May 2013 

NASA/TM–2013-217993

Evaluation of Acoustic Emission SHM of 

PRSEUS Composite Pressure Cube Tests 

Michael R. Horne 

National Institute of Aerospace, Hampton, Virginia 

Eric I. Madaras 

Langley Research Center, Hampton, Virginia 
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May 2013 

NASA/TM–2013-217993 

Evaluation of Acoustic Emission SHM of 

PRSEUS Composite Pressure Cube Tests 

Michael R. Horne 

National Institute of Aerospace, Hampton, Virginia 

Eric I. Madaras 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 
Table of Contents 

Table of Contents ............................................................................................................................ 1

Table of Figures .............................................................................................................................. 2

Symbols and Abbreviations ............................................................................................................ 3

Abstract ........................................................................................................................................... 4

1.0 Introduction ............................................................................................................................... 4

2.0 Acoustic Emission Collection Test Configuration ................................................................... 5

3.0 AE Metrics ................................................................................................................................ 5

4.0 Analysis..................................................................................................................................... 8

4.1 Checkout tests 1 and 2 (0.5P pressure where P=9.2 psi) ..................................................... 9

4.2 Test 3: 1P pressure ............................................................................................................. 10

4.3 Test 4: 2P pressure ............................................................................................................. 11

4.4 Test 5: Failure .................................................................................................................... 12

4.5 Pencil lead breaks: Pre-test and Post 2P ............................................................................ 14

5.0 Lessons Learned, Summary, and Conclusions ....................................................................... 16

6.0 References ............................................................................................................................... 17
2 

Table of Figures 

Figure 1. Evolution of BWB fuselage design [1] ....................................................................................... 18 

Figure 2. Schematic of Pressure cube with approximate locations of some of the AE sensors (red dots) 

mounted on the crown panel and other notations in red by the author. [4] ................................................. 18 

Figure 3. Locations of AE sensors on all the panels of the pressure cube ................................................. 19 

Figure 4. Installation of one AE sensor on the crown panel ..................................................................... 20 

Figure 5. AE Instrumentation Settings ....................................................................................................... 20 

Figure 6. Checkout test 1 (0.5P, P=9.2 psi) Load profile and AE events .................................................. 21 

Figure 7. Checkout test 1 waveforms from AE event #1 captured at all the sensors ................................. 21 

Figure 8. Checkout test 2 (0.5P) Load profile and AE events ................................................................... 22 

Figure 9. Checkout test 2 waveforms from AE event #1 captured at all the sensors ................................. 22 

Figure 10. Pressure 1P Test Load profile and AE events ........................................................................... 23 

Figure 11. Pressure 1P Test: Speckled bulkhead panel AE energy............................................................ 23 

Figure 12. Pressure 1P Test: Gray rib panel AE energy ............................................................................ 24 

Figure 13. Pressure 1P Test: Gray bulkhead panel AE energy .................................................................. 24 

Figure 14. Pressure 1P Test: Speckled rib panel AE energy ...................................................................... 25 

Figure 15. Pressure 1P Test: Crown panel AE energy ............................................................................... 25 

Figure 16. Pressure 2P Test Load profile and AE events ........................................................................... 26 

Figure 17. Pressure 2P Window of increasing strain gage activity ............................................................ 26 

Figure 18. Pressure 2P Test: Speckled bulkhead panel AE energy............................................................ 27 

Figure 19. Pressure 2P Test: Gray rib panel AE energy ............................................................................ 27 

Figure 20. Pressure 2P Test: Gray bulkhead panel AE energy .................................................................. 28 

Figure 21. Pressure 2P Test: Speckled rib panel AE energy ...................................................................... 28 

Figure 22. Pressure 2P Test: Crown panel AE energy ............................................................................... 29 

Figure 23. Pressure 2P Test: Close-up of speckled bulkhead panel AE energy near window of strain gage 

activity ........................................................................................................................................................ 29 

Figure 24. Pressure 2P Test: Close-up of gray rib panel AE energy near window of strain gage activity 30 

Figure 25. Pressure 2P Test: Close-up of gray bulkhead panel AE energy near window of strain gage 

activity ........................................................................................................................................................ 30 

Figure 26. Pressure 2P Test: Close-up of speckled rib panel AE energy near window of strain gage 

activity ........................................................................................................................................................ 31 

Figure 27. Pressure 2P Test: Close-up of crown panel AE energy near window of strain gage activity ... 31 

Figure 28. Failure Pressure Test Load profile and AE events ................................................................... 32 

Figure 29. Failure Pressure Test: Speckled bulkhead panel AE energy .................................................... 32 

Figure 30. Failure Pressure Test: gray rib panel AE energy ...................................................................... 33 

Figure 31. Failure Pressure Test: Gray bulkhead panel AE energy ........................................................... 33 

Figure 32. Failure Pressure Test: Speckled rib panel AE energy ............................................................... 34 

Figure 33. Failure Pressure Test: Crown panel AE energy ........................................................................ 34 

Figure 34. Pencil lead break configuration [9] . ......................................................................................... 35 

Figure 35. Pencil lead break repeatability [7] ............................................................................................ 35 

Figure 36. Change in response with damage accumulation of PLB at sensor 13 ...................................... 36 

Figure 37. Change in response with damage accumulation of PLB at sensor 1 ........................................ 36 

Figure 38. Spectra of initial response of PLB at sensor 13 ........................................................................ 37 

Figure 39. Spectra of response with damage accumulation of PLB at sensor 13 ...................................... 37 

Figure 40. Spectra of initial response of PLB at sensor 1 .......................................................................... 38 

Figure 41. Spectra of response with damage accumulation of PLB at sensor 1 ........................................ 38 
3 

Symbols and Abbreviations 

AE Acoustic Emission 

BWB Blended Wing Body 

COLTS Combined Loads Test facility 

FR Felicity ratio 

LaRC NASA Langley Research Center 

NDE Nondestructive Evaluation 

P Operating pressure (9.2 psi) 

PLB pencil lead break 

PRSEUS Pultruded Rod Stitched Efficient Unitized Structure 

SE Signal Energy 

V Voltage 

Vi Signal voltage 

VIC Visual Image Correlation 
4 

Abstract 

A series of tests of the Pultruded Rod Stitched Efficient Unitized Structure 

(PRSEUS) pressure cube were conducted during third quarter 2011 at NASA 

Langley Research Center (LaRC) in the Combined Loads Test facility (COLTS). 

This is a report of the analysis of the Acoustic Emission (AE) data collected 

during those tests. The AE signals of the later tests are consistent with the final 

failure progression through two of the pressure cube panels. Calibration tests 

and “damage precursor” AE indications, from preliminary checkout 

pressurizations, indicated areas of concern that eventually failed. Hence those 

tests have potential for vehicle health monitoring. 

1.0 Introduction 

A cube constructed of Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) composite 

panels was pressurized pneumatically at NASA Langley Research Center (LaRC) in the 

Combined Loads Test facility (COLTS) to increasing levels during a series of tests. The series 

started with checkout tests to verify the proper operation of all instrumentation and finished with 

the final test in which internal pressure was increased until final failure. This document describes 

the investigation of the Acoustic Emission (AE) data collected during these tests. 

The PRSEUS concept [1] for the composite structure improves on the state of the art as noted in 

figure 1 by eliminating fasteners and providing damage arrest while incorporating integral 

stiffeners for efficient strength to weight ratios. A pultruded rod in a stringer cap perpendicular to 

foam-core frames is a novel way to create a dry carbon fiber stitched preform that supports itself 

during vacuum bagging and resin infusion. Fabrication does not require complex interior mold 

tools [2] so that point-of-fabrication customization for structural changes is easy and 

inexpensive. The dry multi-ply stacks and sub-structures are stitched together using Vectran 

fibers into the near final shape for incorporation into the Blended Wing Body (BWB) design. 

