Terratek, Inc.

Reed Tool Company

Lynn M Barker

Randall V Guest

The present invention involves a new method for measuring plane strain fracture toughness (K.sub.Ic) of a somewhat non-elastic material uniquely suitable for use on specimens that are smaller than have formerly been required for performing valid K.sub.Ic tests. The present invention involves selecting a short-rod specimen configuration in which the specimen material at a crack front or tip therein would be constrained during loading to a plane strain state and, from the measurement and anelastic analysis of a load-displacement curve that includes at least two unloading slopes the energy per unit crack area that is required to slowly advance a steady state crack can be determined, which data, along with measurement of the critical load when the crack passes through a known location, and considering the specimen geometry, is used to determine the fracture toughness, K.sub.Ic, of the specimen.

Method for determining plane strain fracture toughness of non-elastic

The gauge of the present invention consists of a three-pronged spring clip for arrangement between the grips of a conventional grip-loading fracture specimen loading machine, which grips are inserted into the mouth of a standard slotted fracture toughness test specimen, the gauge to monitor the specimen mouth opening displacement thereof during a test for fracture toughness. The spring clip preferably is formed from a single sheet of material that is bent appropriately to produce two aligned legs or prongs with a center leg or prong there between. In operation the outside prongs are essentially stationary while the center prong is arranged to flex with respect to grip movement apart, which center movement is sensed electrically by a strain gauge secured thereto at its point of bending.

Gauge for measuring the mouth opening displacement of fracture toughness

A constant point of load application fracture specimen loading machine for applying a load between grips that engage opposite grip groove faces of a slotted specimen that preferably has as a remainder portion in that slot a 'V' shaped slot root whereby, when the machine grips are moved apart, they rotate so as to apply a constant point of load application to pull the specimen apart at that slot root. The machine of the present invention is useful for measuring the fracture toughness, fatigue-crack growth resistance, or stress corrosion crack growth resistance of slotted specimens providing a unique force application system whereby sliding friction losses are minimized, and other introduced effects, such as from specimen grip machining tolerances, or the like, are largely automatically compensated for.

Constant point of load application fracture specimen loading machine

The present invention consists of a method for accurately measuring the plane strain fracture toughness of ductile and brittle materials, the method to be practiced on a specimen formed such that, when the specimen is appropriately loaded, a crack will initiate at a predetermined point or line, with such crack formed thereat being intrinsically stable such that the load executed on the specimen must be continually increased in order to further advance the crack along a predetermined path through the specimen, until, at some critical location along the predetermined crack path, the load necessary to further advance the crack reaches a maximum, thereafter decrasing as the crack advances beyond the critical location along the remainder of its predetermined path through the specimen, that critical location being essentially independent of the specimen material, it being determined by the specimen geometry alone as long as elastic plane strain conditions prevail in the specimen sufficiently close to the crack tip. The method of the present invention involves taking the specimen through an appropriate loading sequence and determining the critical (or maximum) load that occurs when the crack passes through the critical position. With that data, the plane strain critical stress intensity factor, K.sub.IC, or fracture toughness for the specimen material, can be calculated mathematically considering the critical load, the specimen size and geometry, taking into account the Poisson's ratio of the specimen material, but without necessitating reference to any other standard specimen or fracture toughness test results thereon.

Method for measuring plane strain fracture toughness

An improved method for testing fracture toughness of tungsten carbide, and more particularly sintered tungsten carbide, consists of preparing a specimen from a short rod of sample material by cutting thin longitudinal slots forming a V within the slotted portion of the sample. An initial crack is started at the tip of the V formed by the slots. After the initial crack is started, the specimen is loaded with a force perpendicular to the slot plane and the peak force required in advancing the crack is recorded. The peak force is linearly related to the critical stress intensity factor (fracture toughness) of the specimen material.

Fracture toughness test method

A machine that can be used for loading of a slotted specimen for measuring the fracture toughness, fatigue crack growth resistance, or stress corrosion crack growth resistance of that specimen, the machine having a pressure bag maintained in a frame, which pressure bag can be installed within the slotted portion of the specimen and expanded to produce a desired specimen loading, the machine preferably involving a load pressure sensing device, can optionally include an arrangement for sensing volume enlargement of the specimen slot, and can also include a movable frame portion that expands with the pressure bag during inflation to discourage crimping of that pressure bag during specimen loading.

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Salt Lake City, UT, United States of America

Lynn M Barker