Testing Tech: Compression After Impact Testing
During the late 1960s and most of the 1970s, the composites industry was absorbing the impact of what was then the recent introduction of carbon fiber. The resulting composites exhibited both high strength-to-weight and high stiffness-to-weight ratios. And while it was known that composites reinforced with this new material exhibited limited toughness, there were many more pressing technical concerns to be addressed at the time, including how to implement them in existing designs. However, by the early 1980s, there was increasing interest in the “effects of defects,” as it was popularly termed, particularly in these brittle carbon fiber composites. Among these defects are those that are deliberately introduced, such as fastener holes and access cutouts. These controlled defects create stress concentrations that can significantly degrade the strength of a brittle material. Uncontrolled induced defects also were becoming a concern. The classic cases involved low-velocity impact, such as a wrench dropped on a composite aircraft wing sur-face or a composite leading edge striking a hangar door as an aircraft is being towed.
As I discussed in a previous column (see “Related Content,” at left), open-hole tension and compression test methods were developed to simulate the controlled defects. Test methods also were developed to characterize damage from uncontrolled defects, consisting of a specific impact event followed by an in-plane compression loading to failure. Several organizations defined such test methods, but two methods emerged as the most prominent: one from the NASA-Langley Research Center and another developed subsequently by The Boeing Co. (Seattle, Wash.).
The NASA method1 uses a composite plate specimen, typically a quasi-isotropic laminate between 10 inches and 12.5 inches (254 mm and 317.5 mm) long, 7 inches/177.8 mm wide and nominally 0.25 inch/6.35 mm thick. Usually, a 10-inch-long specimen is clamped over a 5-inch by 5-inch (127-mm by 127-mm) cutout in a base plate and impacted from a drop height of 2 ft/0.61m at a delivered energy of 20 ft-lb by a 0.50-inch/12.7-mm diameter, hemispherically shaped tup (impacter). For the nominal 0.25-inch thick specimen, this corresponds to 960 in-lb per inch of specimen thickness, creating a local damage region. Then the impacted plate was prepared for compression testing by removing 1 inch/25.4 mm of material from each side, reducing the plate width to 5 inches. The compression fixture consisted of four independent edge restraints. Those at the two sides of the specimen were knife edges, intended to provide simply supported edge constraints. Those at the top and bottom were clamps that helped prevent specimen brooming. The NASA fixture, with specimen installed, is shown in Fig. 1.
Interestingly, this same fixture also was used for the Open-Hole Compression test method mentioned above. In both cases, industry resistance to its use arose not because of the fixture itself, but out of concern for the relatively large amount of material consumed per test. In response, Boeing developed new fixtures that required less material for its own Compression After Impact (described below) and Open Hole Compression test methods. These are still in use. The NASA versions have faded into obscurity.
Boeing’s damage-inducing drop-weight impact apparatus is similar to NASA’s, but the cutout in the base is reduced to 3 inches by 5 inches (76.2 mm by 127 mm), the diameter of the impacting tup is increased to 0.625 inch/15.9 mm, and the delivered impact energy is increased to 1,500 in/lb per inch of specimen thickness. The much smaller Boeing specimen is 6 inches/150 mm long, 4 inches/100 mm wide, and nominally 0.20 inch/5 mm thick. After impact, the full specimen is used for the compression test.
The Boeing Compression After Impact (CAI) test method2 (Fig. 2) is quite different from the NASA version. The bottom and both sides of the impacted specimen are supported by a single fixture unit, although the sides are still simply supported and the bottom is clamped. The top of the specimen is restrained by a separate clamp. Although the NASA and Boeing documents refer to the specimen as being “clamped” at the top and bottom edges, the applied mechanics definition of “clamped” says the fixtures should prevent rotation at that edge. In this case, they do not. The restraint is closer to “simply supported” because the end restraint plates are pushed up against the specimen surfaces by hand and anchored with screws, making it impossible to apply significant clamping force.
In 2005, the Boeing version of the Compression After Impact test method with its smaller specimen size was adopted, essentially unchanged, as ASTM Standards D 7136 for the impact event and D 7137 for the compression after impact portion of the test.3,4 It is this ASTM (Boeing) version of the CAI fixture that is used almost exclusively at present.
It should be noted, however, that ASTM D 7137 includes optional gusset plates in the side support angles of the fixture. I do not recommend their use. Because side loads on the fixture during a test are minimal and the fixture is already relatively sturdy, these gussets serve little purpose. They add unnecessary weight and cost to the fixture and tend to be in the way when the fixture screws are tightened during specimen installation.
Of possibly greater concern than fixture stiffness are the high inertial normal forces on the mid-lengths of the side plate knife edges when the specimen (explosively) fails. With repeated use, this can wear and deform the knife edges locally, reducing the specimen support from “simply supported” to “free” in this local (12 mm to 15 mm/~0.5 inch) region. The effect this may have on test reliability remains to be quantified. Recently, another version5 of the CAI fixture, developed by Airbus Industrie (Toulouse, France), has received some attention (see Fig. 3). The specimen, typically quasi-isotropic, is identical in length and width to the ASTM specimen, but slightly thinner at 4 mm/0.16 inch. The top and bottom restraining plates are flat-edged, and the side plates are knife-edged, as in the ASTM fixture. The principal difference is that the Airbus fixture can apply positive clamping force to all four sides of the specimen using screws that forcibly push and hold the restraining plates against the specimen, so the desired boundary conditions can be approached more closely. Whether this fixture will gain in popularity remains to be seen.
1”Standard Tests for Toughened Resin Composites,” NASA Reference Publication 1092, ACEE Composites Project Office, Langley Research Center (Hampton, Va.), 1982.
2”Advanced Composite Compression Tests,” Boeing Specification Support Standard BSS 7260, Rev. C, The Boeing Co. (Seattle, Wash.), December 1988.
3ASTM D 7136, “Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event,” ASTM International, W. Conshohocken, Pa., 2005.
4ASTM D 7137, “Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plates,” ASTM International, W. Conshohocken, Pa., 2005.
5AITM1-0010, “Determination of Compression Strength After Impact,” Airbus S.A.S. (Toulouse, France), Issue 3, October 2005.