Dynamic compressive and splitting tensile tests on mortar using split Hopkinson pressure bar technique

Transcription

1 730 Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique Abtract Dynamic compreive and tenile propertie of mortar under impact loading were invetigated experimentally by uing a plit Hopkinon preure bar (SHPB) apparatu with pule haping technique. Firtly, the baic principle, experimental limitation and ome feaible improvement/modification of SHPB technique ued for dynamic tet on concrete-like material were ummarized briefly. And then the dynamic compreive trength, tre veru train repone, and failure mode of mortar were dicued and analyzed. Finally, a dynamic Brazilian dic tet wa conducted to obtain the plitting tenile property of mortar, and ome typical experimental reult were preented. Both compreive and plitting tenile reult how that mortar i a train-rate enitive material. Either compreive or tenile trength enhance with the increae of train rate, epecially when the train rate i greater than the tranition train rate, which i around 20-1 for the dynamic compreion and for the plitting tenion, repectively. Thee finding are helpful to guide the deign and application of concrete tructure. Keyword Mortar; dynamic propertie; SHPB; impact loading; plitting tenion. Fei Yang a,c Hongwei Ma a Lin Jing b* Longmao Zhao c Zhihua Wang c a College of Science and Engineering, Jinan Univerity, Guangzhou ,China b State Key Laboratory of Traction Power, Southwet Jiaotong Univerity, Chengdu, Sichuan , China c Intitute of Applied Mechanic and Biomedical Engineering, Taiyuan Univerity of Technology, 79 Wet Yingze Street, Taiyuan , China Correponding author: *jinglin_426@163.com Received In revied form Accepted Available online INTRODUCTION Mortar i a common type of concrete-like material, and one of the mot practical application i ued for rehabilitation and repair of reinforced concrete tructure. Thee tructure may be ubjected to variou dynamic loading uch a high-velocity impact, penetration and exploion. Therefore, undertanding better the dynamic propertie of concrete-like material under variou circumtance i a greatly ignificant iue for their engineering application. It i well known that the mechanical behavior of concrete-like material under dynamic loading i trikingly different from that ubjected to quai-tatic loading condition (Hughe, 1978; Grote, 2001; Ro, 1989).

2 731 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique Concrete-like material are generally conidered to be train rate-enitive. Both the tenile and compreive trength increae with train-rate, epecially when the train-rate i greater than a tranition train-rate, which i around 10 0 ~ for the uniaxial tenion and for the uniaxial compreion, repectively (Grote, 2001; Li, 2009; Ro, 1989; Wang, 2012). Dynamic increae factor (DIF) are commonly ued to decribe the dynamic enhancement of concrete-like material under high train rate loading. Baed on a large number of dynamic compreive and tenile experimental reult of concrete material, Bichoff et al. (1991); Malvar et al. (1998) ummarized and analyzed the quantitative relationhip between DIF and train rate, a hown in Fig. 1 and 2, repectively. Figure 1: Effect of train rate on the compreive trength of concrete (Bichoff, 1991). Figure 2: Effect of train rate on the dynamic tenile trength (Malvar, 1998).

