Propagation In Specimens Research Articles

Failure analysis of fatigue crack propagation in specimens of AlSi10Mg aluminum alloy produced by L-PBF: Effect of different heat treatments

Fatigue crack growth behavior of AlSi10Mg aluminum alloy processed by laser powder bed fusion (L-PBF) was studied for three material conditions: as-built, stress relieved and hot isostatic pressed. An improvement in the fatigue crack growth resistance was found for the heat-treated conditions which was attributed to the residual stress relief induced by the heat treatments. The stress ratio did not affect the fatigue crack growth in regime II which was explained by the absence of crack closure. Fatigue crack growth rates were predicted using a model based on the cyclic plasticity of the tested aluminum alloy. The proposed model agreed well with the experimental results.

A coupled thermomechanical peridynamic correspondence model for damage prediction in a freezing rock

This study presents a coupled thermomechanical (TM) bond-associated non-ordinary state-based peridynamic (BA-NOSB PD) (correspondence) model for simulating the thermal behavior and crack propagation in a saturated rock due to freeze. Considering the change in thermal property and the latent water–ice phase transition at low-temperature conditions, the PD form of the nonlinear thermal conduction equation is derived by using the PD differential operator (PDDO). The deformation response is obtained in the framework of BA-NOSB PD model considering the pore ice pressure and thermal expansion. The accuracy of the TM BA-NOSB PD model is verified by simulating the thermal behavior of tuff and sandstone and crack propagation in specimens with pre-existing cracks at low-temperature conditions. The PD predictions of temperature and frost heaving strain are in close agreement with the experimental and previous numerical results. The present model also accurately predicts crack propagation and coalescence in saturated rocks due to the freezing frost.

Evaluating MTS criterion in predicting mixed‐mode crack extension under different loading conditions

AbstractThe maximum tangential stress (MTS) criterion is one of the most widely used criteria for predicting the direction of crack extension. The suitability of this criterion is examined under different loading conditions using the extended finite element method (XFEM). Experimental and numerical results reported in the literature are considered to evaluate the validity and accuracy of the criterion. The results demonstrate that the MTS criterion evaluated by stress intensity factors (SIFs) can accurately predict the direction of crack propagation in specimens under direct tensile loading. This criterion overestimates the angle of crack initiation in the specimen under indirect tensile loading but underestimates the angle in the specimen subjected to 3‐point bending. It is concluded that the MTS criterion based on SIFs could not accurately predict the direction of the crack initiation, which could, however, be determined properly based on the stress distribution around the crack tip obtained by the XFEM numerical models.

Optimal slope angles of the air-coupled transducer from viewpoint of the effective Lamb waves propagation in different materials

Air-coupled transducers (ACT) allow for non-contact generation of acoustic waves which can be later transferred into elastic waves due to interactions with the solid specimen. Proper slope of the ACT in relation to the investigated specimen surface during acoustic waves generation is important as it leads to the possibility of generation of desired Lamb wave modes in the form of elastic waves with selected propagation velocity values. Effective generation of these modes ease the analysis of elastic waves propagation in specimens, simultaneously helping in detection of material defects if they exist. The attempt to determine optimal slope angles was undertaken in case of aluminum (Al), carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) plate-like structure. Due to relatively low excitation frequency of the used ACT (40 kHz) the investigation was focused on the slope angles referred to the generation of zero-order modes - antisymmetric A0 and symmetric S0. In CFRP plate case the issue of efficient generation of shear horizontal (SH0) wave mode was additionally considered. In Al and CFRP plate case optimal slope angles were considered numerically (Comsol, Matlab) and experimentally (lab investigations, Matlab) while in GFRP plate case they were estimated experimentally. Although not all of the slope angles were successfully determined within different approaches, the results helped to form some interesting conclusions. After determination of the slope angles the performance of sloped/non-sloped ACT was compared in full wavefield measurements (comparison of signal energy maps calculated with the usage of root mean square (RMS) formula). Recommendations for further work were depicted within the summary section.

Quasi-static fracture analysis by coupled three-dimensional peridynamics and high order one-dimensional finite elements based on local elasticity.

This work investigates quasi‐static crack propagation in specimens made of brittle materials by combining local and non‐local elasticity models. The portion of the domain where the failure initiates and then propagates is modeled via three‐dimensional bond‐based peridynamics (PD). On the other hand, the remaining regions of the structure are analyzed with high order one‐dimensional finite elements based on the Carrera unified formulation (CUF). The coupling between the two zones is realized by using Lagrange multipliers. Static solutions of different fracture problems are provided by a sequential linear analysis. The proposed approach is demonstrated to combine the advantages of the CUF‐based classical continuum mechanics models and PD by providing, in an efficient manner, both the failure load and the shape of the crack pattern, even for three‐dimensional problems.

