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Transcript of 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
9th International Rope Technology Workshop
Slide 4
Diameter Minimum Break
Strength Braid/twist angle D/d ratio Splices
Definitions
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
Slide 7
Definitions Mechanisms Rope Connections
Single Leg Grommet Cow hitch Eye-to-Eye
Lifting Knots and other hitches Conclusions
Overview
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
Slide 13
D/d Grommet Strength 1/1 1.15 4/1 1.78
Trendline is
similar for both 12strand HMPE and PET
Grommet
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
Slide 16
Cow Hitch
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
Insert picture here of eye-to-eye connection
Slide 18
Definitions Mechanisms Rope Connections
Single Leg Grommet Cow hitch Eye-to-Eye
Lifting Knots and other hitches Conclusions
Overview
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
* Flory/Richards choker testing presented at the Cordage Inst. meeting May 2010
Slide 20
Definitions Mechanisms Rope Connections
Single Leg Grommet Cow hitch Eye-to-Eye
Lifting Knots and other hitches Conclusions
Overview
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
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%
Other Knots, Bends & Hitches
9th International Rope Technology Workshop
Low modulus fibers: 50% guideline OK High modulus fibers: Knots are not recommended
Slide 23
Other Knots, Bends & Hitches
9th International Rope Technology Workshop
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
Definitions Mechanisms Rope Connections
Single Leg Grommet Cow hitch Eye-to-Eye
Lifting Knots and other hitches Conclusions
Overview
9th International Rope Technology Workshop
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
9th International Rope Technology Workshop
Slide 26
Thanks to: John Flory – Tension Technology International Dave Richards – Holloway Houston Inc Kris Volpenhein – Samson Frank Choltco-Devlin – Samson
Acknowledgments
9th International Rope Technology Workshop