Sell Find articles by David R. The authors have declared that no competing interests exist.
Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA One of the more insidious types of failures that can occur in structures are brittle fractures,which are mostly due to either poor quality materials or poor material selection.
Brittle fractures tend to occur suddenly and without much material inelasticity; think of a bone fracture, for example. These failures often occur in situations where there is little ability for the material to develop shear stresses due to three-dimensional loading conditions, where local strain concentrations are high, and where a logical and direct force path was not provided by the designer.
Examples of this type of failure were observed in the aftermath of the Northridge earthquake in multi-story steel structures.
In these buildings, a number of the key welds fractured without displaying any ductile behavior. Fractures tend to occur near connections, or at interfaces between pieces of base materials, as welding tends to introduce local discontinuities in both, materials and geometry, as well as three-dimensional stresses due to cooling.
When specifying materials for a structure that will see very low operating temperatures i. In the civil engineering field that test is the Charpy V-notch test, which is described in this lab.
The Charpy V-notch test is intended to provide a very simplistic measure of the material's ability to absorb energy when subjected to an impact load. Principles In metal structures, one is interested in obtaining ductile behavior, such that there is a sign or forewarning of impending failure.
For example, in a steel beam, this might come in the form of excessive deformation. This performance is quantified through the material toughness, defined as the area under the stress-strain curve, which is the mechanical property most closely associated with ductile or brittle behavior.
Toughness is related to both strength and ductility. While toughness is the ability of the material to plastically deform before failure, ductility is the measure of how much a material can plastically deform before failure.
A material that has high strength but low ductility is not tough, just as a material with low strength and high ductility is not tough. In order for a material to be tough, it must be able to absorb high stress and high strain ductility and strength. The same material, a mild steel, for example, can behave in either a ductile or brittle fashion depending on the actualmaterial chemistry, processing,and loading conditions.
There are at least fivemain drivers for this possible change in performance: The molecular and microstructure of the material, with finer grain sizes resulting in increases in strength and decreases in ductility, and the presence of large quantities of alloys, such as carbon,often resulting in a decrease of the ductility of most steels.
The processing that the material undergoes can result in different toughness in steel plates in the direction of rolling, perpendicular to it, and in the through thickness of the plate.
This latter direction is particularly sensitive as it is hard to develop a consistent microstructure across a thick plate. The loading conditions loading in 3-dimensionswhich often inhibits the development of the shear stresses.
In 1- and 2-dimensional loading,one will generally encounter loading situations that give rise to large shear stresses, and thus a lot of yielding and ductile behavior. In the limit, for a 3D hydrostatic loading, there is no radius to Mohr's circle, and thus there is no shear.
In such cases, the material will not yield but fail suddenly.
The increase in the strain rate, which leads to higher strengths but reduced deformation capacity. A decrease in temperature, which can lead to significant, decreases in toughness. Some materials that might be very ductile at room temperature might become very brittle if the temperature is significantly decreased.
To determine whether a material will behave in a brittle or ductile manner, one typically runs a Charpy V-notch impact test. There are other similar tests, such as Izod impact test,which is the most commonly used toughness test in Europe. These tests intend to measure the energy that a small volume of material can absorb when subjected to a sudden impact load.
As noted earlier, this energy can be considered to be directly related to the area under the stress-strain curve. Each Charpy V-notch specimen to be tested for resistance to impact has standardized dimensions and is designed, supported, and loaded so that it will fail when subjected to a single blow applied in a standardized manner.
It is important to remember that the Charpy measurement is related to the volume and geometry of the specimen, and thus the results are useful for comparing the relative behavior of the materials and not for their absolute value.
To conduct the test, a small, beam-like specimen with a notch on one side Fig. The weight is usually between lbs and lbs, and can be dropped for different heights to produce different amounts of energy.
The V-notch is designed to induce a stress concentration, thus significantly increasing the local stress. When the beam is simply supported on the two sides and struck down the middle, the beam will be bent in tension where the notch is.
As a result, this will create a crack propagation through the specimen when struck.Normalized stress relaxation with σR(t) as the stress at time (t) and σR(0) the stress at start of the relaxation.
Control and 6 h MGO are stress relaxation experiments at similar collagen deformation levels (68 nm) and are taken from the present study.
Based on the experimental and finite element research on prestressed concrete beams with corrugated steel webs, this paper propose a prediction approach for the flexual capacity and deflection of.
This is related to the area under the stress-strain curve, with the toughest materials able to absorb both high stress and high strain. Charpy V-notch impact test values are accurate for specific testing conditions but can also be used to predict the relative behavior of materials.
BEAMS: STRAIN, STRESS, DEFLECTIONS The beam, or flexural member, is frequently encountered in structures and machines, and its elementary stress analysis constitutes one of the more interesting facets of mechanics of materials. A beam is a member subjected to loads applied transverse to the long dimension, causing the .
results to predicted values, and c) to verify certain aspects of stress-strain relations and simple beam theory. STRAINS, DEFLECTIONS AND BEAM BENDING LABORATORY shear and moment diagrams for the beam.
Calculate . • Hooke’s law regarding the proportionality between strain and stress. M = maximum bending moment at some point along the beam length Bending stress f b and each is half of the total load.
R = ½ x (½ x 10’ x lb/ft) = lb Draw the V and M diagrams. M max.