Initial Failure Strength Research Articles

Strengthening Mechanism of Polyurea to Anti-Penetration Performance of Spherical Cell Porous Aluminum.

A composite structure containing a metallic skeleton and polyurea elastomer interpenetrating phase was fabricated, and its anti-penetration performance for low-velocity large mass fragments was experimentally studied. The protection capacity of three polyurea was compared based on the penetration resistance force measurement. Results show that the polyurea coating layer at the backside improves the performance of the polyurea-filled spherical cell porous aluminum (SCPA) plate due to its backside support effect and phase transition effect, which are accompanied by a large amount of energy absorption. The frontal-side-coated polyurea layer failed to shear and provided a very limited strengthening effect on the penetration resistance of the interpenetrating phase composite panel. The filling polyurea in SCPA increased the damage area and formed a compression cone for the backside coating layer, leading to a significant stress diffusion effect. The anti-penetration performance was synergistically improved by the plug block effect of the interpenetrating phase composite and the backside support effect of the PU coating layer. Compared with SCPA, the initial impact failure strength and the average resistance force of the composite plate were improved by 120-200% and 108-274%, respectively.

High-efficient improvement in flexural properties of carbon/Kevlar-fiber hybrid composites by CNT-toughening only between xenogeneic fiber-layers

Hybrid fiber-reinforced polymer (HFRP) composites possess excellent overall performances, but premature delamination between xenogeneic fiber-layers weakens their mechanical properties. Thus, the flexural behaviors of carbon/Kevlar HFRP laminates, toughened by carbon nanotubes (CNT) only between xenogeneic fiber-layers, is studied to reveal the high-efficient improvement. Three stacking sequences (C4/K4, [C2/K2]s, and [C/K2/C]s) are designed, and samples are manufactured through hand lay-up and compression molding. CNT is applied with an areal density of 0–8 gsm to toughen the interfaces only between several xenogeneic fiber-layers in [C/K2/C]s. Three-point bending tests and microscopic characterizations are conducted to evaluate flexural properties and failure modes. Results show that the flexural properties of HFRP with xenogeneic interface toughening display an increasing-then-decreasing trend as the areal density of CNT increases. The flexural modulus, initial failure strength and ultimate strength can be improved by 26.06%, 13.84% and 18.73%, respectively. Importantly, it is found that CNT-toughening only between several xenogeneic fiber-layers is more efficient in reinforcing HFRP than the same fiber-layers. This is due to the fact that interface toughening between xenogeneic fiber-layers can effectively suppress the buckling failure of Kevlar fibers, which has a significant effect on improving flexural properties. Finally, the crack-bridging of xenogeneic fiber-layers and CNT-bridging of cross-layer are also revealed. This work provides some valuable references for the design and application of HFRP.

Fracture strength and energy-dispersive spectroscopy analysis of 3-unit fixed partial dentures fabricated from different monolithic zirconia materials

Statement of problemMastication forces in different regions affect the survival of multiunit posterior restorations. The fracture strength of 3-unit posterior monolithic zirconia fixed partial dentures (FPDs) and their fracture patterns require investigation. PurposeThe purpose of this in vitro study was to evaluate and compare the fracture strength and fracture pattern of 3-unit posterior FPDs fabricated from different monolithic zirconia materials. Material and methodsThirty 3-unit FPDs were fabricated from BruxZir, FireZr, and Upcera (n=10 per group). Energy-dispersive spectroscopy analysis was performed on 2 selected specimens from each group. All specimens were exposed to a mastication simulator for 1.2×106 cycles and then monotonically loaded to fracture at a crosshead speed of 1 mm/min. The surfaces of a selected fractured specimen were examined at magnifications of ×25 and ×500 with scanning electron microscopy. Conformity to normal distribution was evaluated with the Shapiro-Wilk test. One-way analysis of variance was used to compare the normally distributed initial crack formation load F initial (Fi) and catastrophic failure strength F maximum (Fm) means by group. Weibull statistics were calculated by using the maximum likelihood estimation method. The chi-square test was used to compare shape and scale parameters (α=.05). ResultsThe mean Fm values were fail1878.9 N for Upcera, 2177.8 N for BruxZir, and 2229.4 N for FireZr. Upcera and BruxZir showed statistically significant differences for the Fm mean values (P=.039). The differences between the fracture type distributions according to the groups were statistically similar (P>.05). For Fi, Upcera presented the highest Weibull modulus value (2.199), FireZr had the lowest (1.594), while for Fm, BruxZir had the highest Weibull modulus value (9.267) and FireZr the lowest (6.572). ConclusionsUsing the zirconia materials BruxZir, FireZr, and Upcera resulted in high Fm values after aging procedures. With all materials, the fractures were most commonly found in the connector areas in the tested FPDs.

