Failure Mode Changes Research Articles

Study on Damage Characteristics and Failure Modes of Gypsum Rock under Dynamic Impact Load.

The objective of this work was to investigate the damage characteristics and failure modes of gypsum rock under dynamic impact loading. Split Hopkinson pressure bar (SHPB) tests were performed under different strain rates. The strain rate effects on the dynamic peak strength, dynamic elastic modulus, energy density, and crushing size of gypsum rock were analyzed. A numerical model of the SHPB was established using the finite element software, ANSYS 19.0, and its reliability was verified by comparing it to laboratory test results. The results showed that the dynamic peak strength and energy consumption density of gypsum rock increased exponentially with strain rate, and the crushing size decreased exponentially with the strain rate, both findings exhibited an obvious correlation. The dynamic elastic modulus was larger than the static elastic modulus, but did not show a significant correlation. Gypsum rock fracture can be divided into crack compaction, crack initiation, crack propagation, and breaking stages, and is dominated by splitting failure. With increasing strain rate, the interaction between cracks is noticeable, and the failure mode changes from splitting to crushing failure. These results provide theoretical support for improvements of the refinement process in gypsum mines.

Mechanical responses of CFRP/PVC foam sandwich plate impacted by hailstone

Mechanical responses of sandwich plate with carbon fiber reinforced polymer (CFRP) laminate as face-sheets and polyvinyl chloride (PVC) foam as core, which is promising in the lightweight design of high-speed maglev trains, was studied for the case of hailstone impact. Nine 500 × 500 mm sandwich plate specimens of three face-sheet thicknesses 1, 2 and 3 mm and a constant core thickness 30 mm were fabricated and tested under the impact of ice spheres of 50 mm in diameter propelled by the first-stage light gas gun. Three impact speeds, i.e. 30, 60 and 90 m/s, of the ice spheres and three incident angles, i.e. 0°, 30° and 45°, were investigated. For all cases, no macro failures were observed, but invisible damages, i.e. delamination in the upper face-sheets and indentation of the foam core, were found by high resolution ultrasonic testing. The experimental results were then used to calibrate the corresponding numerical model, based on which a thorough parametric analysis was performed to show the full dynamical responses of CFRP/PVC foam sandwich panels under hailstone impact. It is found that the foam cushion could effectively protect the rear face-sheet and increasing the ratio of span to thickness of the panel could reduce the damage degree due to the change of failure mode from local to global.

Crack evolution characteristics of cemented-gangue–fly-ash backfill with different proportions of fly ash and cement

To explore the strength boundaries and corresponding failure characteristics of the backfill body, the basic bearing capacity and failure performance of cemented-gangue–fly-ash backfill (CGFB) with different ratios of fly ash to cement in the binder (F/C) were tested using a uniaxial compressive stress experiment, acoustic emission, and digital image correlation. The results indicate that CGFB has an upper-limit strength of 15.35 MPa, and the strength varies significantly from F/C 0.5 to 2. With the increase in F/C, the brittleness of CGFB decreases. The change in failure mode from tensile accounts for 69.7%, and the change in failure mode to shear accounts for 70.66%. When F/C is low, the crack initiation stress is approximately 0.4–0.5 the peak strength, the crack development is concentrated in the middle of the entire bearing stage, and the scale of the crack is large and develops along the main crack. With the increase in F/C, the initiation stress of the crack decreases to 0.25–0.4 the peak strength, and the development of cracks begins to concentrate in the early and late stages during the loading stage. The scale of the crack is smaller than when F/C is low, and the failure is scattered throughout the specimen.

