Dynamic behaviour and visco-elastic damage model of ultra-high performance cementitious composite

The dynamic behaviour of ultra-high performance cementitious composite (UHPCC) with compressive strength of 200 MPa with different steel fiber volume fractions was studied under impact using the split Hopkinson pressure bar. Three aspects of the testing: a gimbal device, wave shaping and direct strain measurement, were used to increase experimental accuracy. Results indicate that UHPCC has obvious strain rate effects. The peak stress, peak strain, elastic modulus and the area under the stress–strain curve increase with increasing strain rate. When the strain rate exceeds a threshold value, specimens with and without fibers begin to fracture. At high strain rate the unreinforced specimens fracture into small parts while fiber reinforced ones only have fine cracks on the edges. A visco-elastic damage model of UHPCC is proposed based on a nonlinear visco-elastic model (the ZWT model) and the material damage measured by the ultrasonic wave velocity method.     

Energy absorption behaviors of 3D braided composites under impact loadings with frequency domain analysis

This article presented the energy absorption behaviors and damage mechanisms of 3D braided composites under transverse impact and low‐velocity impact with frequency domain analysis method. The transverse impact tests were contracted by modified split Hopkinson pressure bar with the impact velocities of 13.6, 17.8, and 22.8 m/s. The low‐velocity impact tests were performed by Instron 9250 drop‐weight instrument with the impact velocities ranging from 1 to 6 m/s. The experimental results shown that the peak load, displacement to peak load, total energy absorption increased with the increase of impact velocity. The damage morphologies showed the failure mode of 3D braided composite. Increased with the impact velocity, the failure mode changed from resin crack to fiber breakage. The frequency domain analysis results showed that the amplitude of stress wave increased with the increase of impact velocity, but its corresponding frequency was irrelevant to impact velocity. The different amplitude regions corresponded to different failure mode. POLYM. COMPOS., 37:1620–1627, 2016. © 2014 Society of Plastics Engineers     

Dynamic compressive behavior of basalt fiber reinforced concrete after exposure to elevated temperatures

This paper investigated the dynamic behavior of basalt fiber reinforced concrete (BFRC) after elevated temperatures by using a 100‐mm‐diameter split Hopkinson pressure bar apparatus. Changes in weight and ultrasonic pulse velocity (UPV) were also studied. The results indicate that the weight losses of BFRC before cooling increase with temperature, while a reduction in weight loss value is observed after water cooling. The UPV values of BFRC decrease constantly as temperature increases, and the measured velocities under the same temperature increase with fiber content as temperature exceeds 200 °C. For a given temperature, the strain rate, dynamic strength, critical strain, and impact toughness of BFRC increase with impact velocity. For a given impact velocity, the increasing temperature generally leads to an increase in strain rate and critical strain and results in a decrease in dynamic strength and impact toughness except in the case of 200 °C. At 200 °C, however, a marginal reduction, even an improvement in dynamic strength is observed, and the impact toughness initially decreases, then increases with loading rate when compared with that at room temperature. Basalt fiber is effective in improving the strength performance, deformation capacity, and energy absorption property of concrete after high temperature. Copyright © 2015 John Wiley & Sons, Ltd.     

Foreign Object Damage by Spherical Steel Projectiles in an N720/Alumina Oxide/Oxide Ceramic Matrix Composite

Foreign object‐damage (FOD) phenomena of an N720/alumina oxide/oxide ceramic matrix composite (CMC), impacted by 1.59‐mm spherical chrome steel projectiles up to Mach 1, were assessed at ambient temperature at a normal incidence angle in both partial and full supports. The impact damage was in the form of craters, matrix/fiber tow breakage, compaction of the material, delamination and cone cracks, and their occurrence and degree depended on both impact velocity and type of target supports. The partial support resulted in significant damage with increasing impact velocity, accompanying substantial strength degradation. The presence of tensile stress and presumably stress wave interaction at the backside of a target could have been responsible for greater impact damage in partial support. Although the CMC targets impacted at 340 m/s were on the verge of being penetrated, the targets still survived catastrophic failure retaining about 68% of the as‐received strength, indicative of relatively superior FOD resistance as compared to monolithic ceramic counterparts. A quasi‐static analysis of impact force prediction was made based on the energy balance principle and was validated indirectly using the experimental data on frontal impact damage size.     

The ductile-brittle transition of low-density polyethylene

The impact behavior of a low-density polyethylene was studied with an instrumented Charpy tester. A change from elastic or ductile response to brittle fracture was observed over a small temperature interval, usually within 1°C. This characteristic impact transition temperature (ITT) was highly sensitive to shallow, sharp notches. Whereas an unnotched test bar had a very low impact transition temperature of ?94°C, a razor cut with a depth of only 5 percent of the total thickness raised it to ?4°C. The impact transition temperature was effectively reduced by increasing the cooling rate during specimen preparation and by the addition of nonpolar liquids, On the other hand, impact properties were adversely affected by aging, annealing, and adding other thermoplastics.     