Figure 2 illustrates a 3-D schematic of a subsection of the large scale BWB test article being 

realized as the PRSEUS pressure cube test article with a bulkhead panel removed [3]. The cube 

consists of six sides of PRESUS construction. There are two ribs, two bulkheads, a crown and a 

floor. Each bulkhead panel is joined to the adjacent sides by bolting it to T-cross-section integral 

caps. These caps on the surrounding rib, crown, and floor panels capture the panel like a picture 

frame. Every panel has seven or eight rod stiffened PRSEUS stringers (rib or bulkhead panel, 

respectively) spaced 6.0 inches apart. These stringers cross perpendicular to two frames spaced 

24 inches apart. The spaces of unstiffened panel are called bays. At the corner of two contiguous 

panels the stringers on one panel do not intersect the stringers of the other. The outside 

dimensions of the pressure cube are approximately 52 in. x 48 in. x 56 in. [4] 

The AE sensors applied to the crown and one of the rib panels, as well as the section of the frame 

that buckled first during failure, are shown in figure 2. For purposes of illustration the bulkhead 

panel indicated as “blkhd panel removed for clarity” can be considered as the panel that blew out 

during failure, leaving the cube looking much as the figure illustrates, albeit, a little more ragged 

and fractured. 

That bulkhead panel (containing AE sensors 1-6) and the indicated rib panel (containing AE 

sensors 19-26) were painted with black/white speckle pattern. The patterning is for the Visual 

Image Correlation (VIC) non-contacting full-field strain measurement technique developed by 

researchers at the University of South Carolina [5, 6]. These panels are described in this 

document as “VIC” or “speckled” panels. This strain measurement is realized by tracking the 

deformation of the speckling with two location-calibrated and synchronized cameras for each 
5 

panel. The other panels on the cube were painted grey and are described as such in the following 

discussions. 

2.0 Acoustic Emission Collection Test Configuration 

Thirty-two acoustic emission sensors (Digital Wave B225.5) were mounted on the outside of the 

pressure cube as shown in figures 2 and 3. For easy removal all sensors were bonded with Lord 

202 acrylic adhesive onto aluminum metal tape that was attached to the test article as seen in 

figure 4. The diameter of the sensor is approximately 0.75 inches. 

Figure 3 shows the numbering scheme for the sensors. This figure is a multiview orthographic 

projection of all the panels excluding the floor. Each view is a scaled drawing of the stitching 

pattern for the panel. Each panel contains six sensors, except for the speckled rib panel that 

has two extra sensors around an applied impact. A 1-inch spherical impactor applied the dropped 

weight impact of 100 ft-lbs during the test series prior to the final failure pressurization [3]. This 

illustration allows one to note the location of the AE sensors, which were attached to the smooth 

outside of the cube, relative to the structural components (stringers, frames, and integral caps) 

inside the cube. This type of description of the sensor locations is relevant because the significant 

changes in geometry of the cube at the stiffening elements are more likely to be where damage 

occurs first. These stiffeners can also act as waveguides to propagate the AE. 

All sensors were connected to thin coaxial sensor cables, which in turn were connected to Digital 

Wave PA0 preamp/line drivers to buffer the weak signal back to the data acquisition system. The 

preamps were connected via thicker low noise coaxial cables to two Digital Wave FM1 signal 

conditioning 16-channel amplifiers, which were remotely located in the COLTS control room. 

The amplifiers were connected to a computer containing multi-channel Digital Wave data 

acquisition hardware and software. The computer and amplifiers were actively manned during 

the tests. The data was recorded for subsequent processing with the same software. 

A summary of the amplifier gains for the tests is shown in figure 5. Settings did not change until 

the increase in AE rate and signal strength necessitated a reduction in signal gain to reduce signal 

amplitude saturation, and/or a reduction in trigger gain to reduce rate saturation and subsequent 

data loss. A change in signal gain can be compensated for, in post processing under certain 

conditions, by scaling the amplitude as though acquired at a different gain. However, 

comparisons between signals cannot be accurate if one signal is amplitude saturated when 

acquired (i.e. amplitude clipped by a hardware limitation) and the other is not. Calculations such 

as energy or frequency content acquire errors when the signal is clipped. A reduction in trigger 

gain is a change that cannot be compensated for, because it involves loss of potentially relevant 

data that is now below the threshold for triggering data acquisition, but was above the threshold 

at the higher previous gain. When these changes have occurred, comparing parameters such as 

total or cumulative AE/energy or AE event rates is not wise. As noted in figure 5, a trigger gain 

change was implemented when executing AE system check tests (pencil lead break tests) and 

prior to the “2P” and “Failure” pressurization tests. 

3.0 AE Metrics 

AE systems collect structure-borne sound, typically not audible, in the ultrasonic frequency band 
6 

from 50 to 500 kHz, generated by dynamic processes such as damage initiation and growth in a 

structure. The technique can be sensitive to rapidly occurring displacements at scales as small as 

picometers [7]. That sensitivity can monitor pre-cursors to failure in real-time but comes at the 

cost of differentiating types of real damage from other sources of AE. 

For a typical test, data from an array of acoustic sensors located around the structure is acquired. 

The AE data can be used to locate the epicenter or source location of the damage by 

triangulation. This requires using the time-of-flight of the sound from the source to the different 

sensors along with knowledge of the sound wave velocity and sound path. Also typically 

recorded are the event time and various types of test parameters such as load or pressure. 

Therefore, one can plot the event rate (events per unit time) as well as the events as a function of 

those parameters such as load or pressure. 

Knowing the velocity of the AE wave propagation accurately is a major factor affecting the 

accuracy of event location calculations. The simplest unbounded materials are homogeneous and 

isotropic and have three modes of wave travel with differing velocities. Boundaries support 

many other modes of wave propagation (plate, surface, etc.). Wave propagation can also be 

dispersive, particularly in guided waves, where the velocity is a function of frequency. 

Anisotropy introduces velocity that varies with direction even for the same mode of propagation. 

Hence, wave velocity can be a function of where the wave is traveling in a material, what 

direction it is traveling, and its mode of travel. Complex, bounded, inhomogeneous, and 

anisotropic structures made from various materials can potentially create and sustain many 

different modes, frequencies and, hence, velocities of propagation. Damage introduces changes 

in material and mechanical properties that can further affect propagation velocity. 

Knowing the arrival times at the sensors accurately is another major factor affecting the accuracy 

of event location calculations. A low signal to noise ratio makes it more difficult to determine the 

signal arrival time accurately due to the interference of the noise with the signal. Determining the 

wave arrival times can also be complicated when there are numerous overlapping arrivals of 

different wave modes and reflections from different boundaries. Hence, the accuracy of event 

source location is a function of many variables. 

Yet another factor in source location accuracy is the sensor density, or the number of sensors 

near a given AE event. Typically, the more sensors that detect the event and record the arrival 

time, the better the estimate obtained for the source location. However, this comes at the cost of 

requiring additional sensors and instrumentation. For complex structures, one way to limit sensor 

count is to use a regional or zonal location method where each sensor is positioned in a central 

location of a region of critical or structural significance. For this sparse distribution of sensors, 

AE signals from a given source event are typically only detected by a few of the sensors located 

closest to the event. The location estimate is then a region or zone surrounding the first hit sensor 

and bounded by either the zones for surrounding sensors or an estimate of the maximum distance 

a signal might propagate and still be detectable. While this does not provide a pinpoint estimate 

of damage location, it provides a region in which other NDE techniques are used to more 

accurately locate and assess the damage. 

Another metric is the signal energy contained in the detected acoustic signal acquired at each 

sensor. This energy of an event is evaluated by calculating the signal energy, SE, from the signal 

as 
7 





n

i

i tVSE

1

2 , 

where Vi is the signal voltage, i is the time reference point, n is the number of time points in the 

signal, ∆t is the sampling time per point. In other words, the signal energy is the area under the 

voltage2

-time curve, with units such as V2μsec. However, energy in a physical sense is defined 

as Joules with dimensions of Watt·sec or V2

sec/ohm. To relate signal energy to physical energy 

E requires including the impedance in ohms of the load driven by the signal. The calculated AE 

signal energy is related to the energy released in the source event by effects from the transfer 

function of the material (attenuation, filtering, reflection, etc.) during propagation from source to 

sensor as well as from the transfer function of the sensor. So the trends in changes of the energy 

over time, load, or some other experimental variable is more valuable than efforts to get absolute 

calibrated values for each event. 

