Creep‐Induced Residual Stress Strengthening in a Nicalon‐Fiber‐Reinforced BMAS‐Glass‐Ceramic‐Matrix Composite

Abstract

The feasibility of inducing a compressive residual stress in the matrix of a Nicalon-fiber-reinforced BMAS-glassceramic-matrix composite through a creep-load transfer treatment was studied. Specimens were crept at 1100°C under constant tensile load to cause load transfer from the matrix to the fibers, then cooled under load. Upon removal of the load at room temperature, the matrix was put into compression by the elastic recovery of the fibers. This compressive residual stress in the matrix increased the roomtemperature proportional limit stress of the composite. The increase in the proportional limit stress was found to be dependent upon the applied creep stress, with an increase in creep stress resulting in an increase in the proportional limit stress. Acoustic emission results showed that the onset of significant matrix cracking correlated closely to the proportional limit stress. Changes in the state of residual stress in the matrix were supported by X-ray diffraction results. Fracture surfaces of all specimens exhibited fiber pullout behavior, indicating that the creep-load transfer process did not embrittle the fiber/matrix interface.

Design Guidelines for In‐Plane Mechanical Properties of SiC Fiber‐Reinforced Melt‐Infiltrated SiC Composites

“…As reported previously [5], the cause of this increase is due to stress-relaxation in the matrix, particularly the Si-SiC particulate portion. This results in an increased compressive stress in the matrix upon unloading, which must be overcome to form bridged-matrix cracks [19,20]. As a further validation to this hypothesis, a Syl-iBN-3 composite specimen was pre-crept for 50 hours at 1315 o C and 138 MPa.…”

“…This results in an increased compressive stress in the matrix upon unloading, which must be overcome to form bridged-matrix cracks. 19,20 As a further validation to this hypothesis, a Syl-iBN-3 composite specimen was precrept for 50 h at 13151C and 138 MPa. This was then crept at 12001C at 220 MPa (as noted in Fig.…”

Interfacial Behavior of Nicalon-Fiber-Feinforced Glass-Ceramic Matrix Composite Due to Creep-Conditioning

“…The increase in the compressive residual stress was confirmed by the X-ray Diffraction data. 6 The longitudinal residual stress, i.e. in direction parallel to the axis of the applied stress, in the matrix of the as-received specimen was found to be -71 (±8) MPa, and it was -124 (±11) MPa for the specimen that was creep-conditioned at 1100°C for 12 hours at 72 MPa.…”

“…The full description of the creep-conditioning test can be referred from elsewhere. 5 Room-temperature mechanical tests were performed to obtain stress-strain curve for each specimen, i.e. the as-received and crept specimens, using a commercial tensile test machine.…”

“…5 " 6 This compressive residual stress in the matrix increases the roomtemperature proportional limit stress of the composite, and thus delaying the onset of permanent damage and the loss of protection provided by the matrix against environmental degradation of the fibers. A significant increase in compressive residual stress in the matrix has been shown in the analytical model of stress redistribution in a composite during creepload transfer treatment.…”