Ice Screw Analysis

Introduction
3D Models
Material Properties
Analysis
Results
Discussion

icescrews
Screw placement2

Introduction

Ice Screws are devices used by ice climbers to anchor the rope to the ice for protection in the event of a fall.  These screws are hollow tubes made of hardened alloy steel with cutting teeth on one end and a hanger which acts as a crank handle on the other.  The tubes are also threaded with a coarse, high profile thread that facilitates insertion and prevents direct pullout.  Much testing has been done in the lab and in the field to determine the holding power and optimum angle of placement for an ice screw.  Their results show that placing a screw perpendicular to the surface of the ice or at a  slightly down angle holds the highest load.  Long falls can generate tremendous forces on these screws so getting the most out of them is very important.  For more information on the testing results and a little more about ice screws checkout the report from Black Diamond.

3D Models

I created the 3D models based on the dimensions of  Black Diamond ice screws. To somewhat simplify the model, I omitted the slight taper on the outside of the tube and also the teeth on the end of the screw.  The taper reduces the torque required to place a screw in the ice and the teeth do not add or subtract  much from the holding strength.  Because I’m not real interested in the hanger / screw interaction, I simplified the hanger to be a simple tab that I could apply a load to.  The stresses in the ice, I’m assuming, will be very localized.  The 3D model for the ice is a hemisphere with a diameter of 22 inches.  The screw is placed dead center in the face of the hemisphere.

model all
screw 1
screw 2

Material Properties

Ice Screw Steel:
I’m not sure what material BD uses for its screws but I would assume it’s something like AISI 4130.  This is heat treatable alloy steel that gives good hardness, strength and enough ductility that the steel will bend without cracking.  However, knowing the exact alloy is not necessary because I have limited the loads to not exceed a working stress on the screw of 130,000 psi.  This allows for a linear material model for the analysis and all steels have roughly the same Modulus of Elasticity of 29.5X10^6 psi.
Ice:
Ice properties vary from slushy to hard and brittle like glass.  After quite a bit of searching on the web I found some testing data that was done by the U.S. Army Corps of Engineers specifically for Ice Engineering (Chap 6-2).  All of the properties are proportional to strain rate, temperature, grain size, grain structure and porosity and vary greatly.  For strain rates above 10^-3/ sec. ice will behave as a brittle material and the compressive strength will start to decrease.  High strain rates above this limit will most likely occur in the ice directly under the screw by the mouth of the hole.  To help simplify the simulations all Ice will be considered isotropic and ductile.  Compressive strengths for fresh water ice are listed between 72.5 psi and 2900 psi.  The Effective Modulus for ice is is listed between 2.9X10^5 psi at low frequency loading up to 1.3x10^6 psi at high frequency loading.

Analysis

This analysis is meant to show the relative stresses in the ice and ice screw placed at different angles and not to predict pullout loads.  The placement angles that I chose were +15, 0 and -15 degrees.  The screw lengths used are 13, 17 and 22 cm.  For the ice properties I chose an effective modulus of 4.35x10^5 psi. and compressive strengths of 800,1500 and 2900 psi.  The ice will also be modeled as a bilinear isotropic material with a yield strength equivalent to the compressive strength and a tangent modulus of 10,000 psi.  The use of a bilinear model is to try to account for some of the localized ice crushing.  When the calculated stress in the ice exceeds the compressive strength, its stiffness is reduced by a factor of 43.5 times for the rest of the simulation.

Scenarios Analyzed:

Screw Length (cm)

Screw Placement Angle (deg)

Ice Compressive Strength (psi)

17

+15

1,500

17

0

1,500

17

-15

1,500

13

+15

1,500

13

0

1,500

13

-15

1,500

22

+15

1,500

22

0

1,500

22

-15

1,500

17

+15

800

17

0

800

17

-15

800

17

+15

2,900

17

0

2,900

17

-15

2,900

Angle02

The contact surface between the ice and the ice screw was modeled as a frictionless interface and the hanger is bonded to the screw head. For all scenarios, a load of 1,800 lbs was applied to the end of the hanger.  Assuming the flat ice surface is the X,Y plane and the screw hole (-Z axis), the applied load vector is (0,-1738.67,465.87) lbs.  This 15 degree off vertical down (-Y axis) accounts for a slight outward direction of a climber’s fall.

A fixed support constraint was applied to the outer surface of the ice hemisphere and a displacement constraint of x=0 was placed on the end of the hanger to aid in solution convergence.

Results:

The results of the FEA are similar to the results of the field test.  Screws placed at a negative angle  have lower stress both in the ice and the tube of the screw under all scenarios.  Maximum stresses listed in the reports for the ice do not accurately represent the true stress state in the ice.  This was caused by the sliver of ice created by the thread termination point at the mouth of the hole.  For this reason, the ice stress will be displayed in stress plots only with the same contour scale.

