Differences In Fracture Behavior Research Articles

Dynamic Fracture of a Semi-Crystalline Bio-Based Polymer Pipe: Effect of Temperature

The influence of temperature on the resistance to rapid crack propagation of a semi-crystalline bio-based polymer was studied. The experimental results described in this study allow to initiate a first discussion on the role of viscosity and its link with the fracture behaviour and a heterogeneous microstructure such as the semi-crysalline polymer. Dynamic fracture tests on pipes were carried out. It would appear that a temperature decrease of approximately 40 C relative to ambient has no significant influence on the average crack propagation velocity (≈0.6cR), fracture energy and surface roughness. On the contrary, crack propagation paths seem to vary with temperature. The difference in fracture behaviour between the amorphous and crystalline phase varies significantly as a function of temperature. The difference between the initiation resistance and the rapid propagation also varies. This difference seems to be significantly reduced by lowering the temperature. The mechanisms of cavitation damage and plastic flow are increasingly limited by the decrease in temperature (and therefore in macromolecular mobility). Crack propagation in the pipe is more extensive and therefore more critical. This is emphasised in particular by the probability of the material to be macro-branched as the temperature decreases.

Fracture Behavior of Permeable Asphalt Mixtures with Steel Slag under Low Temperature Based on Acoustic Emission Technique.

Acoustic emission (AE), as a nondestructive testing (NDT) and real-time monitoring technique, could characterize the damage evolution and fracture behavior of materials. The primary objective of this paper was to investigate the improvement mechanism of steel slag on the low-temperature fracture behavior of permeable asphalt mixtures (PAM). Firstly, steel slag coarse aggregates were used to replace basalt coarse aggregates with equal volume at different levels (0%, 25%, 50%, 75%, and 100%). Then, the low-temperature splitting test with slow loading was used to obtain steady crack growth, and the crack initiation and propagation of specimens were monitored by AE technique in real time. From the low-temperature splitting test results, SS-100 (permeable asphalt mixtures with 100% steel slag) has the optimal low-temperature cracking resistance. Therefore, the difference of fracture behavior between the control group (permeable asphalt mixtures without steel slag) and SS-100 was mainly discussed. From the AE test results, a slight bottom-up trend of sentinel function was founded in the 0.6–0.9 displacement level for SS-100, which is different from the control group. Furthermore, the fracture stages of the control group and SS-100 could be divided based on cumulative RA and cumulative AF curves. The incorporation of 100% steel slag reduced the shear events and restrained the growth of shear cracking of the specimen in the macro-crack stage. Finally, the considerable drops of three kinds of b-values in the final phase were found in the control group, but significant repeated fluctuations in SS-100. In short, the fracture behavior of PAM under low temperature was significantly improved after adding 100% steel slag.

Investigation on machined surface quality in ultrasonic-assisted grinding of Cf/SiC composites based on fracture mechanism of carbon fibers

As a kind of carbon fiber-rich material, the machined surface quality of Cf/SiC composites is greatly affected by the fracture mode of carbon fibers. Therefore, exploring the material removal mechanism of Cf/SiC composites in grinding through the different fracture mechanisms of carbon fibers from the perspective of microstructure is necessary. In research, the micro-scale fracture mechanism of carbon fibers and the transformation of mechanism were explored. The difference in fracture behavior of carbon fibers caused different energy consumption in material removal, which is instructive for monitoring the machining process with specific grinding energy. The difference of machined surface morphology was directly caused by the difference of material removal mechanism, which can be effectively characterized by the machined surface roughness. The transition of the material removal mechanism was determined by the maximum undeformed chip thicknesses. Ultrasonic-assisted grinding promoted the removal of carbon fibers in nano-scale brittle fractures by reducing the maximum undeformed chip thicknesses, thereby improving the quality of the machined surface.

Investigation on the Electron Beam Welding of Al/Cu Composite Plates

The Al/Cu composite plates composed of 2.5 mm thick Al base plate and 0.5 mm thick Cu cladding plate were joined by electron beam welding (EBW). The butt joints of Al/Cu composite plates were obtained successfully in Modes I (Cu cladding plate was placed upon the Al base plate, welding speed of 1400 mm/min) and II (Al base plate was placed upon the Cu cladding plate, welding speed of 1300 mm/min), respectively. The results showed that microstructures under two modes were similar, but there existed some obvious differences in fracture behavior of the joints and damage behavior of Cu cladding plate. For two butt joints, the (Al2Cu + α-Al) eutectic structure was distributed in continuous networks around the α-Al grains in the weld zone. In addition, the interface between Cu cladding plate and weld zone was composed of Al2Cu intermetallic compound and (Al2Cu + α-Al) eutectic structure. The destruction width of Cu cladding plate was greater in Mode I than that in Mode II. Furthermore, the average loads of the EBW joints were 4.8 kN and 4.5 kN in Modes I and II, respectively.

