Report summarizes the findings of studies made of the influence of both fiber and matrix characteristics upon the mechanics of deformation and fracture of fibrous composites. The major portion of the report is devoted to a study of the mechanics of tensile failure of a fibrous composite

Rosen, B. W.

English

NASA Technical Reports Server

December 1967	 Brief 67-10660 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Study Made of Mechanics of Deformation and Fracture 
of Fibrous Composites 
The existence of many very high strength and stiff-
ness filaments has motivated considerable interest in 
the selection of appropriate matrix materials to obtain 
fibrous composites which can utilize these outstanding 
properties. Studies have been made of the influence of 
both fiber and matrix characteristics upon the me-
chanics of deformation and fracture of fibrous com-
posites. A report summarizing the findings of these 
studies has been prepared presenting the information 
needed to select appropriate constituents for given 
applications. These studies consider the response 
of a matrix reinforced by uniaxially oriented fibers. 
The strength of such a material is treated for the 
cases where the failure criteria are maximum tensile 
and compressive IQads carried by the composite in a 
direction parallel to the fiber orientation. 
When the response of a composite is to be meas-
ured in terms of average stress and average strain, the 
material can be represented by an effective homogene-
ous but anisotropic material having the same average 
response. For the case of a matrix containing uni-
axially oriented fibers, the effective material is 
transversely isotropic and is therefore characterized 
by five elastic constants. Various analytical ap-
proaches to the evaluation of these constants as func-
tions of the constituent properties have been made 
and the relationship between constituent properties 
and elastic moduli is reasonably well understood. 
However, when a fracture criterion is desired, an 
understanding of the average stress-strain response 
is no longer sufficient, and consideration must be 
given to internal irregularities in the state of stress. 
The report treats these problems for both ultimate 
compressive and tensile strengths.
The failure of a fibrous composite under a uniaxial 
compressive load is considered first. A possible failure 
mechanism for this case is the hypothesis that the 
individual fibers buckle in a short wavelength pattern 
in a fashion analogous to the buckling of a column 
or plate on an elastic foundation. An approximate 
evaluation of the influence of fiber geometry and 
fiber and matrix moduli upon the composite compres-
sive strength can then be made. These results are 
compared to the results of an experimental program 
which utilized hollow and solid glass fibers in several 
matrix materials. The test specimens were short 
columns designed to achieve a compressive strength 
failure. 
The major portion of the report is devoted to a 
study of the mechanics of. tensile failure of a fibrous 
composite. The study is based upon consideration of 
the phenomena which occur subsequent to an initial 
internal fracture. The strength of the brittle fibers 
are considered to be defined by a statistical distribu-
tion function. Thus, the initial fracture is likely to 
occur in a fiber. The resulting perturbation of the 
local stress field is treated. An approximate solution 
indicates the nature of the interface stress distribution 
as well as stress concentrations in nearby fibers. This 
stress field could result in adjacent fiber fracture, that 
is, a crack propagation effect, or in separation along 
the interface. A third possibility is that the stresses in 
the vicinity of an initial fiber fracture do not produce 
further fracture and that increasing load produces a 
distribution of fiber fractures corresponding to the 
initial distribution of weak points in the fibers. The 
continued accumulation of these fractures would pro-
duce a weak cross section at which the remaining un-
(contnucd overleaf) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19670000659 2020-03-11T21:39:12+00:00Z
broken fibers could no longer transmit the applied 
load. Instantaneous tensile 'failure would then occur. 
A statistical tensile failure model of a fibrous 
composite is established on this basis, and the me-
chanical characteristics of fiber and matrix are util-
ized to obtain a statistical failure definition. An experi-
mental program to investigate the validity of the 
model is described, and an attempt is made to cor-
relate the tensile properties of constituents and com-
posites. 
Note: 
Additional details of this study are contained in: 
Mechanics of Composite Strengthening, by B. Walter 
Rosen, General Electric Co. Copies of this report are 
available from: 
Technology Utilization Officer 
Headquarters 
National Aeronautics and Space 
Administration 
Washington, D.C. 20546 
Reference:. B67-10660
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: B. W. Rosen
of General Electric 
under contract to
NASA Headquarters
(HQN- 10035)
• 
Brief 67-10660	 Category 03


Study made of mechanics of deformation and fracture of fibrous composites

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