Four-point Bending Fixture Research Articles

An engineering tool for fracture toughness testing

A chevron notch four-point bend geometry fracture specimen is presented which complements the chevron notched short rod and short bar geometry used in ASTM E1304, the newest standard for fracture toughness testing. Although the four-point bend samples require slightly more material, sample preparation is less involved. Therefore total testing costs are lower using this four-point bend geometry when testing materials with low to moderate toughness to yield strength ratios. To test the new four-point bend samples, a stand alone toughness tool has been developed which allows a fracture toughness measurement to be made in about 30 seconds. It is shown that the fracture toughness values, measured using this toughness tool, are identical to the values obtained when loading the samples in a servo hydraulic testing system using a four-point bend fixture in displacement control. The fracture toughness values obtained using either loading method agree with fracture toughness values obtained according to ASTM E399.

Evaluation of piezoresistive coefficient variation in silicon stress sensors using a four-point bending test fixture

The variation of the piezoresistive coefficients from several rosettes on the same die, the same wafer, and finally at different doping levels across a number of wafers was examined. A thorough error analysis of the method of applying a known uniaxial state of stress using a four-point bending (4PB) fixture was completed. A sensor error analysis demonstrated that it is very difficult to determine accurate values for the sum ( pi /sub 11/+ pi /sub 12/) using the common two-element rosette, particularly in p-type material. However, an empirical equation was found that provides an estimate for this coefficient. The second piezoresistive coefficient pi /sub 44/ can be measured accurately. However, the results presented for pi /sub 44/ differ from those of previous authors by some 33%. Thus, it appears necessary to measure this value for a given wafer lot. >

A Microcomputer-Aided Four-Point Bend Test System for Determining Uniaxial Stress-Strain Curves

Abstract A microcomputer-aided four-point bend test system has been developed for determining the tensile and compressive stress-strain curves, as well as elastic constants, for a material from a single bend test. A special four-point bend fixture has been designed and constructed to minimize loading errors which may occur in the tests. Microcomputer software has been developed for processing the large amount of data obtained from each bend test. Two kinds of cast iron (ASTM A48 and A536), which exhibit different stress-strain characteristics in tension and compression, have been tested to examine the validity of the test system for a limited range of strain. It has been shown that the stress-strain relations derived from the bend tests are in good agreement with those obtained directly from the uniaxial tension and compression tests for strains less than about 5%. The advantages and limitations of the bend tests are also discussed.

Shear properties and a stress analysis obtained from vinyl-ester losipescu specimens

The use of losipescu specimens for the determination of the shear properties of a vinyl-ester resin was investigated. The antisymmetric four-point bend and the Adams and Walrath fixtures were studied for their suitability in loading these specimens. Photoelastic and strain-gage data in addition to published finite-element results show that the latter fixture distorts the stress field in the gage section. The antisymmetric four-point bend fixture is found to give the purest shear-stress field in the gage section and to yield the most reliable shear-modulus values. A refined photoelastic analysis shows that the shear-stress distribution between the notch roots is essentially uniform with a relative maximum or minimum at the centroid depending on the depths of the notches. Also, stress risers of up to 30 percent are observed near the notch roots. Except at the roots, finite-element predictions are presented which are in excellent agreement with photoelastic data. The failure mode of this vinyl-ester resin is tensile and the corresponding tensile stress calculated from the average shear stress in the gage section of the losipescu specimen is in excellent agreement with failure data acquired in tension.

Uniaxial stress-strain curves from a bending test

A method which can be used to obtain both tension and compression stress-strain curves from a single bend test was first proposed in 1910. Curiously, this apparently useful technique has been very little application since that time. For this reason, the present investigation was conducted to examine the merits and shortcomings of the test. Three materials which show different stress-strain curves in tension and compression, beryllium, cast iron and prestrained copper, were studied to test the versatility of the method. A special four-point bend fixture, designed to minimize loading errors, was designed for this study and is described. Generally, the stress-strain curves obtained from bending and conventional tests showed good agreement for strains less than about five percent. Finally, some advantages and disadvantages of the bending tests are pointed out.