Shear behavior of reinforced concrete beams subjected to accelerated non-uniform corrosion

In this paper, a modified accelerated corrosion method based on impressing current was adopted to generate non-uniform corrosion of stirrups in reinforced concrete (RC) beams. The shear performance of corroded RC beams was experimentally investigated, aiming to study the influence of stirrup corrosion induced by two different accelerated corrosion methods on the degradation of shear capacity of RC beams. Different corrosion degree of stirrups, corrosion of longitudinal steel bars, as well as shear span ratio were investigated. Results indicates that, the stirrups subjected to the modified accelerated corrosion method demonstrate a considerably higher circumferential and longitudinal non-uniformity, while the classic accelerated corrosion method induces a rather uniform corrosion. The non-uniformity of corrosion aggravates the propagation of both corrosion-induced and load-induced cracks, leading to the change of shear failure mode. The uniform corrosion induced by the classic accelerated corrosion method underestimates the degradation of shear capacity of RC beam, especially in the cases of corrosion degree no less than 15%. The corrosion of longitudinal steel bars mainly correlates to the decrease in the initial stiffness of RC beam, while little influence on the stiffness is observed with corroded stirrups.

Critical effect of heat input on joint quality in resistance element welding of Al and steel

Resistance element welding (REW) has recently attracted increasing interest, especially in the joining of high-strength steels and aluminum alloys. However, the intensity of heat input is a critical factor for the joint quality due to the possibility of excessive melting of the aluminum sheet around the element shank, which often degrades the joint's performance. This study evaluates the influence of heat input on the joint formation and mechanical performance of REW joints between ultra-high strength steel and aluminum alloy. In addition to the experimental investigations, a finite element method (FEM) was used to analyze the temperature distribution, nugget formation, and aluminum sheet melting characteristics as a function of heat input. The results show that the nugget dominates the joint's strength if the heat input is comparatively low. When heat is increased, however, the aluminum sheet plays a significant role in the joint's strength. Excessive melting of aluminum sheets reduces the load-bearing area of the joint. The competition between the weld nugget and aluminum sheet determines the joint's strength. The failure mode changes from interfacial to aluminum sheet failure as the heat input increases. The combined effect of aluminum sheet thickness and heat input on the joint strength was also evaluated. The results indicate that determining the appropriate heat input level is crucial to achieving an excellent performance of the REW joints.

Study on dynamic loading characteristics and energy evolution of sandstone with double cracks

To obtain the mechanical characteristics and energy evolution law of sandstone with double cracks under dynamic load, the test is carried out by using the SHPB test system, and the failure process of the sample is recorded by the high-speed camera system. The stress-strain curves under dynamic load and the failure characteristics of the sample are obtained, and the effects of crack angle and impact pressure on the mechanical characteristics of sandstone with double cracks are analyzed. The results show that the compressive strength of the specimen decreases initially and increases afterwards with the increase of the crack angle under dynamic load, and the compressive strength of the specimen increases gradually with the increase of the impact pressure. At the pressure of 0.03 MPa, the peak strain increases initially, followed by decreases with the increase of crack angle, and shows the opposite law with the increase of impact pressure. Under dynamic loading, the crack tip first starts to crack outwards along the axial direction, and the penetration of the rock bridge lags. The macroscopic cracks of 0° and 90° sandstone are mostly tensile cracks, while those of 30°, 45°, and 60° are mostly shear cracks. With the increase of crack angle, the failure mode changes from tensile failure to shear failure and then to tensile failure. The fractal dimension increases with the increase of impact pressure, and increases and then decreases with the increase of crack angle, and the fractal dimension increases with the increase of energy dissipation per unit volume. In addition, in the process of rock burst prevention and control, the size of the dynamic load and crack angle should be evaluated in advance. When the dynamic load in this area is large, the surrounding rock pressure relief should be carried out in advance to increase the dissipation energy. When there are 60° and 45° crack angles in the rock mass, the stress propagation path should be changed by structural regulation based on grouting filling micro-fracture modification.

