1、<p>　　Experimental Investigation of Bricks Under</p><p>　　Uniaxial Tensile Testing</p><p><b>　　BSTRACT</b></p><p>　　Softening is a gradual decrease of mechanical re

2、sistanceresulting from a continuous increase of deformation imposedon a material specimen or structure. It is a salient feature ofquasi-brittlematerials like clay brick, mortar, ceramics, stoneor concrete which fail due

3、to a process of progressiveInternal crack growth. Such mechanical behaviour iscommonly attributed to the heterogeneity of the material,due to the presence of different phases and materialdefects, such as flaws and voids

4、. For tensil</p><p>　　INTRODUCTION</p><p>　　The tensile behaviour of concrete and other quasi-brittlematerials that have a disordered Internal structure, such asbrick. can be well described by t

5、he cohesive crack modelproposed initially by HILLERBORG [1]. This model has beenwidely used as the fundamental model that describes thenon-linear fracture mechanics of quasi-brittle materials, e.g.[2,3]. According to thi

6、s model and due to crackinglocalization, which is a characteristic of fracture process Inquasi-brittle materials, the tensile beha</p><p>　　There are several experimental methods that have beenused to measur

7、e the fracture properties (tensile strength,fracture energy and ductility Index) that allow the definition ofthe constitutive laws of the material, namely direct tensiletests, indirect tensile tests such as the three-poi

8、nt load test, and the Brazilian splitting test. Although tensile failureresults from a load combination and a multiplicity, of factors.meaning that direct tension is not the only cause of tensilecracking, a direct</p&

9、gt;<p>　　Different issues related to the specimens and the testprocedures have been discussed in the past, namely the testing equipment, the control method, thelocation of theLinear Variable Displacement Transduce

10、rs (LVDTs), thealignment of the specimen and, especially, the attachment of the specimens to the steel platens. The relevance of thelatter Is addressed In Figure 2 [6]. The behaviour inFigure 2a (rotating platens or hing

11、es) Is justified by therotation of the specimen during the loading operation</p><p>　　Tensile fracture parameters of masonry constituents,namely units and the mortar-unit interface, are keyparameters for adv

12、anced numerical modellingof masonryand for a deeper understanding of the behaviour of masonrystructures. in me present paper, an experimentalprogramme using three types of clay brick Is discussed withthe objective

13、of increasing the data available in the literature.</p><p>　　TEST SET-UP AND SPECIMENS</p><p>　　Tensile tests were performed with solid bricks produced byVale da Gandara, Portugal(S), hollow bri

14、cks produced by J.Monteiro e Filhos, Portugal (HP), and hollow bricksproduced by Suceram, Spain (HS). All bricks are extrudedand they were tested in vertical or thickness (V) and inhorizontal or length (H) direction resu

15、lting in six series withthe following notation: SV, SH; HPV, HPH; HSV, HSH.Table 1 gives the dimensions of the bricks and the freewater absorption.The net compressive strength of the</p><p>　　It is noted tha

16、t: (a) bricks HP are extruded with the holesparallel to the larger dimension and bricks HS are extrudedwith the holes parallel to the smaller dimension; (b) bricksHP and HS have small grooves in the upper surface (sideop

17、posite to the facing side) in order to increase adhesionbetween the unit and the backing mortar, see Flgure 3.</p><p>　　Testing equipment and applied measuring devices</p><p>　　The tests were pe

18、rformed in the laboratory of the Civil Engineering Department of University of Minho, using a CS7400 - S shearing testing machine. This machine has twoindependent hydraulic actuators, positioned in vertical andhorizontal

19、 directions. It has a load cell connected to the vertical actuator with a maximum capacity of 25 kN, being particularly suited to small specimens (maximum size of 90 x 150 x 150mm). The adoption of a constant cross secti

20、on for the specimens leads to uncertainty a</p><p>　　control system allows only one Linear Variable Displacement Transducer (LVDT) as displacement control, it was decided to introduce, by means a diamond saw

21、ing machine, two</p><p>　　lateral notches with a depth of 8mm and a thickness of 3mm at mid height of the specimen in order to localize the fracture surface. With the notches, the stress and deformation dist

