Tadanobu Inoue

This study investigated the surface morphology of the thoracic exoskeleton of the coconut crab (Birgus latro) from Okinawa using laser microscopy. While many aquatic crustaceans possess prominent raised surface structures to prevent the attachment of epibionts, the terrestrial coconut crab is thought to exhibit distinct characteristics adapted to its unique environment—one often dominated by limestone and characterized by relatively dry conditions. Three adult coconut crabs, varying in body weight (173 g, 437 g, 964 g), were analyzed. Quantitative surface analysis revealed a uniform micro-bulge structure across both black and yellow color zones of the exoskeleton. The diameter of these bulges consistently ranged from 10 to 17 μm with height between 1.3 and 2.2 μm, regardless of individual body size or coloration. This structural uniformity suggests evolutionary adaptation for durability, moisture retention, friction modulation, and resistance to abrasion and predators, in contrast to aquatic species where larger bulges assist in preventing marine organism attachment. The matte shell texture and micro-bulge arrangement further contribute to minimizing water loss—a crucial factor for terrestrial survival. The consistency of bulge size and distribution, independent of size and carapace pattern, highlights the functional conservation of these microstructures as a fundamental adaptation for terrestrial life.

Overcoming delayed fracture under atmospheric corrosion environment is a critical challenge for the practical application of ultra-high-strength bolts. This study investigated the delayed fracture behavior of 1700 MPa-class quenched and tempered bolts fabricated from a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) by means of outdoor exposure tests, accelerated tests, and thermal desorption spectrometry. In a 10-year outdoor exposure test, the delayed fracture behavior of the ultra-high-strength bolts was categorized into two types: rupture at the thread runout and the first thread root in nut, and fracture at the thread root in nut and the under-head fillet. Outdoor exposure tests using notched bolts could reproduce the delayed fracture at the thread runout. By measuring the strain change for the shank of notched bolt during the outdoor exposure test, we successfully observed the delayed fracture process: the initiation of intergranular fracture along the boundaries of prior-austenite grains, followed by unstable fracture, and culminating in the formation of a shear lip. The delayed fracture of the bolts was attributed to hydrogen embrittlement. Slow-strain-rate testing for hydrogen-pre-charged notched specimens showed a linear relationship between the fraction of intergranular fracture (IGF) area and the peak value of the maximum principal stress σmax* at the notch root. The delayed fracture mechanism of the ultra-high-strength bolts was discussed based on the σmax* and the IGF area fraction at the fracture locations.

Overcoming delayed fracture under atmospheric corrosion environment is a critical challenge for the practical application of ultra-high-strength bolts. This study investigated the delayed fracture behavior of 1700 MPa-class quenched and tempered bolts fabricated from a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) by means of outdoor exposure tests, accelerated tests and thermal desorption spectrometry. In a 10-year outdoor exposure test, the delayed fracture behavior of the ultra-high-strength bolts was categorized into two types: rupture at the thread runout and the first thread root in nut, and fracture at the thread root in nut and the under-head fillet. Outdoor exposure tests using notched bolts could reproduce the delayed fracture at the thread runout. By measuring the stain change for the shank of notched bolt during the outdoor exposure test, we successfully observed the delayed fracture process: the initiation of intergranular fracture along the boundaries of prior-austenite grains, followed by unstable fracture, and culminating in the formation of a shear lip. The delayed fracture of the bolts was attributed to hydrogen embrittlement. Slow-strain-rate testing for hydrogen-pre-charged notched specimens showed a linear relationship between the fraction of intergranular fracture (IGF) area and the peak value of the maximum principal stress σmax* at the notch root. The delayed fracture mechanism of the ultra-high-strength bolts was discussed based on the σmax* and the IGF area fraction at the fracture locations.