Material behavior that is related to change of AE over time and load, such as the Kaiser Effect 

and the Felicity effect, can be used to evaluate damage development. To paraphrase the 

Nondestructive Testing volume of the ASM Metals Handbook [8], initial loading of a virgin 

material typically produces more emission than subsequent loading. An “instantly-plastic” 

material will not produce any emission on later loadings, unless the previous load peak is 

exceeded or unless some type of deterioration occurred between loadings. This is the Kaiser 

effect. This behavior, for metallic materials, is due to AE generated by the 

acceleration/deceleration of large populations of dislocations under load. Hysteresis in these 

motions eventually leads to fracture. For composites, the discontinuities at the boundaries of 

different constituents (i.e., matrix and fibers) are much larger than the dislocations in metals. 

These discontinuities are stress raisers that contribute to degradation by development of 

distributed and distinctly different types of damage than metals. A typical damage progression 

scenario in a laminate composite is matrix microcracking, delaminations, disbonds, and finally 

fiber breakage. In the early stage of this progression, new surface is created internally without a 

significant loss in strength. However, as the specimen is strained, internal friction of these 

surfaces generates AE. If the geometry of the new surface developed at the peak strain is not 

completely locked in that configuration, it will emit AE at strains lower than the peak, when it is 

unloaded and reloaded. However, experience tells us that this typically is not indicative of 

intensive damage progression unless it occurs at a significantly lower fraction of the peak load or 

continues to occur during subsequent loading cycles. To paraphrase [8] again, structurally 

significant defects will tend to emit at loads below previous maximum, or, when the load is held 

constant. However, emission due to structure stabilization will tend not to recur. This is the 

Felicity effect and it is quantified by the Felicity ratio (FR): the load at which emission begins 

divided by the previous maximum load. The Kaiser effect is a FR of 1.0 or greater. For 

composite laminates, FR can systematically decrease under fatigue and FR less than 0.95 has 

been cause for rejection of FRP pressure vessels. [8] 

One of the issues with evaluating material behavior according to the Kaiser effect and Felicity 

ratio is that existence of AE is affected by the signal amplitude. More specifically, the signal to 

noise ratio of a test configuration, as noted previously, governs trigger gains that one uses, which 

affects the size of signal that triggers event acquisition. Therefore when comparing the results of 

different tests one must be cognizant of all the factors affecting each test. 
8 

A zone location approach is used for this analysis by comparing cumulative energy arriving at 

different sensors. In other words, trends in cumulative energy at a sensor indicate proximity and 

intensity of damage. Event locations were calculated, and abandoned, because the complexity of 

the structure and the resultant anisotropy made the results hard to interpret. In addition, because a 

series of loading tests occurred, the behavior is assessed according to the Kaiser Effect. 

4.0 Analysis 

In the following discussion, AE events may be described as occurring at a particular sensor. This 

does not necessarily mean that the source of the AE event is right at the sensor indicated. It is 

shorthand for the scenario that the sensor is likely to be the sensor closest to the AE source. The 

stress wave propagation impinging on that sensor has the largest initial amplitude of all the 

waveforms monitored at all the sensors and therefore has triggered the data acquisition of that 

“event”. 

The panels were almost acoustically independent of each other for several reasons: 

 Gaps between frames force acoustic coupling between frames from panel to panel via the 

bolted metal brackets. Optimum transfer across this joint is dependent upon matched 

acoustic impedances for all the components of the joint. This almost never happens to be 

the case. The presence of reflection and scattering surfaces from the bolt holes further 

tends to attenuate the transmission. 

 Elastomeric sealants reduce acoustic coupling and dampen propagation. 

 Bolted fabrication without adhesive (or with poorly bonded adhesive) reduces acoustic 

coupling. 

Analysis of the sensor verification tests (pencil lead breaks, section 4.5) supported this concept. 

Signal loss was approximately an order of magnitude across a few feet of panel. Subsequently, 

the pressure test data was reduced by splitting the data into five panel files, each containing only 

the channels for each of the five panels. Filtering by energy threshold eliminated events 

occurring on other panels that were not registering on that panel. However, this does not 

eliminate any event that may register on another panel. 

The most likely final failure sequence of the pressure cube, as noted in more detail elsewhere [3] 

is: 

 Metallic frame splice failure: A frame splice cracked. These plates connected one section 

of the frame on the crown panel to another section of the frame on the adjacent rib panel. 

The particular splice in question is the one that connects the section of frame under AE 

sensors 27, 28, and 29 to the section of frame under AE sensors 22 and 23. 

 Frame buckling: Once the splice plates failed, the detached rib frame of the speckled rib 

panel, subjected to increased in-plane bending, buckled. 

 Integral cap failure: Post-frame-buckle load redistribution causes adjacent integral cap to 

fail. 

 Catastrophic decompression in a fraction of a second: During this failure the cube rotated 

counterclockwise, as viewed from above. Sensors on the panel that blew out, were 

thrown up and out, as was the panel itself. This suggests that the panel failure initiated at 
9 

the left edge, air escaping to the left, quickly followed by detaching across the bottom 

and up the other side. 

The following analysis attempts to map the damage development that led to this failure. 

4.1		Checkout tests 1 and 2 (0.5P pressure where P=9.2 psi)

These two tests were performed to exercise all the COLTS testing equipment, systems, and test 

procedures. These tests were not expected to introduce any damage to the structure because the 

load of 0.5P was well below design pressure P. However, a few AE events did occur in both 

tests, but the amplitude was at least 2 orders of magnitude smaller than AE from later higher 

pressure tests. So, no intensive damage occurred, but some of these events possibly foretell the 

locations of the final failure. 

During checkout test 1, only four AE events occurred, near the peak pressure, as seen in the 

“Load profile and AE Events” plot in figure 6. The load profile (vs. time in test) is the red trace 

and AE events are indicated by the blue dots. Three events occurred at sensor 16 on the bulkhead 

panel that did not fail, and one at sensor 31 in the center of the crown panel, between the regions 

later discovered to have delaminated. Please note, as seen in figure 7 of the waveforms captured 

at all the sensors for event 1, there does not seem to be much signal on the sensors other than 16, 

indicating a source very local to that sensor. However, after filtering out frequency content below 

50 kHz, neighboring sensors 11, 14, 17, 18 do indicate some wave content other than noise. 

None of these sensors are near the panel that eventually failed but these events are either much 

smaller than the ones from the second checkout test or, as in the case of the channel 16 signal, 

are not particularly similar to that of a damage induced AE signal. Sensor 20 is showing a type of 

continuous AE not seen on the other sensors, although its frequency content (50-75 kHz) is much 

lower than the 400 kHz seen at sensor 16 and neighbors. 

The high amplifier gains for this test highlight the larger noise amplitude at frequencies below 50 

kHz on all sensors except 1, 13, 15 and 29. As noted previously sensors 1 and 13 are the only 

sensors mounted on a bay panel and may not have as much signal transmitted to them as the 

signal is carried on the stiffeners, hence the lower level of noise. All other sensors, as noted in 

Section 2, are opposite some internal structure such as a frame or stiffener. Channel (sensor) 15 

failed and remained that way for the entire series of tests. Channel 29 showed weak signal for the 

entire test series despite troubleshooting prior to the failure test. The cause of the signal 

weakness for that channel was never ascertained because much sensor and cable damage 

occurred during the failure test. 

