Microscopic Fracture Surface Research Articles

Microscopic characteristics of peri- and postmortem fracture surfaces

This study investigated if microscopic surface features captured with a scanning electron microscope (SEM) effectively discriminate fracture timing. We hypothesized that microscopic fracture characteristics, including delamination, osteon pullout, and microcracks, may vary as bone elasticity decreases, elucidating perimortem and postmortem events more reliably than macroscopic analyses. Thirty-seven unembalmed, defleshed human femoral shafts from males (n=18) and females (n=2) aged 33–81 years were fractured at experimentally simulated postmortem intervals (PMIs) ranging from 1 to 60 warm weather days (250–40,600 ADH). A gravity convection oven was used to approximate tissue decomposition at 37 C and 27 C, and the resulting heat-time unit (accumulated degree hours, or ADH) was used to examine fractures in elastic/wet versus brittle/dry bone. The bones were fractured with a drop test frame using a three-point bending setup, sensors were used to calculate fracture energy, and high-speed photography documented fracture events. The following data were collected to relate fracture appearance to the biomechanical properties of bone: PMI (postmortem interval) length in ADH, temperature, humidity, collagen percentage, water loss, bone mineral density, cortical bone thickness, fracture energy, age, sex, cause of death, and microscopic fracture feature scores. SEM micrographs were collected from the primary tension zones of each fracture surface, and three microscopic fracture characteristics were scored from a region of interest in the center of the tension zone: percentage of delaminated osteons, percent osteon pullout, and number of microcracks. Multiple linear regression showed that microscopic fracture surface features are strong predictors of ADH (adjusted R-squared=0.67 for the 0 – 40,000 ADH samples; adjusted R-squared=0.92 for the 0–16,000 ADH samples). Osteon pullout is the single best predictor of ADH. Additionally, water loss is the primary driver of bone elasticity changes in low ADH samples, while collagen fibers appear to remain intact until later in the postmortem interval (approximately 40,000 ADH in this study). The results of this study indicate microscopic fracture surface analysis detects the biomechanical effects of decreased elasticity more reliably and with greater sensitivity than macroscopic analysis.

Enhanced Mechanical Properties of Ti/Mg Laminated Composites Using a Differential Temperature Rolling Process under a Protective Atmosphere.

Addressing the issue of low bonding strength in Ti/Mg laminated composites due to interfacial oxidation, this study employs a differential temperature rolling method using longitudinal induction heating to fabricate Ti/Mg composite plates. The entire process is conducted under an argon gas protective atmosphere, which prevents interfacial oxidation while achieving uniform deformation. The effects of reduction on the mechanical properties and microstructure of the composite plates are thoroughly investigated. Results indicate that as the reduction increases, the bonding strength gradually increases, mainly attributed to the increased mechanical interlocking area and a broader element diffusion layer. This corresponds to a transition from a brittle to a ductile fracture at the microscopic tensile-shear fracture surface. When the reduction reaches 47.5%, the Ti/Mg interfacial strength reaches 63 MPa, which is approximately a 20% improvement compared to the bonded strength with previous oxidation at the interface. Notably, at a low reduction of 17.5%, the bonding strength is significantly enhanced by about one time. Additionally, it was found that a strong bonded interface at a high reduction is beneficial in hindering the propagation of interfacial cracks during tensile testing, enhancing the ability of the Ti/Mg composite plates to resist interfacial delamination.

