饱和强化粘塑性介质的动态损伤与破碎的细观机制

探讨了饱和强化粘塑性介质中的动态损伤演化规律，给出了损伤演化微分方程。利用动态破碎的能量描述，确定了动态破碎碎块平均尺寸与材料孔隙度的定量关系。最后给出了动态损伤与破碎规律的数值结果。     

孔洞材料动态破碎的细观机理

采用Ｃ－Ｈ－Ｊ动态微孔长大细观模型研究孔洞材料中的动态破碎的细观机理，导出了微孔动态演化方程及动态破碎尺寸与材料孔隙度的关系，利用本文给出的动态破碎的细观机制可对破甲弹的破甲机理给出合理的解释。     

冲击载荷下不同尺寸煤岩动力学分析及损伤特性研究

为探究冲击载荷作用下不同尺寸煤岩的动态力学及损伤特性，利用分离式霍普金森压杆系统对直径50 mm长度分别为15 mm, 20 mm, 25 mm, 30 mm, 35 mm, 40 mm, 45 mm和50 mm的煤岩开展了冲击压缩试验，通过分析应力-应变曲线特征得出应变率、峰值应力和动态弹模随尺寸变化的关系，并基于能量耗散规律提出煤岩受载全过程动态损伤指标K,同时与Weibull分布结合D-P准则得出的损伤变量D_w进行对比，探究不同尺寸煤岩的动态损伤特性。结果表明：不同尺寸煤岩的动态应力-应变曲线包含弹性、塑性和塑性软化3个阶段。随煤岩尺寸的增加，应变率和峰值应力呈减小趋势，动态模量为线性增加；动态损伤指标K是煤岩弹性阶段结束后的瞬时破碎耗能与完全破坏时对应总破碎耗能的比值；动态损伤指标与损伤变量的曲线形态具有相似性，变化趋势具有一致性，表明了K的合理性。指标K的最大损伤值更稳定、更符合破坏状态假设，说明了其具有一定的适用性。     

不同初始静载混凝土轴向拉伸试验研究

为研究混凝土材料的动态性能,利用MTS-810NEW液压伺服试验机对尺寸为100 mm×100 mm×510 mm棱柱体混凝土材料试样进行了初始静态荷载为0~20 k N的动态轴向拉伸试验,研究了混凝土材料经历不同初始静态荷载后的动态拉伸破坏特征、应力应变关系和动态抗拉强度。结果表明:荷载值由静态过渡到动态荷载时,混凝土材料的动弹性模量发生较大变化,且随着初始静态荷载值的增加,混凝土材料动弹性模量有增大趋势;混凝土材料动态应力应变关系曲线中,峰值应力所对应的应变值与初始静态荷载值无关;随着初始静态荷载的增加,混凝土材料动态拉伸破坏断面面积逐渐增大,且粗骨料被拉断的数目随着初始静态荷载的增加而先增加,后趋于平稳;随着初始预加静态荷载值的增加,混凝土材料的动态轴向拉伸强度先增加,然后趋于稳定。     

初始损伤对不同钢组织缺口试样解理断裂韧性的影响

通过对两种钢热处理后得到的三种材料组织的缺口试样在常温下进行了不同预加载荷的四点弯曲正反弯实验，以在缺口前引入不同的微孔洞损伤量，而后通过高温回火处理消除残余应力和加工硬化。随后在-196℃低温下进行弯曲断裂实验，通过力学参数测量和微观观察就初始损伤对不同钢组织缺口试样解理断裂韧性的影响进行了实验研究。研究表明，对于材料A和C，随预加载荷比P0／Pgy的增加，材料中的初始损伤量增加，从而使材料的缺口解理断裂韧性降低。其原因是初始损伤的长条形孔洞附近产生了局部高应力应变集中，促使了解理断裂过程的进行。细晶粒A材料的初始损伤量大，韧性下降幅度大。对于材料B，初始损伤主要为小尺寸的球形孔洞，并且其损伤量远小于材料A和C，故B材料的缺口解理断裂韧性几乎不随预加载荷比变化。     

