Laser Powder Bed Fusion (PBF-LB) is widely used to fabricate complex geometries. However, simple evaluation methods, such as monitoring the warpage of cantilever specimens, are required due to residual stresses generated during the process. In this study, cantilever specimens of Inconel 718 alloy were fabricated by PBF-LB to investigate the relationship between residual stress and warpage. Surface residual stresses were measured using X-ray diffraction. Finite element analysis (FEA) was used to estimate the initial residual stress from the measured warped shapes. The results showed no simple correlation between surface residual stress and maximum deflection. However, under the same volumetric energy density (VED), deflection and residual stress displayed a linear relationship. Further, the through-thickness residual stress distribution, modelled as a fifth-order polynomial, closely matched the experimentally measured deflection. The analysis suggested that the curvature in the stress profile depends on the heat input, characterized by VED. These findings show that cantilever-based warpage measurements can serve as a practical screening method for evaluating residual stress in LPBF parts.

In recent years, engineering plastics have attracted increasing attention as high-strength polymer materials for various industrial applications. With the advancement of composite materials, polymers have been increasingly employed in load-bearing and structural components as substitutes for metallic materials. Consequently, the development of reliable methods for stress evaluation in polymer materials has become an important issue. In this study, the 2θ-sin²ψ method was applied to the evaluation of residual stress in polymer materials. The Committee on X-ray Study of Mechanical Behavior of Materials, the Society of Materials Science, Japan, conducted round-robin X-ray stress measurements using polyphenylene sulfide (PPS) in order to identify essential factors affecting X-ray stress measurement of polymer materials and to establish standardized measurement procedures. This paper reports the results of the round-robin measurements and discusses the determination and validity of the X-ray stress constant required for stress evaluation using the 2θ-sin²ψ method. As a result, the X-ray stress constant for PPS was determined to be K = −140 MPa/deg.

The surface of S45C steel specimen was hardened by uniaxial scanning the semiconductor laser beam. After measuring the burn mark width on the surface, the surface electrical resistance was measured by the 4-point probe method. The hardened layer width and total case depth were determined from the etched micrograph and the hardness distribution of the specimen, respectively. The width of the burn mark was wider than that of the hardened layer. These widths were found to be approximated by linear functions of the laser scanning parameter, √vq₀. Using the four-probe method, the surface resistivity and resistance of the specimen having finite width and thickness were necessary to be measured with the probe spacing less than one-seventh of the specimen width, and then required the correction based on the shape effect. The surface resistance at the center of the hardened layer was larger than before laser hardening and increased with an increase in total case depth. The detectable total case depth was found to be less than one-tenth of the probe spacing. In this study, the surface resistance was measured perpendicular to the laser scanning direction, the change of surface resistance was approximated as a Gaussian distribution. As a result, the total case depth and hardened layer width could be evaluated nondestructively from the peak intensity and distribution width of the fitted Gaussian distribution.

A method is proposed to estimate the near-surface residual stress gradient in the depth direction of shot-peened surface. In the conventional estimation method, the stress gradient is determined by the inverse analysis of nonlinear 2θ versus sin²ψ diagram. The proposed method is useful when clear nonlinearity cannot be observed in the sin²ψ diagram due to the insufficient higher ψ angle. This method includes X-ray dual-axis inclining method, using both the iso-inclination scanning (rotation angle: ψ) and the side-inclination scanning (rotation angle: Ω). Under a certain inclination angle of Ω, “the apparent stress of inclined surface” obtained by the ψ rotation is almost in proportion to cos²Ω. The stress gradient can be determined from the y-intercept of the approximate line, and the residual stress at top surface can be determined from the proportion constant. This method was applied to four steel specimens shot-peened under different conditions. The results of the estimation were compared with the residual stress distribution just below the surface obtained by the conventional X-ray stress measurement and sequential electrochemical polishing performed on the same specimens.