High-temperature and dynamic mechanical characterization of closed-cell aluminum foams

Metallic foams exhibit great application prospects in some extreme working conditions such as high temperatures and/or high-velocity impact environments, but predominantly, their mechanical behavior has remained to be fully understood to date. In this study the uniaxial and biaxial tests are conducted on closed-cell aluminum foams at elevated temperatures. It is indicated that both drop stress and initial failure strength exhibit approximately linear relationship with the applied temperature, while the densification strain keeps almost at a constant value. The higher foam density or lower applied temperature, the larger the initial failure surface characterized in the von Mises - mean stress plane. However, the normalized failure surfaces nearly do not depend on foam density or temperature, and can be well fitted in terms of elliptical or parabolic function. Dynamic compressive tests under constant strain-rates reveal that the compressive strength is correlated to strain-rate positively, but the densification strain negatively. A novel rate-dependent constitutive model is proposed to describe the compressive constitutive behavior of Al foams. Finally, the crushable foam model with the tabular input of yield ratio approach can produce satisfactory numerical predictions compared to dynamic experimental results. This study provides new insights into the intrinsic mechanical properties of metallic foams under some extreme conditions.

Elevated temperature compression behaviors of 3D printed hollow pyramidal lattice sandwich structure reinforced by truncated square honeycomb

This paper focuses on the influence of temperatures on the out-of-plane compressive performance of novel truncated-square honeycomb reinforced hollow pyramidal lattice sandwich structure (TSH-HP) by combining theoretical prediction, experiment and finite element analysis. As the temperature go up from 25 ℃ to 450 ℃, compressive modulus, initial failure strength, peak strength and energy-absorption performance of TSH-HPs prepared by selective laser melting (SLM) techniques decrease by 46.2%, 46.9% 42.3% and 46.3%, respectively, which are in well agreement with experiment and numerical simulation. In addition, the initial failure strength and peak strength of TSH-HPs are 5–10% higher than those of hollow pyramid lattice structure (HP) with same mass under different temperature. Compared with competing cellular materials, TSH-HPs exhibit superiority in the specific strength on the material selection map.

Triaxial tests on the overconsolidated methane hydrate-bearing clayey-silty sediments

Significant experimental efforts have been made to understand the mechanical behaviors of hydrate-bearing sediment; however, studies on the influence of over consolidation conditions on hydrate-bearing sediment are quite rare, while a series of exploratory drilling surveys for hydrate in hydrate-bearing sediment has reported that the phenomenon of over consolidation or apparent over consolidation is widespread. In this study, we present triaxial test results of influence of the over consolidation on hydrate-bearing clayey-silty sediment. The results reveal that both the tested normally consolidated and overconsolidated hydrate-bearing sediment appear to undergo strain-hardening and volumetric contraction, although the increasing over consolidation ratio would weaken the strain-hardening phenomenon and enhance the initial elastic modulus and failure strength. In addition, the increasing hydrate saturation would also increase the initial elastic modulus and failure strength and decrease the volumetric strain of the overconsolidated hydrate-bearing sediment. The mechanism controlling the influence of over consolidation on the hydrate-bearing sediment has been further investigated and discussed by scanning electron microscopy, and it is found that the over consolidation could damage a certain proportion of the cementation structure formed by the hydrate, while increasing the particle occlusion and strengthening the sediment. Under low over consolidation condition, these two mechanisms may almost counteract each other, while under high over consolidation condition, the strengthening mechanism caused by over consolidation is dominant.

