Failure Investigations of Three Unique Brittle Failure Mechanisms in Steel Timber Fasteners

Coated hardened steel fasteners are widely used for construction of wood structures. They conveniently combine high strength, toughness, corrosion resistance, and low cost. However, these fasteners are susceptible to several brittle failure mechanisms. These investigations involved three populations of zinc-coated, martensitic steel fasteners from multiple manufacturers. Different failure modes were identified in the three unique investigation populations: tempered martensite embrittlement (TME), stress corrosion cracking (SCC), and internal hydrogen embrittlement (IHE). Though each material exhibited brittle fracture morphologies, environmental and microstructural clues revealed distinct failure modes for each fastener population. The TME population displayed brittle failure throughout the case-hardened region, a consistent failure location at the shank, and an acute response to picric acid etching. The SCC population displayed extensive corrosion pitting on external and internal (fracture) surfaces, thick corrosion product on the fracture surfaces indicating formation over many months, and brittle fracture initiation at multiple locations in the screw geometry. The IHE population failed within days, contained complex fracture surfaces with all three microstructural crack propagation modes (intergranular, quasi-cleavage, and microvoid coalescence), and contained elevated hydrogen levels. In all three investigations, the documented failure modes were a consequence of susceptibility introduced during the manufacturing process and/or due to environmental factors. For the TME population, the tempering process was performed in an adverse temperature range with a slow cooling rate. For the SCC population, the coating was poorly adhered and flaked off, leading to long-term corrosion. For the IHE population, the microstructure was susceptible to hydrogen pick-up, hydrogen was introduced during zinc electro-plating, and the hydrogen was not baked out after coating. While all three populations of fasteners presented similar brittle fracture morphologies, careful failure analysis and testing showed three different failure mechanisms which could be addressed via quality control during manufacturing, by different material selections, or by controlling installation conditions.