Short Beam Shear Strengths Biology Essay

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This paper presents correlation between viscoelastic and hot/wet and room temperature short beam shear strengths of an auto-of-autoclave carbon-fiber epoxy prepreg, Cycom 5320 8HS. Short beam shear (SBS) test was performed on coupons cut from five panels laid up in [0/45]3s order and cured at post cure temperatures ranging from 210°F to 290°F. The test coupons from each panel were divided into two batches. The first batch was tested at room temperature and the properties of the prepreg were obtained using an encapsulated sample rheometer (ESR). The ESR samples were cured using the same cure cycles as those used for curing the panels. The viscoelastic properties that were used for correlation include glass transition temperature (Tg), storage modulus and loss modulus. It was observed that Tg showed good correlation with the hot/wet and room temperature SBS strengths for the studied cure cycles.

INTRODUCTION

The average service life of an aircraft can be anywhere from 20 to 35 years. In that time it is possible for an aircraft to see as many as 50,000 hours of flight time and 75,000 pressurization cycles. During this service life, composite components on an aircraft are constantly absorbing moisture due to variations in service temperatures and humidity. This moisture variation is a critical environmental factor that can be damaging to the properties of fiber-reinforced polymer composites. (Bullions, Loos and McGrath) The absorption of moisture may reduce the glass transition temperature and mechanical properties by one or all of the following: matrix plasticization, swelling, cracking, and fiber-matrix interface damage. (Bullions, Loos and McGrath) Property characterization and fully understanding the hot/wet thermal and mechanical properties is extremely important for successful and safe applications of components made from composite materials.

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When moisture is combined with high-temperature conditions, composites will behave differently from their original dry state due to hygrothermal degradation. Hot/Wet environmental exposure can reduce the glass transition temperature (Tg) of polymer composites substantially. Material properties, such as short beam shear strength, of the fiber-reinforced polymer composite depend primarily on the matrix resin performance. Due to degradation from water adsorption from the hot/wet environment most epoxy resins and therefore the composite they are incorporated into, will lose their matrix dominated mechanical properties. Degradation of properties can be seen in several ways; the plasticization of the composite material and mechanical damage from debond of the fiber/resin interface will be the main focus of this paper. Plasticization by moisture lowers the glass transition temperature by interrupting the Van der Waals bonds between polymer chains. Water molecules are polar and therefore capable of forming hydrogen bonds with hydroxyl groups. Debonding between the fiber/resin interfaces can occur through hygrothermal cracking or delimitation due to swelling of the resin matrix. As the moisture diffuses through the composite the inner less swollen part and swollen outer part will exert compressive and expansive forces on each other producing internal stresses. Rheological and short beam shear testing were performed to study how hot/wet environments affected the mechanical properties of the composite. Testing was performed on both wet and dry samples so that a fuller understanding could be established.

EXPERIMENTATION

Material

Material used is CYTEC's CYCOM 5320 toughened epoxy resin prepreg system for structural applications. The fiber used in 5320 is CYTEC T650-35 carbon fiber. The properties associated with this fiber are high strength, and oxidation resistance. The typical areal weight of this fiber is 370 g/m2. 8HS is a satin weave produced by the warp yarn crossing over seven fill yarn before going under fill yarn. Satin weave is a very flat, pliable, and has excellent wet out fabric which conforms to complex contours. Mechanical properties are good because of the low amount of crimp and tightness of the weave. However, due to the weave pattern asymmetry, considerations must be taken into account with layup. One side of the fabric fibers are running mainly in the warp direction with the other side fibers is running mainly in the weft direction. Stresses will be built into the composite if all panels are laid up with the same side facing one direction. To minimize the effect half of the panels are placed with the warp side up and half are placed up with the warp side down.

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This material was designed specifically for out-of-autoclave manufacturing to provide a lower cost of processing and flexibility in manufacturing. The suggested combined cure (Figure 1) developed by CYTEC is a ramp rate of 1-5°F/minute with and intermediate cure temperature of 200 ± 10°F for 120-150 minutes and a post cure temperature of 290 ± 10°F for 120-180 minutes.

Figure 1. Recommended Cure Profile for 5320 8HS.

Mechanical Testing

Mechanical properties were obtained from short beam shear testing. Coupons were cut from five panels laid up in [0/45]3s order and cured with an intermediate temperature of 200°F and final cure temperatures ranging from 210°F to 290°F. Table 1 shows the cure profile for each panel. Figure 2 shows the stacking layup of the material. To ensure the quality, defined by the consolidation, of the laminate thickness measurements where obtained by C-scanning the cures laminate. The panels were then cut into coupons and tested according to ASTM D2344 Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates. Samples size was .36 by 1.09 inches.