During checkout test 2, only two AE events occurred, near the peak pressure, as seen in the 

“Load profile and AE events” plot in figure 8. These events show less localized (geographically) 

and longer (in time) activity, indicated by the number of sensors with significant waveform, as 

illustrated in figure 9 of event 1. The event registered significantly on sensors 19 to 26, as 

indicated by the red arrowhead line. These signals look more like typical damage-induced AE 

signals. Also, these sensors are on the speckle-pattern rib panel which suffered a metal splice 

fracture and frame buckling during later tests. Compared to the signals from checkout test 1, 

these signals are more likely to be from a distributed structural “damage” source. The response 

of event 2 is similar and has indications on sensors 1-6, 8, and 10. Note that sensors 1-6 are on 
10 

the panel that eventually blew out entirely during the failure test. Sensors 8 and 10, albeit on 

another panel, are closest to the edge contiguous to the “failure” panel. 

In summary: AE signals from these “checkout” tests are much smaller than failure events 

occurring in later tests, but the general locations of sensors showing AE-like indications are 

consistent with the location of the final catastrophic failures of the speckled rib and bulkhead 

panels. This shows potential for vehicle health monitoring by damage “precursor” AE. 

4.2 Test 3: 1P pressure

The load profile and AE event plot for the 1P test is shown in figure 10. The dotted green lines 

indicate the point in time where the pressure reaches the previous peak pressure achieved during 

the 0.5P tests. A Felicity Ratio of 1 indicates that no structurally destabilizing damage occurred 

during the 0.5 P tests The black vertical line indicates the beginning of the unloading. Events 

after this point indicate that some damage, which created new surface, had occurred during this 

test. As the load reduces, the friction of these surfaces coming together creates acoustic emission 

as they interfere. 

For this analysis, the cumulative energy of the events was calculated for each channel (sensor) 

and is plotted, versus time, for each panel as seen in figures 11-15. Red ovals indicate items of 

interest. In figure 11, the speckled bulkhead panel that blew-out during the failure test, has a 

significant rapid coordinated jump in energy occurring on all sensors around 1680 seconds (~ 8.6 

psi). Sensor 1 with the lowest cumulative energy is bay-located. This bulkhead panel had the 

next-to-the-highest cumulative energies per sensor. Figure 12 shows the cumulative energy plot 

for the gray rib panel, which is opposite the failure initiating speckled rib panel. It has the lowest 

sensor energies of all the panels. Now, note that the cumulative energies of the gray bulkhead 

panel, as seen in figure 13, includes the highest energies, with the largest increases occurring at 

1790 seconds right before unloading. This panel is farthest from the later failure locations, so 

hindsight allows us to suggest that these events were “structurally stabilizing” because their 

location is not involved in the final failure. However, they did occur on a bulkhead panel, 

suggesting that these types of panels may be more likely to fail and should be monitored more 

closely. Significant coordinated increases also occur on most sensors of that panel at 1610 

seconds (~ 8.0 psi) and 1660 seconds (~ 8.5 psi), especially sensors closer to the floor (16, 17, 

and 18). The smallest increases occurred at sensor 13 (bay-located), and 15 (failed). Figure 14 

shows a significant coordinated increase at 1720 seconds (~ 9.0 psi) of the cumulative energies 

of the speckled rib panel which later had initiated the final catastrophic failure. It also has the 

next lowest cumulative energies of the side panels. Comparable cumulative energies are noted 

for the crown panel as seen in figure 15. There is a coordinated increase around 1650 seconds (~ 

8.4 psi) with sensor 27 having the greatest increase by 1700 seconds. Sensor 27 is closest to the 

metal splice failure on the affected frame. The greatest total cumulative energy occurs on sensor 

31. After the 2P test, delaminations were discovered that run along the edges of the crown panel 

contiguous to the rib panels. Sensors 31 and 28 are closer to both of those edges than the other 

crown sensors. An explanation for the large total cumulative energy at 31 may be that the 

delamination damage (or delamination-initiating damage) was already occurring near both 

sensors 30 and 32. Although the energy at sensor 30 or 32 would be greatest from the damage 

nearest to each, the sum of the energy from both damage regions could be greater at the sensor in 

the middle, 31. This may also account for the higher total cumulative energy (vs. others on same 
11 

panel) at sensors 20 and 9 on the rib panels (adjacent to 30 and 32 respectively). In figure 14, 

sensor 20 obviously leads the others on the speckled rib panel. In figure 12 it is harder to see, but 

9 is at the top with 10 and 11 by the end of the test. 

In summary, the highest cumulative energies occurred at symmetric locations: the speckled 

bulkhead panel (that eventually failed) and gray bulkhead panel. The lowest energies (not 

counting the crown) occurred on the two rib panels. This leads one to surmise that even though 

the bulkhead panels have one more stiffening stringer than the rib panels, something about the 

construction of the bulkhead panels contributes more to damage development. As seen in all the 

figures 11-15 the existence of events during unloading indicates that some significant damage 

had occurred during this test. Assuming that the coordinated increases in cumulative energy are 

indicative of primary damage mechanisms the chronology of damage development is as follows: 

 1610 seconds (~ 8.4 psi) Bulkhead, gray: near bottom of panel 

 1650 seconds (~ 8.4 psi) Crown: front left (as if facing speckled bulkhead) 

 1660 seconds (~ 8.5 psi) Bulkhead gray: near bottom of panel 

 1680 seconds (~ 8.6 psi) Bulkhead speckled: left 

 1720 seconds (~ 9.0 psi) Rib speckled: all 

 1790 seconds (~ 9.2 psi) Bulkhead gray: lower left 

Also, evidence suggests that the later-discovered crown panel delaminations were already in 

development during this test. 

4.3		Test 4: 2P pressure

The load profile and AE event plot for the 2P test is shown in figure 16. Some randomness was 

seen in the test pressure values (red dots), so a smoothed version was calculated and is plotted as 

the black curve. The dotted green lines indicate the point in time where the pressure reaches the 

previous peak pressure achieved during the 1P tests. A Felicity Ratio of approximately 0.3 

supports the conclusion that significant damage did occur during the 1P test. Since the trigger 

gain was reduced 3 dB from the 1P test to the 2P test, this FR is especially indicting because AE 

may have been sensed even earlier if the higher gain had been used. The overall larger number 

of events during 2P test compared to 1P test indicates more rapid damage progression. The larger 

number of events during unloading after 1100 seconds (~ 20 psi), than has occurred during 

previous unloadings, also indicates more significant damage had occurred during the 2P test than 

previously. Figure 17 is a closer view of the same data with a region of interest shaded in yellow 

between 16-18 psi (885-950 sec) indicated because of increasing strain gauge activity at that 

time. 

As indicated by the red arrow on the cumulative energy plots for all the panels (figures 18-22), 

all sensors show significant rate increase in cumulative energy around 1060 seconds (~ 19 psi). 

The vertical black lines indicate the beginning of unloading. In figure 21 of the speckled rib 

panel that initiated failure during the failure test, a significant increase occurred during 

unloading; suggesting that the metallic frame splice failure began during the 2P test. Figures 23-

27 are close-ups of the cumulative energy plots with the yellow shaded region of strain gage 

activity-increase between 16 and 18 psi. The red ovals highlight significant increases in 

cumulative energy, most of them occurring well before the strain gages indicated damage. 
12 

During the 16-18 psi window, higher rates of increase (excluding the noted jumps) in the 

cumulative energy, occurred on the speckled bulkhead panel (figure 23) and the speckled rib 

panel (figure 26). Also, on all the panels except the gray bulkhead panel, around 18 psi (950 

seconds) at the end of the yellow shaded region, more rapid increases in cumulative energy 

occurred, especially on speckled rib and crown panels (noted for the delaminations that were 

discovered after this test). The largest cumulative energy at that point (around 1000 seconds, still 

approximately 18 psi) are on the speckled rib panel followed by the speckled bulkhead. Sensors 

19 and 20 leading on the speckled rib panel may be due to delamination growth in the crown 

away from sensor 30 toward 27. On the crown, sensor 27 lead with highest cumulative energy 

for most of the test, followed by sensors 28 and 31. Sensor 27 is the closest crown sensor to the 

metal frame splice that had failed, suggesting damage development that lead to the splice failing. 

