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Four, hot-rolled, plain-carbon steels with varying carbon content were subjected to slow strain-rate tensile (SSRT) tests in a 95-MPa gaseous hydrogen environment at ambient temperature. The influence of pearlite volume fraction on the magnitude of hydrogen-induced degradation of the materials’ strength and ductility was thereby determined. Hydrogen was seen to significantly affect strain-to-failure and reduction-in-area in all four materials, wherein such a loss of tensile ductility was ascribed to the premature initiation and subsequent propagation of surface micro-cracks as revealed by the quantitative damage evolution analyses on the post-fractured specimens. The pearlite grains on sample surfaces manifestly served as the preferential origins of hydrogen-induced micro-cracks, resulting in more considerable embrittlement in materials possessing a higher percentage of pearlite, due to the rapid coalescence of discrete embryonic damage during tensile straining.

After pre-charging at the gaseous phase with a concentration of ~7,000 at. ppm, solute hydrogen was discovered to have an abnormal effect on both the strength and ductility enhancement of a commercially-available, Fe-24Cr-19Ni-based, stable austenitic stainless steel that had been subjected to tensile testing at various strain-rates. Specifically, the impact of hydrogen on material strength was accompanied by amplified yield and flow stresses, as well as tensile strength, while the improvement in ductility featured extended uniform elongation and strain-to-fracture, both of which became more pronounced as hydrogen concentration intensified. The product between tensile strength and uniform elongation served as indicators of the strength-ductility balance, at which hydrogen maximally optimized the indicator at the particular intermediate strain-rate. The yield/flow stress augmentations were interpreted in terms of solid-solution strengthening, whereas the enhanced ductility was primarily ascribed to the facilitation of mechanical twinning, whereby dynamic hydrogen-dislocation interaction exerted a critical influence as was indirectly revealed by supplemental stress-relaxation experiments.

The fatigue limit of a circumferentially-notched austenitic stainless steel exhibited peculiar improvement under fully-reversed loading (R =−1), as compared with that under tension-tension loading (R = 0.1), behavior considered to be unconventional in smooth specimens of similar materials. This was attributed to the different cyclic stress-strain responses at the notch-root, where it was almost elastic at R = 0.1, whereas an elasto-plastic response continued up to later stage of the fatigue process at R =−1. Such cyclic plasticity enhanced martensitic transformation locally and resulted in hardening, thereby restricting fatigue crack initiation from the notch-root.

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2019-08-26, 資料番号: PA2010049000

形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2019-01-26, 資料番号: PA1910027000

Tension-compression fatigue tests were performed on two types of Ni-based superalloy 718 with different microstructures, to which small artificial defects of various shapes and sizes were introduced. Similar tests were also conducted on hydrogen-charged specimens with defects, with a solute hydrogen content ranging from 26.3 to 91.0 mass ppm. In the non-charged specimens in particular, the fatigue strength susceptibility to defects varied significantly according to the type of microstructural morphology, i.e., a smaller grain size made the alloy more vulnerable to defects. The fatigue limit as a small-crack threshold was successfully predicted using the √area parameter model. Depending on the size of defects, the fatigue limit was calculated in relation to three phases: (i) harmless-defect regime, (ii) small-crack regime and (iii) large-crack regime. Such a classification enabled comprehensive fatigue limit evaluation in a wide array of defects, taking into consideration (a) the defect size over a range of small crack and large crack and (b) the characteristics of the matrix represented by grain size and hardness. In addition, the effect of defects and hydrogen on fatigue strength will be comprehensively discussed, based on a series of experimental results.

A brittle-like faceted morphology of a precipitation-strengthened Fe-Ni-Cr-based superalloy after charging via exposure to high-pressure hydrogen gas (100 MPa) at elevated temperature (543 K) was interpreted based on multiple electron microscopy observations: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging. The observation results revealed that the brittle-like facets were derived from intergranular cracking accompanied by hydrogen-assisted microvoid nucleation at the grain boundaries (GBs). Deformation twinning also played a crucial role in triggering the final grain boundary separation due to local stress concentration at its intersection with the GBs after severe strain hardening; such a process has not yet been considered to explain the hydrogen-induced ductility loss of this type of alloy.

