October 2004 Shear Strength as a Function of Test Rate 1913 4 mm notche 20 mm 10.3 mm 据 4 mm (c) Fig 1. Dimensions and configurations of double-notch shear (DNS)test specimens used in this work: (a) interlaminar shear test specimen (b) in-plane shear test specimen, and (c) notch details. The DNS test specimens measuring 4 mm (width)x 4 mm Fig. 2, was used: Because of the merits of the test specimens (depth)x 20 mm (length), as shown in Fig. I, were machined from configuration and tight machining tolerances(e. g.=0.02 mm in the composite laminate. The DNS test specimens had been used parallelism between top and bottom surfaces ) the test specimen reviously for the determination of interlaminar and in-plane shear could stand alone. Percent bending due to misalignment, geomet- trength of the composite at both ambient and elevated tempera rical inaccuracies, or buckling was determined by strain gaging to tures. The square cross section was intentionally made to enable be less than 4%e at a perspective fracture force. In fact, bucking of a one-to-one comparison between the interlaminar and in-plane the test specimens was not an issue since the ratio of buckling shear strength at a given test condition without the interference of force to maximum failure force(1 100 N) was greater than 50 even ssible size effects. Dimensions of DNS test specimens used in for a conservative estimation with both ends hinged. If, for this work were different from those recommended by AStm example, a thin, tall configuration of SiC/BSAS test specimen. C1425"since the test specimens in the standard has a rectangular measuring 15 mm x 30 mm x 2 mm in width, length and cross section and would not be appropriate to determine both to be used in this case. as suggested in ASTM C1425y ouldhn l thickness, respectively, were used, the ratio decreases significantly interlaminar and in-plane shear strength. Two notches were 0.3 to around 3-4, so that appropriate antibuckling guides would have mm wide, 5 mm away from each other, and situated in equal distance from both ends. The two notches were extended to the middle of each specimen within =0.05 mm so that shear failure For both interlaminar and in-plane shear testing in displacement ccurs on the plane between the notch tips. Detailed descripti ontrol, a total of five test rates ranging from 3.3 X 10 to 3.3x 10 mm/s were used. Typically, three test specimens were tested and stress analysis of the DNS test specimens can be found at each rate for a given material direction. Each test specimen was elsewhere. Monotonic shear testing for the SiC /BSAS DNS test pecimens was conducted at 1 100C in ambient air with a relative bration before testing. The shear fracture stress-the average shear umidity of about 45%, using an electromechanical test frame stress at failure-was calculated using the following relation: Model 8562 Instron, Canton. MA). A simple test-fixture config uration consisting of SiC upper and lower fixtures, as shown in (1) where Tr is the shear strength, P, is the fracture force, and W and Ln, are the specimen width and the distance between the two Upper fixture notches, respectively. A limited fractographic analysis was per- formed in an attempt to help understand mechanisms associated with shear failu Extensometer Thermocouple I. Results DNS test specimen Lower fixture Without exception, all test specimens tested failed in shear mode along their perspective shear planes. Typical examples of shear failure at a test rate of 3. x 10- mm/s for both interlaminar nd in-plane test specimens are shown in Fig. 3 Fig. 2. Schematic showing test fixture and test specimen used in this BSAS composite are presented in Fig 4, where shear strength was plotted as a function of applied test rate for both interlaminar andOctober 2004 Shear Strength as a Function of Test Hate 1913 4 mm 20 mm 5 mm 4 mm notches 0.3 mm (a) (b) (c) Fig. 1. Dimensions and configurations of double-notch shear (DNS) test specimens used in this work: |;i| interlaminar shear test specimen, (b) in-plane shear test specimen, and (c) nolch details. The DNS test specimens measuring 4 mm (width) X 4 mm (depth) X 20 mm (length), as shown in Fig. 1. were machined from the composite laminate. The DNS test specimens had been used previously tor the determination of interlaminar and in-plane shear strength of the composite at both ambient and elevated tempera￾tures,*^ The .square cross section was intentionally made to enable a one-lo-one comparison between the interlaminar and in-plane shear strength at a given test condition without the Interterentre of possible si7,e effects. Dimensions of DNS test specimens used in this work were different from those recommended by ASTM C1425"' since the test specimens in the standard has a rectangular cross section and would not be appropriate to determine both interlaminar and in-plane shear strength. Two notches were 0.3 mm wide. 5 mtn away from each other, and situated in equal distance from both ends. The two notches were extended to the middle of each specimen within ±0.05 mm so that shear failure occurs on the plane between the notch tips. Detailed descriptions and stress analysis of the DNS test specimens can be found elsewhere.'^ Monotonic shear testing for the SiC,/BSAS DNS test specimens was conducted at 11()()°C in ambient air with a relative humidity of about 45'7r. using an electromechanical test frame (Model 8562. Instron, Canton. MA). A simple test-fixture config￾uration consisting of SiC upper and lower fixtures, as shown in Fig. 2. was used: Because of the merits of the test specimen's configuration and tight machining tolerances (e.g., ±0.02 mm in parallelism between top and bottom surfaces), the test specimen could stand alone. Percent bending due to misalignment, geomet￾rical inaccuracies, or buckling was determined by strain gaging to he less than 4% at a perspective fracture force. In faet. bucking of the test specimens was not an issue since the ratio of buckling force to maximum failure force (1100 N) was greater than 50 even for a conservative estimation with both ends hinged,'' If. for example, a thin, tall configuration of SiC/BSAS test specimen, measuring 15 mm X 30 mm X 2 mm in width, length, and thickness, respectively, were used, the ratio decreases significantly to around 3-4. so that appropriate antibuckling guides would have to be used in this case, as suggested in ASTM C1425.'' For both interlaminar and in-plane shear testing in displacement control, a total of five test rates ranging from 3,3 X 10"'' to 3,3 X 10"' mm/s were used. Typically, three test specimens were tested at eaeh rate for a given material direction. Each test specimen was held for abuut 20 min at the test temperature for thermal equili￾bration before testing. The shear fracture stress—the average shear stress at failure—was calculated using the following relation: 0) Thermocouple .—. DNS test specimen (self standing) Upper fixture Extensometer Lower fixture Fig. 2. Schematic showing tesi fixture and test specimen used in this work. where T,- is the shear strength. P, is the fracture force, and IV and L^. are the specimen width and the distance between the two notches, respectively, A limited fractographic analysis was per￾formed in an attempt to help understand mechanisms associated with shear failure. 111. Results Without exception, all test specimens tested failed in shear mode along their perspective shear planes. Typical examples of shear failure at a test rate of 3.3 X 10~" mm/s for both interlaminar and in-plane test specimens are shown in Fig, 3, The results of monotonic shear strength testing for the SiC^/ BSAS eomposite are presented in Fig. 4, where shear strength was plotted as a function oi' applied test rate for both interlaminar and