October 2013

NASA/TM–2013-218049

Evaluation of the Behavior of Technova 

Corporation Rod-Stiffened Stitched 

Compression Specimens

Dawn C. Jegley

Langley Research Center, Hampton, Virginia
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October 2013

NASA/TM–2013-218049

Evaluation of the Behavior of Technova 

Corporation Rod-Stiffened Stitched 

Compression Specimens

Dawn C. Jegley

Langley Research Center, Hampton, Virginia
Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076-1320

443-757-5802
1

Abstract 

Under Space Act Agreement 1347 between NASA and Technova Corporation, 

Technova designed and fabricated two carbon-epoxy crippling specimens and NASA 

loaded them to failure in axial compression. Each specimen contained a pultruded rod 

stiffener which was held to the specimen skin with through-the-thickness stitches. One 

of these specimens was designed to be nominally the same as pultruded rod stitched 

specimens fabricated by Boeing under previous programs. In the other specimen, the 

rod was prestressed in a Technova manufacturing process to increase its ability to carry 

compressive loading. Experimental results demonstrated that the specimen without 

prestressing carried approximately the same load as the similar Boeing specimens and 

that the specimen with prestressing carried significantly more load than the specimen 

without prestressing. 

Background 

Under Space Act Agreement 1347 between NASA and Technova Corporation, 

Technova designed and fabricated two carbon-epoxy crippling specimens and NASA 

loaded them to failure in axial compression. Each specimen contained a single 

pultruded carbon rod stiffener aligned in the loading direction. Stiffener flanges were 

stitched to the skin of the specimen so no fasteners were needed. 

One specimen was nominally the same as stringer crippling specimens fabricated by 

Boeing and tested by Boeing and NASA (ref. 1, 2) under the Subsonic Fixed Wing and 

Environmentally Responsible Aviation Projects. The rod in the second specimen was 

prestressed during the manufacturing process in an approach developed by Technova 

(ref. 3). This prestressing was intended to increase the stiffener’s ability to carrying 

compressive loading. 

The pultruded rod stitched concept has been demonstrated to provide excellent 

structural efficiency in tension and compression based on the NASA and Boeing work. 

That work served as a starting point and baseline for the Technova work. The 

prestressing concept, as applied herein, is intended to further increase the structural 

efficiency of carbon panels that would primarily be loaded in compression throughout 

their life. The purpose of this document is to describe the results of the NASA tests on 

the Technova specimens and relate this data to the behavior of Boeing specimens. 

Specimen Description and Preparation 

The cross section geometry of a pultruded rod stitched crippling specimen is shown 

in figure 1. The two specimens had nominally the same geometry. Photographs of the 

Technova specimens are shown in figures 2 and 3. Specimens were approximately 17 

inches long and 6 inches wide. Stiffeners were approximately 1.5 inches tall and the 

skin was approximately 0.12 inches thick. Specimen dimensions are described in Table 

1. 
2

Table 1. Specimen Dimensions 

 Prestressed Non-prestressed 

Average skin thickness (in.) 0.128 0.117 

Skin thickness range (in.) 0.118 to 0.136 0.106 to 0.123 

Width (in.) 6.1 5.9 

Average stiffener diameter 

(in.) 

0.48 0.48 

Length (in.) 16.8 17.0 

Both Technova specimens were delivered to NASA with strain gages attached and 

the ends potted in one inch of an epoxy compound on each loaded end. Ten strain 

gages were applied to each specimen. All strain gages were axial, with gages on the 

skin, flanges, stiffener web, and stiffener rod overwrap as shown in the figures 2 and 3. 

Prior to testing, the unstiffened side of each specimen was painted with a black and 

white speckle pattern to allow the use of a digital image correlation (DIC) system during 

loading to obtain full field displacements and strains of the unstiffened surface. A 

photograph of the prestressed specimen in the test machine is shown in figure 4, where 

the speckle pattern is visible. Four displacement transducers were used with each 

specimen. Three displacement transducers were positioned on the crosshead, as 

shown in figure 4, to monitor the motion of the crosshead so that the uniformity of the 

shortening could be determined and any rotation of the crosshead could be evaluated. 

These transducers were located 1 inch in each direction from the corners of the platen, 

or 11 inches in front of and behind the centerline of the platen and 10.5 inches from the 

centerline of the stiffener web. The fourth displacement transducer was positioned 

perpendicular to the stiffener at the midlength position, parallel to the plane of the 

crosshead. This transducer was used to measure the rolling of the stiffener. 

Side supports used to restrict the motion of the unloaded edges were positioned on 

each unloaded edge 2.5 inches above and below the axial centerline of the specimen, 

as shown in figure 4. A sketch of these side supports is shown in figure 5, where the slot 

for the specimen skin is shown. Supports were clamped to the specimen restricting both 

lateral in-plane motion and out-of-plane motion. Shortening should not have been 

restricted by the side supports. The nuts on the supports were tightened “finger tight.” 

