76 The testing machine applied an increasing tensile load at 0.5 mm/s (0.02 in/s), recording loads at 10 Hz. We photo- graphed failed carabiners and categorized them according to the location of failure: barrel, body, hinge or key (Figure 1). Since Am’D and William carabiners have different strength ratings, we also calculated the relative breaking strength (DR): [1] DR = where RMAX Ρ MAX − ΡRATED respectively. We used the Kolmogorov-Smirnov test to confirm that data and RRATED were normally distributed, with the exception of surface rough- ness measurements for Am’D Ball-Lock carabiners. For com- parisons involving that variable, we used the nonparametric Wilcoxon’s signed rank test to test hypotheses. We used a t-test to compare breaking strength, relative breaking strength, and surface roughness between new and used carabiners of each type, after confirming homogeneity of variance within each comparison by Levene’s test. We used one- and two-way anal- yses of variance (ANOVA) to compare the response variables listed above between failure types and carabiner types (classi- fied by condition), respectively. Where appropriate, we used Tukey’s honestly significant difference to test multiple com- parisons within each ANOVA. We used regression analysis to investigate the effect of surface roughness on breaking strength. Ρ RATED are the breaking strength and rated strength, Kane and Ryan: Carabiner Strength used (Table 1). The relative breaking strength, however, was greater for new William carabiners than new Am’D carabiners (Table 1). There was some evidence of this difference for used Am’D and William carabiners (Table 1). Tri-Act and Ball-Lock carabiners were equally strong when new, but used Tri-Act cara- biners were stronger than used Ball-Lock carabiners (Table 1). Tri-Act and Ball-Lock carabiners had similar relative breaking strengths when new, but when used, Tri-Act carabiners had great- er relative breaking strength than Ball-Lock carabiners (Table 1). There was some evidence that the breaking strength of bar- rel failures was greater than that of key failures (Table 2), but the relative breaking strength was similar for all failure types. Breaking strength diminished somewhat with increasing sur- face roughness, but the relationship was weak (Figure 2). The scatter plot revealed wide variation in roughness for used cara- biners, with roughness values for new carabiners fitting inside that range (Figure 2). Surface roughness did not differ for any comparisons of carabiners (by condition, shape or gate), with the exception that Tri-Act carabiners had marginally less rough surfaces than Ball-Lock carabiners when new (Table 2). Visual inspection did not reveal any extraordinary defects, aside from three carabiners that had gates that were “sticky” and did not close properly. The breaking strength of these carabiners was not different from the mean breaking strength of the matching type of carabiner for each, but they were all Ball-Lock gates. Surface roughness of body failures was greater than barrel failures (Ta- ble 2) but there were no other differences among failure types. DISCUSSION Figure 1. Types of failure of carabiners (clockwise from upper left): key, body, hinge, barrel. RESULTS No carabiner failed below its rated strength, and all exceeded the ANSI Z.133 minimum strength by at least 4.0 kN (899 lbf). Used carabiners were as strong as new carabiners for each type except William Ball-Lock, for which there was some evidence that new carabiners were stronger (Table 1). Used Am’D Ball- Lock carabiners exhibited the smallest relative breaking strength, but were still nearly 10% stronger than rated (Table 1). Am’D carabiners were stronger than William carabiners, both new and ©2009 International Society of Arboriculture Given their current popularity, carabiners will likely continue to used by tree climbers. Thus, two findings in particular were reas- suring. First, used carabiners were, with one exception, as strong as new ones. Second, normal wear and tear (reflected by surface roughness) was only a weak predictor of breaking strength. In both cases, however, the small sample size limits the ability to generalize these conclusions. The F1774 standard (Anonymous 1999) requires a minimum of 5 replicates of each test, which was not achieved for all carabiner types tested. This lack of conformi- ty may not be critical in light of the relatively small variability in the results (the mean coefficient of variation was less than 4% for breaking strength, and it was similar among carabiner types and condition). The conclusions are also limited with respect to time, since many climbers use carabiners for more than one year. Col- lecting longer-term data would be helpful in future investigations, especially considering that Petzl offers a three-year warranty on the carabiners tested. A final limitation is that the climbers come from a potentially biased sample, in that they all worked for com- panies staffed with certified arborists as well as safety programs. Since only three carabiners from the present study had gates that stuck, it is unclear whether gate mechanisms lose functional- ity before the inherent strength of the carabiner is undermined to the point of danger. Loaded when the gate is open, carabin- ers are typically about one-fourth as strong as when the gate is closed. A faulty carabiner loaded with the gate open is a more likely source of strength reduction than normal wear and tear. One reasonably expects that a climber would retire a carabi- ner if the gate no longer functioned properly, but Statham and Roebuck (2004) observed several climbers using carabiners with faulty gates. They speculated that carabiners were being
March 2009
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