循环冲击荷载下红砂岩细观损伤演化及能耗特性研究

为研究循环冲击荷载下红砂岩的细观损伤演化规律和能耗特性，利用带有围压装置的分离式霍普金森压杆(SHPB)装置和核磁共振仪，对红砂岩试件进行不同冲击气压(0.6、0.7、0.8、0.9 MPa)和围压(1、2 MPa)条件下的循环冲击试验，对冲击后岩石孔隙度、T₂谱曲线、核磁共振成像及能耗规律进行分析，同时提出一种新的损伤度计算方法。研究结果表明：总体上看，红砂岩孔隙度随循环冲击次数的增加呈指数增长趋势；前几次冲击孔隙度变化速率较缓，当冲击损伤累积到一定范围，岩石损伤加剧，表现为孔隙度增长幅度迅速增加；随着循环冲击次数的增加，T₂谱曲线起止点向两端移动，新孔隙不断萌生，不同尺度孔隙相互转化，孔隙尺寸和数量明显增长，中型孔隙对孔隙度增长起重要作用；MRI成像显示，随着冲击次数的增加，中心区域明亮斑点由小而分散向大而聚集转变，伴随有明亮条纹出现；提出一种损伤度定义新方法，明确了未损伤和完全损伤2种极端状态；红砂岩累积损伤度与累积比能量吸收值呈指数增长关系，能时密度随冲击次数的增加呈现出先增大，后减小，再增大的趋势，此外，能时密度与损伤度变化值呈正线...     

淀粉/PP基发泡缓冲包装材料动态冲击性能研究

目的 研究不同初始应变率和湿度条件下,淀粉/PP基发泡缓冲包装材料的动态冲击性能,并构建基于湿度及应变率的动态本构模型。方法 应用冲击试验机对淀粉/PP基发泡缓冲包装材料进行不同初始应变率及相对湿度下的动态冲击实验,得到其应力–应变曲线,并构建动态本构模型。结果 动态冲击下,材料的应变率效应较为明显,该材料的应力和能量吸收随着初始应变率的增加而增加。在相对湿度为50%的条件下,当应变为0.6时,随着初始应变率由30 s⁻¹分别增加至34.6、38.6 s⁻¹,材料的应力分别增加了36.1%和50.4%,能量吸收分别增加了25.8%、36.4%。该材料对环境湿度较为敏感,该材料动态冲击力学性能随着相对湿度的增加显著降低,在初始应变率为38.6 s⁻¹条件下,当应变为0.6时,随着相对湿度由50%增加到70%、90%,该材料的应力分别下降了9.7%和11.3%。另外,构建了基于初始应变率和湿度的淀粉/PP基发泡材料的动态冲击本构模型。结论 初始应变率与湿度对材料的缓冲性能有一定的影响。基于初始应变率和相对湿度的动态冲击本构模型,通过实验进行了验证,实验数据和本构模型一致性较好,该本构模型可用于预测该材料的动态冲击应力–应变曲线。     

内部爆炸加载条件下钢管变形与破碎研究

用高速摄影研究了钢管膨胀历程,并对破碎机制进行了分析,结果表明,材料动态塑性是决定破片最终速度的主要因素,圆管膨胀时外表面形成以剪切为主的裂纹,并向内扩展,接近内表面的绝热剪切带伴随着圆管的膨胀而扩展,并未出现失稳现象,但在正应力的作用下成为优先的断裂通道,决定破片的尺寸和形态,中碳Si-Mn贝氏体钢与50SiMnVB钢的破碎性能相近,但贝氏体钢动态塑料性很更好,破碎的膨胀量较大,更有利于提高破片初速,是弹比较理想的候选材料。     