Table 1. Cure cycles for 5320 8HS panels.

Cure Cycle Number

First Ramp-Up (°F/min)

Intermediate Cure Temperature

(°F)

Intermediate Cure Time (min)

Second Ramp-Up (°F/min)

Post Cure Temperature

(°F)

Debulking Time (Hours)

1

3

200

135

3

290

16

2

3

200

135

3

260

16

3

3

200

135

3

240

16

4

3

200

135

3

220

16

5

3

200

135

3

210

16

C:\Documents and Settings\seyed\Desktop\vacuum bag scheme.tif

Figure 2. Layup for 5320 8HS Panels

Shear Rheometer Testing

To obtain viscoelastic properties of 5320 8HS, rheometer samples were cured using the cure profiles from table 2. Rheometer testing was carried out using the ATD CSS 2000 rheometer using 41.3 mm diameter parallel plates. The ATD's plates are designed to prevent high torque slippage by the use of ridges on each plate. There are 20 equally spaced ridges on each side of the plate arranged in a radial fashion. The ridges are 1.5 mm in width, and 15 mm in length with a height of 0.25 mm. On average the sample thickness averaged around 2.68 mm. One important feature of the ATD is that is provides extremely high pressure to the samples (200-4000 KPa), producing consistent laminates with qualities similar that of the autoclave or vacuum bag manufactured parts. The rheometer experiments were carried out at 1 Hz frequency to obtain the rheological properties, and a constant strain of 0.05 degrees was used. For rheometer tests, the samples of prepreg were prepared using 8 plies with an average weight of 5.48 grams and diameter of 1.6 inches. Figure 2 shows the viscoelastic properties for the manufacture's recommended cure cycle.

Table 2. Rheometer Cure Cycles for 5320 8HS

First Ramp-Up

(°F/min)

Intermediate Cure Temperature (°F)

Intermediate Cure Time (min)

Second Ramp-Up (°F/min)

Post Cure Temperature (°F)

Post Cure Time (min)

3

200

135

3

290

180

3

200

135

3

260

180

3

200

135

3

240

180

3

200

135

3

220

180

3

200

135

3

210

180

3

200

315

-

-

-

Figure 2. Viscoelastic Properties of 5320 8HS during Manufacturer Recommended Cure Cycle

RESULTS

Experimental Results

Storage Modulus (G')

The Storage Modulus (G') gives an indication of the state of cure and the final stiffness of the cured sample. When G' from multiple cure cycles are graphed together observations can be made relating to possible values to use for correlation.(S. Alavi-Soltani) Final G' values obtained from the area at the end of the curve that has reached a plateau were chosen for correlation with mechanical properties (see Figure 3). The average final G' of the cured sample for different cured cycles is summarized in Figure 4. While the plateau value indicating completion of cure was relative stable, it was not achieved for specimens with final cure temperature 200°F, therefore making it not ideal for correlation.

Figure 3. Storage Modulus during Cure for Different Cure Cycles for 5320 8HS

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Storage modulus plateau was reached for the samples post cured at 290F, 260F, and 240F.

Figure 4. Average G' values for 5320 8HS

Loss Modulus (G")

The loss modulus, also known as G", represents the energy loss during the cure cycle as heat and viscous damping in the exothermic reactions. Final G" values obtained from the area at the end of the curve that has reached a plateau were chosen for correlation with mechanical properties (see Figure 5). The average final G" of the cured sample for different cured cycles is summarized in Figure 6.

Figure 5. Loss Modulus during Cure for Different Cure Cycles for 5320 8HS

Figure 6. Average Value for Loss Modulus for 5320 8HS material

Glass Transition Temperature (Tg)

Glass transition temperature represents the temperature at which the material state changes from a brittle, glassy solid to either a rubbery elastomeric or viscous fluid. Glass transition temperature can be a good candidate for correlation with mechanical properties as it is highly dependent on the cure cycle and corresponds to an important physical property used in structural design.(S. Alavi-Soltani) To obtain the glass transition temperature, the G' data is used in accordance with the SACMA SRM18R-94 standard. Tg in this study is determined by the inflection point in the modulus curve and is considered less accurate then the Tg determined from tanδ, however the aerospace industry often prefers the former as it provides the first sign of mechanical property change.(Dao, Hodgkin and Mardel) All wet Tg samples were conditioned for 48 hours in boiling water before testing. Figure 7 and 8 show the glass transition temperature data for different cure cycles. Figure 7 is the results for samples that have not undergone conditioning and Figure 8 is for the samples that have undergone hot/wet conditioning.

