李心平，熊 師，耿令新，姬江濤

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含水率對(duì)玉米果穗抗壓特性的影響

李心平，熊 師，耿令新，姬江濤

（河南科技大學(xué)農(nóng)業(yè)裝備工程學(xué)院，洛陽(yáng) 471003）

為研究玉米果穗的抗壓特性及破裂機(jī)理，探索含水率對(duì)玉米果穗抗壓特性的影響，該文先分析了玉米果穗的生物特性，然后選取2個(gè)品種的玉米果穗，含水率處理至5個(gè)標(biāo)準(zhǔn)，在電子萬(wàn)能試驗(yàn)機(jī)上進(jìn)行了靜態(tài)壓縮試驗(yàn)。試驗(yàn)結(jié)果表明：玉米芯含水率對(duì)玉米果穗抗壓特性有極大影響，含水率低于13%時(shí)，隨著含水率的降低果穗抗壓能力小幅度增強(qiáng)；含水率在13%～25%內(nèi)，隨著含水率的增加果穗抗壓能力增強(qiáng)；含水率高于25%后抗壓能力急劇減弱；含水率為25%時(shí)玉米果穗抗壓能力最強(qiáng)；玉米果穗的破裂是由內(nèi)向外逐步破裂的過(guò)程，在受壓過(guò)程中，芯髓最先破裂，隨后木質(zhì)環(huán)形體破裂，木質(zhì)環(huán)形體是玉米果穗抗壓的主要部位；玉米果穗在受壓過(guò)程中存在籽粒脫落的現(xiàn)象，對(duì)玉米果穗施加的載荷值低于610 N不僅可以防止果穗斷裂而且有利于脫粒。該研究結(jié)果可為玉米不斷芯脫粒的進(jìn)一步研究提供參考。

水分；農(nóng)作物；壓力；玉米果穗；抗壓特性；含水率；試驗(yàn)

0 引 言

玉米果穗的不斷芯脫粒是在玉米的脫粒過(guò)程中保持玉米芯完整不斷裂。相對(duì)于市面上斷芯的玉米脫粒機(jī)，不斷芯脫粒能夠簡(jiǎn)化清選環(huán)節(jié)，精簡(jiǎn)清選裝置，降低脫粒機(jī)械的復(fù)雜程度，減少功耗，從而提高脫粒機(jī)械的可靠性同時(shí)降低成本，而且還可以最大程度地回收玉米芯，減少玉米芯在脫粒過(guò)程中的損耗，方便玉米芯后續(xù)的加工利用，玉米果穗的不斷芯脫粒是玉米脫粒機(jī)械的重要發(fā)展方向[1-6]。

國(guó)內(nèi)外對(duì)玉米脫粒的研究主要集中在籽粒特性和脫粒裝置等方面。張新偉等[7]得出了玉米種子內(nèi)部機(jī)械裂紋產(chǎn)生與擴(kuò)展的微觀機(jī)理；蔡超杰等[8]對(duì)發(fā)現(xiàn)種子玉米生物力學(xué)特性對(duì)籽粒破損率、未脫凈率和含雜率有較大影響；高連興等[9]利用籽粒破損強(qiáng)度、果柄強(qiáng)度和脫粒作用力等試驗(yàn)研究含水率對(duì)種子玉米脫粒性能的影響機(jī)理；張翔等[10]對(duì)立式軸流玉米單穗種子脫粒機(jī)進(jìn)行了參數(shù)優(yōu)化；Petkevichius等[11]研究玉米果穗的脫粒過(guò)程發(fā)現(xiàn)高含水率玉米果穗的脫粒損傷明顯高于中等含水率的果穗；Folarin等[12]開(kāi)發(fā)了新型玉米脫粒機(jī)；Steponavicius等[13]通過(guò)脫粒試驗(yàn)發(fā)現(xiàn)脫粒滾筒的慣性矩增大對(duì)脫粒過(guò)程的影響是積極的。玉米果穗在脫粒過(guò)程中會(huì)受到脫粒部件的擠壓、撞擊等外力作用，這些外力會(huì)導(dǎo)致玉米籽粒的損傷和玉米果穗的破裂。果穗的破裂直接影響不斷芯脫粒的效果，而有關(guān)玉米果穗破裂方面的理論研究比較薄弱，國(guó)內(nèi)外鮮有玉米果穗抗壓特性方面的文獻(xiàn)見(jiàn)諸報(bào)道。