The specimen skin thickness was within 0.03 inches of the slot thickness, so only a very 

small amount of local deformation out-of-plane could occur. Because of the contact 

between the supports and the specimen skin included approximately 0.2 inches from the 

edge of the specimen, this condition resembled a clamped edge. In the vertical direction 

gaps were allowed between the support and the potting at each end of the specimen to 

allow the specimen to shorten without loading the supports. 

Experimental Process 

Each specimen was loaded in compression at a rate of 0.008 inches of shortening 

per minute. For both specimens, strain gage and displacement measurements were 

recorded five times per second during loading. Digital image correlation images were 

recorded once each second. 
3

Experimental Results 

The displacements measured by all four transducers are shown in figures 6 and 7, 

respectively, for the non-prestressed and prestressed specimens. The shortening 

results are shown as the red, blue and green curves and the lateral motion is shown as 

the black curve. 

For both specimens, the displacement transducers measuring shortening showed 

significant nonlinearity in the load-shortening curves for loading less than approximately 

15,000 lb. However, for loading greater than 15,000 lb, the load-shortening relationship 

is linear. This initial nonlinearity is believed to be caused by irregularities in the potting 

and load introduction. The three shortening measurements were initially almost identical 

and remained within approximately 10% of each other throughout the test. For both 

specimens, some motion of the stiffener occurred but that motion does not appear to 

indicate stiffener rolling since it remained very gradual motion and never exceeded 0.02 

inches for the non-prestressed specimen and 0.01 inches for the prestressed specimen. 

Digital image correlation full-field out-of-plane displacement results are shown in 

figures 8 and 9, respectively, for the non-prestressed and prestressed specimens. 

White regions within the images are locations where data could not be obtained because 

of hardware between the camera and the specimen surface such as restraints, clamps, 

and strain gage wires. Anything attached to the surface or in the field of view of either 

camera, or casting a shadow on the test article, can limit the area for which DIC data can 

be reliably collected. Load levels of approximately 500, 10,000, 12,100, 41,000, and 

44,000 lb for the non-prestressed specimen are shown. Load levels of approximately 

500, 5,000, 15,000, 61,000, and 65,000 lb are shown for the prestressed specimen. 

Full-field axial strain results are shown in figures 10 and 11, respectively, for the nonprestressed and prestressed specimen for load levels immediately prior to failure. 

Specifically, load levels of approximately 44,300 lb for the non-prestressed specimen 

and approximately 65,000 lb for the prestressed specimen are shown. Again, white 

regions within the images are locations where data could not be obtained because of 

hardware between the camera and the specimen surface such as restraints, clamps, 

and strain gage wires. Isolated peaks in the strains immediately adjacent to the white 

areas are also inaccuracies based on the obstructed areas. Surface strains in the skin 

on the unstiffened side did not exceed a compressive strain of 0.006 in/in., indicating 

that failure did not initiate due to excessive strain in the skin. 

The maximum load for the non-prestressed specimen was 44,339 lb. Failure 

occurred through the skin and stiffener near the potting, as shown in figure 12. The 

maximum recorded load for the prestressed specimen was 64,955 lb. Failure occurred 

through the skin near the potting and through the rod overwrap both near the potting and 

approximately 3.5 inches away from the top failure, as shown in figure 13. 

Comparisons Between Technova and Boeing Specimens 

The average of the measured shortening results are shown in figure 14 for a typical 

Boeing rod-stiffened crippling specimen and both Technova specimens. The Boeing 

specimen was geometrically similar to the Technova specimen with a width of 6.0 

inches, a stiffener diameter of 0.48 inches and a length of 18 inches. The Boeing 

specimens were assumed to be nominally the same as non-prestressed Technova 

specimen. The skin of the Boeing specimens was 0.055 inches thick so the Technova 

specimens were approximately twice the thickness of the skin of the Boeing specimens. 
4

However, relatively little load would typically be carried through the skin, so this 

difference is expected to have little influence on the load carryign capbility of the 

specimens. 

The Boeing specimen displayed a more linear load-shortening behavior than the 

equivalent Technova specimen, but the maximum load values were similar. Two pristine 

Boeing crippling specimens and two fatigued Boeing crippling specimens (ref. 3) have 

been tested with the failure loads shown in Table 2, resulting in an average failure load 

of 45,638 lb. The Technova non-prestressed specimen failed at a load level typical of 

the Boeing specimens. 