Rock fracture initiation and propagation by mechanical and hydraulic impact

Abstract Rock fracture due to high-pressure fluid jetting and insert indentation was experimentally investigated. In the frame of this article, indentation designates the process of pressing a tungsten carbide insert (TCI) for roller cone bits displacement-controlled into the rock surface. Under atmospheric conditions, several crystalline and one sedimentary rock type were tested. Depending on the size, type and bonding of the mineral grains, distinct differences in fracture behaviour were observed. The influence of elevated ambient pressure regimes on the evolving cracks and fractures in the tested granite revealed that microcracking ceases with increasing ambient pressure, while the specific energy is also significantly affected. Under similar conditions, indentation experiments were performed on undamaged samples and on samples containing jetted kerfs. A distinct variation in the measured maximum force and removed volume indicated different underlying rock fracture mechanisms. By applying methods for crack visualization and three-dimensional rendering, it turned out that the dimensions of the kerf significantly affected the mechanism of fracture initiation and propagation as well as the extent of the associated fracture pattern. Furthermore, the common practice of characterizing the rock excavation process only via measurable quantities such as the specific energy, without considering the created fracture pattern, yields misleading results.

The influence of residual stresses on fracture behavior of Pipe Ring Notched Bend specimen (PRNB)

This paper focuses on analysis of the influence of residual stresses on the fracture behavior of PRNB specimens, with the aim to estimate the differences in fracture behavior of PRNB specimens with and without the residual stresses. The residual stresses are estimated experimentally for four different pipe types by using three methods: incremental hole drilling method, X-ray diffraction, and Splitting method. In order to estimate the influence of residual stresses on the fracture behavior of the newly proposed PRNB specimens, there are load-displacement curves obtained experimentally and numerically and then compared. In order to obtain more detailed analysis of residual stresses’ influence, J – integral versus thickness of the specimen curves are presented for the specimen with and without residual stresses and for various loading pin displacements. Obtained results show a very good correlation between experimentally and numerically obtained load-displacement curves. The J – integral analysis leads to the conclusion that residual stresses do not have significant influence on fracture behavior of PRNB specimens made from investigated pipes.

Mechanical characterization of friction drilled internal threads in AZ91 profiles

In this study, the influence of friction drilling tool pre-heating on mechanical properties of chipless manufactured internal threads in thin-walled AZ91 magnesium casting alloy profiles is investigated. In this context, the influence of manufacturing processes on microstructure and the resulting fracture behavior during mechanical loading are in focus for the determination of failure mechanisms. Two batches were investigated, whereas specimens were manufactured without and with pre-heating the friction drilling tool before manufacturing. The mechanical properties were determined in tensile and fatigue tests in tensile loading range. The mechanical results were correlated with the profile qualities in form of computed tomography analyses and hardness mappings. Light and electron microscopic investigations of fractured surfaces were performed to analyze the fracture behavior in cyclic tests. Process-related and stress-related work hardening effects were determined at the edge area of the threads. Differences in fracture behavior under quasi-static and cyclic loads were determined. Turns of internal threads connected to the threaded rod were sheared off in tensile tests without visible cracks on the exterior surface of the flat profile specimens, whereas cyclically tested specimens provided fractured surfaces for fractographic failure analyses. Crack initiation at thread root and two stages of crack propagation until complete failure due to overload fracture were investigated. Pre-heating of the friction drilling tool during manufacturing of the threads had no influence on quasi-static and fatigue properties, respectively.

