1、I l MIL-HDBK-787 ND m 7779770 0037887 5 I I I I l I l 1. F- d8“5 MIL-HRBK-787 1 APRtL 1988 MILITARY STANDARDIZATION HAND,BOO). 1 the attenuation due to voids k v) becomes, where k is the wave number (W/.A), v is the void content, a is the void radius, d is the scattering cross section and g is a fun
2、ction of the shear and longitudinal wave velocities. He modified the scattering cross section (3) as proposed by Ying and Truell(l0) by considering the sound velocities as a function of the void contett. It is interesting to note that the functional dependency of the void diameter and wavelength are
3、 the same as for the Rayleigh grain scattering (kD 1) in polycrystalline metals where the average .grain size (D) has been replaced by the void radius. 8.7.1.1 Results. A comparison of Martins calculated attenuation response for the data display of figure 7 is shown in figure 9 where the attenuation
4、 response at zero void content has been eliminated. It should be noted that the 5.NHz and 7 MHz curves are above and below the associated.experimenta1 data, respectively. Martin attributes this to two possible sources of error: void content and void size. The former under the best of conditions is +
5、1/2$ while the latter can be particularly significant. - 8.8 Use of velocity, The use of velocity to monitor the influence of the presence of voids or porosity upon strength related properties in homogeneous materials is numerous. For example, tensile strength correlations have been established by Z
6、iegler and Gertner for cast iron (ll-1; Lockyer (12) for a r14) for sintered materials. articulate resin composite; Serabian (13) for concrete and Brockelman 8.8.1 Propagation directions. .In utilieing the velocity as a means for characterizing fiber reinforced composites, propagation directions per
7、pendicular and parallel to the fiber plane( s) are used. Propagating ultrasound within the fiber plane can provide. anisotropic elastic constant measurements. However, dispersive effects can hamper the measurement of velocity and its subsequent interpretation. The propagation perpendicular to the fi
8、bers is by far the most suitable for nondestructive interrogation purposes and will be considered in this presentation. 8.8.1.1 Theory of Propagation, In unidirectional fiber reinforced composites the accepted theory of propagation is that due to Musgrave (15) and supposes a hexagonal symmetry. The
9、five elastic constants can be determined experimentally - e.g., by rotating a specimen in an immersion bath, and using a through-transmission measuring technique. Martin( 16) and Reynolds and Wilkinson (17) have proposed models. for the velocity of propagation. The void content is limited to the mat
10、rix. In any model it is necessary to account for the effect of voids upon the elastic constants involved. Por this purpose, the former used the work of Hasin (18) while the latter adopted the work of Bouc her ( 19). - - 10 Provided by IHSNot for ResaleNo reproduction or networking permitted without
11、license from IHS-,-,-MIL-HDBK-787 ND m 9999970 0039906 5 m I I MIL-HDBK-787 O 8.8.2 Velocity vs void content. The results-of Martins model is shown in figure 10 for glass fiber (a) and carbon fiber (b); the effect of the fiber content is also considered. As to be expected, the longitudinal velocity
12、(VL) and the shear velocities with polarization perpendicular (VSJ) and parallel (Vs,) to the fiber plane all decrease with void content. Also, increasing the fiber content increases the velocity at any given void content as well as producing a more pronounced velocity decrease. It must be remembere
13、d that since it was assumed that the void content is limited to the matrix, any increase in the fiber content would appropriately limit the void content. Also, the elastic constants of the fiber are much larger than those of the matrix. It is significant to note that from the standpoint of sensitivi
14、ty,the longitudinal wave velocity is desirable for measuring the void content. There is very little velocity-void content data to confirm Martins model. For example, for a carbon reinforced material, Stone and Clarke (2) indicate a 6% decrease in the velocity for a void content of 5% while Martins m
15、odel indicates an 1s decrease. 11 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-787 ND m 9999970 0037907 7 W MIL-HDBK-787 9. DETECTION OF DELAMINATIONS 9.1 General. Interply delaminations are perhaps the most straightforward anomalies to d
16、etect. Using the scanning technique of figure 2.(a) an indication would be noted between the first interface and first back reflection while in figure 2(b) and (c) a delamination would mean total reflection and hence 5ero transmission. By proper C-scan procedures one can obsemre these events to map
17、out the delamination. For example, figure 11 indicates the C-scan detection of a delaminate in a 16 ply carbon epoxy reinforced material. The delamination was simulated by embedding a interply square film patch. Of prime importance is the size of the beam at the detection plane of interest. It is fo
