Carabiner Testing

Analysis of Fatigue Failure in D-shaped Carabiners

Massachusetts Institute of Technology Center for Sports Innovation

K Blair, D Custer, J Graham, M Okal Introduction • Current standard: Single pull to failure test (SPTF) • Climbers need rating reflecting in-field use – Cyclic & Dynamic loads result from falling, hanging and lowering – Typical Load Range: 2- 10 kN – Only most severe falls approach minimum SPTF ratings • Continued cyclic loading can result in fatigue failure of carabiners • Current carabiner retirement guidelines do not address fatigue life

April 5, 2002 © MIT Center for Sports Innovation 2 Objective

• This study characterizes the lifetime of carabiners under cyclic loads – Loads reflect in-field use – Controlled laboratory environment

April 5, 2002 © MIT Center for Sports Innovation 3 Carabiner Load Analysis

Carabiners 20 kN • Worst case scenario is factor 2 fall

12 kN – Factor = Distance climber falls/length of belayed rope Falling Climber • Dynamic rope stretches to absorb 1/3 of the force of the Dynamic Rope climber’s fall for the belayer 8 kN • Top carabiner loaded to 20 kN

Belayer

April 5, 2002 © MIT Center for Sports Innovation 4 Background: Climbing Loads

• Empirical studies have shown close correlation between in-field loads and those predicted by models • Single cycle period (0.5 seconds) is in the middle of typical field-load duration • Forces used in study are in the middle to high range of expected field loading – Low forces unlikely to pose danger to climbers – Testing at low forces prohibitively time consuming

April 5, 2002 © MIT Center for Sports Innovation 5 Carabiners

• All carabiners from same manufacturer • D-Shaped 7075 Aluminum • SPTF rating – 24 kN Closed gate – 7 kN Open gate • Each carabiner loaded with 12 kN proof load as part of manufacturing process

April 5, 2002 © MIT Center for Sports Innovation 6 Approach • Test Design – ASTM Test Set-up: – Carabiners clipped around 2 steel dowels – Dowels connected to grips • Testing: Evaluate through cyclic, dynamic loading – Cycles to failure – Carabiner Deformation – Crack Formation (X-ray photography)

April 5, 2002 © MIT Center for Sports Innovation 7 MTS Test System

Steel Pin Carabiner

Machined Steel Grip

Applied Load

April 5, 2002 © MIT Center for Sports Innovation 8 Equipment Details

• Load and deflection – Measured directly from the MTS machine – LabView computer based data acquisition system – Load error ± 13 N – Displacement error ± 0.01 mm • Fracture surface observations – X-Ray photos: Torrex 150D X-Ray – Photos: Zeiss Stemi 2000-C microscope

April 5, 2002 © MIT Center for Sports Innovation 9 Test Matrix

Cyclic Load Number Tested Range, kN 0.5 - 4 3 Open 0.5 - 5 3 Gate 0.5 - 6 3 0.5 - 8 3 0.5 - 10 3 0.5 - 12 4 Closed 0.5 - 14 4 Gate 0.5 - 16 4 0.5 - 18 4 0.5 - 20 4

April 5, 2002 © MIT Center for Sports Innovation 10 Experimental Approach

• Fatigue tests run cyclically from 0.5 kN to indicated maximum load • Gate gap length measured periodically with micrometer throughout test • Short exposure X-Ray photographs take periodically in 8, 10 and 12 kN tests – Photos copied to transparencies – Compared to determine deformation as a function of number of cycles

April 5, 2002 © MIT Center for Sports Innovation 11 Overview of Results

• Determination of Load vs Cycles to failure curve (L-N curve) • Carabiner deformation apparent only in high load cases – Majority of deformation occurs in first few load cycles • Post failure analysis of crack surface provides information on critical crack length • Not able to find evidence of crack formation before failure

April 5, 2002 © MIT Center for Sports Innovation 12 Cycles to Failure vs. Load

Closed Gate Open Gate 25

10 Max kN Load, 5

0 0 2000 4000 6000 8000 10000 12000 14000

Cycles to Failure

April 5, 2002 © MIT Center for Sports Innovation 13 Cycles to Failure vs. Load, Log Plot

