Initial Trials – Breaking Soundboard Samples

Rick Kemper, Sligo Harp Shop

 

A sample of redwood in deflection, 55.8 lb load


In this experiment, I broke 18 sample sections of Sitka Spruce, Western Cedar and Redwood.  Each sample was build into a fixture that approximated a cross section of the soundboard near the bass end of the harp.   With a one inch pine liner on each side, each 17” sample had an open span (liner to liner) of 15 inches.  Each sample was made from clear, tight grain stock with minimal run-out.*

 

The fixture was suspended from an overhead beam, a dial indicator was attached to measure the deflection at the soundboard at the center of the sample, and progressive loads added to the eye bolt to simulate string tension. 

 

 

This experiment had four goals:

1.   It attempted to determine how soundboard deflection corresponds to its load

2.   It attempted to measure whether screws through the soundboard and into the liners measurably improve the sound board’s load bearing capacity

3.   It attempted to measure the relative breaking strengths and failure modes of Spruce, Western Cedar and Redwood.

4.   To Determine whether soundboard samples broken in this type of fixture fail the same way as real sound boards do

 

 

 

A few practical tips for anyone wanting to try this in their own shop:

·  When sound board sample fails, it can be a sudden explosive bang.  Work carefully.

·  Suspend the weights so they only fall in inch or two to the floor.  Wear steel toed shoes and keep your feet toes out of the way.

·  Make sure the tie- ins to the overhead beam are secure and rated to carry the loads you are testing

·  You can get a 110 lb fish scale and a 2” dial indicator on E-bay for ~$35 each

·  You can use plastic buckets of Sand calibrated to the nearest tenth of a pound for a load

·  Set up the dial indicator so the tip is pre-loaded.  The sample should break away from the calipers delicate tip

·  Use a secondary lanyard to tie the dial indicator and holder to the overhead beam so it does not fall four feet to the floor


 

 

DATA:

For each material, samples #1-3 were simply glued to pine liners.  Samples #4-6 were glued and secured at the edge with a .210" cherry wood batten and number six by ¾ inch steel square drive screw.  Testing stopped when the sample broke.  For example, the #1, #2 and #3 Redwood samples were able to sustain a 25.8 lb load, but failed when they were loaded with 35.8 lbs of weight.  There are no deflection data points for loads greater than 25.8 lbs for glued redwood samples.  

 

The samples all broke at some common failure points. 

 

It is interesting to note that the glue was never a point of failure in this trial.**   The samples would often break near the glue line, but they always left a fine “beard” of wood fibers on top of the glue boundary.  

 

Most Frequent

 

Left: Redwood splitting along the grain, just above the liner near the glue line.  Most glued (only) samples failed in this mode

 

 

Right: Rupture across the grain at the bending point near the string rib.  Many of the samples that were glued and screwed  failed in this mode. 


 

 

Less Frequent

 

Left: Screwed Cedar Sample failed when the liner cracked away from oak bass Plate

 

Right: Screwed Sitka Sample, Liner cracking below the screw tip

 

 


 

 

Many of the glued and screwed samples had an interesting dual failure mode.  At 25-45 loads the sound board would audibly crack just above the liners but the sample would not fall apart as the screws held the wood being tested to the fixture.  After an addition 10-30 lb load had been added, the sample would then rupture or break off the liner below the tip of the screw.

 

Tabular Data:

Deflection Tables with detail on failure modes. 

Sitka

 

 

 

 

 

 

 

Load

#1

#2

#3

#4

#5

#6

Average

0

0.000

0.000

0.000

0.000

0.000

0.000

0.000

5.8

0.056

0.061

0.062

0.067

0.065

0.064

0.063

15.8

0.151

0.153

0.158

0.166

0.160

0.153

0.157

25.8

0.231

0.250

0.245

0.250

0.260

0.231

0.245

35.8

0.300

0.326

0.322

0.320

0.335

0.299

0.317

45.8

0.365

0.400

0.388

0.394

0.405

0.348

0.383

55.8

0.430

 

 

0.460

0.467

0.419

0.444

65.8

0.489

 

 

0.520

0.535

0.494

0.510

#1 Sitka failed (in the soundboard) just above the glue line, the liner broke just above the base plate on the other side at when 10 lbs was added to the 65.8 lb load.

#2 Failed as #1, but at 55.8 lbs load

#3 The Sitka failed above the liner on both sides.

