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Harpur Hill, Buxton Derbyshire, SK17 9JN T: +44 (0)1298 218000
F: +44 (0)1298 218590 W: www.hsl.gov.uk © Crown copyright
(2006)
Development of a technique to measure the
dynamic loading of safety harness and lanyard webbing
HSL/2006/37
Louise Robinson B.Eng (Hons) GradIMMM Project Leader:
Author(s): Louise Robinson B.Eng (Hons) GradIMMM
Science Group: Engineering Control Group
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CONTENTS
1
INTRODUCTION.........................................................................................
1 1.1 Background
.............................................................................................
1 1.2 Aim
..........................................................................................................
1 1.3 Test material
............................................................................................
2
2 DESIGN AND DEVELOPMENT
................................................................. 3
2.1 Design Requirements
..............................................................................
3 2.2 Rig Design and Production
......................................................................
3 2.3
Instrumentation........................................................................................
6
3
TESTING.....................................................................................................
9 3.1 Development of test
method....................................................................
9 3.2 Data
Analysis.........................................................................................
10 3.3 Line-scan image analysis
......................................................................
11
4 RESULTS AND DISCUSSION
.................................................................
14 4.1
Results...................................................................................................
14 4.2 Discussion of
results..............................................................................
16
5
CONCLUSIONS........................................................................................
18
6
RECOMMENDATIONS.............................................................................
19
7
REFERENCES..........................................................................................
20
APPENDIX.......................................................................................................
21 Drop Testing of Webbing –
Method..................................................................
21 Graphs showing load against time
...................................................................
23 Graphs showing extension against
time........................................................... 27
Graphs showing load against
extension...........................................................
31
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EXECUTIVE SUMMARY
Objectives
The objective of this study was to develop a method for
evaluating the dynamic performance of safety harness and lanyard
webbing materials. The performance of webbing materials prior to
their use in the manufacture of safety harnesses and lanyards is
measured in terms of their static properties. Their breaking
strength is determined statically or quasi-statically, however in a
fall situation these materials would behave in a different manner.
During a fall the webbing materials would be dynamically loaded and
so the dynamic breaking strength is an important parameter, which
may or may not be related to the static breaking strength.
Main Findings
A test rig was developed comprising of two parts that separated
to dynamically load the webbing material. This provided consistent
results, comparable with those obtained by static testing. Four
loadcells built into the rig provided load data, with elongation of
the samples measured using a line-scan camera attached to a data
capture device. Ten samples in all were tested. Two of the webbing
types tested failed; two of the webbing types tested did not break.
The dynamic breaking loads achieved for the webbings were up to 38
% lower than the equivalent maximum breaking strength resulting
from static tensile testing. The dynamic elongations to failure
were up to 28 % lower than the equivalent static elongation to
failure. The energies absorbed to failure were up to 44 % lower
than those resulting from static testing. Two of the four webbings
tested were able to withstand a 127 kg drop mass. This mass was
identified as being typical of the 99th percentile of workers at
height in a recent body size study commissioned by HSE [Haines,
Elton and Hussey, 2005].
Recommendations
The test method has been very successful and could be adapted
for other strengths of webbing material. The rig was
over-complicated and could be simplified by the removal of the load
cells on the top member, as these proved to be unnecessary. The rig
generates a lot of energy during a drop test and the energy
absorbers used within it at present are not sufficient. A re-design
of the rig will be necessary as a future development, to improve
robustness and provide better energy absorption.
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1 INTRODUCTION
1.1 BACKGROUND The performance of webbing materials prior to
their use in the manufacture of safety harnesses and lanyards is
currently measured in terms of their static properties. Their
breaking strength is determined statically or quasi-statically,
however in a fall situation these materials would behave in a
different manner. During a fall the webbing materials would be
dynamically loaded and so the dynamic breaking strength is an
important parameter. The dynamic performance of these woven
materials may or may not be related to their static performance and
the nature of their weave. Currently there are no requirements to
measure the dynamic strength of webbing materials, or other
components used to manufacture safety harnesses and lanyards. The
dynamic testing that is carried out currently uses a standard 100
kg drop mass on the finished product as whole, either a safety
harness or a lanyard. BS EN 361:2002 “Personal protective equipment
against falls from height – Full body harnesses” states that
harnesses should withstand two dynamic drop tests, using a standard
100 kg torso dummy, with a free fall distance of 4 metres. The
standard also states that the harness should withstand a force of
15 kN applied to it by pulling the torso dummy, whilst in the
harness, suspended by each attachment point in turn. BS EN 354:
2002 “Personal protective equipment against falls from height –
Lanyards” states that all textile lanyards must withstand a static
force of at least 22 kN. Lanyards are only tested dynamically in
the same way as harnesses, to determine whether they can withstand
a drop test using a 100kg test mass dropped from a height of 2
metres above the attachment point (the lanyard and connectors
having been adjusted to 2 metres in length, resulting in a free
fall distance of 4 metres). It could be of great benefit to the
manufacturers of these materials to know how their product performs
dynamically before being incorporated into a harness or lanyard,
for future development. If significant differences arose between
dynamic and static performance of the webbings this would have a
strong impact as it would reduce the factors of safety on products
currently on the market. I was asked by Mr Martin Holden, formerly
of Health and Safety Executive (HSE) Construction Division
Technology Unit, to develop a method of dynamically testing webbing
samples. The research into webbing performance contributes towards
HSE’s commitment to reduce fatal and major injuries resulting from
falls from height under the Fit3 programme (Fit for Work, Fit for
Life, Fit for Tomorrow)
[http://www.hse.gov.uk/aboutus/plans/hscplans/0506/fitfor.htm].
