Source: 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 1994 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.e., the Alaska pipeline) many cycles of loading (a bridge on an interstate highway), or where welding is used extensively, it is necessary to have a simple test that characterizes the material's robustness, or resistance to fracture. 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.

In this experiment we will test several Charpy specimens at different temperatures to illustrate the temperature effect on impact resistance of mild steel.

1. To prepare the testing machine, first make sure the path of the hammer is clear of any obstructions. Once the path is clear, lift the hammer until it latches and secure the lock to prevent accidental release of the hammer.
2. To prepare the specimens, use the cold box to cool one specimen of each metal to a temperature well below freezing.Use a ho

After repeating the experiment for may specimens and temperature values, you can plot the temperature dependence of the energy absorbed and clearly see the existence of an upper and lower shelf (or flat horizontal portions). These shelves indicate that there are clear minima and maxima that can be achieved for a given material and processing. The main interest is in carefully quantifying the transition temperatures to minimize the risk that these fall within the operating temperatures of the structure being designed. Sim

Impact testing, in the form of Charpy and Izod tests, is commonly used to measure the resistance of metallic materials to brittle fracture. The Charpy test uses a small beam specimen with a notch. The beam is loaded by a large hammer attached to a frictionless pendulum. The combination of the strain rate from this loading sequence and the presence of the V-notch that creates a local large stress concentration result in fast crack propagation and splitting of the specimen.

The test determines t