#4 Failed as #1

#5 Failed in the liner on both sides, liner cracking just below screw tips

#6 Failed as #5

 

 

Redwood

#1

#2

#3

#4

#5

#6

Average

0

0.000

0.000

0.000

0.000

0.000

0.000

0.000

5.8

0.088

0.133

0.193

0.128

0.089

0.115

0.124

15.8

0.347

0.288

0.373

0.274

0.369

0.264

0.319

25.8

0.459

0.397

0.505

0.388

0.498

0.380

0.438

35.8

 

 

 

0.503

0.600

0.472

0.525

45.8

 

 

 

0.583

0.695

0.554

0.611

55.8

 

 

 

0.653

0.807

0.664

0.708

65.8

 

 

 

0.778

 

0.754

0.766

75.8

 

 

 

0.857

 

0.854

0.856

#1 Redwood failed at liner at both sides. 

#2 Failed as #1

#3 Redwood ruptured, cracking near center at edge of “string rib”

#4 Failed when the screw pulled out of liner on one side

#5 Sample ruptured into three pieces, failing ¼” in from each liner and at Center near “string rib” 

#6 Sample ruptured into two pieces, failing ¼” in from one liner and at Center near “string rib” 

 

 

Cedar

#1

#2

#3

#4

#5

#6

Average

0

0.000

0.000

0.000

0.000

0.000

0.000

0.000

5.8

0.105

0.099

0.142

0.103

0.146

0.103

0.116

15.8

0.243

0.230

0.307

0.251

0.319

0.235

0.264

25.8

0.355

0.335

0.442

0.378

0.438

0.347

0.383

35.8

0.454

0.427

0.544

 

0.531

0.438

0.479

45.8

0.537

0.504

0.657

 

0.613

0.537

0.570

55.8

 

 

 

 

0.718

0.619

0.669

65.8

 

 

 

 

0.806

0.728

0.767

75.8

 

 

 

 

0.874

0.798

0.836

#1 Failed Cedar at liner, both sides 

#2 Failed Cedar at liner, one side

#3 As did #1

#4 Failed when liner separated from oak base plate

#5 Ruptured at center (near string rib), then within each of the liners pulling each of the screws out

#6 As did #5

 

 

The data tables shown in stress/strain graphical format

 

 

 

 


Comparative Averages for the different kinds of woods:

 

 

Breaking load for samples tested (in lbs)

Glued Only

Screwed and Glued

 

 

Specie average

 

 

Specie average

SS1

75.8

62

SS4

75.8

76

SS2

55.8

SS5

75.8

SS3

55.8

SS6

75.8

RW1

35.8

36

RW4

85.8

79

RW2

35.8

RW5

65.8

RW3

35.8

RW6

85.8

WC1

55.8

56

WC4

35.8

69

WC2

55.8

WC5

85.8

WC3

55.8

WC6

85.8

 

 

 

 

 

 

Average

51

 

Average

75

 

Deviation

13.3

 

Deviation

16.2

 

SS – Sitka Spruce RW – Redwood      WC – Western Red Cedar

 

 

Some useful findings

1.   The amount of deflection for a given load is fairly consistent within each species, regardless of whether the sample was screwed down or not.   Sitka Spruce had the most tightly grouped (consistent) load/deflection data points.  The standards deviation values for any given load averaged 5%.  Cedar and redwood demonstrated greater variations of 11% and 12% respectively.

2.   When the Sitka samples were loaded within 70-100% of their breaking strength, they were 43% stiffer than redwood or cedar.  Sitka deflects less.  This result does not necessarily mean Sitka will sound better than the other woods.  It does not mean that the Sitka boards are less likely to break.  It simply means a harp with a Sitka board is likely to have less belly.

3.   For glued-only construction, Sitka was the strongest sample in the fixture at 62 lbs, followed closely by Cedar (56 lbs), and redwood in a distant third (36 lbs).

4.   Reinforcing the attachment to the fixture with screws only increased the load capacity of the Sitka Samples by 21%.  Screwing the redwood samples to the fixture dramatically increased their load bearing capacity (up 121%).

5.   The glued and screwed Sitka samples broke at a 76 lbs load on average.  Screwed redwood and cedar samples broke at 79 lb and 69 lb loads (respectively) on average.  Screws allowed the alternative species to bear loads about as well as the Sitka.

