Ultrasonic fatigue testing on notched and smooth specimens of ultrafine-grained steel

Ultrasonic and conventional electromagnetic resonance fatigue tests were conducted on notched and smooth specimens of ultrafine-grained steels. The notched specimens developed no internal fractures but showed a fatigue limit. In this case, ultrasonic fatigue testing showed good agreement with the electromagnetic resonance. The smooth specimens showed internal fractures after ultrasonic fatigue testing, but not during electromagnetic resonance testing. In the smooth specimens, ultrasonic fatigue testing showed a longer fatigue life. These results show that the frequency effect is negligible in surface fractures of notched specimens but not in smooth specimens. The frequency effect causes a change in type of fracture from surface fracture to internal fracture. Concerning the gigacycle fatigue properties, the ultrafine-grained steel shows a fatigue limit with notched specimens, whereas the fatigue limit can disappear with smooth specimens because of the internal fracture.     

Failure analysis of a diesel engine piston-pin

A diesel engine piston-pin used in a truck was smashed in four when servicing. The longitudinal and transverse cracking happened on the failed piston-pin. The cracks initiated from the internal hole surface and propagated toward the external circle. The occurrence of beach marks or fatigue striations on the fracture surfaces of all crack origin regions indicates that fatigue fracture is the dominant failure mechanism of the piston-pin. The internal hole and external circle surfaces are specified to be carburized. The microstructure and the microhardness profiles on the external circle and internal hole surface regions were examined to determine the depth of the carburized layer. However, not only was no carburized layer found on the internal hole surface, but also the serious decarburization occurred on the surface region of the internal hole. Appearance of decarburization in the internal hole surface decreases intensely the fatigue strength of the internal surface so that the crack initiated from the internal surface and propagated toward the external circle, at last the fatigue fracture occurred. Improper carburizing technology is responsible for the appearance of the decarburization on the internal hole surface.     

Some consequences of a fracture criterion for oriented polymers based on electron spin resonance spectroscopy

Electron spin resonance (ESR) spectroscopy, which is able to monitor covalent bond rupture during polymer fracture, has provided an experimental basis on which to construct an atomistic fracture model which takes proper cognizance of the distribution of stress over the material's internal structure. Using a computer model developed by DeVries and Lloyd, estimations of times to fracture were obtained for several different temperatures, stresses, and stress rates. The predicted rupture times agreed satisfactorily with experimental observations, which indicates the practical utility of the ESR-based model. Further, the effective activation parameters which result when the Zhurkov macroscopic fracture model is correlated with the ESR predictions provide an insight as to the role of the internal stress distribution in determining the kinetics of the fracture process.     

The effect of heat treatment on plane strain fracture of glassy polymers

The effect of annealing on the plane strain fracture of round-notched polycarbonate and poly(methyl methacrylate) bars has been investigated. Morphological observations of thin sections and fracture surfaces revealed that the fracture initiated from internal crazes which were nucleated at the tip of a local plastic zone. The critical hydrostatic stresses for internal craze nucleation were nearly constant regardless of annealing, while the shear yield stress increased with increasing annealing time. The reduction in toughness by annealing can be ascribed to the decrease of the maximum extent of the plastic zone which gives the critical hydrostatic stress for craze nucleation.     

Microstructure and fracture in three dimensions

A high resolution three dimensional (3D) scanning technique called X-ray microtomography was used to measure internal crack growth in small mortar cylinders under compressive loading. Tomographic scans were made at different load increments in the same specimen. 3D image analysis was used to measure internal crack growth during each load increment. Load–deformation curves were used to measure the corresponding work of the external load on the specimen. Fracture energy was calculated using a linear elastic fracture mechanics approach using the measured surface area of the internal cracks created. The measured fracture energy was of the same magnitude that is typically measured in concrete tensile fracture. A nominally bilinear incremental fracture energy curve was measured. Separate components for crack formation energy and secondary toughening mechanisms are proposed. The secondary toughening mechanisms were found to be about three times the initial crack formation energy.     

