Penetration and failure of lead and borosilicate glass against rod impact

This paper presents the experimental design and results for gold rod impact on DEDF (5.19 g/cm³) and Borofloat (2.2 g/cm³) glass by visualizing simultaneously failure propagation in the glass with a high-speed camera and rod penetration with flash radiography. At a given impact velocity, the velocity of the failure front is significantly higher during early penetration than during steady-state penetration of the rod. For equal pressures but different stress states, the failure front velocities determined from Taylor tests or planar-impact tests are greater than those observed during steady-state rod penetration. The ratio of average failure front velocity to rod penetration velocity decreases with increasing impact velocity (v_p) in the range of v_p=0.4–2.8 km/s. As a consequence, the distance between the rod tip and the failure front is reduced with increasing _vp$v_{\mathrm{p}}$. The Tate term R_T increases with impact velocity.     

Study on energy properties and failure behaviors of heat-treated granite under static and dynamic compression

Abstract

The material testing machine and the split Hopkinson pressure bar (SHPB) were adopted, respectively, to conduct the static and dynamic compression tests on granite specimens heat treated by different temperatures. The effects of strain rate and heat-treatment temperature on the mechanism of energy evolution of the specimen during deformation and failure process were studied. The results show a significant strain rate effect on the granite, with the energy dissipation density increasing with increasing impact velocity (or strain rate), regardless of the treatment temperature. The specimens heat treated at 300?°C and 700?°C have the minimum and maximum energy dissipation densities, respectively. The specimen in the SHPB tests easily broke into pieces or even powder; while under static compression, only macroscopic fracture surfaces and spalling phenomenon on the specimen were detected. The energy dissipation density is inversely proportional to the compressive strength of the specimen. The rate of energy dissipation change is defined, which can be used to identify the stages in the deformation process of rock and to determine the position of the failure point in the stress-strain curve. For both the dynamic and static compression tests, the value of energy utilization ratio is relatively low, with a maximum value of about 35%.     

Failure and penetration response of borosilicate glass during short-rod impact

A series of reverse ballistic experiments reveal further properties of the failure front (FF) associated with the penetration of borosilicate glass by a gold rod. Importantly, the FF ceases to propagate a short time after the rod is fully eroded. The rods, 1 mm in diameter, were short (5–11 mm). The glass targets were 20-mm diameter cylinders, 60-mm or 100-mm long. Impact velocities varied between 1 and 2 km/s. The impact and penetration process was observed with five flash X-rays and a 16-frame high-speed optical camera. A FF propagates from the impact region. The velocity of FF propagation is an increasing function of the impact velocity. The termination of the FF can reasonably be predicted in most cases with a simple model that assumes a rarefaction wave, originating at the time of complete rod erosion, propagates from the bottom of the penetration channel to the FF at a speed equal to the bulk wave speed of undamaged glass.     

Characterization of heterogeneous materials under shear loading at finite strain

This article presents a homogenization procedure to predict the effective shear response of heterogeneous materials at large deformation. Assuming local periodicity, heterogeneous microstructure is identified by a representative volume element that is subjected to an equivalent macroscopic deformation field. The energy balance and periodicity conditions are considered to relate macro and micro-stress fields. Based on the symmetrical planes of the microstructure and local periodicity, it is shown that the analysis of one-quarter of three-dimensional representative volume element is enough to evaluate the effective shear response at finite deformation. A computational method is subsequently developed to obtain the shear response of heterogeneous microstructures. The homogenization procedure is implemented to evaluate shear response of two specific heterogeneous materials, elastomeric composite and reinforced viscoelastic fluid. The performance is successfully verified by comparison of the deformation in the macroscopic level to the response of a homogenized cell.     

Failure criterion for laminated glass under impact loading and its application in finite element simulation

A failure criterion for laminated glass in case of impact is presented. The main idea of this criterion is that a critical energy threshold must be reached over a finite region before failure can occur. Afterwards crack initiation and growth is based on a local Rankine (maximum stress) criterion. The criterion was implemented in an explicit finite element solver. Different strategies for modeling laminated glass are also discussed.To calibrate the criterion and evaluate its accuracy, a wide range of experiments with plane and curved specimens of laminated glass were done. For all experiments finite element simulations were performed. The comparison between measured and simulated results shows that the criterion works very well.     