Mechanical response and damage evolution of CF/PEEK‐reinforced Al metal‐composite hybrid structure at high strain rate

AbstractA CF/PEEK‐reinforced Al metal‐composite hybrid structure (CF/PEEK/Al) and CF/PEEK bar were subjected to high strain rate compression tests with a split Hopkinson pressure bar (SHPB). With the increase in strain rates, both CF/PEEK and CF/PEEK/Al showed the strain rate strengthening effect, and the microscopic fracture surface of CF/PEEK changes from rough to smooth. In the plastic rheological stage of stress–strain curve, CF/PEEK shows a plateau effect, while CF/PEEK/Al shows the strain weakening effect. When the strain rate exceeds 3800 s−1, the stress–strain curve exhibits a secondary wave peak. The peak values of the secondary peak in CF/PEEK/Al are larger than CF/PEEK. In terms of peak stress, yield stress, and specific energy absorption, CF/PEEK/Al outperforms CF/PEEK. Compared with other structures, the CF/PEEK/Al metal‐composite hybrid structure has certain advantages in terms of yield strength and yield strain. The finite element method (FEM) and DIC system were applied to observe the dynamic damage process in the compression impact test. It was found that in CF/PEEK/A1, the Al thin‐walled tube was damaged and cracked at the contact interface, and the inner CF/PEEK began to be damaged at the contact center with the input bar. And at 335 μs after contact began, the shear band ran through to the side of the samples.Highlights A CF/PEEK‐reinforced Al metal‐composite hybrid structure (CF/PEEK/Al) was proposed, which is superior to CF/PEEK. When the strain rate exceeds 3800 s−1, the stress–strain curves of CF/PEEK and CF/PEEK/Al exhibit a secondary wave peak. The peak values of the secondary peak in CF/PEEK/Al are larger than CF/PEEK. With the increase in strain rates, both CF/PEEK and CF/PEEK/Al showed the strain rate strengthening effect, and the microscopic fracture surface of CF/PEEK changes from rough to smooth. In CF/PEEK/A1, the Al thin‐walled tube was damaged and cracked at the contact interface, and the inner CF/PEEK began to be damaged at the contact center with the input bar. And at 335 μs after contact began, the shear band ran through to the side of the samples.

A novel voxel-particle energy approach to predict 3D microscopic fracture surface of porous geomaterials and fracture permeability modeling

Characterizing and predicting 3D microscopic fractures are rarely reported due to the challengeable difficulty to determine its spatial trajectories. In this study, a double threshold segmentation algorithm is proposed to obtain 3D microstructure models of porous geomaterials by X-ray computing tomography (CT) images. A novel voxel-particle energy (VPE) approach is proposed to predict 3D fracture characteristics, and fracture permeability modeling is conducted. Results show that excellent agreements are found between the experimental and numerical results, showing the accuracy of proposed VPE method. As dynamic viscosity increases, microscopic fracture permeability increases. Microscopic fracture permeability decreases with increasing input pressure, and it increases with increasing voxel scale. The proposed VPE method provides an effective tool to predict 3D cracks and its spatial trajectories, which is helpful for the deep resource and underground space development.

Shear Transformation Zone and Its Correlation with Fracture Characteristics for Fe-Based Amorphous Ribbons in Different Structural States

Fe-based amorphous alloys often exhibit severe brittleness induced by annealing treatment, which increases the difficulties in handling and application in the industry. In this work, the shear transformation zone and its correlation with fracture characteristics for FeSiB amorphous alloy ribbons in different structural states were investigated. The results show that the bending strain decreases sharply with the annealing temperature increase, accompanied by decreased shear band density and the induced plastic deformation zone. Furthermore, the microscopic fracture surface features transform from a micron-scale dimple pattern to nano-scale dimples and periodic corrugations. According to nano-indentation results, the strain rate sensitivity and shear transformation zone volume change significantly upon annealing treatment, which is responsible for the deterioration of bending ductility and the transition of microscopic fracture surface features.