Fatigue crack growth (FCG) tests were performed with two types of metastable austenitic stainless steels having different austenite phase stabilities under hydrogen-precharged conditions (internal hydrogen) and in gaseous hydrogen environments (external hydrogen). The materials showed a peculiarly slower FCG rate with internal hydrogen than with external hydrogen even though the hydrogen concentration was much higher under the internal hydrogen conditions. The results are interpreted in terms of hydrogen-modified plastic deformation character comprising inhibited cross-slipping or enhanced deformation twinning in combination with the sequence of hydrogen penetration and strain-induced α′ martensite formation in the local region surrounding the fatigue crack tip.

Accepted: 2020-06-11, 資料番号: PA2110038000

The impact of hydrogen on crack-tip plastic-zone development was revisited via a novel approach, utilizing the measurement of fatigue crack-growth retardation in a medium-strength martensitic steel after a single overloading in laboratory air and in 90-MPa-hydrogen gas. The plastic zone can be characterized according to the crack-propagation length for reverting from the retardation caused by plasticity-induced crack-closure ascribed to overloading (overloading-affected, crack-growth distance). Hydrogen sharpened the shape of overloaded crack-tip and suppressed the extension of the severely-deformed zone in the crack proximity. Besides, it enhanced frequent crack-tip branching, giving rise to a slower crack growth rate than the in-air situation at the initial stage of retardation. However, no change in the overloading-affected, crack-growth distance was detected between the in-air and hydrogen-gas conditions. Ultimately, hydrogen barely altered the overall plastic-zone size.

Fatigue crack growth of two carbon steels with different pearlite volume fractions were studied in pressurized gaseous hydrogen environment. Notably, pearlite was found to mitigate hydrogen-assisted fatigue crack acceleration. This positive impact of pearlite was ascribed to ferrite/cementite lamellar aligned perpendicularly to the cracking direction, which functioned as barriers to intermittently arrest the crack propagation. Meanwhile, brittle delamination fracture ensued in the pearlite lamellar lying parallel to the crack-plane increased the crack growth rate and compromised the above positive effect to some extent. The material behavior is rationalized in light of fractographical observations and microstructural analyses of the crack-wake.

The fatigue limit properties of a carbon steel and a low-alloy Cr Mo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the Cr Mo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained S N data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.

Hydrogen-assisted fatigue crack growth (HAFCG) in pure iron at a relatively low stress intensity range exhibits brittle-like intergranular (IG) fracture, while the macroscopic crack acceleration is not significant. The present study focuses on the mechanism of IG fracture in terms of the microscopic deformation structures near the crack propagation paths. We found that the IG fracture is attributed to hydrogen-enhanced dislocation structure evolution and subsequent microvoid formation along the grain boundaries. The impact of such IG cracking on the macroscopic fatigue crack growth (FCG) acceleration is evaluated according to the dependency of IG fracture tendency on the hydrogen gas pressure during testing. It is demonstrated for the first time that increased hydrogen pressure results in a larger fraction of IG fracture and correspondingly faster FCG. On the other hand, the gaseous hydrogen environment also has a positive role in decelerating the FCG rate relative to air due to the absence of oxygen and water vapor. The macroscopic crack propagation rate in hydrogen gas is eventually determined by the competition between the said positive and negative influences.

