Nonvisible impact damage effects on composite hardware

Due to its simple procedures and relative low cost, visual inspection (VI) has been often employed as the only means of nondestructive inspection (NDI) method to be performed on composite hardware. The assumption usually is that if no damage is found visually, the structure is sound to be used. This assumption, however, has been challenged many times by the primary users and safety authorities. To resolve this issue, numerous studies have been conducted to investigate the effects of nonvisible impact damage to space flight composite hardware including aircraft wings, tails, spacecraft adapters, launch vehicle solid motor cases and composite pressure vessels. This paper presents a brief survey of these studies. In addition, the impact damage test data that have been generated in a research and development (R&D) program conducted on composite overwrapped pressure vessels (COPVs) are also presented. Introduction In recent years, graphite/epoxy (Gr/Ep) composite materials have been widely used for primary structures such as aircraft wings, spacecraft bus and adaptors, launch vehicle solid motor cases, and for critical hardware including pressure vessels. This expanded use results from Gr/Ep composite materials having high specific strength and modulus. However, Gr/Ep composites are known to be susceptible to impact damage. Static strengths of structures made of this type of composite materials have shown degradation as the results of impact manifesting a few types of damage mechanisms: fibers breaking, matrice cracking and composite layers delaminating. Broken fibers can be detrimental to tensile (burst) strength, and delamination can be detrimental to compressive strength. In aircraft structures, the upper wing skins are mostly in compressive state during potential foreign object damage (FOD) impact events such as hail and bird strikes. The degradation of compressive strength as the result of impact damage is important for aircraft wings. Thus the compressive strength after impact (CAI) data are important for the wing design. On the other hand, the tensile strength after impact (TAI) and the burst strength after impact (BAI) are *Senior Project Engineer, Am Member. This paper is declared a work of the U.S. Government and is . important for the design of composite structures primarily loaded in tension. This include pressurized composite hardware such as COPVs and composite solid motor cases. To prevent catastrophic failure due to impact, it is important to know the effect of the impact event quantitatively so that the structures’s flightworthiness can be properly assessed. To achieve this goal, ND1 is often needed. However, not like in metallic structures, ND1 methods for detecting impact damage in composite structures are not yet well developed. There are no well established criteria that could be used reliably to accept or reject the hardware if damage was detected. The current practice in the space industry is to detect impact damages by VI. Experimental data have shown that at certain impact energy levels and conditions, damages induced by impact are not reliably detected by VI. The minimum impact energy level creating a damage size that can be detected by VI is referred as visible damage threshold (VDT) energy level. The minimum impact energy level that creates damage barely visible is referred to as barely visible impact damage (BVID). Test data shown in Figures 1 and 2 illustrate that even at the VDT energy level, the static strengths of GriEp composite materials could be significantly degraded. For a specific composite hardware, the values of CAI, TAI, and BAI depend on many parameters, including ply orientation, stacking sequence, structural geometry, stiffhess, preload conditions and above all, impact conditions. Consequently, CAI, TAI, and BAI at VDT (or BVID) energy level also change from structure to structure. However, in some applications, engineers tend to use the data generated from other so-called “similar” hardware to avoid the costly effort in generating the values for a specific design, thus creating a lot of concerns and endless argument. To settle this issue, a survey on the existing data available in the open literature has been conducted. Although there are many test data documented in the open literature, only a few typical ones are presented here for the purpose of illustration. In addition, ‘the impact damage test data that have been generated recently by a team led by The Aerospace Corporation for COPVs in an Air Force and NASA jointly funded R&D program are also presented here for the purpose T~1 of discussion. not sut?ject to copyright protection in the United States. “American Institute of A’e%?autics and Astronautics” Examples from Open Literature Effects on Thin Laminates The influence of impact on the strength of thin laminates