1.2 AIM The purpose of this research programme was to develop a
test method to measure the dynamic performance of webbing
materials. The data required from the test programme were breaking
strength and elongation to failure. A purpose-made test rig was
required to be designed, built and commissioned with a small number
of samples tested to refine the technique and identify further
areas for development. The development of a reliable technique
could lead to further research into the dynamic performance of
materials affected by various types of damaging effects such as
ultraviolet (UV) degradation, ingress of dirt effects, abrasion and
weathering.
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1.3 TEST MATERIAL The samples to be used were to be taken from
the same selection as used in the previous study carried out by HSL
entitled “Assessment of the factors that influence the tensile
strength of safety harness and lanyard webbings” [Parkin and
Robinson, 2002]. In that work each webbing was statically tensile
tested to failure in the as received condition, so the breaking
load, elongation to failure and energy absorbed to failure were
known. These results were used for comparison with the results
obtained by dynamic testing. The webbing materials chosen for
testing were from different suppliers, made from either polyamide
or polyester, and manufactured in different ways with different
additives. This resulted in webbings with different colours,
weaves, dimensions and mechanical properties. Each webbing type was
supplied in a reel, and was cut into one metre sample lengths, the
ends of which were heat sealed to prevent fraying during testing.
The webbings were originally designated numbers from 1 to 10 in
previous research [Parkin and Robinson, 2002]. Since then, however,
some of these webbings are no longer manufactured and those
remaining available for testing were numbers 1, 2, 3 and 8. The
webbing samples in this study were all tested in the ‘as received’
condition and were all dry.
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2 DESIGN AND DEVELOPMENT
2.1 DESIGN REQUIREMENTS A test rig was required which would
dynamically load a webbing specimen, and which would be
instrumented to measure the breaking load and extension to failure.
Slotted cylindrical grips, of the same type used for static testing
of webbing were required, to avoid samples breaking due to damage
caused by conventional grips. The samples were required to be
dynamically loaded to failure, and so the rig needed to be designed
to withstand the shock loading which would occur during a drop. In
order to ensure true dynamic loading of the specimen, the rig was
to be comprised of two parts, linked by the webbing sample under
test. One part of the rig would act as a carrier and be arrested by
a catching device, while the other part comprising the weight pack
would continue to drop, uninterrupted, causing dynamic loading of
the webbing. Means of arresting both the top section of the rig
before the webbing had broken, and the falling weight pack after
the webbing had broken were required, incorporating energy
absorbers to minimise ‘ringing’. Ringing is the generation of
harmonic resonances within the rig as a result of the impact,
causing noise to be generated in the data collected by load cells.
At its worst, this could totally obscure the data being collected.
Both the drop height and the weight needed to be adjustable in
order to vary the drop forces the specimens experience. In that
way, tests could be performed to specific maximum forces, as well
as allowing the determination of breaking forces to cause failure.
The drop tests needed to be performed in a controlled manner in
order to give consistent results and to ensure safety during the
drop. A guided system therefore was required. Because the rig was
designed to separate, a safe means of temporarily holding the rig
together while samples were being loaded was required.
2.2 RIG DESIGN AND PRODUCTION
The initial design for a separating drop rig was conceived by
myself, in conjunction with Mr. Carl Wilson, formerly of Field
Engineering Section, HSL, and a draft design produced (Figure 1).
After discussion with R.A. Engineering consultants, this design was
refined and finalised. The test rig was designed to fit into the
existing drop tower facility in the Field Engineering Section of
HSL. The rig was designed such that both the drop height and the
drop mass could be altered.
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Figure 1: Showing the original sketch and specifications of the
drop rig
The design comprised of a lightweight carrier section for the
top of the rig, with two load cells holding slotted cylindrical
grips. The bottom section comprised of a fixed lower plate onto
which slotted weights could be stacked, with an upper clamping
plate. Twelve rectangular plate slotted weights, each of 20 kg were
included in the design, to vary the drop forces which could be
produced. Two more load cells were attached to the upper clamping
plate, and then held another cylindrical grip. Two threaded bars
were fixed to the bottom plate, passed through the weight pack,
through holes in the clamping plate where the weight pack could be
secured by nuts, and through the top section of the rig, allowing
the whole unit to be fastened together for ease of loading samples.