22、ribution is no longer uniform, with stress and strain gradients occurring very localized near the notch tips. Since three-dimensional npn-uniform crack opening can occur on tensile tests [10], the tensile test control us

23、ing the average of the deformations registered on the four corners of the specimen is the most appr</p><p>　　After preparation of the specimens' ends, glue adhesion conditions were enhanced by making a s

24、eries of superficial slots with a saw. Then, the specimens were carefully fixed to the steel platens using an epoxy resin (DEVCOM) in such a way that the platens were kept perfectly parallel. Here, It Is noted that the s

25、teel platens are fixed (non-rotating), meaning that load eccentricity Is not specimens. The only source of an issue for pnsmadc eccentricity is parallelism between the steel platens wh</p><p>　　Specimen di

26、mensions</p><p>　　Taking into consideration the brick dimensions and the test set-up, 40 x 40 x 70mm S brick specimens were extracted as shown In Figure 5. HP and HS bricks are hollow and, therefore, the spe

27、cimens extracted from the bricks must be representative of the brick shell, a channel or U specimens,and the brick web 1 specimens, see Figure 6. Here, it is noted that the usage of channel specimens in questionable beca

28、use a load eccentricity is introduced by the fact the top and bottom flanges are fully glue</p><p><b>　　RESULTS</b></p><p>　　From the force-elongation relationship obtained in the te

29、nsile tests, the following parameters were evaluated: tensile strength, fracture energy, and residual stress at ultimate scan reading. The notches reduce the Young's modulus of the brick (Eb) by about 20% - 40% [11].

30、 As the measure of</p><p>　　Eb is questionable, it is not shown here.</p><p>　　Figure 7 illustrates the procedure adopted for evaluating the fracture energy, G,. In the cohesive crack model addr

31、essed above, the crack opening u is equal to the total elongation, subtracted from the elastic deformation (u,, = v x lmaes / E0) and the irreversible deformation u;,,, which accounts for inelastic effects during materia

32、l unloading, in the vicinity of the macro-crack. Here, /means is the distance between the measuring points of the LVDT.</p><p>　　The maximum force recorded by the load cell was divided by the effective area

33、of each specimen (notched cross-section), in order to determine the tensile strength.</p><p>　　The fracture energy is identified with the work that is carried out to complete the separation of the two faces

34、of the macro-crack, per unit of area. It is not possible to determine the exact crack opening for which the stress value transferred becomes zero, due to long tail exhibited by the softening branch of the stress-opening

35、crack. For the calculation of the fracture energy, the value of the fracture energy Is usually calculated as the result of the sum of two quantities, one quantity being</p><p>　　Here, taking into account the

36、 force-elongation diagrams and the internal friction of the testing equipment, the fracture energy was simply evaluated up to a deflection of 60pm or up to a deflection corresponding to a force of 200N (if the deflection

37、 is less than 60pm). For the tests aborted before these limit conditions, the energy dissipated was not evaluated.</p><p>　　S specimens</p><p>　　The stress-elongation relationships for specimens

38、 SV Figure 8. For specimens SV (in the extrusion direction), the average values Were 3.48N/mm2(42%) for the tensile strength and 0.0575N/mm (39%) for the fracture energy. The ductility index, again given by the ratio Gf/

39、ft, was 0.0165mm. The values inside brackets Indicate the values of the coefficients(CV)for the sixteen successful tests.</p><p>　　For specimens SH(perpendiclar to the extrusion direction), the average val

40、ues were 2.96N/mm(63%) for the tensile strength and 0.0508N/mm(41%)for the fracture energy. The values</p><p>　　inside brackets indicate the values of the coefficients of variation for the fourteen successfu

41、l tests. The ductility index was 0.0172mm..The tensile strength in the extrusion direction was 4.5% of the compressive strength. The tensile strength in the extrusion direction was18% higher and the fracture energy is 15

42、% higher than the values obtained in the perpendicular direction, due to the alignment of the microstructure. The ductility was similar in both directions. Therefore, brick type</p><p>　　S exhibited only mod

43、erate anisotropy.</p><p>　　All the results exhibit very a large scatter, though the scatter was higher in the direction perpendicular to the extrusion direction. The reason for this seems to be flaws,micro-c

