Tensile fracture behavior of two types of silicon nitride specimen geometries conducted by ten US groups

The work reported was conducted to provide a basis for structural ceramic m echanical property standardization activities under way in the United State s, Germany, Japan, and Sweden, All measurements reported here were conducte d by 10 US. groups on GN-10 silicon nitride within an International Energy Agency program including the United States, Germany, Japan, and Sweden. Thi s cooperative work included tensile strength studies of two geometries of b utton-head tensile specimens, The authors conducted some of the measurement s and performed data analyses and interpretation. The tensile fracture beha vior of GN-10 silicon nitride was studied at room temperature. A total of 1 50 strain-gaged button-head tensile specimens were measured, One hundred of a straight collet design and 50 of a tapered collet design were fractured. All specimens were highly strain gaged and the outputs for each were: meas ured during loading to fracture, Bending moments were calculated, Each part icipating laboratory group fractured 15 tensile specimens, 10 of the straig ht collet design and 5 of the tapered collet design under rigorously contro lled testing conditions. Of 100 straight collet specimens 75 broke,in the g age section. Of 50 tapered collet specimens 34 broke within the gage sectio n. Analysis of the Weibull m and a, estimators at upper and lower confidenc e bounds of 95% and 5% did not indicate a clear choice between the two desi gns. For specimens which fractured in the gage section, the unbiased maximu m likelihood Weibull estimators for in and a, were 12.5 and 730 and 10.4 an d 716, for the straight and tapered collet configurations, respectively. Th ese are not statistically different at the 95% and 5% confidence levels. St rengths were also analyzed in terms of a three-parameter Weibull model. The straight collet specimen data fitted the three parameter model well with a threshold stress estimator gamma of 506 MPa, while the tapered collet spec imens provided a poorer fit to the model and had a threshold stress estimat or of 432 MPa, a difference of about 15%. Regression analysis indicated tha t the straight collet grip provided less bias of strength as a function of bending moment. The straight collet specimens showed essentially little dep endence of tensile strength upon bending moment in the range of 0% to 6%, w hile the tapered collet specimens showed a decrease in strength as the bend ing moment increased from 0% to 4%, However, the regression parameter was l ow and no significant statistical conclusion could be made regarding the su periority of either of the grip designs.