Fracture Energy for Short Brittle Fiber/Brittle Matrix Composites With Three-Dimensional Fiber Orientation

1990 ◽  
Vol 112 (4) ◽  
pp. 502-506 ◽  
Author(s):  
R. C. Wetherhold
Keyword(s):  
Fracture Energy ◽  
Fiber Orientation ◽  
Interfacial Shear Stress ◽  
Three Dimensional ◽  
Matrix Composites ◽  
Limited Data ◽  
Brittle Matrix ◽  
Pull Out ◽  
Brittle Fiber ◽  
Parametric Studies

Adding brittle fibers to a brittle matrix can create a composite that is substantially tougher than the monolithic matrix by providing mechanisms for energy dissipation during crack propagation. A model based on probabilistic principles has been developed to calculate the increased energy absorption during fracture for a brittle matrix reinforced with very short, poorly bonded fibers. This model, previously developed for planar fiber orientations, is extended to consider the three-dimensional fiber orientations that may occur during composite fabrication. The fiber pull-out energy is assumed to dominate other fracture energy terms, and simple parametric studies are performed to demonstrate the effect of fiber orientation, fiber length, fiber diameter, and fiber-matrix interfacial shear stress. In particular, the fiber orientation effects may be grouped into an effective “orientation parameter.” The model predictions compare satisfactorily with the limited data available, and offer a conceptual framework for considering the effect of changing the physical variables on the fracture energy of the composite.

scholarly journals Fracture Energy for Short Brittle Fiber/Brittle Matrix Composites With Three-Dimensional Fiber Orientation

1989 ◽  
Author(s):  
Robert C. Wetherhold
Keyword(s):  
Fracture Energy ◽  
Fiber Orientation ◽  
Interfacial Shear Stress ◽  
Three Dimensional ◽  
Matrix Composites ◽  
Limited Data ◽  
Brittle Matrix ◽  
Pull Out ◽  
Brittle Fiber ◽  
Parametric Studies

Adding brittle fibers to a brittle matrix can create a composite which is substantially tougher than the monolithic matrix by providing mechanisms for energy dissipation during crack propagation. A model based on probabilistic principles has been developed to calculate the increased energy absorption during fracture for a brittle matrix reinforced with very short, poorly bonded fibers. This model, previously developed for planar fiber orientations, is extended to consider the three-dimensional fiber orientations which may occur during composite fabrication. The fiber pull-out energy is assumed to dominate other fracture energy terms, and simple parametric studies are given to demonstrate the effect of fiber orientation, fiber length, fiber diameter, and fiber-matrix interfacial shear stress. In particular, the fiber orientation effects may be grouped into an effective “orientation parameter”. The model predictions compare satisfactorily with the limited data available, and offer a conceptual framework for considering the effect of changing the physical variables on the fracture energy of the composite.

Effect of Fiber Extensibility on the Fracture Toughness of Short Fiber or Brittle Matrix Composites

1992 ◽  
Vol 45 (8) ◽  
pp. 377-389 ◽  
Author(s):  
L. K. Jain ◽  
R. C. Wetherhold
Keyword(s):  
Fracture Toughness ◽  
Fracture Energy ◽  
Fiber Orientation ◽  
Crack Opening ◽  
Matrix Composites ◽  
Brittle Matrix ◽  
Pull Out ◽  
General Terms ◽  
Brittle Matrix Composites ◽  
Fiber Orientation Distribution

A micromechanical model based on probabilistic principles is proposed to determine the effective fracture toughness increment and the bridging stress-crack opening displacement relationship for brittle matrix composites reinforced with short, poorly bonded fibers. Emphasis is placed on studying the effect of fiber extensibility on the bridging stress and the bridging fracture energy, and to determine its importance in cementitious matrix composites. Since the fibers may not be in an ideal aligned or random state, the analysis is placed in sufficiently general terms to consider any prescribable fiber orientation distribution. The model incorporates the snubbing effect observed during pull-out of fibers inclined at an angle to the crack face normal. In addition, the model allows the fibers to break; any fiber whose load meets or exceeds a single-valued failure stress will fracture rather than pull out. The crack bridging results may be expressed as the sum of results for inextensible fibers and an additional term due to fiber extensibility. An exact analysis is given which gives the steady-state bridging toughness G directly, but presents a non-linear problem for the bridging stress-crack opening (σb – δ) relationship. An approximate analysis is then presented which gives both G and σb – δ directly. To illustrate the effect of extensibility on bridging stress and fracture energy increment due to bridging fibers, a comparison with the inextensible fiber case is provided. It is found that effect of extensibility on fracture energy is negligible for common materials systems. However, extensibility may have a significant effect on the bridging stress-crack opening relationship. The effect of other physical and material parameters such as fiber length, fiber orientation and snubbing friction coefficient is also studied.