對(duì)玉米果穗抗壓特性進(jìn)行研究，可以為脫粒部件的形狀尺寸、脫粒部件對(duì)果穗作用力的大小等參數(shù)設(shè)計(jì)提供依據(jù)[14-18]。本文主要研究玉米果穗的抗壓特性及破裂機(jī)理，分析應(yīng)力—應(yīng)變規(guī)律以及壓縮破壞特征[19-24]。通過(guò)不同含水率下的壓縮試驗(yàn)，探究含水率對(duì)玉米果穗抗壓特性的影響，分析適合玉米果穗不斷芯脫粒的含水率最優(yōu)值、玉米果穗的抗壓峰值力等參數(shù)特性。

1 玉米果穗生物特性

玉米果穗是由玉米芯和玉米籽粒組成，玉米芯是玉米脫去籽粒后的果軸，一般占玉米果穗總干重的20%～30%。玉米芯自內(nèi)向外由芯髓、木質(zhì)環(huán)形體和穎殼三部分組成[25]。玉米果穗是一種由多胞層次狀生物復(fù)合材料組成的多結(jié)構(gòu)物體，其物理力學(xué)性質(zhì)取決于化學(xué)組成與物理構(gòu)造特點(diǎn)。圖1為玉米果穗構(gòu)造形成示意圖。

如圖1所示，玉米芯主要由纖維素、淀粉、木質(zhì)素和少量的灰分組成[26]，木質(zhì)環(huán)形體由纖維素鏈狀分子形成單位晶胞，單位晶胞又形成纖維素基本纖絲，基本纖絲再聚集形成微纖絲后鑲嵌在木質(zhì)素和果膠所組成的基體中，然后形成片層結(jié)構(gòu)，再由多個(gè)片層同心地形成木質(zhì)環(huán)形體[27]，結(jié)構(gòu)密實(shí)，質(zhì)地堅(jiān)硬，因此木質(zhì)環(huán)形體堅(jiān)固而且強(qiáng)度大。芯髓是由纖維素形成的纖絲與淀粉顆粒相互嵌套組成基體，然后形成網(wǎng)狀結(jié)構(gòu)，再由多個(gè)網(wǎng)狀體相互聚集形成芯髓，網(wǎng)狀結(jié)構(gòu)的特征導(dǎo)致其結(jié)構(gòu)疏松，內(nèi)部間的連接不緊密。穎殼是由淀粉顆粒包裹在纖維素周圍組成基體，然后形成絲狀結(jié)構(gòu)，再由多條絲狀體相互排列聚集形成穎殼，穎殼則比較松軟，是一個(gè)一個(gè)地連接在木質(zhì)環(huán)形體表面，相互之間獨(dú)立沒(méi)有連接，穎殼之間通過(guò)緊密地排列形成了玉米芯穩(wěn)定的表面。

圖1 玉米果穗的構(gòu)造

2 材料與方法

2.1 試驗(yàn)材料與設(shè)備

選取2個(gè)玉米品種作為試驗(yàn)材料，手工采摘，通過(guò)自然晾曬將籽粒含水率處理至5個(gè)標(biāo)準(zhǔn)[28]，每個(gè)籽粒含水率均允許上下浮動(dòng)3個(gè)單位（±0.3%），同時(shí)測(cè)出對(duì)應(yīng)的玉米芯含水率，結(jié)果見(jiàn)表1。每個(gè)品種每一含水率均準(zhǔn)備8個(gè)完好無(wú)損的玉米果穗，從中隨機(jī)抽取6個(gè)果穗作為試驗(yàn)樣本。