3 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 732 Split Hopkinon preure bar technique, which decouple cleverly the inertia effect in tructure and the train rate effect in material, ha been ued widely to characterize the dynamic compreive performance of variou engineering material at high train rate in the region of 10 2 ~ (Kolky, 1949). With the ue of large diameter Hopkinon bar to invetigate the dynamic propertie of concrete-like material, ome key problem uch a high-frequency ocillation and diperion of tre wave, and non-uniform tre/train and non-contant train rate deformation in the pecimen, may be met in the tet (Gary, 1998; Zhao, 1998). In recent year, there ha been increaing interet in employing pule-haping technique to determine the dynamic propertie of concrete-like material, aiming to attenuate high-frequency ocillation and increae the rie-time of the incident pule, and achieve tre equilibrium and nearly contant train-rate in the pecimen (Chen, 2003; Duffy, 1971; Frew, 2002). Uing the pule-haping SHPB apparatu, a large number of tudie were conducted to invetigate the dynamic mechanical propertie of normal concrete (Zhang, 2009), high-trength concrete (Wang, 2012), and fiber-reinforced concrete (Li, 2009), and o on. However, few tudie have been reported to invetigate the dynamic repone of mortar to impact loading, although it i ignificant for the engineering application, a tated above. In thi tudy, the dynamic compreive and tenile tet on mortar were therefore conducted uing a SHPB et-up with a pule haper, to ae and undertand the dynamic repone of mortar to impact loading. 2 SHPB TECHNIQUE FOR CONCRETE-LIKE MATERIAL TESTS 2.1 Baic principle A typical SHPB tet ytem generally conit of a triker (which i propelled by a ga gun), input bar, output bar, hock aborber and a data acquiition ytem, a hown in Fig. 3. With the impact of a triker at the free end of the input bar, a compreive longitudinal incident wave wa created and then travel along the bar. When the tre wave reache the pecimen-bar interface, due to the mimatch of mechanical impedance between the pecimen and preure bar, it i partially reflected back into the input bar while the ret i tranmitted into the output bar. The incident, reflected and tranmitted pule in the preure bar were recorded by the reitance train gauge attached to the input and output bar urface, repectively. Figure 3: Schematic diagram of a typical SHPB apparatu. Baed on the one-dimenional elatic wave propagation theory, the tre, train and train rate of the pecimen can be calculated by

4 733 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique EA b A b t 2c t 0 0 r l 2c l 0 r dt (1) (2) (3) where E b and A b are the Young modulu and cro-ectional area of the preure bar, c 0 i the 1-D longitudinal elatic wave peed, A and l are the original cro-ectional area and length of the pecimen, repectively. 2.2 Experimental limitation and improvement Experimental limitation A valid SHPB tet i baed on the following aumption: (i) one-dimenional tre wave propagation in preure bar; (ii) tre/train uniformity within the pecimen; and (iii) radial-inertia and friction effect of the pecimen are negligible. For the SHPB tet of concrete-like material, the brittle nature of material and relatively large diameter of pecimen and preure bar, may reult in violation of the above baic aumption and affect the validity/accuracy of experiment. Firtly, the mall failure train (le than 1%) of concrete-like material often caue the pecimen fail before tre uniformity within the pecimen achieved, a a reult of a harp trapezoidhaped incident wave in the conventional SHPB tet. Secondly, ince concrete pecimen are required to be large enough to contain ufficient micro-tructure in order to be repreentative a a macrocopic material tet (in the ASTM tandard [2007], the minimum cro-ectional dimenion of a rectangular ection i at leat three time the nominal maximum ize of the coare aggregate in the concrete pecimen), the axial and radial inertia effect have greater influence on the actual tre-train repone of concrete-like material. Radial inertia confinement may caue an apparent dynamic trength enhancement intead of the train-rate enitivity of the teted material (Bichoff, 1991; Grote, 2001; Li, 2003). Thirdly, with the larger diameter of preure bar, the tre wave propagation in the bar may not meet the one-dimenional wave aumption in nature; and 2-D effect caued by radial inertia become non-negligible, reulting in evere wave diperion. Moreover, the experimental reult are influenced by the complex boundary condition (e.g. mialignment and interfacial friction at the bar-pecimen urface), wave diperion, pecimen ize effect on trength, and o on Improvement/modification To overcome thee limitation and obtain the reliable experimental reult, ome feaible modification have been developed and ued in the SHPB tet for concrete-like material. For example, the pule-haping technique wa employed to increae the riing-time of incident wave, guaranteeing the reverberation time of the tre wave in the pecimen i greater than 3 before the failure of the pecimen, in order to achieve tre uniformity within the pecimen.