44、racks and inclusions in the burnt clay. It is well known that the fracture process is a three-dimensional process [10] and Figure 9a illustrates the typical superficial cracking patterns of brick specimens. It is clear t

45、hat both straight and pronounced S-shaped cracks appear, meaning that a large scatter must be found.</p><p>　　Finally, the results of the fracture energy vs. the tensile strength were plotted in Figure 10, w

46、here it can be seen that there was a weak correlation between fracture energy and tensile strength, although a clear trend for fracture energy to increase with an increase of tensile strength was found. </p><

47、p>　　CUNGLUSION</p><p>　　The present paper aims to discuss the tensile behaviour of bricks and provide data for advanced numerical simulations. For this purpose, three different producers were selected inc

48、luding solid and hollow bricks from Portugal and Spain. Direct tensile tests on a servo-controlled machine were carried out in order to obtain the tensile strength, the fracture energy and the shape of the stress-elongat

49、ion diagram. </p><p>　　All bricks were tested in two orthogonal directions, namely along and normal to the direction of extrusion. For the hollow bricks, two different types of specimen were extracted so tha

50、t the shell and the web could be characterized. Due to the presence of voids and internal firing cracks, the complete stress-elongation diagram could not be obtained in several of the specimens.</p><p>　　The

51、 results indicate a large scatter for the tensile strength and fracture energy. The folldwing conclusions with respect to the tensile strength are possible: (a) bricks possess anisotropy with higher strength in the direc

52、tion parallel to extrusion; (b) in hollow bricks, the tensile strength of the shell is higher than that of the web. Moreover, the average results in the brick specimens are fairly constant taking into consideration that

53、three different brick manufacturers were involved. Theref</p><p>　　ACKNOWLEDGMENTS</p><p>　　The present work was partially supported by project GROW- 1999-70420 "Industrialised solutions fo

54、r construction of reinforced brick masonry shell roofs" funded by European Commission.</p><p>　　單軸拉伸試驗(yàn)下磚的實(shí)驗(yàn)研究</p><p><b>　　摘要</b></p><p>　　轉(zhuǎn)化是來自在一個(gè)材料樣本和結(jié)構(gòu)逐步減少機(jī)械阻力的過程

55、，這是粘土磚、砂漿、石材等準(zhǔn)脆性材料具體到一個(gè)漸進(jìn)過程的顯著特點(diǎn)。其破壞的原因是內(nèi)部裂紋的增長(zhǎng)。由于缺陷和空洞的存在，這些特性通常材料的異質(zhì)性。在混凝土中，拉伸破壞現(xiàn)象已得到確定，但是這種破壞很少存在粘土磚中。在目前的論文中，米尼奧大學(xué)進(jìn)行了一系列拉伸試驗(yàn)，改試驗(yàn)還包括三個(gè)不同類型磚的單軸拉伸。這三種試驗(yàn)保過抗拉強(qiáng)度、斷裂能量的量化和實(shí)用價(jià)值采納的建議。</p><p><b>　　引言</b&g

56、t;</p><p>　　混凝土和其它準(zhǔn)脆性材料懶神行為有一個(gè)無序的內(nèi)部結(jié)構(gòu)材料，如磚。改</p><p>　　象可以很好地描述最初有希勒勒提出的去裂紋模型，改模型已經(jīng)作為最基本的模型用于解釋準(zhǔn)脆性材料的非線性斷裂。依據(jù)這個(gè)模型，準(zhǔn)脆性材料的一個(gè)特點(diǎn)就是開裂的位置不同，這是拉伸材料在不同部位的拉伸特點(diǎn)，見圖1。直到達(dá)到高峰負(fù)荷，彈塑性應(yīng)力應(yīng)變關(guān)系圖是有效的。據(jù)悉，非彈性行為的高峰值發(fā)生是由