has been investigated by Rhodes’. Impact tests were conducted on four-point beam-bending sandwich type specimens with 4-ply [O/90], Gr/Ep facesheets loaded in tension or compression. The impact test results are shown in Figures 1 and 2. The results clearly showed that the TAI and CA1 had significant loss even when the applied impact energy levels were below the VDT which is around 0.35 ftlb for this thin laminate. For tension strength, the average TAI is 45% of the undamaged tensile strength. For compression strength, the average CA1 is 50% of the undamaged compression strength. Effects on Thick Laminates The effect of impact damage on thick laminates that represent the typical composite laminate cons@uction of aircraft wings has been studied by McCarty et al’. Figure 3 shows the impact test results of the 32 ply [+45/90/0/+45/~~45/0a]o GrlEp composite laminates. Impacts were done using a drop weight impactor with a 0.5 in. diameter hemispherical head. Two impact levels were employed: 16.7 ft-lb. and 25-ft-lb. At the 16.7 fi-lb impact energy level, the external damage was barely visible, thus identified as BVID. Nevertheless the internal damage area, as indicated by through transmission ultrasound (TTU), varied from 1.45 in.’ to 2.40 in.2 and as shown in Figure 3, static compression strength degraded by 45-65%. Effects on Thick Filament Wound Motor Case An investigation has been undertaken by Poe3 to determine the relevance of impact shape to nonvisible impact damage and tensile strength of the filament wound case (FWC) of the solid rocket motors planned at one time for the Space Shuttle. Figure 4 presents the impact damage test results of 30 in. diameter, 12 in. long rings, I.4 in. thick. These rings were cut from an 84 in. long cylinder made by Hercules Inc. The fiber was {(k 56.5),/0/[(+56.5~/0]~/~(%6~5~/0],/(+56, where the 0” layers are the hoops and the f56.5” AS4W-12K graphite and the winding resin was Hercules HBRF-55A epoxy. The orientation was layers are the helical. Open symbols in the figure designate nonvisible impact damage and the filled symbols representing the visible impact damage. These results show that the NV wound with IM-6 graphite fibers subjected to a 10 ft-lb impact energy barely showed any discernible damage, yet, on average, their BAI was 45% of the undamaged vessel. Effects on Thin Filament Wound Pressure Vessels Without Liners An impact damage testing program has been onducted by Adler et al” on filament wound pressure vessels (FWPVs) without liners. Figure 5 shows the impact damage test results for the 6 in. diameter FWPVs with different fibers including Kevlar (K-29 and K-49 and a high performance Kevlar) and graphite (AS-4 and IM-6). A significant degradation of the burst strength of the Graphite FWPVs was shown. When subjected to a 10 ft-lb impact, the FWPVs wound with IM-6 graphite fibers barely showed any discemiable damage, yet, their average BAI was 45% of the undamaged vessels. Impact Damage Testing of COPVs COPVs have been widely used in the space flight systems such as spacecraft and launch vehicles primarily due to their light weight. These COPVs usually contain gaseous fluids at very high pressure (around 5,000 psi) or liquid propellent which are toxic. Failure of any of these COPVs will be devastated. However, there is no specific impact damage control requirements that could be implemented on these COPVs to prevent catastrophical failure. The lack of impact damage control requirements have generated serious concerns from safety communities. In order to develop the needed requirements, the Air Force and NASA have jointly funded an R&D program entitled, “Enhanced Technology for Composite Overwrapped Pressure Vessels”4 conducted by a government-industry team let by Aerospace. An impact damage testing program was conducted as a major part of this R&D effort. Specimens used in the impact damage tests were different size flight qualified cylindrical and spherical COPVs built by three major pressure vessel manufacturers in the United States. Majority of the tests were conducted using small spherical and cylindrical COPVs to save cost. The small spherical COPVs were 10.25 in. in diameter. The metallic liners were made of 0.05 in. thick 5086 aluminum and the composite wraps were Armco T-40 graphite fiber and epoxy resin, 0.18 in. thick. This COPV design was qualified for a maximum expected operating pressure (MEOP) at 6,000 psi. All the COPVs were proof tesed at 7,500 psi (1.25x MEOP) level. The design burst pressure is 9,000 psi (1.5x MEOP). The average real burst pressure is 10,600 psi. The small cylindrical COPVs were 6.5 in. in diameter, 20 in. long. They were made of 6061-T62 aluminum liners overwrapped with Toray T1000 graphite fibers with epoxy resin. In the cylinder section, the liner is 0.035 in. thick and the thickness of the composite overwrap is 0.109 in. This COPV