I've been goofing off in Merrick Laboratories and have learned a little bit about Black Locust wood. This was prompted by a talk I had with a guy that makes hammers, a blacksmith, about handles. Outside the blacksmith shop were several Locust trees so I told him I make handles of Black Locust. He said he would never do that because it is too brittle.
Anyway, not being one to believe what people say without supporting evidence, I went to work. Post truth I am not.
Part 1: ResearchMy primary source here is the Wood Handbook published since 1910 by the US Department of Agriculture, Forest Service, Wood Products Laboratory. A link to this document is below. I looked up the three things that were of interest; bending strength (modulus of rupture), shear strength parallel to the grain and impact resistance. Modulus of rupture is a pseudo bending ultimate stress. I call it pseudo because the tension and compression behavior of wood is not symmetric and classic beam theory does not exactly apply. Shear strength is just the ultimate shear stress parallel to the grain. Parallel to grain because wood is very weak in shear parallel to the grain and very strong perpendicular to the grain so wood shear failure is always parallel to the grain. Impact resistance is expressed in inches and is the 50 lb weight drop distance that will break a 2" square beam spanning 28 inches. There are several species of Hickory so the values vary a bit. I have added Red Oak, Black Maple and Douglas Fir for comparison
Modulus of Rupture
Hickory: 18,100 to 20200 psi
Locust: 19,400 psi
Red Oak: 13,900 psi
B Maple: 13,300 psi
D Fir: 12,400 psi
Shear:
Hickory: 1740 to 2430 psi
Locust: 2480 psi
Red Oak: 1910 psi
B Maple: 1820 psi
D Fir: 1130 psi
Impact:
Hickory: 67 to 88 inches
Locust: 57 inches
Red Oak: 41 inches
B Maple: 40 inches
D Fir: 31 inches
Part 2: TestingI used smaller pieces than the wood handbook values which shouldn't have much effect on bending and shear. My impact test used a 15 lb piece of 3/4" steel plate edgewise for a drop weight. My impact beams were 1" x 0.75" in section and tested flatwise. The span was 6 inches. My impact tests are not comparable to the published values but are comparable for the two materials tested.
Modulus of Rupture
Hickory: 24,800 psi
Locust: 19,600 psi
Shear:
Hickory: 1700 psi without failure during bending test
Locust: 1200 psi without failure during bending test
Impact:
Hickory: 24 inches
Locust: 24 inches
Part 3: Calculated Demand:My calculations find that my current hammer handle design has the following working stresses based on the design forces specified by the ASME standard B107.400 for striking tools. I used the prying forces specified in that standard for claw hammers.
Design:
Bending: 10,700 psi
Shear: 210 psi
Part 4: Conclusion:I think carefully selected pieces of Black Locust will perform very well for making hammer handles. Locust has higher bending strength than some hickory species and higher shear strength than any hickory species. Black locust impact resistance is similar to hickory and in my tests, the same. Hickory has the added advantages of better decay resistance and much less drying shrinkage which should help keep the head tight if the hammer is exposed to moisture changes.
Black Locust may have somewhat less impact strength than hickory but the biggest risk of failure in a climbing hammer is probably when prying with the pick where the failure mode is static bending and black locust is stronger than hickory in static bending. For a purely striking (impact) tool (blacksmith's hammer) a thinner handle made from hickory might be adequate compared to locust.
Lastly, it is possible to break any hammer. Hammer abuse is punishable by banishment.
Wood Handbook:
https://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100