Both sensors 28 and 31 being high indicate that both delaminations in the crown were growing. 

As noted previously sensor 29 was weak during the entire series of tests. 

All the noted indications suggest that the critical damage on the “failure initiation” panel 

(speckled rib) and the “blow-out” panel (speckled bulkhead) was already occurring and that 

structural integrity and remaining fatigue life had been severely compromised. 

Summary: 

The highest accumulated energies are from the speckled rib panel, followed by the speckled 

bulkhead panel. 

Chronology of indications: 

 820 seconds (~14 psi): Rib speckled, Ch. 19, 21, 22, 23, 24 rate increases (mid to upper 

right around impact) 

 820 seconds (~14 psi): Crown, Ch. 27, 30, 31, (left edge around back left corner, if facing 

bulkhead speckled) 

 835 seconds (~15 psi): Bulkhead speckled, Ch. 2-6 especially 3 and 5 

 835 seconds (~15 psi): Rib gray, Ch. 8, 10-12 especially 10 and 12 (lower right corner) 

 800-840 seconds (~13.5-15 psi): Bulkhead gray, no jumps but energy rate Ch. 16-18 

increasing past Ch 14 (bottom). 

 875 seconds (~15.5 psi): Rib speckled, Large energy release especially Ch 25 (bottom). 

 940 seconds (~16.5 psi): Rib gray and Rib VIC 

 950 seconds (~18 psi): Rib speckled, all and crown all except 29 and 32 

 950 seconds to end of test: rate increases at most all sensors, again especially on the 

speckled rib panel followed by the speckled bulkhead panel. 

4.4		Test 5: Failure

The load profile and AE event plot for the failure test is shown in figure 28. The dotted green 

lines indicate the points in time when AE starts and the time when the pressure reaches the 

previous peak pressure achieved during the 2P tests. A Felicity Ratio of approximately 0.6 

indicates that more damage did occur during the previous 2P test Failure occurs a little after 2000 

seconds at a pressure of approximately 48 psi (approximately 5 times the design pressure P=9.2 

psi). Figures 29-33 are the cumulative energy plots. All of the plots show a leveling off or 

reduction of AE rate around 1550-1600 seconds, which coincides with a pressure hold of 

approximately a minute. This illustrates the extent that AE event rate is affected by load rate. 
13 

Variations in load rate should be kept to a minimum during testing, especially when analyzing 

the AE for onset of different failure mechanisms. The overall behavior, more obvious in this test, 

but also occurring during previous tests, is that the sensors nearer the floor (especially 6, 12, 18) 

tend to lead in cumulative energy for each side panel. This could be due to the hatch or the 

different construction of the floor (relative to the other panels) either making more noise or 

increasing stresses at the edge of the other panels. 

The raw average AE data rate was much higher for this test than for the previous 2P test: 

 2P: 8209 events over 1300 sec = 6.3 event/sec 

 Failure: 50001events over 2000 sec = 25 events/sec (approx. 4X higher) 

Because of the increased AE rates, even after filtering, there does not seem to be many easily 

identified jumps in cumulative energy for this test as opposed to previous tests. This suggests a 

new regime of damage development. Previous tests had large events followed by periods that 

were more quiescent. In those tests, stress redistribution after stress relief allowed structural 

stability for a period of time before stress buildup exceeded local weakness. In this test, large and 

small events are more evenly distributed in time and location. Dynamic load redistribution 

almost immediately causes more damage. Failure is accelerating. 

Some sense of failure progression can be illustrated by tracking increases of cumulative energy 

rates. The activity on the right side of the speckled bulkhead (sensors 3 and 5) panel picks up 

around 1700 seconds (~38 psi) compared to the left side. The rates during the pressure hold, 

around 1600 sec (~ 36 psi), do not flatten as much as in the other panels. This suggests the panel 

is weaker or the stresses higher than the other panels, especially since there was also a short 

pressure hold at 1700 seconds. The entire gray rib panel also seems to be ramping up about the 

same time with sensors 10-12 (on the lower half) leading. Both the gray rib panel and gray 

bulkhead panel are more quiescent than the other panels, relatively speaking, if not including the 

near-floor sensors. Looking at the speckled rib panel where the frame buckled, four sensors were 

located equidistant from the prescribed impact on the integral cap: 21, 22, 23, and 24. We see a 

distinct increase at sensors 22 and 23 (located on that frame) at 1840 seconds (~ 41 psi) but not 

at sensors, 24 and 21, which are near the integral cap. Expecting the impact to have a consistent 

effect on the damage development and hence AE received at all four sensors, 21-24, the increase 

suggests that this is when either the frame metal splices started cracking and/or the frame was 

weakening. As to the question of whether the impact had any effect at all on initiating the failure, 

one should note that earlier, between 1400 and 1500 seconds (28-32 psi) the sensors around the 

impact (21-24) are starting to lead the others (not including the floor sensor 25). This is not 

definitive evidence, but it raises the possibility that the impact contributed to local weakening. 

This could have led to the frame or frame/panel interface weakening before the splice failure. For 

the crown, the central edge sensors (28 and 31) have highest cumulative energy. These sensors 

are closest to both of the crown delaminations, which already existed before this test, as noted 

previously. This suggests that the delaminations are either growing or rubbing to create AE. 

Summary: 

Damage Chronology 

 ~20 psi (1200 sec): Cumulative energy at bottom of gray bulkhead leads others. 

 ~24 psi (1300 sec): Cumulative energy of other bottom sensors (6, 12, 25) also leading. 
14 

 ~32 psi (1550-1600 seconds): Most all sensor energies start to level off due to pressure 

hold, except for the sensors of the two bulkhead. 

 ~38 psi (1700 sec): Most all sensor energies ramping up again, even though there was 

another pressure hold at 1700 seconds. 

 ~41 psi (1840 sec): Energy increase beginning on frame that buckled at sensors 22 and 

23. 

4.5		Pencil lead breaks: Pre‐test and Post 2P

The pencil lead break (PLB) is a standardized method [9] for testing AE sensor and 

instrumentation response. It consists of loading a graphite pencil lead in bending at a particular 

angle to a surface (governed by the addition of a guide ring to a mechanical pencil) as illustrated 

in figure 34. When the lead breaks, a step-unload of the surface generates stress waves 

propagating in the structure away from the point of loading. Figure 35 shows the resultant load 

traces of 10 PLB’s overlaid to illustrate the repeatability of the step unload []. The peak loads at 

breakage (notation 2) are within approximately ± 8% of -0.75 N (-0.17 lb) illustrating that it is 

reasonably consistent. One can also see that the load rate of the lead on the surface (notation 1) is 

much slower than the unload. The resultant wideband AE is more of a function of the response of 

the material, as illustrated by the reverberation of the load cell (notation 3) used to measure the 

PLB’s, than the application of the load. This is much more analogous to the stress release of 

damage development in the material than the classic coin tap test (and later variants) where the 

impact of the coin governs the response. In plate-like structures, the resultant AE plate wave 

propagation is very similar to that generated by microstructural damage development. 

 For the PRSEUS pressure cube, tests two series of PLB’s with the same data acquisition settings 

were executed: one series prior to any pressurization tests at Langley and one after the 2P test, 

but before the final failure test. The PLB’s were performed adjacent to every AE sensor. Data 

acquisition is the same as in a regular AE test such that wave propagation arriving at all the 

sensors was captured for each PLB at a single sensor. Prior to the failure test, the cube had to be 

tipped on its side to apply a damage initiating impact. Sensors 1 and 14 had been dislodged and 

reattached, hence, initiating the second series of PLB’s. The original intent of the PLB’s was 

only to check out the sensor attachment and instrumentation, so no particular care was taken to 

conduct the PLB’s in exactly the same locations for each series. However, they were conducted 

approximately an inch away from each sensor in the same manner as noted in the standard. 

The energy propagating to distant sensors is less likely to be affected by local variations in PLB 

location than by the types of damaged structure the wave propagation would have to traverse. 