Influence of single or multi-factor coupling of temperature, humidity and load on the aging failure of adhesively bonded CFRP / aluminum alloy composite joints for automobile applications

Applying materials such as CFRP (Carbon Fiber Reinforced Plastics) and aluminum alloys to automobiles is an effective way to accomplish lightweight design, thereby bringing energy conservation and emission reduction. Bonding structure is the key joining technology for dissimilar materials, but it will degrade during the automotive service by environmental temperature, humidity and load. Scholars mainly carry out accelerated aging tests by increasing the temperature or humidity, but it is hard to compare the degree of influence of temperature, humidity and load owing to the aging environments and adhesive materials are various and different. In this study, to comprehensively compare the effects of various aging environments coupled with load that may be experienced during the service life of an automobile on the aging failure of composite bonded joints, seven typical aging environments were selected, namely room temperature (RT), high temperature (HT), low temperature (LT), high/low temperature cycle (TC), immersed in water at room temperature (IRT), high temperature and high humidity (HTHH), and hygrothermal cycles (HC). Aging environments were coupled with static load to form single or multi-factor aging conditions of temperature, humidity and load. Accelerated aging tests were conducted to analyze the failure strength, failure mode and SEM microscopic fracture surface of bonded joints made of aluminum alloy and CFRP. The degree of influence of single or multi-factor coupling of temperature, humidity and load on the aging failure of bonded joints was analyzed by visual analysis and ANOVA (Analysis of Variance). The results show that single temperature, humidity or load had limited effects on the degradation of the failure strength of butt joints, while coupling effects of multiple factors are more obvious. When the aging environment was coupled with the load, the aging environment played a major role, and the load accelerated the aging rate without changing the decreasing trend caused by the aging environment. Both the adhesive and CFRP underwent different degrees of aging, the effect of load on the degree of aging of the adhesive was significantly greater than the CFRP, but the effect of aging environment on the degree of aging of adhesive and CFRP was related to the aging time. Normally, aging environment, aging time and load had significant effects on the failure strength, and the degree of influence decreased successively. As for the selection of the accelerated aging environments for the car, HC and TC environments should be preferred and the influence of load should be taken into consideration in accelerated aging tests.

Fracture characteristics and damage prediction in flat steel beams under contact explosions

Research to predict and mitigate local damage of steel elements subjected to contact and close-in explosion remains a topic of interest. In this study, the effects of charge mass, specimen geometry, and explosive charge position on the damage and fracture of flat, mild-steel specimens under contact charges are investigated. From macroscopic and microscopic observations of the fracture surfaces, rupture properties of the flat steel specimen under contact explosion are examined. Based on these observations, the rupture process of flat steel specimens under contact explosion is considered as follows: 1) the opposite surface of a blast -loaded specimen was damaged by spalling due to tensile stress wave, and 2) the rest of the cross-section of a specimen was stretched and ruptured by blast pressure into two pieces. On the microscopic fracture surface observed by a scanning electron microscope demonstrates cleavage, which is caused by brittle spalling and rupture. Their microscopic fracture surfaces are different based on charge positions, specimen thicknesses, and observation points. Furthermore, the effects of charge mass and specimen cross-section on damage to steel specimens under contact explosions are evaluated, and empirical equations to predict thickness limits are developed for spalling and rupture of flat, mild-steel specimens.

Study on Microscopic Fracture Surface Analysis based on Deep Learning (1) : Fracture Surface Classification by Convolutional Neural Network

Study on Microscopic Fracture Surface Analysis based on Deep Learning (1) : Fracture Surface Classification by Convolutional Neural Network

Research on Macro-Micro Failure Mechanism of Interbedded Sandstone in the Deep of Lannigou Gold Mine

Abstract The surrounding rock of the Lannigou gold mine roadway is mainly sandstone, most of which contains thin interlayers of argillaceous and carbonaceous minerals. To explore the deformation and failure mechanism of interbedded sandstones under static load caused by surrounding rock stress, the macroscopic failure modes, macroscopic physical properties, and microscopic fracture surface characteristics of sandstones were studied from different inclination angles. Triaxial compression tests and scanning electron microscopy tests were conducted. The results show that the peak strength of the specimen changes in a “spoon”-shaped pattern with increasing inclination. With increasing confining pressure, the peak strength, peak strain, residual strength, and elastic modulus of specimens increase, which damages interlayers. The macroscopic failure mode is mainly affected by the interlayer without confining pressure, and both tensile-shear failure (0° and 90°) and composite failure (30° and 60°) occur. Under higher confining pressure, the effect of the interlayer is smaller, and the specimen shows shear-tensile failure (0°, 30°, 60°, and 90°). Furthermore, the propagation mode of microcracks and the microscopic failure mode of mineral crystals differ for different macroscopic failure modes. In this study, the microscopic mechanism of macroscopic failure of interbedded sandstone was identified, which is significant for practical engineering guidance.