Transverse shear modulus and strength of honeycomb cores

The transverse shear mechanical behavior and failure mechanism of aluminum alloy honeycomb cores are investigated by the single block shear test in this paper. The transverse shear deformation process of honeycomb cores may be approximately categorized into four stages, namely elastic deformation, plastic deformation, fracture of cell walls and debonding of honeycomb cores/facesheets. The elastic deformation of unit cell under transverse shear displacement is also investigated by the finite element method, and the result shows that the bending deformation of the cell walls is similar to that of the cantilever beam. In order to precisely predict the equivalent transverse shear modulus and strength, not only shear deformation but also bending deformation of cell walls should be considered. Therefore, in the present paper, the equivalent transverse shear modulus and strength are predicted by application of the cantilever beam theory and thin plate shear buckling theory in conjunction with simplifying assumption as to the displacement in the cores. It is concluded that the contribution of bending deformation of cell walls to equivalent transverse shear modulus and strength is obvious with the decreasing height of cell walls.     

Modes of dynamic shear failure in solids

The technique of loading edge cracks by edge impact (LECEI) for generating high rates of crack tip shear (mode-II) loading is presented. The LECEI-technique in combination with a gas gun for accelerating the impactor is used to study the high rate shear failure behaviour of three types of materials. Epoxy resin (Araldite B) shows failure by tensile cracks up to the highest experimentally achievable loading rate; steel (high strength maraging steel X2 NiCoMo 18 9 5) shows a failure mode transition: at low rates failure occurs by tensile cracks, at higher rates, above a certain limit velocity, failure by adiabatic shear bands is observed; aluminum alloy (Al 7075) shows failure due to shear band processes in the high rate regime, but this failure mode is observed over the entire range of lower loading rates, even down to quasi-static conditions. Characteristics of the failure modes are presented. When transitions are observed in the failure process from tensile cracks to shear bands the limit velocity for failure mode transition depends on the bluntness of the starter crack the failure is initiated from: The larger the bluntness of the starter crack the higher the critical limit velocity for failure mode transition. The data indicate that adiabatic shear bands require and absorb more energy for propagation than tensile cracks. Aspects of the energy balance controlling mode-II instability processes in general are considered. Effects very different than for the mode-I instability process are observed: When failure by a tensile crack occurs under mode-II initiation conditions, a notch is formed between the initiated kinked crack and the original starter crack, and, at this notch a compressive stress concentration builds up. The energy for building up this stress concentration field is not available for propagation of the initiated kinked crack. The energy density of a mode-II crack tip stress field, however, when compared to an equivalent mode-I crack tip field, is considerably larger, and, consequently, the remaining driving energy for any mode-II initiated failure process, nevertheless, is higher than for the case of equivalent mode-I initiation conditions. Furthermore, mode-II crack tip plastic zones are considerably larger than equivalent mode-I crack tip plastic zones. Consequently, validity conditions for linear-elastic or small scale yielding failure behaviour are harder to fulfill and possibilities for the activation of nonlinear high energy ductile type failure processes are enhanced. Speculations on how these effects might favour failure by high energy processes in general and by shear bands processes in particular for conditions of high rate shear mode-II loading are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental studies on shear failure of freeze-bonds in saline ice: Part I. Set-up, failure mode and freeze-bond strength

The strength of freeze-bonds in thin saline ice has been investigated through two series (in 2008 and 2009) of experiments in the Hamburg Ship Model Basin (HSVA) as a function of the normal confinement (σ), the submersion time (Δt) and the initial ice temperature (T_i). The freeze-bonds were mostly formed in a submerged state, but some were also formed in air. The experimental set-up was improved in the 2009 experiments. In 2008 a ductile-like failure mode dominated (78%), whereas in 2009 the brittle-like dominated (93%). We suggest that this is a combined ice and test set-up effect. The 2009 experimental procedures allowed for careful sample handling giving higher strength and it was softer. Both these things should provoke a more brittle-like force-time response. The average freeze-bond strength in brittle-like samples was around 9 kPa while in ductile-like samples was around 2 kPa. The maximum freeze-bonds strength were measured for short submersion times, from 1 to 20 min, and reached a maximum value of 30 kPa.A Mohr-Coulomb like failure model was found appropriate to represent the freeze-bond shear strength as function of the normal confinement. Saline freeze-bonds in saline water had cohesion/friction angle around 4 and 1.4 kPa/25° for the brittle- and ductile-like samples respectively, which fitted well with previously published data.A bell-shape dependence for τ_c vs. Δt was found, which agreed with the predictions by Shafrova and Høyland (2007). We suggest that this is essentially a freeze-bond porosity effect and propose three phases in time with subsequent cooling, heating and equilibrium to account for this trend. Qualitative experiments showed that the submersion time and the initial ice temperature were strongly coupled.To account for the connection between contact time, block dimensions and ice properties and the freeze-bond strength, dimensionless number were used. Fourier scaling was more appropriate than Froude scaling to scale freeze-bonds.The freeze-bonding made in air developed fast (in less than 30 s) when the ice was cold and dry, but no freeze-bonding occurred for the same contact times when the ice was warm and wet.     