Closed Gate Open Gate 100

10 Log (Max Load)

1 100 1000 10000 100000

Log (Cycles to Failure)

April 5, 2002 © MIT Center for Sports Innovation 14 Statistical Data

Cyclic Load Mean Cycles Standard % Variation Range, kN to Failure Deviation 0.5 - 4 7,849 1,598 20% Open Gate 0.5 - 5 3,350 384 11% 0.5 - 6 1,774 413 23% 0.5 - 8 10,939 1,657 15% 0.5 - 10 5,533 722 13% 0.5 - 12 2,958 439 15% Closed Gate 0.5 - 14 1,556 297 19% 0.5 - 16 1,451 209 43% 0.5 - 18 750 200 24% 0.5 - 20 263 51 20%

April 5, 2002 © MIT Center for Sports Innovation 15 Deformation Observations

• Gate gap measurement and X-Ray photographs failed to detect deformations • Careful measurement of carabiner length shows small deformation for large load cases (20 kN) • Majority of carabiner deformation for large loads occurs early in life

April 5, 2002 © MIT Center for Sports Innovation 16 Load vs Stroke First Cycle of 0.5-20kN Cyclic Test

Plastic deformation begins 15

Load (kN) Load 10 Carabiner plastically deforms 2.7mm along major axis after first cycle 5

0 01234567 Gate engages Stroke (mm) (slope increases by factor of ~3) April 5, 2002 © MIT Center for Sports Innovation 17 Two Cycles of Loading 0.5 -20 kN Case 25

Gate engages 10 Load (kN)

5 Cycle 1 Cycle 200 0 01234567

d Stroke (mm)

April 5, 2002 © MIT Center for Sports Innovation 18 Two Cycles of Loading 0.5 - 8 kN Case

9 8 7 Cycle 233 6 Cycle 9291 5 4

Load [kN] 3 2 1 0 -33.5 -33 -32.5 -32 -31.5-1 -31 Stroke [mm]

April 5, 2002 © MIT Center for Sports Innovation 19 Spine Strain for 0.5 – 20 kN Test

10 Load (kN)

Gate engages 5

0 0 1000 2000 3000 4000 5000 Strain in Spine (microinches/inch)

April 5, 2002 © MIT Center for Sports Innovation 20 Spine Strain for 0.5 – 8 kN Test

9 8 Cycle 233 Cycle 9291 7 6 5

Load [kN] 3 Gate engages 2 1 0 -4000 -3500 -3000 -2500 -2000 -1500 -1000 -500-1 0

Strain [mm/mm]

April 5, 2002 © MIT Center for Sports Innovation 21 Surface Crack Formation

• Carabiners cycled at 0.5-8 kN range were X-Rayed to search for surface cracks • X-Rays take about every 500 cycles • No surface cracks were detected

April 5, 2002 © MIT Center for Sports Innovation 22 Fracture Observations

• All carabiners break at “elbow” – Fits prediction made by Finite Element Model – Consistent with in-field failure characteristics • Observed cross-section under microscope

April 5, 2002 © MIT Center for Sports Innovation 23 Fracture Surface Pictures

0.5 - 8 kN load cycle 0.5 - 14 kN load cycle

April 5, 2002Magnification © MIT Center for Sports =Innovation 5x 24 Crack Size on Fracture Surface

20 -0.7641 Closed Gate y = 4.6871x R2 = 0.912 Open Gate 15

[3] 10

[2] Maximum Load [kN]

5 [2] y = 3.1038x-0.3476 R2 = 0.8569

0 0.1 Crack0.2 Length0.3 [cm] 0.4 0.5 April 5, 2002 © MIT Center for Sports Innovation 25 Discussion

• Cyclic Testing – Even at loads representing extreme falls, this specific carabiner has long life – Result should be encouraging for climbers • Deformation – Carabiner deformation very small and not detectable, especially for loads below the manufacturer’s proof test of 50% SPTF – Any plastic deformation occurs in first few loading cycles – Data suggests that deformation can be detected using a mold