Effect of plastic anisotropy on microscale ductile fracture and microformability of stainless steel foil

The thin foil undergoing extensive plastic deformation during the rolling process usually exhibits a significant anisotropy. In microforming, due to the downscaling effect, the influence of plastic anisotropy on ductile fracture behavior and material formability is size-dependent. To clarify the interaction of size effect and plastic anisotropy on the formability of thin foil, the tensile tests and forming limit experiments of SUS304 foils with different thicknesses and grain sizes were performed. It is found that the deformation behavior changes from polycrystalline to the single crystal as the reduction of λ, the number of grains across the thickness direction. This transformation significantly affects the fracture behavior including limit strain, ultimate stress, and fracture mechanism. Furthermore, the difference in fracture behavior among diverse deformation orientations is aggravated with the reduction of foil thickness and the increase of grain size. Due to the interaction of size effect and plastic anisotropy, the microformability of metal foils is shifted down with the reduction of λ, and the severe scatter in the forming limit curve (FLC) of 20 μm thick foil is observed. Besides, the deviation of strain path from linearity is increased with decreasing λ, which is caused by the coupled effect of the intensified anisotropy and the declined compatible deformation capability. Unlike the common V-type FLC, the microscale FLCs of the specimens with the thicknesses of 200 and 100 μm are characterized by a continued drop of the limit strain along the strain paths from uniaxial tension, planar strain to biaxial stretching. The phenomenon is ascribed to the multiple effects of the deviation of strain path, material anisotropy and the transformation of fracture pattern at the microscale level.

Atomistic origins of the differences in anisotropic fracture behaviour of LiTaO3 and LiNbO3 single crystals

The search of new designs for communication technologies is driven by the required high data transfer rates. Piezoelectric single crystals such as LiTaO3 and LiNbO3 have qualified as materials for efficient and precise frequency filters also considered for the new 5G standards. To ensure optimal high frequency functionality, single crystal wafers are grown and cut in particular directions. However, due to the high anisotropic physical properties of these brittle materials, the structural integrity and reliability of devices are affected by the crystal orientation with respect to the occurring thermo-mechanical stresses. In this work, the anisotropic fracture response of LiTaO3 and LiNbO3 single crystals is investigated through combination of (i) toughness measurements in notched micro-cantilevers along weak cleavage planes and (ii) atomistic modelling of cleavage fracture energies using density functional theory. It is demonstrated that differences in fracture behaviour between LiTaO3 and LiNbO3 can be explained by the stronger chemical bonding in LiTaO3 as compared to LiNbO3 within the loaded crystallographic planes. The knowledge on the alignment of tough as well as weak planes (i.e. cleavage planes) can be used to tailor the design of single crystal based functional components, aiming to exhibit enhanced mechanical reliability without compromising the functionality.

Ductility of steel fibre/epoxy composites in function of their microstructure

Recently we showed that continuous stainless steel fibres deliver structural composites with high stiffness and high failure strain at the same time. In the current research we further investigate these new materials focusing on the effect of the microstructure on their tensile facture behaviour. The steel fibre composites are produced from quasi-unidirectional woven (Q-UD) and purely unidirectional (P-UD) fibre architectures in combination with epoxy resin. The main difference in their microstructures is in the fibre distribution. In the Q-UD composite, steel fibres are tightly packed in bundles with 85% local fibre volume fraction and these bundles are separated by resin rich areas. In P-UD composite, steel fibres are homogeneously distributed through the composite with an average volume fraction of 50%. With the highly packed fibres, Q-UD composites achieve a higher strain to failure. In-situ characterization of damage development using acoustic emission registration and crack observations on the specimen surface were used to propose a hypothesis to explain the difference in fracture behaviour, caused by the different microstructures.

Wire bond degradation under thermo- and pure mechanical loading

This paper presents a fundamental study on degradation of heavy Al bond wires typically used in high power modules. Customized samples are designed to only consist of Al bond wires on standard Si diodes. These samples are subjected to pure mechanical and passive thermal cycling to investigate the bond degradation behavior on a simple system as well as compare these two test methods. Although an appreciable difference in fracture behavior is observed between these two methods, both provide correlation between the number of cycles and degree of degradation, especially in the case of the passive thermal test. To enable investigation of degradation rate a large number of bond interfaces is analyzed and they are found to follow conventional accepted fracture laws like Paris-Erdogan. With additional work this could enable the possibility of obtaining empirical parameters to be used in actual physics based lifetime laws.