57、于微裂過程中消耗的能量通常被忽略。應(yīng)力開裂張拉位移關(guān)系圖1b介紹了在斷裂過程區(qū)的應(yīng)變后峰轉(zhuǎn)化行為。凝聚力應(yīng)力張開位移座高峰壓力逐漸減少直到為零，與其相對(duì)應(yīng)的裂紋的兩個(gè)邊之間距離增加從零到關(guān)鍵的開裂點(diǎn)。軟化圖在描述假設(shè)的基礎(chǔ)性作用斷裂過程抗拉強(qiáng)度特點(diǎn)的斷裂能量，即由該地區(qū)給予的軟化圖，簡(jiǎn)圖16.關(guān)鍵性裂紋張拉可以代替延性指數(shù)D；其代表了能源正?；目剐詮?qiáng)度。此參數(shù)允許脆性材料的表征和和降部分的形狀直接關(guān)系到應(yīng)力變形圖。</p>

58、<p>　　已經(jīng)有幾個(gè)用于測(cè)量斷裂性能的實(shí)驗(yàn)方法對(duì)材料直接拉伸實(shí)驗(yàn)和間接拉伸實(shí)驗(yàn)本構(gòu)關(guān)系，這意味著直接拉伸不是破壞的唯一原因。直接拉伸實(shí)驗(yàn)似乎是最適合的測(cè)試表征準(zhǔn)脆性材料的實(shí)效機(jī)理。這個(gè)測(cè)試定義為可參考的方法。樣本組織和測(cè)試程序已經(jīng)在過去發(fā)表過，即測(cè)試設(shè)備，控制方法，線性可變位移傳感器的安放位置。后者在圖2中心理問題的相關(guān)性，在圖2的案例中，用固定壓板，彎矩和多個(gè)裂縫會(huì)出現(xiàn)。這樣的結(jié)果產(chǎn)生于一個(gè)稍大的抗拉強(qiáng)度和更高的能量值

59、消散。最后，其指出雖然抗拉強(qiáng)度和斷裂在屬性材料內(nèi)考慮，但是，眾所周知，砌體成分?jǐn)嗔岩蕾囉诖笮『鸵?guī)模，即單位砂漿設(shè)備接口一個(gè)實(shí)驗(yàn)程序使用三種類型磚在文獻(xiàn)中體現(xiàn)目標(biāo)數(shù)據(jù)的增加。</p><p>　　拉伸斷裂參數(shù)的磚石成分，即單位和砂漿設(shè)備接口，是關(guān)鍵參數(shù)先進(jìn)的磚石結(jié)構(gòu)的數(shù)值模擬并為磚石結(jié)構(gòu)的特性有更深入的了解。我在本論文中，實(shí)驗(yàn)程序使用三種類型的粘土磚討論，文獻(xiàn)提供的目標(biāo)數(shù)據(jù)的增加的。</p><

60、p><b>　　測(cè)試設(shè)置的標(biāo)本</b></p><p>　　由河谷達(dá)拉進(jìn)行的實(shí)心磚的拉伸實(shí)驗(yàn)，左右的磚都是擠壓的，他們測(cè)試是</p><p>　　直的，厚的，水平的和長(zhǎng)度方向六大系列。表1給出了磚的尺寸和自由水吸收。磚的凈抗壓強(qiáng)度在沿?cái)D出方向分別是78N/mm2，82N/mm2和5882N/mm2。在這里需指出：這些指標(biāo)僅僅是指標(biāo)性的，正如前兩個(gè)值是從相互獨(dú)立的

61、不同研究者和不充足信息的實(shí)驗(yàn)程序得到的，第三個(gè)抗拉強(qiáng)度值是制造商提供的。值得注意的是：HP磚平行較大的尺寸，HP和HS磚在表面上有小槽以增加附著力之間的單位和支持結(jié)構(gòu)。</p><p>　　圖1一般的凝聚力模型：（a）彈性應(yīng)力應(yīng)變圖；（b）應(yīng)力裂紋張開位移圖</p><p>　　圖2邊界條件的影響：（a）針截邊界；（b）夾緊邊界：（c）軟化形狀的影響</p><p>