Modelling of damage and failure of glass/epoxy composite plates subject to impact fatigue

An investigation has been carried out to study the impact fatigue damage of glass/epoxy laminated composites. Accumulation of damage, such as matrix cracking, delamination and fibre breakage, with repeated impact of the composite material may reduce the overall stiffness. These damage modes have been combined in a very complicated way to describe damage growth and fracture. A model is proposed for characterising the damage as a function of the normalised impact number. The scalar variable D, which characterises the material damage, is written as a function of the life duration β, using a modified form of the Mankowsky empirical law [Int J Solids Struct 32(11) (1995) 1607]. The macroscopic failure mode and the internal damage in laminated specimens of glass/epoxy as a consequence of impact fatigue are analysed at different levels of incident impact energy. The impact fatigue tests have been conducted on an apparatus built in our laboratory.     

DEM investigations of failure mode of sands under oedometric loading

It will be practically useful to explore the evolutions of the failure modes of sand grains within a sand specimen subject to compression for the particle breakage research. This paper attempts to deal with this challenge by conducting a discrete element method (DEM) simulation study on oedometric compression of two kinds of sands (spherical and non-spherical particles). In this study, particle morphologies reconstructed by the spherical harmonic (SH) analysis were created using the agglomerate method, and the micro-parameters used to define the contact model and the properties of walls and balls were adopted based on the single particle crushing tests. The effects of particle shape on the crushing behavior of granular materials and on the evolutions of failure modes of sand grains were captured, and the experimental data was used to evaluate the feasibility and reliability of the proposed DEM modelling strategy. The simulation results show that particle shape affects not only the number, type and orientation of cracks but also the evolution of the particle failure modes. The failure mode of chipping is the most common way to crush for both spherical and non-spherical particles. The particles that have less aspect ratio, sphericity and convexity are more likely to experience the failure mode of comminution. These findings shed light on the key role of particle shape in the investigation of the failure mode of sand grains and facilitate a better understanding of grain-scale behavior of granular materials.     

二维C/SiC复合材料在冲击拉伸下的损伤过程

利用分离式霍普金森拉杆实验装置(SHTB)和超高速照相机,对二维C/SiC复合材料进行了冲击拉伸力学性能实验研究,同时结合其宏观力学行为,分析了在冲击拉伸载荷作用下的损伤破坏过程。结果表明:材料的应力-应变曲线呈明显的非线性特征,其内部损伤破坏和裂纹扩展过程分为四个阶段:损伤积累于第一阶段,裂纹起源于第二阶段,屈服失效于第三个阶段,快速扩展于第四个阶段。     

Analysis of composite skin/stiffener debounding and failure under uniaxial loading

In this paper, damage mechanisms in the composite bounded skin/stiffener constructions under monotonic tension loading are investigated. The approach uses experiments to detect the failure mechanisms, two and three-dimensional stress analysis to determine the location of first matrix cracking and computational fracture mechanics to investigate the potential for cracks and delamination growth. The laminates strength and damage mechanisms obtained from both experimental and finite elements analysis are presented for several laminates lay-up configurations. Observations on the performed experiments show matrix crack initiation and propagation in the skin and near the flange tip, causing the flange to almost fully debounded from the skin in some cases, interlaminar debounding and fiber breakage up to the failure of the components. The finite elements analysis is also show that the matrix cracks are initiated in the first skin layer for most of the cases. With increasing the applied load the matrix cracks are propagated through the thickness to reach the next layer and causes delamination between the two layers. With increasing the applied load this delamination is propagated up to the occurrence of unstable delamination growth or the first fiber breakage known as the final failure of the component. The obtained experimental failure loads are compared with those calculated by the finite elements analysis.     

Experimental and numerical study of cylindrical triaxial test on mono-sized glass beads under quasi-static loading condition

This paper aims at studying the shear behavior of homogeneous granular materials by conventional triaxial test. The work is performed both in laboratory tests and by discrete element method simulations. Conventional triaxial tests are performed on glass beads packing, while a cylindrical rigid wall boundary condition based on lame formula and a series of procedures are proposed to simulate the conventional triaxial test. The experimental results on dry and saturated glass beads samples have been studied to find out the effect of saturation condition on the shear behavior. The comparisons between experimental and numerical results show that the numerical model can reproduce deviatoric curves satisfactorily in experimental conditions as long as experimental sample remains cylindrical. It correctly describes the volumetric strains of a numerical sample up to the peak value. Additionally, a parametric study on the influence of main micromechanical parameters has been carried out, which has been compared to experimental tests with glass beads of different textures. The comparison highlights the significant effect of friction coefficients and rolling resistance coefficients on global behavior of granular materials.     