Design and assembly of chain-oriented-crystalline multilayered composite with largely improved mechanical strength

Design and assembly of organic/inorganic multilayered composite consisting of alternatively stacking of stiff inorganic platelets and soft organic polymers have great potential for developing next-generation lightweight high-strength materials. Beyond previous designs about the improvement of inorganic nanoplatelet alignment and inter-platelet bonding, a new design concept of control over the physical aggregation state of organic polymer chains within the multilayered structure for mechanical reinforcement is proposed. Based on a typical assembly system of nanoclay platelets and poly (vinyl alcohol) (PVA), a facile three-step process of gelation, uniaxial pre-stretching and evaporation is developed to construct chain-oriented-crystalline multilayered composite, in which the aggregation state of PVA including chain orientation (from 0.020 to 0.897), crystallinity (from 42.8 to 45.9%) and crystallite orientation (from 0.007 to 0.937%) is regulated successfully. These structural changes bring a 4.1-fold increase in the tensile strength of the multilayered composite because of substantial improvement in the strength of PVA layer that enlarges stress transfer from PVA to nanoclay platelets and leads to a failure mode change from the pull-out of nanoclay1 platelets to their fracture. These findings open an avenue for fabricating lightweight high-strength nanocomposites based on inorganic nanoplatelets and crystalline polymers.

Dynamic mechanical response and fracture mechanism of rock-like materials with concealed structural planes of different filling thicknesses

Structural planes are the primary variables determining the stability of rock masses. Numerous non-penetrating structural planes exist in nature and are concealed inside the slopes or underground rock formations. The concealed structural planes (CSP) pose a possible hazard to the construction safety of rock mass engineering, particularly for the rock mass subjected to dynamic load. Using rock-like materials, intact specimens and specimens with CSP of varying filling thicknesses are constructed in order to examine the dynamic mechanical response and fracture mechanism of rock masses with CSP. Single impact experiments and cyclic impact experiments are carried out at various speeds using the Split Hopkinson Pressure Bar experimental system (SHPB). The results demonstrate that the specimens' peak stress is negatively correlated with the filling thickness, and that as the reflection coefficient increases and the transmission coefficient decreases, the filling thickness increases. Also, the energy dissipation rate of the specimens is positively correlated with the increase in thickness of filling at impact velocities of 3.7 m/s and 4.5 m/s, with the rate showing an increasing trend followed by a decreasing trend at impact velocities of 5.5 m/s and 6.5 m/s. The main surface and internal cracks of specimens can be categorized into seven types, which are combined into three failure modes. As the filling thickness increases, the failure mode changes from transverse tensile failure to combined tensile and shear failure to bi-directional tensile failure under combined transverse and axial tensile action. The crack propagation behavior and the fracture mechanism from the interior to the exterior of the CSP specimens are investigated using the impact test results, finite element model, and mechanical theory analysis, which can be used as a reference to reveal the destabilization process of the rock mass with CSP.

Reinforcement Corrosion in RC Hollow Piers: Destructive and Non-Destructive Tests

In this work, cyclic-load tests on reduced-scale corroded reinforced-concrete hollow cross-section bridge piers have been experimentally performed and compared to the results of similar non-corroded piers. Piers were aged by using an imposed electric current and sodium chloride water solution before performing a mechanical cyclic-load test. The corrosion process has been detected with Non-Destructive Evaluation techniques by means of SonReb method (to check concrete degradation) and by measuring corrosion potential (to check steel degradation). The crack pattern was recorded by dedicated cameras, and an LVDT system was set up to monitor the cyclic-load test. Experimental results focused on degradation monitoring and mechanical performance under cyclic loads. During the cyclic-load mechanical test, the first cracks on the piers surface occurred diagonally, inclined at about 45°. This is the consequence of the failure mode change from ductile failure, as expected for slender designed piers, to brittle shear failure. The flexural failure occurred in the case of non-corroded piers. Presented tests can provide a useful contribution of experimental data to analyse the behaviour of corroded reinforced concrete hollow bridge piers, scarcely tested. In particular, the cyclic response can be a useful reference for the proposition/validation of nonlinear capacity models for the evaluation of the seismic capacity of corroded bridge piers.