Distributed damage that occurs early in the damage progression of composites tends to attenuate 

signal strength so typically energy received at a sensor would be expected to be lower postdamage. 

Two scenarios could explain the inverse behavior of energy being higher post-damage. There is 

an increased potential for unstable residual stresses, due to load cycling, that could release and 

add to the stress wave when tripped by that passing wave energy. Also, the cube is a combination 

of different geometries and local stiffness. In the virgin condition, the stress wave propagation 

away from a PLB has a particular distribution due to these variations. This distribution will be 
15 

affected, with damage localization, as many smaller cracks coalescence into larger 

discontinuities, possibly focusing the energy at some sensors. In particular, damage along the 

stiffener/panel interface could reduce the transfer of wave propagation into the stiffener, thereby 

increasing the amplitude at the sensor, given everything else equal. 

For these tests, an indication of the energies collected at all sensors due to one PLB is made by 

calculating the RMS (root-mean-square) value for each collected waveform. This is plotted in 

figures 36-37 with respect to each sensor. Figure 36, for a PLB at sensor 13, shows the expected 

behavior of attenuation from damage development because all the RMS values prior to testing 

(blue X) are higher than after the 2P test (red X). To accentuate the sometimes small differences 

in RMS value, the scale for RMS is such that the values collected at the PLB sensor are off the 

graph. Figure 37 illustrates the opposite behavior where the post test RMS values are larger than 

the pretest. Figures 38-41 are spectra of the waveforms collected at a PLB sensor and its nearest 

neighbors. The pre-test to post comparison of spectra of the “expected-behavior” responses, seen 

in figures 38 and 39, respectively, shows a decrease in energy post-2P across most all 

frequencies. Figures 40 and 41, illustrating the inverse behavior for the PLB at sensor 1, show 

the increase occurring predominantly in the frequencies below 150 kHz. Note that sensor 1 is on 

the panel that blew out during failure. 

Therefore, comparing the number of times sensors had signal energy increased (i.e. more energy 

received post-2P than before testing) and tabulating by panel we have the following results: 

 Bulkhead (VIC, failed) 34 

 Rib (VIC, impacted) 10 

 Crown 9 

 Bulkhead (gray) 6 

 Rib (gray) 4 

Note the interesting correspondence of behavior with damage. The bulkhead panel that blew out 

had the most “increased energy” behavior and the rib panel that initiated failure had the second 

most. In third is the crown panel, which had major delaminations. 

Summary: 

 The locations of the sensors with “increasing” energy PLB behavior seem consistent with 

major structural failures of the speckled rib and bulkhead. 

 PLB response may be useful as an indicator of structural damage on a large scale. 

 Identification of particular frequency bands that may be more sensitive to damage merits 

further investigation. 
16 

5.0 Lessons Learned, Summary, and Conclusions 

Lessons Learned: 

 The AE event rate is affected by load rate. Variations in load rate should be kept to a 

minimum during testing, especially when trying to analyze the AE for onset of different 

failure mechanisms. 

Summary and Conclusions: 

 The AE signals from the 0.5P “checkout” tests are much smaller than events occurring in 

later tests, but the locations of sensors showing AE-like indications are generally 

consistent with the location of the final catastrophic failures of the speckled rib and 

bulkhead panels. 

 In the 1P test, the highest cumulative energies occurred at symmetric locations: the 

speckled bulkhead panel (that eventually failed) and the gray bulkhead panel. The lowest 

energies (not counting the crown) occurred on the two rib panels. This leads one to 

surmise that even though the bulkhead panels have one more stiffening stringer than the 

rib panels, something about the construction of the bulkhead panels contributes more to 

damage development. A Felicity Ratio of 1 indicates that no structurally destabilizing 

damage occurred during the 0.5 P tests. However, the existence of events during 

unloading indicates that some significant damage had occurred during the 1P test. Also, 

evidence suggests that the later-discovered crown panel delaminations were already in 

development during this test. 

 For the 2P test, in general, a Felicity Ratio of approximately 0.3 supports the conclusion 

that significant damage did occur during the 1P test. The larger number of events during 

unloading, than had occurred during previous unloadings, indicates more significant 

damage had occurred during the 2P test than previously. All indications suggest that 

critical damage on the “failure initiation” panel (speckled rib), metal frame splice, and the 

“blow-out” panel (speckled bulkhead) was occurring. Crown panel delaminations were 

growing. 

 For the Failure test, again, a Felicity Ratio of approximately 0.6 indicates that more 

damage did occur during the previous 2P test. The crown delaminations are continuing to 

emit AE. Indications raise the possibility that the prescribed impact contributed to 

damage which led to the frame weakening preceding the splice failure. 

 PLB’s & AE in early tests (“precursor” AE): These types of tests may indicate the 

appropriate regions that will ultimately fail. Closely monitoring those regions may prove 

to be an efficient and effective means of structural health evaluation. 
17 

6.0 References 

[1] Vicroy, D.D. Jegley D.C., Cosentino G. “NASA Blended-Wing-Body Research Overview”. 

67th Annual International Conference on Mass Properties Engineering. Seattle, Washington 2008 

[2] Velicki, A. “Damage Arresting Composites for Shaped Vehicles,” NASA CR-2009-215932, 

Sept. 2009. 

[3] Yovanof, N.P., and Lovejoy, A.E.,.Baraja, J., Gould, K, "Design, Analysis and Testing of a 

PRSEUS Pressure Cube to Investigate Assembly Joints," 2012 Aircraft Airworthiness and 

Sustainment Conference, April 3, 2012, Baltimore, MD. 

[4] Yovanof, N.P. et al, “Pressure Cube Test Specification” Boeing Research and Technology, 

Document ZA153690, 2011 

[5] Sutton, M., Cheng, M., Peters, W., Chao, Y., and McNeill, S.,“Application of an Optimized 

Digital Correlation Method to Planar Analysis,” Image and Vision Computing, Vol. 4, No. 3, 

1986, pp. 143–151. doi:10.1016/0262-8856(86)90057-0 

[6] Schreier, H., Braasch, J., and Sutton, M., “Systematic Errors in Digital Image Correlation 

Caused by Intensity Interpolation,” Optical Engineering, Vol. 39, No. 11, 2000, pp. 2915–2921. 

doi:10.1117/1.1314593

[7] Horne, M. R. “Rayleigh Wave Acoustic Emission during Crack Propagation in Steel”. PhD 

Dissertation, Virginia Polytechnic Institute and State University. 2003. 

[8] ASM handbook Vol. 17 Nondestructive Evaluation and Quality Control: Acoustic Emission 

Inspection. 

[9] ASTM E976 "Standard Guide for Determining the Reproducibility of Acoustic Emission 

Sensor Response“ 
18 

Figure 1. Evolution of BWB fuselage design [1]

Figure 2. Schematic of Pressure cube with approximate locations of some of the AE sensors (red dots) mounted on

the crown panel and other notations in red by the author. [4]

Stitched Hat

Infused Concept

PRSEUS

Stitched Pultruded Rods

787 Baseline

Co-bonded Skin/Stringer

Design Philosophy

No Damage Arrestment

No-Growth Design Criteria

Lower Operating Strains

Leads to Higher Weight

Design Philosophy

Stitching Arrests Damage

Arrest Damage at Interfaces

Higher Operating Strains

Leads to Lighter Airframe

Design Philosophy

Stitching Arrests Damage

Able to Design Beyond No-Growth 

Design Criteria (Like Metals)

Damage Growth not Critical

Higher 0-deg Fiber Orientation

(Higher Operating Stresses)

Better Neutral Axis Location

Leads to Lighter Airframe

29

27

28

30

31

32

Sensors 19-26 on this panel

Frame rail that buckled
19 

Figure 3. Locations of AE sensors on the panels of the pressure cube

Rib Panel

(Speckled)

Bulkhead panel

(Speckled)

Rib Panel

(Gray)

Bulkhead panel

(Gray)