表1 試驗(yàn)材料參數(shù)

主要試驗(yàn)設(shè)備有DNS系列電子萬(wàn)能試驗(yàn)機(jī)，美國(guó)帝強(qiáng)十二型水分測(cè)試儀和DSC-W630型相機(jī)（索尼有限公司生產(chǎn)）。DNS系列電子萬(wàn)能試驗(yàn)機(jī)由試驗(yàn)機(jī)主機(jī)和計(jì)算機(jī)組成，示值誤差為±0.5%，試驗(yàn)機(jī)主機(jī)結(jié)構(gòu)如圖2所示；負(fù)荷傳感器采用CLY型、精度為0.02、量程為10 kN的力傳感器。電子萬(wàn)能試驗(yàn)機(jī)和負(fù)荷傳感器均由長(zhǎng)春機(jī)械科學(xué)研究院有限公司生產(chǎn)。美國(guó)帝強(qiáng)十二型水分儀由DICKEY-john公司生產(chǎn)，示值誤差為±0.5%。

1.上橫梁 2.上壓盤 3.玉米果穗 4.下壓盤 5.負(fù)荷傳感器 6.活動(dòng)橫梁

2.2 試驗(yàn)方法

試驗(yàn)時(shí)，將玉米果穗放在壓縮夾具下壓盤工作面中央，點(diǎn)擊開(kāi)始按鈕后，活動(dòng)橫梁開(kāi)始向上移動(dòng)。當(dāng)壓縮夾具上壓盤接觸到玉米果穗后，活動(dòng)橫梁以5 mm/min的速度均勻緩慢地向上移動(dòng)，同時(shí)計(jì)算機(jī)開(kāi)始記錄位移-力、位移-應(yīng)力的數(shù)據(jù)。試驗(yàn)設(shè)定的斷裂敏感度為最大值的80%，因?yàn)閿?shù)值下降至最大值的80%時(shí)，玉米果穗已經(jīng)完全破裂，滿足試驗(yàn)結(jié)束的要求。當(dāng)達(dá)到斷裂敏感度時(shí)，活動(dòng)橫梁停止，試驗(yàn)自動(dòng)結(jié)束，計(jì)算機(jī)輸出載荷、應(yīng)力的數(shù)據(jù)以及相應(yīng)的圖像。用相機(jī)拍下玉米果穗破裂過(guò)程中外部變化的照片，然后將果穗沿徑向切開(kāi)，觀察其內(nèi)部變化，記錄玉米果穗的破壞形式，然后進(jìn)行下一次試驗(yàn)，每個(gè)品種每一含水率均進(jìn)行6次試驗(yàn)。為觀察記錄玉米果穗的內(nèi)部破裂過(guò)程，每個(gè)品種每一玉米芯含水率均額外選取一個(gè)果穗從中間徑向切開(kāi)，把切開(kāi)的玉米果穗放到下壓盤上進(jìn)行試驗(yàn)，以果穗徑向截面為觀測(cè)窗口，用相機(jī)連續(xù)拍下果穗壓縮試驗(yàn)的全過(guò)程。

2.3 試驗(yàn)因素和指標(biāo)

選取玉米品種、玉米芯含水率為試驗(yàn)因素，由于最大載荷能夠直觀顯示玉米果穗所能承受的最大壓力，最大應(yīng)力是玉米果穗抗壓強(qiáng)度的量化表示，因此把玉米果穗受到的最大載荷和最大應(yīng)力作為試驗(yàn)指標(biāo)，采用雙因素隨機(jī)區(qū)組試驗(yàn)，如表2所示。