Experimental and numerical study on truss string structure considering hysteretic performance asymmetry

Aiming at the characteristics of the asymmetric hysteretic performance of truss string structure, and considering the plastic development of truss string structure. The experimental model of the truss string structure was established, and the experimental study was carried out under cyclic loading with vertical amplitude of 1:2 and 1:4. The difference in hysteretic behavior of truss string structure under different loading systems and the weak position of the structure under cyclic loading is obtained, and the results were compared with the previous study of cyclic loading test. The results show that with the increase of cyclic downward loading ratio, the downward energy consumption of the truss string structure increases gradually, and the failure mode changes from the buckling of the lower chords of mid-span to the buckling of the upper chords of mid-span, which eventually leads to the overall instability of the structure. The weak positions of the truss string structure are distributed in the upper and lower chord members of the truss string structure in mid-span.

The temperature-dependent shear strength of ice-filled joints in rock mass considering the effect of joint roughness, opening and shear rates

Abstract. Global warming causes many rockfall activities in alpine mountains, especially when ice-filled joints in the rock mass become thawed. The warming and thawing of frozen soils and intact rocks have been widely studied in the past several decades; however, the variation of shear strengths of ice-filled joints is not fully understood. In this study, a series of compression–shear experiments were conducted to investigate the shear strength of ice-filled rock joints by considering the effects of joint roughness, temperature, opening, shear rates and normal stress. The joint roughness can improve the shear strength of ice-filled joints. However, the contribution of joint roughness is controlled by some noticeable bulges instead of the joint roughness coefficient (JRC) index. The peak shear strength of the ice-filled joint linearly increases with the increasing aggregation of rupture ice area before these noticeable bulges. As the joint opening increases, the effect of joint roughness decreases, and the shear strength of ice-filled joints tends to be equal to the shear strength of pure ice. In addition, the shear strength quickly reduces with increasing temperature from −15 to −0.5 ∘C. The shear failure mode changes from shear cracking of joint ice to the shear debonding of the ice–rock interface above −1 ∘C. Increasing shear rate decreases the shear strength of ice-filled joints because the joint ice displays the brittle failure phenomenon at a high shear rate. The peak shear strength of the ice-filled joint linearly increases with the increase in normal stress. Moreover, it is also proved that the Mohr–Coulomb criterion can be used to characterize the shear strength of ice-filled joints under different normal stresses. This research can provide a better understanding of the warming degradation mechanism of ice-filled joints by considering the abovementioned important influencing factors.

Fracture Fractal and Energy Transfer Characteristics of Deep-Mine Marble under an Impact Load

With changes in mining depth, the dynamic mechanical characteristics of the same type of rock also change, so that the blasting excavation process must be constantly adjusted and optimized to meet the requirements of safe and efficient mining. To study the energy evolution law and fractal characteristics of deep marble during the destruction process, dynamic impact tests under different strain rates were carried out on a deep marble sample using the three-axis dynamic combination Hopkinson pressure bar system. The experimental results show that the larger the incident energy is, the more energy is transmitted. The proportion of absorbed energy does not increase with the increase in the incident energy. Only 30%–38% of the incident energy is absorbed by the impacted rock. With an increase in the strain rate, the energy consumption density gradually increases, the fragmentation degree is intensified, the fractal dimension gradually increases, and the failure mode changes from compression–shear failure to crushing failure.

Flexural Performance and End Debonding Prediction of NSM Carbon FRP-Strengthened Reinforced Concrete Beams under Different Service Temperatures

This paper aims to evaluate the influence of relatively high service temperatures (near or beyond the glass transition temperature (Tg) of epoxy adhesive) on the flexural performance and end debonding phenomenon in near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP)-strengthened, reinforced concrete (RC) beams. To this end, an experimental program consisting of 24 beams (divided into four groups) was performed, where different parameters was combined (i.e., service temperature, steel reinforcement ratio, CFRP ratio, and concrete compressive strength). In addition, the effect of the testing temperature on the end debonding phenomenon was investigated with an analytical procedure according to fib Bulletin 90, and the predictions were compared to experimental results. Taking specimens tested at 20 °C as a reference, no considerable change was observed in the ultimate load of the specimens tested below 60 °C (being in the range of epoxy Tg), and all specimens failed by FRP rupture. On the other hand, the increase in testing temperature up to 70 and 85 °C was followed by a decrease in the capacity of the strengthened beams and a change in failure mode, moving from FRP rupture to end debonding and concrete crushing. The analytical procedure successfully predicted the occurrence of premature end debonding failure and demonstrated that the effect of temperature on the mechanical properties of materials can be a key factor when predicting the premature end debonding in a NSM joint.