62、;　　圖3為測(cè)試選擇的磚：（a）磚瓦；（b）惠普磚；（c）恒生轉(zhuǎn)</p><p>　　表1 磚標(biāo)本系列：尺寸和吸收</p><p>　　檢測(cè)設(shè)備和應(yīng)用測(cè)量設(shè)備</p><p>　　在米尼奧大學(xué)土木工程系實(shí)驗(yàn)室進(jìn)行的實(shí)驗(yàn)，使用了CS7400-S剪</p><p>　　測(cè)試機(jī)器，這個(gè)機(jī)器有兩個(gè)獨(dú)立的液壓執(zhí)行機(jī)構(gòu)，垂直位置和水平位置。其有一個(gè)小荷載

63、單元，，該單元連接到帶有最大25kN容量的垂直驅(qū)動(dòng)器，該驅(qū)動(dòng)器特別適用于小樣本。</p><p>　　采用恒定截面為標(biāo)本的位置的不確定性微裂。自動(dòng)控制系統(tǒng)只允許有一個(gè)線性可變位移空，8mm深度，3mm厚意味著磚石鋸切機(jī)的表面。伴隨著缺口，應(yīng)力和變形分布的不均勻，應(yīng)力和應(yīng)變發(fā)生缺口提示。</p><p>　　然而，現(xiàn)有的準(zhǔn)備只能控制一個(gè)位移傳感器，見圖4。在線性全過程的的0.17%的基礎(chǔ)上，

64、該傳感器有一個(gè)1mm的措施。0.5mm/s的變形率在測(cè)試中被使用。經(jīng)測(cè)量，施加的力衡量一個(gè)25kN最大承重傳感器的0.03%準(zhǔn)確性。</p><p>　　在準(zhǔn)備這些式樣的最后兩端，通過一系列專業(yè)的鋸的槽，用膠水粘合情況下得到了擔(dān)離。接著，在環(huán)氧樹脂壓板保持完全平行時(shí)固定標(biāo)本鋼壓板。這意味著荷載偏心標(biāo)本是否為鋼壓板固定的主要原因。</p><p><b>　　樣本尺寸</b&

65、gt;</p><p>　　考慮磚的尺寸和測(cè)量，測(cè)定40×40×70mm磚瓦標(biāo)本，見圖表5。HP和HS磚石空心的，因此從磚提取的標(biāo)本應(yīng)是磚殼的典型，通道或樣本磚見表6。這兒，使用渠道標(biāo)本是有疑問的，應(yīng)為荷載偏心介紹 的事實(shí)頂端和底部是完全粘鋼標(biāo)本，因?yàn)檎车桶逋耆潭ǖ模云穆适欠浅５偷?。FEM的計(jì)算顯示正常荷載偏心率只有0.03。</p><p><b>

66、　　結(jié)果</b></p><p>　　從拉伸關(guān)系得到的拉伸測(cè)試，對(duì)下面的關(guān)系進(jìn)行評(píng)價(jià)：拉伸強(qiáng)度，斷裂能，最終殘余應(yīng)力。缺口減少量約為40%，在此不會(huì)顯示。</p><p>　　圖標(biāo)7說明了評(píng)估程序斷裂能力。在上文娶紋張開模型上，裂紋張開口等于中伸長(zhǎng)率。彈性變形和不可逆轉(zhuǎn)的變形。荷載單元最大承載力分為每個(gè)試樣的有效面積和確定其拉伸強(qiáng)度。</p><p>　

67、　斷裂能力得工作原理就是確定完成分離兩副面孔的宏觀裂紋和單元面積。然而這不可能確定確切的裂紋張開的應(yīng)力值。對(duì)于斷裂能的計(jì)算，脆性的判斷通常是兩個(gè)數(shù)量總和的計(jì)算結(jié)果。一個(gè)測(cè)量的數(shù)量和其他數(shù)量的后計(jì)值。直接壓力下面積計(jì)算從開放圖到標(biāo)稱值的峰值強(qiáng)度。后計(jì)值下獲得的線性由線面積計(jì)算的線性和非線性調(diào)整見下圖。</p><p>　　在考慮到力伸圖和內(nèi)耗的檢測(cè)設(shè)備下，脆性能量只是評(píng)估到60pm偏轉(zhuǎn)或偏轉(zhuǎn)到200N的力量。消散能