Equivalent maturity for ambient temperature effect on fracture parameters of site-casting dam concrete

This paper presents the influence of ambient temperature on the fracture properties of site-casting (or cast in-situ in other words) dam concrete. By virtue of the mixing tower system in the site of a super-high arch dam, the wedge splitting specimens with the maximum aggregate size of 150mm were poured in summer and in winter seasons, respectively. The variations of fracture parameters obtained by experiment with different age were revealed, and the results show that the dam concretes poured in summer and in winter seasons show much difference in fracture characteristics at the age of 28days, however, with the increase of age, the difference in fracture behavior decreased, and was almost zero at the age of 180days. On top of that, by introducing the concept of equivalent maturity, the relationships between fracture parameters of site-casting dam concrete and equivalent maturity were obtained.

Mechanical properties and fracture behaviour of multilayer alumina composites

Adopting a ceramic/polymer multilayer structure design to simulate the structure of nacre is usually believed to be an effective way to increase the toughness of ceramic composites at the expense of the material's bending strength. However, in this study, we found that both the bending strength and the toughness could be improved simultaneously when using a certain Al2O3/Kevlar multilayer composite design compared to pure alumina samples with the same dimensions. The fracture behaviour of the Al2O3/Kevlar multilayer composite was studied to find a reason for this improvement. The results showed that the complex and asymmetrical stresses occurring in the Kevlar-reinforced layers were the main reason for the differences in fracture behaviour. We expect our results to open up new ways for the design of future high performance ceramic composites.

Meso-scopic deformation in brittle granular materials

Compaction is the process of removing void-space from a porous material. In brittle particulate systems, the majority of densification is caused by particle fracture. This preliminary study aimed to investigate the differences in fracture behaviour between quasi-statically and shock loaded glass-microsphere beds. Macro-scale quasi-static (20 μm s−1) and dynamic compaction curves were measured that show subtle qualitative differences in stress-density space. Samples were recovered from a quasi-static and dynamic experiment at a similar order of stress. Differences in fracture behaviour were observed that may explain the differences in crush curves. Results suggest that the primary total-fracture process occurs relatively instantaneously at low stresses in the quasi-static regime. The sphere fracture process is slow relative to the stress-wave therefore causing a different fracture pattern in the shock regime.

Fracture Behaviour of Thin Sheet Stainless Steel

The differences in fracture behavior between the compact tension C(T) and the middle tensile M(T) specimens make structure integrity assessment uncertain. Two different types of specimens C(T) and M(T) specimens made from stainless steel have been used for fracture toughness testing at the room temperature by the principles of the ASTM 1820-05 standard procedure. Stable crack initiation and crack propagation occurred for the C(T) specimens at lower values of crack driving force than for the M(T) specimens. Crack tip opening displacement-CTOD has been directly measured on the surface of specimens by using a stereo-optical grading method. The critical crack tip opening displacement at crack initiation CTODi has been measured as a plastic Stretch Zone Width (SZW) during a post test fractographic inspection. Comparison between the CTOD-R curves of both types of specimens shows some difference between the C(T) and the M(T) specimens, but a more significant difference appeared in the crack driving force, as consequence of different constraint (triaxiality) of the C(T) versus the M(T) specimens. Therefore, the result obtained by test on laboratory C(T) specimens cannot be directly used as fracture toughness material properties in a structure integrity assessment, except as a conservative lower bound estimate.

Oxidation behavior of 3D Hi-Nicalon/SiC composite exposed in wet and simulated air environments

Oxidation behavior of a 3D Hi-Nicalon/SiC composite was investigated in wet and simulated air with a flow rate of 3.5cms−1 at temperatures above 1000°C. Results showed that oxidation was occurred mainly on the specimens surface and rate-controlled by oxygen diffusion for both oxidation environments. Silica formed in wet air was viscous and thicker. After oxidation, strength deterioration of the composite was associated mainly with the degradation of Hi-Nicalon fiber. Meanwhile, the difference in fracture behavior could be attributed to the change of fiber/matrix interface bonding caused by high temperature oxidation.

