Critical Failure Load Research Articles

Combinatorial Materials Design Approach to Investigate Adhesion Layer Chemistry for Optimal Interfacial Adhesion Strength

A combinatorial material adhesion study was used to optimize the composition of an adhesion promoting layer for a nanocrystalline diamond (NCD) coating on silicon. Three different adhesion promoting metals, namely W, Cr, and Ta, were selected to fabricate arrays of co-sputtered binary alloy films, with patches of seven different, distinct alloy compositions for each combination, and single element reference films on a single Si wafer (three wafers in total; W–Cr, Cr–Ta, Ta–W). Scratch testing was used to determine the critical failure load and practical work of adhesion for the NCD coatings as a function of adhesion layer chemistry. All tested samples eventually exhibit delamination of the NCD coating, with buckles radiating perpendicularly away from the scratch track. Application of any of the presented adhesion layers yields an increase of the critical failure load for delamination as compared to NCD on Si. While the influence of adhesion layers on the maximum buckle length is less pronounced, shorter buckles are obtained with pure W and Cr–Ta alloy layers. As a general rule, the addition of an adhesion layer showed a 75% improvement in the measured adhesion energies of the NCD films compared to the NCD coating without an adhesion layer, with specific alloys and compositions showing up to 125% increase in calculated practical work of adhesion.

Laboratory Study on the Stability of Large-Size Graded Crushed Stone under Cyclic Rotating Axial Compression.

In this paper, the stability of large-size graded crushed stone used for road base or cushioning under repeated load is investigated. Using an in-house developed device, large-size crushed stone mix was compacted and molded by the vibration and rotary compaction method. Cyclic rotating axial compression was applied, and the shakedown theory was used to study the cumulative deformation of the large-size crushed stone specimens. The effects of gradation parameters on the cumulative strain and stability behavior were analyzed, and the critical stability and failure loads were determined according to the shakedown theory. The test results indicate that there are three obvious instability behavior stages of large-size graded crushed stone under cyclic rotating axial compression: elastic stability, plastic creep, and incremental plastic failure. Large-size graded crushed stone has a higher critical stability load stiffness than conventional-size graded crushed stone. The critical shakedown load of the specimen is mainly affected by the skeleton structure performance, and the critical failure load by the properties of the crushed stone material. Increasing the content and compactness of large-size crushed stone in the specimen can improve the stiffness and stability performance, and to achieve improvements, the content of large-size crushed stone should be controlled between 22% and 26%. The critical shakedown load increases with the increase in the California bearing ratio (CBR) value, while, on the other hand, the CBR value has little relationship with the critical failure load.

Instability Mechanism of Pillar Burst in Asymmetric Mining Based on Cusp Catastrophe Model

Pillars are often reserved asymmetrically in the mining process. The roof deflection curve under non-equal span conditions of adjacent stopes is derived by considering the roof-pillar system as a rock beam-pillar model. The pillar instability condition under asymmetric mining is determined based on instability theory and cusp catastrophe theory. Pillar burst represents the equilibrium stability of the roof-pillar system. The pillar failure may be in a violent manner or a gentle manner, depending on the post-peak stiffness ratio of the roof-pillar system. By calculating the factor of safety (FOS) and roof-pillar stiffness ratio K, the pillar stability with different stope spans can be evaluated. The theoretical results are validated by comparison with a case study and numerical simulation. When the stope spans are not equal, the pillar is affected by small-eccentric compression. Four pillar failure patterns under eccentric compression are proposed and explained. The main factors affecting pillar burst appear to include the geometric parameters and mechanical properties of the roof-pillar system. It is difficult to change the mechanical properties, but the stiffness ratio K can be increased by improving the geometric parameters, so as to minimize the burst tendency. Once K < 1 and the critical compression failure load is reached, the pillar on the larger stope span side fails first, and then, the whole pillar loses its stability. Considering the external work during the pillar unstable failure, the rockburst energy index is optimized.

