The Effect of Bending on the Tensile Strength of Statically Loaded ... Papers/TP_The Effect...

Slide 1

The Effect of Bending on the Tensile Strength of Statically Loaded

Synthetic Ropes

*Mark Pederson, Greg Mozsgai and Danielle Stenvers

2011 MTS/OIPEEC 9th International Rope Technology Workshop

Slide 2

General overview of tensile strength reductions

of terminated ropes when bending is involved

Static cases examined only No shock, fatigue, creep, etc

Introduction

9th International Rope Technology Workshop

Slide 3

Definitions Mechanisms Rope Connections

Single Leg Grommet Cow hitch Eye-to-Eye

Lifting Knots and other hitches Conclusions

Overview


Slide 4

Diameter Minimum Break

Strength Braid/twist angle D/d ratio Splices

Definitions


Slide 5

Unequal loading of strands resulting in tensile failure

Exacerbated by: High modulus fiber Low twist Looser braid Low D/d

Mechanisms of Strength Loss


Slide 6

“Cutting” or “Pinching” of strands from high contact and strand-on strand pressure Fewer strands Larger strands Tighter braid

Mechanisms of Strength Loss


Slide 7




Overview


Slide 8

Single leg slings are used to baseline most rope strengths

Theoretical load distributions

There is a true loss in bend around pin but each leg around pin only carries ~½ T

Single Leg


Slide 9

Failure types

Base of splice Closest to a true rope strength

Crotch splice Tearing open

Back of pin Failure due to tight bend

Single Leg


Slide 10

Current practice-minimum testing requirements: D/d:

Cordage Institute (CI 1500) 2 ASTM (D4268) 1.5 ISO (2307:2010) 2

Eye Length: Cordage Institute (CI 1500) N/A ASTM (D4268) 2x Pin Dia. ISO (2307:2010) 3x Pin Dia.

Single Leg


Slide 11

Recommendations: Min D/d = 2

D/d = 1 did not show

significant strength loss, however break occurred at the back of the eye (on the pin)

Single Leg


0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

8 4 1

Brea

king

Stre

ngth

(lbs

)

D/d (Pin Diameter vs Rope Diameter)

Single Leg Efficiency Comparison 18mm dia 12 Strand HMPE

Base

of S

plic

e

Base

of S

plic

e

Apex

of E

ye

Slide 12

Grommet configuration highlights bending losses

Grommet strength is actually <2x rope strength

Current industry recommendations range from 1.6-1.8x rope strength

Grommet


Slide 13

D/d Grommet Strength 1/1 1.15 4/1 1.78

Trendline is

similar for both 12strand HMPE and PET

Grommet


1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

0 1 2 3 4 5 6 7 8 9

Effic

ienc

y (x

Sing

le L

eg B

reak

Stre

ngth

)

D/d (Pin Diameter vs Rope Diameter)

Grommet Efficiency FactorsASB Grommets

Amsteel Blue (vs Pub MBS)

DNV Model

Slide 14

Splice Placement: Splice area centered

on the pin allows the legs to more evenly adjust upon loading.

Two splices do not produce a more efficient sling.

Grommet: Other Considerations


A 1.6-1.8x B 1.7-1.9x C 1.7x D 1.75x

Slide 15

Common configuration with mooring lines and pendants

OCIMF guideline of 85% efficiency based on same-size ropes Lower modulus materials (nylon, polyester)

Strength efficiency considerations: Materials Construction Rope sizes

Cow Hitch


Slide 16

Cow Hitch


58%

64%

70%

86% 86%

81%

85%

75%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Pare

llel c

ore

Laye

d PE

T

8 st

rand

N

ylon

Pare

llel c

ore

Brai

ded

PET

Brai

ded

PET

(8s,

12s

, 6s

2epc

, 12s

2e

pc)

8 St

rand

Po

lyol

efin

8 St

rand

Po

lyol

efin

/PET

12 St

rand

Po

lyol

efin

/PET

ASB-

ASB

(sam

e si

ze)

Asse

mbl

y Str

engt

h Ef

ficie

ncy v

s. M

ainl

ine

MBS

Cowhitch Efficiency Vs Amsteel Blue MBS

Slide 17

Common configuration for tug and mooring line connections (mainline to pendant , HMPE to HMPE)

Connection efficiency per rope size ratio: 1 : 1 90% 1.6 : 1 100%*

*(based on the smaller rope’s strength)

Eye-to-Eye


Insert picture here of eye-to-eye connection

Slide 18




Overview


Slide 19

Current recommendations: ASME B30.9 80% (PET roundslings)

Flory/Richards* 60% (HMPE slings)

Test Results*: 12-strand, HMPE fiber

Ranged from 60% - 75% (varied by manufacturer)

Choke Angle is critical Chocker strength

Choker Hitch


* Flory/Richards choker testing presented at the Cordage Inst. meeting May 2010

Slide 20




Overview


Slide 21

Knots involve tight bends on itself, lowering the rope strength

Traditional rule of thumb has been 50% Factors affecting strength:

Fiber Construction Type of knot

Other Knots, Bends & Hitches


Slide 22

Fiber Construction Knot/Hitch Retained Strength HMPE 12-strand Bowline 35% Technora 12-strand Bowline 25% Nylon Jacketed Bowline 57% Nylon 3-strand Bowline 55% Nylon/Polyester Kernmantle Bowline 63-67% Polypropylene 3-strand Bowline 53-60% Polyester Jacketed Square-knot 45-65%



Low modulus fibers: 50% guideline OK High modulus fibers: Knots are not recommended

Slide 23



100%

63.1%

69.4%

69.9%

70.8%

49.9%

60.3%

54.6%

73.4%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Brk. Str.

Bowline

Fig. 8 end

Fig. 8 bight

Butterfly

Sheet Bend

Fish knot

Dbl Sheet Bnd

Dbl Fish Knot

% of break Strength

Kno

t Typ

es

10.5 mm Dynamic Climbing Rope Knot Comparison

*Data from Dave Richards Testing

Slide 24




Overview


Slide 25

Termination ordinarily is the weakest part of the rope Rope should be designed into a system taking into

account strength loss due to bending Strength losses associated with bending vary

with: Configuration (grommet, knot, hitch, etc) Eye size Pin size Fiber Type

Conclusions


Slide 26

Thanks to: John Flory – Tension Technology International Dave Richards – Holloway Houston Inc Kris Volpenhein – Samson Frank Choltco-Devlin – Samson

Acknowledgments


Slide 27

Questions?


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The Effect of Bending on the Tensile Strength of Statically Loaded ... Papers/TP_The Effect...

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