Figure 7. Dry Glass Transition Temperature (Tg) for Different Cure Cycles for 5320 8HS

Figure 8. Wet Tg for Different Cure Cycles for 5320 8HS

For post cure temperatures ranging from 240oF to 290oF, wet Tg was less than dry Tg indicating that the material's properties have been effected by the hydrothermal condition. In addition, the post cure temperature of 240 oF to 290 oF, for both wet Tg and dry Tg showed a gradual increase in value as the post cure temperature increased. However, for post cure temperatures ranging from 200oF to 220oF, wet Tg had no significant variation and was greater than dry Tg. This suggests that for post cure temperatures ranging from 200oF to 220oF, wet Tg was affected predominantly by the conditioning temperature (209oF) rather than the curing temperature.

Short Beam Shear

Short Beam Shear is a generalized shear strength test which used to characterize the interlaminar shear. It is generally used for quality control and comparison of composite materials. Multiple failure modes are acceptable according to the ASTM D2344 standard. Refer to Figure 9 for acceptable failure modes. Changes in the failure mode were observed over the different post cure temperatures. Failure mode for the specimen post cures at 290°F showed interlaminar shear and compression failure as seen in Figure 10. All other samples showed only interlaminar shear as seen in Figure 11. Compressive failure shows that the bond between the fibers and the matrix was strong, therefore the fibers fractured. This is an indication that there was interaction between the fibers and that the mode was not entirely dominated by resin. Interlaminar shear indicated by delamination in the center and edges of the samples was found in all samples. This failure mode clearly indicates that the bond strength between lamina layers was insufficient to hold the lamina together.

Figure 9. Acceptable Failure Modes According to ASTM D2344

Figure 10. Failure mode for the specimens post cured at 290oF: Interlaminar Shear and Compression

Figure 11. Failure mode for all other specimens: Interlaminar Shear

Average values for short beam strength along with standard deviation are shown in Figure 12. The results show a significant drop in the average SBS strength as the post cure temperature decreases.

Figure 12. Average Room Temperature Short Beam Shear (SBS) Strength for Different Cure Cycles

Statistical analysis in Figure 13 shows that since there was no overlap in the conference interval there was significant difference between the cured panels.

C:\Documents and Settings\seyed\My Documents\Work Files\ADMRC\11052010 meeting\SBS Strength.bmp

Figure 13. One-Way Analysis of Variance (ANOVA) with Minitabâ„¢

Correlation Results

Short beam shear strength data showed a standard deviation of 0.12 for Dry Tg, 0.22 for Average G', and 0.25 for Wet Tg, and 0.64 for Average G" as seen in table . The small amount of deviation is an expected result as these properties are resin-dominated. As such, the trends seen in relationship to cure properties are similar. Dry Tg, and Average G' showed the greatest potential for direct correlation due to their very similar shape and magnitude as seen in Figure 14.

Table #3. Standard deviation of tested properties from

Standard Deviation for Strength vs. Tested Properties

Dry Tg

0.12

 

 

 

Ave G'

0.22

Wet Tg

0.25

Ave G"

0.64

 

 

 

strength.

Figure 14. Normalized Values for Strength, Average G', Average G", Dry Tg, and Wet Tg properties overlaid to observe similarities in trends.

The data results for the dry glass transition temperature versus the wet glass transition temperature show that the material was affected by the hot/wet conditioning. With non-conditioned result there is a gradual increase with the increase in all post cure temperature. The samples that have been conditioned with a hot/wet environment the results seen are a constant glass transition temperature of around 257° for the post cure temperatures of 200°F, 210°F, 220°F indicating that the material's final Tg was affected by the conditioning process rather than the curing temperature. For post cure temperatures of 240°F, 260°F, and 290°F there is a gradual increase in the glass transition temperature though it is greatly reduced compared the dry or unconditioned samples.

CONCLUSIONS

The understanding of the connection between viscoelastic properties and mechanical properties of thermosetting prepreg composite has been improved by this effort. This investigation also showed that hygrothermal exposure influenced the properties of the composite. The viscoelastic properties of the prepreg, average final storage modulus, average loss modulus, and wet and dry glass transition temperature of the cured samples, were obtained using an Encapsulated Sample Rheometer (ESR). Short beam shear (SBS) test which is quickly becoming a standard among composite testing experts was performed to obtain the shear strength of the cured specimens. It was found that the short beam shear strength showed a strong correlation to the dry glass transition temperature, and average storage modulus (G') respectively after the cure was complete. It was also seen that the wet glass transition temperature from samples cured between 220°F and 200°F were affected more by the conditioning process than by the post cure temperatures. Additionally, that the samples cured between 240°F and 290°F showed a drastic reduction in their glass transition temperature.