Failure mechanism of geopolymer composite lightweight sandwich panel under flexural and edgewise compressive loads

This paper proposes a new lightweight sandwich panel manufactured with Fiber-Reinforced Geopolymer Composite (FRGC) skin layers and PUR foam core for prefabricated buildings. The failure mechanisms of the developed sandwich panel are investigated under flexural bending and edgewise compressive loading to validate the performance of the developed panel. Digital Image Correlation (DIC) technique is used to obtain the deformation and strain during the tests. Two typical failure mechanisms, i.e. shear failure of the core and tensile failure of the skin layers of the one-way sandwich panel under flexural loading are studied by conducting quasi-static three-point bending tests. The influence of the core thickness-to-span ratio of the sandwich panel on shear failure of the core is studied. The test results show that the failure mode transforms from the shear failure of the PUR foam core to delamination between the FRGC skin and PUR foam core, when the thickness-to-span ratio increases from 0.1 to 0.3. The influence of skin thickness-to-span ratio on the skin tensile failure is also studied. Analytical models for the shear failure of the core and the tensile failure of the skin layer are proposed. Furthermore, the failure mechanism of the developed sandwich panel with different thickness-to-length ratios under edgewise compressive loading is studied. It is found that the failure mode changes from the material failure of the FRGC skin to the global buckling when the thickness-to-length ratio reduces from 0.90 to 0.30. Analytical models for predicting the critical failure load under edgewise compressive loading are also proposed.

Ananalytical and Experimental Study of T - Shaped Composite Stiffened Panels: Effect of 90° Plies in Stringers on Curing and Buckling Performance

The effect on curing deformation and strength of 90° plies in stringers for composite T-shaped stiffened panel were presented by numerical analysis and experimental measurement. In this work, the finite element model (FEM) for curing deformation based on cure kinetics formulation was utilized. The model for buckling and failure based on Tsai-Wu failure criteria and incremental/ Newton -Raphson mixed iteration equation was established. The curing, buckling and post-buckling analysis were performed on composite T-shaped stiffened panels to obtain the deformation, the critical load and mode of failure, with different 90° plies-ratios in stringers. Furthermore, the experimental investigation has been carried out for specimens with different plies-ratios of 90° plies in stringers. The good agreement between the numerical results obtained by finite element method and the experimental ones demonstrated that this kind of numerical analysis could estimate appropriately the behavior of the structure. In addition, it was found that the 90° plies in stringers play an important role in reducing the curing deformation, but it has little effect on the axial compression loading capacity for composite T-shaped stiffened panel, which is useful for application in civil aviation aircraft.

Postbuckling response of functionally graded hybrid plates with cutouts under in-plane shear load

The present study deals with the investigation of buckling and postbuckling responses of functionally graded hybrid square plates which are symmetric about its mid plane. The cutouts of various sizes and shapes located at the center of the plate are considered. The plate is subjected to positive and negative in-plane shear loads. The boundary conditions used are simply supported on all the four edges of the plate. Three stacking sequences are considered to investigate the optimum stacking sequence at which the critical buckling and first failure loads are highest. Functionally graded hybrid plates (FH) subjected to negative in-plane shear load is more effective compared to plates subjected to positive in-plane shear load. Diamond shaped cutout with small sized perforation has the highest critical buckling and first ply failure loads amongst the FH plates with cutouts. It is observed that the direction of applied in-plane shear load, fiber stacking sequence; cutout shape and size substantially influence the strength and failure characteristics of functionally graded hybrid composite plates.

