Fracture Mechanics Investigation Research Articles

Fatigue Safety Assessment of Existing Steel Bridges in China

Based on investigation of linear fatigue-damage accumulation theory and fracture mechanics, a deterministic assessment approach was first proposed, in which the fatigue failure behavior of existing steel bridges was reflected. Furthermore, according to the stochastic fatigue failure mechanism of existing steel bridges, member fatigue reliability assessment models were proposed using probabilistic fracture mechanics. The deterministic and probabilistic assessment methods proposed in this paper were applied to evaluate the remaining fatigue life and service safety of two case study bridges. According to assessment results, the safe inspection intervals and maintenance strategies were determined, which can control and avoid fatigue failure accidents and unnecessary repairs or replacements.

Fractographic Investigations of Ordinary Ceramic Refractory Materials with Reduced Brittleness

A fractographic procedure was developed and applied for ordinary ceramic refractory materials with a rather large size of heterogeneities and defects. It is based on a stereooptical method for generation of digital surface profiles which are evaluated by an autocorrelation function. Furthermore, a lateral correlation length is derived. A group of seven refractory materials was characterised by mechanical and fracture mechanical investigations, and the same specimens had been characterized by the fractographic procedure. Correlations have been tested. The results show a relation between the lateral correlation length and two fracture mechanical characteristics which are significant for the material brittleness and the elastic strain energy stored at maximum load. These relations are contributed to the dependence of the crack path on brittleness. With decreasing brittleness the amount of the crack path proceeding along the grain/matrix boundary increases for the materials investigated.

Investigation of Fracture Mechanism of 6061 Aluminum Alloy by Means of In-situ Observation

Fracture processes of 6061 aluminum alloy specimens under different stress states are investigated with in-situ tensile experiments.The results indicate that a lot of slip bands are produced in all specimens' surfaces under different stress states,but the fracture mechanism is different.With stress triaxiality decreasing,the fracture modes vary from dimple-coalescence mixing shear fracture to pure shear fracture and the fracture surfaces also vary from dimple fracture mode to shear fracture mode.The grain boundaries of 6061 aluminum alloy are the weakest area and microcracks initiate at the grain boundaries.With loading increasing,the microcracks extend and coalesce.The specimens fracture due to coalescence or shearing between the microcracks.The less the stress triaxiality is,the more obvious the orientation of the dimple on the two surfaces of fracture surface tends to be,and the smoother the fracture surfaces will be.

Acrylic Nanocomposite Resins for Use in Stereolithography and Structural Light Modulation Based Rapid Prototyping and Rapid Manufacturing Technologies

AbstractA novel family of optically transparent acrylic nanocomposites containing up to 30 wt % silica nanoparticles with an average diameter of 20 nm was developed for application in structural light modulation (SLM) and stereolithography (SL) technologies. The uniform dispersion of nanoparticles affords a significantly improved toughness/stiffness‐balance of the photopolymerized and postcured nanocomposites. It is possible to increase stiffness, as expressed by Young's modulus, from 1290 to 1700 MPa without encountering the embrittlement typical for many other conventional filled polymers. Fracture behaviour is examined by means of fracture mechanics investigation and SEM analyses of fracture surfaces. According to TEM analyses and measurement of optical transmittance remarkable uniform dispersion of silica nanoparticles was achieved. The silica nanoparticle concentrations up to 17 wt % give only marginally higher viscosities and do not affect transmittance, while slightly increasing the exposure times needed in photopolymerization. Moreover, the silica nanoparticles afford materials with reduced shrinkage and improved properties. The green effective ankle splay out (EASO) measured on H‐shaped diagnostic specimens, is significantly reduced for the nanocomposite materials from 1.38 mm for the unfilled material to 0.82 mm for nanocomposites containing 30 wt % nanosilica. The building accuracy is increased significantly with increasing content of silica nanofillers.

Investigation of fracture mechanism of 6063 aluminum alloy under different stress states

Fracture mechanisms in a 6063 aluminum alloy were investigated and analyzed carefully by in-situ tensile tests in SEM with a vacuum chamber. Specimens used were designed to produce different stress states. Studies indicated that with stress triaxiality (σ m/σ e) decreasing, the fracture modes changed from normal fracture to shear fracture and the fracture surfaces changed from the dimples and intragranular dominated fracture mode to the shear dominated fracture mode. The grain boundaries of the 6063 aluminum alloy were the weakest positions. In the case of high stress triaxiality, the grain boundary cracks were produced by normal stress or by the incompatibility of deformation between neighboring grains, and the normal stress dominated the crack propagation. In the case of low stress triaxiality, the boundary cracks were produced by the relative slipping of grains against neighboring grains, and the shear stress dominated the crack propagation. The final fracture of the specimens occurred by connections of cracks through transgranular cracking of the ligaments among these cracks.