6.   The batten/screw reinforcement increases the samples’ load bearing capacity by about 45% on average.

7.   The samples and pine liners in the fixtures had significant variations in their breaking strength, even though these samples were sawn from top quality boards, adjacent to each other.  Wood is a natural product and even when one selects top grade spruce for the sound board, there is significant variation.  As harp builders we should not be surprised when some soundboards break sooner (and later) than the average. 

 

Shortcomings of this Experimental Approach:

Critics would note that string loads are not perpendicular to the plane of the soundboard.  String-to-sound board angles typically range from 25-35 degrees, so the perpendicular component of the string loads are going to be significantly lower than they were in this experiment.    That is true.  In many designs, the string rib will take much of the load along the length of the sound board, and the sound board itself must resist the normal component of the string’s tension, especially where the belly is the greatest, about 2/3rd of the way towards the bass end. 

 

In these tests, loads were rapidly increased from 0-85 lbs in a minute or two.  Most REAL soundboards fail at sustained loads several days or years after they have a sub-critical load imposed on them. Since wood deforms plastically over large time spans, the abbreviated nature of the test may not accurately reflect what is actually happening. 

 

The thick oak base plate used in the fixture was quite stiff. This imposed end conditions on the sample that replicate a very stiffly braced sound box.  Many harp designs have sound boxes that are much more flexible, they are built lightly with little or no bracing. 

 

Statisticians will point out that the number of samples are small, and 3-5 times more trials would need to be completed to produce reliable (statistically significant) results.

 

To all these critics, I say phooey!  If you think you can do better, go do your own tests and publish YOUR resul. . . . (Editors note: Please excuse Rick, he is taking a short break and has promised us he will resume his medications post hoc.  He will also complete two weeks of mandatory sensitivity training). 

 

A model is only useful if it can accurately reflect (even better, predict) real world conditions.  Do the results on the samples correspond to real soundboard failures?   Based on two dozen of my own soundboard failures and repair jobs brought in by my clients, I would estimate that about half the boards fail when the soundboard splits at the edge, just above the glue line at the liner.   Most of these have been old pedal harps or boards that were secured only with glue.  The other half fail in rupture, usually near the string rib, about 2/3rds of the way down at the bass end.  In 15 of the 18 trials, the samples broke in one of these two failure modes. 

 

 

You can learn where the soundboard is going to break through sad experience or through predictive methods like Finite Element Analysis (FEA).  These pictures are courtesy of Patrick Jordan, a fellow Uilleann piper who a genuine rocket scientist for his Day job.  The light green areas in the second panel (showing Von Mises Stress) are the areas most likely to break as predicted by the same kind of fancy pants computer program they use at NASA.  Pretty slick, eh?

 


 

 

In the other 3 trials, the pine liners failed.   Each of those three failures happened in a sample with screw reinforcements.  Failure in the liner is not something I have encountered on my own harps or through repair jobs.   I have not seen a sound boards that  failed in the top half of the sound board or at the very bass end.  

 

I have never had seen a harp soundboard that failed when the string ribs cracked either.  There are a few reasons this happened in the trial.  The short 1¼” grain in the pine “liner” used in the fixture is much more likely to split than a full length liner is would be.  Also, sound boards are glued over the sound box edge, and in most of the designs I have examined, this is a composite component, made from a strip pine on the iside (the liner) and the hardwood or plywood of the sound box side.   The composite structure is much less likely to split too. 

 

 

Conclusions:  

Returning to our original goals for this experiment:

Q: How does soundboard deflection correspond to load?

       

It is a fairly shallow concave curve, with notable differences between species.  Within the small set of samples tested, Sitka had the least amount of variation in deflections (5%).  Redwood and Cedar were about twice as variable.  The Sitka samples were 43% stiffer in the fixtures than the other materials at service load levels (70-100% of the sample’s breaking strength).  There were significant variations in the breaking (ultimate) strengths among species and among fastening regimes (fixtures that were glued only vs. fixtures reinforced with screws).   More work on actual soundboards in service would need to be done to determine whether deflection measurements can accurately predict the service life of a specific sound board. 

 

Q. Does a batten/screw reinforcement measurably improve the sound board’s load bearing capacity?

Yes, by about 45% on average, but the limited number of tests indicate that that figure is likely to vary significantly depending on the species used.