Fracture loads prediction of the modified 3D-printed ABS specimens under mixed-mode I/II loading

The fracture features of the 3D-printed samples are assessed employing Semi-Circular Bending (SCB) specimens. The critical failure loads are experimentally measured and have been predicted using both Maximum Tangential Stress (MTS) and Generalized Maximum Tangential Stress (GMTS) criteria. The 3D-printing process is improved with several approaches to decline the voids volume and escalate the weld lines strength of the 3D-printed specimens to boost their mechanical specifications. The Acrylonitrile Butadiene Styrene (ABS) SCB samples with numerous pre-crack angles ranged from pure mode I to pure mode II are produced utilizing a modified 3D-printing process. The results of the tensile tests confirmed that the properties of the specimens created by the modified 3D-printing process are independent of the raster of printing orientation. The predicted failure loads with the GMTS criterion had a better correlation with the experiment compared to the MTS criterion owing to the impact of the T-stress term in the 3D-printed SCB samples. Furthermore, the cross laminar fracture augmented fracture toughness and declined crack kinking behavior of the 3D- printed ABS specimens.

Tribological analysis of electrostatically developed multi (YSZ, TiN, SiC) nanocomposite coated cutting tool material

The rapid wear of materials and inferior surface quality due to friction is a critical issue to address the high strength materials. A promising route to overcome the drawbacks by virtue of nanocomposite coating materials and adopt new coating techniques that can enhance the performance than the existing coating technology. An approach providing high performance with better coating property needs to be developed. This approach has the potential to produce high performance nanocomposite coating deposited by a technique designed, termed as electrostatic spray coating (ESC). Further, mechanical and tribological characteristics of nanocomposite (YSZ, TiN, SiC) coating produced by ESC technique were studied. The microstructure, EDAX, mapping and wear mechanism morphology were investigated by means of FE-SEM (field emission scanning electron microscopy). Micro hardness of coating at room and elevated temperatures were measured as part of mechanical tests to figure out the interrelation between properties of nanocomposite coating and mechanism of wear behavior. The dry sliding wear test and scratch resistant tests were performed under ambient temperature to evaluate the tribological properties. To evaluate the scratch resistance of nanocomposite coating scratch tester was utilizing, the critical load for failure of coating on tungsten carbide was determined. With the objective to analyze the wear behavior of nanocomposite coating and compare the results with uncoated surface, wear test was performed utilizing pin on disc configuration. In an effort to perform the wear test, the pin specimen being uncoated and nanocomposite coated tungsten carbide along with the counterpart disc specimen material as titanium alloy (Ti–6Al–4V). The response of wear test was analyzed with reference to wear mechanisms, friction coefficient and temperature of uncoated and nanocomposite coating. This approach was effective in enhancing the wear resistance properties of nanocomposite coating. Interestingly, these tests revealed that the nanocomposite coating deposited by electrostatic spray coating technique exhibited remarkable characteristics beneficial to diverse industrial applications.

Bending behavior of lightweight C/SiC pyramidal lattice core sandwich panels

Due to the excellent mechanical and chemical properties at ultra-high temperature, ceramic matrix composite structures have potential applications for thermal protection systems of hypersonic vehicles. This paper presented a PIP method to fabricate C/SiC pyramidal lattice core sandwich panels (LCSPs). Their bending behaviors with different core angles were experimentally studied firstly. Then the critical failure loads of different failure modes, including facesheet crushing or wrinkling, core member crushing, core shear failure and interlayer delamination, were theoretically established to construct the failure mechanism maps. The influence of geometric parameters, such as the length and the core angle of the core strut, and the thickness of the facesheet, on the failure mode was also analyzed. In addition, further failure process under bending load was investigated by finite element analysis (FEA) method. Elastic-damage constitutive model and cohesive element were applied to modeling the behavior of C/SiC composite and interlayer delamination between the sheet of the core and the facesheet. Theoretical and FEA results agreed well with experimental data. This work is believed to be helpful to understand the bending mechanism C/SiC pyramidal LCSPs.