Editorial

Elastomeric materials show a wide range of different elastic and inelastic properties. Additionally, this class of materials is subjected to large deformations. Considering all these effects, fracture mechanical investigations are very challenging tasks and cannot be performed with standard approaches. Effects of inhomogeneities and discontinuities such as cracks can be investigated with the so-called material force approach in an efficient and elegant way. For comprehensive investigations of inelastic materials, the complete balance of the material motion problem has to be formulated. In this case, the material volume forces depend on the internal history variables which are required for the inelastic constitutive model. This paper derives a general formulation for rate-dependent and rate-independent inelastic materials based on a multiplicative split of the deformation gradient to cover viscoelastic and elastoplastic materials at finite deformations.

Experimental and numerical investigation of wood fracture mechanisms at different humidity levels

Abstract Differences in fracture patterns and properties at various moisture levels are experimentally and numerically evaluated and discussed. Experiments were performed on spruce, pine, oak and beech. The influence of moisture at 98%, 80%, 65% and 30% RH and the mechanisms involved were investigated for softwoods and hardwoods subjected to opening mode I fracture using in situ and ex situ real-time environmental scanning electron microscopy (ESEM). The wedge-splitting technique was employed. To quantify the effect of humidity, fracture toughness values were obtained from ex situ tests and finite element analysis of the contact problem in wedge-splitting. In addition, lattice fracture model simulations were performed for numerical investigation of the fracture mechanisms. Distinct changes in wood fracture behaviour were observed as a function of moisture content. Fracture toughness was highest at 30% RH for all species except for oak, and showed higher values in the radial-longitudinal than in the tangential-radial direction. In green wood, water droplets moved away from the cell lumens around the crack tip. Drying of wood promotes microcracking and crack bridging as toughening mechanisms. The findings reported may be useful for further research into the interaction between moisture transfer and stress gradients in wood accompanied by moisture-crack phenomena.

Influence of Nanoscale Morphology on the Micro- and Macromechanical Behaviour of Polymers and Polymer Nanocomposites

There is enormous scientific and economic interest in the development and evaluation of polymer nanocomposites due to the fact that the properties of a material become increasingly insensitive to flaws at the nanoscale, enabling the exploitation of the unique physical and mechanical properties of very small objects in large-scale components. However, the successful industrial implementation of such novel materials poses unique challenges which are not only related to the small size of the reinforcements. Decades of intensive research have shown that polymer nanocomposites differ from their counterparts based on traditional reinforcements in many ways and a fundamental understanding of the structure-propertyrelationships of such novel materials is only slowly emerging. Although issues such as the intrinsic properties of the nanoscale constituent as well as the degree of dispersion and orientation of individual filler particles clearly appear to be important factors, molecular interactions between the filler and the matrix during processing can lead to pronounced variations in the matrix microstructure. These variations in themselves lead to pronounced changes in the micro- and macromechanical deformation behaviour of the nanocomposites. A detailed investigation of fatigue crack growth behaviour of such novel materials for example is essential in order to understand the fracture mechanical performance and the transition from a ductile to a brittle behaviour which is often observed experimentally, especially in the case of amorphous matrices. However, as the size of the filler particles approaches the molecular level, the novel interactions at the interface or even in the interphase can lead to significant changes in the micromechanical deformation behaviour. Significant work has been carried out regarding the fracture mechanical investigation of polymer blends with both micro- and nanoscale morphologies and much can be learned by comparing the results of polymer nanocomposites to these more established polymer blends.

Material forces for inelastic models at large strains: application to fracture mechanics

Elastomeric materials show a wide range of different elastic and inelastic properties. Additionally, this class of materials is subjected to large deformations. Considering all these effects, fracture mechanical investigations are very challenging tasks and cannot be performed with standard approaches. Effects of inhomogeneities and discontinuities such as cracks can be investigated with the so-called material force approach in an efficient and elegant way. For comprehensive investigations of inelastic materials, the complete balance of the material motion problem has to be formulated. In this case, the material volume forces depend on the internal history variables which are required for the inelastic constitutive model. This paper derives a general formulation for rate-dependent and rate-independent inelastic materials based on a multiplicative split of the deformation gradient to cover viscoelastic and elastoplastic materials at finite deformations.

