INFLUENE OF THE STRIN RTE ON THE TENSILE STRENGTH IN LUMINS OF DIFFERENT PURITY F Gálvez, JRodríguez and V Sánchez Gálvez Department of Materials Science ETSI aminos anales y Puertos Polytechnic University of Madrid iudad Universitaria s/n 284 Madrid, Spain e-mail: fgalvez@materupmes bstract It is well known that the properties of the materials may be different as the strain rate increases dvanced ceramic materials such as aluminas, could present an increase in their strength as the strain rate becomes higher In this paper the investigation is focused on the influence of the strain rate on the tensile strength of alumina The influence of this variable on this property is experimentally analysed by means of two different kind of tests carried out from low to high strain rates The splitting test of brittle materials is a testing technique widely used at low strain rates It has been recently extended to dynamic conditions using the Hopkinson split pressure bar In this work this method is used both in static and dynamic conditions with servohydraulic machines and a Hopkinson bar The tensile strength of alumina has been measured at three different strain rates The spalling test of long bars is an additional technique that provides the dynamic tensile strength of brittle materials in uniaxial conditions The test procedure and the experimental details are also presented and discussed in a separate paper This technique has also been used to measure dynamic tensile strength of alumina at higher strain rates The influence of the strain rate on the tensile strength is presented and a comparison between the two kind of tests is also discussed To identify the physical mechanisms causing the failure, a microscope analysis of fracture surfaces using SEM has also been performed The study has been applied to the different specimens tested at low and high strain rates with the two different kind of tests The results of the fractographic analysis are presented and discussed 1 INTRODUTION The effect of the strain rate on the mechanical properties of materials is well known Mechanical properties of ceramic materials are affected by the strain rate, and its effect on the compressive strength has been studied [1], [2], showing an increase in strength with strain rate ut the effect of strain rate under tensile loads has not been widely studied In this paper, an experimental programme of tensile tests of alumina based ceramics with different testing devices covering strain rates from 1-6 s -1 to 1 3 s -1 is presented The splitting tests of short discs and the spalling tests of long bars are the basis of the programme The splitting tests are performed both in static machines as well as in a Hopkinson bar This technique is widely used since years in brittle materials as concrete [3] and ceramics [4] at low strain rates, and has been recently introduced to high strain rates [5], [6], [7] The spalling test of long bars is a novel technique that provides the tensile strength of brittle materials at high strain rates under uniaxial stress conditions This technique is based on the wave propagation in long bars and has been used by some authors like Johnstone [7], Najar [8] or Gálvez [9], but the procedure to obtain the tensile strength has not been still verified The correct method to obtain the test results with this method has also been studied in detail by Gálvez [1], and is presented in a separated paper
2 MTERILS To analyse the effect of the strain rate on the tensile strength of ceramic materials, four different ceramic materials were selected, three aluminas with different purity, 94% (94), 98% (98) and 995% (995) and an alumina reinforced with zirconia (ZR) The density and the elastic modulus were measured in order to obtain the elastic wave velocity and the results are summarised in Table 1 To measure the elastic modulus an impulse excitation technique has been used To determine the average grain size, a polish of the specimen followed by a chemical etching was done and the data obtained is included in the same table The materials, manufactured by Morgan Matroc, were directly supplied with the actual specimen geometry except in the case of 98% alumina and ZR, where the specimens were mechanised from 1-mm square tiles The specimen geometry is discs for the splitting tests and rods for