Dynamic crack propagation in specimens with a surface irregularity

Initiation of cracks and their propagation in prescribed orientations on tubular specimens has been made possible on a dynamic fatigue testing rig developed at the University of Aberdeen. This rig was originally designed to perform experiments on single edge notched beams (SENB) [1,2]. Modifications have recently been made so as to accommodate experimental tests on tubular specimens with a range of sizes and other cross sections. Crack initiation at grooves on such specimens has been followed by crack growth. At the same time, lateral oscillations of cracked specimen have been measured, as well as accelerations of base excitation, masses above and below cracked specimen. Forces on these two masses have been observed by two load cells attached at positions close to specimen. These load cells facilitated the measurement of stresses in experiment. Crack length time histories have also been constructed by applying an alternating current potential difference (ACPD) method. Fatigue cracks were initiated at the pre-cut grooves in aluminium tubular specimens. Three specimens with different groove sizes were tested in fifteen individual experiments. A three-dimensional Finite Element model was established for each specimen so as to calculate the stress concentration factor (SCF). This formed the basis of determining the amplitude of forcing input made possible by an electromechanical shaker. The phase shifts of acquired time histories has provided some indication of energy transfer mechanism during fatigue and system dynamic response. Observations of cracked specimen displacement during experiments was compared against calculated displacement from elastic theory. Nonlinear responses were observed, suggesting nonlinear stiffness characteristics of the specimen due to discontinuities introduced by crack growth and plasticity effects. Comparison of damage combinations in experimental observation was made with predictions from BS EN 1999-1-3. All experimental observations of total damage combination were higher than predicted values by the latter code.

Dynamics of destruction of epoxy composites filled with ultra-dispersed diamond under impact conditions

Dynamics of destruction of epoxy composites with different contents of nanoparticles of ultra-dispersed diamond is investigated. Epoxy diane oligomer ED-20 was employed as the basic bonding agent in composite formation. It is characterized by its ability of providing high adhesion and cohesive strengths, easy processibility, as well as low coating shrinkage on deposition onto long-length surfaces of complex profile parts. Polyethylene polyamine (PEPA) was used for cross-linking of the epoxy composites, which made it possible to carry out the curing process at room temperature. Formation of interfacial bonds ⟨⟨polymer-filler⟩⟩ due to the improved chemical activity of dispersed particles surface is determined by the method of IR-spectral analysis. The nanoparticles introduction into epoxy binder of an optimal content of q = 0.025 to 0.075 pts.wt. allows to increase the impact resilience 2 times relative to the epoxy matrix. Speed of crack propagation in specimens in time to materials destruction on pendulum is found. Fracture surface of nanocomposite materials is investigated by method of optical and electron microscopy. Based on the analysis of REM image of nanocomposite destruction surface, materials with an ordered structure are found. That allows to state about thermodynamic equilibrium in systems after cross-linking action and about stability of their mechanical strength during the operation process.

Thickness determination of dual-layer coatings based on ultrasonic spectral filtering

In this paper, a non-destructive method for determining the thicknesses of dual-layer coatings is developed using an ultrasonic spectral filtering technique based on the theoretical investigation of wave propagation in specimens with two layers and three interfaces. The acoustic reflection coefficient of dual-layer coatings was separated from the measured signal through Fourier transformation and Wiener deconvolution. Utilising the multi-frequency windows autoregressive spectral extrapolation (MARSE) technique, the cosine signals (sine signals), including the time-of-flight (TOF) of each layer, were split from the real part (imaginary part) of the acoustic reflection coefficient. The thicknesses of the dual-layer coatings were calculated utilising the periods of the cosine signals (sine signals) and the longitudinal wave velocities of each layer. Ultrasonic experiments were carried out on the dual-layer coatings, where the thicknesses of layer 1, layer 2 and the aluminium substrate were 230 μm, 450 μm and 5 mm, respectively. The thicknesses measured by the proposed ultrasonic method were in good agreement with those observed by optical microscopy (OM) and the relative errors of layers 1 and 2 were both less than 5.0%.

