霍銀磊,李夢(mèng)瑤,王惠
簡(jiǎn)支曲梁結(jié)構(gòu)的大變形及吸能分析
霍銀磊,李夢(mèng)瑤,王惠
(河南科技大學(xué) 包裝工程系,河南 洛陽(yáng) 471000)
通過(guò)對(duì)簡(jiǎn)支曲梁緩沖器的非線性大變形及能量吸收特性的理論研究,為其緩沖設(shè)計(jì)與應(yīng)用提供理論參考?;贓uler–Bernoulli梁理論,以曲梁的曲率半徑及截面角為基本參數(shù)推導(dǎo)簡(jiǎn)支圓形曲梁大變形控制方程,考慮壓板作用下曲梁的多種變形情況給出曲梁的大變形及變形能的解析表達(dá),進(jìn)而計(jì)算不同外力及初始安裝角下緩沖器的變形情況及變形能,并與數(shù)值計(jì)算結(jié)果進(jìn)行對(duì)比。理論計(jì)算結(jié)果與數(shù)值解高度吻合,表明計(jì)算方法的可靠性,緩沖器的緩沖系數(shù)取決于曲梁材料、初始曲率半徑及安裝角度,與其數(shù)量無(wú)關(guān);當(dāng)初始安裝角為時(shí),緩沖器的最小緩沖系數(shù)可取到6.12。所討論曲梁緩沖器具有明顯的非線性大變形特性和良好的緩沖吸能特性,能夠替代傳統(tǒng)緩沖材料,方便地用于運(yùn)輸系統(tǒng)的緩沖設(shè)計(jì)中,給出了簡(jiǎn)支曲梁緩沖器的基本設(shè)計(jì)方法。
簡(jiǎn)支曲梁;緩沖結(jié)構(gòu);大變形;緩沖性能
目前廣泛應(yīng)用的緩沖材料主要有塑料基的泡沫塑料、氣泡膜、氣柱袋等以及紙基的蜂窩紙板、瓦楞紙板、紙漿模塑等。此2類材料要么難以降解,要么以結(jié)構(gòu)的不可逆破壞來(lái)吸收沖擊能量而難以重復(fù)使用[1],因此,具有良好回彈性的緩沖機(jī)構(gòu)或結(jié)構(gòu)受到越來(lái)越多的關(guān)注,例如應(yīng)用于精密儀器及高速重復(fù)沖擊緩沖場(chǎng)合的氣囊類緩沖系統(tǒng)[2]、油氣緩沖器[3]、彈性支撐緩沖系統(tǒng)[4]、恒力緩沖裝置[5]以及以磁流變阻尼[6]為代表的半主動(dòng)式緩沖技術(shù)等。此類緩沖結(jié)構(gòu)或機(jī)構(gòu)大都具有較強(qiáng)的非線性變形特性,具有較大的平臺(tái)應(yīng)力或者恒力區(qū)間,能夠在變形過(guò)程中吸收大量的能量,但其組成一般較為復(fù)雜,廣泛應(yīng)用受到限制。相對(duì)而言,Pham等[7]曾經(jīng)利用曲梁設(shè)計(jì)了一個(gè)可在區(qū)間內(nèi)輸出恒力的穩(wěn)態(tài)機(jī)構(gòu)用于過(guò)載保護(hù)和力控制系統(tǒng),為曲梁機(jī)構(gòu)的緩沖應(yīng)用提供了借鑒。松田技術(shù)研究所[8]基于曲梁開(kāi)發(fā)了用于大型精密儀器運(yùn)輸減震的金屬球狀減震器,其試驗(yàn)結(jié)果表明減震器實(shí)現(xiàn)了減震98.5%的效果,但研究沒(méi)有涉及到對(duì)其大變形及沖擊能量吸收特性的理論或者實(shí)驗(yàn)分析。
對(duì)于曲梁的大變形分析已取得大量可喜的成果:趙躍宇等[9]詳細(xì)地評(píng)述了國(guó)內(nèi)外曲線梁的研究進(jìn)展情況,概述了曲梁靜動(dòng)力學(xué)的基本理論、建模及分析方法、面內(nèi)面外振動(dòng)及分析方法、非線性問(wèn)題及分析方法。近年來(lái),曾森等[10]分析和總結(jié)了前人研究成果,對(duì)曲梁相關(guān)方程做了更具普遍性的分析,給出了數(shù)學(xué)上更嚴(yán)密的結(jié)果。李卓庭等[11]考慮曲梁微段的面內(nèi)變形和面外變形,對(duì)曲梁的幾何方程進(jìn)行了嚴(yán)格的推導(dǎo)和闡述。Lin等[12-14]分別利用拉格朗日和歐拉描述分析了層合曲梁的有限變形,給出了圓曲線和螺旋曲線疊合梁的解析解?;陬愃品椒ǎ苡碌萚15]對(duì)壓電層合曲梁在力電載荷下的有限變形進(jìn)行了分析,以梁曲率半徑和弧切角為基本參數(shù)推導(dǎo)了壓電曲梁在外載荷作用下的控制方程,計(jì)算了圓弧層合曲梁的非線性變形。Batista[16]給出了自由端受到力矩和傾斜力作用的懸臂直梁的精確Jacobi橢圓函數(shù)的解析解。萬(wàn)澤青等[17]基于一階剪切變形理論和軸線可伸長(zhǎng)的精確幾何非線性理論,推導(dǎo)了變曲率曲梁在熱機(jī)載荷作用下的幾何非線性控制方程。
