Stress-free Sample Research Articles

Spherical Indentation Method for Measuring Biaxial Residual Stresses Without Using Stress-Free Sample

Spherical Indentation Method for Measuring Biaxial Residual Stresses Without Using Stress-Free Sample

Novel bulk triaxial residual stress mapping in an additive manufactured bridge sample by coupling energy dispersive X-ray diffraction and contour method measurements

A novel approach for determining triaxial residual stress states by coupling energy dispersive X-ray diffraction and contour method measurements is provided and validated in a Ti‐5Al‐5V‐5Mo‐3Cr additive manufactured (AM) bridge sample. Synchrotron X-ray diffraction (SXRD) can provide relatively fine spatial resolution (on the order of 10–100 µm) for mapping 3D elastic strain fields within a sample. However, for samples with dimensions larger than one or two centimeters the path length can get prohibitive as both constant wavelength and energy dispersive SXRD are based upon transmission measurements. As an example, for a plate-like sample geometry where the thickness is limited (less than a centimeter) it is trivial to measure the longitudinal and height direction elastic strain components with excellent in-plane spatial resolution (∼100 µm) and a somewhat lower through-thickness resolution (10–15 times larger) but obtaining the through thickness component is often not possible as the beam path must be parallel to one of the large dimensions of the plate. While small samples of low Z-number alloys (e.g., Ti or Al) allow determination of the three elastic strain components, this is often not the case for higher Z alloys (e.g., Fe or Ni) or large samples where some strain components can be indeterminable. To overcome these limitations, this work applies a combination of synchrotron X-ray diffraction and the contour method (a mechanical relaxation technique), to determine the triaxial stress state. This novel combination is demonstrated and validated in a relatively small additive manufactured sample where all three orthogonal strain components are accessible via X-ray diffraction for stress determination. The paper also explores methods for determining the strain-free lattice parameter, typically obtained from a small stress-free reference sample. This work shows that a small size AM sample is not stress free, producing unreliable magnitudes of strain and stress. Instead, a strain-free lattice parameter is determined using residual stress equilibrium conditions, which gives consistent strain and stress trends for both X-ray diffraction (alone) and the new diffraction-contour coupling technique. This demonstrates that the coupling technique can be confidently applied to samples to determine stress when path length (size or Z-number) prohibit determination of three orthogonal strain components via diffraction.

X-ray stress analysis with a 2D detector. Part 2. Goniometer alignment

The use of 2D detectors for stress/strain analysis enables new strategies for data acquisition and treatment to be devised, thus enabling faster measurements to be performed, including stress mapping without goniometer rotations. These advantages were developed in Part 1 of this work [François (2023). J. Appl. Cryst. 56, 48–60], but they imply the occurrence of additional alignment issues that need to be addressed carefully. The aim of this second part is to describe the misalignment errors geometrically using the general formalism introduced in Part 1. Under the assumption that the defects of the equipment are small, closed-form analytical expressions are provided for each type of error. It is found that the trigonometric functions describing each defect are independent. Thus, it becomes possible to identify and quantify the type of error from measurements performed on a stress-free sample such as a powder specimen. This is achieved by a linear least-squares regression analysis on the measured peak positions for various values of the goniometric angles. The proposed method is then applied to an example.

Effects of tensile loading during annealing of alnico melt spun ribbons

Conventional magnetic annealing (MA) of the permanent magnet alloy alnico involves application of an external magnetic field at temperatures within the spinodal decomposition range. This field biases the growth of the Fe-Co rich, ferromagnetic α1-phase in an energetically favorable 〈001〉 direction in alignment with the applied field within an Al-Ni rich, paramagnetic α2-phase. Utilizing a magnetic field to bias the α1-phase may limit alnico from reaching theoretical coercivity due to (1) the field having maximum biasing ability at temperatures near the Curie temperature where large α1-phase nanorods form and (2) connectivity of the α1-phase occurs unavoidably during MA. Both decrease the effective shape anisotropy of the α1-phase, thereby reducing coercivity. Herein, we explore tensile-loading as a biasing mechanism to control and optimize the final alnico nanostructure beyond that achieved by MA. Two samples of melt-spun alnico were heat-treated at 860 °C for 5 minutes: one sample was subjected to 10 MPa tensile stress for comparison with a stress-free control sample. Structural and magnetic characterization revealed that the stress-annealed ribbon sample possessed expected phase assemblages, but was distinguished by a ∼2× larger grain diameter and an elongated anisotropic α1-phase within grains that were oriented to a shear stress along 〈001〉 directions at an angle of ∼45° relative to the loading direction. Both types of annealing produced a similar increase in the coercivity and remanence, but a decrease in saturation magnetization.