68、在終止前得測(cè)試不給予評(píng)估。</p><p>　　圖4 測(cè)試設(shè)置：（a）拉力測(cè)試LVDT的首選位置；（b）前視圖包括LVDT位置的細(xì)節(jié)</p><p>　　圖5 實(shí)心粘土磚：（a）SH標(biāo)本；（b）SV標(biāo)本</p><p>　　圖6 空心磚的典型樣本：（a）殼、通道或U標(biāo)本；（b）網(wǎng)站或1標(biāo)本</p><p>　　圖7 斷裂能評(píng)估過程示意圖<

69、;/p><p><b>　　S樣本</b></p><p>　　SV和SH樣本拉伸關(guān)系描繪在圖8中。對(duì)于樣本SV，抗拉強(qiáng)度平均值是3.48N/mm2，塑性為0.0575N/mm2（39%）。延性指數(shù)是0.0165mm。括號(hào)內(nèi)的值表示16個(gè)成功測(cè)試的系數(shù)值。對(duì)于樣本SH，抗拉強(qiáng)度平均值是2.96N/mm2，塑性為0.0508N/mm?？箟簭?qiáng)度在擠壓方向的拉伸強(qiáng)度為15%。&

70、lt;/p><p>　　抗壓強(qiáng)度在擠壓方向的拉伸強(qiáng)度為4.5%。在擠壓方向的抗拉強(qiáng)度為18%，塑性在垂直方向上為15%。由于微觀結(jié)構(gòu)的對(duì)齊方式，延性在兩個(gè)方向上市相似的，因此，S混凝土樣本只體現(xiàn)出有溫和的各向異性。所有的結(jié)果都表現(xiàn)出非常大的分散性，雖然垂直方向的擠壓是分撒的。很顯然，無論直線和明顯的S形出現(xiàn)裂縫，這意味著分散的程度一定要打。</p><p>　　所有的結(jié)果表現(xiàn)出非常大的分散，雖

71、然，垂直方向的分散要比擠壓方向的分散高一些。在燒制粘土中，產(chǎn)生此現(xiàn)象的原因似乎是裂縫、微裂紋和夾雜物。眾所周知，斷裂過程是一個(gè)三維過程10，在圖9a中顯示了典型的膚淺開裂的磚標(biāo)本模式。很顯然，無論直線和明顯的S形出現(xiàn)裂縫，這意味著必須找到一個(gè)大的分散、在所有情況下，開裂表面是曲折的，周圍聚集之間的聚合和接口矩陣。</p><p>　　最后，斷裂能與拉伸的結(jié)果可以看出有一個(gè)塑性能源和弱相關(guān)抗拉強(qiáng)度，雖然斷裂明顯的趨

72、勢(shì)增加拉伸強(qiáng)度深度的增加就會(huì)被發(fā)現(xiàn)。</p><p>　　圖8 應(yīng)力伸長(zhǎng)磚瓦標(biāo)本：（a）SV;(b)SH.較粗的線是所有樣本的平均</p><p><b>　　結(jié)論</b></p><p>　　本文旨在探討了拉伸行為的混凝土，并提供先進(jìn)的模擬數(shù)據(jù)值。為此，選擇了包括來自葡萄牙和西班牙的實(shí)心和空心磚的三個(gè)不同生產(chǎn)者。直接拉伸實(shí)驗(yàn)的開展以取得拉伸強(qiáng)

73、度，烈性及應(yīng)力伸長(zhǎng)的形狀圖。</p><p>　　所有的混凝土都是沿著正常的擠壓方向進(jìn)行測(cè)試的。對(duì)于空心磚，提取的兩種不同類型標(biāo)本外殼網(wǎng)絡(luò)特點(diǎn)（a）具有各向異性與高強(qiáng)度方向平行擠壓的混凝土。（b）由于網(wǎng)絡(luò)拉伸強(qiáng)度的實(shí)心磚。此外，采樣的磚標(biāo)本是相當(dāng)恒定的，在考慮到三種不同的磚制造商的情況下。因此相應(yīng)地以實(shí)驗(yàn)為目的的如下規(guī)律是可行的：（a）約5%的壓縮強(qiáng)度的拉伸磚的抗壓強(qiáng)度是0.018mm，塑性大約是0.08和0.0