為簡(jiǎn)化計(jì)算,諸多數(shù)值方法也被應(yīng)用于曲梁大變形問(wèn)題的求解中:Surana[18]利用有限節(jié)點(diǎn)法和全拉格朗日方法給出了二維曲梁?jiǎn)卧膸缀畏蔷€性公式。呂和祥等[19]借助Lagrange(T.L.)法、修正的Lagrange(U.L.)法及帶有動(dòng)坐標(biāo)的迭代法求解梁的幾何非線性問(wèn)題。蔡松柏等[20]首次采用共旋坐標(biāo)法導(dǎo)出了平面梁?jiǎn)卧l(fā)生大轉(zhuǎn)動(dòng)小應(yīng)變時(shí)的非對(duì)稱單元切線剛度矩陣,由Newton-Raphson迭代法獲得了大轉(zhuǎn)動(dòng)梁、方形和圓形框架的高精度數(shù)值解。李世榮團(tuán)隊(duì)[21-23]基于打靶法分別研究了框架結(jié)構(gòu)的大變形平衡構(gòu)形、功能梯度變曲率曲梁在機(jī)械和熱載荷共同作用下彎曲變形、沿軸線均布切向隨動(dòng)載荷作用下的非線性平面彎曲問(wèn)題以及懸臂半圓形曲梁在沿軸線均布的切向隨動(dòng)載荷作用下的非線性平面彎曲問(wèn)題。Sharifnia[24]采用梁截面斜角和軸線長(zhǎng)度作為主要參數(shù),提出了一種簡(jiǎn)單而有效的有限元方法來(lái)分析平面靜力問(wèn)題中直線型和曲線歐拉伯努利梁的大撓度。
文中針對(duì)松田技術(shù)研究所設(shè)計(jì)的球形減震器,討論其受壓時(shí)的大變形情況并探討其緩沖應(yīng)用。文中將基于Euler–Bernoulli梁理論建立曲梁的大變形平衡方程,考慮準(zhǔn)靜態(tài)壓力作用下曲梁的非線性大變形特性,分析端部簡(jiǎn)支的曲梁球形緩沖結(jié)構(gòu)發(fā)生大變形時(shí)的位形及能量吸收特性。以期為曲梁結(jié)構(gòu)的緩沖設(shè)計(jì)及應(yīng)用提供參考。
考慮圖1a的松田曲梁結(jié)構(gòu)球形緩沖器,置于上下壓板間的曲梁結(jié)構(gòu)高度為,兩端簡(jiǎn)支單根細(xì)長(zhǎng)曲梁(圖1b)截面厚度,寬度,安裝端未變形截面角(軸線切向與軸的夾角)為L(zhǎng),簡(jiǎn)支曲梁兩端約束于軸并在壓板上的豎直外力作用下發(fā)生彎曲變形。
(1)
根據(jù)Euler?Bernoulli梁理論,忽略曲梁的軸向伸長(zhǎng),處曲梁微元可表示為:
(2)
式中:、分別為曲梁變形前后的曲率半徑。
曲梁變形后的幾何關(guān)系:
,
,
(3)
,
(4)
圖1 松田的球形緩沖器及曲梁力學(xué)模型
Fig.1 Spherical shock absorber of Matsuda and mechanical model of simply supported curved beam
圖2 一般曲梁的變形分析
,
,
(5)
由式(5)得曲梁變形控制方程:
(6)
(7)
式(7)有通解:
(8)
對(duì)于末端簡(jiǎn)支曲梁,末端彎矩恒為0,因此有邊界條件:
,
(9)
由式(8)及邊界條件(9)即可求得:
(10)
式(10)分離變量并兩邊積分得到關(guān)系:
(11)
(12)
及曲梁的變形能:
(13)
忽略壓板的作用,假設(shè)豎直外力直接作用于曲梁末端??紤]一般簡(jiǎn)支半圓曲梁的大變形問(wèn)題(),得到不同豎直外力作用下變形后曲梁的截面角及整根曲梁的大變形位形圖(圖3),由圖3可見(jiàn)隨著壓力的增大,曲梁末端截面角逐漸增大,曲梁末端縱向位移也逐漸增大;當(dāng)壓力不為0時(shí),變形后的曲梁截面角隨著的增大而非線性增大,體現(xiàn)了大變形曲梁的非線性特性?;诖虬蟹╗19-23]的曲梁大變形數(shù)值解一并在圖3中給出??梢?jiàn)文中解析解與數(shù)值解吻合的很好,證明了文中解析解的可靠性。
圖3 一般半圓簡(jiǎn)支曲梁的壓縮特性(,)
對(duì)于安裝在上下壓板之間的曲梁結(jié)構(gòu),曲梁在壓板壓力作用下的高度變?yōu)椋▓D4),當(dāng)壓力較小時(shí),,壓力作用點(diǎn)位于曲梁端部,曲梁變形情況與1.2節(jié)中情況相同;隨著壓力的增大,當(dāng)時(shí),此時(shí)曲梁受力點(diǎn)離開(kāi)其末端向其中部轉(zhuǎn)移;隨著壓力的進(jìn)一步增大,曲梁兩末端接觸并相互擠壓,因此,對(duì)于壓板間曲梁緩沖結(jié)構(gòu)的大變形分析應(yīng)分別考慮3種不同的情況。
(14)
此時(shí)曲梁位形及受力情況見(jiàn)圖5,曲梁段發(fā)生彎曲變形,段自由。
圖4 壓板間曲梁的變形情況
圖5 情況2曲梁變形及微段受力分析
,
(15)
,(16)