Investigation of creep behaviours of gypsum specimens with flaws under different uniaxial loads

The aim of this study is to identify the influence of the dip angle of a pre-existing macrocrack on the lifetime and ultimate deformation of rock-like material. Prediction of lifetime has been studied for three groups of specimens under axial static compressive load levels. The specimens were investigated from 65% to 85% of UCS (uniaxial compressive strength) at an interval of 10% of UCS for the groups of specimens with a single modelled open flaw with a dip angle to the loading direction of 30° (first group), at an interval of 5% of UCS increment for the groups of specimens with single (second group), and double sequential open flaws with a dip angle to the loading direction of 60° (third group). This study shows that crack propagation in specimens with a single flaw follows the same sequences. At first, wing cracks appear, and then shear crack develops from the existing wing cracks. Shear cracking is responsible for specimen failure in all three groups. A slip is expected in specimens from the third group which connects two individual modelled open flaws. The moment of the slip is noticed as a characteristic rise in the axial deformation at a constant load level. It is also observed that axial deformation versus time follows the same pattern, irrespective of local geometry. Specimens from the first group exhibit higher axial deformation under different load levels in comparison with the specimens from the second and third groups.

Mode III interlaminar fracture of carbon/epoxy laminates using the Six-Point Edge Crack Torsion (6ECT)

The recently proposed Six-Point Edge Crack Torsion (6ECT) test was used to evaluate the mode III interlaminar fracture of carbon/epoxy laminates. Plate specimens with starter delaminations in 0/0, 0/90 and 0/45 interfaces were tested. Data reduction was performed with an effective crack scheme validated in a previous numerical study. The tests allowed the evaluation of fairly unambiguous initiation G IIIC values and of subsequent R-curves. Examinations of specimen cross-section showed considerable lengths of pure interlaminar propagation in specimens with starter delaminations in 0/90 and 0/45 interfaces. The latter specimens had the lowest initiation G IIIC values.

Tensile Strength versus Toughness of Cement-Based Materials against High-Velocity Projectile Impact

This paper presents a laboratory experimental study on the effect of tensile strength and toughness on the resistance of cement-based materials to high-velocity projectile impact. Three types of cement-based materials with similar compressive strength but different tensile strengths and toughnesses were examined in terms of the penetration depth, crater diameter, and crack propagation in specimens subjected to impact of ogive-nosed projectile with a diameter of 13.35 mm and a mass of 19 grams traveling at velocities of about 650 m/s. The materials evaluated include: (i) a plain high-strength concrete (HSC) with low tensile strength and toughness, (ii) a fiber-reinforced concrete (FRHSC) with high toughness but similar flexural tensile strength as the HSC, and (iii) a polymer cement composite (PCC) with high flexural tensile strength but low toughness. The PCC was developed from a combination of inorganic cements with water-soluble polymers, and it had a 28-day flexural tensile strength of 25 MPa. Results suggest that the toughness was a more important parameter that contributes to the impact resistance of the cement-based materials. Increasing the tensile strength alone, however, did not seem to contribute to the impact resistance of the cement-based materials.

Effect of trabecular orientation on mechanical resistance and ultrasound propagation in specimens of equine vertebrae

Osteoporosis involves alterations, not only in density, but also in the architectural organisation of the bone; in particular, trabecular orientation, following the skeletal load directions, lends a high degree of stiffness to the whole bone. We investigated the relationship between trabecular orientation, density, stiffness and ultrasound (US) propagation in two orthogonal directions (par. = parallel to, and ort. = orthogonal to the main orientation of the trabeculae) in cylindrical equine bone specimens (thoracic vertebrae) where a preferential orientation is present. A total of 15 cylinders were progressively decalcified with 0.2 mol/L ethylenediaminetetraacetic acid (EDTA). At different levels of decalcification, we measured the apparent density (g/cm 3), bone mineral density or BMD (g/cm 2), stiffness coefficient (MPa) and various US parameters. Before decalcification, stiffness values were the same in all directions. As the decalcification proceeded, the stiffness declined and, at low BMD values, it was significantly different in the two directions, being the highest in the par. one. Different behaviours of US parameters were observed in the two directions: SoS (speed of sound) was closely related to apparent density, BMD and stiffness in the par. direction (r = 0.88, 0.92 and 0.88, respectively, p < 0.0001). In the ort. direction, no significant association has been found between SoS and apparent density, BMD or stiffness. In the same experimental setup, US fast wave amplitude (FWA) was related to apparent density and BMD in the par. direction (r = 0.72 and 0.67, respectively, p < 0.0001) and in the ort. direction (r = 0.83 and r = 0.84 respectively, p < 0.0001). FWA was also correlated to stiffness in both directions (r = 0.61 par.; 0.81 ort., p < 0.0001). These results show that trabecular orientation strongly influences both mechanical properties of bone and US propagation. Furthermore, we found that US parameters can be predictors of mechanical properties of the bone independent of bone density. (E-mail: fcavani@unimore.it)

