the hope you will correct me where I have gone wrong.

 

Being ever motivated by the practical, the reason for my particular interest here is, should it be possible to define a continuum of metallurgical characteristics which would range between a "fast" reed and a "slow" reed of the same pitch, such that one could easily select characteristics you want in the final sound without having to compromise on clearances or restrictive woodwork to remove higher partials, it would be easier to build concertinas with a wider range of tones without losing speed.

 

I find myself suddenly very interested in Young's M of E..!

 

Cheers

 

Chris

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I take it the pure sinusoidal swing you mentioned earlier would be a theoretical movement with absolutely no decel and accel, just an instant change of direction?

 

 

No that is exactly incorrect!;)

 

A sinusoidal swing describes a continuous acceleration from zero velocity at the extreme displacement to maximum velocity near the centre of the swing.

Edited by Theo
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I take it the pure sinusoidal swing you mentioned earlier would be a theoretical movement with absolutely no decel and accel, just an instant change of direction?

 

 

No that is exactly incorrect!;)

 

A sinusoidal swing describes a continuous acceleration from zero velocity at the extreme displacement to maximum velocity near the centre of the swing.

Quite right, I knew the shape but was suffering from brain fade. I have often wondered what the true shape would be. All of the down accelerations and speeds would be faster than the up ones, and there would be a substantial speed increase when the reed broaches the frame on the way down and a slowing when it does so on the way up.

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