Little research has been done on the post-processing (aging) of rapid prototyped (RP)
polymers at temperatures below 123K (–238˚F). Test specimens of RP thermosetting resin
(DSM-Somos 8110) were fabricated and cryogenically aged from 10-25 hours. The tensile
strength and impact toughness were measured. This work will study the effect of cryogenic
aging on yield strength of Somos 8110. This paper will also discuss our interpretation of the data
based on fractography.The work was funded by an NSF Grant under Research Experiences for Undergraduates
(REU).Mechanical Engineerin

Bullman, J.

Foyos, J.

Fritz, B.

Fuentes, F.

Gallardo Jr., A.

Martinez, J.

Mendelson, M.

Noorani, R.

English

Texas ScholarWorks

EFFECTS OF CRYOGENIC AGING ON A RAPID PROTOTYPED (RP) POLYMERF. Fuentes1, A.  Gallardo Jr1., J.  Martinez1, J. Bulman1, J. Foyos1,M. Mendelson1, R. Noorani1, B. Fritz21NSF Research Experiences for the Undergraduates (REU)Loyola Marymount University, Los Angeles, CA - 900452Northrop Grumman , El Segundo, CA - 91355ABSTRACTLittle research has been done on the post-processing (aging) of rapid prototyped (RP)polymers at temperatures below 123K (–238˚F).  Test specimens of RP thermosetting resin(DSM-Somos 8110) were fabricated and cryogenically aged from 10-25 hours.  The tensilestrength and impact toughness were measured.  This work will study the effect of cryogenicaging on yield strength of Somos 8110.  This paper will also discuss our interpretation of the databased on fractography.INTRODUCTION    As industry continues to increase its use of rapid prototyping (RP) materials, the necessityfor enhancing the properties of RP materials is becoming more important.  Past studies haveconcluded that by reducing the temperature below 123K (–238˚F), cryogenic processingincreased the strength of many materials, particularly metals.  But while the strength of thematerials increased, the cryogenic treatments also reduced ductility.Though relatively unstudied, the results of cryogenic tempering on certain polymers willprove highly valuable to industry.  This study hopes to increase the usage of RP materialsthrough cryogenic processing by optimizing the combination of strength and ductility for thestereolithography photopolymer epoxy resin, DSM Somos 8110 [1].The cryogenic test procedure began with the construction of the tensile and impact testsamples using the RP materials.  In order to increase the mechanical properties, these sampleswere then subjected to 88K (-300˚F) using several different holding and ramping times.  Next,the samples were examined using tensile and impact tests.  Finally, the data obtained from thetests was analyzed. Based on metals, it was expected that the tensile strength of the RP materialswould increase, and the toughness would decrease after cryogenic processing.  However, wehoped there would be a combination of cryogenic aging treatments that would improve both thestrength and toughness.The purpose of this paper is to discuss the cryogenic method for post-processing rapidprototyped DSM-Somos 8110 epoxy resin.  In addition, this paper will examine the effect ofcryogenic aging on the tensile strength and impact energy.115BACKGROUNDCryogenicsCryogenics is the science and art of producing a low temperature environment.  It started in1877 when two scientists, Cailletet in Paris and Pictet in Geneva, developed a procedure toliquefy oxygen in the laboratory [2].  Now, liquid nitrogen is one of the most common coolingmedia.  Since the normal boiling points of nitrogen and other permanent gases such as helium,oxygen, and argon is about 84.8K (≈-307˚F), the cryogenic temperature is generally consideredto be 84.8K or below [2].Cryogenic processing is an important field in industry today.  For example, an increase of195% to 817% in wear resistance has been reported for standard steel that was cryogenicallytreated [3].  The cryogenic process consists of three stages that are time and temperaturedependent to the aging temperature.  This process starts with the gradual ramping down oftemperature for a pre-determined time range, to a specific point.   The temperature is then heldfor a period of time.  Finally, a gradual process of temperature ramp-up is executed.  As a resultof cryogenic processing, some unknown molecular changes occur, modifying the structure of thematerial [3].Conventionally, heat treating is used to strengthen steel and other alloys.  Upon quenchingsteel below its eutectoid decomposition temperature, some retained austenite remains [3].However, cryogenic processing transforms virtually all of the retained austenite to martensite,along with a precipitation of fine carbide particles.  