為研究玉米芯含水率對(duì)玉米果穗受壓過(guò)程中載荷與應(yīng)力的影響，以玉米各品種為母體、以玉米芯含水率為因素用MATLAB軟件對(duì)表2進(jìn)行一元非線性回歸分析，對(duì)數(shù)據(jù)進(jìn)行擬合，回歸擬合后得到（玉米芯含水率）與（玉米果穗最大載荷）和（玉米果穗最大應(yīng)力）的對(duì)應(yīng)函數(shù)關(guān)系，回歸分析結(jié)果見(jiàn)表3。

由表3可知，玉米芯含水率對(duì)2個(gè)品種玉米果穗的最大載荷和最大應(yīng)力影響很大，回歸擬合的決定系數(shù)均在0.97以上。對(duì)回歸方程的顯著性及回歸系數(shù)的顯著性檢驗(yàn)，其檢驗(yàn)結(jié)果均為顯著或極顯著。

表2 玉米果穗壓縮試驗(yàn)結(jié)果與處理

表3 回歸分析結(jié)果

注：為玉米芯含水率，為玉米果穗最大載荷，為玉米果穗最大應(yīng)力。

Note:is water content of corn cob,is maximum load of corn ear,is maximum stress of corn ear.

3 結(jié)果與分析

3.1 玉米芯含水率對(duì)玉米果穗抗壓特性的影響分析

3.1.1 玉米芯含水率對(duì)玉米果穗塑性的影響

玉米果穗完全破裂時(shí)的應(yīng)變值為果穗的破裂應(yīng)變，2個(gè)玉米品種的果穗破裂應(yīng)變與玉米芯含水率的關(guān)系如圖3所示。為方便表述，2個(gè)品種的玉米芯含水率均按10.5%、16.5%、24.5%、29.5%、33.5%區(qū)分。玉米芯含水率10.5%的玉米果穗破裂應(yīng)變最小，表明其塑性變形過(guò)程最短，說(shuō)明低含水率下的玉米果穗塑性弱；玉米芯含水率24.5%的玉米果穗破裂應(yīng)變最大，表明其塑性變形過(guò)程最長(zhǎng)，說(shuō)明玉米芯含水率為24.5%時(shí)玉米果穗塑性最強(qiáng)；隨著含水率的增加，破裂應(yīng)變值先增加后減小，說(shuō)明隨著玉米芯含水率的增加，玉米果穗的塑性先增強(qiáng)后減弱。

圖3 玉米芯含水率與玉米果穗破裂應(yīng)變的關(guān)系

3.1.2 玉米芯含水率對(duì)玉米果穗應(yīng)力與載荷的影響

圖4a表明含水率低于13%時(shí)，玉米果穗最大應(yīng)力隨含水率的減小而增大，且曲線較為平緩，說(shuō)明其抗壓強(qiáng)度隨含水率的減小而緩慢的增大；圖4b表明含水率低于13%時(shí)，玉米果穗所能承受的最大載荷隨含水率的減小而緩慢的增大。由于在低含水率下，玉米果穗塑性弱，其抵抗外力作用的形變能力弱，因此低含水率下的抗壓性能總體較弱。但隨著含水率的減小，玉米果穗的抗壓強(qiáng)度緩慢增強(qiáng)，導(dǎo)致玉米果穗抗壓能力小幅度增強(qiáng)。

圖4a表明在13%～25%的含水率區(qū)間內(nèi)，玉米果穗最大應(yīng)力隨含水率的增加而增大，說(shuō)明玉米果穗抗壓強(qiáng)度隨含水率的增加而提高、但變化速度較緩；圖4b表明在13%～25%的含水率區(qū)間內(nèi)，玉米果穗所能承受的最大載荷隨含水率的增加而提高、但變化速度較緩。

a. 玉米芯含水率與玉米果穗破裂最大應(yīng)力的關(guān)系

a. Relationship between water content of corn cob and maximum stress in the rupture of corn ear

b. 玉米芯含水率與玉米果穗破裂最大載荷的關(guān)系

研究發(fā)現(xiàn)，較低含水率段內(nèi)的含水率對(duì)其塑性影響較大，隨著含水率的增加其塑性增強(qiáng)，使玉米果穗抵抗外力作用的形變能力增強(qiáng)，壓縮過(guò)程中產(chǎn)生了較為明顯的塑性變形；同時(shí)果穗抗壓強(qiáng)度也隨含水率的增加而增加，從而提高了玉米果穗的抗壓能力。