WHY COARSE TOASTED RUSK ROLLS ARE CRISPIER THAN FINE ONES

ABSTRACTToasted rusk rolls with a coarser structure are sensed crispier than those with a fine structure, at the same water activity (aw). The present paper shows that this difference in crispness perception is related to differences in fracture behavior and accompanying acoustic emission. Both sensory and instrumentally determined crispness decreased gradually with increasing aw in roughly the same manner for both coarse and fine products. Nevertheless, the coarse rusk roll was perceived as being crispier than the fine one. Typically, in the coarse structure the measured “Number of Force Drops” of a relatively large size and the “Number of Sound Events per cross section area” of relatively large intensity were more numerous than in the fine one. Our data show that relatively large force drops and sound events are related to the more intense crispness perception and stronger sound sensation for the coarse structure toasted rusk roll. We propose the “Total Sound Energy per cross section area” and the “Mean Sound Event Intensity” to be primarily responsible for the higher crispness perception of the coarse rusk over the whole aw range.PRACTICAL APPLICATIONSCoarse structured toasted rusk rolls are sensed crispier than fine structured ones (same aw). Difference in crispness is related to fracture behavior and accompanying acoustic emission. Crispness decreased gradually with increasing aw similarly for coarse and fine products. Sound energy cq. Intensity seems primarily responsible for the higher crispness perception.

Influence of water sorption of the underlying abutment on fracture resistance of zirconia copings

Objective. To investigate the influences of abutment water sorption and various aging parameters on the fracture resistance of zirconia copings. Methods. Using a master die, identical replicas were made from three resin materials. The first was a melamine resin with very high water sorption (n = 48), the second an experimental resin core build-up composite with moderate water sorption (n = 40) and the third a commercially available core build-up composite with low water sorption (n = 40). On the abutment replicas, zirconia copings (n = 128) were made using a computer-aided design–computer-aided manufacturing system. The copings were luted onto the abutments using zinc oxide phosphate cement. In the melamine group, a subgroup of samples (n = 8) was cemented with a composite cement as controls. The forty specimens in every abutment material group were randomly divided into one of five subgroups, as follows: (i) not aged; (ii) mechanically (dry) loaded only (50 N; 1.2 × 106 cycles); (iii) stored for 10 days in water; (iv) thermally cycled (TC; 6000 × 5/55°C); and (v) TC and mechanically loaded (TCML; 50 N, 1.2 × 106; 6000 × 5/55°C). After aging, all copings were loaded to fracture. Results. A statistically significant difference was found between the three abutment–die groups if the samples were aged by TCML. The zirconia copings cemented on abutments with high water sorption fractured during TCML, and the subgroup with moderate water sorption had significantly lower fracture resistance. A change of luting material had no impact on this behavior. Conclusion. Only the simultaneous combination of all chosen aging factors (TCML) was able to detect a difference in fracture behavior of a zirconia coping luted on abutments with varying water sorption.

Anisotropic Fracture Behaviour of Progressively Drawn Pearlitic Steel

This paper analyses the differences in fracture behaviour of two cold drawn pearlitic steels with different degree of strain hardening: a slightly drawn bar and a heavily drawn wire. The load-displacement curve F-u was analysed in the two cases, with special emphasis in the characteristic points of the plot: the load defining the end of linear behaviour (Fe), the final fracture load (Fmax) and, in the heavily drawn steel, the pop-in load (FY). Results demonstrate that slightly drawn steels exhibit isotropic fracture behaviour with crack propagation along its own plane (mode I propagation). On the other hand, heavily drawn steels exhibit a markedly anisotropic fracture behaviour with crack deflection (mixed mode propagation), and vertical fracture embryos suddenly appear associated with the pop-in instant. This special fracture mode is a consequence of the markedly oriented microstructure of the heavily drawn steel.

Fracture mechanisms in biopolymer films using coupling of mechanical analysis and high speed visualization technique

The aim of this study was to provide a detailed description of the fracture mechanisms in three different biopolymer thin materials: gelatin, hydroxypropyl cellulose (HPC) and cassava starch films. That was achieved by using a combination of fracture mechanics methodology and in situ visualization with high speed imaging technique. By analyzing the unstable and stable crack propagation that occurs in these materials, the brittle to ductile transition induced by water plasticization was assessed and discussed. Each biopolymer material exhibited a distinctive brittle and ductile fracture mode, which were described in this work. In order to evaluate the occurrence of permanent deformation in the proximity of the crack propagation front, FTIR maps with polarized light microscopy were employed, which enabled the visualization of structural homogeneity and cohesivity. The differences in fracture behaviour were interpreted taking into account the structural organization within these materials during stretching. The coupling of mechanical data and advanced visualization techniques was a particularly effective method to describe and interpret the complex fracture mechanisms in biopolymer materials which is strongly affected by their interaction with water molecules.