18、r this reason that highly focused beams are used. According to Jones and Stone(20) this can be accomplished by using a simple diameter stop such as a metal washer in front of a flat or unfocused transducer. The hcrease in resolution is made at the expense of interrogation power. In effect, this prod
19、uces a smaller source with an attendant highly divergent beam. Broadband focused transducers (see figure 12), or better still, a conical transducer would be more advantageous. Calibration of an interrogation systems ability to com2letely detail a given she delamination can be made by the use of flat
20、 bottomholes drilled perpendicular to the interrogation surface. For the through-transmission immersion mode it is necessary to consider a taping procedure on the drilled side such that the ultrasound encounters a material/air interface. 9.2 Photographic recording system. Knollman et al (21) suggest
21、 the use of a photographic recording system to take advantage of the wide tonal range available. Using the double focused transducers shown in figure I3( a), the receiver modulates a light source to indicate the transmitted ultrasound. The noted variations in the light source would then appropriatel
22、y expose a photographic emulsion. For example, a totally unbonded delamination would be dark on the resulting photographic image while a totally bonded region would appear light. More than 60 shades of intervening gray scales are available. Figure 13(b) and (c) indicates the difference between conve
23、ntional C-scan and the photographic process presented above. The delaminations were simuldted by interply Teflon film inserts in a 14-ply unidirectional graphite/epoxy composite. A comparison reveals the substantial increased clarity of the delamination of .the photographic recording. Also, the vari
24、ous gray-shades suggest the presence of voids,-thickness variations and fiber loading variations. O Chang et al (22) developed a spectrographic technique to detect and locate delaminations. Essentially, the technique is based upon the resonance effect in that destructive interference occurs when the
25、 depth of the delamination is an integral multiple of the half wavelength of the ultrasound. Figure 14 shows their apparatus and data where an area without delaminations is used as a standard. Flat bottom holes of 6, 3 and 1.5 mm diameters were used to simulate delaminations. The straight line relat
26、ionship between the points of frequency minima and the attendant order number is indicative of the existence of the interferences phenomenon. As displayed in table 1, it was possible to calculate the depth of the delamination. It is significant to note that the sensitivity of the procedure is suffic
27、ient tojdepth locate a delamination of 1.5 mm in diameter. With such sensitivities, it is possible to note crack-like defects as well. The size of any detected delaminations can be found by standard scanning procedures for composites. 12 Provided by IHSNot for ResaleNo reproduction or networking per
28、mitted without license from IHS-,-,-MIL-HDBK-787 ND m 7779770 0037708 7 m MIL-HDBK-787 10. MEASUREMENT OF STRENGTH RELATED PROPERTIES 10.1 General. The ultrasonic stress wave factor proposed by Vary and Bowles (23) lends itself to studying strength related- properties. The crux of the technique is s
29、hown in figure 15. The pulse from the longitudinal wave transmitter reaches the receiver in a complex fashion involving interacting phenomena of reflection, mode -conversion at the fiber/matrix interfaces and diffraction of waves generated at anomaly sites such as produced By voids and fiber and mat
30、rix microcracking. Typical transducer separation range is 1 to 4 inches. The transmitting transducer operates in the high kHz, to low MHz frequency range while the receiver is a medium KHz, resonant type transducer normally used in acoustic emission work. The transducers are usually clamped on; howe
31、ver, Rodgers (24) describes a commercially available , self-contained, portable equipment line which incorporates a tandem set of free rolling contact transducers that have no couplant requirements. Therefore, the stress wave factor measurements take on the full implications of an interrogation tech
32、nique. 10.2 Stress wave factor (SWF) . The stress wave factor (SWF) is taken as , SWF = GRN (17.1) where N is the number of cycles above a threshold amplitude, R is the pulse repetition rate and G is the sampling time interval. A modified SWF has been proposed by Williams and Lampert (25) which util
33、izes a single pulse (N = 1) and sums all the amplitudes of the peaks above a threshold apmplitude, thus in effect G =a. Rogers (24) worked with the RMS values of the voltage peaks and states that this is insensitive to minor changesin waveform shape and eliminates threshold level effects. The stress
