CRACK TIP MECHANICS BASED ON PROGRESSIVE DAMAGE OF ARROW: HIERARCHY OF SINGULARITIES AND MULTISCALE SEGMENTS
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摘要: 在适度的空间和时间尺度组合下, 裂纹既可在几个月中蠕变几个纳米,也能在几秒钟内扩展10\,km. 虽然裂纹的尖端没有实际的质量,但是它能通过激活周围的物质而处于高能量状态. 依赖于材料的损伤方向,激活质量的减少和增加可发生在尺度转变之前或之后.每个尺度区的分段阈值被假定为与裂纹尖端速度的平方$\dot{a}^2$和激活质量密度$\cal {M}$的乘积有关: ${\cal {W}} = {\cal{M}}_{ \downarrow \uparrow } \dot {a}_{ \uparrow \downarrow }^2 $和 ${\cal {D}} = {\cal {M}}^{ \downarrow \uparrow }\dot {a}_{\uparrow \downarrow }^2$. ${\cal {W}}$和${\cal{D}}$分别被称为直接吸收和自耗散能量密度. 正如下标/上标符号所示,激活的质量密度${\cal {M}}_{ \downarrow \uparrow } $和 ${\cal{M}}^{ \downarrow \uparrow }$与裂纹尖端速度$\dot {a}$变化趋势相反,既可增加也可减少. $\dot {a}^2$和$\cal{M}$的互补效应隐含着常用于宇宙物理学建模的膨胀和/或收缩的物理过程.在用于尺度敏感的裂纹尖端的行为时, 激活的质量密度有相同的解释.分段时的多尺度可以由$\cdots$皮观、纳观、微观和宏观$\cdots$组成.因此, 形象地说,材料损伤过程可以通过裂纹扩展过程中非均匀的总体和局部能量的传递来模拟.疲劳裂纹扩展引起的材料损伤被用来阐释由大到小和由慢到快的尺度/时间序,热力学中的冷$\to $热和有序$ \to $无序转换.这一过程正巧与宇宙演化的箭形方向相反, 宇宙演化遵循小$ \to $大和快$\to $慢, 而热力学相反,遵循热$ \to $冷和无序$ \to $有序.为了表示由损伤萌生所造成的类裂缝型缺陷的不均匀性,提出了一个被称为裂纹尖端力学(crack tip mechanics, CTM)的新模式.涉及的范围是模拟原子列之间的界面裂纹或连续体中分叉的切口.假如需要的话, 尺寸和时间的范围可以复盖从皮观到宏观甚至更大.虽然采用疲劳裂纹来说明CTM的基本原理,在宇宙物理学背景中与直接吸收和自耗散相关的膨胀和收缩的情况可以描述裂纹周围激活质量的行为,它们可看为能量的汇或源.奇异性被用来捕获能量的源或汇的特性, 物理上, 两者作为界面的一部分,从数学上看则是不连续的线的一部分. 能量从一种形式变为另一种形式取决于能量吸收或耗散的箭形损伤时间,这之中牵涉到尺度分段和奇异性强度的联合应用.材料组分随时间的劣化是根据指定的设计寿命导出的,从而使材料的响应与加载率的时间历史匹配.2024-T3铝板的皮观/纳观/微观/宏观开裂模型用来说明什么地方可以增加结构的寿命部分.皮观/纳观/微观/宏观/结构系统的性能随时间劣化可以用9个尺度转变物理参数来描述:纳观/微观区有3个($\mu _{na / mi}^\ast ,$ $\sigma _{na / mi}^\ast,d_{na / mi}^\ast )$,微观/宏观区有3个($\mu _{mi / ma}^\ast ,\sigma_{mi / ma}^\ast ,d_{mi / ma}^\ast )$,皮观/纳观区有3个($\mu_{pi/na}^\ast ,\sigma _{pi/na}^\ast ,d_{pi/na}^\ast)$.下标$pi,na,mi,ma$ 和$struc$分别表示皮观、纳观、微观、宏观和结构.只要知道两个相连的尺度敏感参数,在较低尺度的时间相关的局部物理参数就完成了分析连续体的形式论,虽然它们并不需要用实验来知道.更具体地说, 根据皮观 $\to$ 纳观 $\to$ 微观 $\to$宏观分别有1.25/1.00/0.75/0.50的$\lambda $奇异性强度,皮观裂纹、纳观裂纹、微观裂纹和宏观裂纹的转变特征是从时间箭形的指定的寿命预期来确定的.附加的0.25强度的奇异性可用于结构元件. 回想起来, $\lambda =0.5$相应于断裂力学中的应力分量与$r^{0.5}$成反比,$r$是与宏观裂纹尖端的距离.微观裂纹、纳观裂纹和皮观裂纹分别赋予$r^{ - 0.75},r^{ - 1.0},r^{ -1.25}$的奇异性. 箭形时间(以年为单位)取决于问题的定义.设备的关键部件可用$1.5^\pm / 2.5^\pm / 3.5^\pm / 5.5^\pm$寿命分布和总寿命为$13^\pm$年(a)的皮观/纳观/微观/宏观尺度来设计运行. 上标$\pm$表示多于或少于实际运行的时间. 累进损伤被假定为发生在皮观$ \to$纳观$ \to $微观$ \to $宏观方向.同样的方案用于20年总寿命的2024-T3铝板的疲劳损伤, 按照$1.5^\pm /2.5^\pm / 3.5^\pm / 5.5^\pm / 7.0^\pm$的方式将它的寿命分布在皮观、纳观、微观、宏观和结构的尺度上,这样的指定只是满足在每个尺度范围内损伤内部材料结构所用的能量匹配,因此可以强制执行在总寿命的跨度内精确的时间相关的材料性能劣化过程.Abstract: Appropriatecombination of size and time scale can accommodate a crack tocreep a few nano meters in months or to propagate ten kilometersin a couple of seconds. The tip does not have a real mass so tospeak of, but it can pack a high energy state by activating thesurrounding matter. Decrease and increase of activated mass ispresumed to occur before and after scale transition depending onthe direction of arrow of