Mechanics and fracture of structured pillar interfaces

Material architecture and geometry provide an opportunity to alter the fracture response of materials without changing the composition or bonding. Here, concepts for using geometry to enhance fracture resistance are established through experiments and analysis of the fracture of elastic-brittle, polymer specimens with pillar-structures along the fracture plane. Specifically, we investigate the fracture response of double cantilever beam specimens with an array of pillars between the upper and lower beams. In the absence of pillars, unstable crack growth and rapid catastrophic failure occur in the double cantilever specimens tested in displacement control. Introducing pillars at the interface by removing material via laser cutting yields a discontinuous interface and leads to a more gradual fracture process and an increase in the work of fracture. The pillar geometry affects the failure load and, notably, increasing the slenderness of the pillars leads to higher critical failure loads due to greater load sharing. The effect of pillar geometry on fracture is established through experiments and analysis, including analytical modeling and finite element simulations. An analytical model that includes the macro-scale response of the beam and the micro-scale response of the pillars is presented and describes the key effects of pillar geometry on fracture response.

Bilayer graphene film synthesized by hot filament chemical vapor deposition as a nanoscale solid lubricant

In the present study, we synthesized homogeneous bilayer graphene (BLG) films on the Cu substrate by adopting the hot filament chemical vapor deposition (HFCVD) method. The precise control of the layer number of fabricated graphene films could be obtained by adjusting the flow rate of hydrogen or reactive pressure. Either increasing the flow rate of hydrogen under a fixed reactive pressure or decreasing the reactive pressure with a fixed CH4:H2 ratio resulted in an increase in the layer number of the synthesized graphene film. We further examined the nanotribological properties of BLG by Atomic Force Microscopy (AFM). The BLG film resulted in an 83%–92.5% reduction in friction when applied on the surface of either copper or SiO2/Si substrates. The obtained coefficients of friction (COFs) were as low as 0.025–0.085. Besides, the critical failure load of the BLG film on the SiO2/Si substrate was measured as 1266 ± 28 nN in a progressive-force scratching test, in which we used a diamond tip with a radius of 10 nm. The corresponding actual contact pressure was estimated at as high as 14.15 GPa, according to Derjaguin-Muller-Toporov model, which was enhanced by over one order of magnitude compared with that obtained for the bare SiO2/Si substrates (1.2 GPa). The results achieved in the present study demonstrate that homogeneous BLG films with promising lubricity and superior wear resistance could be synthesized in the HFCVD process, which, we believe, provides an effective and controllable approach for promoting the application of BLG films as a nanoscale solid lubricant in a variety of Nano/Micro Electromechanical Systems devices.

A comparative study of the mechanical and tribological properties of PECVD single layer and compositionally graded TiSiCN coatings

In this study, TiSiCN coatings, with and without a Ti/TiN/TiCN interlayer, are deposited on tempered and plasma-nitrided H13 hot-working tool steel substrate by pulsed direct current plasma enhanced chemical vapor deposition. The self-lubricant super hard TiSiCN coatings have a nanocomposite structure consisting of an amorphous SiCN matrix embedding TiCN nanocrystals. In the presence of the graded interlayer, coating adhesion improves significantly, as revealed by a 40% increase of the critical load for adhesion failure in scratch testing of the coated samples. The formation of shorter radial cracks around the indentation zone during Rockwell C indentation test reveals that the coating with the graded interlayer has a higher toughness compared to the single layer coating. The wear rate of the graded coating is found to be about three times lower than that of the single layer coating, which can be attributed to the higher hardness, better adhesion to the substrate and higher toughness of the graded coating.