准脆性材料中椭圆形微裂纹的生长与演化

微裂纹的生长与演化是导致准脆性材料损伤及破坏的本质,本工作对承载过程中准脆性材料内部的椭圆形微裂纹的生长与演化规律进行研究。采用复势函数求解受远场载荷作用下代表性单元中椭圆形微裂纹的变形,讨论了椭圆形微裂纹初始取向的变化对微裂纹尺寸增长的影响,并结合微裂纹扩展准则推导出损伤开始时的临界应力条件。基于翼型裂纹扩展过程的能量守恒关系,建立了损伤阶段的本构关系。     

炸药单耗对赤铁矿爆破块度的影响规律数值模拟研究

通过在连续-非连续单元方法(CDEM)中引入朗道点火爆炸模型及岩体塑性-损伤-断裂模型,实现了赤铁矿爆破破碎过程的模拟。提出了5个评价爆破后块度分布特征的指标,分别为平均破碎尺寸(d50)、极限破碎尺寸(d90)、块体不均匀系数(d90/d50)、系统破裂度(Fr)及大块率(Br)。基于CDEM方法及上述5个评价指标,分析了改变炮孔直径、改变间排距等两种改变炸药单耗的方式对赤铁矿爆破块度的影响规律。数值计算结果表明:随着炸药单耗的增大,赤铁矿的破碎尺寸逐渐减小;相同炸药单耗情况下,改变炮孔直径的破碎效果略优于改变间排距的破碎效果。在双对数坐标下,随着炸药单耗的增大,平均破碎尺寸(d50)及极限破碎尺寸(d90)均线性减小;采用衰减型幂函数进行了拟合,给出了平均破碎尺寸(d50)及极限破碎尺寸(d90)与炸药单耗间的函数关系。随着炸药单耗的增加,块体不均匀系数(d90/d50)及系统破裂度(Fr)均逐渐增大,而大块率(Br)则迅速减小。当炸药单耗大于0.25 kg/t时,大块率(Br)已经小于0.5%;当炸药单耗超过0.6 kg/t时,大块率(Br)为0.0%。     

Anisotropic dynamic damage and fragmentation of rock materials under explosive loading

This paper describes the development of a constitutive model for predicting dynamic anisotropic damage and fragmentation of rock materials under blast loading. In order to take account of the anisotropy of damage, a second rank symmetric damage tensor is introduced in the present model. Based on the mechanics of microcrack nucleation, growth and coalescence, the evolution of damage is formulated. The model provides a quantitative method to estimate the fragment distribution and fragment size generated by crack coalescence in the dynamic fragmentation process. It takes account of the experimental facts that a brittle rock material does not fail if the applied stress is lower than its static strength and certain time duration is needed for fracture to take place when it is subjected to a stress higher than its static strength. Numerical results are compared with those from independent field tests.     

Prediction of fragment size and ejection distance of masonry wall under blast load using homogenized masonry material properties

The fragment hazard resulting from a nearby explosion is a major concern in the design of structures which may be subjected to blast loads. This paper presents a predictive method based on the theories of continuum damage mechanics and mechanics of micro-crack development, and numerical simulation to determine the probabilistic fragment size distribution and the launch distances. Theoretical derivations are presented to calculate fragment distribution. The fragmentation process is modeled according to the crack initiation and propagation, which depend on the material damage levels and are estimated using continuum damage mechanics theory. The proposed method involves two steps. First a finite element model is developed to estimate the material damage, fragment distribution and the ejection velocity. Then a simple algorithm is used to predict the fragment trajectory and the launch distance based on the fragment size and the ejection velocity. A masonry wall is used as an example in this study. The wall is modeled with both the distinctive consideration of the brick and mortar material properties and the homogenized masonry material properties. The reliability and efficiency of using the homogenized masonry material model in predicting the masonry wall damage and fragmentation are proven. The program AUTODYN is used in this study to conduct the numerical simulations with the proposed models linked to it as user subroutines. The numerical results indicate that the masonry fragments approximately follow the Weibull distribution, which is consistent with some empirical fragment distributions. The proposed method avoids using erosion technique, which inevitably results in a loss of fragment mass, and avoids discretizing the structure into particles or predefining the failure planes, which may lead to unrealistic prediction of damage propagation and evolution and therefore inaccurate fragmentation process and fragment size distributions.     