Removal of the nuts put the specimen under tension, and released
the bottom section of the rig from the top, separating both parts,
making it ready to drop.
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1 2 3
Figure 2: Showing the three stages of the drop, from left to
right: 1) release; 2) top section arrested, dynamic loading of
sample;
3) bottom section arrested after sample failure
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In order to control the drop a frame was designed to fit into
the drop tower, above the drop rig to support guide wires. These
wires were secured to a base plate using eye bolts, and tensioned
using turnbuckles until just taut. Both top and bottom sections of
the rig ran through the guide wires ensuring they would impact in
the correct positions on the arresting devices. In order to prevent
damage to the guide wires, they were designed to be fully enclosed
in tubes attached to the top section of the rig. These tubes passed
through holes in the bottom of the rig, so as to allow unhindered
separation of the two parts.
The base plate incorporated arresting devices for both parts of
the rig; two impact columns positioned one metre apart, and one
metre tall arrested the top section, and two energy absorbing pads
in the centre of the plate formed an arrest buffer for the bottom
section. Both catchers had extra energy absorbing pads added to
them, made from dense rubber.
The rig was designed to be raised and lowered on the existing
winch in the drop tower, allowing a drop from any height up to two
metres. It was lifted using wire strops attached to the fixed lower
plate of the bottom section. The rig was dropped from the winch
hook using an electronic bomb release.
The dynamic action of the drop rig can be seen in Figure 2, and
a photograph of the finished rig can be seen in Figure 3. It was
manufactured from steel, and powder coated red to protect it from
corrosion. Twelve weights, of approximately 20 kg each were also
provided, manufactured from steel and painted yellow to protect
them from corrosion.
2.3 INSTRUMENTATION
Four calibrated 5000 kg capacity S-Beam Type 620 load cells,
manufactured by Tedea-Huntleigh were built into the rig, two in the
top section and two in the bottom section. The data from the load
cells was acquired by a VHS Spectra data logger, manufactured by
Intercole.
The data from the load cells was logged at a speed of 25 kHz in
order to ensure a suitable resolution.
Three techniques were trialled to measure the extension of the
samples. These were high-speed video recording, laser tracking and
line-scan camera measurement.
2.3.1 High Speed Video
This is a system where a digital video camera captures images
direct to solid state memory. Unlike conventional video recording,
which captures 25 frames per second, high speed video captures up
to 4500 frames per second. This can then be played back at a rate
to allow the event to be analysed at an appropriate speed.
The high-speed video system available at the time of the test
programme at HSL was a monochrome system, recording grayscale
images. To provide a means of measurement of the images, black and
white markers were placed on the middle of the top and bottom
scroll clamps. These markers can be seen in Figure 3. Four drops
were recorded at four increasing speeds, however, the faster the
record speed, the lower the resolution of the recording. This
system, although successful for measuring the displacement of the
scroll clamps, did not have a fast
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enough recording rate to capture the point of failure of the
webbing. This meant that a definite end point for the failure could
not be determined, and so an accurate elongation to failure could
not be determined by this method.
Figure 3. Finished drop rig, with lifting modification and
line-scan camera targets.
(Shown with non-test webbing sample to demonstrate loading)
2.3.2 Laser tracking
A small laser unit with detector was attached to the back of the
top section of the rig, with a sliding black and white marker
strip, on which it focused. The strip was attached to the lower
section of the rig and was free to slide vertically past the laser
during a drop test. The strip comprised of black bands 2 mm wide
spaced 2mm apart on a white background. This formed a regular strip
of black and white bands, similar in appearance to a barcode. As
this strip passed the laser, a detector recorded the change in
contrast as electrical pulses. The rate of passage of these bands
could be seen as different width pulses on a trace of voltage
against time. The wider the recorded pulse, the slower the marker
had travelled past the laser at that point. Analysis of these
pulses can be used to calculate drop displacement.
The shock loading imparted by the rig caused the laser tracking
system to vibrate, which resulted in poor resolution of the bands,
and multiple reading of the same bands. This caused poor resolution
of the pulses in the recorded data trace, meaning no useful results
could be extracted.
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2.3.3 Line-scan Measurement
The line-scan method of measuring displacement was originally
developed in the Field Engineering Section. The line-scan camera
was designed for precise measurement of components for quality
control. It was originally used to measure the distance between two
points, by scanning across an item, recording the data as a line of
1024 greyscale pixels. Measurements can then be taken between areas
of high contrast, either naturally present on the item, or created
by placement of markers. In order to create a time history,
software was developed by Software and Control Section, HSL, to
store individual lines of pixels as a single bitmap image. Images
captured by a line-scan camera have a much higher resolution than
those of a conventional video camera, and the image is captured
much faster.