A new methodology for measuring residual stress using a modified Berkovich nano-indenter

In this paper, a new methodology, based on nanoindentation, is proposed to estimate the magnitude and orientation of the principal residual stress components, in a non-equi-biaxial residual stress field at the nanoscale. The proposed method exploits the force-penetration response provided by a modified Berkovich tip that, compared to the most traditional one, is characterized by a preferential elongated size. This feature allows in estimating non-equibiaxial residual stress field since the force-penetration response changes according to the indenter orientation. No observation of the residual imprint is required. Principal stress components and orientations are estimated thanks to theoretical and finite element analysis, establishing a linear relation between stress components and load variations respect to the stress-free sample. Finite element analysis is used to calibrate the model coefficients and to verify the accuracy of this approach for isotropic metals. Obtained results revealed that the method is well suitable in estimating both tensile and compressive residual stresses with an average error lower than 6% for the first case and lower than 10% for the second one.

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

Investigations of internal stresses in high-voltage devices with deep trenches

Deep trenches, as essential elements of silicon chips used in electronic high-power and high-frequency devices, are known as starting points for dislocation generation under the influence of internal mechanical stresses resulting mainly from the difference in the thermal expansion coefficients between silicon and silicon dioxide. Since the electrical insulation of the devices requires a sequence of mechanical, chemical, and high-temperature processes during the preparation of the deep trenches, including the formation of an amorphous SiO2 edge layer, the emergence of the internal stresses is hardly avoidable. The method of cross correlation backscattered electron diffraction in the scanning electron microscope is used here to quantitatively determine the magnitude and local distribution of internal stresses in silicon around the deep trenches after four different process steps. For this purpose, Kikuchi diffraction images are recorded of the wafer cross section areas along lines perpendicular and parallel to the deep trenches. After Fourier transformation, these images are cross correlated with the Fourier transform of the diffraction image from a stressfree reference sample site. The well-established numerical evaluation of cross correlation functions provides the complete distortion tensor for each measuring point of the line scan, from which the stress tensor can be calculated using Hooke's law. It is found that the in-plane normal stress component σ11 perpendicular to the long edges of the deep trench is larger than the other stress components. That means it essentially determines the magnitude of the von-Mises stress, which was determined as a general stress indicator for all measuring points, too. A characteristic feature is the local distribution of the stress component σ11 with maximum tensile stresses of some hundred megapascals at transition between Si and amorphous SiO2 on the long edges of the deep trench, and with even higher maximum compressive stresses immediately below the bottom of the deep trench. At a distance of about 2 μm from the edges of a single deep trench, all stress components decrease to negligibly small values so that steep stress gradients occur. The range and distribution of tensile and compressive stresses are in accordance with finite element simulations; however, the measured stresses are higher than expected for all investigated states so that dislocation formation seems to be possible. The influence of the electron acceleration voltage on the determination of the internal stresses is discussed as well.

Two-Directional Stress-Free Comb Sample for Weld Study by Neutron Diffraction

Neutron diffraction is a powerful tool for investigations of residual stresses in bulk polycrystalline samples. The method is based on measuring the relative change in the spacing, d, in the stressed lattice with respect to the reference, or stress-free, spacing d0. Therefore, the precision of measuring the stress strongly depends on the precision of measuring d0. Conventional stress-free comb sample with teeth in the normal direction considerably simplifies measurement of d0 in welds. However, it has been shown that in a thick weld, a “too long tooth” may contain residual stress. In this study, the idea of two-directional comb sample is proposed. Experiments have shown more complete stress relief in this comb type.