Fatigue crack propagation behaviors in Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with STA and BASCA heat treatments

Fatigue crack propagation behaviors of a novel high strength Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with equiaxed and lamellar microstructures were systematically investigated. Fatigue crack propagation tests at both increasing and decreasing stress intensity factor range values were carried out with a frequency of 15 HZ and stress radio R of 0.1 by using compact tension samples. The sample with lamellar microstructure displayed the faster fatigue crack propagation rates in near-threshold regime compared to the sample with equiaxed microstructure. The fatigue threshold (ΔKth) was 2.7 MPa·m1/2 for lamellar microstructure, and 3.2 MPa·m1/2 for equiaxed microstructure. The steady state crack growth rate in lamellar microstructure was slightly slower than that in equiaxed microstructure in the Paris regime, while much faster in the rapid growth regime. The interactions between crack path and microstructures as well as the microscopic fracture surface morphology in both microstructures were detected and analyzed by scanning electron microscope (SEM), revealing the different crack propagation behaviors in three regimes. The results showed that the crack propagation behaviors were a result of two contributing factors: interface obstacles and crack front profile. The increased fatigue crack propagation resistance of equiaxed microstructure in near-threshold regime can be attributed to the interface obstacles which dominated over the rougher crack front profile. While the higher fatigue crack growth resistance for lamellar microstructure in Paris regime was attributed to the more positive effect of rougher crack front profile. In the rapid growth regime, the crack in lamellar microstructure was found to be extended along the grain boundary α, which was regarded as a low energy crack path and led to the faster crack propagation rate.

Modified mixed-mode caustics interpretation to study a running crack subjected to obliquely incident blast stress waves

Crack-wave interaction is common in blast, producing mixed-mode cracks when stress waves are oblique to cracks. How to measure mixed-mode crack-tip stress under blast stress waves is difficult. To this end, optical caustics method was employed to study a running crack in a PMMA plate subjected to obliquely incident blast stress waves, by which stress problem was transformed into optical problem. Incident P and S waves, as well as reflected waves, were visualized as fringe patterns, and mixed-mode crack-tip stress was represented by a caustics pattern (shadow spot). However, mixed-mode caustics patterns under P waves were not consistent with the classical interpretation, hence a modified mixed-mode caustics interpretation was proposed and then verified by P-wave compression and tension loading cases. In modified interpretation, an iteration procedure was proposed to obtain modified stress intensity factors, i.e. KI and KII, and crack-tip positions which were more precise than those by the classical interpretation. P-wave compression phase decreased KI and crack velocity sharply but produced the largest KII, while the following P-wave tension phase had an opposite effect. S waves and reflected waves produced higher KI and crack velocity with lower KII. KII and crack velocity histories explained curved crack path and microscopic fracture surface. Finally, the reason for the success of the modified interpretation was discussed, and it is indicated that transient stress superposition and delayed redistribution in the crack tip are the mechanism underlying the modified interpretation.

Microstructure and Properties of 304 Stainless Steel Specimen Manufactured by Cold Metal Transfer + Pulse Composite Arc Additive

304 stainless steel test block was fabricated by continuous melting wire with CMT and pulse mixed mode, and the path of additive manufacturing is layered slice S-shaped. The relationship between microstructure and properties of the specimen was investigated by microscope, SEM, EBSD, XRD, tensile, impact and electrochemical experiments. The results show that molding between weld and weld is very good, and the microstructure is mainly Austenite, Ferrite and a little of σ, and there are three kinds of Ferrite morphology: cellular, wormlike and lath. σ phase precipitates easily in regions with high ferrite content and is distributed at the boundary between austenite and ferrite. The specimen has good low temperature toughness. The microscopic fracture surface is mainly dimple, and the precipitates in the fracture surface are mainly fine carbide particles. The tensile strength of the additive manufacturing 304 specimen is higher than the forged specimen, and the type of fracture is ductile fracture. The electrochemical analysis of 304 stainless steel specimens and forgings shows that CMT and pulse arc additive manufacturing specimen has excellent corrosion resistance and its corrosion current is slightly lower than the forging.