13 17 22 cm med ice
17 hard med soft

17 cm Ice Screw in Medium Strength Ice (1,500 psi Compressive Strength)

click to view report

screw 17 med +1503 screw 17 med 0
screw 17 med -15

Figures 1, 2 and 3 show the Ice screw stress in medium strength ice (1,500 psi crush) with an applied load of 1,800 lb.  Contour scales have been adjusted to a maximum of 81000 psi in the contour plots for the screws.  Placement angles are +15, 0 and -15 degrees respectively.

Figures 4, 5 and 6 show the Ice stress in medium strength ice (1,500 psi crush) with an applied load of 1,800 lb.  Contour scales have been adjusted to show red where the ice stress has exceeded the crush strength.  Placement angles are +15, 0 and -15 degrees respectively.

ice 17 med +15 face ice 17 med +15
ice 17 med 0 face ice 17 med 0
ice 17 med -15 face ice 17 med -15

17 cm Ice Screw in High Strength Ice (2,900 psi Compressive Strength)

click to view report

screw 17 high +15
screw 17 high -15
screw 17 high 0

Figures 7, 8 and 9 show the Ice screw stress in high strength ice (2,900 psi crush).  Contour scales have been adjusted to a maximum of 81000 psi in the contour plots for the screws.  Placement angles are +15, 0 and -15 degrees respectively.

Figures 10, 11 and 12 show the Ice stress in medium strength ice (1,500 psi crush) with an applied load of 1,800 lb.  Contour scales have been adjusted to show red where the ice stress has exceeded the crush strength.  Placement angles are +15, 0 and -15 degrees respectively.

ice 17 high +15 face ice 17 high +15
ice 17 high 0 face ice 17 high 0
ice 17 high -15 face ice 17 high -15

17 cm Ice Screw in Low Strength Ice (800 psi Compressive Strength)

click to view report

screw 17 low +15
screw 17 low -15
screw 17 low 0

Figures 13, 14 and 15 show the Ice screw stress in low strength ice (800 psi crush).  Contour scales have been adjusted to a maximum of 81,000 psi in the contour plots for the screws.  Placement angles are +15, 0 and -15 degrees respectively.

Figures 16, 17 and 18 show the Ice stress in low strength ice (800 psi crush) with an applied load of 1,800 lb.  Contour scales have been adjusted to show red where the ice stress has exceeded the crush strength.  Placement angles are +15, 0 and -15 degrees respectively.

ice 17 low +15 face ice 17 low +15
ice 17 low 0 face ice 17 low 0
ice 17 low -15 face ice 17 low -15

13 cm Ice Screw in Medium Strength Ice (1,500 psi Compressive Strength)

click to view report

screw 13 med +15
screw 13 med -15
screw 13 med 0

Figures 19, 20 and 21 show the Ice screw stress in low strength ice (1,500 psi crush).  Contour scales have been adjusted to a maximum of 81,000 psi in the contour plots for the screws.  Placement angles are +15, 0 and -15 degrees respectively.

Figures 22, 23 and 24 show the Ice stress in medium strength ice (1,500 psi crush) with an applied load of 1,800 lb.  Contour scales have been adjusted to show red where the ice stress has exceeded the crush strength.  Placement angles are +15, 0 and -15 degrees respectively.

ice 13 med +15 face ice 13 med +15
ice 13 med 0 face ice 13 med 0
ice 13 med -15 face ice 13 med -15

22 cm Ice Screw in Medium Strength Ice (1,500 psi Compressive Strength)

click to view report

screw 22 med +15
screw 22 med -15
screw 22 med 0

Figures 25, 26 and 27 show the Ice screw stress in low strength ice (1,500 psi crush).  Contour scales have been adjusted to a maximum of 81,000 psi in the contour plots for the screws.  Placement angles are +15, 0 and -15 degrees respectively.

Figures 28, 29 and 30 show the Ice stress in medium strength ice (1,500 psi crush) with an applied load of 1,800 lb.  Contour scales have been adjusted to show red where the ice stress has exceeded the crush strength.  Placement angles are +15, 0 and -15 degrees respectively.

ice 22 med +15 face ice 22 med +15
ice 22 med 0 face ice 22 med 0
ice 22 med -15 face ice 22 med -15

Discussion:

As the above figures show, placing your ice screws at a negative angle will reduce the stress in both the ice screw and the ice.  Reducing these stresses will allow the ice screw placement to hold more load during a fall.  Results for the 22 cm and the 17 cm ice screws were almost identical.  I believe that a longer ice screw will hold better in softer ice however, in good ice they will hold about the same.  The 13 cm ice screw appears to hold as well as the 17 cm screw in good ice.  In soft ice or very brittle ice this short screw length would be mostly in the crush zone by the surface and would be more likely to pull out under load.  As with every high risk sport, there is no substitute for experience.  This information has been provided as a comparison of different scenarios and not as a definitive load prediction for ice screw failure.  Ice conditions change daily and hidden cracks and air pockets can greatly reduce an ice screws holding strength.  Use your best judgment when placing screws and use screamers to help reduce the impact load from falls.

 

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