Neutron Diffraction Research of Residual Stress in Metal Plates Deposited on Rigid Substrate by DLMD Method

One of the features of direct laser metal deposition (DLMD) method is presence of high residual stress in produced parts. The information about residual stress distribution is critically important for the competent organization of production processes in additive manufacturing. To study residual stress distribution in large thin-walled parts produced by DLMD, two model samples were manufactured. Small thickness of the samples allowed using the calculation technique that does not require measurement of the lattice parameter of stress-free sample. It is shown that stress distribution in thin-walled plate built on a rigid substrate considerably differs from distribution in plate built on a flexible substrate.

Saliva resistin as a screening marker of gestational diabetes mellitus

Objective The aim of this study was to evaluate the resistin concentrations in saliva; which is a noninvasive and stress-free diagnostic sample, and to investigate the significance of salivary resistin concentrations in screening GDM. Methods This cross-sectional case-control study included 41 newly diagnosed GDM patients and 40 healthy pregnant. The participants were consecutively included in the study among eligible pregnant women; who were in the age range from 18 to 40 years of age and at the gestational age between 24 and 28 weeks. The levels of serum and salivary resistin were determined using an enzyme-linked immunosorbent assay method. Results Maternal serum resistin and salivary resistin concentrations were significantly higher in the patients with GDM compared to the individuals in the control group. The data were evaluated by the receiver–operator curve analysis; which revealed that serum and saliva resistin concentrations were moderately successful markers to differentiate subjects with GDM from healthy pregnant women. Conclusions The results indicate that the determination of saliva resistin levels at the gestational age between 24 to 28 weeks may be used as an alternative, stress-free, and noninvasive technique that may be used in GDM screening.

Measuring Nonequibiaxial Residual Stresses and Mechanical Properties Using Knoop Indentation

Abstract A proper knowledge of the mechanical properties and residual stresses of materials has a significant role in the prediction of engineering failures. Indentation is a simple, nondestructive test that is capable of estimating both residual stresses and mechanical properties. In frequent studies, the response of materials during the indentation process has been used as a key parameter to distinguish different substances. Here, a state-of-the-art method with no undesirable restriction is suggested to attain the work hardening exponent, yield strength, and planar nonequibiaxial residual stresses. In the current work, an extensive series of Knoop indentation simulations were performed using two indenter angles. Subsequently, a precise observation was made in order to find existent relationships. A local method was employed based on characteristics of similar materials to obtain stress-free sample parameters through a genetic algorithm, and then another error function was defined in order to measure the yield strength and work hardening exponent. After the determination of the mechanical properties and the stress-free sample’s parameters, a particular and precise categorization was made. Then, neural network analysis was employed to derive planar residual stresses. Experimental validation was conducted using six types of aluminum and steel specimens. The results confirmed a good agreement between the test data and those predicted using the suggested procedure.

Effects of segregation, quenching rate, and external stress on precipitation behaviors of Al-Cu-Cd alloys

The addition of Cd to AlCu alloys alters their precipitation behaviors and improves their mechanical properties. Previous studies focused primarily on the aging process. However, the inhomogeneous conditions caused by segregation and quenching rate and the external stress during aging affect the precipitation behaviors, which is unavoidable for large-scale components. In this work, the effects of these factors on the precipitation behaviors of Al-Cu-Cd alloys were studied. The obtained results indicated segregation and a low quenching rate delayed the precipitation process, and reduced the 8 h-aged yield strength from 459 MPa to 389 MPa and 312 MPa, respectively. External stress had two opposite effects on the precipitation behavior: (i) trapping Cd atoms and impeding the motion of Cd atoms and (ii) introducing dislocations and accelerating nucleation of the precipitates. The 15 h-aged yield strength subjected to the external stress of 60 MPa was 432 MPa, and the 9 h-aged elongation subjected to the external stress of 130 MPa was 6.7%. Both were lower than those of the stress-free sample. Therefore, minimizing the inhomogeneous conditions caused by segregation and quenching rate and avoiding the external stress during aging was the optimum route of alloy fabrication.