On multiaxial failure behavior of closed-cell aluminum foams under medium strain rates

Metal foams are commonly used to withstand impact loading from different directions in real life applications attributable to their superior performance of energy absorption to weight. It has been of special interest to explore the complex yield/failure behavior of metal foams under dynamic impact recently. However, existing studies have mainly focused on crushing behavior under simple quasi-static loading conditions. Hence, this study aims to explore the multiaxial dynamic failure behavior of closed-cell aluminium foams. First, the uniaxial failure behavior of foams with three different apparent densities is experimentally investigated at varying strain rates (10−3-150 s−1) through quasi-static compression and drop weight tests, respectively. It is found that the initial failure strength exhibits evident strain-rate dependency. An empirical relation is established to associate the initial failure strength with relative density and nominal strain rate. Second, the multi-axial failure behavior is characterized based upon the developed microscopic computed tomography (micro-CT) image foam models. It is revealed that at low and medium strain rates, the hardening of the uniaxial failure strength is rate-dependent on the foam cell-wall base material. Under compression-shear and triaxial compression loadings, the initial failure surface in the von Mises - mean stress plane significantly expands with increasing strain rates. After normalized by the uniaxial failure strength at the corresponding strain rates, the failure surface is almost independent of loading or strain rates. The normalized failure surface can be well depicted by an elliptic or a parabolic function. Moreover, with use of suitable plastic Poisson's ratio, the normalized failure surface could be well characterized by Deshpande and Fleck (D&F) model and Miller model. The modified rate-dependent constitutive models are suggested for foam materials subject to quasi-static and dynamic loadings.

Reinforcement Effect of C and R Layer on Initial Failure Strength of CFRP Bolted Joints

Reinforcement Effect of C and R Layer on Initial Failure Strength of CFRP Bolted Joints

Enhanced mechanical performance of brazed sandwich panels with high density square honeycomb-corrugation hybrid cores

A novel lightweight sandwich construction with square honeycomb-corrugation hybrid core (SHCH) was fabricated by honeycomb-corrugation assembly and vacuum brazing. The out-of-plane quasi-static compression behavior of the hybrid structures was explored in detail by experimental measurement, numerical calculation and theoretical prediction. The proposed brazed structures in high density region are found still to have strength and energy absorption capacity significantly greater than the sum of those of empty corrugated sandwich and square honeycomb-cored sandwich. In addition, the compressive strengths of the proposed square honeycomb–corrugation hybrid cores exhibit obvious superiority over those of competing core topologies in the high-density region. • Novel lightweight sandwich constructions with square honeycomb-filled corrugated hybrid cores are proposed. • The out-of-plane compressive behavior of the hybrid cores is enhanced. • Compressive stiffness, initial failure strength, and failure modes are predicted. • The strength and energy absorption capacity of honeycomb and corrugation in hybrid cores are significantly enhanced. • The compressive strengths of such hybrid cores exhibit obvious superiority in the high-density region.

Progressive Damage Analysis (PDA) of Carbon Fiber Plates with Out-of-Plane Fold under Pressure

The out-of-plane fold is a common defect of composite materials during the manufacturing process and will greatly affect the compressive strength as well as the service life. Making it of great importance to investigate the influence of out-of-plane defects to the compressive strength of laminate plates of composite materials, and to understand the patterns of defect evolution. Therefore, the strip method is applied in this article to create out-of-plane defects with different aspect ratios in laminated plates of composite materials, and a compressive performance test is conducted to quantify the influence of out-of-plane defects. The result shows that the compressive strength becomes weaker with a greater aspect ratio. When the highest aspect ratio is set to 0.12 in the experiment, the compressive strength reduces by 36.1%. Then we establish a 3-D progressive damage model based on continuum mechanics, and write it into the UMAT subroutine together with the 3-D Hashin criteria and the non-linear degradation criteria of materials. 3-D solid modeling is performed for the samples with an out-of-plane fold based on ABAQUS, and progressive damage analysis is conducted to acquire the inplane evolution process of initial failure strength with different laminates. The experimental results agree well with the simulation results.