material damage. The segmentationthreshold for each scale range is postulated to depend on theproduct of the squared of the crack tip velocity $\dot {a}^2$ and activated mass density $\cal {M}$. as ${\cal {W}} = {\cal {M}_{\downarrow \uparrow}}{\dot {a}_{ \uparrow \downarrow }^2}$ and ${\cal {D}} = {\cal{M}^{ \downarrow \uparrow}}{\dot {a}_{ \uparrow \downarrow }^2}$.The quantities $\cal {W}$ and $\cal {D}$ are referred to,respectively, as the direct-absorption and self-dissipation energydensity. The activated mass densities $\cal {M}_{ \downarrow\uparrow } $ and $\cal {M}^{ \downarrow \uparrow }$ can increaseor decrease in opposition to the crack tip velocity $\dot {a}$ asindicated by the subscript/superscript notation. The compensatingeffects of $\dot {a}^2$ and $M$. are implicit to the physicalprocess of expansion and/or contraction often used in cosmophysics modeling. The activated mass density has the sameinterpretation when applied to the scale sensitive crack tipbehavior. Multiscaling when segmented may consist of $\cdots$pico, nano, micro and macro $\cdots$. The material damage processcan thus be simulated figuratively speaking by crack growthentailing non-uniform global and local energy transfer. Materialdamage by fatigue crack growth is used to illustrate the size/timearrow of large$\rightarrow$small and slow$\rightarrow$fast asadvocated, respectively, by the thermodynamics ofcold$\rightarrow$hot and order$\rightarrow$disorder. Thisincidentally is opposite to the direction of arrow in cosmicevolution such that the events follow small$\rightarrow$large andfast$\rightarrow$slow while the thermodynamics reverses,respectively, to hot$\rightarrow$cold anddisorder$\rightarrow$order. A new paradigm referred to as Crack Tip Mechanics (CTM) is proposed torepresent inhomogeneity by crack-like defects as the cause ofdamage initiation. The closed ended line is depicted forsimulating the interfacial gap between rows of atoms or a branchcut in a continuum. The range of the size time scale can coverfrom pico to macro or even wider range if necessary. Although thefatigue crack is used for demonstrating the basic principles ofCTM, the scenarios of expansion and contraction associated withthe direct-absorption and self-dissipation energy density in thecontext of cosmo physics can describe the behavior of theactivated or energized mass around the crack tip which can beviewed as an energy sink or source. Singularity is used to capture the character of the energy source or sink, bothphysically as part of an interface or mathematically as part of aline of discontinuity. Energy exchange from one form to another isassumed to depend on the damage time of arrow of energy absorptionor dissipation that involve the combine use of scale segmentationand singularity strength. Time