由圖4a可知，含水率高于25%后，玉米果穗最大應(yīng)力隨含水率的增加而降低且變化速度較快，說(shuō)明玉米果穗抗壓強(qiáng)度隨含水率的增加而急劇減小；由圖4b可知，含水率高于25%后，玉米果穗所能承受的最大載荷隨含水率的增加而降低且變化速度較快。高含水率段玉米果穗抗壓強(qiáng)度減弱嚴(yán)重，同時(shí)玉米果穗的塑性也在降低，導(dǎo)致玉米果穗抵抗破裂的能力變?nèi)?，降低了玉米果穗的抗壓能力?/p>

由圖4可知，在含水率為25%時(shí)，玉米果穗的最大應(yīng)力和最大載荷均達(dá)到峰值，說(shuō)明玉米芯含水率25%的玉米果穗抗壓能力最好。

3.2 玉米果穗受壓過(guò)程中的破裂分析

3.2.1 玉米果穗內(nèi)部變化與破裂機(jī)理

分析比較果穗在試驗(yàn)過(guò)程中的徑向切面照片可知，不同品種不同含水率的玉米果穗在壓力作用下破裂的內(nèi)部過(guò)程均相似，破裂過(guò)程如圖5所示。

圖5a顯示壓盤開(kāi)始接觸果穗；隨著壓力的不斷增大，如圖5b所示，玉米果穗開(kāi)始破裂，芯髓最先出現(xiàn)裂紋；圖5c～圖5d顯示裂紋逐漸變長(zhǎng)變寬，裂紋從芯髓部分向上、下2個(gè)方向同時(shí)擴(kuò)展，逐漸接近木質(zhì)環(huán)形體；圖5e～圖5f顯示順著裂紋擴(kuò)展的方向，木質(zhì)環(huán)形體逐漸破裂，裂縫快速變大，果穗被壓扁，變形明顯。

圖5 玉米果穗破裂過(guò)程

在玉米果穗豎直方向施加壓力后，如圖6所示，外加壓力作用在籽粒上后分為指向玉米芯的力1和側(cè)向壓力2。1順著籽粒傳遞到穎殼深處，并繼續(xù)傳遞到木質(zhì)環(huán)形體。在初始階段，壓力一直由木質(zhì)環(huán)形體承受，并沒(méi)有作用到芯髓上，芯髓受到了木質(zhì)環(huán)形體的保護(hù)。但隨著壓力的逐漸增加，玉米果穗開(kāi)始出現(xiàn)變形，同時(shí)木質(zhì)環(huán)形體也開(kāi)始出現(xiàn)微弱變形，逐漸地由圓形變?yōu)闄E圓狀。此時(shí)木質(zhì)環(huán)形體傳遞的力環(huán)向作用于芯髓四周，該環(huán)向分布的力可正交分解為豎直方向的壓力3和水平方向的張力4。在3的擠壓和4的拉伸作用下，芯髓內(nèi)部間的連接遭到破壞，芯髓中部開(kāi)始出現(xiàn)裂紋，隨著力的增大，裂紋縫隙變寬，向上下2個(gè)方向擴(kuò)展，因此裂紋一般都是豎直方向的。裂紋擴(kuò)展接近木質(zhì)環(huán)形體后，木質(zhì)環(huán)形體與裂紋接觸處最為脆弱，當(dāng)壓力超過(guò)木質(zhì)環(huán)形體的破裂極限值時(shí)，木質(zhì)環(huán)形體與裂紋接觸的地方破裂，從而使裂紋從內(nèi)部一直擴(kuò)展到果穗外部。