内燃机车用柴油机连杆螺钉断裂分析

内燃机车在检修重组后运行约一个月,其柴油机的两根连杆螺钉发生断裂。采用化学成分分析、硬度测试、断口分析和金相检验等方法,对两螺钉断裂的原因进行了分析。结果表明:两螺钉的断裂均为疲劳型断裂,其中一根螺钉先行断裂,其断裂的根本原因可能是螺钉紧固处于不合理的状态所致。     

Numerical modelling of fracture initiation and propagation in biaxial tests on rock samples

A two-dimensional boundary element code, based on the displacement discontinuity method is used to simulate a confined compression test. The method takes account of the granular nature of the rock and of the presence of pre-existing defects. Fracture propagation is thought to depend, amongst other factors, on the crack orientation, the residual friction angle, the dilation angle, and the confining pressure. To obtain a more precise understanding of the influence of these properties on the crack growth process, their influence on the normal stress and the excess shear stress on potential fracture planes ahead of the crack tip is investigated for a single crack configuration. The orientation of the potential fracture planes proves to be the most important parameter determining fracture growth. A series of numerical experiments is carried out to determine the influence of the tessellation pattern used to represent the granular nature of the rock. Both the influence of the type of tessellation and the tessellation density are evaluated, and reasons for the differences in behaviour are presented. The results of the simulations with the Delaunay and a Voronoi tessellation with internal fracture paths compare well with the fracture pattern obtained in laboratory tests. The pre-peak non-linearity in the stress-strain response obtained with the Voronoi tessellation and the post-peak strain softening obtained with the Delaunay tessellation are combined in one model. For that purpose, a Voronoi tessellation with internal fracture paths is used, whereby the properties of the elements of the polygons and of the internal fracture paths are assigned different values. The role that is played by shear failure and the influence of dilation on the localisation process is determined by means of some further numerical experiments. It is shown that at the scale, at which the material is modelled, shear failure is required for a shear band to develop.     

Internal friction,crack length of fracture origin and fracture surface energy in alumina-zirconia composites

Two series of alumina-zirconia composites, i.e. alumina-unstabilized zirconia and alumina-partially stabilized zirconia with 3 mol % Y₂O₃, with different zirconia content were slip casted and fired at 1550°C for 3 h. Elastic constant, bending strength and fracture toughness were measured. Internal friction was determined to follow the formation of cracks, nondestructively, which could be one of the fracture origins. The crack length of the fracture origin and the fracture surface energy were calculated by applying Griffith's fracture theory. Microstructures of the fracture surfaces were observed using a scanning electron microscope. For the unstabilized zirconia system, the increase in the internal friction of the order from 10⁻⁴ to 10⁻³ was a guide to find the formation of cracks which lead to the fracture. The increase in the cracks becoming a fracture origin lead to the increase inK _lc and also to the apparent increase in the fracture surface energy. For the partially stabilized zirconia system, the increase in the fracture surface energy with an increase in zirconia content, keeping low internal frictions of the order of 10⁻⁴, indicates the intrinsic strengthening of the grain boundaries in comparison to the unstabilized zirconia system. Internal friction is the most suitable nondestructive physical quantity to find the microcracks which leads to the fracture.     

Embedded representation of fracture in concrete with mixed finite elements

As an alternative to the smeared and discrete crack representations, an embedded representation of fracture for finite element analysis of concrete structures is presented. The three-field Hu–Washizu variational statement is extended to bodies with internal discontinuities. The extended variational statement is then utilized for formulating elements with a discontinuous displacement field. The new elements are capable of modelling different deformation modes of an internal discontinuity at the element level. The satisfactory performance of the embedded crack representation is verified by several case studies on concrete fracture.     

