Failure Modes Of Materials Research Articles

Structural Static/Fatigue/Damage Tolerance Comprehensive Design Optimization of Composite Laminate

This paper develops a structural static/fatigue/damage tolerance comprehensive design method for composite laminated structures under mechanical–thermal coupling environments. The developed design method can finely quantify the additional stress and deformation caused by the mechanical–thermal coupling effect. In addition, a new damage evolution-based fatigue reliability evaluation method is proposed, and the structural fatigue behavior can be described and predicted. The different failure modes of composite material and their correlations are considered during the damage evolution analysis. The structural possible accidental damage can also be taken into account by applying the global preset damage in design stage. Thus, the static requirement, fatigue requirement, and damage tolerance requirement can all be considered simultaneously during the design process. The structural comprehensive design of composite laminate is conducted by virtue of the two-level optimization method. A case of a notched composite laminate is employed to verify the proposed method. In addition, a case of a civil aircraft wing structure is used to explain the feasibility of the developed method in dealing with structural design problems in engineering.

Concepts and definitions related to mechanical behavior of fiber reinforced composite materials

It is common in the composites literature to find usage of terminology that is a legacy of monolithic materials such as metals, ceramics and polymers. With the aim of alleviating ambiguity and misinterpretations that can result from such usage, this article attempts to clarify concepts and definitions related to the mechanical behavior of fiber reinforced composites. Focus is placed on failure mechanisms and modes in composite materials, and their comparison with metals. Of special interest is the limit to a material's load-carrying capacity, commonly described as “strength”, which is a subject of major confusion in the literature. The various versions of strength, e.g., residual strength, open-hole tensile strength, in-situ strength and compression after impact strength are scrutinized from the point of view of their being material properties. It is recommended that the practice of loose and ill-defined usage of the strength terminology should be abandoned in favor of resorting to proper use of critical stress states for relating those to the load-carrying capacity under consideration.

Phase field modeling scheme with mesostructure for crack propagation in concrete composite

Tracking the crack propagation in concrete at the mesoscopic level is of great importance for revealing the crack pattern and failure mechanism, due to the huge threat of cracking to structure safety. This paper develops a phase field modeling scheme with mesostructure of concrete composite to investigate its crack propagation behavior. With the classical phase field model and the staggered algorithm implementation by three-dimensional (3D) finite element method (FEM), crack propagation within concrete’s mesostructure composed of coarse aggregate, mortar matrix, interfacial transition zone (ITZ) and initial defects including micropores and microcracks is modeled. The proposed modeling scheme is tested on single-aggregate samples and complex mesostructured concrete samples subjected to uniaxial loading, showing reasonable accordance with the experimental observation and common failure modes of concrete material. The results indicate that mesostructural configurations are important factors affecting the crack pattern of concrete. The presence of coarse aggregates is prone to initiate and induce interfacial cracking in the ITZ effect, but also exerts the blocking effect on the crack growth. And the initial defects (micropores and microcracks) are also the important influential factor on crack initiation, propagation and the formation of the major crack. It is found that the higher the volume fractions of aggregates and initial defects, the cracking of concrete initiates earlier, propagates faster and penetrates more easily when subjected to the same loading condition.

Dynamic mechanical behaviour of three-dimensional random fibrous materials at high temperatures

We studied the dynamic mechanical behaviour of three-dimensional (3D) random fibrous (RF) materials at various strain rates using a high-temperature split Hopkinson pressure bar (SHPB) system from 288 to 1423 K. The failure mode of 3D RF materials in the in-plane (IP) direction exhibits a shear fracture mode with an angle of ~30° at a strain rate of 300/s, and the bonding between fibres remains intact at 288 K despite the fracture in fibres. The variation of dynamic strength at a strain rate of 500/s (from 288 to 773 K, the strength increases from 4.72 ± 0.61 to 5.27 ± 1.03 MPa; from 773 to 1423 K, the strength decreases from 5.27 ± 1.03 to 3.36 ± 0.93 MPa) aligned well with observations of the static experiment with increasing temperature. We determined the strain rate sensitivity factor by fitting strength versus strain rate at various temperatures and observed a significant effect of the strain rate sensitivity on strength at 288, 773, and 1273 K. However, the effect of strain rate on the strength is low at the transition temperature of viscous flow and brittle deformation of 3D RF materials. These results can be used as the theoretical basis for the preparation and performance evaluation of 3D RF materials.

The Influence of Temperature in the Al 2024-T3 Aluminum Plates Subjected to Impact: Experimental and Numerical Approaches.