The coconut crab, Birgus latro, has black protrusions on the tops of its walking legs and claw fingers. In addition, there are regularly aligned small black protrusions on parts of the exoskeleton surface of the claws and leg. In this study, the elemental composition, crystal structure, tissue structure, and mechanical properties of these protrusions were studied using a materials science approach, and the results were compared with those of mineralized cuticle. These leg tips were found to be a non-calcified fibrous tissue of α-chitin connected to the mineralized cuticle. The tip of the second walking leg was elongated and had a pointed shape with an oval cavity at its center that was more than 1000 times larger than the pore tubes (100–350 nm) of the mineralized cuticle. It was very soft, with a hardness of 0.4 GPa, corresponding to 11–12% of the hardness of the hard exocuticle and 55–57% of the hardness of the soft endocuticle. The elastic modulus of 8.0 GPa obtained by means of nanoindentation testing was consistent with that of α-chitin fibers of shrimp shells obtained by means of tensile testing. These soft protrusions provide a secure grip on the surfaces of trees or rocks and protect the claw fingertips. It was concluded that the black protrusions are related to a unique ecological (engaging in vertical movements, entering and exiting limestone caves, and escape behavior) aspect of the coconut crab, the largest terrestrial crustacean.

The Lüders phenomenon is one type of inhomogeneous plastic deformation occurring in the elastic-to-plastic transition region, and it is an undesirable plastic deformation behavior. Although conventional measures based on the chemical composition design, plasticity processing principle, or utilization of composited microstructures are used to suppress this phenomenon in engineering, demerits are present, such as high cost and low fracture behavior. The Lüders phenomenon begins with the formation of plastic bands (inhomogeneous yielding) at one or several local sites. If yielding simultaneously occurs everywhere rather than at several local sites, the formation of local plastic bands will be inhibited; as a result, the Lüders deformation will be suppressed. Based on this idea, a new approach was proposed in which the number of local yield sites was increased by heat treatments. A medium-carbon tempered martensite steel (Fe-0.3C-1.5Mn, in wt%) was used to verify the validity of the new approach, and the optimum heat-treatment conditions for the balance of mechanical property and deformation behavior were determined.

Decapod crustaceans have tooth-like white denticles that are present only on the pinching side of the claws. In the mud crab, Scylla serrata, a huge denticle exists on the movable finger of the dominant claw. This is mainly used to crush the shells of the crab’s staple food. The local mechanical properties, hardness (H_IT) and elastic modulus (E_r), of the peak and valley areas of the largest denticle were examined via a nanoindentation test. The microstructure and elemental composition were characterized using a scanning electron microscope and energy-dispersive X-ray spectroscopy. The striation patterns originating from a twisted plywood structure parallel to the surface were visible over the entire denticle. Most of the largest denticle was occupied by a hard area without phosphorus, and there was a soft layer corresponding to the endocuticle with phosphorus in the innermost part. The H_IT of the denticle valley was about 40% lower than that of the denticle peak, and the thickness of the soft endocuticle of the denticle valley was five times thicker than that of the denticle peak. The H_IT–E_r map showed that the abrasion resistance of the denticle surface was vastly superior and was in the top class among organisms. The claw denticles were designed with the necessary characteristics in the necessary places, as related to the ecology of the mud crab.

Decapod crustaceans have tooth-like denticles on their claw fingers, which come into direct contact with predators and prey. Since the denticles are subject to more frequent and intense stress than other parts of the exoskeleton, they must be especially resistant to wear and abrasion. We clarified the mechanical resistance and tissue structure of the denticles arranged in a line on the fixed finger of the mud crab, which has huge claws. The denticles of the mud crab are small at the fingertip and become larger closer to the palm. The denticles have a twisted-plywood-pattern structure stacked parallel to the surface regardless of size, but the abrasion resistance strongly depends on the size of the denticles. Due to the dense tissue structure and calcification, the abrasion resistance increases as the denticle size increases, reaching its maximum at the denticle surface. The denticles of the mud crab have a tissue structure that prevents them from breaking when pinched. The high abrasion resistance of the large denticle surface is an essential feature for the frequent crushing of shellfish, which is the mud crab’s staple food. The characteristics and tissue structure of the claw denticles on the mud crab may provide ideas for developing stronger, tougher materials.