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TECHNICAL COMMITTEE REPORT TCM 18/01

Failed bolt, San Vito Sicily Incident Ref. 01/18/O.FRE

SUMMARY This report details the failure of a bolt on a sport climbing route in San Vito lo Capo, Sicily.

Author: Isabel Hadley

Checker: Wil Treasure

Date 29/01/2019

Approved by the Technical Committee

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1. INTRODUCTION A BMC member has made the committee aware (see incident report in the Annex) of the failure of a bolt as a result of a leader fall on the route ‘Non ė main troppo tardi’ at San Vito lo Capo, Sicily. The leader sustained minor injuries, and it is understood that a local climber later replaced the casualty bolt with a p-bolt. The retrievable part of the bolt was sent to the BMC Technical committee for examination. 2. EXAMINATION One part of the fractured bolt, along with the associated hanger and a maillon, was received for this investigation, as shown in Figure 1. The bolt was visually inspected, and photographed in the as-received condition. The fracture surface was examined after degreasing ultrasonically in acetone, using scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) micro-analysis. The fracture surface was further examined by SEM/EDX, after being cleaned in an inhibited solution (pyrene) to remove corrosion products. One metallographic section was prepared by cutting the bolt in half longitudinally, Figure 1b. The section was examined, using light microscopy and SEM/EDX. Vickers hardness measurements were carried out on the same metallographic section, at a load of 1kg, and compared with the specification for a widely-used austenitic stainless steel, ie type 304. The chemical composition of the bolt was also determined quantitatively, using ICP (inductively coupled plasma) spectroscopy and compared with the specification for a type 304 stainless steel. 3. RESULTS & DISCUSSION Figure 2a) shows the fracture appearance in the SEM, whilst Figures 2b)-2d) show examples of the EDX spectra obtained from various regions of the fracture surface. Figure 3 shows SEM images and Figure 4 the light micrographs. The results of the chemical analysis (Table 1) and the hardness test (table 2) show the composition and properties of the bolt to be broadly in line with those of a type 304 stainless steel, although it should be noted that there is no documentation or marking to suggest that the bolt claimed to meet this specification. What is clear from the chemical analysis is that the Mo content (0.01wt%) is outside the specification limits for type 316 stainless steel, which contains around 2% Mo, and generally confers higher resistance to pitting corrosion compared with the more commonly-used type 304 grade. The micrographs in Figure 4 show branched cracking, characteristic of stress corrosion cracking (SCC). This is a very common time-dependent cracking mechanism in austenitic stainless steels subjected to an environment containing sodium chloride and warm temperatures, and is entirely consistent with the environment in which the failure occurred. It is understood that several such failures

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have occurred in climbing equipment elsewhere, especially in tropical coastal environments, and that replacement by a resistant alloy (such as Ti) is the best solution – if the bolt is replaced with a similar bolt, there is nothing to prevent SCC occurring again. 4. CONCLUSIONS & RECOMMENDATIONS 1. The composition and properties of the broken bolt were broadly consistent with those of a type 304 austenitic stainless steel. 2. Failure occurred by a process of stress corrosion cracking, which is consistent with the environment (warm coastal cliff) in which the failure occurred. 5. ACKNOWLEDGEMENTS The author is grateful to her colleague Dr Qing Lu for carrying out the work described in this report.

Table 1 Results of chemical composition analysis of the bolt (TWI analysis No.

S/18/63), and the relevant material specification.

Specification/

test sample

Element, wt%

C Mn Si P S Cr Ni Mo N

S30400:

ASTM A240/240M-17,

type 304

0.07

max

2.00

max

0.75

max

0.045

max

0.030

max

17.5-

19.5

8.0-

10.5

NS 0.10

max

Test sample 0.040 1.45 0.34 0.019 0.034 18.11 8.28 0.01 0.079

Max = maximum; NS=not specified.

Table 2 Results of Vickers hardnesses measured from the metallographic section,

using a 1kg load

Specification/ test sample Hardness

UNS S30400: ASTM A240/240M-17, type 304

Max.: 201HBW/92HRBW

UNS S30400: ASTM A269/A269M-15, TP304

Max.: 200HV/192HBW/90HRB

Test specimen 208HV1.0 (min.) - 285HV1.0 (max.) 248/HV1.0 (average)

Note: min.=minimum; max.=maximum.

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a) D016050_003

b) D016050_007

c) D016050_005

Figure 1 Photographs of the bolt in the as-received condition:

a) The bolt and associated hanger and maillon;

b) Fracture face of the bolt and adjacent threads, the bolt was cut in half longitudinally along

the yellow dashed line for metallographic examinations;

c) View of the bolt in b) after rotating 180°.

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a)

b)

Figure 2 SEM micrograph (nominal scale shown) of the fracture surface and EDX spectra

obtained from the fracture surface after degreasing ultrasonically in acetone for 10 minutes:

a) Overall view of the fracture surface;

b) EDX spectrum obtained from region 7 (a relatively clean area) indicated in a);

7

2

3

4

5 6

1

A

D

E

C

B

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c)

d)

Figure 2 (continued) SEM micrograph (nominal scale shown) of the fracture surface and EDX

spectra obtained from the fracture surface after degreasing ultrasonically in acetone for 10

minutes:

c) EDX spectrum obtained from region 2 indicated in a), similar results were obtained from

regions 3-5 indicated in a);

d) EDX spectrum obtained from region 1 indicated in a), a similar result was obtained from

region 6 indicated in a).

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a)

b)

Figure 3 SEM micrograph (nominal scale shown) of the fracture surface and EDX spectra

obtained from the fracture surface after degreasing ultrasonically in acetone for 10 minutes:

a) Region A in Figure 2a at a higher magnification, showing a mixture of ductile and brittle

fracture modes, and similar results were found at regions B, C;

b) The circled region in a) at a higher magnification;

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c)

d)

Figure 3 (continued) SEM micrograph (nominal scale shown) of the fracture surface and EDX

spectra obtained from the fracture surface after degreasing ultrasonically in acetone for 10

minutes:

c) Region D in Figure 2a at a higher magnification, showing a region with brittle fracture;

d) The circled region in c) at a higher magnification;

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e)

f)

Figure 3 (continued) SEM micrograph (nominal scale shown) of the fracture surface and EDX

spectra obtained from the fracture surface after degreasing ultrasonically in acetone for 10

minutes:

e) Region E in Figure 2a at a higher magnification, showing a region with brittle fracture;

f) The circled region in e) at a higher magnification.

Wet

corrosion

product

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a) Macro22-03-2018_(007)_T14.00.26

b)

Figure 4 Light macrograph and micrographs of the metallographic section prepared (Figure

1b), etched electrolytically in 20% H2SO4 with 0.1g/l NH4CNS:

a) Macro view, a millimetre scale is shown;

b) Region A in a) at a higher magnification;

A B

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c)

d)

Figure 4 (continued) Light macrograph and micrographs of the metallographic section prepared

(Figure 1b), etched electrolytically in 20% H2SO4 with 0.1g/l NH4CNS:

c) Region B in a) at a higher magnification;

d) Region B1 in c) at a higher magnification.

B-1