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Research Papers: Gas Turbines: Structures and Dynamics

Stress, Strain, and Energy at Fracture of Degraded Surfaces: Study of Replicates of Rough Surfaces

[+] Author and Article Information
Hector E. Medina

e-mail: hedmedina@yahoo.com

Brian Hinderliter

e-mail: bhinderl@d.umn.edu

Virginia Commonwealth University,
Richmond, VA 23226

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 29, 2013; final manuscript received September 2, 2013; published online November 21, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(3), 032502 (Nov 21, 2013) (5 pages) Paper No: GTP-13-1330; doi: 10.1115/1.4025660 History: Received August 29, 2013; Revised September 02, 2013

Due to the aging of structures, the issues of plant life management and license extension are receiving increasing emphasis in many countries. Understanding failure of structures due to random roughness on surfaces at early stages of degradation is therefore crucial. It has been shown that even slightly sinusoidal roughness can increase stress concentration by a factor of two or three, which can be critical for a brittle component due to the significant reduction of its load-carrying capacity, even with slight roughness. A more in-depth fracture analysis of surfaces possessing random roughness is needed in order to more profoundly understand, and, hence, develop models that will predict more accurately, failure of structural materials exposed to degrading, in-service conditions. Using a technique previously developed and successfully applied, replicates of random rough surfaces, imprinted with various levels of degradation, and at three distinct auto correlation lengths, were realized and mechanical testing was performed on them. The stress, strain, and energy at fracture are reported. Finite element analysis was carried out to elucidate experimental results. Besides the expected reduction of energy at fracture with degradation, a relaxation region was observed where the energy slightly increases. This phenomenon implies that even after degradation has progressed there is a local maximum of energy at fracture due to the competing effect of tendons and growth of pits. The results find applications on the early stage of maintenance of surfaces of structures in service.

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References

Gao, H., 1991, “Fracture Analysis of Nonhomogeneous Materials Via a Moduli-Perturbation Method,” Int. J. Solid Struct., 27, pp. 1663–1682. [CrossRef]
Koch, G. H., Brongers, M. P. H., Thompson, N. G., Virmani, Y. P., and Payer, J. H., 2002, “Corrosion Costs and Preventive Strategies in the USA. Supplement to Materials Performance,” U.S. Federal Highway Administration, McLean, VA, Report No. FHWA-RD-01-156.
Medina, H., and Hinderliter, B., 2012, “Use of Poly (Methyl Methacrylate) in the Study of Randomly Damaged Surfaces: I. Experimental Approach,” Polymer, 53(20), pp. 4525–4532. [CrossRef]
Ogilvie, I. R., Sieben, V. J., Floquet, V. J., Zmijan, R. Z., and Mowlem, M. C., 2010, “Reduction of Surface Roughness for Optical Quality Microfluidic Devices in PMMA and COC,” J. Micromech. Microeng., 20, p.065016. [CrossRef]
Chung, C. K., Lin, Y. C., and Huang, G. R., 2005, “Bulge Formation and Improvement of the Polymer in CO2 Laser Micromachining,” J. Micromech. Microeng., 15, pp. 1878–1884. [CrossRef]
Choudhury, I. A., and Shirley, S., 2010, “Laser Cutting of Polymeric Materials: An Experimental Investigation,” Opt. Laser Tech., 42, pp. 503–508. [CrossRef]
Nichols, M. E., and Peters, C. A., 2002, “The Effect of Weathering on the Fracture Energy of Hardcoats Over Polycarbonate,” Polym. Degrad. Stab., 75, p. 439. [CrossRef]
Medina, H., and Hinderliter, B., 2013 “Where Do Random Rough Surfaces Fail: Fracture Loci Safety Envelopes at Early Stages of Degradation,” J. Energy Power Eng., 7, pp. 907–916.
Heripre, E., Sun, L., Dubouski, S., and Durand, J., 2010, Crack Propagation, Wear and Rough Surfaces: Contact Mechanics II, Solid Mechanics Laboratory, Ecole Polytechnique ParisTech. Paris.
Martin, J. W., Nguyen, T., and Wood, K. A., 2005, “Unresolved Issues Related to Predicting the Service Life of Polymeric Materials,”Service Life Prediction: Challenging the Status Quo, J. W.Martin, R. A.Ryntz, and R. A.Dickie, eds., Federation of Societies for Coatings Technology, Blue Bell, PA, Chap. 1.
Medina, H., and HinderliterB., 2013, “Method to Generating and Realizing Replicates of Randomly Roughened Surfaces, Tested on Poly Methyl Methacrylate,” Exp. Tech. (in press). [CrossRef]
Malvern, L., 1969, Introduction to the Mechanics of a Continuous Medium, Prentice-Hall, Inc., Englewood Cliffs, NJ.
Pearson, K., 1997, Stress Concentration Factors, 2nd ed., John Wiley and Sons., New York.

Figures

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Fig. 1

Notable linear-elastic behavior at room temperature of a typical specimen used in the experiments

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Fig. 2

Visual schematic of equal-stress lines distribution due to (a) a single discontinuity and (b) multiple discontinuities

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Fig. 3

Cartoon plot of stress concentration factor for tension of a flat member with semicircular notches, for r/H = 18. Based on data from Ref. [13].

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Fig. 4

Three grooves on a beam under bending modeled using FEM. (a) Stress concentration is distributed. (b) A thick (tendon) interruption in the center of middle groove. (c) A thin tendon in the center of middle groove.

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Fig. 5

Dimensions of an actual PMMA specimen. Note squared ablated area in the center.

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Fig. 6

Stress distribution on a random rough surface having ACL = 45. The plot shown corresponds to the average roughness at locations along ablated width (Wa). Note that there is extra material not ablated to the right and left of rough surface. Compare lines x0 thru x1 with those in Fig. 7.

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Fig. 7

Top view of plot of surface whose stress distribution is shown in Fig. 6. Darkest blue means deepest. The values are expressed in meters. Compare lines x0 thru x6 with those shown in Fig. 6.

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Fig. 8

Strain energy at fracture for 12 replicates of each surface (varying level of degradation), for ACL = 45. Degradation goes from D = 0 (initial half-Gaussian roughness) to D = 135 (a Gaussian roughness with mean three times larger than that of D = 45.

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Fig. 9

Strain energy at fracture for 12 replicates of each surface (varying level of degradation), for ACL = 10. Degradation goes from D = 0 (initial half-Gaussian roughness) to D = 45 (A Gaussian roughness).

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Fig. 10

Strain energy at fracture for 12 replicates of each surface (varying level of degradation), for ACL = 45. Degradation goes from D = 0 (initial half-Gaussian roughness) to D = 45 (a Gaussian roughness).

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Fig. 11

Strain energy at fracture for 12 replicates of each surface (varying level of degradation), for ACL = 90. Degradation goes from D = 0 (initial half-Gaussian roughness) to D = 45 (a Gaussian roughness).

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