34、 wave factor is a valid propagation parameter as long as salient items such as transducer activation, transducer characteristics, system gain and transducer separation are held constant. In actuality, .the stress wave factor is a measure-of attenuation or the ability of the material to propagate ult
35、rasound. It represents a unique attenuation scheme in that only a single specimen side is required. The latter is a particularly desirable feature for examining thin materials such as composites. When the pah of the ultrasound is hampered by anomalies then one can expect the stress wave factor to be
36、 affected. Since such anomalies can affect strength related properties, a correlation with the stress wave factor should be evident. 10.2.1 Application of SWF. Vary and Lark (1) describe a study using the stress wave factor to investigate the influence of fiber/resin bonding and fiber orientation. T
37、he composite consisted of type AS graphite and PR-288 epoxy resin. Fiber orientations studied were O0 , 100, 9001 , Oo; +45OIs and +45OIs. Two different fiber treatments were used: with nd without polyvinyl alcohol (PVA) coating. Tabs were attached to each specimen to facilitate tensile loading. Eac
38、h specimen was scanned by conventional ultrasonic interrogation techniques to eliminate those specimens with serious flaws. Stress wave factor measurements were made at 15 equally spaced points within the gage length of the specimen. Figure 16 indicates the results along with the points of fracture
39、due to tensile loading. It can be seen that fracture occurred at the point of the minimum stress wave factor. The latter was noted regardless of the fiber orientation. It should be noted Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-MIL-HDBK-787 ND
40、 m 9799970 0039704 O m MIL-HDBK-787 that in the case of multiple fractures there were corresponding Stress wave minima. Since such ultrasonic measurements were made prior to tensile loading, it is obvious that the failures may be attributed to the initial state of the microstructural anomaly content
41、 of the material. It is noteworthy that such anomalie-s were undetectable by conventional Ultrasonic interrogation techniques. 10.2.2 Stress wave factor vs ultimate strength. Figure 17(a) shows the relationship of the stress wave factor and the ultimate strength ( figure 19 displays the crack format
42、ion and the acoustic emission results. The similarity of the general shapes of crack formation and attenuation suggests that they are related. 11.4 Cracking. As to be expected it was found that the 900 plies failed first by transverse cracking at load levels approximately one-third of the ultimate s
43、trength of the laminate. The entire laminate does not fail catastrophically, but additional transverse cracks continue to appear up to load levels of approximately two-thirds of the ultimate load. At this load level, the transverse cracks have reached an almost regular saturation spacing. (27) Spaci
44、ngs in the 900 layers of O. 024-0.059 inches (0.6 - 1.4 mm) and 0.023-0.043 inches (0.6 - 1.1 mm) were observed for the go9 - +450, 9OOs and OO, 900, t450s laminates, respectively. It is proposed that the increasing attenuation with increasing load and crack formation is due to the diffraction or be
45、am spreading of the incident ultrasound by the cracks. (28) In effect, the cracks form a regularly spaced diffraction grating. It is important to note that the number of cracks and hence their spacing depend upon the applied load level. Also, as the loads approach the failure strength of the laminat
46、e the crack spacing reaches a Characteristic spacing unique to the laminate. 15 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-. ._ MIL-HDBK-787 12. FATIGUE DAMAGE 12.1 General. When materials are subje-cted to cyclio or fatigue loading it is well k
47、nown that they can fail even though the ultimate static strength has not been exceeded. This is true for metals, plastics and composite materials. Within the service life of a structure such fatigue loads are unavoidable, therefore all design criteria must include fatigue analysis as well as static
48、strength requirements. However, for composites the effects of fatigue and the resulting damage are not as well understood as those pertaining to metals. Well established design criteria are not available and one must consider every new material/structure in its own light. Salkind(29) indicates that
49、the growth of fatigue damage-in a metal can be much more abrupt (also see figure 20). It is interesting to note that a larger tolerant and inspection threshold damage sise exists for the composite material. It would appear that the propagation of damage is arrested by the internal structure of a composite material (see figure 21) . Thus, one would expect more desirable fatigue characteristics for the composite material. This is shown in figure 22 for a number of un
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