5 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 734 Meanwhile, a theoretical optimal method for determining the length-to-diameter ratio of the SHPB pecimen wa modified by Samanta (1971) for metal material to eliminate the effect of axial and radial inertia, where the material rate of change (i.e., the rate-of-change of a quantity that i defined with reference to pecific particle of the moving continuum) wa conidered. He concluded that the influence of radial and longitudinal inertial tree i the minimum for the mall deformation pecimen in the contant train-rate SHPB tet, if the pecimen dimenion atifying l r 4 3 (4) Similarly, Klepczko and Malinewki (1978) developed a modified formula for friction effect, that i, r 3l (5) where i the Coulomb friction coefficient, and the friction effect can be neglected for 2 r 3l 1. However, different from thoe metal material (e.g. teel), concrete i hydrotatic tre dependent and the pecimen ize ha great influence on it compreive/tenile trength. Zencker and Clo (1999) pointed out that the accurate dynamic tre veru train curve can be obtained for the pecimen with the lenderne ratio l d 0.5 in the one-dimenional tre tate; and it can be alo achieved for a relatively long pecimen with l d 1 if both the 1-D tre tate and uniformity of axial tre ditribution are guaranteed. Therefore, an optional range l d 0.5~1.0 i widely ued in the tet. 3 DYNAMIC COMPRESSIVE TESTS 3.1 Experimental proce Specimen The pecimen were made of mortar which i a mixture of PO 42.5 cement, water and medium fine and with average finene modulu of The ma radio of the three material i 533:302:1600. The cylindrical SHPB pecimen of diameter D 70 mm and length L 55 mm, and 100 x 100 x 100 mm cubic pecimen and D75 x L150 cylinder for quai-tatic tet were cat into the deigned tainle teel mould and placed into the conervation room for curing 28 day according to the technical tandard. The quai-tatic compreive trength of 100 mm cube at an approximate train rate of i 28.7 MPa. The Young modulu and Poion ratio determined from the tandard tet on D75 x L150 mm cylinder are E 19 GPa and 0.13, repectively. For the SHPB tet, the planene of pecimen wa controlled below 0.02 mm to enure the experimental preciion.

6 735 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique Experimental et-up The dynamic compreive tet of cement mortar ample were conducted uing the conic variable cro-ectional SHPB with diameter of 74 mm. The length of the projectile, incident and tranmitter bar, which are made of teel with Young modulu equal to 200 GPa, are 800 mm, 3200 mm and 1800 mm, repectively. The ketch of the overall experimental et-up i hown in Fig. 4. Copper dic with diameter of 10 mm and thickne of 1.0 mm, which i with yield trength of 300 MPa, Young modulu of 128 GPa and Tangent modulu of 1.28 GPa, were ued a pule haper in the preent tudy. Figure 4: Sketch of the typical SHPB experimental et-up. Two high dynamic train amplifier were ued for calibration and to amplify the ignal from the train gauge. The original reading were acquired uing a digital ocillocope (TDS 420A, Tektronix.com, America). The impact velocity of the projectile, which i controlled by ga preure, i meaured by two parallel light gate and an electronic time counter. A decribed in Section 2.1, the incident, reflect and tranmitted pule in the preure bar were recorded by the reitance train gauge attached to the incident and tranmitted bar urface, repectively. The dynamic compreive tre-train relationhip of pecimen can be calculated uing the recorded train-time ignal baed on the one-dimenional tre wave theory. 3.2 Experimental Reult The dynamic compreive experiment of cement mortar ample were conducted at everal different train rate by uing Split Hopkinon preure bar with pule-haping technique. The detail of the geometric dimenion of pecimen and pule-haper, experimental condition and reult are lited in the Table Improvement on tre wave hape The high-frequency ocillation of incident wave may caue large ocillation of tre-train curve for the concrete-like material with low trength and low Young modulu, o that it i difficult to determine the upper and lower yield limit of teted material. Moreover, the effect of diperion and high-frequency ocillation of tre wave are evident for the dynamic tet of nonhomogeneou material with large diameter Hopkinon bar. Pule haper wa employed to expect to improve the tre wave hape, and therefore a comparion of tre wave generated with/without pule haper i made in thi ection.

7 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 736 Specimen number Diameter (mm) Length (mm) Size of pulehaper (mm) Riing-time of incident wave ( ) ( -1 ) f cd (MPa) Failure mode No cracking d 10 h Edge-cracking d 10 h No cracking d 10 h Edge-cracking d 10 h Edge-cracking d 10 h No cracking d 10 h No cracking d 10 h Edge-cracking d 10 h Edge fracture d 10 h Edge-cracking d 10 h Hourgla-hape damage d 10 h Edge-cracking d 10 h Edge fracture d 10 h Hourgla-hape damage d 10 h Hourgla-hape damage d 10 h Hourgla-hape damage Table 1: Dynamic compreive experimental reult of mortar under different train rate. Fig. 5 how the tre wave recorded by the train gauge on the incident bar with/without pule haper. It i hown that a proper pule haper can attenuate high-frequency ocillation of incident tre wave to improve the tre wave hape. Uually, a nearly flat plateau in the reflected pule in a SHPB tet i ued to judge the nearly contant train rate in the pecimen. It can be alo found from Fig. 5 that the reflected wave could be improved to trend to generate a poible nearly flat plateau, a marked by the dotted rectangle. Surely, the achievement of pulehaping function i highly dependent of a good matching between dynamic propertie of the pule-haper material and the teted material and proper geometrical dimenion of the pulehaper for a given impact velocity. Figure 5: Stre wave hape with/without pule haper.