Numerical investigation on the debonding of cement anchorage in clay rocks based on a hybrid FEM/DEM model

In this study, the failure process of the cement mortar-clay rock (C–C) binary was analyzed using a composite hybrid FEM/DEM model. First, an improved pressure-related cohesive element was used to simulate the mechanical properties of the binary interface. Then, a numerical model of the cement anchorage body in clay rocks was constructed based on the new interfacial cohesive element and FDEM model of clay rock. Finally, the debonding and slip failure process of the anchorage was analyzed by the numerical model and a corresponding physical model test. The results show that: except for the debonding of the interface, the fracture of weaker side material (clay rock) is also a potential failure mode of the C–C binary. The clay rocks have a larger internal friction angle but smaller cohesion, so the failure mode changes from the shear failure of clay rock to the debonding of the interface when the normal pressure is up to 200 kPa. The failure of the anchor may also cause by the generation of the shear failure zone in soft rock instead of the interface debonding. Therefore, the shear strength of the rock should also be considered in the calculation of anchorage resistance.

Processing and properties of powder metallurgy Ti-Cu-Nb alloys

Binary Ti-Cu and Ti-Nb alloys have been extensively studied for biomedical applications, but the potential of ternary Ti-Cu-Nb alloys, where the combined benefits of adding Cu and Nb on the performance of biomedical Ti alloys is harvested, has not been fully exploited. Therefore, this study aims to investigate the processing and properties of Ti-Cu-Nb alloys obtained via powder metallurgy. Independently of their actual chemistry, the studied Ti-Cu-Nb alloys are characterised by a lamellar microstructure, which is refined by the incremental addition of Cu+Nb, and a eutectoid substructure is formed for sufficiently high contents of Cu. The balance between slightly higher residual porosity and the microstructural changes, including the stabilisation of a greater amount of β phase, a lamellar structure with refined features, and the precipitation of the Ti2Cu intermetallic phase, leads to a monotonic linear increment of the resistance to plastic deformation. However, the ductility initially increases for small additions of Cu+Nb and subsequently decreases due to the change in failure mode induced by the progressively higher impact of the residual pores. The studied Ti-Cu-Nb alloys share the same strengthening mechanism and have better strength/ductility pairs with respect to binary Ti-Cu and Ti-Nb alloys and better elongation compared to cast Ti-Cu-Nb alloys.

Numerical analysis of dynamic response: Fatigue behaviour analysis of Al alloy-CFRP riveted single-shear lap joints

Fatigue tests were carried out on Al alloy-carbon fibre reinforced plastic (CFRP) riveted single-shear lap joints. Through the signal processing of dynamic response, the relationship between the evolution of displacement, stiffness, dissipated energy and different failure behaviours is studied, and the service state evolution and fatigue failure process are revealed. Results show that higher load amplitudes lead to changes in the interface slip state, which increases the dissipated power. The weak point of the joint is changed from the aluminium alloy lap plate to the CFRP lap plate, which is finally reflected in the change of failure mode.

Carbon fibre surface modification facilitated by silver-catalysed radical decarboxylation.

A silver catalysed radical decarboxylation process was used to graft a copolymer (4 : 1; methylacrylate/acrylic acid) onto short carbon fibres. Surface grafting was confirmed by XPS, SEM and TGA, suggesting that the polymer accounted for 10% of the modified materials mass. Incorporation of these surface enhanced carbon fibres into an epoxy resin gave composites demonstrating an increase in ductility and a clear change in failure mode from adhesive, at the fibre-matrix interface, to cohesive, within the matrix polymer itself.