The camera was set up to measure two points on the test rig, one
on the top scroll clamp, and one on the bottom (Figure 3). These
two points were identified with large markers, comprised of a black
line between two white lines. This gave a high contrast area which
could be clearly seen by the camera from the distance required to
fit the whole rig in the frame. The camera was set-up with a line
rate of 2, a gain of 8 and a capture period of 2000. Each bitmap
showed a period of 2 seconds, with each row of pixels being
equivalent to 0.5 ms.
The separation of the two sample grips was measured before each
test, to calibrate each image, and allow calculation of the
extension to failure.
This system proved very successful, but was difficult to use as
it had to be triggered manually at the start of each drop.
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3 TESTING
3.1 DEVELOPMENT OF TEST METHOD
Three commissioning tests were carried out, without
instrumentation, using samples of webbing which were not part of
the main test programme. The rig was set up with only one slotted
weight in the weight pack. The first drop was from a height of 0.5
metres, the second from 1 metre and the third test from a height of
2 metres. The webbing samples were cut to one metre lengths to
allow sufficient material to wrap around the clamp arrangement. The
gauge length of webbing tested was between 120 mm and 150 mm. The
gauge length was taken as the distance between the centre line of
the top and bottom clamps. This was measured before each test using
a vernier caliper.
After the first drop a problem was noted with the way in which
the webbing was loaded in the clamp arrangement. The webbing tended
to pull out of the clamps rather than elongate in the gauge length.
Part of the reason for this was that the scroll arrangement had
been fixed in the vertical position. Rotating the clamps through
90° resulted in a considerable improvement in the holding
properties of the clamps. The webbing was fed back over itself,
wrapping in an s shape around the grips (Figure 4). The grips were
fixed in line with each other, resulting in the sample having to be
loaded in an s-type arrangement to ensure centre line loading,
however this was only at the mid-point of the specimen.
Figure 4: Showing the sample scrolling arrangement
The first two drops did not break the webbing, the third drop
however did break the webbing. It was decided to increase the
weight in the pack to 60 kg, and drop from a height of 2 metres,
throughout the test programme, to ensure that the samples would
break. The total drop weight on the webbing was 127 kg.
This test procedure was developed, which was followed throughout
the test programme and can be seen in the Appendix.
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3.2 DATA ANALYSIS
The loadcell data collected was converted into ASCII format
using a custom-written extraction program and input into a
numerical analysis software package. A conversion was applied to
the data to convert it from millivolt (mV) output to load in
kilo-Newtons (kN), using the data obtained from calibration of the
load cells. Since the data were recorded at 25 kHz, each data point
represented 1/25000th of a second, so time data were generated, to
allow a load history trace to be plotted.
Sample number
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Load
cel
l out
put,
mV
-800
-600
-400
-200
0
200
400
600
800
1000
1200
Loadcell 1Loadcell 2Loadcell 3Loadcell 4
Figure 5: Raw loadcell data from test 4
Data from all four loadcells were recorded, and plotted against
time (Figure 5). The top two loadcells recorded the load seen in
the top section of the rig, the bottom two loadcells recorded the
load seen in the bottom section of the rig. In order to obtain the
load experienced by the webbing, the data from the two top
loadcells were added together. The data from the bottom two load
cells, which was equal but opposite, were also added together and
inverted. The two resulting load-time traces were compared (Figure
6).
Sample number
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Load
cel
l out
put,
mV
-400
-200
0
200
400
600
800
1000
1200
1400
1600
Top loadcell dataBottom loadcell data
Figure 6: Raw loadcell data from test 4, combined to give data
for the top and bottom
sections of the rig
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The data from the top loadcells showed the same overall trend as
that from the bottom, but showed a sharp peak at the beginning of
loading. This resulted from the impact of the top section of the
rig on the arrester arms. It can also be seen that the data trace
from the top section was noisier than that from the bottom section.
This was due to ringing in the top section of the rig after arrest.
The bottom section of the rig, however, fell uninterrupted,
breaking the sample before it hit the arresters and resulting in a
data trace with no impact peaks occurring until after the sample
had broken. Overall, there was good agreement between the data from
the top section and that from the bottom section. Because of this,
all breaking loads were taken from the bottom loadcell data only
(Figure 7).
The time to failure was determined as the time from the onset of
loading to the load falling to zero.