degradation of the materialconstituents are derived according to specified design life suchthat the material response is matched with the time the history ofthe loading rate.A pico/nano/micro/macro fatigue cracking model of a 2024-T3aluminum panel will be used for demonstration where the structurelife portion may be added. Time degradation of thepico/nano/micro/macro/struc system behavior can be described byusing nine scale transitional physical parameters: three for thenano/micro range ($\mu _{na / mi}^\ast ,$ $\sigma _{na / mi}^\ast,d_{na / mi}^\ast )$, three for the micro/macro range ($\mu _{mi/ ma}^\ast ,\sigma _{mi / ma}^\ast ,d_{mi / ma}^\ast )$, and threefor the pico/nano range ($\mu _{pi / na}^\ast ,\sigma _{pi /na}^\ast ,d_{pi / na}^\ast )$. The subscripts pi, na, mi, ma andna/pi na/pi na/pi struc designate, respectively, pico, nano,micro, macro and structure. Only the ratios of two successivescale sensitive parameters need to be known. The time dependentlocal physical parameters at the lower scale completes theformalism of analytical continuation though they need not be madeknown by tests.More specifically, the transitional character of picocracks,nanocracks, micro- cracks and macrocracks are determined from thespecified life expectancy of time arrow according topico$\rightarrow$nano$\rightarrow$micro$\rightarrow$macro with therespective singularity strength of $\lambda$ given by1.25/1.00/0.75/0.50. An additional singularity of strength 0.25may be added for the structural components. Recall that$\lambda$=0.5 corresponds to the inverse square root r$^{-0.5}$in fracture mechanics with r being the distance from themacrocrack tip. The microcrack, nanocrack and picocrack tip areassigned with the singularities r$^{-0.75}$ , r$^{-1.00}$ andr$^{-1.25}$ , respectively. The time of arrow in years will dependon the problem definition. A critical device component may bedesigned to operate at the pico/nano/micro/macro scale with a lifedistribution of 1.5$^{\pm}$ /2.5$^{\pm}$ /3.5$^{\pm}$ /5.5$^{\pm}$and total life of 13$^{\pm}$ years. The superscript ${\pm}$indicates more or less the actual time elapsed. Progressive damageis assumed to occur in the direction ofpico$\rightarrow$nano$\rightarrow$micro$\rightarrow$ macro. Thesame scheme is applied to the fatigue damage of a 2024-T3 panelwith a total life time of 20 years that may be distributed overthe pico, nano, micro, macro and struc scale according to1.5$^{\pm}$ /2.5$^{\pm}$ /3.5$^{\pm}$ /5.5$^{\pm}$ /7.0$^{\pm}$ .Such a specification can only be satisfied by matching the energyused in damaging the internal material structure at each scalerange. Hence, the precise time dependent material propertydegradation process over the total life span can be enforced.
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