8 737 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique Another important function of the pule haper i to increae the riing-time of the incident pule to facilitate tre equilibrium in the pecimen. The riing-time of incident wave i the mot important parameter for the validity aement of tre equilibrium, which i cloely related to the tranit time t 0 required for the incident wave to travel a ditance of the pecimen length, given by t0 l c (6) where l and c are the pecimen length and the longitudinal wave peed in the pecimen, repectively. If the time duration required for the axial tre equilibrium within the pecimen i, then the required reverberation time of the tre wave in the pecimen i n 2t 0 (7) For a typical SHPB tet for concrete-like material, the tre uniformity tate can be conidered to achieve for n 3 (Ravichandran, 1994). Therefore, to enure the axial tre equilibrium within the pecimen in an SHPB tet, the riing-time of the incident pule hould be at leat no le than, i.e. t r 2nt 0 2nL C (8) In the preent tudy, the longitudinal elatic wave peed, c E, for the teted mortar material i 2982 m/, o the required value of the riing-time of incident wave hould be greater than 6t 0, that i, μ for the pecimen length of 55 mm. It can be found from Table 1 that all the riing-time of incident wave after uing the pule-haper are greater than the required value tended to achieve axial tre equilibrium in the pecimen Dynamic compreive trength The dynamic tre-train curve of pecimen under different train rate are hown in Fig. 6. It i found that cement mortar i a train-rate enitive material; both the uniaxial dynamic compreive trength and train of pecimen increae obervably with the increae of train rate. The peak train of teted pecimen eem to preent an approximate increcent tendency with the rie of train rate, although there i an abnormal peak train for the pecimen at the train rate of ; thi may be caued by the manufacture defect or experimental error. A tated earlier, dynamic increae factor (DIF) i commonly ued to decribe the train-rate effect on the compreive trength of brittle material. The DIF value are calculated by DIF f f (9) cd c where cd f and c f are the dynamic and quai-tatic compreive trength of pecimen, repectively. Fig. 7 give the relationhip between compreive DIF and train rate in a emi-logarithmic form. It i obviou that the dynamic compreive trength of mortar enhance with the increae of

9 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 738 train rate, epecially when the train rate exceed ~20-1 ; thi train rate i alo named a tranition train rate. However, there i no conitent concluion on the phyical mechanim interpretation of the dynamic trength enhancement of concrete-like material. The majority of reearcher agree that uch a trength increae i related with a material vicoity due to the preence of free water in the pore of concrete at the low train rate (Roi, 1991; 1996). For the high train rate, ome author believe it may attribute to be of tructural origin (Ragueneau, 2003), which eem a non-homogeneou tre tate within the pecimen produced by inertia generate a large radial contraint imilar with a confining preure (Kotovo, 1983). Other alo explained thi enhancement in trength i due to a delayed formation of the micro-crack at increaing loading rate (Roi, 1996). Figure 6: Stre-train curve of cement mortar under different train rate. Figure 7: Relationhip between compreive DIF of mortar and train rate Dynamic failure mode The impact failure mode of cement mortar pecimen ubjected to different train rate are hown in Fig. 8. It can be oberved from damage degree and fracture hape of ample that with the increae of train rate, the damage of pecimen accelerate and the number of fragment with