(17)
(18)
(19)
因此,利用式(12)—(13),曲梁上任意點(diǎn)的位置及變形能可分別寫(xiě)為:
(20)
(21)
(22)
此時(shí)曲梁位形及受力情況如圖6所示,曲梁段、段均發(fā)生彎曲變形。
圖6 情況c曲梁變形及受力情況
,
,
(23)
(24)
(25)
(26)
由式(26)及邊界條件(23)的第3式得:
(27)
(28)
再加上關(guān)系:
(29)
圖7給出了半圓曲梁在壓板作用下的變形情況,其中曲梁初始安裝角,由式(14、22)分別可得、。由圖7可見(jiàn)變形后曲梁的截面角隨著的增大而增大:當(dāng)時(shí),,隨著的增大而非線性增大,壓力越大的增大也越快,對(duì)應(yīng)的末端截面角也越大;此時(shí)壓板對(duì)曲梁壓力的作用點(diǎn)位于曲梁端部。
圖7壓板間簡(jiǎn)支曲梁的壓縮變形特性(,)
如果不考慮機(jī)械能的損失,曲梁結(jié)構(gòu)組成的緩沖器在受到?jīng)_擊作用時(shí),全部沖擊能量都轉(zhuǎn)化為曲梁的變形能。因此,在一定的外力作用下曲梁緩沖器的變形能越大代表其所吸收的沖擊能量也就越大。沖擊過(guò)程中作用在曲梁緩沖器上的壓力及在此壓力下曲梁結(jié)構(gòu)的變形量(上壓板位移)及變形能可分別表示為:
,
,
(30)
其中:分別為組成曲梁緩沖器的曲梁數(shù)目。
對(duì)于高度為的曲梁緩沖器,借鑒對(duì)實(shí)體材料的能量吸收評(píng)價(jià)方法[25],曲梁緩沖器的緩沖系數(shù)可表示為:
(31)
其中為作用于單根曲梁上的沖擊力,由式(31)可見(jiàn)緩沖器的緩沖系數(shù)與曲梁數(shù)量m無(wú)關(guān),僅取決于曲梁的初始高度H和曲率半徑R(或初始安裝角αL);對(duì)于給定的沖擊力,緩沖器的變形能越大,緩沖系數(shù)C就越小。圖8給出了具有不同初始安裝角αL的曲梁緩沖器的力–變形曲線及緩沖系數(shù)曲線,可見(jiàn)曲梁緩沖器受壓時(shí)體現(xiàn)出類似于實(shí)體緩沖材料的明顯的非線性變形特性,力–變形曲線具有較為明顯的平臺(tái)階段;隨著沖擊壓力的增大緩沖系數(shù)曲線也有明顯的極小值出現(xiàn)。曲梁末端初始安裝角αL越小,緩沖器的平臺(tái)階段越明顯,緩沖系數(shù)的極小值也越?。寒?dāng)時(shí),緩沖系數(shù)的極小值取到。
圖8 壓板間曲梁的壓縮特性及緩沖系數(shù)()
Fig.8 Compression characteristics and cushioning coefficients of simply supported curved beams between platens ()
在產(chǎn)品的緩沖設(shè)計(jì)中,如果已知產(chǎn)品不發(fā)生破壞所能夠承受的最大沖擊力max以及緩沖器的安裝空間(即緩沖器的高度),再利用圖8b的最小極小值點(diǎn)即可快速確定曲梁的最佳安裝角L及的作用在單根曲梁上的沖擊力,進(jìn)而確定曲梁的數(shù)目。當(dāng)然,也可以根據(jù)緩沖器初始高度及安裝角L由圖8b的對(duì)應(yīng)曲線的最低點(diǎn)找到單根曲梁承受的沖擊力,進(jìn)而確定曲梁的數(shù)目。
文中針對(duì)曲梁結(jié)構(gòu)緩沖裝置,基于Euler–Bernoulli梁理論建立了以曲率半徑和截面角為基本參數(shù)的平板壓力作用下曲梁的大變形平衡方程,給出了壓板作用下曲梁發(fā)生大變形時(shí)的位形的解析表達(dá),并與打靶法數(shù)值解進(jìn)行了對(duì)比,解析解與數(shù)值解吻合較好。壓板間曲梁的變形情況較為復(fù)雜,隨著壓力的增大,曲梁所受壓力作用點(diǎn)逐漸由端部向中部移動(dòng),其力–變形曲線具有明顯的非線性特性。隨著安裝角的減小,在相同的平板壓力作用下,結(jié)構(gòu)的變形也越小,力–變形曲線表現(xiàn)出更明顯的平臺(tái)階段,相對(duì)應(yīng)的最小緩沖系數(shù)也越小,緩沖性能越好。在明確了產(chǎn)品能夠承受的最大沖擊力及允許的緩沖空間的情況下,可方便地利用曲梁的緩沖系數(shù)曲線進(jìn)行曲梁緩沖器的各參數(shù)設(shè)計(jì)。
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Analysis of Large Deformation and Energy Absorption of Simply Supported Curved Beam
HUO Yin-lei,LI Meng-yao, WANG Hui