These effects have resulted in increased wearresistance [3].Polymers and StressesPolymers can be classified according to their chain structure and chain arrangement.  Chainstructure of polymers is further characterized by the chemical composition of monomers andtheir relative arrangement within a chain.  The common chain arrangements are linear chains,branched chains, and cross-linked networks (e.g. epoxy resins).The general characterization of polymers is very difficult due to the complicated structures,the great variety of chemical compositions and the transfer of molecular groups [4].  However,the fracture strength of polymers can be explained from a microstructural point of view.  Thesefactors can include molecular stress concentrations and residual thermal stresses [5].The twisting and kinking of polymer chains contributes to local stress concentrations due tothe uneven stress distribution between the molecules.  When the entangled polymer chains areunder stress, the chain segments in the load direction provide most of the load-carrying capacity.When polymers are cooled too quickly during cryogenic processing, thermal stresses are built upwithin the polymer, and these act to reduce the tensile strength [4].116EXPERIMENTAL PROCEDUREEquipment and ProcessThe following major equipment was used in this research.1. Northrop Grumman SLA-250 RP Machine2. Cryogenic Treatment Equipment3. Instron Tensile Testing Machine4. Izod Impact Tester5. Scanning Electron Microscope (SEM)The experimental setup if shown in Figure 1.Figure 1. Experimental setup for cryogenic processing.The following outlines the steps used in the experimental process that was used to design,fabricate, test and analyze the samples.1. Design Dog Bone and V-Notch Samples2. RP Machine Builds Parts from Design3. Expose Parts to Cryogenic Treatment with Liquid He.4. Tensile and Impact Testing5. SEM FractographyDesigning and Prototyping the SamplesDrawings of the dog bone and v-notch shaped samples were created using AutoCAD (Figs.2a and 2b) and these were saved as separate .DWG files.  These files were then converted intoSTL format for use with stereolithography software.  SLA-250 (Northrop Grumman) RPmachines was then used to rapid prototype the parts.Cryogenic ChamberTemp ControlVacuumPumpCryogenicPump117Figure 2a. Two dog bone-shaped samples. Figure 2b. Two v-notch-shaped samples.Cryogenic TreatmentThe following process was used to cryogenically treat each sample.1. All samples except the baseline went through cryogenic treatment before testing.  Thesamples were prepared at Northrop Grumman, and cryogenically aged at Loyola MarymountUniversity.2. The cryogenic process is characterized by three parameters: ramp-down time from roomtemperature to 88K (–300oF), hold time at 88K, and ramp-up time from 88K to roomtemperature.3. Preliminary experiments were performed on cryogenic treatment.  The ramp-down time of14 hours was used.  Three holding times of 10, 15 and 20 hours, and ramp up times of 14hours were used.4. Samples are labeled as follows: XX-XX-XX. The numbers represent the ramp-down time,holding time and ramp up time, respectively, in hours. (e.g: 14-10-14 means that the sampleswere ramped-down in 14 hours, cryogenically aged 10 hours, and then ramped-up in 14hours.)Tensile TestingThe yield and ultimate strengths of the samples were measured using the Instron UniversalTesting Instrument 4500.   The cross-head speed of the test machine was 0.0212  mm/s.  Theinterface with the machine was performed using the front panel and a software program runningon a desktop computer. The test specimens were designed in the shape of a dog-bone, similar  toASTM D638-97.  The end pads of the specimen were tightened and aligned between the twogrips of the frame unit.    A computerized load cell located inside the frame unit measured theforce applied to the gauge section of the sample. The software calculated the tensile stress in thegauge section.  Strain was measured with an extensometer, and the stress vs. strain curves wereplotted as the specimen was continuously loaded.  The yield stress was measured at 0.2% offsetstrain, and the ultimate strength was measured at the maximum stress the material couldwithstand.   Four to six specimens were tested for each cryogenic aging treatment (0-20 hours).The tensile strength data were statistically analyzed to determine the effect of aging treatment onthe yield and ultimate strengths.Izod Impact TestingIzod impact testing is performed to determine the toughness of a material.  