10 739 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique maller ize increae, repectively. Cement mortar pecimen under quai-tatic and low train rate tend to axial plit failure mode, while thoe often preent hourgla-hape and hatter mode at higher train rate. It hould be pointed out that thee failure mode of mortar pecimen hown in Fig. 8 may be reulted by multiple pule during the tet; the actual dynamic failure proce and failure mode of pecimen only ubjected to ingle tre pule loading, need to be invetigated further by uing high-peed camera Figure 8: Failure mode of cement mortar ample under different train rate. 4 DYNAMIC TENSILE TESTS Compared to compreion, concrete-like material are much weaker in tenion (their tenile trength i 1/20 1/10 of the compreive value), which reult that the failure of concrete-like material often occur via tenion for the engineering tructure. Therefore, undertanding of the dynamic tenile propertie of concrete-like material i important for their application. Uually, the dynamic propertie of concrete-like material can be meaured uing direct dynamic tenile tet (Reinhardt, 1982; Zielinki, 1982), dynamic bending tet (Tanaka, 1980), plitting (or Brazilian dic) tet (Lu, 2011; Neville, 1995) and palling tet (Rong, 2012). In thi tudy, the third method (i.e., Brazilian dic tet) wa adopted for mortar. 4.1 Fundamental theory The Brazilian dic tet i a imple indirect tet method to meaure the plitting trength of brittle material. In thi method, a thin circular dic i compreed along it diameter until it failure, a hown in Fig. 9. Baed on elaticity theory, the tre ditribution along the diametrical loading line of the dic pecimen can be derived from the two-dimenional tre field, which i determined by the following equation (Neville, 1995; Timohenko, 1951).

11 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 740 x 2P DL (10) y 2P D DL y D 2 y 1 (11) where P i the force on the pecimen at failure; D and L are the diameter and thickne of pecimen, repectively; y i the ordinate of the point of interet in Fig. 9. Figure 9: Schematic diagram of the Brazilian dic tet. In the dynamic Brazilian dic tet uing SHPB apparatu, if the dynamic tre equilibrium tate i achieved, it i uually aumed that the engineering tenile tre near the center of the dic pecimen i proportional to the peak value of the tranmitted wave. So the dynamic tenile trength of pecimen can be written in the quai-tatic form: td 2 Pt ( ), ( ) 2 P t R ( t ) DL (12) where td i the dynamic tenile trength of pecimen; P repreent the force that i tranmitted through the pecimen; R i the radiu of the preure bar; () t i the peak tre of the tranmitted wave. Correpondingly, the train rate in the pecimen can be etimated by td E (13) where i the time lag between the tart and the maximum of the tranmitted tre wave, and E i Young modulu of the pecimen, where the tatic value i uually ued for calculation due to the weak train rate enitivity. In an actual tet, the load i applied over a mall zone, thu bearing trip are uually employed to control thi zone and pread the load over the actual load-bearing width. ASTM (1986) recommend the width of the trip ( w 0 ) hould be approximately 1/12 of the diameter of the cylindrical pecimen ( d ), a modified expreion ha been alo propoed to etimate the tenile trength for a non-tandard trip width tet (Galvez, 2003), given by

12 741 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique td 2P ld 32 2 ( ) 1 (14) where w0 d i the relative width of the load-bearing trip. 4.2 Experimental arrangement The dynamic Brazilian dic tet on mortar were conducted uing the ame SHPB apparatu a that in the compreive tet, but with the different placement of pecimen, a hown in Fig. 10. The dimenion of pecimen (diameter of 70 mm and thickne of 55 mm) i alo the ame. A pair of ingly curved urface load-bearing trip with the width of 13 mm and thickne of 10 mm wa ued in the tet. Five train gauge were mounted uniformly on the end-urface of pecimen to explore the crack proce, a hown in Fig. 11. Average quai-tatic plitting tenile trength of D70 L55 cylindrical mortar pecimen with the ame dimenion and load-bearing trip i 4.7 MPa. Figure 10: Schematic diagram of the dynamic Brazilian dic tet. Figure 11: Photograph of the pecimen mounted with train gauge. 4.3 Experimental reult The information of pecimen, experimental condition and the correponding tet reult are ummarized in Table 2. The typical dynamic plitting failure mode, and dynamic plitting tenile trength of mortar determined from equation (14) are preented and dicued in the following ubection. Here, a typical tre hitory curve obtained from the pecimen SP12 wa hown in Fig. 12. It i clear that the dynamic tre repone hitory i nearly linear during the whole loading proce; the calculation of train rate by uing Eq. (13) i therefore conidered to be accepted, although thi method may be underetimated lightly the actual train rate.