Initial failure strength prediction of woven composites using a new yarn failure criterion constructed by deep learning

A new failure criterion for fiber tows (i.e. yarns) is developed based on a micromechanical model using the mechanics of structure genome (MSG) and a deep learning neural network model. The proposed failure criterion can be applied to yarns in mesoscale textile composites modeling while capturing the failure initiation at the fiber and matrix level. A plain weave fiber reinforced composite material example is used to compute the initial failure strength constants of a woven lamina based on the proposed yarn failure criterion. To study the accuracy and efficiency of the failure criterion, a comparison to a meso-micro coupled model explicitly capturing the failure initiation at fiber and matrix level is performed. Moreover, the differences between the mesoscale modeling results based on the proposed criterion and other yarn failure criteria (i.e. maximum stress, Tsai-Wu, and Hashin) are studied. Lastly, the failure envelope analysis of the mesoscale plain weave example is carried out using the MSG solid model to further demonstrate the accuracy and efficiency of the new yarn failure criterion under combined loading conditions.

An elastoplastic model for gas flow characteristics around drainage borehole considering post-peak failure and elastic compaction

The excavation and drainage drilling for underground mining induces stress redistribution around the gas drainage borehole, thus forming three physical zones: residual state zone, strain softening zone and elastic zone. The formation process of these zones contains complex interactions among deformation, natural gas flow, and coal seam damage. A better understanding of these interactions could provide better guidance for the gas drainage engineering. Extensive studies have focused on the effect of effective stress or effective strain on permeability variation based on the poroelastic theory. Meanwhile, as there is few permeability models taking the post-peak failure effect into account, previous permeability variation analysis seldom commonly considered the elastoplastic characteristic of coal seam, which results in the permeability misestimation. Therefore, this study proposes a new approach to analyze this interaction process. The innovation of this approach is that it takes into account the influence of coal permeability enhancement in failure zone and the volumetric compaction in elastic zone around the drainage borehole. In this approach, analytical solutions of stress and strain are developed to include both the strain softening around a gas drainage borehole and the compaction in elastic zone. These solutions thus remove the flaws that previous studies did not consider the compaction in elastic zone. Further, a new permeability model is proposed by the introduction of damage enhancement coefficient for post-peak failure. Third, the permeability distribution of coal around a gas drainage borehole is calculated based on the analytical solutions and the new permeability model. Fourth, the gas flow equation is numerically solved to obtain gas pressure profiles. The gas content computed by this approach is verified by field data. Finally, parametric study is carried out to investigate the effect of the damage enhancement coefficient, initial geo-stress, drilling volume, and uniaxial strength on the gas pressure and the permeability around the gas drainage borehole. Based on these numerical analyses, it is found that the evolution of permeability is closely related to the physical properties of coal and the geological condition of coal seam. Higher initial geo-stress and lower failure strength have larger unloading zone and higher permeability enhancement. This compaction helps the coal seam form a flow-shielding zone near the interface between plastic zone and elastic zone. The gas flow in the coal around the drainage borehole can be divided into four different zones.

Biomechanical Analysis of Latarjet Screw Fixation: Comparison of Screw Types and Fixation Methods

To compare the initial fixation stability, failure strength, and mode of failure of 5 different screw types and fixation methods commonly used for the classic Latarjet procedure. Thirty-five fresh-frozen cadaveric shoulder specimens were allocated into 5 groups. A 25% anteroinferior glenoid defect was created, and a classic Latarjet coracoid transfer procedure was performed. All grafts were fixed with 2 screws, differing by screw type and/or fixation method. The groups included partially threaded solid 4.0-mm cancellous screws with bicortical fixation, partially threaded solid 4.0-mm cancellous screws with unicortical fixation, fully threaded solid 3.5-mm cortical screws with bicortical fixation, partially threaded cannulated 4.0-mm cancellous screws with bicortical fixation, and partially threaded cannulated 4.0-mm captured screws with bicortical fixation. All screws were stainless steel. Outcomes included cyclic creep and secant stiffness during cyclic loading, as well as load and work to failure during the failure test. Intergroup comparisons were made by a 1-way analysis of variance. There were no significant differences among different screw types or fixation methods in cyclic creep or secant stiffness after cyclic loading or in load to failure or work to failure during the failure test. Post-failure radiographs showed evidence of screw bending in only 1 specimen that underwent the Latarjet procedure with partially threaded solid cancellous screws with bicortical fixation. The mode of failure for all specimens analyzed was screw cutout. In this biomechanical study, screw type and fixation method did not significantly influence biomechanical performance in a classic Latarjet procedure. When performing this procedure, surgeons may continue to select the screw type and method of fixation (unicortical or bicortical) based on preference; however, further studies are required to determine the optimal method of treatment. Surgeons may choose the screw type and fixation method based on preference when performing the Latarjet procedure.