Time, s
0.0 0.1 0.2 0.3 0.4 0.5
Load
, kN
-10
-5
0
5
10
15
20
25
Figure 7: Data from test 4 after analysis, using bottom loadcell
data only
3.3 LINE-SCAN IMAGE ANALYSIS
The data from the line-scan camera was recorded in the form of a
bitmap, measuring 1024 pixels wide by 4000 pixels in length (Figure
8). The width of the bitmap corresponds to the size of the image
the camera is focussed on, in this case the resting position of the
drop rig. The length of the bitmap corresponds to the recording
time, which in this case was set to 2 seconds, therefore each pixel
represented 0.5 milliseconds.
Figure 8: Original image captured by line-scan camera
The bitmaps were analysed using a software package for drawing
from JASC Ltd. called Corel Draw, version 8. The bitmap image was
rotated through 90° anticlockwise for ease of analysis. Each image
consisted of a grey background with two diagonal lines passing
through it, consisting of a white band, with a black central band.
These bands were made up of single line- 11
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scan images of the changing position of the marker strips, on
the sample grips of the rig, with each 0.5 ms. The line to the top
right of each image represented the position of the top section of
the rig, and the line to the left represented the position of the
bottom section of the rig.
Figure 9: Analysis conducted by measuring the separation of the
lines at pixel intervals
on a cropped and stretched area of the image
The distance between the centre point of each black central band
on the image was measured in pixels, at intervals of 2 pixels
horizontally (Figure 9). The measurements were taken at intervals
of one pixel horizontally around a change in direction, where they
occurred. Before each test, the separation of the black marker
strips was measured using a vernier calliper, and this measurement
was then applied to the line-scan image data to calculate the
elongation. A plot of extension versus time was then generated,
from which extension to failure could be determined (Figure
10).
Time, s
0.00 0.01 0.02 0.03 0.04 0.05
Ext
ensi
on, m
m
0
20
40
60
80
100
120
140
160
180
Figure 10: A graph of extension (change in separation) against
time, created from the
measured data.
The point of failure could be seen on the plot as a change in
velocity, but sometimes this was difficult to determine. Some
traces show several changes in velocity, but in some this change is
barely perceptible. For this reason it was found to be easier to
determine the time to failure from the load data, and then apply
this to the line-scan data to determine the extension to
failure.
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Using both the data from the line-scan images and the loadcell
data, graphs of load against extension were plotted, allowing the
energy to failure to be determined. This was the area under the
curve, and was calculated in kilojoules, kJ.
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4 RESULTS AND DISCUSSION
4.1 RESULTS
The results of the drop tests are shown in the Table 1,
below.
Table 1. Results of dynamic testing of webbing samples
Test number Webbing designated
number
Breaking load, kN Elongation to failure, m
Energy to Failure, kJ
1 3 Pulled out of grips No data -
2 3 20.545 0.115 1.470
3 3 20.417 0.106 1.318
4 3 20.190 0.106 1.295
5 8 Did not break
(withstood 37 kN) No data -
6 2 Did not break
(withstood 37 kN) No data -
7 1 No data 0.100 No data
8 1 18.713 0.095 0.972
9 1 18.915 0.092 1.135
10 1 17.377 0.091 0.987
Table 2 shows the average breaking loads, extension to failure
and energy to failure, compared to that obtained for the
‘as-received’ material tested statically in previous research. All
the results presented have been anonymised and are referred to as
webbing 1, 2, 3 and 8. Table 2 also shows the percentage
differences between the dynamic and static tensile performance of
the webbing materials.
The graphs plotted from both the loadcell data and the line-scan
camera data, along with graphs of load against extension can be
seen in the Appendix. Graphs of load against extension were plotted
and the area under the curve calculated to obtain the energy to
failure, in kilojoules (kJ).
In test number 1 the line-scan camera failed to trigger, and so
no elongation data was recorded, and therefore no energy to failure
could be calculated. In tests 5 and 6, the webbing samples used did
not break, and so a maximum load withstood was recorded. Since the
samples did not break, no elongation to failure was determined, and
therefore no energy absorbed to failure was calculated. On test
number 7 the software running the data logger crashed during the
extraction phase of the data recording.
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Table 2. Comparing results obtained dynamically with those
obtained statically
Dynamic Testing Results Static Testing ResultsWebbing,
designated number
Average breaking load, kN
Average elongation to failure, m
Average energy to failure, kJ
Average breaking load, kN
Average elongation to failure, m
Average energy to failure, kJ
1 18.34 0.093 1.03 29.27 0.129 1.83
didn't fail2 withstood 37 kN - - 40.54 0.193 3.02
3 20.38 0.109 1.36 25.49 0.133 1.58
didn't fail8 withstood 37 kN - - 28.67 0.192 2.08
Dynamic results as a percentage of static results
Breaking load, % Elongation to failure, % Energy to failure,
%
62.70 72.10 56.30
- - -
79.95 81.95 86.10
- - -
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Tests 1 to 4 were carried out on webbing 1. Tests 5 and 6 were
conducted on two different types of webbing, designated webbings 2
and 8 respectively. Since these webbings did not fail, tests 7 to
10 were carried out on webbing 3.