Fatigue strength of Ni-based superalloy 718 was investigated relative to shear-mode crack-growth thresholds, to evaluate the influence of crack-opening/-closing stresses perpendicular to shear-mode crack-planes. The impact of precipitation-hardening was also examined. Using solution-treated and precipitation-hardened materials, three different types of fatigue tests were performed: (i) push–pull; (ii) pure-torsion; (iii) torsion with superposed static tension. All tests revealed non-propagation of small, shear-mode cracks, while confirming the negligible impact of precipitation-hardening on shear-mode crack-growth threshold. Additionally, crack-opening/-closing stresses were determined to only imperceptibly alter shear-mode resistance. Based on these findings, a novel strategy was proposed to quantify the alloy’s fatigue strength.

To investigate the effect of hydrogen on fatigue life characteristics and crack growth behaviors through the entire fatigue life of a carbon steel, tension-compression fatigue tests and elasto-plastic fracture toughness tests were conducted in a hydrogen gas environment under the pressures of 0.7 and 115 MPa. The fatigue tests revealed that the fatigue life and fracture morphology vary drastically with the hydrogen gas pressure. This study demonstrates that such differences can be explained by the combination of fatigue crack growth properties and fracture toughness properties in hydrogen gas at each pressure.

In order to develop safer and more energy-efficient, hydrogen pre-cooling systems for use in hydrogen refueling stations, it is necessary to identify a high-strength metallic material with greater thermal conductivity and lower susceptibility to hydrogen embrittlement, as compared with ordinary, stable austenitic stainless steels. To accomplish this task, the hydrogen compatibility of a precipitation-hardened, high-strength, copper-based alloy was investigated by slow-strain-rate tensile (SSRT), fatigue-life, fatigue-crack-growth (FCG) and fracture toughness tests in 115-MPa hydrogen gas at room temperature. The hydrogen solubility and diffusivity of the alloy were also determined. The hydrogen solubility of the alloy was two or three orders of magnitude lower than that of austenitic stainless steels. The alloy also demonstrated absolutely no hydrogen-induced degradation of its strength properties, a factor which could contribute to the reduction of costs related to the construction and maintenance of hydrogen refueling stations, owing to the downsizing and improved cooling performance of the pre-cooling systems.

The influences of internal and external hydrogen on the tensile ductility loss and fracture behaviors of a precipitation-hardened Ni-based superalloy 718 were investigated via slow strain rate tensile (SSRT) testing under hydrogen pre-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen) . Severe degradation of tensile ductility was confirmed both in internal and external hydrogen conditions, and the degree of such degradation became more significant with increasing hydrogen content or hydrogen gas pressures. Moreover, the loss of tensile ductility was more pronounced in internal hydrogen conditions than external hydrogen environments. In association with such degradation of macroscopic tensile ductility, hydrogen also altered fracture mode from ductile microvoid coalescence to some brittle appearances. Whereas typical intergranular fracture combined with a decent fraction of quasi-cleavage fracture appeared on the fracture surface formed in external hydrogen environments, several types of unique faceted characteristics were found on the fracture surfaces in internal hydrogen conditions. The detailed observation of the mid-sectioned lateral surfaces of post-mortem samples successfully revealed that the observed distinctions consisted of the fracture along grain boundaries and {111} crystallographic planes including annealing twin boundaries, besides the frequency of the cracking along twin boundaries evidently increased at higher hydrogen concentration. On the basis of the series of experimental results, the initiation and propagation mechanisms of those hydrogen-induced cracks are discussed in terms of hydrogen distribution, intrinsic deformation character of the material itself as well as the alteration of plastic deformation mode caused by dissolved hydrogen.