Wave propagation and strain localization in dynamically loaded specimen

The aim of the presentation is focused on two aspects which are the influance of the waves propagation in specimens on the choise of places of strain localization and the discussion of numerical models that serve the recognition of this phenomenon. There are some experimental tests which are under consideration (thin plate, axisymmetric bar, 3-D specimen) which are numericaly studied. The experiments are made in the Hopkinson tube with the typical velocity of deformations of order 10 4 s -1 so the processes could be treated as adiabatic. For ductile materials under such conditions to avoid the mathematical consequences due to thermal softening (ill-posedness) the viscoplastic constitutive description is used. In numerical simulations we have shown the well-posedness of the solution of governing equations by showing the insensitivity of the results to spatial discretization. The set of numerical examples proofs that it is possible to estimate the places of strain localization by observing the velocities of particles. Intensive plastic zones appear in the places where the local velocities are close to zero. In the formulation we have accepted the constitutive viscoplastic model, finite deformations and the evolution of porosity. The computations were performed in the environment of ABAQUS finite element program.

Fatigue-Crack Propagation in Heat-Resistant Alloys under Thermomechanical Loading. Part 1. Experimental Method and Results of the Investigation of Crack-Propagation Rates

We propose a calculation and experimental method for studying the influence of cyclic thermomechanical loading, wherein the object under investigation is subjected to the simultaneous action of a cyclically varying mechanical load and temperature varying with time and resulting in the occurrence of thermal stresses, on fatigue-crack propagation in specimens of triangular cross section. We present results of the calculation of the stress-intensity factors in elastic formulation for an angular mode-I crack in the case of loading by pure bending and temperature varying with time. We studied crack-growth resistance of alloys KhN70VMTYu (Eacute;I617) and KhN73MBTYu (Eacute;I698) at a constant temperature and under thermomechanical loading. Kinetic fracture diagrams da/dN vs KI have been constructed for the alloys at a constant temperature and for two regimes of thermal cycling within a near-threshold and mean-amplitude portions of the curve da/dN vs KI .

Numerical analysis of crack propagation in tension specimens of concrete considered as a 2D multicracked granular composite

A numerical model is presented to study the mechanisms of microcrack propagation in concrete, the load-displacement response and the trajectories of the crack propagation in specimens under displacement control. The microstructure of concrete in this model is represented by a matrix, inclusions and pre-existing microcracks introduced around the inclusions. Both the matrix and the inclusions are assumed to be elastic, homogeneous brittle materials. The stiffness of the inclusions is considered to be three times higher than that of the matrix. Crack propagation in the numerically-generated concrete is controlled by fracture mechanics-based criteria and is calculated through the finite element method. The influence of the microstructure of concrete, the size and the distribution of grains, the properties of the interfacial zone between grains and the matrix, as well as the boundary conditions, on final crack patterns and load-displacement responses are investigated. The different appearance of the numerical load-displacement curves exhibiting the quasi-brittle behaviour observed in experiments is explained in terms of several points of view, especially the dynamic crack propagation.

Strength characteristics and fatigue crack growth in a composite with long aligned fibers