Sampling, identification and sensory evaluation of odors of a newborn baby\u2019s head and amniotic fluid

For baby odor analyses, noninvasive, stress-free sample collection is important. Using a simple method, we succeeded in obtaining fresh odors from the head of five newborn babies. These odors were chemically analyzed by two-dimensional gas chromatography coupled with mass spectrometry (GC × GC-MS), and compared with each other or with the odor of amniotic fluid from the baby’s mother. We identified 31 chemical components of the volatile odors from neonate heads and 21 from amniotic fluid. Although 15 of these components were common to both sources, there was an apparent difference in the GC × GC patterns between the head and amniotic fluid odors, so the neonate head odor might be individually distinct immediately after birth. Therefore, we made artificial mixtures of the major odor components of the neonate head and maternal amniotic fluid, and used psychological tests to examine whether or not these odors could be distinguished from each other. Our data show that the artificial odor of a neonate head could be distinguished from that of amniotic fluid, and that the odors of artificial head odor mixtures could be correctly discriminated for neonates within an hour after birth and at 2 or 3 days of age.

Investigation of AlGaN/GaN HEMTs degradation with gate pulse stressing at cryogenic temperature

Degradation on DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) after applying pulsed gate stress at cryogenic temperatures is presented in this paper. The nitrogen vacancy near to the AlGaN/GaN interface leads to threshold voltage of stress-free sample shifting positively at low temperature. The anomalous behavior of threshold voltage variation (decrease first and then increase) under gate stressing as compared to stress-free sample is observed when lowing temperature. This can be correlated with the pre-existing electron traps in SiNX layer or at SiNX/AlGaN interface which can be de-activated and the captured electrons inject back to channel with lowering temperature, which counterbalances the influence of nitrogen vacancy on threshold voltage shift.

Stress-free deposition of [001] preferentially oriented titanium thin film by Kaufman ion-beam source

We proposed a method to control and minimize residual stress in [001] preferentially oriented Ti thin films deposited by a Kaufman ion-beam source using a substrate temperature during deposition (T) as the parameter. We determined the residual stress, corresponding lattice parameters, and thickness of deposited films using X-ray diffraction and X-ray reflectivity measurements. We showed that the Ti film deposited at T≈273°C was stress-free with corresponding lattice parameters a0 and c0 of (2.954±0.003)Å and (4.695±0.001)Å, respectively. The stress-free sample has the superior crystallographic quality and pure [001] orientation. The Ti thin films were oriented with the c-axis parallel to the surface normal. We also investigated root mean square of surface roughness of deposited films by atomic force microscopy and it was in the range from ≈0.58nm to ≈0.71nm. Such smooth and stress-free layers are suitable for microelectromechanical systems.

Determination of salivary α-amylase and cortisol in psoriatic subjects undergoing the Trier Social Stress Test

The study was aimed to investigate the response in psoriatic patients of the two primary neuroendocrine systems, namely the Autonomic Nervous System and the Hypothalamic-Pituitary-Adrenal (HPA) axis, whose main biomarkers of activation are salivary α-amylase and cortisol, respectively. Reliable analytical procedures for the determination of salivary α-amylase enzymatic activity and cortisol concentration in oral fluid were developed. The non-invasive sampling that does not require trained personnel represents one of the main advantages of oral fluid analysis compared to the analysis of blood and its derivatives (e.g. plasma). Since oral fluid offers the advantage of a simple and stress free sample collection, it is particularly useful to quantify stress related molecules.α-amylase activity was measured by a spectrometric method, whereas salivary cortisol by Reversed-Phase High-Performance Liquid Chromatography coupled to Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry. The effect of salivary pH and flow rate on salivary α-amylase activity and cortisol were also investigated.Finally, both methods were preliminary tested on oral fluid samples of psoriatic volunteers underwent to a psychosocial stress task (the Trier Social Stress Test), as a first step of a more comprehensive study on the responsiveness of psoriatic subjects to stressors.