Table 2. Failure Loads 

Specimen ID Manufacturer Test type Failure load (lb) 

Rod 1* Boeing Static 48,500 

Rod 2* Boeing Static 41,400 

R3* Boeing Fatigue/static 45,000 

R4* Boeing Fatigue/static 47,654 

Non-prestressed Technova Static 44,339 

Prestressed Technova Static 64,994 

*Reference 3 

The failure load of the prestressed Technova specimen was significantly greater than 

that of the non-prestressed specimens. Comparing the prestressed specimen to the 

average of the Boeing specimens or to the Technova non-prestressed specimen, the 

prestressed specimen supported over 40 percent more load than specimens without 

prestressing, as shown in figure 15. Boeing specimens are represented in the figures as 

solid green bars and their average is the green hatched bar. Technova specimens are 

represented by the solid red bars. With only one prestressed specimen, definitive 

conclusions cannot be drawn, however, the prestressing appears to have a significant 

impact on the load-carrying ability of the specimen. 

Concluding Remarks 

Two carbon-epoxy pultruded rod stitched specimens fabricated by Technova 

Corporation were loaded to failure in axial compression at NASA Langley Research 

Center. The loads supported by the Technova specimens indicate that the Technova 

specimens are of high quality. The non-prestressed specimen supported a load level 

equivalent to comparable Boeing specimens. The difference in load-carrying capability 

between the prestressed and non-prestressed specimens indicates that the prestressing 

technique has a potential to improve the structural efficiency in selected structures. 

Since most aircraft structures are not loaded exclusively in one direction, prestressing is 

not appropriate everywhere, however, selective application of this technique could be 

warranted for aircraft, launch vehicle or other structures. 

References 

1. Velicki, A., “Damage Arresting Composites for Shaped Vehicles,” NASA CR2009-21532, Sep. 2009. 

2. Jegley, Dawn C., “Experimental Behavior of Fatigued Single Stiffener PRSEUS 

Specimens, NASA TM 2009-215955, Dec. 2009. 

3. Technova Corporation, “Prestressing of Composite Structures for Enhanced 

Efficiency,” Phase II SBIR, U.S. Air Force contract FA8650-09-C-3908. 


Figures 

a) Stiffened side b) Unstiffened side 

Fig. 2. Technova non-prestressed specimen. 




		

	



Fig. 1. Cross section geometry. Dimensions are in inches. 

		






a) Stiffened side b) Unstiffened side 

Fig. 3. Technova prestressed specimen. 

Fig. 4. Specimen in test machine and displacement measurement locations. 








	





Fig. 5. Edge supports. Dimensions are in inches.

		

		







Fig. 6. Displacement transducer measurements for Technova non-prestressed specimen. 

		






a) 500 lb 

Fig. 8. Full-field displacements for Technova non-prestressed specimen.

Fig. 7. Displacement transducer measurements for Technova prestressed specimen. 


b) 10,000 lb 

c) 12,100 lb 

Fig. 8. Continued. 


d) 41,000 lb 

e) 44,300 lb 

Fig. 8. Concluded 

 


a) 500 lb 

b) 5,000 lb 

Fig. 9. Full-field displacements for Technova prestressed specimen. 


c) 15,000 lb 

d) 61,000 lb 

Fig. 9. Continued. 


e) 65,000 lb 

Fig. 9. Concluded. 

Fig. 10. Full-field strains for non-prestressed specimen at 44,300 lb. 

	





b) 65,000 lb 

Fig. 11. Full-field strains for Technova prestressed specimen. 

	



a) Stiffened side 

b) Unstiffened side 

Fig. 12. Post-test photographs of Technova non-prestressed specimen. 


a) Stiffened side 

b) Unstiffened side 

Fig. 13. Post-test photographs of Technova prestressed specimen. 

 


Fig. 14. Load-shortening comparison with Boeing specimen. 

Fig. 15. Failure load comparison with Boeing specimens. 

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

Technova

Prestressed

Boeing

Load, lb

Boeing

Boeing-average

Technova 

Non-prestressed
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Evaluation of the Behavior of Technova Corporation Rod-Stiffened 

Stitched Compression Specimens 

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Jegley, Dawn C.

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NASA Langley Research Center

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14. ABSTRACT

Under Space Act Agreement 1347 between NASA and Technova Corporation, Technova designed and fabricated two 

carbon-epoxy crippling specimens and NASA loaded them to failure in axial compression. Each specimen contained a 

pultruded rod stiffener which was held to the specimen skin with through-the-thickness stitches. One of these specimens was 

designed to be nominally the same as pultruded rod stitched specimens fabricated by Boeing under previous programs. In the 

other specimen, the rod was prestressed in a Technova manufacturing process to increase its ability to carrying compressive 

loading. Experimental results demonstrated that the specimen without prestressing carried approximately the same load as the 

similar Boeing specimens and that the specimen with prestressing carried significantly more load than the specimen without 

prestressing. 

15. SUBJECT TERMS

Buckling; Composites; Graphite epoxy; Nonlinear anaysis; PRSEUS; STAGS; Stitching

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