Strength values and fatigue crack growth on a specially made composite material are reported. The composite specimen consisted of an epoxy matrix and one layer of long aligned glass fibers that were equally spaced. The results on strength showed that for a range of fiber spacing λ, the composite's strength σc, scaled with a fiber spacing in the form of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdmhaaa!39C2!\[\sigma \] c % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% qG7oaaleqaaaaa!395B!\[\sqrt {\text{\lambda }} \] = % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOUdaaa!393F!\[{\text{\kappa }}\]. Based on dimensional arguments the constant κ was found proportional to the fracture toughness of the matrix. Fatigue crack propagation experiments were performed on specimens with different fiber spacing and applied loads. The crack speed reached a steady mode of propagation in specimens where relation % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdmhaaa!39C2!\[\sigma \] c % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% qG7oaaleqaaaaa!395B!\[\sqrt {\text{\lambda }} \] = % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOUdaaa!393F!\[{\text{\kappa }}\] was satisfied. The same mode of propagation was reached for the debonding along fibers in the bridging zone as well as the crack opening displacement. The crack opening displacement at a fiber location and the corresponding debonding were linearly related. Within the resolution of the observations, no fiber fracture was seen in the bridging zone. Using a standard Green's function, stress intensity factor simulations were carried out for different types of tractions on the fibers in the bridging zone. When the fibers in the bridging zone were under a uniform load, the total stress intensity factor Kt, at the crack tip, was found constant at the steady state and proportional to % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdmhaaa!39C2!\[\sigma \] % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyOhIu6aaO% aaaeaacqaH7oaBaSqabaaaaa!3B3F!\[\infty \sqrt \lambda \]. Assuming that Kt is constant during steady crack growth, the results of the simulations were used to correlate steady crack speed in three sets of data. Dimensional analysis of the steady crack speed was carried out as an attempt to identify important parameters and the role of the fiber spacing in the fracture of the composite specimens. The steady crack speeds were correlated with the total stress intensity factor for each fiber spacing. The resulting exponents were found to be about 20 percent different. Assuming that at steady state the energy release rate for an interfacial crack is proportional to tt2r, where t is the stress carried by a fiber [20], a power expression for the rate of debonding with t % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaacq% aHepaDaSqabaaaaa!39DF!\[\sqrt \tau \] was found to have an exponent approximately equal to that for the steady crack speed.

Effect of heat treatment on the resistance of a dispersion hardening nickel alloy to hydrogen embrittlement

1. Prolonged immersion of the alloy in an atmosphere of hydrogen and the magnitude of loading both influence the nature of fracture and critical crack propagation in specimens of 03Kh20N60MVYu. 2. In order to evaluate the influence of hydrogen on the crack resistance of alloys, determination of the property ratios KcH/Kc and KcH/σ0.2 is proposed, and also the threshold value of KIcH defining the "safe working" level for parts. 3. Heat treatment according to the regime: austenize at 1050°C, 1 h; air cool + age at 75°C, 3 h; water cool, produces the formation of a structure in alloy 03Kh20N60MVYu with uniformly distributed particles of γ′-phase whose sizes deviate only slightly from the average.

Strain gauging for accurate determination of K and G in impact tests

Impact testing of materials is becoming increasingly important as a wide range of new materials are being developed for demanding high loading-rate working conditions. Charpy pendulum and many other impact-testing machines are being better instrumented to provide more information about the forces acting on a specimen up to and during fracture. Mostly, the force sensors are near the points of contact on the striker or the support and these can provide well for recording the overall forces acting on the specimen to be monitored. Of increasing interest is the distribution of stress and strain within the specimen during the initiation and propagation of fracture. This paper reports research using on-specimen strain-gauge sensors for impact testing of non-metallic specimens. Comparisons are made between force-time traces from sensors on the specimen and those located on the striker. Observations are made as to how the stresses relate to the fast crack in the core of the material specimen and those acting on the surface of the material about the crack, and also those acting on the plastic hinge formed on the compression side of the specimen. Optical and scanning electron microscopic studies are made of the crack surfaces and high-speed photography is used to observe the crack propagation in specimens with and without side-grooves to guide the crack and increase constraint.

The measurement of fatigue crack propagation in specimens immersed in liquid sodium at elevated temperatures: Technique and preliminary results

The measurement of fatigue crack propagation in specimens immersed in liquid sodium at elevated temperatures: Technique and preliminary results

The effect of environment on the mechanism of Stage I fatigue fracture

Fatigue crack propagation in nickel-base superalloys at low and intermediate temperatures occurs predominantly in the Stage I mode, along {111} slip planes. Cracking normally starts at an external surface and the Stage I fracture surface has a cleavage appearance. Both of these factors indicate that the environment may play an important role in this mode of propagation. To assess the role of environment in Stage I fracture and to determine the mechanism of failure, fatigue tests were run in air and vacuum on single crystals of low-carbon MAR-M200. The fatigue life at room temperature is significantly greater in vacuum than in air, and the improvement in life increases as the stress range is reduced. Fatigue crack propagation in specimens tested in air and in vacuum is entirely in the Stage I mode, but only the specimens tested at low stress ranges in air have a cleavage appearance. In vacuum and at high applied stress levels in air, fracture surfaces have a matte appearance with fewer fracture steps and river lines. At high magnifications, a dimpled structure is observed on these fracture surfaces. The fatigue life in air can be attributed to a faster rate of crack growth resulting from oxygen adsorption at the crack tip. A model for Stage I fatigue crack propagation in planar slip materials is presented which is an extension of the Griffith-Orowan criterion to cases where localized cleavage occurs at a crack tip in fatigue.