The failure of the webbing occurred very rapidly, with an
average time to failure of 0.0225 seconds for webbing type 1 and
0.0165 seconds for webbing 3. The two webbings which did not fail
experienced a peak load after approximately 0.0025 seconds from the
onset of load.
The failures of the samples all occurred perpendicular to the
applied force, unlike in static testing where diagonal failures
(approximately 45°) are also seen. All the failures occurred within
the gauge length. A typical failure of a webbing sample can be seen
in figure 11. The webbing colour in the photograph has been
digitally altered to protect the manufacturers anonymity.
Figure 11. Typical dynamic failure of webbing
4.2 DISCUSSION OF RESULTS
It can be seen from both the results in Table 1 and the graphs
in the Appendix, that the breaking loads obtained for webbing 1
were in good agreement, as were the extensions obtained. This was
also the case for the tests conducted on webbing 3. This shows that
the testing technique produces consistent results and is a suitable
technique for dynamic testing of webbing samples.
Table 2 shows the comparison between dynamic test results and
those obtained by static testing for the same webbing types in the
same, as received, condition.
Two of the webbings used in this study failed when subjected to
a mass of 127 kg from a height of 2 metres, two other webbings
tested did not fail. The mass was greater than 95 percentile 100kg
standard drop mass specified by BS EN 364:1993 “Personal protective
equipment against falls from a height – Test methods” for dynamic
testing of lanyards, but this is not out of the range of body
weights seen in the construction industry. A recent study by
Loughborough University on behalf of HSE found that, out of a study
of workers across various industries
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involved with working at height, the 99th percentile of the
sample group weight was 127.6 kg [Haines, Elton and Hussey, 2005].
This man-weight was measured without tools and equipment being
carried, which could add up to 10 kg to the total weight of the
worker. Indeed, a 95th percentile value of 122 kg was suggested as
a more accurate representation of the weight of workers involved in
working at height, close to the weight used in this test programme.
As the rig has been designed for varying loads and drop heights,
100kg drops could be performed to allow a direct comparison with
the current EN standard methods.
The average dynamic breaking load of webbing 1 was 18.34 kN; the
average elongation to failure was 0.093 m and the average energy
1.03 kJ. When tested statically, the same webbing had an average
breaking load of 29.27 kN, an average elongation to failure of
0.129 m, and an average energy of 1.83 kJ.
The average dynamic breaking load of webbing 3 was 20.38 kN; the
average elongation to failure was 0.109 m and the average energy
1.36 kJ. When tested statically, the same webbing had an average
breaking load of 25.49 kN, an average elongation to failure of
0.133 m, and an average energy of 1.58 kJ.
Webbings 2 and 8 did not break when subjected to drop testing.
The load withstood by each webbing was 37 kN. No data was gathered
for elongation, so energy absorbed could not be calculated. The
static breaking strengths of these webbings were 40.54 kN and 28.67
kN respectively. They were however of different dimensions to the
other webbings tested, being thicker, but narrower. It is possible
that thicker webbing may perform better dynamically than thinner,
but further testing would need to be carried out to determine this.
Differences in weave could also account for the difference in
performance.
Table 2 also shows the differences between the average dynamic
test results and the average static results as a percentage of the
higher, static results. The smallest difference in performance was
seen in the energy absorbed to failure for webbing 3, where the
dynamic result was approximately 14% lower than the static result.
The largest difference in performance was seen in the energy
absorbed to failure of webbing 1, where the dynamic result was
approximately 44 % lower than the static value. Webbings 1 and 3
failed at less than this 22 kN value.
Dynamic testing of both webbings 1 and 3 resulted in lower
breaking loads, lower elongations to failure and lower energy
absorbed to failure than those obtained by static testing. This
would be expected because the sudden application of load on the
webbing gives little time for the fibres in the weave of the
webbing to move and stretch, lowering the energy absorption
capabilities.
The time to failure of webbing 1 was 0.0225 seconds; the time to
failure of webbing 3 was shorter, at 0.0165 seconds. Both webbing
types 2 and 8, which did not fail, reached a peak load in the
webbing after 2.5 milliseconds.
All of the webbing samples showed failures perpendicular to the
applied load. Static tensile testing can often result in failures
at angles. This difference in failure path may be the result of the
way the webbing is woven, rather than a feature of testing. More
tests would need to be carried out using different types of
webbings to investigate whether straight failures perpendicular to
the load are a feature of dynamic failures.
The relationship between the dynamic test results obtained for
150 mm gauge length samples and results obtained by standard drop
testing of 2 metre lanyards, as detailed in BS EN 364:1993, has not
been explored in this work. The results may be similar, or may need
a scaling factor; this would need to be explored in further work.