考虑时间和温度影响的复合材料开孔层板压缩失效分析

鉴于复合材料性能的时间-温度相关性,对固化后的树脂基体(5228A)进行了动力学分析(Dynamic Mechanical Analysis,DMA),得到了试验窗口中不同温度下的储能模量曲线片段,利用封闭平移(Closed Form Shifting,CFS)方法对其进行扩展,建立了主曲线并得到了曲线片段对应的平移因子。根据加速试验方法(Accelerated Testing Methodology,ATM),分别以两种铺层的准各向同性开孔层板(CCF300/5228A)为研究对象,建立了匀应变率(Constant Strain Rate,CSR)压缩强度主曲线。借助微距拍摄和超声波C扫描,对其渐进损伤过程和不同温度下的破坏形貌进行了观测。结果表明:即使温度低于玻璃态转变温度,树脂基体动态力学性能也会随时间的增加而降低;单层较厚的开孔层板压缩强度对时间和温度更加敏感,而单层较薄的开孔层板则具有更好的损伤容限性能;温度升高、加载速率降低时,开孔层板压缩最终破坏主导因素从分层损伤趋于纤维屈曲。     

Damage mechanisms and failure modes in cross-ply laminates under monotonic tensile loading: The influence of fiber sizing

In this study, the effect of fiber-matrix interphase on the damage modes and failure mechanisms in (0, 90₃), cross-ply graphite-toughened epoxy laminates is investigated. Two material systems (designated as 810 A and 810 O) with the same fiber and same matrix, but with different fiber sizings, were used to study the effect of the interphase. The system designated as 810 A contained an unreacted Bisphenol-A (epoxy) sizing, while a thermoplastic polyvinylpyrrolidone (PVP) sizing was used in the 810 O system. Damage accumulation in the cross-ply laminates under monotonic tensile loading was monitored using edge replication, x-ray radiography, acoustic emission, optical and scanning electron microscopy. Results indicate that the fiber-matrix bond strength is lower in the 810 O system compared to the 810 A system. Transverse matrix cracking initiates at a significantly lower stress level in the 810 O laminate. The 810 O laminates also exhibit longitudinal splitting, while the stronger bonding suppress this damage mode in the 810 A laminates. Numerous local delamination occur on the 0/90 interface at the intersection of 0 and 90 degree ply cracks, in the 810 O laminates. These are absent in the 810 A laminates. The failure modes are also different in the two material systems used in this study. The 810 A laminate exhibits a brittle failure, controlled by the local stress concentration effects near broken fibers. In the 810 O laminates, the presence of longitudinal splits result in the reduction of stress concentration effects near fibe fractures. This results in a global strain controlled failure in the 810 O system. It is concluded that the presence of different fiber sizings result in different damage modes and failure mechanisms in the cross-ply laminates used in this study.Research Associate, Research Assistant, Alexander Giacco Professor and Professor respectively.     

Experimental fracture investigation of CNT/epoxy nanocomposite under mixed mode II/III loading conditions

For the first time, the brittle fracture of epoxy‐based nanocomposite reinforced with MWCNTs (multi‐walled carbon nanotubes) and subjected to mixed mode II/III loading conditions is investigated. This experimental investigation is carried out using a newly developed test configuration. Araldite LY 5052 epoxy, which is a resin frequently used in aerospace industry, is utilized to fabricate pure epoxy and nanocomposite test specimens with two different MWCNTs contents of 0.1 and 0.5 wt%. The obtained experimental results reveal that adding MWCNTs to epoxy resin up to 0.5 wt% improves the fracture toughness under pure mode II and pure mode III loading with an increasing trend. This is while the improvement under mixed mode II/III loading is reduced by adding nanotubes more than 0.1 wt%. To justify the variations of fracture toughness in terms of nanoparticles content, SEM (scanning electron microscopy) photographs of the fracture surfaces of the specimens in the vicinity of the initial crack front are prepared. Additional fracture mechanisms caused by adding carbon nanotubes are discussed in detail based on the provided SEM images.     

