This section of the guidelines covers analysis procedures for determination of structural joint properties. Reference to the following related sections may be useful:
Test Methods
9.2.3.6 Mechanically Fastened Joints
9.2.3.7 Fusion-Welded Joints
Data Requirements
9.2.4.6 Mechanically Fastened Joints
9.2.4.7 Fusion-Welded Joints
Examples of Data Analyses and Data Presentation
9.9.5 Mechanically Fastened Joints
9.9.6 Fusion-Welded Joints
It is important to recognize that these guidelines for the analysis and presentation of fastener design allowable properties in MIL-HDBK-5 are substantially different than the version that has been used for at least 20 years. These new guidelines are based on standardized statistical procedures, and involve the development of B-basis yield and ultimate load fastener allowables. Fastener tables included in MIL-HDBK-5 prior to Revision J will not be systematically reviewed or updated in accordance with these new guidelines. However, new fastener data proposals, or revisions to existing fastener allowable tables, will be based on the statistical procedures described in this section of the Handbook.
These new procedures were adopted to:
- • Migrate toward a consistent level of statistical confidence in tabulated fastener design properties.
- • Provide a method that accounts for (but is not driven by) singularity points in the data.
- • Allow for greater confidence and accuracy in fastener design allowables as sample sizes increase.
- • Ensure that fastener data analysis procedures will provide repeatable, unbiased results when used by different analysts.
Fastener tables approved prior to Revision J of MIL-HDBK-5 include ultimate load design allowables that are approximately equivalent to B-basis design properties. The yield properties shown in these same tables cannot realistically be equated with B-basis design properties; these previously established yield properties should be treated as conservative average fastener yield loads. To avoid confusion the basis of all fastener properties presented in Chapter 8 of MIL-HDBK-5 must be clearly delineated, as illustrated in Section 9.9.5.
9.7.1
MECHANICALLY FASTENED JOINTS — Some mechanical fasteners will not develop full bearing strengths of materials in which they are installed. Joint allowables for these fasteners must therefore be determined from test data. Fasteners for which allowable loads must be determined are:
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9.7.1.1
Definitions — Terms used in Section 9.7.1 vary among users of this Handbook. To provide consistency, these terms are defined herein in accordance with the intent of MIL-HDBK-5.
- (a) Deformable Shank Fasteners—A fastener whose shank is deformed in the grip area during normal installation processes.
- (b) Nominal Hole Diameters—Nominal hole diameters for deformable shank solid, blind rivet and blind fasteners will be according to Table 9.7.1.1. When tests are made with hole diameters other than those tabulated, hole sizes used will be noted in the report and on the proposed joint allowables table.
- (c) Nondeformable Shank Fasteners—A fastener whose shank does not deform in the grip area during normal installation processes.
- (d) Nominal Shank Diameter—Nominal shank diameter of fasteners with shank diameters equal to those used for standard size bolts and screws (NAS 618 sizes) will be the decimal equivalents of stated fractional or numbered sizes. These diameters are those listed in the fourth column of Table 9.7.1.1. Nominal shank diameters for nondeformable shank blind
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9.7.1.2
Yield Load Determination — The preferred method of determining yield load is by the secondary modulus method.** To obtain secondary modulus line, during the test the joint is unloaded from a load close to, and preferably above, estimated yield load to a load value in the range of about 10 to 20 percent of estimated yield load. The joint then is reloaded and secondary modulus is the slope of this second loading line. This procedure is described in NASM 1312-4 and is illustrated in Figures 9.7.1.2(a) through (e).
If curves similar to Curves A and B in Figure 9.7.1.2(b) are obtained early in the test program, strain hardening will be presumed. In that case, unloading should be delayed in subsequent tests until after anticipated yield load. Curves showing strain hardening may be extrapolated a reasonable amount to determine yield load by the secondary modulus method as shown.
The initial loading line is used to establish the intersection with the abscissa from which to measure yield offset. At times, minor irregularities occur on initial loading which necessitates redrawing of the lower part of the curve as a continuation of the normal curve, as shown in Curves C and D of Figure 9.7.1.2(c).
Unusually shaped curves are sometimes obtained. Typical of these are the illustrations in Figure 9.7.1.2(d). Data which are typified by Curves A or B are unacceptable for analysis. When the secondary modulus has a straight-line portion of recognizable length, do as shown in Curve C. When the secondary curve has two straight parts, but is more in question (as in Curve D), and there are satisfactory curves available from similar group test specimens, use the slope which approximates other curves. Otherwise, the more conservative (steepest) will be used. An acceptable alternate is to draw a straight
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9.7.1.3
Shear Strength of Fastener — Each group of double-shear or single-shear results for a specific fastener type, size, and material will be analyzed to determine an A-value, except driven rivets which will be analyzed to obtain a B-value. Data will be checked for their conformance to a Pearson distribution through use of the Anderson-Darling test described in Section 9.5.4.4. If the assumption of a Pearson distribution is not rejected:
- (a) For solid driven rivets, compute the B-value as shown in Section 9.5.5.1 and select the next lower shear strength from Table 8.1.1.1, if it is within 2 ksi of the computed value. If the computed B value is more than 2 ksi above the next lower value in Table 8.1.1.1, a new value may be proposed.
- (b) For other fasteners, compute the A-value as shown in Section 9.5.5.1 and select the next lower shear strength from Table 8.1.1.1. If the computed A-value is more than 5 ksi above the next lower value in Table 8.1.1.1, a new value may be proposed.