Spherical Indentation, Part II: Experimental Validation for Measuring Equibiaxial Residual Stresses

Abstract It is occasionally important or even inevitable to measure residual stresses without harming engineering components. Spherical indentation can be categorized as a nondestructive method to measure residual stresses, as it causes very little damage to the surface of the sample. Furthermore, it is cheap and quick. Spherical indentation has gained the interest of a number of researchers in recent years. Although, this technique has been used in the literature to measure residual stresses, it has mostly been employed to determine material characteristics. In the current study, the application of the spherical indentation to measure residual stresses was examined and verified experimentally. The instrumented indentation tests resulted in load-displacement curves for different materials with a variety of ranges of equibiaxial stresses, which were then used to train an artificial neural network (ANN). The stress measurements were carried out for both residual and applied stresses. A quenched sample and a cross-shaped bending sample were considered as a source of residual stresses and in-plane applied stresses, respectively. An important benefit of this method was that it could be used with no requirement for a reference stress-free sample. The results of stress measurements illustrated a reasonable success.

In-situ residual stress analysis during annealing treatments using neutron diffraction in combination with a novel furnace design

A method for in-situ monitoring of residual strain/stress evolution during annealing treatments has been developed using a new induction heating setup designed for neutron strain-scanning instruments. First, time-of-flight neutron diffraction was used for determining the location and level of maximum residual stresses induced by quenching in disc shape samples of Ni-base superalloy Inconel 718. These were found to be in the centre of the samples being 700MPa in tension in radial/hoop directions and −300MPa in axial direction. Subsequently, an in-situ annealing experiment was undertaken at 750°C for 8h in order to monitor the d-spacing evolution in this particular location in the three principal directions and in an attached stress-free sample. The measurements revealed that most stress relaxation occurs during the heating stage as a result of a combination of plasticity and early stage creep relaxation. During the isothermal stage the d-spacing changes measured in the stressed sample were dominated by the evolution of the stress-free d-spacing resulting in only very limited further stress relaxation. Ex-situ measurements after annealing confirmed the in-situ observations at the 8h mark of 60% residual stress reduction validating the in-situ experimental protocol.

A cut‐off fracture approach for residual stress estimation in thick epoxies

The use of thick carbon fibre reinforced thermosets appears recently for naval and offshore efficient design. However internal residual stresses are developed during the curing of thermosets, as shown by defects like fibre wavinesses, cracks and bubbles, especially with increasing thickness. The understanding of phenomena involved during the curing of thick epoxy was described in a previous work by a by a finite element modeling of the thermal, chemical and mechanical couplings . To enable a better quality control, pulsed lasers have recently been used in non‐destructive inspection and characterization of composites and enable quality comparison from a reference stress free sample [2]. The experimental set‐up of the laser shock technique was assessed by simulation approach. Description of material parameters required is presented in the first part of the paper, based on previous experiments and models to identify material parameters for any degree of cure, assuming a dependency to the degree of cure only at room temperature. In this framework, this study presents the effects of epoxy heterogeneity, induced by the curing process of a thick matrix, for internal stress level and damage prediction. The plasticity of the matrix was described by a Ramberg‐Osgood model and an hydrodynamic constitutive law. As a first approach, a simple cut‐off fracture model based on the maximal tensile stress is used to identify the weakest parts of the solid matrix. Simulation comparison with a homogeneous fully cured stress free sample enables a first average residual stress level estimation.

The precipitation behavior of Al–Cu–Mg–Ag single crystal during aging under elevated compression stresses

The Al–Cu–Mg–Ag single crystal is prepared to study its precipitation behavior during aging under elevated compression stresses. Transmission electron microscopy (TEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and hardness test are used for this investigation. The results show that a compression stress of 50MPa leads to more uniform length distribution but slightly lower number density of Ω-phase in the stress-aging sample, as compared to that of the stress-free aging sample. When a higher compressive stress of 200MPa is applied, the number density of Ω-phase is significantly decreased while a large number density of θ’-phase are precipitated along the dislocation lines introduced by the applied stress. Furthermore, the stress-orientating effect on Ω precipitation can not be observed in Al–Cu–Mg–Ag single crystal during stress aging even under the compressive stress of 200MPa loading along the [2¯15]Al direction. This result signifies that the applied stress magnitude is not the only critical factor in determining whether the stress-orienting effect of Ω-phase can be generated, as this effect may be also associated with the loading orientation during stress aging.