The results obtained, however, are comparable to the static tensile
test results, since the same gauge length was used.
17
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5 CONCLUSIONS
• The final design of the separating drop rig for webbing
testing performed well, collecting consistent, good quality
data.
• Only the two load cells in the bottom section of the test rig
were required to generate load data.
• Of the three methods used to measure elongation a line-scan
camera was the most suitable method of measuring elongation of the
samples.
• The dynamic breaking loads achieved for the webbings were up
to 38% lower than the equivalent maximum breaking strength
resulting from tensile testing. They also showed up to 28 % lower
extension to failure and up to 44 % lower energy absorption.
• The failures in the webbing samples all occurred perpendicular
to the applied load, in the gauge length.
• The webbing failures occurred in 0.0225 second for webbing
type 1 and 0.0165 s for webbing type 3.
• Two of the webbings tested did not fail, withstanding a peak
load of 37 kN.
• Two of the four webbings tested were able to withstand a 127
kg drop mass. This mass was identified as being typical of the 99th
percentile of workers at height in a recent body size study.
18
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6 RECOMMENDATIONS
The overall system has been very successful and generated
consistent results. The system does need some refinement before
further testing takes place, but has proven to be fairly robust
under shock loading. Some fine-tuning of the design would be
beneficial, as the rig is subject to more wear and tear than
expected and has suffered somewhat during the test programme. The
rig has a tendency to move sideways when it rebounds after a drop
has taken place, so a system based on four guide wires rather than
two may help to prevent this.
Better damping of the system would also result in less wear and
tear on the rig and would reduce noise in the recorded data.
Ideally a pneumatic system would be used, however this is costly,
and a significant improvement on the current system could probably
be achieved by using other, cheaper damping materials.
Four load cells were found to be unnecessary and so the top set,
which generate more noise than the bottom set, can be removed from
the system. The data from the bottom two load cells have proved to
be sufficient during testing.
Off-setting the top and bottom clamps would improve the way in
which the specimens were loaded during testing, ensuring centre
line loading down the whole gauge length of the specimen.
Currently, only the middle of the gauge length is loaded along the
centre line.
The line-scan camera currently has to be manually triggered,
which has proven to be quite difficult, as it relies on the
reactions of the operator observing the test to press the button at
the same time as the rig is released. Integrating the camera
trigger with the bomb release trigger would provide more reliable
recording of the data.
The drop test rig has proven a suitable method for measuring the
dynamic performance of webbing, and its use could be extended to
include measuring the performance of rope and safety nettings. It
could be used for measuring the dynamic properties of webbing
materials which have been subjected to various types of damage,
such as weathering, UV degradation, edge damage and ingress of dirt
damage. This would provide a valuable comparison to the research
data already available about the static properties of such
materials in these conditions [Parkin and Robinson, 2002].
Further research would need to be conducted to examine the
relationship between the results obtained by this test method and
those obtained by testing in accordance with BS EN 364:1993 using 2
metre long lanyards. This would enable a better comparison to be
drawn between testing of short gauge length specimens and full
length, commercially available lanyards.
19
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7 REFERENCES
R Parkin and L Robinson, Assessment of the factors that
influence the tensile strength of safety harness and lanyard
webbings, HSL/2002/16, 2002.
http://www.hse.gov.uk/research/hsl/hsl02-16.htm
V Haines, E Elton and M Hussey, Revision of body size criteria
in standards – Protecting people who work at height , RR 342, HSE
Books 2005. ISBN 0 7176 6102 4.
http://www.hse.gov.uk/research/rrhtm/rr342.htm
20
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APPENDIX
DROP TESTING OF WEBBING – METHOD The dynamic webbing testing
drop rig is installed in the drop tower of Field Engineering
Section. The bomb release is controlled by the drop tower control
system, which has safety interlocks on the gate, PIR sensors to
ensure no personnel are in the exclusion zone prior to testing and
a warning system.
Pre-test inspection Inspect the rig before use for signs of
structural damage. If damage is present do not proceed with
testing. Inspect guide-wires prior to testing for damage and
replace if there are any signs of damage. Attach and tension the
guide-wires using turnbuckles so that there is no sideways movement
of the wires. Inspect wire strops prior to use for any signs of
damage. Replace them if there are any signs of damage. Attach wire
strops to the eyebolts at the base of the weight pack, using
d-shackles. Ensure that the attenuating material is present on the
impact columns and arrest buffers before testing. Ensure that four
nuts are present on the threaded bars, two on each bar. Two nuts
are needed to secure the weight pack; two nuts are needed to
maintain the separation between the carrier section and the weight
pack during a drop test. A further two nuts are needed to secure
the weight pack to the carrier section in order to safely load a
sample into the clamps.
Setting up the test
Lift the weight pack section of the rig onto the arrest buffers.