In this paper, perforation experiments were carried out and numerically modelled in order to analyze the response of 2024-T3 aluminum alloy plates under different initial temperatures T0. This alloy has a particular relevance since it is widely used as a structural component in aircrafts, but it is also interesting for other sectors of industry. A gas gun projectile launcher was used to perform impacts within initial velocities V0 from 40 m/s to 120 m/s and at temperatures varying from 293 K to 573 K. A temperature softening of the material was observed which was manifested in the reduction in the ballistic limit by 10% within the temperature range studied. Changes in the material failure mode were also observed at different test conditions. Additionally, a finite element model was developed to predict the material response at high velocities and to confirm the temperature softening that was observed experimentally. An optimization of the failure criterion resulted in a reliable model for such mild aluminum alloys. The results reported here may be used for different applications in the automotive and military sectors.

Study on Mechanical Bearing Strength and Failure Modes of Composite Materials for Marine Structures

With the gradual application of composite materials to ships and offshore structures, the structural strength of composites that can replace steel should be explored. In this study, the mechanical bearing strength and failure modes of a composite-to-metal joining structure connected by mechanically fastened joints were experimentally analyzed. The effects of the fiber tensile strength and stress concentration on the static bearing strength and failure modes of the composite structures were investigated. For the experiment, quasi-isotropic [45°/0°/–45°/90°]2S carbon fiber-reinforced plastic (CFRP) and glass fiber-reinforced plastic (GFRP) specimens were prepared with hole diameters of 5, 6, 8, and 10 mm. The experimental results showed that the average static bearing strength of the CFRP specimen was 30% or higher than that of the GFRP specimen. In terms of the failure mode of the mechanically fastened joint, a cleavage failure mode was observed in the GFRP specimen for hole diameters of 5 mm and 6 mm, whereas a net-tension failure mode was observed for hole diameters of 8 mm and 10 mm. Bearing failure occurred in the CFRP specimens.

A rate-dependent phase-field framework for the dynamic failure of quasi-brittle materials

In recent years, phase-field theory has become an efficient approach for predicting damage and fracture of engineering materials. However, the rate-dependence of quasi-brittle materials is not considered in most exiting models. To address this issue, we develop a new phase-field damage model for the dynamic failure of quasi-brittle solids based on the microforce balance law, within which a linear viscoelastic constitutive relation in effective stress space and a hyperbolic phase-field evolution equation are incorporated. Then several representative numerical examples are presented to demonstrate the ability of the proposed model in characterizing the dynamic failure process of quasi-brittle solids. Good agreements are achieved between numerical predictions and theoretical and experimental results. In particular, the increase of tensile strength and the transition of failure modes of concrete-like materials with the increase of loading rates can be well reproduced.

Design of cold-formed stainless steel circular hollow section columns using machine learning methods

Most existing design methods for the bearing capacity of stainless steel circular hollow section (CHS) columns were developed for a specific grade considering merely the global buckling failure mode. However, a variety of stainless steel grades with significant differences in material properties exist, and CHS columns may undergo local buckling, global buckling and global–local interactive buckling. To develop a unified design method suitable for various stainless steel grades and failure modes, this study adopted a machine learning based framework. First, 39 tests were conducted on cold-formed stainless steel CHS columns. Material properties, imperfections, load-deformation curves, and failure modes were reported in detail. Then, test data on stainless steel CHS columns in literature were collected and formed a database with 280 columns. Afterwards, two machine learning algorithms, Random Forest and Extreme Gradient Boosting, were used to predict the bearing capacity of column based on four types of input parameters. The Random Forest algorithm obtained the highest prediction accuracy when using all the design parameters as input. The accuracy of Random Forest algorithm based on the Comprehensive parameters (i.e., the non-dimensional slenderness of cross-section and member) is improved considerably when including the ratio of yield strength over the Young’s modulus as the input parameter. Finally, the prediction of machine learning method was compared with that of the design method in Design manual for structural stainless steel, and the proposed method shows a better accuracy.

Study on high-cycle fatigue fracture mechanism and strength prediction of RuT450

Fatigue failure is the most common failure mode of structural materials. In this study, the high-cycle fatigue properties at different temperatures, fracture surface morphologies and corresponding damage mechanisms of a widely used vermicular graphite cast iron RuT450 were investigated. It is found that the fatigue strength values are 150 MPa, 143 MPa and 108 MPa for room temperature, 400 °C and 500 °C, respectively. The decrease rate of fatigue strength is affected by the change of morphology. At high temperature, the proportion of pearlite decreases and graphite becomes blunt. In general, the cracks initiate from the graphite phase boundary and propagate through the pearlite lamellae. In addition, according to the change of matrix micro-structure and crack growth processes under different temperature conditions, and by the quantitative analysis of micro-structure, the effect of temperature can be transformed into related parameters of matrix. Based on the changes, a fatigue strength prediction model for vermicular graphite cast iron at different temperatures was proposed, which predicts the trend well.