Dependencies between internal structure and mechanical properties of spray dried granules – Experimental study and DEM simulation

The mechanical properties of spray dried granules are decisive with regard to further applications and can be modified via internal granule structure. To obtain the correlations between structural and mechanical properties, necessary experiments are often time and resource consuming. The simulation of varied granule structures and their effect on resulting mechanical properties seems to be a promising approach.In this paper, a model of the particulate internal structure of a spray dried granule was generated with the Discrete Element Method (DEM) based on real structure parameters. The model considers real primary particle number, particle size distribution and radial granule inhomogeneity, what results in the implementation of granule shell thickness and macro void. The internal structure of simulated granules showed significant influence on their mechanical properties. An increase of granule shell thickness and packing density of the primary particles within the shell results in fracture strength increase accompanied by decreasing fracture strain. The simulated reduction of the solid bridge bond size between the primary particles representing the decreasing binder amount leads to decreasing fracture strength and strain as previously determined experimentally (Eckhard et al., 2014). Consequently, the DEM is appropriate for evaluating the effect of changed real internal structure parameters on resulting mechanical granule properties.     

Stress states in RENi5 powders during hydrogen charging and discharging cycles

In this study, the evolution of the stress states in RENi ₅ particles during hydrogen charging and discharging cycles were investigated using coupled diffusion–deformation finite-element method analyses. The results indicate that large tensile stresses, of the order of 20–30% of the modulus of elasticity, develop in the particles even in the absence of both internal and external crack-like defects. The internal and external cracks behave differently from each other during hydrogen charging and discharging cycles. Therefore, the fracture resistance of the particles containing external cracks will be different from the particles having internal cracks. The disc-shaped particles, in addition to having faster charging–discharging cycles, may offer better resistance to fracture than the spherical particles.     

An experimental verification of nonlocal fracture criterion

By using a nonlocal field theory, Eringer et al. [6] obtained a finite solution for the stress at the tip of a sharp crack. This solution permitted the development of a nonlocal fracture criterion for crystalline materials that is given in terms of atomic distance and theoretical cohesive strength.

The nonlocal fracture criterion is generalized for application to real materials by the introduction of a characteristic dimension (a measure of the size of the internal structures). Particleboard, a wood-based composite with controllable internal characteristics (particle dimensions and amount of resin), is used to substantiate the nonlocal fracture criterion.     

Aircraft engine bleed system tubes: Material and failure mode analysis

The failures of T-ducts made of Ti A40 with 50 mm in diameter and 0.52 mm in thickness, used in the pneumatic system of a commercial aircraft, are examined as part of a research program aimed to improve the development of such systems. The investigation included the fractographic analysis of the burst parts, microstructure and internal duct surface observations and mechanical tests performed in samples taken from the failed ducts and from unused material. The failed duct fracture surfaces showed typical features of transgranular fatigue cracking. Delamination and oxide particle deposits were found in their internal surfaces. Tensile and fatigue tests revealed some effects of aging, but with no evidence of embrittlement. The fracture mechanisms were the same for the failed ducts and for the unused material. The internal surface damage, associated to the geometry of the duct and to the solid-solution hardening increases the local stress concentration. These facts suggest that cracks had initiated at multiple sites around the inside surface, so that further increments of cracking occurred on subsequent cyclic pressurization during service life until the occurrence of sudden final fast fracture.     

Phase‐field modeling of ductile fracture at finite strains: A robust variational‐based numerical implementation of a gradient‐extended theory by micromorphic regularization