注：F為施加的壓力，kN；F1為指向玉米芯的力，kN；F2為側(cè)向壓力，kN；F3為豎直壓力，kN；F4為水平張力，kN。

圖7反映了不同玉米芯含水率下應(yīng)力與應(yīng)變的關(guān)系，5種含水率的曲線走勢(shì)比較相似，分析應(yīng)力-應(yīng)變曲線可知[29-30]：隨著應(yīng)變的增加，應(yīng)力逐漸增大，玉米果穗發(fā)生塑形變形，應(yīng)力到達(dá)峰值后開(kāi)始減小，曲線呈鋸齒形波動(dòng)，進(jìn)入屈服階段。在玉米果穗徑向切面的壓縮試驗(yàn)過(guò)程中，當(dāng)芯髓出現(xiàn)裂紋時(shí)觀察發(fā)現(xiàn)計(jì)算機(jī)輸出的應(yīng)力曲線并沒(méi)有下降，反而繼續(xù)上升，直到木質(zhì)環(huán)形體出現(xiàn)裂紋時(shí)，曲線才開(kāi)始下降。這說(shuō)明芯髓的破裂對(duì)果穗的抗壓性能沒(méi)有影響，木質(zhì)環(huán)形體的破裂導(dǎo)致了果穗內(nèi)部穩(wěn)定性的明顯破壞。由于木質(zhì)環(huán)形體堅(jiān)固且強(qiáng)度大，在受壓過(guò)程中能一直承受主要的載荷，因此木質(zhì)環(huán)形體是玉米果穗抗壓的主要部位。

注：玉米芯含水率：1. 10.5%2. 16.5%3. 24.5%4. 29.5%5. 33.5%

3.2.2 玉米果穗外部變化與分析

圖8a記錄了在壓縮試驗(yàn)中完整玉米果穗受壓破裂時(shí)果穗側(cè)面的變化情況，由圖8a可知，玉米果穗出現(xiàn)變形，側(cè)面籽粒出現(xiàn)松動(dòng)、脫落現(xiàn)象，籽粒排列雜亂，但并無(wú)籽粒破裂的情況。圖8b記錄了果穗受壓破裂時(shí)果穗頂部的變化情況，圖中黑線圈出的區(qū)域?yàn)楣氤惺苷龎旱淖蚜?，圖片顯示承受正壓的籽粒排列完好、無(wú)脫落現(xiàn)象，籽粒也沒(méi)有破裂。

圖8 玉米果穗外部變化

對(duì)玉米果穗施加壓力后，壓力的一部分分解為籽粒的側(cè)向擠壓力2，上下兩處的側(cè)向擠壓力同時(shí)向果穗側(cè)面籽粒傳遞，玉米果穗側(cè)面單個(gè)籽粒的受力分析如圖9所示。

籽粒受到外力后的徑向平衡方程如式（1）所示。

式中1為的最小值；2為的最大值。

果穗受外力作用時(shí)，籽粒與玉米芯連接力c的方向?yàn)橹赶蜉S負(fù)方向，穎殼對(duì)籽粒的摩擦力變?yōu)橄騼?nèi)，玉米果穗側(cè)上方籽粒受力均滿足平衡方程（1），而側(cè)下方籽粒的受力與側(cè)上方籽粒不同，由于籽粒位置的變化導(dǎo)致籽粒重力在軸上分力方向變?yōu)橹赶蜉S正方向，因此側(cè)下方籽粒受力滿足徑向平衡方程（2）。

1.籽粒 2.穎殼 3.木質(zhì)環(huán)形體 4.芯髓

1.Grain 2.Glume shell 3.Wooden ring form 4.Core pulp

注：為籽粒重力，kN；c為籽粒與玉米芯連接力，kN；為支撐力與軸的夾角；為擠壓力與軸的夾角；為重力與軸的夾角；N為角度為時(shí)穎殼對(duì)籽粒的支撐力，kN；f為角度為時(shí)穎殼對(duì)籽粒的摩擦力，kN；5為來(lái)自上側(cè)相鄰籽粒的壓力，kN；6為來(lái)自下側(cè)相鄰籽粒的壓力，kN。