Energy-based ductile failure predictions in cracked friction-stir welded joints

This research aims to study the fracture behaviour in dissimilar aluminum alloys adjoined by friction stir welding (FSW). In this way, experimental data dealing with this topic was taken from the recent literature. In those experimental results, two metal sheets made of Al 7075-T6 and Al 6061-T6 were adjoined together by FSW in the form of well-known specimen, namely the cracked semi-circular bend (CSCB) and then they are tested under mixed mode I/II loading condition. Due to the fact that substantial plastic behavior exist in the welded material and consequently significant plastic deformations were observed around the crack tip, failure prediction of the mentioned specimens needs failure prediction models in the basis of the elastic-plastic fracture mechanics which can be realized as sophisticated operations inquiring long time. In this way, the Equivalent Material Concept (EMC) is utilized in this research and then coupled with two eligible energy-based criteria, namely the averaged strain energy density (ASED) and J-integral criteria. Thus, the critical failure load of the welded samples is predicted. Comparison between the empirical data and theoretical predictions from energy-based evaluations showed that this model has enough capability in estimating the critical failure load of the CSCB samples.

Acoustic Emission-Based Study to Characterize the Crack Initiation Point of Wood Fiber/HDPE Composites.

The crack initiation point can be regarded as a sign of composite failure and plays a vital role in the evaluation of fracture toughness. Wood-plastic composites (WPCs) are viscoelastic materials and the evaluation of fracture mechanism and toughness has a great significance in their applications. Therefore, we used the acoustic emission (AE) technique to measure the crack initiation point of the WPCs and evaluate their fracture toughness. The results show that the novel AE-based methods were more effective than the conventional standard methods for characterization of the crack initiation point. Using the relationship of cumulative AE events with time and load, the critical failure load was quickly determined, and then the critical stress intensity factor and fracture toughness were calculated. The fracture toughness of the WPCs increased with an increase in the wood fiber content.

Modelling of cracking of the ceramic foam specimen with a central notch under the tensile load

In this contribution, open cell ceramic foam structures composed of regular/irregular shape cells and containing a macroscopic central notch/crack were investigated in terms of their cracking upon the tensile loading and their strength was predicted using the FE simulations. The developed model of the real ceramic foam specimen containing a central notch/crack was subjected to a tension, resulting in a failure beginning at the tip of the notch. The work discusses an approach how to predict the critical failure load and also the crack path in the foam structure. Various cell irregularity and size of the macroscopic notch was considered in the investigations. Predictions of the foam structure cracking were performed using the beam element based FE model having characteristics of the real foam structure and by utilization of the stress criterion which considers failure of particular struts, when the maximum stress on them reaches the tensile strength of the ceramic material. Outputs from simulations were compared with available experimental observations and with an estimation of the critical failure load calculated using the developed analytical model.

Effects of grain boundary on wear of graphene at the nanoscale: A molecular dynamics study

Graphene has shown excellent tribological behaviors, enabling its potential applications as lubricating and anti-wear coatings, however, the grain boundaries (GBs) formed during the preparation process may deteriorate the performance of graphene. Using large-scale molecular dynamics simulations, we study the wear mechanism of graphene GBs with various misorientation angles between two grains. Compared with pure nanoindentation at the GBs, the critical load of wear failure upon nanoscratching across the GBs is much lower due to the synergetic actions of interlocking and pushing between the tip and graphene atoms. The misorientation angle between the adjacent grains significantly effects the onset and fashion of atomic-scale wear. Results show that wear resistance of the graphene with large-angle GBs is slightly lower than that of pristine graphene. Nevertheless, a number of the long bonds emerge in the vicinity of the low-angle GBs during scratching, leading to wear failure at much smaller load than the large-angle GBs. Furthermore, wear resistance of the low-angle GBs can be enhanced by increasing the interfacial strength between graphene and substrate due to the reduced number of the long bonds at the GB. This study sheds light on improving wear resistance of graphene coating by properly controlling its microstructures.