(Department of Packaging Engineering, Henan University of Science and Technology, Henan Luoyang 471000, China)
The work aims to study the nonlinear large deformation and energy absorption characteristics of the shock absorber formedby simply supported curved beam to provide theoretical reference for shock absorber design and application. Based on Euler-Bernoulli beam theory, the control equation of large deformation of simply supported circular curved beam was derived, in which the curvature radius and section angle were selected as the basic parameters of the control equation. The large deformation and deformation energy of the simply supported curved beam were analytically expressed considering various deformation conditions of the curved beam under the action of the platens, and then the deformation conditions and deformation energy of the shock absorber under different external forces and initial installation angles were calculated and compared with the numerical results. The high agreement between the theoretical results and the numerical solution showed the reliability of the calculation method. The cushion coefficient depended on the material, radius of curvature and initial installation angle, and had nothing to do with the quantity of beam. When the initial installation angle was, the minimum cushion coefficient of the shock absorber could reach 6.12. The simply supported curved beam shock absorber has obvious nonlinear large deformation characteristics and good energy absorption characteristics. It can be used to replace the traditional shock absorption materials and is convenient to be used in shock absorption design of transport system. The basic design method of simply supported curved beam shock absorber is given.
simply supported curved beam; cushion structure; large deformation; cushioning performance
TB121;TB485.1
A
1001-3563(2022)19-0190-08
10.19554/j.cnki.1001-3563.2022.19.022
2022–01–09
國(guó)家自然科學(xué)基金(11972286)
霍銀磊(1979—),男,博士,講師,主要研究方向?yàn)榘b動(dòng)力學(xué)、機(jī)械動(dòng)力學(xué)。
責(zé)任編輯:曾鈺嬋