The sample is made118with a centered v-shaped notch.  During the impact testing, the sample is subjected to a quickand intense blow by a hammer pendulum.  The impact test evaluates the material’s resistance tocrack propagation.  The impact energy absorbed by the sample during failure is determined bycalculating the difference in potential energy of the hammer [5].  SEM FractographyThe fracture surfaces of the failed tensile samples of Somos 8110 resin were examined undera scanning electron microscope (SEM).  SEM fractography was used to interpret the results ofthe tensile strength data for both the untreated (baseline) and cryogenically treated samples [6].The SEM used an acceleration voltage of 1.2kV.  The fracture surfaces were observedsequentially at magnifications of 15x, 40x and 150x.The stretching of the polymer chains and fracture initiation were correlated with the yieldstrength data.  Here critical-size cracks were initiated from defects and propagated when theyield strength of the polymer was exceeded.  The propagation of the crack front was correlatedwith the ultimate strength.  Then the crack propagation patterns could be observed on the fracturesurface of the failed specimens.RESULTS AND DISCUSSIONTensile and Impact TestingThe results of the yield strength and ultimate strength vs. cryogenic treatment time (0-20hours) are shown in Figures 3A, 3B.  In both cases, the strengths appear to be affected by agingtime.  To verify these results, the data were statistically analyzed using multiple t-testing, whichcompared the means of two treatments at-a-time at a 0.05 level of significance [7].  A one-wayanalysis of variance (ANOVA) could not be used, because the variances of the treatments wereunequal.  For the yield strength data (Figure 3A), it was determined with 95% confidence thatthe means of the treatments were statistically insignificant.  Due to the large variances at 15 and20 hours, we can say that there was no significant increase in the yield strength with aging time.For the ultimate strength (Figure 3B) vs. aging time, the data were analyzed in the sameway.  Again, one-way ANOVA could be not be used.  Multiple t-testing on the treatment meansshowed that for 95% confidence two conclusions were reached: (1) the mean strengths at 0, 10and 20 hours were equivalent, and (2) the mean strength at 15 hours was significantly lower thanthat at 10 and 20 hours.  Hence, aging time produced a drop in the ultimate strength between 10and 15 hours.  Otherwise, there was no significant effect of cryogenic aging on the ultimatestress.When the impact energy data vs. aging time (Figure 3C) were statistically analyzed, thelarge variance at 10 hours indicated that the treatment means were insignificant with 95%confidence.  Hence, there was no effect of cryogenic treatment time on the impact energy.119SEM FractographySince all of the tensile specimens failed near the fillet radius, failure analysis was performed onthe tested specimens.  The fracture surfaces of the tensile specimens were examined under threedifferent treatment conditions: (1) Sample A, treated for 15 hours and having the highest yieldstrength (Figure 3A), (2) Sample B, treated for 15 hours and having the lowest yield strength(Figure 3A) and (3) Sample C, untreated.The following findings were made from observing the fracture surfaces of Samples A, B, Cin Figures 4A, 4B and 4C, respectively.  In all cases, the fracture originated from surface defectson the fillet radius (between the gauge length and grips) of the tensile specimens.  Fracturepropagated from left to right, as shown by the 'river markings' on the fracture surface [6].Samples B and C showed similar fracture patterns that consisted of mixed mode I and III fracture[6].  Here, fracture initiated from the stepped fillet radius in Figures 4B, 4C and 5B, which wasformed during stereo-lithographic processing. Sample A exhibited primarily mode I fracturefrom the fillet surface, and fracture did not originate from steps on the fillet radius (Figures 4Aand 5A).Samples B and C had low yield strengths and similar fracture patterns.  The low strengthswere interpreted as being caused by the stepped fillet radius.  The steps probably caused a highstress concentration at the fillet surface. In addition, the steps appeared to have surface defects onthe fillet radius (Figures 4B, 4C and 5B).  Sample A had the highest yield strength, and therewere no steps or surface defects on the fillet radius where fracture originated.  