In this study, we measured the type II residual stress in a welded austenitic stainless steel specimen. The dimensions of the incident X-ray beam were 0.4 × 0.4 mm², the X-ray beam was transmitted into the specimen, and the diffraction image was measured using the double exposure method. The specimen was moved with a step of 25 μm. The diffraction angles were determined from the diffraction spots. As a result, we were able to successfully measure the type II residual strain of each crystal grain. When we examined the type II residual strain in the base material of the specimen, its standard deviation was 0.00156, which was just under 300 MPa. When we measured the residual stress distribution near the weld boundary, it was found that the type II residual stress had a significant influence on the residual stress.

In this paper, we considered what the specimen thickness and measurement conditions should be in residual stress measurement using the double exposure method. Two plate specimens with thicknesses of 3.0 and 1.7 mm were cut off from a welded specimen with a thickness of 5 mm. The residual stresses of these specimens were measured using the double exposure method, and residual stress maps were created. Comparing the residual stress maps, it was found that a plate thickness in the range of 2 to 3 mm was sufficient, as the residual stress characteristics were maintained between the two specimens. In addition, for efficient measurement, it is preferable to prepare two specimens with a plate thickness of 2 or 3 mm and compare the residual stress maps to understand the macroscopic residual stress distribution and the state of microscopic residual stress. It is also advisable to measure a wide area efficiently with as high-energy X-rays as possible and high spatial resolution. Each microscopic residual stress has its own unique location and diffraction pattern. Therefore, to create a residual stress map for a thick plate by stacking multiple plates, the diffraction patterns are multiplied by an attenuation factor and then superimposed. The diffraction angle is determined using the crosscorrelation algorithm, and then the residual stress map of the stacking plate specimen can be obtained.

Damage to the ceramic layer is a problem in thermal barrier coatings (TBCs) used in high-temperature components of gas turbines. Changes in the stress state within the TBC due to exposure to high-temperature heat are thought to be one of the causes of TBC delamination and cracking. Therefore, developing a method for measuring stress components in each direction, such as in-plane and out-of-plane stress, is important for elucidating the damage mechanism and for realizing nondestructive inspection and life prediction. While in-plane stress can be measured using the sin²𝜓 method, obtaining out-of-plane stress requires cutting the sample or combining synchrotron X-rays and laboratory X-rays while removing the sample surface. This report proposes a nondestructive method for evaluating out-of-plane stress in TBCs using only high-energy synchrotron X-rays. The sin²𝜓 method was performed using high-energy synchrotron X-rays on the surface of TBC test specimens thermally exposed to high-temperature atmosphere for different times. By focusing on the physical meaning of the intercept of the 2𝜃-sin²𝜓 diagram, which had not previously received attention, and advancing the analysis, in-plane and out-of-plane stresses were separated. The out-of-plane stress measured on the TBC topcoat surface after heat exposure for different times was tensile, which is consistent with the trend of out-of-plane stress measured by the hybrid method using both synchrotron X-rays and laboratory X-rays. This method can be applied to both non-destructive measurements from the TBC surface and cross-sections, and is applicable to materials other than TBCs.

To obtain the fracture toughness and the fracture energy of materials, edge chipping tests have been conducted with loading an indenter onto the surface near an edge of a sample and forming a fragment. The edge chipping test on rock is not only a simple method to investigate the cutting mechanism but also a simulation of undercutting for effective hard rock excavation. In previous studies, the relation between maximum force and cutting distance in the edge chipping test on rock has been formulated, but the cutting process has been poorly modeled. In this study, the variable-compliance-type model based on non-linear viscoelastic theory was applied to the edge chipping tests to reproduce the cutting process of rock. First, the original model for stress and strain was modified to be represented by force and penetration in the edge chipping test. Then, the values of the constants in the model were determined with the aid of the previous edge chipping tests using button tips. The calculation with a single set of the constants for a single rock type succeeded in reproducing the experimental results under various button tip diameters and cutting distances. In addition, a relation between those constants and uniaxial compressive strength was found for brittle rocks. The rate dependence of force-penetration curves, which is essential for practical rock excavation, was examined with the aid of the calculation under various tip penetration rates.