Effect of resin crosslink density on the impact damage resistance of laminated composites

It is generally recognized that fiber-reinforced laminated composites are susceptible to damage resulting from low-velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composite's overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing the molecular weight between crosslinks, which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to low-velocity impacts. A mechanistic study shows that at least two distinct processes occur during an impact event. First, the laminate experiences a local failure, which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Hertzian fracture occurs once at a critical threshold level initiates laminate damage through the development of a spatially configured array of matrix microcracks, which resemble that of a Hertzian cone together with radial cracks. Further damage accumulates in the laminate by inter-ply delamination with the size of delaminated area increasing coincident with the impact load. Systematic changes in resin crosslink density show that both damage initiation and accumulation are affected. However, the maximum resistance for damage initiation occurs at a much higher crosslink density than that measured for damage accumulation.     

Thermal shock effects on the impact resistance,tolerance and flexural strength of alumina based oxide/oxide ceramic matrix composites

In this study the effects of thermal shock on the impact damage resistance, damage tolerance and flexural strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates with balanced and symmetric layup were gradually heated to 1200°C in an air-based furnace and held for at least 30 min before being removed and immersed in water at room temperature. The laminates were then subjected to low velocity impacts via a hemispherical steel impactor. The resultant damage was characterized non-destructively, following which the laminates were subjected to compression tests. Three-point bend tests were also performed to evaluate the effect of thermal shock on the flexural strength and related failure modes of the laminates. Thermally shocked laminates showed smaller internal damage and larger external damage areas in comparison to their pristine counterparts. For the impact energy and resultant damage size considered, the residual compressive strengths for the thermally shocked and pristine laminates were similar.     

Attrition of limestones by impact loading in fluidized beds: The influence of reaction conditions

The extent of attrition associated with impact loading was studied for five different limestones pre-processed in fluidized bed under different reaction conditions. The experimental procedure was based on the measurement of the amount and the particle size distribution of the debris generated upon impact of sorbent samples against a target at velocities between 10 and 45 m/s. The effect of calcination, sulfation and calcination/re-carbonation on impact damage was assessed. Fragmentation by impact loading of the limestones was significant and increased with the impact velocity. Lime samples displayed the largest propensity to undergo impact damage, followed by sulfated, re-carbonated and raw limestones. Fragmentation of the sulfated samples followed a pattern typical of the failure of brittle materials. On the other hand, the behaviour of lime samples better conformed to a disintegration failure mode, with extensive generation of very fine fragments. Raw limestone and re-carbonated lime samples followed either of the two patterns depending on the sorbent nature. The extent of particle fragmentation increased after multiple impacts, but the incremental amount of fragments generated upon one impact decreased with the number of successive impacts.     

高温下钢筋与混凝土粘结锚固性能试验研究

为研究温度对钢筋与混凝土粘结性能的影响,通过制作30个标准立方体试块、8个温度场试件及15个中心拉拔试件,分别完成了室温(20℃)、100℃、200℃、400℃、600℃下标准立方体试块抗压试验与抗拉劈裂试验、拉拔试件温度场试验及中心拉拔试验.分析了高温作用对混凝土抗压强度与抗拉强度的影响,根据温度场试验研究结果,提出一种简易的高温下中心拉拔试验方法,在此基础上研究了高温下钢筋与混凝土粘结性能退化规律.基于Harajli模型综合考虑温度对粘结强度、峰值滑移及试件破坏模式的影响,提出了高温下钢筋与混凝土粘结-滑移本构模型.试验结果表明:高温下混凝土强度、钢筋与混凝土的粘结强度随温度升高整体呈下降趋势,但在100℃时发生陡降现象,高温下钢筋与混凝土的粘结强度变化趋势与混凝土抗拉强度相近.最后提出了高温下钢筋与混凝土的粘结-滑移本构模型,并验证了模型的适用性.     

Effect of the impact angle on the breakage of agglomerates: a numerical study using DEM

The study of agglomerate strength is of vital importance in several industrial applications such as pharmaceutical, detergent and food manufacturing. Agglomerates could experience a size reduction during the production and handling processes due to collisions with other agglomerates or with the moving components and walls as well as during bulk flow due to shear deformation. In this analysis, we focus on the agglomerate damage due to oblique impact on walls, as this is a common damage process during, for example, pneumatic conveying and size reduction in pin mills.