Note:is grain gravity, kN;cis connection force of grain and corn con, kN; is angle of support force andaxis;is angle of pressure andaxis;is angle of gravity and y axis;Nis support force of glume shell on grain when angle is, kN;fis friction of glume shell on grain when angle is, kN;5is pressure from upper adjacent grain, kN;6is pressure from basiscopic adjacent grain, kN.

圖9 玉米果穗受壓時(shí)徑向受力分析

Fig.9 Force analysis of corn ear in radial direction

由上述平衡方程可知，在壓力施加的初始階段，果穗側(cè)面籽粒能夠保持徑向受力的平衡，籽粒不會(huì)脫落，但隨著壓力的增大，籽粒間的相互擠壓力增大，導(dǎo)致籽粒與玉米芯連接力逐漸增加到最大值，也就是果柄的斷裂極限值，隨后平衡方程會(huì)被破壞，果柄斷裂，籽粒出現(xiàn)脫落現(xiàn)象。果穗頂部和底部的籽粒受到壓盤的直接壓力，隨著壓力不斷增大，果柄會(huì)斷裂，籽粒逐漸被壓入穎殼深處，穎殼對(duì)籽粒的緊密包裹是籽粒沒(méi)有脫落的原因，但承受正壓的籽粒與果穗實(shí)際已經(jīng)分離。機(jī)械脫粒時(shí)確保脫粒裝置對(duì)玉米果穗施加的載荷值低于610 N不僅可以防止果穗斷裂而且有利于脫粒。

4 結(jié) 論

1）玉米芯含水率對(duì)玉米果穗抗壓特性有極大影響，含水率低于13%時(shí)，隨著含水率的降低，果穗塑性減弱，抗壓強(qiáng)度緩慢增加，導(dǎo)致抗壓能力小幅度增強(qiáng)；含水率在13%～25%內(nèi)，隨著含水率的增加，果穗塑性增強(qiáng)，抗壓強(qiáng)度增加，導(dǎo)致抗壓能力增強(qiáng)；含水率高于25%后，果穗塑性減弱，抗壓強(qiáng)度減小，使其抗壓能力急劇減弱；玉米芯含水率為25%時(shí)的玉米果穗抗壓能力最強(qiáng)，最適合玉米的不斷芯脫粒。

2）玉米果穗的破裂是由內(nèi)向外逐步破裂的過(guò)程，在受壓過(guò)程中，芯髓最先破裂，然后木質(zhì)環(huán)形體破裂；裂紋是在果穗內(nèi)部豎直方向壓力和水平方向張力的共同作用下產(chǎn)生的，并在豎直方向擴(kuò)展；木質(zhì)環(huán)形體結(jié)構(gòu)密實(shí)堅(jiān)固，是玉米果穗抗壓的主要部位。

3）玉米果穗在受壓過(guò)程中，壓力會(huì)傳遞給周向分布的籽粒，籽粒間擠壓力的增大會(huì)破壞籽粒的徑向受力平衡，導(dǎo)致果柄斷裂，籽粒脫落。機(jī)械脫粒時(shí)確保脫粒裝置對(duì)玉米果穗施加的載荷值低于610 N不僅可以防止果穗斷裂而且有利于脫粒。

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Influence of water content on anti-pressing properties of corn ear

Li Xinping, Xiong Shi, Geng Lingxin, Ji Jiangtao

(,471003,)