Molecular dynamics simulations of ultrafast radiation induced melting at metal–semiconductor interfaces

Metal–semiconductor contacts in silicon carbide (SiC) diodes endure damages at the interface when exposed to harsh radiation environments. Due to the rapid rise in temperature and ultrafast cooling that follows the radiation impact, the structural properties of the materials can be altered through melting, recrystallization, and amorphization. A detailed understanding of the material failure modes at the interface is lacking, specifically at the nanoscale. We use molecular simulations to investigate the ultrafast melting at tungsten (W)–SiC interfaces following radiation damage and apply deep learning techniques to track the transient evolution of the local molecular structures. We show that W near the radiation track undergoes melting and, eventually, most of it recrystallizes with a noticeable degree of undercooling, while SiC is rendered permanently amorphous. The observation of local undercooling in W films is important as it can affect the device performance even before the bulk melting temperature of the material is reached. We also show that at high temperatures, the interface undergoes a fracture-like failure. The results presented here are significant in understating the different failure modes of SiC diode materials.

Study on static lateral load\u2013slip behavior of single-shear stapled connections in plywood for upholstered furniture frame construction

The static lateral load–slip behavior of a single-shear plywood-to-plywood single-staple connection (SPSC) was investigated experimentally. A mechanics-based approach was used to develop mechanical models for deriving estimation equations for critical lateral loads of SPSCs based on failure modes of staple legs and connection member materials developed during static lateral loading process. Experimental results indicated that the static lateral load–slip behavior of SPSCs can be characterized with three major stages. This experiment provided the evidence that the ultimate lateral load capacity of SPSCs was partially governed by staple direct withdrawal load capacity in main members. The proposed mechanical models were verified experimentally as a valid means for deriving estimation equations for critical lateral loads of SPSCs evaluated in this study.

Experimental and Numerical Study of Fracture Behavior of Rock-Like Material Specimens with Single Pre-Set Joint under Dynamic Loading.

The Split Hopkinson Pressure Bar (SHPB) is an apparatus for testing the dynamic stress-strain response of the cement mortar specimen with pre-set joints at different angles to explore the influence of joint attitudes of underground rock engineering on the failure characteristics of rock mass structure. The nuclear magnetic resonance (NMR) has also been used to measure the pore distribution and internal cracks of the specimen before and after the testing. In combination with numerical analysis, the paper systematically discusses the influence of joint angles on the failure mode of rock-like materials from three aspects of energy dissipation, microscopic damage, and stress field characteristics. The result indicates that the impact energy structure of the SHPB is greatly affected by the pre-set joint angle of the specimen. With the joint angle increasing, the proportion of reflected energy moves in fluctuation, while the ratio of transmitted energy to dissipated energy varies from one to the other. NMR analysis reveals the structural variation of the pores in those cement specimens before and after the impact. Crack propagation direction is correlated with pre-set joint angles of the specimens. With the increase of the pre-set joint angles, the crack initiation angle decreases gradually. When the joint angles are around 30°–75°, the specimens develop obvious cracks. The crushing process of the specimens is simulated by LS-DYNA software. It is concluded that the stresses at the crack initiation time are concentrated between 20 and 40 MPa. The instantaneous stress curve first increases and then decreases with crack propagation, peaking at different times under various joint angles; but most of them occur when the crack penetration ratio reaches 80–90%. With the increment of joint angles in specimens through the simulation software, the changing trend of peak stress is consistent with the test results.

Special review: Mechanical investigation on failure modes of reticular porous metal foams under different loadings in engineering applications

PurposeThe purpose of this paper is to provide a summarization and review of the present author's main investigations on failure modes of reticular metal foams under different loadings in engineering applications.Design/methodology/approachWith the octahedral structure model proposed by the present authors themselves, the fundamentally mechanical relations have been systematically studied for reticular metal foams with open cells in their previous works. On this basis, such model theory is continually used to investigate the failure mode of this kind of porous materials under compression, bending, torsion and shearing, which are common loading forms in engineering applications.FindingsThe pore-strut of metal foams under different compressive loadings will fail in the tensile breaking mode when it is brittle. While it is ductile, it will tend to the shearing failure mode when the shearing strength is half or nearly half of the tensile strength for the corresponding dense material and to the tensile breaking mode when the shearing strength is higher than half of the tensile strength to a certain value. The failure modes of such porous materials under bending, torsional and shearing loads are also similarly related to their material species.Originality/valueThis paper presents a distinctive method to conveniently analyze and estimate the failure mode of metal foams under different loadings in engineering applications.