the spalling tests as indicated in Table 2 Table 1 Properties of materials measured before testing Material ρ (Kg/m 3 ) E (GPa) c (m/s) Grain size (µm) 94 3658 33 918 83 98 3877 366 9717 24 995 395 391 14 14 ZR 427 348 9292 2 Table 2 Geometry of specimens for the different tests Specimen Geometry Diameter (mm) Length (mm) Splitting 94 & 995 Splitting 98 & ZR 8 4 12 6 Spalling 8 1 3 EXPERIMENTL To cover a wide range of strain rate, two kinds of tests have been employed The diametral compression of short cylinders, called splitting tests, and the spalling test of long bars The splitting tests were performed in two different loading devices Tests at low and medium strain rates were performed in a servohydraulic testing machine with low and fast displacement control respectively, and tests at high strain rates were performed in a Hopkinson bar The spalling tests were used to achieve higher strain rates and to ensure a uniaxial stress state in the specimen during the tests The machine used for the splitting tests at low and medium strain rates was an Instron 851 with a 25 kn and a 1 kn load cells The test was performed with displacement control at velocities of 2 µm/s and 2 µm/s, and the load history was recorded To ensure that the load is applied in a loading point, the specimen is positioned between two ceramic supports and the load plates were protected with two steel discs The testing method has been previously described by the authors [9] The tensile stress in the loading plane is obtained from the following expression: σ t 2P = πld (1) where P is the load applied, D the specimen diameter and L the specimen length The strain rate in each test is obtained from the history of stresses in the loading plane with the following expression: σt ε 1 Spliting = E t LoadPlane (2)
The mean strain rates achieved were 1-6 s -1 in the slower tests, and 1-2 s -1 in the faster ones The splitting tests at high strain rates were performed in a Hopkinson bar The specimen was positioned between two small ceramic blocks to ensure loading in a point, and two steel discs of the same material of the bars were employed to protect the bars The force transmitted to the specimen was recorded by means of strain gauges attached to the output bar The tensile stresses and the strain rate in the specimen were derived with the same expression than that used in the static tests The strain rate achieved using the Hopkinson bar has been 1 2 s -1 for the splitting tests The spalling tests were used to achieve higher strain rates and to ensure a uniaxial stress state in the specimen during the tests The principles of this testing method have been previously presented [9] and the procedure to obtain the tensile strength is fully described in a separate paper The mean strain rate reached in the spalling tests has been up to 1 3 s -1 and has been obtained from the stresses derived in the fracture plane with the following expression: σt ε 1 Spalling = E t FracturePl ane (3) 4 RESULTS ND DISUSSION The number of tests performed in each condition is shown in Table 3 and the results of tensile strength and its standard deviation are presented in Table 4 The data obtained for the different materials exhibit a wide scatter, which can be inherent of ceramics Weibull approach could be recommended, but not enough specimens have been tested Nevertheless the results obtained are enough to show the tendency of the tensile strength with the strain rate The results for 94% alumina are shown in Figure 1, for 98% alumina in Figure 2, for 995% alumina in Figure 3 and for alumina reinforced with zirconia in Figure 4 In all cases no changes in the tensile strength in the range 1-6 to 1-2 s -1 have been observed When testing with the Hopkinson bar a strain rate of 1 2 s -1 has been obtained and the tensile strength presents now an increment compared to static tests The strength is increased in 4% for the 94, 37% for the 98, 5% for the 995 and 84% for the ZR When the spalling technique is employed, the results are greater compared to splitting Hopkinson tests The increase of strain rate is about an order of magnitude from 1 2 to 1 3 s -1 and the increase in the strength is 23% for the 94, 2% for the 98, 12% for the 995 and 13% for the ZR These results show a clear dependence of the tensile strength with the strain rate, but