Hence, the steppedfillet surfaces with their associated surface defects probably caused the specimens to failprematurely at lower strengths. The steps and surface defects in Samples B and C also accountedfor the higher variance (scatter) in the data.The differences in the fracture patterns between Sample A and Samples B, C could have alsoaccounted for their difference in yield strength.  In Samples B and C, it appeared that the crackfront propagated at slower velocities than in Sample A.  The crack had more time to separate onto different levels and develop mode III shear lips (Figures 4B and 4C).  In Sample A, therewere no steps and surface defects at the fillet radius.  Consequently, a critical-size crack had tobe nucleated on the fillet surface, and this required extra stress, which caused the strength to behigher.  Once a critical-size crack was formed, the extra stress caused the crack to propagatecatastrophically at a higher velocity.  The higher velocity fracture in Sample A is indicative ofprimarily mode I fracture patterns with little mode III shearing (Figure 4A).120  Figure 4A. 40x SEM micrograph of Sample A  in Figure 3A.     Figure 4B. 40x SEM micrograph of Sample B    Figure 4C. 40x SEM micrograph of Sample Cin Figure 3A.        (Baseline) in Figure 3A.  1011121314150 5 10 15 20 25Aging Time (HoursUltimate Strength (MPa)Experimental Data Average012345678-5 0 5 10 15 20 25Aging Time (HoursYield Strength (MPa)Experimental Data AverageFigure 3B.  Ultimate Strength vs. Cryogenic AgingTime.01234560 5 1 0 1 5 2 0 2 5Aging Time (HoursImpact Strength (N)Experimental Data AverageFigure 3C. Impact Energy vs. Cryogenic AgingTime.Figure 3A. Yield Strength vs. Cryogenic AgingTime.ABC121CONCLUSIONS AND RECOMMENDATIONSBased on the findings from our research, the following conclusions and recommendationscan be made:1. DSL-Somos 8110 test specimens were fabricated by stereo-lithography using SLA-250 RPequipment.  The specimens were laser cured by pulling the specimens parallel to the tensile axis.2. The test specimens were cryogenically treated by a ramp-down cycle from room temperatureto 88K in 14 hours, a hold cycle of 10 - 20 hours, and then a ramp-up cycle from 88K to roomtemperature in 14 hours.3. Due to large data scatter, the yield strength and impact energy were not affected by cryogenicaging treatment.  Only the ultimate strength exhibited a significant decrease with aging treatmentfrom 10 to 15 hours.4. It was expected the wide data scatter was caused by microscopic steps and their associateddefects at the fillet radius of the tensile specimens.  Samples with steps and defects had lowerstrengths, and those without steps and flaws had high strengths.  The variability in the surfacetexture of the fillet radius caused a wide variability in the tensile strength and impact energy.5. For future work, it is recommended that the SLA specimens be rapidly prototyped normal tothe face of the dog-bone specimen.  This will hopefully reduce the data scatter, which currentlymasks any potential effect of cryogenic aging time on the tensile strength and impact energy.ACKNOWLEDGEMENTSThe work was funded by an NSF Grant under Research Experiences for Undergraduates(REU).  The authors also appreciate the support provided by the Northrop GrummanCorporation.  We also acknowledge the contributions toward the development of this paper byour students, J.C. Jackson and Joey Weinmann.Figure 5A. 15x SEM micrograph of Sample Afrom Figure 4A.Figure 5B. 15x SEM micrograph of Sample C(Baseline) from Figure 4C.122REFERENCES1.  Stratasys, 1996, “FDM System Documentation”. F1650-5, F1650-6, F1650-11, G-8, G-9, and Q79.2. Scurlock, R.G., 1992, “History and Origins of Cryogenics,” Institute of Cryogenics,University of Southampton, UK.  Oxford: Clarendon Press.3. Chhabra, V., “Ingenious Cryogenics: Gainsville Inventor Frank Masyada Brings theFuture to Our Fingertips,” Business, July, 1994.4. Hartwig, G., “Polymer Properties at Room and Cryogenic Temperatures,” Plenum Press,New York and London.5. Askeland, D.R., 1995, “The Science and Engineering of Materials,” Third Edition, PWSPublishing Company, Boston6. Hull, D., 1999, “Fractography: Observing, Measuring and Interpreting Fracture SurfaceTopography,” Cambridge University Press, UK.7. Montgomery, D.C., Runger, G.C., and Nubele, N.R., 2000, “Engineering Statistics,” 2ndEdition, John Wiley & Sons Inc., New York.123

Effects of Cryogenic Aging on a Rapid Prototyped (RP) Polymer

https://repositories.lib.utexas.edu/bitstream/2152/76333/2/2002-14-Fuentes.pdf

Effects of Cryogenic Aging on a Rapid Prototyped (RP) Polymer

Abstract

Similar works

Full text

Available Versions

Texas ScholarWorks