Multiaxial fatigue and failure analysis of helical compression springs

Multiaxial fatigue criteria are applied to the analysis of helical compression springs. The critical plane approaches, Fatemi–Socie and Wang–Brown, and the Coffin–Manson method based on shear deformation, were used to predict fatigue lives of the springs under constant amplitude loading. Experimental fatigue lives are compared with the multiaxial fatigue criteria predictions. The stress analysis was carried out in the finite element code ANSYS, and the multiaxial fatigue study was performed using the fatigue software nCode. A failure analysis was conducted in order to determine the fatigue crack initiation point and a comparison of that location with the most damaged zone predicted by the numerical analysis is made. The Fatemi–Socie critical plane approach gives a good prediction of fatigue life. While the Wang–Brown criterion overestimates spring fatigue life, the Coffin–Mason model gives conservative results.     

Damage development in carbon/epoxy laminates under quasi-static and dynamic loading

The contrasting characteristics of damage evolution have been examined in a multidirectional carbon/epoxy composite laminate (IM7/8551-7) subjected to both quasi-static and dynamic loading. Our experiments were performed on bend-test bars that were loaded either in ‘supported' four-point bending or under ‘unsupported' conditions with a Hopkinson pressure bar to induce dynamic loading. We found differences in the damage that occurred in specimens loaded by the two techniques, in terms of the number of cracks and the length of the cracks. In the case of quasi-static loading, there were many matrix cracks within individual plies and only a few delamination cracks between plies; the maximum ratio of numbers of matrix to delamination cracks observed was 6:1. Despite their small number, the delamination cracks had a greater total length than the matrix cracks, and specimen failure occurred as a result of delamination crack propagation. During dynamic loading, the ratio between numbers of matrix and delamination cracks was 3:1, and in this case the ratio between the total crack lengths was unity. A quantitative assessment of damage induced during quasi-static bending was made from specimen stiffness results. Using simple beam theory and knowing the location of the damage, we correlated beam stiffness to the materials effective elastic modulus. We found that the composite's effective modulus decreased rapidly with small amounts of initial damage, but that subsequent increases in damage decreased the effective modulus at a much lower rate.     

Fracture mechanisms of wood/polyester laminates under quasi-static compression and shear loading

There is increasing use of natural fiber/polymer composites as alternatives to traditional structural materials like concrete and metals and to the inorganic fibers like carbon. While the fracture mechanisms during crushing of synthetic fiber/polymer composites have been thoroughly studied, limited information is available on post-fracture investigation and identification of the dominant fracture mechanisms of wood/polyester composites. In this study laminates of Douglas-fir veneer were fabricated using a catalyzed polyester resin and their potentials as energy absorbers have been investigated and discussed. Factors for this study were (i) laminates symmetry (face layers of 0° or 90°), (ii) lay-up balance (balanced and unbalanced) and (iii) number of lamina (8, 11, and 12). Samples were tested under quasi-static Combined Loading Compression (CLC) and their compressive performances were compared to control specimens using glass fiber as reinforcement. Results indicated that the effect of symmetry on compressive properties of wood veneer/polyester laminates was significant with laminates with face layers of 90° and core layers of 0° had the highest deflection to failure. Increasing the wood/polyester laminate thickness enhanced their energy absorbing ability by bringing more fracture mechanisms into play but it noticeably reduced the laminates compressive modulus. Despite the brittle failure of glass fiber composites wood laminates exhibited a progressive fracture mechanisms with shear buckling as the dominant mode of failure in symmetric samples. This progressive failure with high energy absorbing ability make wood/polyester laminates a good candidate to be used as an energy absorber structure where high deflection to failure and longer failure time are required.     

Fatigue characterization and modeling of 30CrNiMo8HH under multiaxial loading

Fatigue of 30CrNiMo8HH steel alloy has been studied thoroughly. Uniaxial cyclic tension-compression, cyclic torsion, proportional tension-torsion, and non-proportional tension-torsion at various strain ratios have been considered. Tests were performed at standard laboratory conditions on solid and tubular specimens machined from an actual driveline component. Fractography was conducted on the tested samples to investigate the fatigue mechanisms involved. Under torsion, large numbers of early micro cracks were found to emanate from the sample's surface, with a few propagating into very long longitudinal cracks. In biaxial tests, cracks tend to propagate into the gauge reducing the cross section area. A strain energy density fatigue parameter has been employed for life prediction of the material under uniaxial and biaxial loading. The life prediction method is based on two different cracking mechanisms that agree with the observed cracking mechanisms in torsion and biaxial loading of 30CrNiMo8HH steel alloy studied here. Energy-based properties are obtained and the predicted lives are compared to experimental results. The results obtained agree well with experiments.