The Stress Concentration Mechanism of Pores Affecting the Tensile Properties in Vacuum Die Casting Metals.

The absolute pressure strongly affects the porosity and mechanical properties of castings produced by vacuum high-pressure die casting (V-HPDC) technology. The pore size, quantity and distribution of AlSi9Cu3 samples under three absolute pressures were evaluated by X-ray tomography and optical and electron microscopy. The paper presents an elaboration the stress concentration mechanism of pores affecting the tensile properties. According to a mathematical analysis of a sample under uniaxial stress, the greater the radius of the pore, the higher the stress value is at the pore perimeter. When the absolute pressure drops from 1013 mbar to 100 mbar, the porosity decreases from 6.8% to 2.8%, and the pore number and mean size decreases. In tensile tests, the pore sizes of the fracture surface decrease with decreasing absolute pressure, and the pore distribution becomes uniform. The tensile properties and extensibility of the sample are improved, and the microscopic fracture surface of the sample changes from cleavage fracture to quasi-cleavage fracture. The number, size and distribution of pores in die casting collectively affect the properties of the sample. Large-size or complex pores or pores with concentrated distributions produce large stress concentrations, decreasing the strength of the metal.

Low temperature impact toughness of high strength structural steel

Impact toughness properties of four kinds of high strength steel (HSS) were investigated with the nominal yield strength 460, 690, 800 and 960 MPa, respectively. The specialty of impact toughness among base metal, heat affected zone (HAZ) and weld metal were also compared and discussed. The correlations between the impact toughness of HSS and their nominal yield strength, plate thicknesses, testing temperature, welding methods were further studied. Combined with the scanning electron microscope (SEM) fracture surface observation, the microstructural features and corresponding fracture mechanisms have been analyzed. The results show that the impact toughness for HSS base metal deteriorated with the increasing nominal yield strength. The impact toughness of weld metal for HSS is much lower than that of the corresponding base metal. The impact performance of HAZ is discretized due to the welding thermal cycling on the HAZ where significant changes in grain structure and properties occur. The microcosmic fracture surface of HSS is examined to explain the reduction in impact performance under low temperature.

Crack propagation mechanisms of AISI 4340 steels with different strength and toughness

In this paper, the crack propagation characteristics in AISI 4340 steel with different strength and toughness have been investigated in quasi-situ via 3D X-ray tomography during fracture toughness testing. Three kinds of crack propagation characteristics were observed by the 3D image analysis, which were determined by the competition between cleavage fracture and voids growth/coalescence. It is found that the main crack would connect the micro-voids near the crack tip by cleavage fracture if the local stress achieves the cleavage fracture strength, resulting in rapid crack propagation. However, the growth and coalescence of micro-voids ahead of the crack tip would be linked with the main crack by strain-controlled, leading to the ductile fracture. The above two mechanisms would exist simultaneously in materials, which might be explained by the competition balance between strain and stress. Studying on transition of crack propagation mechanisms in materials would further understand the essence of fracture toughness. It will provide new insight into the fracture behavior of engineering materials and some new materials such as nanostructured materials and metallic glasses. Additionally, based on the energy principle, we propose a quantitative relationship between the microscope fracture surface morphology and fracture toughness of materials.