The corn threshing without cob fracture can greatly simplify cleaning link. The rupture of corn ear can directly affect the effect of the core threshing without cob fracture. In order to study the anti-pressing properties and rupture laws of corn ear and explore the effect of water content on anti-pressing properties, the static compress experiments were carried out on the electronic universal testing machine. The analysis of the biological characteristics of corn ear showed that the wooden ring form had compact structure and its strength was big. And the core pulp and the glume shell had loose structure and small strength. The corn varieties selected by experiment were Zhongdan 868 and Zhenghuangnuo No. 2. The water content of each variety was treated as 5 criteria. Four tests were done for each test level. The corn variety and the water content of corn cob were used as test factors. The maximum load and the maximum stress of corn ear were used as the test indices. The experimental data were analyzed by MATLAB software and the function relation between water content of corn cob and test indices was obtained. The relationship between water content of corn cob and fracture strain of corn ear indicated that the plasticity of corn ear was firstly enhanced and then declined with the increase of the water content of corn cob. Through the analysis of the fitting curves between the water content of corn cob andthe test indices, it was known that the water content of corn cob had a tremendous effect on the anti-pressing properties of corn ear. When the water content was below 13%, with the decrease of water content, the plasticity of corn ear decreased and the compressive strength increased slowly, which resulted in a slow enhancement of the anti-pressing ability of corn ear. When the range of water content was 13%-25%, with the increase of water content, the plasticity and the compressive strength of corn ear both increased, which resulted in the enhancement of the anti-pressing ability. When the water content was higher than 25%, the plasticity of corn ear was weakened and the compressive strength decreased with the increase of water content, which resulted in a sharp decline of the anti-pressing ability. To observe the internal fracture process of corn ear, samples were cut in radial direction and then the cut samples were put on the test bench to do experiment. Using the radial section of corn ear as the observation window, the whole process of corn ear compression test was recorded by camera. Through the analysis of the photos of corn ear in the process of compression, it was known that the core pulp firstly ruptured and the crack was extended in vertical direction. Then the crack gradually approached to the wooden ring form. Finally, the wooden ring form ruptured and corn ear presented obvious deformation. The analysis of the strain-stress curves indicated that stress gradually increased and corn ear showed plastic deformation with the increase of strain. Stress began to decline after peak. Curves showed jagged wave and the yield stage was entered. The rupture of the wooden ring form led to the decline of stress curves. So the wooden ring form was the main anti-pressing part of corn ear. In the process of compression, corn ear showed the phenomenon of grains falling. The radial balance equations of grain were established by force analysis. Analysis indicated that pressure would be transmitted to the grains in the circumferential distribution. The increase of the pressure between grains would destroy the radial force balance of grains, resulting in the fracture of the carpopodium. Test results showed that the anti-pressing ability of corn ear was the strongest when the water content of corn cob was 25%, and this water content of corn cob was the most suitable for the corn threshing without cob fracture. The rupture of corn ear was a gradual rupture process by the inside-out. Applying pressure less than 610 N to corn ear could not only prevent the fracture of corn ear but facilitate threshing. The research results can provide data reference and theoretical support for the further research of the corn threshing without cob fracture.

moisture; crops; pressure; corn ear; anti-pressing properties; water content; experiment

10.11975/j.issn.1002-6819.2018.02.004

S513

A

1002-6819(2018)-02-0025-07

2017-08-07

2018-01-04

國(guó)家自然科學(xué)基金與河南人才培養(yǎng)聯(lián)合基金資助項(xiàng)目（U1204514）

李心平，教授，博士，主要從事農(nóng)產(chǎn)品收獲與加工機(jī)械研究。Email：aaalxp@126.com

李心平，熊 師，耿令新，姬江濤. 含水率對(duì)玉米果穗抗壓特性的影響[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2018，34(2)：25－31. doi：10.11975/j.issn.1002-6819.2018.02.004 http://www.tcsae.org

Li Xinping, Xiong Shi, Geng Lingxin, Ji Jiangtao. Influence of water content on anti-pressing properties of corn ear[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(2): 25－31. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2018.02.004 http://www.tcsae.org