the changes in the testing technique cannot be neglected To analyse the possible changes in the fracture mode, a fractographic analysis with a scanning electron microscope was done The specimens were metalised in the fracture surface and examined in detail The results have been the same in all materials and the micrographs for 94 are shown in next figures The fracture surface of a splitting specimen tested at a strain rate of 1-6 s -1 is shown in Figure 5, a splitting test in Hopkinson bar at 1 2 s -1 in Figure 6, a spalling test in Figure 7 and a detail of a spalling test in Figure 8 In all cases a brittle fracture is shown and an absence of plasticity is clearly observed ll specimens show a predominant transgranular cracking, but with some intergranular borders leavage marks can be easily identified in all cases Nevertheless, at different tests and different conditions no changes in fracture mode can be assumed, and no change on the fracture mode with the strain rate has been identified Table 3 Number of tests done with each technique Number of tests
Testing device Material 1 2 D 94% l 2 O 3 5 4 9 8 98% l 2 O 3 6 6 5 995% l 2 O 3 1 5 8 7 ZR 7 1 6 5 Table 4 Results of tests with each loading method Mean tensile strength Testing device Material 1 2 94% l 2 O 3 161 (23) 181 (8) 278 (28) 358 (51) 98% l 2 O 3 179 (21) - 285 (31) 329 (78) 995% l 2 O 3 161 (26) 163 (29) 243 (43) 271 (38) ZR 155 (12) 172 288 (3) 322 (35) Note to Table 3 and Table 4: 1 represents splitting tests at very low strain rate, 2 splitting tests at intermediate strain rate, splitting tests in Hopkinson bar and spalling tests of long bars 5 5 Material: 94 Material: 98 4 3 2 4 3 2 1 1 1-8 1-6 1-4 1-2 1 1 2 1 4 Figure 1: Tensile strength and its standard deviation versus strain rate in 94% alumina 1-8 1-6 1-4 1-2 1 1 2 1 4 Figure 3: Tensile strength and its standard deviation versus strain rate in 98% alumina 5 5 Material: 99 Material: ZR 4 4 3 2 3 2 1 1 1-8 1-6 1-4 1-2 1 1 2 1 4 Figure 2: Tensile strength and its standard deviation versus strain rate in 995% alumina 1-8 1-6 1-4 1-2 1 1 2 1 4 Figure 4: Ttensile strength and its standard deviation versus strain rate in alumina reinforced with zirconia Note to Figure 1, Figure 2, Figure 3 and Figure 4: Results of splitting tests in servohydraulic machines (), the same type of tests in Hopkinson bar () and spalling tests of long bars ()
Figure 5 1x fracture surface of a splitting test of 94% alumina carried out in static machine Strain rate 1-6 s -1 Figure 7 1x fracture surface of a spalling test of 94% alumina Strain rate 1 3 s -1 Figure 6 1x fracture surface of a splitting test of 94% alumina carried out in Hopkinson bar Strain rate 1 2 s -1 Figure 8 2x fracture surface of a spalling test of 94% alumina Strain rate 1 3 s -1 References [1] Nemat-Nasser S, Deng H Strain rate effect on brittle failure in compression cta metall mater Vol 42, No 3, pp 113-124, 1994 [2] Lankford J Temperature-strain rate dependance of compressive strength and damage mechanisms in aluminium oxide Jof Mat Sci 16 pp 1567-1578, 1981 [3] Neville M Properties of concrete Pitman publishing 1973 [4] Ovri JEO, Davies T J Diametral compression of silicon nitride Materials science and engineering 96, pp 19-116, 1987 [5] Rodríguez J, Navarro, Sánchez-Gálvez, V Numerical assessment of the dynamic tension test using the Split Hopkinson ar J of Testing and Evaluation Vol22 No4 pp 335-342, 1994
[6] Rodríguez J, Navarro, Sánchez-Gálvez, V Splitting tests: an alternative to determine the dynamic tensile strenth of ceramic materials Journal de Physique IV, olloque 8 Supplément au Journal de Physique III, nº4, pp11-16 septembre 1994 [7] Johnstone, Ruiz, Dynamic Testing of eramics under Tensile Stress, Int J Solids Structures, Vol 32, No 17/18, pp 2647-2656, 1995 [8] Najar J Dynamic tensile fracture phenomena at wave propagation in ceramic bars Journal de Physique IV, olloque 8 Supplément au Journal de Physique III, nº4, pp647-652 septembre 1994 [9] Gálvez Díaz-Rubio F, Rodríguez J, Sánchez-Gálvez, V Tensile Strength Measurements of eramic Materials at High Rates of Strain Journal de Physique IV, olloque 3 Supplément au Journal de Physique III, nº7, pp151-156 d oût 1997 [1] Gálvez F aracterización mecánica de materiales cerámicos avanzados a altas velocidades de deformación Ph D Thesis 1999