Berk Korucu - 030080104 - 10th Week Definitions Part-2

3) Compliance-Based Fracture Toughness Testing (Material Testing)

There is no previous definition.

      Compliance-Based Fracture Toughness Testing. Laboratory testing for fracture toughness has become increasingly reliant on servohydraulic equipment, and a synthesis of mechanical test apparatus with sophisticated computer data acquisition and controls is becoming the fracture toughness test standard. Compliance-based fracture testing employs a displacement (CMOD) gage. DC signals are amplified and conditioned to control and monitor the test, as shown schematically in Fig. 11. The load is generally monitored by the use of a load cell, mounted within the test frame in the load train.

     

      

     Compliance-based fracture testing employs the relationship between compliance, which is the reciprocal of the loaddisplacement plot generated during testing, and the crack length. For example, the equation for the crack mouth opening compliance of a compact tension specimen for LEFM (following ASTM E 399) can be expressed as follows (Ref 20):

      

      where C is the compliance, v is the displacement of the clip gage, B is the specimen thickness, and P is the load.The relationship between compliance and crack length has been verified in the literature (Ref 64), and these relationships are generally good for specific specimen geometries, such as compact tension (C(T)) and single-edged notch bend specimens (SE(B)), as found in ASTM E 399, E 813, and E 1737 (Ref 20, 26, 17). With reference to Fig. 12, and using the C(T) specimen for an example, the elastic compliance relationship (following ASTM designation E 399) works as follows:

      1. The clip gage records a displacement and the load cell records the load, both values of which are input into the compliance equation (Eq 26).

      2. The compliance is input into an equation of the form:

      

      3. where ux is a value to use for a polynomial expression.

      4. Continuing, ux is input into a fifth-order polynomial expression with various constants as follows:

      

      5. where W is the specimen width and ci are various constants. Multiplying both sides of the equation by W obtains the crack length.

      6. Finally, the stress intensity factor can be determined by the following expression:

      7.                                     

      It should be noted that the compliance-based procedure just developed describes only LEFM testing in detail. For EPFM testing the procedure is essentially the same, except that plasticity must be accounted for in the determination of J and CMOD.

       (P. Andresen et al. , ASM Handbook Vol 20 Materials Selection And Design , p.1273-1274)

4) Design for Wear Resistance (Design)

There is no previous definition.

Wear is damage to a solid surface as a result of relative motion between it and another surface or substance. The damage usually results in the progressive loss of material. The scientific measure used for wear is volume loss. However, in engineering the concern with wear is usually associated with dimensional or appearance changes that eventually affect performance and not with volume loss. As a result other measures are often used in practice, such as depth of the wear scar on a mechanical component or the degree of haze with optical components.


For any material, wear can occur by a variety of mechanisms, depending on the properties of the material and the situation in which it is being used. Wear resistance is, therefore, not an intrinsic material property like hardness or elastic modulus. Both wear and wear resistance are system properties or responses.


Collection of all the mechanical, chemical, and environmental elements that can affect wear and wear behavior is referred to as the tribosystem. Typical factors that can affect wear behavior are the properties of the materials, the nature of the relative motion, the nature of the loading, the shape of the surface(s), the surface roughness, the ambient temperature, and the composition of the environment in which the wear occurs. Tribosystem design parameters are those parameters that affect wear and that the designer can specify and alter. Designing for the control of wear involves selecting values for tribosystem design parameters in order to obtain acceptable wear behavior or life. The process for doing this is called wear design.


Fundamentally, wear design consists of identifying those design factors that can affect wear and then determining values for them on the basis of their effect on wear rate. The aim is to achieve an acceptably low wear rate. Common tribological design parameters are materials, surface contours, lubrication, and roughness. However, these are not the only ones that can be considered design parameters. For example, loading, type of relative motion, and various environmental parameters may be utilized as design parameters in some situations.


Different design parameters influence wear rate in different ways. There are four fundamental ways to reduce wear rates: by modifying the surface to make it more wear resistant, by using a more wear-resistant material or material pair, by increasing the separation between the surfaces, and by reducing the severity of the contact (i.e., by modifying those features of the wear situation that tend to increase wear rate). Often, the wear rate reduction associated with a design parameter comes from a combination of these methods.


For example, one common method for reducing wear rate is to use a lubricant. A lubricant can reduce wear rate by reducing surface shear forces, by reacting with the surface to form a more wear-resistant surface layer, or by acting as an interposing layer that decreases the amount of contact between the contacting surfaces. In most situations all three elements are present. A lubricant can also conduct heat away from the contact region. In situations where temperature of the contacting surfaces is a factor in wear behavior, this is another way by which a lubricant can affect wear.


(R. G. Bayer, ASM Handbook Vol 20 Materials Selection And Design, p.1424)