Computer simulations have been carried out using Distinct Element Analysis, where the breakage characteristics of oblique impacts and the effect of the interparticle bond strength have been analysed. The procedure adopted here provides an isotropic and spherical agglomerate (uniform mass distribution and coordination number within radial segments of the agglomerate). The results indicate that the damage ratio (i.e. the number fraction of the broken bonds) depends on the normal component of the impact velocity only, i.e. the tangential component has little effect. However, the position of the clusters produced on impact does depend on the impact angle, which influences the pattern of breakage and in turn the size distribution of the large clusters.     

湿热环境对碳纤维/环氧复合材料的冲击破坏特性影响

为了研究湿热环境对碳纤维/环氧树脂(CFRP)复合材料抗冲击性能的影响,对碳纤维/环氧复合材料层合板进行70℃水浴处理,采用锥头圆柱形弹体对湿热饱和试样和干燥室温试样进行速度分别为45 m/s、68 m/s、86 m/s的冲击,采用激光测速仪测量冲击前后的速度,然后采用超声C扫描检测系统、超景深三维显微系统、扫描电镜(SEM)等方法对试样的冲击破坏进行检测。实验结果表明:随着冲击速度的增加,试样的破坏投影面积增加;在速度较低时,湿热环境对碳纤维/环氧树脂层合板的损伤孔洞面积影响更大;湿热处理之后的碳纤维/环氧树脂层合板层间性能明显降低。     

碳纤维复合材料曲梁冲击后四点弯曲强度及损伤失效模式研究

通过四点弯曲试验和落锤冲击试验,研究了复合材料层合曲梁冲击前后四点弯曲强度及其破坏模式。不仅通过超声C扫描分析了不同内径复合材料层合曲梁试件冲击后的损伤特征,而且分析了冲击损伤对层合曲梁强度及层间最大应力的影响;同时,通过数字散斑相关方法得到复合材料层合曲梁在四点弯曲载荷作用下的变形场以及失效模式。研究结果将为复合材料层合曲梁在飞行器结构中的应用提供有价值的实验依据。     

Impact breakage of spherical, cuboidal and cylindrical agglomerates

A numerical study of the micro-mechanics of breakage of agglomerates impacting with a target wall has been carried out using discrete element simulations. Three agglomerates of different shapes are examined, namely spherical, cuboidal and cylindrical. Each agglomerate consists of 10,000 polydisperse auto-adhesive elastic spheres with a normal size distribution. The effect of agglomerate shape and impact site on the damage of the agglomerates under an impact velocity of 1.0 m/s for an interface energy of 1.0 J/m² is reported. It is found from the simulations that cuboidal edge, cylindrical rim and cuboidal corner impacts generate less damage than spherical agglomerate impacts. The cuboidal face, cylindrical side and cylindrical end impacts fracture the agglomerates into several fragments. Detailed examinations of the evolutions of damage ratio, number of wall contacts and total wall force indicate that the size of the contact area and the rate of change of the contact area play important roles in agglomerate breakage behaviour. Internal damage to the agglomerate is closely related to the particle deceleration adjacent to the impact site. However, the local microstructure may not be a decisive factor in terms of the breakage mode for non-spherical agglomerates.     

板坯连铸电磁制动下结晶器内金属流场的研究

以水银为实验工质,通过物理模拟实验研究电磁制动下,板坯结晶器内钢水流动规律. 实验通过超声多普勒测速仪获取不同电磁制动方式[单条型电磁制动(EMBr-Ruler)和流动控制结晶器(FC Mold)]、水口浸入深度和结晶器宽度下的液流特征. 结果表明,在EMBr-Ruler或FC Mold II电磁制动下,结晶器窄壁处形成"液流通道",不利于结晶器内快速形成向下的活塞流. 水口出口距下区磁场越近或被磁场所覆盖,则通道效应减弱; 结晶器宽度增大,亦可降低该通道内液流的冲击效果. FC Mold II下,液面流速和湍流度比EMBr-Ruler时低[液面水平流速最大值分别为0.155 (Case 1)和0.134 m/s (Case 2)],且加大水口浸入深度,可降低液流撞击结晶器窄壁的水平流速[其最大值0.071 (Case 2)和0.068 m/s (Case 3)]. 结晶器宽度的变化不改变水口浸入深度对结晶器流场的影响规律.     