Failure analysis and strengthening mechanism of Z-pinned composite T-joints under tensile loading

Abstract Z-pinning technology is applied to composite T-joints in this paper to explore the strengthening mechanisms. The structural properties of T-joints with skin thickness values at 2 and 4 mm were determined under stiffener tensile load. The results revealed that the initial failure strength and failure modes of T-joints were affected by the thickness of skin. Under stiffener tensile load, the actual stress situation of Z-pin tends to be complicated with the deflection of skin. As a result, a mixed mode I/II stress condition exists at the delamination crack tip during tensile loading, and an enhanced frictional zone is located near the delamination surface. When the deflection of skin is indispensable, the snubbing effect acts as a significant factor to strengthening. As the pullout process of pins from the flange is limited by snubbing, pins could fail by split. This paper discusses the strengthening mechanism of Z-pinning and elaborates a potential enhancing effect of Z-pins resulting from the complicated loading condition due to different structure dimensions.

Static and fatigue tensile properties of cross-ply laminates containing vascules for self-healing applications

The effect of including hollow channels (vascules) within cross-ply laminates on static tensile properties and fatigue performance is investigated. No change in mechanical properties or damage formation is observed when a single vascule is included in the 0/90 interface, representing 0.5% of the cross sectional area within the specimen. During tensile loading, matrix cracks develop in the 90° layers leading to a reduction of stiffness and strength (defined as the loss of linearity) and a healing agent is injected through the vascules in order to heal them and mitigate the caused degradation. Two different healing agents, a commercial low viscosity epoxy resin (RT151, Resintech) and a toughened epoxy blend (bespoke, in-house formulation) have been used to successfully recover stiffness under static loading conditions. The RT151 system recovered 75% of the initial failure strength, whereas the toughened epoxy blend achieved a recovery of 67%. Under fatigue conditions, post healing, a rapid decay of stiffness was observed as the healed damage re-opened within the first 2500 cycles. This was caused by the high fatigue loading intensity, which was near the static failure strength of the healing resin. However, the potential for ameliorating (via self-healing or autonomous repair) more diffuse transverse matrix damage via a vascular network has been shown.

Experimental investigation of incipient shear failure in foliated rock

It has long been known that rock fabric plays a key role in dictating rock strength and rheology throughout Earth's crust; however the processes and conditions under which rock fabric impacts brittle failure and frictional strength are still under investigation. Here, we report on laboratory experiments designed to analyze the effect of foliation orientation on the mechanical behavior and associated microstructures of simulated fault rock sheared at constant normal stress of 50 MPa. Intact samples of Pennsylvania slate were sheared with a range of initial fabric orientations from 0 to 165° with respect to the imposed shear direction. Foliation orientations of 0–30° produce initial failure at the lowest shear stress; while samples oriented at higher angles are less favorable resulting in higher peak failure strength. In all samples, post-peak development of through-going deformation zones in the R1 Riedel orientation results in a residual strength that is lower than that observed for powdered gouge from the same material. As shear strain increases, all samples approach a residual apparent friction, defined by the ratio of shear strength to normal stress, of ∼0.4. However, the final deformation microstructure depends strongly on the orientation of the pre-existing foliation. When fabric is oriented at low angles to the shear direction (<60°) slip occurs on the pre-existing foliation. Passive rotation of spectator regions becomes apparent for samples with foliation orientation of 60–90°. At higher foliation angles, deformation typically occurs in wide, through-going zones at an R1 orientation that cross-cuts the fabric or in a P-orientation along the fabric. These samples typically exhibit greater geometric layer thinning per unit shear strain. Our results document how pre-existing foliation, in any orientation, can lower the residual shear strength of rock. In contrast, the initial yield strength and peak failure strength of foliated rock is often higher in comparison with powdered samples of the same material, due to strength anisotropy and because activating slip on pre-existing foliation requires dilation and associated work against normal stress. The relationship between rock fabric orientation and frictional strength evolution that we document can explain how incipient faulting and/or flexural slip occur in foliated rock, especially when the orientations of fractures and original fabric vary widely.