The effect of hydrogen on the deformation and fracture behavior in pure Cu, pure Ni and Cu–Ni alloy was studied via tensile tests of H-charged, smooth and circumferentially-notched specimens at room temperature (RT) and 77 K. Hydrogen-diffusion properties were determined by the desorption method. To obtain a uniform hydrogen concentration in the H-charged specimens, specimens were exposed to 100-MPa hydrogen gas at 543 K for 200 h, based on the determined hydrogen diffusivity. In tensile tests of smooth pure Ni and Cu–Ni alloy specimens at RT, common hydrogen effects were detected, namely, an increase in yield and flow stresses — a hardening effect; and a ductility loss that was accompanied by a change in fracture surface from ductile to brittle feature — an embrittling effect. With regard to the embrittling effect, the pure Ni and Cu–Ni alloy showed different fracture-surface morphologies at RT; the pure Ni showed an intergranular (IG) surface and the Cu–Ni alloy surface was flat. However, a number of IG cracks were detected beneath the fracture surfaces on the smooth Cu-Ni alloy. The tensile tests of the H-charged smooth specimens at 77 K yielded an IG surface for the pure Ni and a ductile fracture surface with dimples in the Cu–Ni alloy. In contrast, tensile tests of the H-charged, notched specimens at RT demonstrated clear IG fractures for the pure Ni and Cu–Ni alloy. These facts indicate that IG cracking was the first step in the embrittling process for the pure Ni and Cu–Ni alloy, and IG cracking was accompanied by a large plastic deformation that formed the flat surface (unclear IG surface) for the smooth Cu–Ni alloy. Considering that the HE of both pure Ni and Cu–Ni alloy was related to IG cracking, possible mechanisms were discussed and tensile tests performed at 77 K suggested two possibilities: (I) interaction between hydrogen-moving dislocation is more important in the HE process of the Cu-Ni alloy compared to the pure Ni; (II) hydrogen transportation towards grain boundaries are required to cause the IG fracture in the Cu-Ni alloy.

Elasto-plastic fracture toughness tests of a commercially pure iron were performed in air and in hydrogen gas at two different pressures. Some unique characteristics of hydrogen-enhanced cracking were exhibited at both the macroscopic and microscopic length scales, based on the observation of fracture surface, fracture plane, plasticity distribution and dislocation structure. The possible mechanisms responsible for the hydrogen-induced degradation of fracture toughness are discussed.

Hydrogen-assisted, fatigue crack-growth in pure iron within a low stress-intensity range was ascribed to the emergence of intergranular fracture, the significance of which was emphasized by increased hydrogen-gas pressure, while conversely mitigated by an elevation of test temperature. Based on conventional thermodynamic theory, a single parameter, GB hydrogen-coverage (θx), was used to derive a systematic, unified evaluation of such a complex reliance on the dual environmental variables. Furthermore, post-mortem microscopic analyses of the crack-wake deformation microstructures were employed to elucidate the contribution of dislocation activity regarding the triggering of IG fracture, which also varied significantly with the alteration of θx.

Fatigue strength of precipitation-hardened materials is well known to be lower than that expected from their high yield and tensile strengths. To elucidate the reasons behind such anomaly, torsional fatigue tests, in addition to uniaxial fatigue tests, were conducted on samples of the Ni-based superalloy 718, subjected to two different heat treatments (solution-annealing and precipitation-hardening). Despite the significant differences in tensile and yield strengths between the two samples, the torsional fatigue strengths were mutually equivalent. This result was interpreted in light of the resistance against fatigue crack-initiation and -propagation in shear-mode, which cannot be enhanced by precipitation-hardening due to the depletion of precipitates by persistent dislocations motion within localized glide-bands.

Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompanied by a significant acceleration of fatigue crack growth. However, in hydrogen-gas at 423 K, the crack-wake plasticity was well-evolved and analogous to that observed in an inert environment. This apparent recovery of deformation micro structure coincided with suppressed crack growth acceleration, the rationale for which can be interpreted by the trapping/de-trapping equilibrium between hydrogen and dislocations.