Crown

30 3231

27 28 29

1

2 3

4

6

5

7

8

9

10

12

11

18

17 16

15 14

13

20

19

26

21

25

24

23 22

Stringer stitches

Floor cap stitches

Frame stitches

Integral cap stitches

Applied Impact

Regions of delamination found by UT post‐2P test

Frame that buckled

Metal splice failure

Panel that blew out

Bulkhead panel notches for

rib panel stringer fitment

AE sensor location color code:

Mounted on bay

Mounted at stringer

Mounted at stringer‐integral cap intersection

Mounted at stringer‐frame intersection
20 

Figure 4.  Installation of one AE sensor on the crown panel

Figure 5. AE Instrumentation Settings  

Test Signal Trigger Data Acquisition (Sampling) Parametric 

(Sampling)

Description [and ID] Date Gain 

(dB)

HP Filter 

(kHz)

Gain 

(dB)

BP Filter 

(kHz)

Rate 

(MhZ)

Range 

(V) Points Pretrig Rate 

(Hz)

Range 

(V)

Pencil lead breaks to 

check AE channels 7/29/2011 24 20 21 50 - 750 1 +/- 1 2048 256

Pneumatic pressure 0.5P

(P = 9.2 psi) [Checkout 1] 7/29/2011 48 20 33 50 - 750 1 +/- 1 2048 256 0.5-1* +/- 10

Pneumatic pressure 0.5P 

[Checkout 2] 7/29/2011 48 20 33 50 - 750 1 +/- 1 4096 256 0.5-1* +/- 10

Pneumatic pressure 1P 

[Shaker 1P] 7/29/2011 48 20 33 50 - 750 1 +/- 1 4096 256 0.5-1* +/- 10

Pneumatic pressure 2P 

[Pressure 2P] 7/29/2011 36 20 30 50 - 750 1 +/- 1 4096 256 0.5-1* +/- 10

Post-“applied impact” 

PLB’s 8/9/2011 24 20 21 50 - 750 1 +/- 1 2048 256

Failure @ 48 psi 

[Pressure failure] 8/10/2011 24 20 30 50 - 750 1 +/- 1 4096 256 0.5-1* +/- 10
21 

Checkout 1 test: PRSEUS cube 

Time (sec)

800 1000 1200 1400 1600 1800 2000 2200 2400

Pressure (psi)

0

1

2

3

4

5

6

Event #

0

1

2

3

4

5

Pressure

Event

Figure 6. Checkout test 1 (0.5P, P=9.2 psi) Load profile and AE events

Figure 7. Checkout test 1 waveforms from AE event #1 captured at all the sensors

Sensor 1

Sensor 13

Sensor 32

Sensor 9 Sensor 16

Etc.

Sensor 17

Sensor 25

Sensor 8
22 

Checkout 2 test: PRSEUS cube 

Time (sec)

400 600 800 1000 1200 1400 1600

Pressure (psi)

0

1

2

3

4

5

6

Event #

0

1

2

3

Pressure

Event

Figure 8. Checkout test 2 (0.5P) Load profile and AE events

Figure 9. Checkout test 2 waveforms from AE event #1 captured at all the sensors

Sensor 1

Sensor 32

Sensor 9 Sensor 16

Etc.

Sensor 17

Sensor 25

Sensor 8

Sensor 26

Sensor 19 Sensor 24
23 

Test 1P Pressure and AE

Time (sec)

500 1000 1500 2000

Pressure (psi)

0

2

4

6

8

10

Event #

100

200

300

400

500

600

700

Time vs Pressure (psi) 

Event Time vs Events 

0.5P pressure

Begin unload

Figure 10. Pressure 1P Test Load profile and AE events

Bulkhead (VIC) panel AE Cumulative Energy

Time (sec)

1200 1400 1600 1800 2000

Energy (V2

sec)

0

2000

4000

6000

8000

10000

Ch 1 

Ch 2 

Ch 3 

Ch 4 

Ch 5 

Ch 6 

Begin unload

Figure 11. Pressure 1P Test: Speckled bulkhead panel AE energy
24 

Rib (gray) panel AE Cumulative Energy

Time (sec)

1200 1400 1600 1800 2000

Energy (V2

sec)

0

2000

4000

6000

8000

10000

Ch 7 

Ch 8 

Ch 9 

Ch 10 

Ch 11 

Ch 12 

Begin unload

Figure 12. Pressure 1P Test: Gray rib panel AE energy

Bulkhead (gray) panel AE Cumulative Energy

Time (sec)

1200 1400 1600 1800 2000

Energy (V2

sec)

0

2000

4000

6000

8000

10000

Ch 13 

Ch 14 

Ch 15 

Ch 16 

Ch 17 

Ch 18 

Begin unload

Figure 13. Pressure 1P Test: Gray bulkhead panel AE energy
25 

Rib (VIC) panel AE Cumulative Energy

Time (sec)

1200 1400 1600 1800 2000

Energy (V2

sec)

0

2000

4000

6000

8000

10000

Ch 19 

Ch 20 

Ch 21 

Ch 22 

Ch 23 

Ch 24 

Ch 25 

Ch 26 

Begin unload

Figure 14. Pressure 1P Test: Speckled rib panel AE energy

Crown panel AE Cumulative Energy

Time (sec)

1200 1400 1600 1800 2000

Energy (V2

sec)

0

2000

4000

6000

8000

10000

Ch 27 

Ch 28 

Ch 29 

Ch 30 

Ch 31 

Ch 32 

Begin unload

Figure 15. Pressure 1P Test: Crown panel AE energy
26 

PRSEUS Cube Test 4 "2P": Pressure

Time (sec)

0 200 400 600 800 1000 1200 1400

Pressure (psi)

0

5

10

15

20

25

Event #

0

2000

4000

6000

8000

10000

Test Pressure (psi) 

Smoothed Pressure

Event Time vs Events Begin unload

Figure 16. Pressure 2P Test Load profile and AE events

PRSEUS Cube Test 4 "2P": Pressure

Time (sec)

800 850 900 950 1000

Pressure (psi)

0

5

10

15

20

25

Test Pressure (psi) 

Smoothed Pressure

Figure 17. Pressure 2P Window of increasing strain gage activity
27 

Bulkhead (speckled) panel AE Cumulative Energy

Time (sec)

400 600 800 1000 1200 1400

Energy (V2

sec)

0

20000

40000

60000

80000

Ch 1 

Ch 2 

Ch 3 

Ch 4 

Ch 5 

Ch 6 

Begin unload

Figure 18. Pressure 2P Test: Speckled bulkhead panel AE energy

Rib (gray) panel AE Cumulative Energy

Time (sec)

400 600 800 1000 1200 1400

Energy (V2

sec)

0

20000

40000

60000

80000

Ch 7 

Ch 8 

Ch 9 

Ch 10 

Ch 11 

Ch 12 

Begin unload

Figure 19. Pressure 2P Test: Gray rib panel AE energy
28 

Bulkhead (gray) panel AE Cumulative Energy

Time (sec)

400 600 800 1000 1200 1400

Energy (V2

sec)

0

20000

40000

60000

80000

Ch 13 

Ch 14 

Inactive

Ch 16 

Ch 17 

Ch 18 

Begin unload

Figure 20. Pressure 2P Test: Gray bulkhead panel AE energy

Rib (speckled) panel AE Cumulative Energy

Time (sec)

400 600 800 1000 1200 1400

Energy (V2

sec)

0

20000

40000

60000

80000

Ch 19 

Ch 20 

Ch 21 

Ch 22 

Ch 23 

Ch 24 

Ch 25 

Ch 26

Begin unload

Figure 21. Pressure 2P Test: Speckled rib panel AE energy
29 

Crown panel AE Cumulative Energy

Time (sec)

400 600 800 1000 1200 1400

Energy (V2

sec)

0

20000

40000

60000

80000

Ch 27 

Ch 28 

Ch 29 

Ch 30 

Ch 31 

Ch 32 

Begin unload

Figure 22. Pressure 2P Test: Crown panel AE energy

Bulkhead (speckled) panel AE Cumulative Energy

Time (sec)