Investigating the Material Properties and Microstructural Changes of Fused Filament Fabricated PLA and Tough-PLA Parts.

Fused deposition modelling (FDM) is a popular but complex additive manufacturing process that works with many process parameters which are crucial to investigate. In this study, 3D parts were fabricated by placing each filament layer in opposite direction to the others; for this, two combinations of raster angles, (45° −45°) and (0° 90°), along with three different infill speeds were used. In this study, two 3D printing material types—Polylactic Acid (PLA) and tough-PLA were used. The material properties of each 3D part were investigated to identify the best combination of these parameters. A microstructural analysis was also performed on outer and inner surfaces along with fracture interface of the parts after tensile testing using a scanning-electron-microscopy (SEM) to explain material failure modes and reasons. The results suggest that for both the material types, a raster angle of 45° −45° produces stronger parts than to a raster angle of 0° 90°. This study also suggests that a slow infill speed improves tensile properties by providing a better inner-connection between two contiguous roasters. Thus, the detailed analysis of microstructural defects correlated with tensile test results provides insight into the optimisation of raster angle and infill speed, and scope for improvement of mechanical properties.

Mechanical behavior of nano-hybrid composite in comparison to lithium disilicate as posterior cement-retained implant-supported crowns restoring different abutments

ObjectiveResin-based materials are gaining popularity in implant dentistry due to their shock absorption capacity. Therefore, the aim of this study was to evaluate the fracture strength and failure mode of resilient materials for both crowns and abutments and compare them to the most widely used materials in different combinations after subjection to long-term fatigue loading. MethodsForty-eight cement-retained implant-restorations were assembled on titanium implants. Identical custom-made CAD/CAM abutments were milled out of 3 different materials (n = 16); T: titanium, Z: zirconia and P: ceramic-reinforced PEEK. Each group was subdivided, according to the restorative crown material, into two subgroups (n = 8); C: nano-hybrid composite and L: Lithium disilicate. Specimens were subjected to dynamic load of 98 N for 1,200,000 cycles with integrated thermal cycling. The surviving specimens were subjected to quasi-static loading until failure. Shapiro–Wilk test was used to test for normality. One-way ANOVA followed by Tukey’s post-hoc test was used to detect statistically significant differences between groups. ResultsAll specimens withstood 1,200,000 load cycles. The fracture strength values varied from a minimum of 1639 ± 205 N for group PL to a maximum of 2949 ± 478 N for group ZL. SignificanceThe abutment material influenced the fracture strength and failure mode of the restoration. A combination of zirconia abutments and nano-hybrid composite showed the most favorable mode of failure within the test groups. Therefore, this combination might be recommended as an alternative for restoring single implants in the posterior area.

Analysis of Strength and Microstructural Characteristics of Mine Backfills Containing Fly Ash and Desulfurized Gypsum

The utilization of solid wastes (SWs) as a potential resource for backfilling is not only conducive to environmental protection but also reduces the surface storage of waste. Two types of SWs, including fly ash (FA) and desulfurized gypsum (DG), were used to prepare cementitious backfilling materials for underground mined-out areas. Ordinary Portland cement (OPC) was used as cement in mine backfill. To better investigate the feasibility of preparing backfill materials, some laboratory tests, such as uniaxial compressive strength (UCS), scanning electron microscopy (SEM), and energy dissipation theory, were conducted to explore both strength and microstructural properties of backfilling. Results have demonstrated that the main components of FA and DG in this study are oxides, with few toxic and heavy metal components. The ideal ratio of OPC:FA:DG is 1:6:2 and the corresponding UCS values are 2.5 and 4.2 MPa when the curing time are 7 days and 14 days, respectively. Moreover, the average UCS value of backfilling samples gradually decreased when the proportion of DG in the mixture increased. The main failure modes of various backfilling materials are tensile and shearing cracks. In addition, the corresponding relations among total input energy, dissipated energy and strain energy, and stress–strain curve were investigated. The spatial distribution of oxygen, aluminum, silicon, calcium, iron and magnesium elements, and hydration product are explored from the microstructure’s perspective. The findings of this study provide both invaluable information and industrial applications for the efficient management of solid waste, based on sustainable development and circular economy.