Hydrogen effect on fatigue performance of commercially pure BCC iron has been studied with a combination of various electron microscopy techniques. The fatigue crack growth (FCG) in gaseous hydrogen was found to consist of two regimes corresponding to a slightly accelerated regime at relatively low stress intensity factor range, ΔK, (Stage I) and the highly accelerated regime at relatively high ΔK (Stage II). These regimes were manifested by the intergranular and quasicleavage types of fractures respectively. Scanning electron microscopy (SEM) observations demonstrated an increase in plastic deformation around the crack wake in the Stage I, but considerably lower amount of plasticity around the crack path in the Stage II. Transmission electron microscopy (TEM) results identified dislocation cell structure immediately beneath the fracture surface of the Stage I sample, and dislocation tangles in the Stage II sample corresponding to fracture at high and low plastic strain amplitudes respectively.

To develop safer and more cost-effective high-pressure hydrogen tanks used in fuel cell vehicles (FCVs), the metallic materials with the following three key properties, i.e. lightweight, high strength and excellent resistance to hydrogen embrittlement should be explored. In this study, the compatibility of high-strength, precipitation-hardened aluminum alloy 7075-T6 was evaluated according to the four types of mechanical testing including slow-strain rate tensile (SSRT), fatigue life, fatigue crack growth (FCG) and fracture toughness tests in high-pressure gaseous hydrogen environments (95 ∼ 115 MPa) at room temperature. Even though numerous publications have previously reported significant degradation of the mechanical properties of 7075-T6 in some hydrogenating environments, such as moist atmosphere, the understanding with regards to the performance of this alloy in high-pressure gaseous hydrogen environments is still lacking. In SSRT tests, the alloy showed no degradation of tensile strength and ductility. Furthermore, fatigue life, fatigue crack growth and fracture toughness properties were also not degraded in hydrogen gas. Namely, it was first demonstrated that the material has big potential to be used for hydrogen storage tanks for FCVs, according to its excellent resistance to high-pressure gaseous hydrogen.

A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip.

In order to develop more energy-efficient and safer, hydrogen pre-cooling systems destined for use in hydrogen refueling stations, a metallic material must first be researched and found to possess three excellent material properties: high strength, high thermal conductivity and low susceptibility to hydrogen embrittlement (HE). This study investigated the hydrogen compatibility of a beryllium-copper alloy 25 (UNS-C17200), fabricated by a solution annealing at 1053 K and via subsequent aging treatment at 588 K. After these thermal processes, the tensile strength exceeded 1200 MPa, due to the precipitation of nano-sized CuBe compounds (γ’ phase). Slow strain rate tensile (SSRT) and tension-compression fatigue tests were performed using this material, in addition to fatigue crack growth and fracture toughness tests, in laboratory air and in gaseous hydrogen with a pressure of 115 MPa at room temperature. After the SSRT test, the material showed no hydrogen-induced degradation of strength or ductility and, surprisingly, there was also no degradation of fatigue resistance or fracture toughness values in high-pressure gaseous hydrogen. Specifically, it was revealed that the material demonstrated an excellent HE resistance, despite having such a high tensile strength.

In order to study the influence of hydrogen on plastic deformation behavior in the vicinity of the fatigue crack-tip in a pure iron, a multi-scale observation technique was employed, comprising electron channeling contrast imaging (ECCI), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The analyses successfully demonstrated that hydrogen greatly reduces the dislocation structure evolution around the fracture path and localizes the plastic flow in the crack-tip region. Such clear evidence can reinforce the existing model in which this type of localized plasticity contributes to crack-growth acceleration in metals in hydrogen atmosphere, which has not yet been experimentally elucidated.

In order to clarify the contribution of dislocation‒hydrogen interaction on the hydrogen embrittlement (HE) of pure Ni and of Cu‒55 wt% Ni binary alloy, slow strain rate tensile (SSRT) tests were conducted at room temperature (RT) and at 77 K on hydrogen-precharged specimens. Regarding the SSRT test at RT, hydrogen increased the flow stress and induced intergranular fracture in both pure Ni and Cu–Ni alloy. Furthermore, based on scanning transmission electron microscopy investigations, it was suggested that the evolution of dislocation structures had been enhanced by hydrogen, but only in the case of pure Ni. At 77 K, the ductility of pure Ni was degraded by hydrogen, whereas that of Cu–Ni alloy was not. The difference in temperature dependence of the dislocation‒hydrogen interaction between pure Ni and Cu–Ni alloy was discussed, based on the previously proposed HE mechanisms.