三维正交机织复合材料弹道冲击实验及破坏模式

本文用钢芯弹对三维机织复合材料作弹道冲击测试。得到了弹体的入射速度和剩余速度,比较了常见几种材料的弹道性能评价参数的差异,并考察侵彻破坏模式和靶体最后的损伤破坏形态。在300-800m/s冲击速度范围下观测了材料的冲击破坏形态,发现机织复合材料受弹面和子弹出射面破坏形态不一样,受弹面主要是以纤维的压缩、剪切破坏以及基体开裂为主,出射面以纤维的拉伸、厚度方向的纱线断裂为主要破坏模式。通过对破坏模式和形态的分析,可以帮助建立更加准确的破坏准则,从而在设计抗弹材料时起到一定的作用。     

Field indicator of chloride penetration depth

Chloride-induced corrosion of steel reinforcement is causing serious damage to many concrete structures. A number of methods to evaluate the chloride penetration into concrete have been developed. The most common practice in measuring the chloride profile is very time consuming. A simple colourimetric method of measuring the depth of chloride penetration into concrete by spraying a 0.1-N AgNO₃ solution is very attractive. But some questions have been raised about its sensitivity. In this article, the results from colourimetric tests and the corresponding quantity of chloride detected at the colour-change boundary, determined for more than 70 concrete samples, are given. The magnitude and variation of chloride concentrations are compared with those reported in the literature and evaluated in relation to typical chloride threshold values. In addition, a relationship between the colourimetric penetration depth and the charge passed during testing to ASTM C1202 is shown.     

Numerical modelling of the breakage of loose agglomerates of fine particles

This paper presents a numerical study of the breakage of loose agglomerates based on the discrete element method. Agglomerates of fine mannitol particles were impacted with a target wall at different velocities and angles. It was observed that the agglomerates on impact experienced large plastic deformation before disintegrating into small fragments. The velocity field of the agglomerates showed a clear shear zone during the impacts. The final breakage pattern was characterised by the damage ratio of agglomerates and the size distribution of fragments. While increasing impact velocity improves agglomerate breakage, a 45-degree impact angle provides the maximum breakage for a given velocity. The analysis of impact energy exerted from the wall indicated that impact energy in both normal and tangential directions should be considered to characterise the effects of impact velocity and angle.     

Fragmentation of a Steel Sphere by a High‐Velocity Impact on a Highly Porous Thin Bumper

The number‐size distribution of fragments arising during fragmentation of a steel projectile upon its normal impact on a thin bumper with a velocity of 2.5–7.3 km/sec is obtained. High‐porous sheets made of fine copper powder are used to reduce the number of large fragments of the bumper in the debris cloud behind the bumper. As the impact pressure increases from 30 to 160 GPa, the character of fragmentation of the steel sphere changes from irregular fragmentation near the projectile‐failure threshold to more uniform fragmentation with the maximum impact velocity. The main result of the experiment is the observation that the density distribution of the number of fragments increases and does not tend to zero as the size of fragments decreases. The data are analyzed within the framework of the Grady and Gilvarry exponential distributions.     

Ball Impact Response of Unstrengthened and Chemically Strengthened Glass Bars

Ball impact experiments were conducted at impact velocities of 52–345 m/s on unstrengthened and chemically strengthened lithium aluminosilicate glass bars to assess the damage propagation characteristics. The damage was captured by high‐speed imaging at frame rates up to 500 000 frames per second (fps). Upon impact, the damage front in the unstrengthened glass reached a maximum velocity of 1626–2135 m/s, and rapidly fell to zero within a short distance. On the contrary, the damage front in the strengthened glass reached an initial velocity of 1791–2275 m/s, but then stabilized to a constant velocity of 1920 m/s until the entire glass bar was consumed. In addition, the cascading release of stored energy due to strengthening led to self‐sustained damage propagation preferentially within the outer layers of the glass bar (predominantly within the compressive zones) at a higher rate than in the interior region (dominated by residual tension). This is counter to what has been traditionally reported in the literature. Stress wave reflection from the rear of the unstrengthened bar caused tensile cracking (i.e., spallation), but the ultrahigh residual compression in the strengthened bar prevented similar damage initiation. Finally, it is suggested that strengthened glasses are most suitable as intermediate layers in laminate window panels for impact applications.     

七个泉油田套损机理研究

七个泉油田所处的柴达木盆地为干燥的内陆盐湖盆地,地层普遍具有“高盐、高矿化度”特点,受各种因素影响造成油水井套损严重.截止到2011年6月,套损井38口,占开发井总数15.7％.这些井的损坏已影响到油田的正常生产.通过对七个泉油田多臂井径成像、井壁超声电视测井和铅模测量成果对套损类型进行了研究,采用通过套损综合图与多臂并径叠合图对地质因素进行了分析,最后对影响套损的工程和开发因素进行研究.研究表明,可将七个泉油田套损形态分为套管缩径、剪切变形和剪切错断两类,井身“狗腿”处对套损有一定关系,加强固井和提高套管钢级对套损影响不大,而注水压力是造成套损主要开发因素.