Effect of Load Direction on Failure of Out-of-Plane Composite Joints

The mechanical properties of fabric composite π joints under stretch-bending load are studied using the progressive damage method. The failure mode of fillers is evaluated by the modified maximum stress failure criterion due to their structural particularity, while other parts of the joint by the maximum stress failure criterion. Compared with the results of joints under simple tensile load, the numerical simulation result of the joint under stretch-bending loads with varied directions reveals that the flexion can cause significant reduction in the initial and ultimate failure strength although it has little effect on the structure stiffness before initial failure. In addition, the base laminate and the skin are more easily damaged under stretch-bending loads.

Initial and final failure strength analysis of composites based on a micromechanical method

In this paper, a micromechanical model for investigating initial and final failure surface of fiber-reinforced composites laminate is established. The Generalized Method of Cells, which effectively predicts the thermal stresses influence on stress–strain behaviors of fiber-reinforced composites, is combined with Von-Mises yield criterion to describe nonlinear stress–strain behaviors of composites laminate. The Tsai–Hill criterion is incorporated into the model for predicting failure strength of lamina and typical laminate in uniaxial tension and biaxial loading, respectively. The numerical simulations based on a micromechanical method are performed to study the effects of thermal residual stresses on initial and final failure strength. Results revealed that thermal residual stresses influence on failure strength of the UD lamina is closely dependent on fiber off-axis angles. The contribution of thermal residual stresses in the final failure surface of [0°/±30°/90°]2 laminate and [90°/±45°/0°]2 laminate can be ignored. The results in longitudinal for lamina and biaxial tensile loadings for [90°/±45°/0°]2 laminate show good agreement with the experimental data.

Humeral Head Reconstruction for Hill-Sachs Defects: A Biomechanical Comparison of 2 Fixation Techniques for Bone Grafting

The purpose of this biomechanical study was to compare anterograde with retrograde screw fixation for allograft reconstruction of Hill-Sachs defects. In 8 pairs of fresh-frozen humeral heads, a 40% Hill-Sachs defect was created. The resultant wedge-shaped osteochondral fragment was used as allograft. For each technique, two 3.75-mm screws were used for fixation. To test the strength of fixation, a custom tool was used that would apply load to the graft. By use of a materials testing machine, a staircase cyclic loading protocol was performed (500 cycles at 10, 20, 30, and 40 N) and then load to failure. Graft displacement was measured by an optical tracking system. For the 2 techniques, graft displacement increased with increasing load and increasing number of cycles up to a mean of 0.9 ± 0.42 mm for anterograde fixation and 1.1 ± 0.79 mm for retrograde fixation. This increase was significant within each technique across all 4 loading levels (P < .05). However, there were no significant differences in graft displacement between the 2 techniques at any loading level or number of cycles (P = .16 to P = .96). In addition, the load to failure between the anterograde and retrograde techniques (98.5 N and 95.6 N, respectively) was not significantly different (P = .706). The initial fixation and failure strength of anterograde and retrograde graft fixation techniques for substantial Hill-Sachs defects do not significantly differ in a biomechanical cadaveric model. This biomechanical study supports that in an engaging Hill-Sachs defect, both anterograde and retrograde screw fixation techniques can be used for fixation of humeral head allografts.

Tensile Failure Strength Analysis and Experimental Confirmation of Stitch Reinforced Composite of T-stiffened Structure

Based on the progressive failure analysis method, taking account of 7 kinds of failure modes: stitch thread failure mode, resin extend (or compress) failure mode, fibre extend (or compress) failure mode, extend (or compress) delamination failure mode, a new failure analysis FE model was proposed for predicting the tensile strength of stitch-reinforced composite T-stiffened structures. The 3D solid element and truss element were used to simulate laminate and stitch thread individually in the FE model. The Hashin failure criterion and stiffness degradation technology are used to analyze damage failure of the stitch T-stiffened structure. Comparing the initial and final damage failure strength with those of experiments on stitch composite T-stiffened structures validated the Python secondary development program for the FE model presented in the paper. There was good agreement between the failure strength from the damage failure analysis and that of the experiment. This indicated that the model was correct, and that it could be a reference for stitch-reinforced composite design and analysis.