Tension-compression fatigue tests using smooth specimens of low carbon steel JIS-SM490B were carried out in air and hydrogen gas environment under the pressure of 0.7 and 115 MPa at room temperature. In 0.7 MPa hydrogen gas, fatigue life curve was nearly equivalent to that in air. On the other hand, in 115 MPa hydrogen gas, fatigue life was significantly degraded in the relatively short fatigue life regime (e.g. Nf < 105). To clarify the effect of hydrogen environment on fracture process, fracture surfaces of these specimens were observed. In general, fatigue fracture process of steels with low or moderate strength is macroscopically divided into 3 stages. In the first stage (stage I), fatigue cracks initiate in some crystalline grains. In the second stage (stage II), the cracks propagate stably on a cycle-by-cycle basis. In the final stage (stage III), a tilted fracture surface, shear-lip, is formed by ductile tearing. In SM490B steel, this general fracture process was confirmed in air and 0.7 MPa hydrogen gas. In contrast, in 115 MPa hydrogen gas, there was no tilted portion in the stage III region, and the fracture surface was totally flat. Observation with scanning electron microscope revealed that dimples were formed by ductile tearing in the tilted fracture region in air and 0.7 MPa hydrogen gas. On the other hand, only a quasi-cleavage fracture surface existed in the final fracture region in 115 MPa hydrogen gas. To understand the cause of this peculiar fracture morphology, we conducted elasto-plastic fracture toughness tests in each environment, and investigated the fracture morphology. As a result of fracture toughness tests, crack growth rate in air and 0.7 MPa hydrogen gas was approximately equal to each other, and both the fracture surfaces were covered by dimples. This fracture morphology was in accordance with that of stage III morphology in fatigue specimen tested in air and 0.7 MPa hydrogen gas. However, in 115 MPa hydrogen gas, the crack growth was significantly accelerated, and the whole fracture surface was covered by quasi-cleavage. In this paper, firstly, the similarity of fracture surface between two test methods, i.e. fatigue test and fracture toughness test, is investigated. And then, the formation mechanism of the flat fracture surface is discussed by paying attention to the crack-growth acceleration in high-pressure hydrogen gas.

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.

To diagnose the activities and progressions of dental caries lesions, we developed nondestructive measuring device to detect and quantify the mineral loss in the early stage of caries. This novel device with ultrasonic technology measures acoustic frequency characteristics of the tooth surface. Extracted human tooth were embedded in polymethylmethacrylate and polished. The results of the repeated measurements indicated the sufficient reproducibility of acoustic measurements in same samples. Since this acoustic measurement was influenced by the moist condition of tooth surfaces, tooth surfaces should be air-dried and quickly measured for the reproducibility in clinics. We developed the nondestructive ultrasonic measuring device to detect the mineral loss from tooth surfaces and diagnose the initial change of progression of dental caries lesions in enamel and dentine.

To reduce the errors of the periodontal probing measurements, we developed a novel constant load periodontal probe, Tohoku University-type constant load periodontal probe (TUCL probe), which has a similar shape with the one used commonly. The tip of the TUCL probe buckles at the hinge of the handle by the excess load on the tip. The probing forces performed by six new resident dentists were recorded by using the model tooth with pressure sensor mounted on the dummy jaw. The maximal force probed upper right first incisor showed 28 to 109 g (average 56 g) when using normal CPI probes. When they used TUCL probes, however, their maximal probing force showed 16 to 33 g (average 20 g). This indicated TUCL probe could exclude the exceed load of probing, which depends on the examiner’s skill. The novel TUCL probe with constant probing load will standardize the periodontal probing examination.