600 700 800 900 1000

Energy (V2

sec)

0

10000

20000

30000

40000

50000

Ch 1 

Ch 2 

Ch 3 

Ch 4 

Ch 5 

Ch 6 

Figure 23. Pressure 2P Test: Close‐up of speckled bulkhead panel AE energy near window of strain gage activity  
30 

Rib (gray) panel AE Cumulative Energy

Time (sec)

600 700 800 900 1000

Energy (V2

sec)

0

10000

20000

30000

40000

50000

Ch 7 

Ch 8 

Ch 9 

Ch 10 

Ch 11 

Ch 12 

Figure 24. Pressure 2P Test: Close‐up of gray rib panel AE energy near window of strain gage activity

Bulkhead (gray) panel AE Cumulative Energy

Time (sec)

600 700 800 900 1000

Energy (V2

sec)

0

10000

20000

30000

40000

50000

Ch 13 

Ch 14 

Inactive

Ch 16 

Ch 17 

Ch 18 

Figure 25. Pressure 2P Test: Close‐up of gray bulkhead panel AE energy near window of strain gage activity
31 

Rib (speckled) panel AE Cumulative Energy

Time (sec)

600 700 800 900 1000

Energy (V2

sec)

0

10000

20000

30000

40000

50000

Ch 19 

Ch 20 

Ch 21 

Ch 22 

Ch 23 

Ch 24 

Ch 25 

Ch 26

Figure 26. Pressure 2P Test: Close‐up of speckled rib panel AE energy near window of strain gage activity

Crown panel AE Cumulative Energy

Time (sec)

600 700 800 900 1000

Energy (V2

sec)

0

10000

20000

30000

40000

50000

Ch 27 

Ch 28 

Ch 29 

Ch 30 

Ch 31 

Ch 32 

Figure 27. Pressure 2P Test: Close‐up of crown panel AE energy near window of strain gage activity
32 

PRSEUS Cube Failure test: Pressure and AE

Time (sec)

0 500 1000 1500 2000 2500

Pressure (psi)

0

10

20

30

40

50

60

Event #

0

10000

20000

30000

40000

50000

60000

Pressure

Events 

Figure 28. Failure Pressure Test Load profile and AE events

Bulkhead (speckled) panel AE Cumulative Energy

Time (sec)

800 1000 1200 1400 1600 1800 2000 2200

Energy (V2

sec)

0

2e+4

4e+4

6e+4

8e+4

1e+5

Ch 1 

Ch 2 

Ch 3 

Ch 4 

Ch 5 

Ch 6 

Failure

Figure 29. Failure Pressure Test: Speckled bulkhead panel AE energy
33 

Rib (gray) panel AE Cumulative Energy

Time (sec)

800 1000 1200 1400 1600 1800 2000 2200

Energy (V2

sec)

0

2e+4

4e+4

6e+4

8e+4

1e+5

Ch 7 

Ch 8 

Ch 9 

Ch 10 

Ch 11 

Ch 12 

Failure

Figure 30. Failure Pressure Test: gray rib panel AE energy

Bulkhead (gray) panel AE Cumulative Energy

Time (sec)

800 1000 1200 1400 1600 1800 2000 2200

Energy (V2

sec)

0

2e+4

4e+4

6e+4

8e+4

1e+5

Ch 13 

Ch 14 

Inactive

Ch 16 

Ch 17 

Ch 18 

Failure

Figure 31. Failure Pressure Test: Gray bulkhead panel AE energy
34 

Rib (speckled) panel AE Cumulative Energy

Time (sec)

800 1000 1200 1400 1600 1800 2000 2200

Energy (V2

sec)

0

2e+4

4e+4

6e+4

8e+4

1e+5

Ch 19 

Ch 20 

Ch 21 

Ch 22 

Ch 23 

Ch 24 

Ch 25 

Ch 26 

Failure

Figure 32. Failure Pressure Test: Speckled rib panel AE energy

Crown panel AE Cumulative Energy

Time (sec)

800 1000 1200 1400 1600 1800 2000 2200

Energy (V2

sec)

0

2e+4

4e+4

6e+4

8e+4

1e+5

Ch 27 

Ch 28 

Ch 29 

Ch 30 

Ch 31 

Ch 32 

Failure

Figure 33. Failure Pressure Test: Crown panel AE energy
35 

Figure 34. Pencil lead break configuration [9]

Figure 35. Pencil lead break repeatability [7]
36 

PRSEUS cube PLB sensor 13

Sensor

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

RMS (zero baseline) = St. Dev.

0.000

0.002

0.004

0.006

0.008

0.010

Pretest

Post 2P vs Co

Figure 36. Change in response with damage accumulation of PLB at sensor 13  

PRSEUS cube PLB sensor 01

Sensor

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

RMS (zero baseline) = St. Dev.

0.000

0.002

0.004

0.006

0.008

0.010

Pretest 

Post 2P 

Figure 37. Change in response with damage accumulation of PLB at sensor 1
37 

Spectrum of PLB @ sensor 13 pretest

Frequency (kHz)

0 100 200 300 400 500

Amplitude (V)

0.00

0.05

0.10

0.15

0.20

Freq.(kHz) vs Channel 13 

Freq.(kHz) vs Channel 14 

Freq.(kHz) vs Channel 15 

Freq.(kHz) vs Channel 16 

Freq.(kHz) vs Channel 17 

Freq.(kHz) vs Channel 18 

Figure 38. Spectra of initial response of PLB at sensor 13

Spectrum of PLB @ sensor 13 post 2P

Frequency (kHz)

0 100 200 300 400 500

Amplitude (V)

0.00

0.05

0.10

0.15

0.20

Freq.(kHz) vs Channel 13 

Freq.(kHz) vs Channel 14 

Freq.(kHz) vs Channel 15 

Freq.(kHz) vs Channel 16 

Freq.(kHz) vs Channel 17 

Freq.(kHz) vs Channel 18 

Figure 39. Spectra of response with damage accumulation of PLB at sensor 13
38 

Spectrum of PLB @ sensor 01 pretest

Frequency (kHz)

0 100 200 300 400 500

Amplitude (V)

0.00

0.05

0.10

0.15

0.20

Freq.(kHz) vs Channel 1 

Freq.(kHz) vs Channel 2 

Freq.(kHz) vs Channel 3 

Freq.(kHz) vs Channel 4 

Freq.(kHz) vs Channel 5 

Freq.(kHz) vs Channel 6 

Figure 40. Spectra of initial response of PLB at sensor 1

Spectrum of PLB @ sensor 01 post 2P

Frequency (kHz)

0 100 200 300 400 500

Amplitude (V)

0.00

0.05

0.10

0.15

0.20

Freq.(kHz) vs Channel 1 

Freq.(kHz) vs Channel 2 

Freq.(kHz) vs Channel 3 

Freq.(kHz) vs Channel 4 

Freq.(kHz) vs Channel 5 

Freq.(kHz) vs Channel 6 

Figure 41. Spectra of response with damage accumulation of PLB at sensor 1
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Evaluation of Acoustic Emission SHM of PRSEUS Composite Pressure 

Cube Tests 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Horne, Michael R.; Madaras, Eric I.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

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National Aeronautics and Space Administration

Washington, DC 20546-0001

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 REPORT NUMBER

L-20227

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NASA

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14. ABSTRACT

A series of tests of the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) pressure cube were conducted during 

third quarter 2011 at NASA Langley Research Center in the Combined Loads Test facility (COLTS). This is a report of the 

analysis of the Acoustic Emission (AE) data collected during those tests. The AE signals of the later tests are consistent with

the final failure progression through two of the pressure cube panels. Calibration tests and “damage precursor” AE indications,

from preliminary checkout pressurizations, indicated areas of concern that eventually failed. Hence those tests have potential 

for vehicle health monitoring. 

15. SUBJECT TERMS

Acoustic emission; Blended wing body; Composite pressure cube; PRSEUS; Signal energy rate

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01- 05 - 2013