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1、Experimental Investigation of Bricks UnderUniaxial Tensile TestingBSTRACTSoftening is a gradual decrease of mechanical resistanceresulting from a continuous increase of deformation imposedon a material specimen or structure. It is a salient feature ofquasi-brittlematerials like clay brick, mortar, c
2、eramics, stoneor concrete which fail due 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. For tensile failure thisphenomenon has
3、 been well identified for concrete but veryfew results exists for clay brick. In the present paper, theresults of an extensive set of tests carried out at Universityof Minho and including three different types of back underniaxial tension will be presented. Both tensile strength andfracture energy a
4、re quantified, with recommendations for theadoption of practical values.INTRODUCTIONThe tensile behaviour of concrete and other quasi-brittlematerials that have a disordered Internal structure, such asbrick. can be well described by the cohesive crack modelproposed initially by HILLERBORG 1. This mo
5、del has beenwidely used as the fundamental model that describes thenon-linear fracture mechanics of quasi-brittle materials, e.g.2,3. According to this model and due to crackinglocalization, which is a characteristic of fracture process Inquasi-brittle materials, the tensile behaviour Is characteriz
6、edby two constitutive laws associated with different zones ofthe material during the loading process.see Figure 1. Theelastic-plastic stress-strain relationship of Figure la is validuntil the peak load is reached. It is noted that before thepeak Inelastic behaviour occurs due to micro-cracking andth
7、e energy dissipated in this process is usually neglected forthe calculation of the fracture energy. The stress-crackopening displacement relationship of Figure lb describesthe strain softening behaviour in the fracture process zoneafter the peak. The cohesive stress-opening displacementdiagram Is ch
8、aracterized by the gradual decrease of stressfrom ft maximum value, to zero, corresponding to theIncrease of the distance between the two edges of the crackfrom zero to the critical opening, u, The softening diagramassumes a fundamental role In the description of thefracture process and Is character
9、ized by the tensile strength,fr, and the fracture energy, Gr, which Is given by the areaunder the softening diagram, see Figure 1b. The criticalcrack opening, ue, can be replaced by the ductility index d,4 given as the ratio Grl fr, which represents the fractureenergy normalized by the tensile stren
10、gth. This parameterallows the characterization of the brittleness of the materialand is directly related to the shape of the descending portionof the stress-deformation diagram. There are several experimental methods that have beenused to measure the fracture properties (tensile strength,fracture en
11、ergy and ductility Index) that allow the definition ofthe constitutive laws of the material, namely direct tensiletests, indirect tensile tests such as the three-point load test, and the Brazilian splitting test. Although tensile failureresults from a load combination and a multiplicity, of factors.
12、meaning that direct tension is not the only cause of tensilecracking, a direct tensile test seems to be the moslappropriate test to characterize the basic failure mechanism(mode I) of quasi-brittle materials. This test Is defined as thereference method to follow (5j being adoptedin this work farthe
13、characterization of the tensile behaviour of bricks.Different issues related to the specimens and the testprocedures have been discussed in the past, namely thetesting equipment, the control method, thelocation of theLinear Variable Displacement Transducers (LVDTs), thealignment of the specimen and,
14、 especially, the attachment ofthe specimens to the steel platens. The relevance of thelatter Is addressed In Figure 2 6. The behaviour inFigure 2a (rotating platens or hinges) Is justified by therotation of the specimen during the loading operation, wherethe crack proceeds from one side of the speci
15、men to theother side. In the case of Figure 2b using fixed (non-rotating)platens, a bending moment is introduced and multiple crackswill appear. This results in a slightly larger tensile strengthand a higher value of energy dissipated (fracture energy).Finally, It is noted that although the tensile
16、strength andfracture energy are considered Intrinsic properties of thematerial, it Is well known that fracture properties are size andscale dependent 6,7.Tensile fracture parameters of masonry constituents,namely units and the mortar-unit interface, are keyparameters for advanced 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 di