Effect of prior cyclic damage on rock failure exposed to triaxial multistage unloading confining pressure and cyclic loads

AbstractA series of triaxial alternative unloading and damage‐controlled cyclic loading tests were conducted on deeply buried granodiorite. The testing results reveal that the prior cyclic loading damage influences rock volumetric deformation, stiffness degradation, damage evolution, and failure modes. Rock volumetric deformation and hysteresis ratio increase with increasing prior cyclic damage, and the subsequent damage propagation is the most severe for a sample subjected to high cyclic damage. A larger hysteresis ratio indicates large plastic deformation and energy loss. In addition, a damage evolution model was proposed based on the irreversible radial strain, and it well describes the damage propagation at the respective cyclic loading stage (CLS) and unloading confining pressure stage (UCPS). Moreover, as prior damage increases, the dominant failure mode changes from shear failure to mixed shear–tensile failure and then to tensile failure. It is suggested that rock failure is cyclic damage dependent; damage fully develops under high cyclic damage.

Synthetical effect of material inhomogeneity and welding defects on fatigue behavior of 2205 duplex stainless steel cruciform welded Joints: Experiments and Life-prediction model

This paper performed experimental and analytical investigations on the synthetical effect of material inhomogeneity and welding defects on the fatigue behavior of 2205 duplex stainless steel cruciform welded joints. A series of experiments were performed, including microstructure characterization by the electron back-scattered diffraction, fatigue mechanical tests assisted by digital image correlation technique and fracture morphology observations by scanning electron microscope. Results show that fatigue life and failure location of cruciform welded joints depended heavily on their geometric configurations for lower cyclic amplitudes. However, with the elevation of loading, material inhomogeneity played a greater roles, leading to the change of fatigue failure mode with both geometric configuration and cyclic stress amplitude. To quantize the effects of welding defects and material inhomogeneity, a modified structure strain method was proposed by considering the heterogeneous plastic properties, to accurately predict both fatigue lifetime and failure location of cruciform welded joints at both low and high cycles fatigue. Lastly, a damage tolerance-based fatigue design method was discussed to guide the highly-reliable manufacture and damage detection of duplex stainless steel cruciform welded joints in the engineering.

Elemental Gains and Losses during Hydrothermal Alteration in Awak Mas Gold Deposit, Sulawesi Island, Indonesia: Constraints from Balanced Mineral Reactions

Hydrothermal gold mineralization is commonly associated with metasomatic processes resulting from interaction of hostrock with infiltrating hot aqueous fluids. Understanding of the alteration mechanism requires quantification of element changes in altered rock, relative to the unaltered or least-altered rock, representing the protolith. Balanced mineral reactions are used to gain quantitative insight into the alteration process associated with gold mineralization at the Awak Mas deposit. Three representative samples were carefully selected from the least-altered pyllite and the two alteration zones bordering the mineralization. Mineral mode, textural features, and mineral compositions were studied by microscopy and electron microprobe analyzer (EMPA). Quantitative modal analysis was performed with a Quanta 650 F QEMSCAN® system. The hydrothermal alteration sequence around the mineralization starts with the proximal albite–ankerite–pyrite alteration zone via the distal albite–chlorite alteration zone to the least-altered phyllite wall-rock. Balanced mineral reaction calculations were performed to evaluate elemental gains and losses. Most noticeable is the addition of Si, Na and Ca to each alteration zone. This alteration is represented by the almost complete replacement of muscovite by albite. The addition of Fe and S was highest in the albite–ankerite–pyrite alteration zone. Alteration of the least altered phyllite to the albite–chlorite zone involved a mass increase of 14.5% and a neglectable volume increase of 0.6%. The mass and a volume increase from the least altered phyllite to the albite–ankerite–pyrite zone was 40.5% and 0.47%, respectively. The very low volume change during alteration is also corroborated by the textural preservation indicating isovolumetric metasomatic reactions. The replacement of muscovite by albite may have had an important effect on the change of the rock failure mode from ductile to brittle, with consequences for the focusing of fluid flow.