An experimental investigation on manifold failure and material deterioration

Carbon monoxide reformer furnaces were used in the petrochemical industry to produce kinds of gases such as CO, OXO and H2 from a mixture of natural gas and steam at high temperature. However, the degradation of material microstructure was frequently encountered in the outlet manifolds due to the service environment undertaking elevated temperature and complex stress state, leading to their premature failure. In this paper, some properties were compared between premature failure manifold and virgin pipe material, such as chemical compositions, mechanical properties at room temperature and elevated temperatures, etc. The deterioration trend of manifold material 20Cr33NiNb alloy was obtained. Microscopic fracture surfaces were observed. The microstructure of failure manifold was analyzed by SEM and EDS. The results indicated that elevated temperature and high stress level were considered to be the failure reasons. Some meaningful suggestions were proposed.

Explosive compact-coating of tungsten–copper alloy to a copper surface

This study proposed a new method for coating tungsten–copper alloy to copper surface. First, the tungsten–copper alloy powder was pre-compacted to the copper surface. Then, the powder in the hydrogen atmosphere was sintered, and the pre-compacted powder was compacted by explosive compact-coating. Finally, diffusion sintering was conducted to improve the density of the coating layer. The theoretical density of the coating reached 99.3%. Microstructure characteristics indicated that tungsten and copper powders were well mixed. Tungsten particles were larger than copper particles. Scanning electron microscope (SEM) fracture surface analysis was different from the traditional fracture of metals. Coating and substrate joint surfaces, which were analyzed by SEM, indicated that the tungsten–copper alloy was sintered on the copper surface. The hardness of the coating layer was 197.6–245.2 HV, and the hardness of the substrate was approximately 55 HV.

High temperature embrittled duplex stainless steels: influence of the chemical composition on the fatigue crack propagation

Abstract: Austenitic-ferritic (duplex) stainless steels are successfully used in chemical, nuclear, oil and gas industries, due to their good mechanical properties and excellent generalized and localized corrosion resistance in many environments and operating conditions (for example, chloride induced stress corrosion). The aim of this work is the analysis of the influence of the chemical composition and of the high temperature embrittlement processes in three different duplex stainless steels. Scanning electron microscope (SEM) fracture surface analysis was performed to investigate the fatigue crack propagation micromechanisms.

Effect of heat treatment on the microstructures and mechanical properties of Al-5.5Zn-2.5Mg alloy

The Al-5.5 Zn-2.5 Mg (wt%) ternary alloy was prepared using a vacuum melting furnace and a casting furnace. Microstructural and mechanical properties of the alloy were investigated as-cast and under heat-treated conditions. To investigate the effect of heat treatment, numerous designed Al-5.5 Zn-2.5 Mg samples were homogenized under different conditions and then aged under different regimes. The effects of heat treatment on the microstructures were examined by OM, SEM, and TEM, and mechanical properties of the Al-Zn-Mg alloy were studied. A good combination of high microhardness and reasonable tensile strength were obtained by successive and suitable heat treatments. After aging for 24h at 150°C, the peak microhardnes and tensile strength values were achieved as 157MPa and 188.8MPa, respectively. The microscopic fracture surfaces of the aged samples under different homogenization and aging conditions were observed using scanning electron microscopy. Fractographic analysis of the tensile fracture surfaces shows that the type of fracture changed significantly from ductile to more ductile depending on the aging regime.

Fracture Surface Morphology of Delamination Failure of Polymer Fiber Composites Under Different Failure Modes

The delamination of fiber reinforced polymer composites is one of the most common failures encountered in industrial applications. The most unique macroscopic and microscopic fracture surface features of the delaminations under different failure modes are of interests not only for practical failure analysis investigations but also it helps to reveal the physics behind the delamination phenomenon. In this work, fracture surface morphology of the delaminated carbon fiber polymer composites under mode I, mode II, and mixed-mode I/II loading conditions is investigated mainly with scanning electron microscopy. The unique fractographic features are identified and discussed. The results on ductile and brittle matrix composites have shown their own features, and most important of all the alignment angle of fibrils in the resin-rich ductile matrix could be correlated with the delamination mode.