Investigation of ballistic impact properties and fracture mechanisms of Ti 3SiC 2 ternary ceramics

The deformation response of Ti 3SiC 2 ternary ceramics have been investigated by performing ballistic impact experiments at a range of low velocities. The reactively hot pressing compacts with an off-stoichiometric ratio were predominantly Ti 3SiC 2 phase (99.13 vol.%) and the grains were elongated in shape with a grain size of 3–8 μm. The bulk modulus (196 GPa) was slightly lower than that calculated by fitting the pressure versus lattice volume data with the Birch–Murnaghan equation (206 GPa), and that predicted by using the first-principle computational technique (204 GPa). The typical fracture surface of the fragments was failed by transgranular fracture combined with intergranular fracture. Most fracture mechanisms were similar to energy-absorbing mechanisms observed in the vicinity of the indentation. The dynamically impacted fragments of Ti 3SiC 2 contained dislocations and stacking faults, grouped in bands extending from the grain boundaries and ending in the grain. The Si plane was depleted and the remaining thin lamella consisted of six close-packed Ti planes, with five C planes in-between, showing identical stacking as {1 1 1} planes in TiC. The Hugoniot elastic limit and the hydronamic pressure at the HEL point were estimated to be 6 and 3 GPa, respectively.

A numerical analysis for cracks emanating from a surface semi-spherical cavity in an infinite elastic body by FRANC3D

This note is specifically concerned with cracks emanating from a surface semi-spherical cavity in an infinite body (see Fig. 1) by using the boundary element software FRANC3D developed by a fracture mechanics investigation group of Cornell University. The numerical results can reveal the effect of the geometry of the surface cavity on the stress intensity factors.

A new numerical analysis for a semi-circular surface crack

This note is concerned with a surface semi-circular crack by using the fracture analysis software FRANC3D developed by a fracture mechanics investigation group of Cornell University. Attention is specifically paid to the effect of the boundaries parallel to the crack on the stress intensity factors (SIFs).

Fracture mechanical investigation of steel grade S890 used in Cleuson–Dixence hydropower plant shaft

As a consequence of the accident at the Cleuson–Dixence hydropower plant in December 2000, several investigations were started. In a material qualification programme for the steel S890, as applied in the Cleuson–Dixence plant, strength investigations and small scale fracture mechanic tests in weld metal, heat affected zone and base metal have been performed. Additionally, wide plate tests were performed to verify the results. With the obtained material properties, a fracture mechanics assessment of the critical crack size of the shaft in the area of the pipe rupture was performed using the R6 procedure. This procedure was verified for the given conditions, i.e. material strength and toughness including the welding quality, by the comparison of the calculational and experimental results of the tested wide plates with cracks in base metal and weld metal.

Mechanical properties of talc- and CaCO3-reinforced high-crystallinity polypropylene composites

Toughening mechanisms and mechanical properties of two high-crystallinity polypropylene (hcPP)-based composite systems, hcPP/talc and hcPP/CaCO3, are investigated. Significant improvement in tensile modulus is observed in the PP/talc composite, but only a moderate improvement is found for hcPP/CaCO3. The introduction of CaCO3 nanoparticles to hcPP helps nucleate a measurable amount of β-phase crystals and results in a significant drop in crystallization temperature, suggesting a possible retardation of hcPP crystallization. In addition, the hcPP/CaCO3 nanocomposite shows more pronounced damping characteristics than that of hcPP/talc, throughout the temperature range studied. A detailed investigation of fracture mechanisms suggests that well-dispersed, highly oriented talc particles cause embrittlement of hcPP. Only when the crack extends toward the edges of the specimen will the crack deflection/bifurcation and microcracking mechanisms initiate. In the case of hcPP/CaCO3, the CaCO3 nanoparticles help trigger massive crazing and shear yielding if the testing speed is in quasi-static. The presence of β-phase crystals around the CaCO3 particles could facilitate the formation of crazes throughout the hcPP matrix. Approaches for toughening hcPP are discussed.

