T. Liber

New method for testing composites at very high strain rates - A method was developed for testing and characterizing unidirectional and angle-ply composite laminates at strain rates in the 100 to 500 s-1 regime

A method was developed for testing and characterizing composite materials at strain rates in the 100 to 500 s−1 regime. The method utilizes a thin ring specimen, 10.16 cm (4 in.) in diameter, 2.54 cm (1 in.) wide and 6–8 plies thick. This specimen is loaded by an internal pressure pulse applied explosively through a liquid. Pressure is measured by means of a calibrated steel ring instrumented with strain gages. Strains in the composite specimen are measured with strain gages. Strains in the calibration and specimen rings are recorded with a digital processing oscilloscope. The equation of motion is solved numerically and the data processed by the mini-computer attached to the oscilloscope. Results are obtained, and plotted by an X-Y plotter in the form of a dynamic stress-strain curve. Unidirectional 0-deg, 90-deg and 10-deg off-axis graphite/epoxy rings were tested at strain rates up to 690 s−1. Times to failure ranged between 30 and 60 μs. The 0-deg properties which are governed by the fibers do not vary much from the static ones with only small increases in modulus. The 90-deg properties show much higher than static modulus and strength. The dynamic in-plane shear properties, obtained from the 10-deg off-axis specimens, are noticeably higher than static ones. In all cases the dynamic ultimate strains do not vary much from the static values.

Effect of laminate construction on residual stresses in graphite/polyimide composites

The influence of ply-stacking sequence and ply orientation on the magnitude of lamination residual stresses in graphite/polyimide angle-ply laminates was investigated. The effect of stacking sequence was investigated with laminates of [02/±45]s, [±45/02]s, [0/+45/0/−45]s and [+45/02/−45]s layup. The effect of ply orientation was evaluated with additional specimens of [02/±15]s and [02/902]s layup. Thermal strains were measured using embedded-gage techniques. Residual strains were determined by comparing thermal strains in the angle-ply laminates with those of a unidirectional laminate. The ply-stacking sequence did not have an influence on the magnitude of residual strains. The highest residual strains occur in the [02/902]s laminate and the lowest, approximately one-fourth in magnitude, occur in the [02/±15]s laminate. The maximum residual strains in the [02/±45]s group are slightly lower than those in the [02/902]s laminate. Residual stress computations show that, at room temperature, the transverse-to-the-fibers stresses in all plies, except those of the [02/±15]s laminate, exceed the transverse tensile strength of the unidirectional material.

Higher-order numerical differentiation of experimental information

Cubic-spline and discrete-quadratic polynomial techniques are presented for reliably computing up to third-order derivatives of experimental information. The concept is demonstrated by stress analyzing from measured displacements a transversely loaded plate and a beam under four-point bending. The respective displacement fields were recorded using holography and moiré. The accuracy of the employed numerical-differentiation techniques is indicated.

Wave propagation in transversely impacted composite laminates

An experimental investigation was conducted to determine wave-propagation characteristics, transient-strain distributions and residual properties for unidirectional and angle-ply boron/epoxy and graphite/epoxy laminates impacted with silicon-rubber projectiles at velocities up to 250 ms−1 (820 ft/s). Tests were conducted at normal and 45-deg oblique impact. Strain signals obtained from surface and embedded strain gages were recorded and analyzed to determine the types of waves, propagation velocities, peak strains, strain rates and attenuation characteristics. The predominant wave is a flexural on propagating at different velocities in different directions. The flexural wave velocity is higher in the higher-modulus direction. In general, measured wave velocities were higher than theoretically predicted. The amplitude of the in-plane wave is less than ten percent of that of the flexural wave. Peak strains and strain rates in the transverse to the (outer) fiber direction are much higher than those in the direction of the fibers. Strain rates up to 640 s−1 were measured. Under oblique 45-deg impact, the flexural wave is still the predominant one. Peak strains under this oblique impact range between 36 and 56 percent of those under normal impact of the same velocity. Residual elastic properties and strength were measured around the point of impact. The most significant result was a reduction in the transverse strength of the unidirectional laminates. The dynamics of impact were also studied with high-speed photography. The projectile is completely flattened within 50–70 μs and the total contact time is of the order of 300 μs.

Stress Analysis of Anisotropic Laminated Plates

Membrane and interlaminar stresses in statically loaded and vibrating composite structures are obtained by determining optically and numerically the respective partial derivatives of the holographically recorded transverse displacement field. A clamped, circular, orthotropic boron-epoxy plate under a uniformly applied static pressure and vibration are considered separately. Under uniform static pressure, yr0 = 0, while the anisotropy of the laminated composite results in ir9 ^ 0, which is unlike the corresponding isotropic case. The vibrating plate has complicated, unsymmetrical displacements producing both in-plane shearing strains and stresses. The higher order derivatives are obtained optically by holographic-moire and numerically by employing cubic-spline and discretequadratic differentiate functions.

Lamination Residual Strains and Stresses in Hybrid Laminates

The influence of hybridization in angle-ply laminates on curing residual strains and stresses was investigated. The specimens were graphite/Kevlar 49/epoxy and graphite/S-glass/epoxy laminates of various stacking sequencevariations of the basic [′45/0 2 ]; construction. The ′45-deg plies and, in some cases, half the 0-deg plies were graphite/epoxy. Residual stains were determined experimentally following previously developed techniques by taking the difference between the unrestrained thermal strain of an individual ply and the restrained strain of that ply within the laminate. It was found that stacking sequence did not have an influence on measured residual strains. In the graphite/Kevlar/epoxy group, the transverse (∈ 9 0 ) strain in the 0-deg Kevlar plies is the highest, exceeding 9 x 10 - 3 , reflecting the high-transverse thermal expansion of unidirectional Kevlar. The transverse strain in the 0-deg graphite plies is much lower, reaching a peak value of 5.6 x 10 - 3 . In the graphite/ S-glass/epoxy group the transverse strains in the 0-deg S-glass plies are relatively low, compared to similar strains in the 0-deg graphite plies. Residual strains in the graphite plies do not seem to be influenced much by the type and number of hybridizing plies. Residual stress computations show that at room temperature the transverse to the fibers residual stresses exceed the static strength for the graphite plies and reach values up to 75 percent of the transverse strength for the glass plies. The residual stresses in the Kevlar plies are high enough to cause early transverse failure of these plies before the laminate is cooled down to room temperature.

Ultrasonic Techniques for Inspecting Flat and Cylindrical Composite Specimens

Ultrasonic techniques are discussed for inspection and evaluation of flat and cylindrical composite specimens. Immersion ultrasonic techniques are described using a 5-MHz broad-band focused transducer in the pulse-echo mode. Two types of recording methods are employed, conventional pen-lift C-scanning and analog scanning, augmented with photographs of oscilloscope traces of the pulse at selected locations. The complete scanning system is described, including a fixture for scanning tubular specimens. The techniques discussed are applied to monitoring flaw growth in graphite/epoxy coupons of [(0/′45/90) s ] 2 and [0 2 /′45] 2 s layups with four types of initial flaws subjected to fully reversed spectrum fatigue loading. The flaws investigated are (1) circular hole, (2) embedded film patch, (3) internal ply gap, and (4) surface scratches. It was found that, in general, flaw growth is greater in specimens of [0/′45/90) s ] 2 layup than in those of [0 2 /′45] 2 s layup. The residual tensile strengths for the preceding specimens, determined after four lifetimes of fatigue testing, are not significantly lower than the initial strengths.