Abstract :
The venerable problem of the origins of cracking and failure of a brittle heterogeneous
solid (heterogen) under compression is analyzed here from a new point of view. with the cause 01
its microcracking and atrophy (degeneration) under load. found, first of all. in the differences in
Poisson’s ratio of its components.
Four fundamental phenomena, found in experiments, are the basis of the new approach to the
behavior and failure of a het~rogen under compression. (I) The non-linear part of the ascending
branch of SSc is due to formation and accumulation of stahk microcracks. (2) The intrinsic elastic
modulus re~1~u111U.s VZS~U~~up to the peak point. (3) Concrete and rock fail ill sp/itrb!y. (4) The
pattern of SSc i.s rile suww for different types of concrete and rock under different types of loading.
It is found that some kinds of’gradien/ rnecham.ws can induce loctil IIZ~.YL’W.W.st ruin.c of tensiott
and cause microcracking in a heterogen under cornprrssion. The firsr creates local strain gradients
among the components due to difference in their Poisson extension, when the components with
lower Poisson’s ratio are tensioned in the lateral direction. There is also a mechanism which creates
an internal thrust due to gradir~it ilz /he elastic moduuli of the component.
It is shown that. in a brittle solid built from randomly oriented crystals. a population of lute&/J,
tc~vioned cr~,stals, called “acrons”. are created due to &radients in Poisson’s ratio of a single crystal
along its three axes. The models of gradient strain in the acronq are given. Including the equation
of critical strains. The problem of crystals acting as “pistons” due to a process of sliding is also
discussed.
The gradient models explain the appearance of microcracks and their stochasticity and why.
instead of grooving, into n~~rocraclis. they are stable. in good accordance with a vast number of
experiments, Gradient mechanisms. especially that of Poisson, are rrnircrsol, descriptiw und bawd
on measurable parameters. They suffice to exhaust the bearing capacity of a heterogen under
inci-easing compression without recourse to shear stresses. They affect every heterogen under compression
: rock materials, concrete. ceramics -and do not need initial microcracking to initiate and
realize the process of atrophy (degradation) of the brittle solid. ( 1997 Elsevier Science Ltd.