 

Q. What are the relative breaking strengths and failure modes of Spruce, Western Cedar and Redwood?

Species tended to fail in similar modes on similar fixture conditions.  Glued samples tended to fail when cracks formed along the grain in the sample just above the glue line that attaches them to the liner.  Samples that were glued and screwed usually failed with the sample material rupturing at a bending point near the string rib, with a smaller group rupturing near the edge of the sound board. 

 

In samples that were only glued to the fixture, the Sitka sustained the highest load on average at 62 lbs.  Cedar was next at 56lbs and Redwood the weakest at 36 lbs.  In screwed fixtures, the averages between species were much less marked.  Redwood broke at 79 lbs. on average, Sitka Spruce 76 lbs. and Western Red Cedar, 69 lbs).  

 

Q. Do these samples fail the same way as real sound boards do?

In general, yes.  There were a small number of samples that failed in the liner or when the liner cracked away from the oak base plate.  This is an unusual failure, but is likely a result of the fixture not replicating the composite construction (typically pine and hardwood) where the sound box is joined to the sound board.

 

 

 

 

End Note

 

In writing up this experiment, I tried to avoid getting preachy about how other builders should build their harps.  I’m not saying Sitka is more suitable than Redwood, or that Cedar should replace endangered old growth Sitka.  I’m not out to prove that all soundboards should be glued and screwed.  The hope is that the results of these tests will provide some clues that point to promising avenues a frustrated builder may want to explore as he tries to solve a particular problem.  The solution he selects is gong to depend on the harp’s design, the building practices he uses, the problem he is trying to solve and (most importantly) his willingness to experiment.   For example;

 

·        A builder is having trouble with a design where the soundboards are splitting away at the edge of the sound box, the screw batten edge treatment may be a good way to address that weakness. 

·        If he believes a lighter, thinner soundboard will improve the harp's performance, and isn't screwing his boards down, he can reduce the thickness of his boards, secure the edge with battens and screws and the soundboard is quite likely to stay together.

·        If he has been insisting on using hard to find (expensive) Sitka because it is "stronger" than the other woods, these tests suggest he could substitute Redwood or Cedar with the appropriate reinforcement.  They will probably deflect more than top grade Sitka. 

 

Hopefully, harp repairpersons will also take a closer look at blown boards to see how they fail and what went wrong.  They may be able to take some proactive steps to affect a longer lasting repair.

 

Next Steps

When I have the time and funds, I plan to conduct an expanded battery of tests that would incorporate many of these improvements;

·        Double the sample sizes (6-8 samples instead of three)

·        Test tapered soundboards that are profiled to 70, 55 and 40% of their thickness at each edge.

·        Test the stiffness and breaking strength of the various aircraft birch plywoods used for sound boards

·        Test some common domestic hardwoods samples – cherry, tulip poplar, and aspen

·        Use a composite (pine and hardwood, split resistant) liners on the fixture

·        Verify whether the same trends hold when the fixture holding the sample is not as rigid

·        Test string ribs with feathered to see if it mitigate bending fractures

·        Add oriented strand composites at center and edge of the sound board samples to see if they improve the breaking strength of the sound board.

 

 

 

Are you still reading!?  Nobody reads the footnotes anymore do they?

 

* Wood selection: any impoverished harp builder would be crazy to use his best stock for destructive testing, right?  I was sorely tempted to skimp.  I am as likely to give in temptation as the next guy, but in this case I remembered, hey, this is for Science, my little contribution to the harp building brain trust!  For once I let my conscience prevail over selfish, base instincts.  The Sitka spruce was procured from Sitka Sales in Edgewood Washington and had a specific gravity of 0.39.  The redwood and cedar planks were cut from recycled old growth timbers.  The redwood has a specific gravity of 0.41, the cedar 0.39.   All the samples were milled from the same three prime AAA grade quarter sawn planks with ring counts of 24-30/inch.

 

** Yellow PVA glue (Titebond II) was the sole adhesive used in the fixture and to attach the soundboards samples.   Many builders have had significant problems with yellow glue for attaching soundboards because it can slowly creep over long periods of time when subject to high sheer loads.  This tendency is exacerbated by elevated temperatures or prolonged humidity.  The Titebond II  was not a point of failure in any of the samples tested, but the duration of these loads was very short.  Titebond is a great glue for many other applications, but I cannot recommend it for the critical soundboard to sound box joint.  I use Raka 1:3 epoxy for that and would recommend new builders use something more creep resistant than the common PVA glues like Titebond for their sound boards.

 

 

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