Matrix cracking and the mechanical behaviour of SiC─CAS composites

1992 ◽  
Vol 437 (1899) ◽  
pp. 109-131 ◽  
Keyword(s):  
Elastic Properties ◽  
Three Dimensional ◽  
Matrix Cracking ◽  
Repeated Loading ◽  
Matrix Composites ◽  
Pull Out ◽  
Matrix Cracks ◽  
Brittle Matrix Composites ◽  
Dimensional Network ◽  
The Matrix

An experimental investigation has been carried out on the mechanical properties of unidirectional (0) 12 , (0, 90) 3S , (±45, 0 2 ) S , and (±45) 3S composites consisting of CAS glass ceramic reinforced with Nicalon SiC fibres. Measurements have been made of the elastic properties and of the tensile, compression and shear strengths of the composites, and these have been supported by a detailed study of the damage which occurs during monotonic and repeated loading. These damage studies have been carried out by means of edge replication microscopy and acoustic emission monitoring. The elastic properties of the composites are, by and large, close to the values that would be predicted from the constituent properties and lay-up sequences, but their strengths are lower than expected, and it appears that the Nicalon reinforcing fibre has been seriously degraded during manufacture. The fracture energy is much higher than predicted from observations of fibre pull-out, and it is suggested that the energy required to form a close three-dimensional network of matrix cracks could account for the high apparent toughness. The matrix cracking stress can be predicted reasonably closely by the Aveston, Cooper and Kelly model of cracking in brittle matrix composites, but it is shown that subcritical microcracks can form and/or grow at stresses well below the predicted critical values without affecting composite properties.

Improving Fracture Toughness of Brittle Matrix Composites Using End-Shaped Ductile Fibers: The Effects of Adhesion and Matrix Shrinkage

Materials ◽  
2003 ◽  
Author(s):  
Robert C. Wetherhold ◽  
Renee M. Bagwell
Keyword(s):  
Fracture Toughness ◽  
Bond Strength ◽  
Surface Chemistry ◽  
Matrix Composites ◽  
Release Agent ◽  
Brittle Matrix ◽  
Matrix Shrinkage ◽  
Pull Out ◽  
Brittle Matrix Composites ◽  
The Matrix

Ductile fibers are added to brittle matrix composites to increase the fracture toughness. To further improve fracture toughness, end shaped ductile fibers are added to act as anchors to utilize more of the fibers’ plasticity. Previous research focused on optimizing the volume of the shaped end for a given end shape family. Results indicate that for a given end shape family there is an optimum volume; above or below this volume results in a lower fracture toughness contribution. This research investigates two additional factors, adhesion of the matrix to the fiber and matrix shrinkage, and determines their effects on the fracture toughening of brittle matrix composites. The fiber was an annealed copper and the matrices used were a low shrinkage epoxy, a high shrinkage epoxy, and polyester. Results indicate that controlling the surface chemistry of the fiber can give an additional degree of freedom to the utilization of the fiber plasticity, although the importance of this control depends on the particular system. The fiber surface chemistry affects the bond strength and the adhesion; if the fiber cannot debond from the matrix, then shaping the end does not permit use of the plastic potential. Depending on the system, the adhesion and bond strength of the matrix to the fiber significantly affects the amount of fiber plasticity utilized. To determine the effects of friction and matrix shrinkage on the utilization of the fiber plasticity, release agent was applied to the end shaped fibers to reduce the adhesion, bond strength, and friction during pull out. Results indicate that frictional work and adhesion has a large impact on the utilization of the fiber plasticity; with release agent, the end shaped fiber utilizes little of the fiber plasticity. Furthermore, this indicates that for the matrices investigated, matrix shrinkage has a minor influence on the utilization of the fiber plasticity.

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