A Novel, Needle-Array Dry-Electrode With Stainless Steel Micro-Tips, for Electroencephalography Monitoring

A novel, needle array dry electrode consisting of 10 × 10 array of stainless steel (SS) Microtips was developed for electroencephalography (EEG) monitoring. The developed dry electrode uses commercially available, inexpensive, SS acupuncture needles certified for invasive use, to collect the EEG signal. The microtips of the acupuncture needles project out of a flat Teflon base by approximately 150 μm. Mechanical failure analysis was carried out, with theoretical calculations for individual needles and experimental measurements with a universal testing machine (UTM). The theoretically calculated critical load for failure for individual needle was 0.88 N, while the UTM measurements show the failure occurring at 0.95 N; this difference is probably due to the simplified assumptions used in calculations. The UTM measurements of the individual needle applied against a Silicone elastomer reveal that the force required for the penetration of the needle of the electrode into skin maybe as low as 0.01 N. Needle array insertion into silicone elastomer sheet and its optical inspection was carried out to assess the ability of the microneedles to penetrate the skin. The impedance of the electrode, measured in three electrode configuration in 0.9% NaCl solution, was approximately 6.8KΩ at 20 Hz, which is sufficiently low to fulfill the requirements of biopotential measurement. The construction and characteristics of the developed needle array dry electrode show that they are suitable for penetrating the stratum corneum of the skin and acquire the EEG signal directly from the interstitial fluidic layer underneath. The construction of the electrode and its mechanical and electrical characteristics show that it is a promising dry electrode for long duration EEG Monitoring.

Experimental and numerical study of hybrid steel-FRP balcony overhang of ships under shear and bending

A model of a composite-to-steel hybrid balcony overhang is tested to find its strength and stiffness under imposed shear and bending loads. The configuration tested is representative of a solution being considered for the balconies of cruise ships, where the substitution of steel by composites aims at weight saving. The steel component is a channel type of structure made of two plates that serve as external supports of the sandwich plate. A finite element model is used to compare with the experimental results and to predict the critical regions of stress concentration. The results include load-deflection and load-strain curves with the maximum value of the shear and bending loads. The critical locations and failure loads have been investigated too.

Investigation on Flexural Behaviour of Cold Formed Steel Latticed Built-Up I Section with Drop Web

This paper deals with the study of flexural behaviour of cold formed latticed built-up I beam with drop web for various sections by varying depth, thickness and span length of the section. The numerical analysis was carried out by FEM software ABAQUS 6.13[11]. The experimental works carried out by Cheng Yu and Benjamin W. Schafer (2006) [1] were taken for validating the numerical analysis procedure. It provides a cost effective way to explore the performance of products in a virtual environment. The ultimate capacity is obtained by performing both linear and non-linear analysis. Theoretical investigation as per AISI specification was carried out for all the specimens to check the accuracy of the design proposal. Numerical finite element procedure is used to instruct the effect of various parameters on behavior of beams. The critical load and different modes of failure are studied by this numerical investigation. Comparison between deformed shape and load vs. displacement curves of I section with drop web were presented.

Indentation failure of foam cored sandwich structures subjected to elevated temperatures

This research explores the detrimental effect of elevated temperatures on local indentation failure of polymer foam cored sandwich structures with laminated glass fibre reinforced epoxy face sheets. A simple analytical model to predict the critical indentation failure load at elevated temperatures is presented and validated against experimental observations. For this purpose, a sandwich beam loaded in three-point bending with an induced through-thickness elevated temperature profile was studied experimentally and by means of analytical and finite element models. In the experiment, the through-thickness temperature gradient was induced with an infrared lamp pointing on the top face sheet, while the local displacement and strain fields near the applied point load were recorded by digital image correlation. The analytical model proposed, which accounts for temperature degraded/reduced foam core properties, superimposes the local response, approximated by the classical Winkler foundation model, and the global response obtained by sandwich beam theory. The comparative study has shown that the critical load causing core crushing failure reduces significantly with elevated temperatures by as much as 50% at an elevated temperature of 90℃. It is shown that the simple analytical model can predict the local deflections and core stresses of foam cored composite sandwich structures subjected to simultaneous localized mechanical loading and elevated temperatures. Thus, the analytical model can be used as a preliminary design tool to determine the critical core crushing load at elevated temperatures.