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Research Papers: Design and Analysis

Effect of Helix Angle on the Stress Intensity Factor of a Cracked Threaded Bolt

[+] Author and Article Information
Raghu V. Prakash

e-mail: raghuprakash@iitm.ac.in
Department of Mechanical Engineering,
Indian Institute of Technology Madras,
Chennai 600 036, India

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the Journal of Pressure Vessel Technology. Manuscript received July 30, 2011; final manuscript received January 11, 2012; published online March 18, 2013. Assoc. Editor: Noel O'Dowd.

J. Pressure Vessel Technol 135(2), 021202 (Mar 18, 2013) (12 pages) Paper No: PVT-11-1160; doi: 10.1115/1.4007290 History: Received July 30, 2011; Revised January 11, 2012

The fracture behavior of a crack in a threaded bolt depends on the stress intensity factor (SIF). Available SIF solutions have approximated the threaded bolt as a circular groove, thus, the SIF predominantly corresponds to the opening mode, mode-I. As a thread in a bolt has a helix angle, the crack propagates under mixed mode conditions (opening, sliding and tearing), esp. when the crack sizes are small. This paper presents the results of SIF solutions for a part-through crack emanating from a Metric threaded bolt. A 3D finite element model with preexisting flaws was generated to calculate the SIF values along the crack front. Crack aspect ratios in the range of (0.2 < (a/c) < 1) and crack depth ratios in the range of (0.1 < (a/d) < 0.5) (where “a” is crack length, “c” is semi major axis of ellipse and “d” is minor diameter of the bolt) were considered along the crack plane for the SIF estimation. The SIF values at the midregion decreases with an increase in aspect ratio (a/c), and SIF increases when the crack depth ratio (a/d) increases in the midregion. Close to the free edges, higher SIF values was observed for crack depth and aspect ratios ranging between 0.2 and 0.6 compared to midregion. In the crack surface region, up to a crack depth ratio of 0.25, significant influence of mode-II and mode-III fracture was noted for shallow cracks (a/c < 0.2). Significant influence of mode-II and mode-III fracture was observed for semicircular cracks (a/c = 1) beyond the crack depth ratio of 0.3.

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Figures

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Fig. 1

Surface crack nomenclature at the thread root region

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Fig. 2

Stress distributions around thread root region

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Fig. 3

Stress concentration factor versus crack depth ratio (a/d)

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Fig. 4

(a) 3D cracked bolt model, (b) symmetric bolt model, (c) part-through crack in a threaded bolt, (d) meshed bolt model with crack, and (e) 3D Cracked bolt model with boundary conditions

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Fig. 5

(a) 3D bolt and nut assembly with surface crack and (b) meshed contact region with surface crack

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Fig. 6

Mesh convergence test results around the crack region

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Fig. 7

Singular elements around crack front

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Fig. 8

Influence of helix angle—variation of crack depth ratio (a/d): (a) (a/d = 0.1), (b) (a/d = 0.2), (a/d = 0.3), (c) (a/d = 0.4), and (d) (a/d = 0.5)

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Fig. 9

Influence of helix angle—variation of crack aspect ratio (a/c): (a) (a/c = 0.2), (b) (a/c = 0.4), (c) (a/c = 0.6), (d) (a/c = 0.8), and (e) (a/c = 1.0)

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Fig. 10

Crack depth ratio versus correction factor—middle region

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Fig. 11

Crack depth ratio versus correction factor—middle region: (a) (a/c = 0.2)—middle, (b) (a/c = 0.4)—middle, (c) (a/c = 0.6)—middle, (d) (a/c = 0.8)—middle, and (e) (a/c = 1.0)—middle

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Fig. 12

Crack depth ratio versus correction factor—surface region

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Fig. 13

Crack depth ratio versus correction factor—surface region: (a) (a/c = 0.2)—surface, (b) (a/c = 0.4)—surface, (c) a/c = 0.6)—surface, (d) (a/c = 0.8)—surface, and (e) (a/c = 1.0)—surface

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Fig. 14

Normalized SIF for short (a/d = 0.1) and deep (a/d = 0.5) cracks—Influence of mixed modes: (a) Influence of mode-I SIF, (b) Influence of mode-II SIF, and (c) Influence of mode-III SIF

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Fig. 15

Influence of mixed mode fracture—middle region (elliptic crack)

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Fig. 16

Influence of mixed mode fracture—elliptic crack surface region (Left side)

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Fig. 17

Influence of mixed mode fracture—elliptic crack surface region (right side)

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Fig. 18

Influence of mixed mode fracture—middle region (semi circular crack)

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Fig. 19

Influence of mixed mode fracture—semi circular crack surface region (left side)

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Fig. 20

Influence of mixed mode fracture—semi circular crack surface region (right side)

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Fig. 21

Influence of loading condition on SIF estimation—short crack (a/d = 0.1): (a) (a/d = 0.1), and (a/c = 0.2), (b) (a/d = 0.1) and (a/c = 0.6), (c) (a/d = 0.1) and (a/c = 1.0)

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Fig. 22

Influence of loading condition in SIF estimation—deep crack (a/d = 0.5): (a) (a/d = 0.5) and (a/c = 0.2), (b) (a/d = 0.5) and (a/c = 0.6), (c) (a/d = 0.5) and (a/c = 1.0)

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