13 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 742 Specimen number Diameter (mm) Length (mm) Size of pulehaper (mm) Stre rate (GPa/) Strain rate ( -1 ) Dynamic plitting trength (MPa) SP SP SP SP SP SP SP SP SP d 10 h SP d 10 h SP d 10 h SP d 10 h SP d 10 h Table 2: Dynamic plitting tenile experimental reult of mortar. Figure 12: A typical tre hitory curve in the dynamic plitting tet Typical plitting failure mode Fig. 13 how the typical failure pattern of mortar pecimen after dynamic plitting tet, compared to that under quai-tatic tet. It i hown that all the pecimen plit into two main halve along the loading path a expected. The damage degree of pecimen under dynamic loading i more eriou than that under quai-tatic loading, and increae with the train rate. In the dynamic cae, with the major macro-crack develop, other crack occur at the loading end, reulting in the wedge-haped local failure around the contact point. Two edge wedge of pecimen are uually totally cruhed into very mall fragment at higher train rate Dynamic plitting tenile trength Similar to dynamic compreive tet, the tenile DIF wa employed to decribe the enitivity of mortar to train rate, a hown in Fig. 14. It i hown that the dynamic tenile trength alo in-

14 743 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique creae with train rate, where the tranition train rate i around Unlike the dynamic uniaxial compreion, the dynamic tenile tet i often conidered a giving the puret information on mechanical behavior of concrete-like material becaue the inertial effect doe not reproduce a tenile trength (the inertia effect of traction produce a tre tate cloe to a multiaxial tate of traction) (Ragueneau, 2003). The dynamic trength enhancement in tenion wa may be attributed to the preence of free water and crack growth in concrete (Roi, 1996) Figure 13: Splitting failure pattern of mortar pecimen. Figure 14: The quantitative relationhip between tenile DIF and train rate Crack growth proce A mentioned above, the crack growth may contribute to the dynamic tenile trength enhancement of mortar. Thi i becaue the micro-crack propagating in the pecimen need more energy to initiate a new crack than to grow the old one, reulting that thee crack may be forced to propagate through the tronger fine aggregate rather than the weaker pate-aggregate interface. Therefore, it i intereting to explore the crack growth proce of the dynamic plitting tenile pecimen. Fig. 15 how a typical et of train-time hitory curve of the pecimen SP10 at train rate of It can be found from Fig. 14 that the initial crack i oberved at the approximate central

15 F. Yang et al. / Dynamic compreive and plitting tenile tet on mortar uing plit Hopkinon preure bar technique 744 point of the dic pecimen, a expected. Several micro-crack along the loading path are formed at the early tage, and they develop eparately with time but they aemble together later to become a main crack. According to the fracture time of two adjacent train gauge, the velocitie of crack growth can be etimated by dividing the length between thee two train gauge by the time interval. The value of the velocitie calculated are m/ (for train gauge 1 and 2), m/ (for train gauge 2 and 3), m/ (for train gauge 3 and 4) and m/ (for train gauge 4 and 5), repectively. An average velocity of the crack growth with the value of m/ i therefore obtained for the pecimen SP10. It hould be noted that, due to the irregularity of crack, pecimen defect, and the complexity of experimental condition, the crack velocity obtained i only an approximate value; and a further and ytematic tudy need to be conducted. Figure 15: Typical train-time hitory curve in tenile plitting tet (pecimen SP10). 5 CONCLUSIONS Dynamic compreive and plitting tenile tet on mortar under impact loading were conducted by uing plitting Hopkinon bar technique with a pule haper. With regard to experimental limitation of SHPB tet on concrete-like material, ome feaible improvement and modification were ummarized. Reult indicate that a proper pule haper can attenuate high-frequency ocillation of incident tre wave to improve the tre wave hape. Mortar i a train-rate enitive material; both compreive and tenile trength enhance with the increae of train rate, epecially when the train rate i greater than the tranition train rate, which i around 20-1 for the dynamic compreion and for the plitting tenion, repectively. An approximate crack velocity of m/ wa obtained for the teted mortar at train rate of Acknowledgment The author wih to acknowledge the financial upport provided by the China National Natural Science Funding (grant number ) and opening foundation for State Key Laboratory of Exploion Science and Technology (grant number ).

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