Simulation of Stable Crack Growth for Welded Joints Including Strength Mismatching

1.IotroductionStrengthmismatchingeffectsonfracturetough-nessofferritesteelsremainakeyissueforthesafetyassessmentofstructures.Structuralandpressureves-selsteelsgenerallyexhibitincreaseinfracturetough-nessoverthefirstfewdistanceofstablecrackgrowth.Laboratorytestingoffracturespecimenstomeasureresistancecurves(R-curves)consistentlyrevealsamarkedeffectofstrengthmismatchingonR-curves.Fortheweldedjoint,whoseweldmetalstrengthishigherthanthatofthebasemetal(overmatching),yieldahigherRcurvecomparingwit…     

Dynamic damage and failure in visco-plastic materials

In this paper, a dynamic damage model in ductile solids under the application of a dynamic mean tensile stress is developed. The proposed model considers void nucleation and growth as parts of the damage process under intense dynamic loading (strain rates ε 10³ s⁻¹). The evolution equation of the ductile void has the closed form, in which work-hardening behavior, rate-dependent contribution and inertial effects are taken into account. Meanwhile, a plate impact test is performed for simulating the dynamic fracture process in LY12 aluminum alloy. The damage model is incorporated in a hydrodynamic computer code, to simulate the first few stress reverberations in the target as it spalls and postimpact porosity in the specimen. Fair agreement between computed and experimental results is obtained. Numerical analysis shows that the influence of inertial resistance on the initial void growth in the case of high loading rate can not be neglected. It is also indicated that the dynamic growth of voids is highly sensitive to the strain rates.     

圆柱形长杆超高速正碰撞薄板结构破碎效应

超高速碰撞多层板结构破碎效应研究对空间碎片防护及动能武器毁伤效应研究有着重要意义。采用ANSYS/AUTODYN程序的SPH方法,对超高速碰撞碎片云的形成过程进行了数值模拟,某典型时刻一次及二次碎片云形貌的数值模拟结果与实验结果吻合较好,验证了计算方法和模型参数的正确性。在此基础上采用数值模拟方法,对钨合金、轧制均质装甲(Rolled Homogeneous Armor,RHA)及LY12铝三种材料的圆柱形弹体超高速碰撞薄板的破碎规律进行了研究,基于量纲分析方法得出了弹体破碎长度随弹靶材料特性、弹靶尺寸及初始撞击速度变化的关系式。并研究了钨合金及RHA两种材料的长杆弹对八层RHA板结构的超高速碰撞效应。     

Mesoscale modelling and analysis of damage and fragmentation of concrete slab under contact detonation

It is interesting and important for researchers to understand the damage process in order to reliably predict fragment distribution of concrete material under blast loading. In the present study, a mesoscale concrete model is developed to simulate the dynamic failure process of a concrete slab under contact detonation. In the mesoscale model, the concrete material is assumed to consist of two phases, that is, the high strength coarse aggregates and the low strength mortar matrix, randomly distributed in the structure components. Each coarse aggregate is assumed to be circular with a random radius in a given distribution range following the Fuller's curve. The mesoscale model together with a dynamic plastic damage material model is incorporated into the hydrocode AUTODYN. The dynamic damage process of the concrete slab under contact detonation is numerically simulated. Based on the numerical results, the fragment size distribution is estimated by an image analysis program. Two different random aggregate distributions are assumed in the present simulations. Numerical results from the two different cases are compared, and the results from the mesoscale model are compared with that from the homogeneous concrete material model. The fragment size distributions obtained from numerical simulations are also compared with those from the empirical statistic formulae.