Impact‐toughening mechanisms of calcium carbonate‐reinforced polypropylene nanocomposite

AbstractThe impact fracture mechanisms of polypropylene (PP), containing 9.2 vol % of calcium carbonate (CaCO3) nanoparticles, were investigated using optical microscopy and transmission electron microscopy. The incorporation of CaCO3 nanoparticles reduces the size of spherulites and induces the formation of β‐phase crystallites, which leads to a more ductile PP matrix. Double‐notch four‐point bending (DN‐4PB) Charpy impact specimens and notched Izod impact specimens were utilized to study the fracture mechanism(s) responsible for the observed toughening effect. A detailed investigation reveals that the CaCO3 nanoparticles act as stress concentrators to initiate massive crazes, followed by shear banding in PP matrix. These toughening mechanisms are responsible for the observed, improved impact strength. A comparison of the fracture mechanisms observed between DN‐4PB Charpy and Izod impact tests is also made to show the effectiveness of DN‐4PB for investigation of impact fracture mechanisms of polymeric systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3070–3076, 2006

Investigation of fracture mechanism on liquid crystalline epoxy networks arranged by a magnetic field

A liquid crystalline epoxy resin was cured under non- and 10T-magnetic fields, and polydomain and monodomain networks were obtained, respectively. The fracture toughness of these systems was evaluated and it was clarified that the toughness of the magnetic field system showed a higher value. To investigate the toughening mechanism, polarized micro FTIR measurements were carried out. As a result, it was clarified that their mechanisms were quiet different. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1406–1412, 2006

Non-uniform Compressive Strength of Debonded Sandwich Panels – II. Fracture Mechanics Investigation

This article describes the development, validation and application of a FEM based numerical model for prediction of residual strength of damaged sandwich panels. The core of the theoretical method is a newly developed procedure for prediction of the propagation of a face-core debond. As demonstrated, the method can predict the maximum load carrying capacity of real-life panels with debond damages, where the failure is governed by face-sheet buckling followed by debond growth. The developed theoretical procedure is an extension of the as Crack Surface Displacement method, here denoted as the Crack Surface Displacement Extrapolation method. The method is first developed in 2D and then extended to 3D by use of a number of realistic assumptions for the considered configurations. Comparison of the theoretical predictions to a series of large-scale experiments, described in [17], shows that the model is indeed able to predict the failure modes and the residual strength of damaged panels with accuracy sufficient for practical applications. This opens up for a number of important engineering applications, for example risk-based inspection and repair schemes.

Mechanical and fracture mechanical characterization of building materials used for external thermal insulation composite systems

Fracture mechanical investigations of building materials applied for external insulation composite systems have been performed in order to provide material data for the numerical simulation of mechanical failure. For this purpose a wedge splitting procedure according to Tschegg has been employed and modified for the investigation of material layers with a thickness of approx. 5 mm. The testing method proved to be suitable for investigation of both the materials themselves and the compound of two layers. Results of commercially available building materials for external insulation composite systems are shown. From their dependence on different preparation procedures, it may be concluded how temperature and humidity may affect material properties under conditions of actual service.

A Fracture Mechanics Investigation on Crack Growth in Massive Forming

To understand the crack growth in massive forming and to consequentially avoid crack growth in workpieces, it is necessary to investigate its dependence on the crack depth and thus on the state of hydrostatic stress. Prior work shows that the crack opening displacement (COD) for shallow cracked tension specimens with low stress triaxiality is twice as high as for deep cracked specimens with high stress triaxiality. This work examines the crack growth in compression specimens with pre-cracked cylindrical upsetting samples. The compression samples were cut in the stress symmetry plane in order to observe crack initiation and crack growth by a single specimen technique. In this way it is possible to observe blunting, crack initiation and crack growth inside the upsetting specimens. The resulting COD does not differ significantly from the values achieved in tension samples with short surface cracks.

Investigation of fracture mechanisms in resin concrete using spatially multiplexed digital Fresnel holograms

A digital twin Mach-Zehnder holographic interferometer is presented and applied to the investigation of fracture mechanisms in resin concrete submitted to a three-point flexural loading. The double interferometric setup allows spatial multiplexing for a simultaneous double sensitivity measurement. The component determination is obtained from two different illumination directions giving two sensitivity vectors. The numerical reconstruction of the object is performed following the discrete Fresnel transform. We investigate the image formation by merging analogic and discrete Fourier calculations in a smooth reference plane wave scheme. We examine the complex relation between the original object and the reconstructed one and the object localization is derived. Experimental results are presented and exploited to obtain the evolution of the crack tip propagation during the test.