Toward an In-Depth Material Model for Cermet Nuclear Thermal Rocket Fuel Elements

The development and qualification of nuclear thermal propulsion (NTP) fuel element technologies would be aided by an in-depth model of material response and failure modes at operating conditions. Integrated computational materials engineering techniques have the potential to provide such a model, as demonstrated here through three case studies focused on a tungsten–uranium mononitride (UN) cermet fuel. The first case focuses on the erosion of tungsten (also named wolfram), a nominal coating/cladding and fuel element matrix material, in hot hydrogen. Ab initio techniques are used to calculate erosion rates and thermal expansion at NTP operating conditions. The second focuses on the stability of UN fuels at high temperature and in the presence of hydrogen. Phase diagram techniques augmented with ab initio thermodynamic data reveal potential instabilities and decomposition pathways at high hydrogen concentrations. The third focuses on using microstructure information to predict high-temperature mechanical response and failure of tungsten. Combined finite element and discrete dislocation dynamics techniques provide mechanical properties in agreement with experimental methods. The integration of these techniques for an all-encompassing material model is discussed.

NASA Ames Thermophysics Ground Test Facilities Supporting Future Planetary Atmospheric Entry

A review of the current facility capabilities for testing Thermal Protection Systems and quantifying their entry environments at NASA Ames Research Center is presented based on the expected targets of interest to the Planetary Science and Astrobiology communities. While the operational capabilities of these facilities are generally considered sufficient for supporting future missions to targets of interest, expanded ground test capabilities such as larger sample sizes, flight-relevant gas mixtures, dusty environments, and flight-relevant shear/pressure combinations would reduce future entry vehicle design uncertainties and applied margins. These reduced uncertainties may translate into reduced entry vehicle masses, increased robustness, and decreased operational risks during entry phases for science missions. Expanded ground test capabilities would also offer the ability to study material failure modes in environments even more representative of flight than are currently achievable. A list of desired future test capabilities is presented along with suggestions of possible methods of achieving each. The main recommendation of this paper is the undertaking of a detailed study of the benefits and associated costs of each of these expanded capabilities to determine the best future path.

The use of the empirical crack orientation tensor to characterize the damage anisotropy

In practice, a dominant failure mode of brittle materials, such as fiber-reinforced thermosets is the initiation and propagation of cracks under cyclic loading. In general, the damage in composites with irregular microstructure do not occur in regular patterns. There are several state-of-the-art continuum mechanical models which take the anisotropic damage for predicting the material behavior into account. Since damage generally occurs in an anisotropic way, methods that experimentally characterize the damage anisotropy are needed.Micro-Computed Tomography systems (μCT) acquire volumetric images in a non-destructive way. By combining μCT scanning and mechanical in-situ testing, detailed microstructure data of specimens under load are generated. Based on image processing methods, the damage propagation from crack initiation to fracture is analyzed. In practice, cyclic load is a common load case for many components. Consequently, fatigue and cycle load tests are essential for a comprehensive material characterization.In this contribution interrupted in-situ μCT tests with cyclic tensile load are performed on Sheet Molding Compounds (SMC), a discontinuous glass fiber-reinforced thermset composite. Image processing methods are introduced for the experimental determination of the anisotropic damage characteristic based on volumetric images. The methods enable analyzing the spatial crack orientation distribution. In order to quantify the damage anisotropy, empirical formulations of the crack density distribution and second-order crack orientation tensor are applied. The presented work in this contribution extends the results of preliminary experimental damage investigations. In addition to previous studies, the damage anisotropy is quantified and analyzed.

Experimental Study on Axial Compression Behavior on Circular Timber Columns Strengthened with CFRP Strips and Near-Surface Mounted Steel Bars

To investigate the working mechanism and reinforcement effectiveness of the ancient timber columns strengthened with carbon fiber reinforced polymer (CFRP) strips and near-surface mounted steel bars, in this paper, 42 timber columns have been tested under axial compression, which considered the arrangement modes of CFRP strips, number of near-surface mounted steel bars, and different strengthening ways as the main influencing factors. Based on the test results, failure modes of specimens and materials were analyzed, compressive bearing capacity and ultimate deformation of each specimen were obtained, and load-displacement curves of timber columns were diagramed. Moreover, the load-strain curves of specimens were obtained through the data collected using an extensometer, and the deformation compatibility performance of three different materials was evaluated according to the strain comparison. The test results indicate that the wrapped CFRP strips way and near-surface mounted method can work together to mutually boost the reinforcement effectiveness. The proposed composite reinforcement method can significantly improve the bearing capacity and deformation performance of timber columns. Moreover, the increase in the number of CFRP strips and steel bars generates a marked enhancement in timber columns’ compression performance. The load-strain curves of timber, steel bars, and CFRP strips are in good agreement, and the percentage differences of three different materials’ strains are almost lower than 20%, illustrating that three types of materials have acceptable deformation compatibility performance.