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9.7.1.4
Sheet Critical and Transition Critical Strengths — The analysis of data in the bearing and transitional regions provides design allowable curves for yield and ultimate strength where sheet or plate material of the joint is generally critical. To accomplish the analysis, tables and graphs are required as detailed in this subsection. The use of computer programs to analyze data and to prepare tables of
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9.7.1.5
Calculation of Allowable Loads — Allowable yield and ultimate loads will be calculated for each thickness and diameter combination using the B-basis lower bound curves described above. Allowable loads will not be calculated for thickness/diameter combinations below the t/D range tested, or for diameters not tested.
In these calculations, thickness (per Section 9.9.5, Note 11), and diameters to be used will be the nominal shank diameter (per Section 9.7.1.1) for S-Type fasteners and recommended nominal hole diameters (per Section 9.7.1.1) for H-type fasteners. Figure 9.7.1.5 shows a suggested format for this set of calculations.
Computation of Allowables from Design Curves
| D | D2 | t | t/D | Py/104D2 | Py | Pu/104D2 | Pu |
|---|
D, t, P_{u}, and P_{y}, as described in 9.7.1.4
Figure 9.7.1.5. Suggested tabular layout for computing allowables from design curves
The analysis of joint allowable load data for the case where data are required for procuring or regulatory agency (not for use in MIL-HDBK-5) for a limited range of sheet thickness and fastener diameter is as follows. An analysis similar to that described in Section 9.7.1.4 is required for data over the limited t/D range evaluated. In the special case where one sheet thickness and one fastener diameter have been tested in accordance with the requirements of Section 9.7.1.3, data will be analyzed as follows: the ultimate-load calculations will be made utilizing the statistical formulas listed in Section 9.7.1.3, where the k value is obtained from Table 9.10.1 for the appropriate number of test values (n) and 90 percent probability (B) value at a 95% confidence level. These ultimate-load values will be compared with values computed from bearing ultimate strengths of the joint material. In each comparison, the lower of either (1) statistical value computed from joint test data, (2) computed B-basis ultimate value from regression analysis, (3) computed bearing ultimate strength, or (4) fastener shear ultimate strength, will be the ultimate-load design allowable.
Similarly, the yield-load values will be compared with values computed from bearing yield strengths of the joint material. These yield-load values will be compared with values computed from bearing yield strengths of the joint material. In each comparison, the lower of either (1) statistical value computed from joint test data, (2) computed B-basis yield value from regression analysis, (3) computed bearing yield strength, or (4) fastener shear yield strength, will be the yield-load design allowable.
The load values so calculated will be rounded to three or four significant figures as follows:
- (1) Load values less than 1000 will be rounded to 3 figures (load values less than 100, 2 figures).
- (2) Load values greater than 1000 will be rounded to 4 figures. The fourth figure will be a 0 or 5.
9.7.2
FUSION-WELDED JOINT DATA — The purpose of this section of the guidelines is to provide a uniform procedure by which reliable design data on welded joints can be developed for use within the aerospace industry. Unlike most other guidelines procedures, for which reasonably complete concurrence has been found among the users of MIL-HDBK-5, those relating to fusion-welding allowables are still subject to interpretation by users in view of their own welding processes. An additional consideration is that fusion-welding allowables are highly process-dependent. Design values will not be presented in MIL-HDBK-5 since their application will be limited to the process represented by data from which the allowables were derived.
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9.7.2.1
Data Collection and Interpretation — Determination and presentation of properties of weldments requires adequate definition of pertinent welding parameters, including a description of base materials, welding process variables, and weld character. The most significant variables considered are divided into three basic categories: base materials, welding process variables, and weld character (see Figure 9.7.2.1). Variables listed are the minimum that must be identified and required by the specification.
In summary, the primary concern of population definition for weldments is to describe welding conditions in a manner that will assure reproducibility of this same population and will be sufficiently detailed to allow proper data analysis.
9.7.2.1.1
Base Materials — Base material variables include appropriate stipulation of alloy, composition form, preweld and postweld heat treat conditions, filler material, and material thickness.
9.7.2.1.2
Welding Process Variables — The most difficult aspect is establishing welding variables. The variables must be sufficiently detailed to represent the population of weldments produced, as well as to assure reproducibility of welds within this population. Appropriate selection of variables to be stipulated must be based on an interpretation of their effect on weldment properties and desirability of control.
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9.7.2.1.3
Weld Character — Appropriate levels of weld character must be prescribed in order to define a population of weldments. This includes a description of internal and external quality levels, as well as minimum joint strength requirements. In most specifications there are several weld classes which identify in detail the quality level requirements. In addition, means of determining weldment characteristics are established by stipulation of both nondestructive and destructive test methods.
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9.7.2.2
Data Analysis — Some concepts used for base-metal analyses lend themselves to analysis techniques for weldments. The procedures described in other sections of the guidelines may be used as a basis for analysis of mechanical property data for weldments in order to obtain A- and B-values. The procedures involve either direct statistical analysis of weldment data when sufficient data exist, or an indirect statistical analysis of ratios of paired properties.
The data samples required for direct statistical analysis will usually limit its use to tensile ultimate strength of weldment coupons. The indirect analysis may be used to derive other properties of interest using smaller samples. One example is to derive the minimum shear strength for the cases where only tensile distribution is known; one would operate on the ratio SUS/TUS in this case.
The indirect computation method also provides a tool for rational development of weld factors to be used in translating coupon-derived minimum properties to hardware design. In this case, ratio of hardware failure stress to control coupon failure stress is used.