This work provides a robust variational‐based numerical implementation of a phase field model of ductile fracture in elastic–plastic solids undergoing large strains. This covers a computationally efficient micromorphic regularization of the coupled gradient plasticity‐damage formulation. The phase field approach regularizes sharp crack surfaces within a pure continuum setting by a specific gradient damage modeling with geometric features rooted in fracture mechanics. It has proven immensely successful with regard to the analysis of complex crack topologies without the need for fracture‐specific computational structures such as finite element design of crack discontinuities or intricate crack‐tracking algorithms. The proposed gradient‐extended plasticity‐damage formulation includes two independent length scales that regularize both the plastic response as well as the crack discontinuities. This ensures that the damage zones of ductile fracture are inside of plastic zones or vice versa and guarantees on the computational side a mesh objectivity in post‐critical ranges. The proposed setting is rooted in a canonical variational principle. The coupling of gradient plasticity to gradient damage is realized by a constitutive work density function that includes the stored elastic energy and the dissipated work due to plasticity and fracture. The latter represents a coupled resistance to plasticity and damage, depending on the gradient‐extended internal variables that enter plastic yield functions and fracture threshold functions. With this viewpoint on the generalized internal variables at hand, the thermodynamic formulation is outlined for gradient‐extended dissipative solids with generalized internal variables that are passive in nature. It is specified for a conceptual model of von Mises‐type elasto‐plasticity at finite strains coupled with fracture. The canonical theory proposed is shown to be governed by a rate‐type minimization principle, which fully determines the coupled multi‐field evolution problem. This is exploited on the numerical side by a fully symmetric monolithic finite element implementation. An important aspect of this work is the regularization towards a micromorphic gradient plasticity‐damage setting by taking into account additional internal variable fields linked to the original ones by penalty terms. This enhances the robustness of the finite element implementation, in particular, on the side of gradient plasticity. The performance of the formulation is demonstrated by means of some representative examples. Copyright © 2016 John Wiley & Sons, Ltd.     

The effect of poling treatment and crystal structure of PZT on fracture toughness and fatigue resistance

Empirical measurements of fracture toughness and fatigue strength were conducted for piezoelectric Pb(Zr _x ,Ti_{1 – x} )O₃ of various compositions such as tetragonal, MPB, and rhombohedral. Before the poling treatment the rhombohedral showed the highest fracture toughness, while the tetragonal revealed the lowest fracture toughness. After poling treatment, the fracture toughness measured by the pre-cracked SENB method decreased in all three compositions. The most remarkable decrease was observed in the tetragonal composition. However, when the indentation strength method was used the highest fracture toughness was observed in the tetragonal. The stress intensity factor relief due to microcracks around the indentation marks and the anisotropic internal stresses caused by domain alignment during the poling treatment were proposed as explanations for the comflicting results. Fatigue resistance was lowered by the compressive stress introduced during the poling treatment. The highest fatigue resistance was observed in the rhombohedral composition of low tetragonality, which exhibited low internal stress.     

Constraint effect on the ductile crack growth resistance of circumferentially cracked pipes

A systematic study has been carried out by using 2D axisymmetric models to understand the ductile fracture behavior of pipes with internal and external circumferential cracks. Crack growth resistance curves have been computed using the complete Gurson model. Pipes with various diameter-to-thickness ratios, internal pressure, crack depths and material properties are analyzed. The results have been compared with those of corresponding SENT and standard SENB specimens. It clearly indicates that the SENT specimen is a good representation of circumferentially flawed pipes and an alternative to the conventional standard SENB specimen for the fracture mechanics testing in engineering critical assessment of pipes.     

Internal pressure test for characterizing fast-fracture reliability of grinding wheels

A constant danger associated with the use of most grinding wheels (vitrified-bond alumina and silicon carbide wheels) is the possibility of fracture during operation. A standard practice is to subject newly manufactured wheels to a spin test and accept wheels that survive. We propose an internal pressure test which offers a simpler, more economical alternative to the spin test for testing grinding wheels. Probabilities of failure in the internal pressure test are correlated with failure probabilities in the spin test using probabilistic fracture mechanics. Results indicate a reasonably good correlation between the two tests, thus demonstrating their equivalence. A scheme for the easy implementation of the internal pressure test to detect damage in grinding wheels is outlined.     

Fracture analysis in micropolar elasticity: anti-plane crack

Anti-plane fracture analysis is performed for a micropolar material. The crack problem is reduced as Cauchy singular integral equations by Fourier integral transform. Numerical solutions of the stress intensity factors are obtained by the Lobatto–Chebyshev collocation method. Parametric studies indicate that the fracture behavior of the micropolar material depends on the coupling factor and the internal length scale. Larger coupling factor and internal length scale lead to stronger micropolar effect. The micropolarity is beneficial to reducing the driving force of a micro-crack, however, it may also prompt the propagation of a macro-crack.