April 5, 2002 © MIT Center for Sports Innovation 26 Discussion

•Crack Growth – No surface cracks were found during testing – Appears that when the carabiner is un-loaded, all surface cracks completely close – Crack length vs. cycles to failure trends agree with theoretical models, but direct comparison cannot be made due to complicated geometry of the carabiner

April 5, 2002 © MIT Center for Sports Innovation 27 Conclusions

• Carabiner failure can be characterized with L-N data • The carabiner tested exceeds reasonable expectations of carabiner fatigue life • Decreasing carabiner weight will likely result in decreased life forcing the need for fatigue ratings • Deformation cannot be used to predict fatigue failure • Deformation can be used to detect plastic deformation due to excessive loads

April 5, 2002 © MIT Center for Sports Innovation 28 Future Work • Testing other types of carabiners would allow for more general conclusions • Effect of load history should be studied • Effects of surface damage on the speed of crack initiation should be investigated • Crack initiation and propagation life should be characterized – Cycle carabiner at low load levels – Pull carabiner apart on a single load – Measure the length of the crack front

April 5, 2002 © MIT Center for Sports Innovation 29 April 5, 2002 © MIT Center for Sports Innovation 30 BACKUPS New Testing and Rating Standard • Based on in-field conditions, results, and current ASTM standard – In-field conditions • 0.5 sec. average loading period • Dynamic/sinusoidal loading • 20 kN maximum load (worst case scenario) –Results • Trend line for Cycles to Failure vs. Load – ASTM standard • Test minimum of 5 for 20 kN test • Factor of Safety = 1.2 • Rate by number of cycles to failure for 20 kN case – Black Diamond Light D Carabiner: ~200 cycles April 5, 2002 © MIT Center for Sports Innovation 32 21 Carabiner Fatigue Safety Margin

Recorded fatigue failure characteristics (standard of 13 1.2 factor of safety above minimum) y = -3.3508Ln(x) + 39.139 2 11 R = 0.9516 Maximum Load [kN] 9 Minimum fatigue failure requirements (based on maximum in-field carabiner use) 7

0 2000 4000 6000 8000 10000 12000 14000 5 Cycles to Failure

Introduction Motivation Objective Approach Results Conclusions/Future Work Questions

Webbing What Is a Carabiner?

• Metal link connecting climber to rope and rope to mountain side via webbing • Features gate that climber opens to insert/remove rope or webbing under loaded & unloaded conditions • Most common type Climber attaching rope to carabiner. – D-shaped – Aluminum

• MTS error load Error: ±13N 13N => 100 ∗ = 0.16% error 8000N • Carabiner manufacturing • Negligible errors: – Strain Gauge – Temperature – Deformation of steel pins (see next slide)

l Grip Pin modeled as b Pin

Carabiner

•From Crandall, Dahl, Lagner: 2/3 −bLPb 22 )( δ = ∗= 1097.4 −6 m max 39 LEI •Smallest deflection observed: 0.0015 m at 8kN

e − 697.4 •This represents a 100 ∗ = 0.33% error e − 35.1 (at most) April 5, 2002 © MIT Center for Sports Innovation 37 Critical Stress Intensity Factor (Kc)

210

175 Closed Gate Open Gate 140

105

Stress [kPa] Stress 70

• Too difficult to accurately run tests at desired frequency – Desired test frequency and max. load: 2 Hz, 8 kN – MTS machine capabilities: 1.3 Hz, 7.3 kN • MTS machine unable to account for stretch in webbing • 1 test completed at lowest load, results inconclusive

Before Plastic After Plastic Deformation Deformation

Gate Pin

Motion Resulting from Red Plastic Deformation Latch Paint Displacement Opening

Close-up of gate latch on carabiner depicting the resulting displacement due to plastic deformation of the carabiner in an unloaded condition both before and after the carabiner is cyclically loaded.

• Used to evaluate stress concentrations in carabiner to predict failure area.

• Max Stress at top and bottom of carabiner

• Stress concentrations also evident at bends

• MTS Tensile Loading Machine – Program load conditions – Mating of machined grips to vice clamps of MTS – Tare out zero displacement conditions • Strain Gauges – Attachment to carabiner – Tare out zero load condition