The material tests provide the basic materials data for conducting structural crack growth life and residual strength analyses.  The tests are relatively simple to conduct compared to many of the tests in the other categories.  Typically, a large number of material tests are conducted in the early part of the design phase so that the appropriate materials can be selected to meet design objectives.  The materials selection process may concentrate on specific design criteria relative to requirements of cost, weight, strength, stiffness, fracture toughness, corrosion resistance, and crack growth resistance to fatigue loading.  The damage tolerance materials tests discussed in this section must, of course, be supplemented by other tests, e.g. tensile tests, exfoliation tests, etc., in order to ensure that preliminary material trade studies result in the appropriate choices for the given application.  Typically, before the final bill of materials for the structure is signed off, additional in-depth structural tests must be accomplished to verify initial material choices and to identify additional criteria not initially considered.

Residual strength and crack growth life analyses are supported by a damage integration package that requires the definition of fracture toughness and crack growth rate properties for the materials being considered (See Section 2 for a discussion of the damage integration package).  As indicated in Section 4 on Residual Strength and in Section 5 on Crack Growth, a material’s crack growth behavior is a function of a wide number of different factors such as anisotropy, environment, loading rate, processing variables, product form, thickness, etc.  The damage integration package accounts for these effects by utilizing data collected from specimens (a) that are representative of the material variables of interest, (b) that contain cracks which grow in the appropriate direction, and (c) that are loaded in the manner representative of operational conditions.

Standardization of test methodologies, data reduction and reporting procedures are to a large part responsible for the success of the current life prediction models.  The predictive accuracy of any lifing model is only as good as the quality of the baseline crack growth and fracture data inputs.  The American Society for Testing and Materials (ASTM) is the world leader in producing consensus testing standards to accurately identify materials behavior in general – and most important to the DTDH – have been the leader in developing procedures usable for damage tolerance applications.  The ASTM Standards applicable to the DTDH are listed in Table 7.2.1.

The ASTM Book of Standards is published yearly to give all users of the test methods and analytical procedures the latest versions available.  Within this section, whenever an ASTM Standard Test Method is referenced (i.e. ASTM E399), the ASTM Book of Standards for the current year should be consulted.

Table 7.2.1.  ASTM Standards for Damage Tolerant Testing

Each of the Standard Test Methods used for damage tolerance testing have a selection of test specimens that are preferred for each test.  Figure 7.2.1 shows the most common types of specimens and includes the preferred specimen ratios of width/thickness (W/B) for each type.  The thickness B is the dominant geometric consideration for determining if the specimen crack tip geometry is in a plane strain or a plane stress (or intermediate) condition.  An asterisk denotes the most common W/B ratio for damage tolerance testing.

Figure 7.2.1.  Specimens for Damage Tolerance Testing

Figure 7.2.1.  Specimens for Damage Tolerance Testing (Continued)

As a result of the concerns about the effects of anistropy on material fracture toughness and crack growth resistance properties, standard nomenclature relative to directions of mechanical working (grain flow) has evolved.  Figure 7.2.1 shows drawings of specimens which will be oriented in different directions relative to the product form.  The orientation of the crack plane should be identified whenever possible in accordance with the systems shown in Figure 7.2.2.

For rectangular sections, the reference directions are identified in parts a and b of Figure 7.2.2 where an example of a rolled plate is used.  The same system would be useful for sheet, extrusions, and forgings with non-symmetrical grain flow:

L – direction of principal deformation (maximum grain flow)
T – direction of least deformation
S – third orthogonal direction.

Figure 7.2.2.  Crack Plane Orientation Code for Rectangular Sections and for Bar and Hollow Cylinders [ASTM 2001]

When reporting crack orientation in rectangular sections, the two letter code, such as T-L in Figure 7.2.2a, is used when both the loading direction and direction of crack propagation are aligned with the axes of deformation.

For specimens tilted with respect to two of the reference axes (Figure 7.2.2b), a three-letter code, e.g. L-TS, is used.  The designation used can be interpreted by considering the codes as a composite pair in which the first element in the pair designates the direction normal to the crack plane and the second element designates the expected direction of crack propagation.  The code T-L for a cracked specimen indicates that the fracture plane has a stress application normal in the T direction (width direction of the plate) and the expected direction of propagation in the L direction (in the longitudinal direction of the plate), see Figure 7.2.2a.  The code L-TS means that the crack plane is perpendicular to the L direction (principal deformation) and the expected crack direction is intermediate between T and S, see Figure 7.2.2b.

For cylindrical sections where the direction of principal deformation is parallel with the longitudinal axis of the cylinder, such as for drawn bar stock and for extrusions or forged parts having a circular cross section, the specimen reference directions are described in Figure 7.2.2c.  The three directions used here are:

L – directional of maximum grain flow (axial)
R – radial direction, and
C – circumferential or tangential direction

Interpretation of the specimen designations relative to the location of the crack plane and crack path is similar to that employed for the rectangular sections.

In the remainder of this section, attention will be given to those tests which are utilized to collect data that support the material selection function and the damage integration package.  The first of these subsections covers those tests which are used to establish the fracture toughness of materials.  The other subsections cover tests utilized to collect sub-critical crack growth data.