Lift the carrier section of the rig onto the impact columns. Add
the required number of weights to the weight pack, lifting up the
red clamping plate and slotting the yellow weights on to the bottom
plate. Ensure that the weight with two holes is always on the top
of the weight pack, and immediately underneath the red clamping
plate. Adjust the nuts nearest the weight pack on the threaded bars
so that they secure it by holding the red clamping plate in place.
Using the wire strops, lift the weight pack so that threaded bars
pass through the holes in carrier section. The carrier section
should remain resting on the impact columns. Lift the weight pack
so that there is 100mm between the slots in the sample holders.
Secure the threaded bars externally with nuts to take up the weight
of the weight pack.
21
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Pass the webbing sample through the slotted half bar clamps so
that it wraps over itself around the clamps. The webbing must be
loaded so that it scrolls between the grips, ensuring centre line
loading in the centre of the gauge length. Load the sample by
slowly loosening the nuts on the top of the carrier section,
allowing the sample to take the weight of the pack. Remove these
nuts from the threaded bar. Adjust the uppermost nuts on the
threaded bars so that they are tightened against the carrier
section to maintain the separation during the drop test. Measure
and record the gauge length. Attach a bomb release between the
winch the wire strops pass, so that the strops will fall free when
the bomb release is activated.
Running the test Close and lock the safety gate on the drop
tower. Lift the rig by the winch and bomb release to the test
height required. Zero recording equipment attached to the load
cells. Activate other recording media (high speed video, line-scan
camera). Activate the loadcell data logger. Activate the bomb
release.
After testing The rig should be checked carefully after testing
for signs of damage, especially to the wire strops and tensioned
guide wires. Remove the broken sample from the sample clamps. Lift
the rig slowly using the wire strops, so that the threaded bars
pass through the carrier section, until the internal nuts butt up
against the lower side of the carrier section. Place nuts on the
threaded bars on the upper side of the carrier section and tighten
to secure the weight pack. Lift the rig off the impact columns
using the wire strops and check, and replace if necessary, the
attenuating material. Replace the drop rig on the impact columns
and remove the tension from the wire strops. Slacken off the
guide-wires using the turnbuckles.
22
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GRAPHS SHOWING LOAD AGAINST TIME
Test 2
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
0
5
10
15
20
25
Test 3
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
0
5
10
15
20
25
23
-
Test 4
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
0
5
10
15
20
25
Test 5
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
-10
0
10
20
30
40
24
-
Test 6
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
-10
0
10
20
30
40
Test 8
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
0
5
10
15
20
25
-
Test 9
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
0
5
10
15
20
Test 10
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Load
, kN
0
5
10
15
20
26
-
GRAPHS SHOWING EXTENSION AGAINST TIME Test 2
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Ext
ensi
on, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Test 3
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Ext
ensi
on, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
27
-
Test 4
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Exte
nsio
n, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
Test 7
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Exte
nsio
n, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
28
-
Test 8
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Ext
ensi
on, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Test 9
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Exte
nsio
n, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
29
-
Test 10
Time, s
0.000 0.005 0.010 0.015 0.020 0.025
Ext
ensi
on, m
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
30
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GRAPHS SHOWING LOAD AGAINST EXTENSION
Test 2
Extension, m
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Load
, kN
0
5
10
15
20
25
Test 3
Extension, m
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Load
, kN
0
5
10
15
20
25
31
-
Test 4
Extension, m
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Load
, kN
0
5
10
15
20
25
Test 8
Extension, m
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Load
, kN
0
5
10
15
20
25
32
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Test 9
Extension, m
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Load
, kN
0
5
10
15
20
25
Test 10
Extension, m
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Load
, kN
0
5
10
15
20
25
33
1 INTRODUCTION 1.1 BACKGROUND 1.2 AIM 1.3 TEST MATERIAL 2 DESIGN
AND DEVELOPMENT 2.1 DESIGN REQUIREMENTS 2.2 RIG DESIGN AND
PRODUCTION 2.3 INSTRUMENTATION 2.3.1 High Speed Video 2.3.2 Laser
tracking 2.3.3 Line-scan Measurement
3 TESTING 3.1 DEVELOPMENT OF TEST METHOD 3.2 DATA ANALYSIS 3.3
LINE-SCAN IMAGE ANALYSIS
4 RESULTS AND DISCUSSION 4.1 RESULTS Table 2. Comparing results
obtained dynamically with those obtained statically
4.2 DISCUSSION OF RESULTS
5 CONCLUSIONS 6 RECOMMENDATIONS 7 REFERENCES APPENDIX DROP
TESTING OF WEBBING – METHOD Pre-test inspection Setting up the test
Running the test After testing
GRAPHS SHOWING LOAD AGAINST TIME GRAPHS SHOWING EXTENSION
AGAINST TIME GRAPHS SHOWING LOAD AGAINST EXTENSION
