Instrumented impact testing of CFRP composite laminated plates.

Dogaru, Florin ; Udroiu, Razvan

1. INTRODUCTION

The CFRP material presents an increased susceptibility to damage due to impact. A first stage in understanding the causes and in limiting the damage due to impact is to find out the dynamic response and the influence of various parameters upon the response. In this paper the authors give a simple block diagram which can be used in instrumented impact test, using Lab VIEW program, for obtaining all results that characterize the response due to impact on the composite laminated plate.

2. MATHEMATICAL FOUNDATION

A standardized impact test procedure was used for these investigations (Fuoss et. al. 1998, Lifshitz et al. 1995), see fig.(1). In this instrumented impact test is measured only one parameter that is the acceleration of the projectile.

The velocity and the displacement of the projectile during the impact are calculated through integration of the measured acceleration curve using LabVIEW program, see fig.(2), (Dogaru 2005). The following equations were used (Nesttles & Douglas 2000):

F(t) = [M.sub.1] [d.sup.2]x/[dt.sup.2] = [M.sub.1] x a(t) (1)

where a(t) is the deceleration due to impact between the impactor and the plate, [M.sub.1] is the mass of the impactor, F(t) contact force. The acceleration measured on the accelerometer sensor is:

[a.sub.m](t) = g - F(t)/[M.sub.1] = g - a(t) (2)

where g is gravity. The velocity and deflection are calculated taking into account only the portion of the acceleration curve during contact time:

v(t)= V + [[integral].sup.t.sub.0][a.sub.m](t')dt', x(t) = [x.sub.0] + [[integral].sup.t.sub.0] v(t')dt' (3)

with initial conditions v(t = 0) = V, x(t = 0) = [x.sub.0] = 0, where V is the initial velocity of the impactor and x(t) is the total traverse deflection of the plate in the contact point due to global deformation and contact deformation. The energy transferred to the plate during the impact time can be evaluated using the equation:

E(t) = [[integral].sup.t.sub.0]F(t')dx = [[integral].sup.t.sub.0]F(t')v(t')dt', (5)

where F(t) is the contact force due to impact and can be evaluated knowing the mass of the projectile's and using Eq.(1). Trapezoid formula is used to solve Eq.(5).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

For any continuous function f(x), supposing the interval ([x.sub.0], [x.sub.n]) is divided in n equidistant intervals, [t.sub.0], [x.sub.0], [x.sub.1], ..., [x.sub.i], [x.sub.i+1], ..., [x.sub.n], [x.sub.i+1] - [x.sub.i] = [t.sub.0], the result is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

If we replace f(t') = F(t') x v(t') in Eq.(6) for any time t = [t.sub.n] = [t.sub.0] x n we obtain:

E(t) = E([t.sub.0] x n) = E(n) = [t.sub.0]/2 [f(0) + f(n) + 2 [n-1.summation over (i=1)]f(i)] (7)

The Eq.(7) was calculated numerically for every time t using MATLAB program.

3. RESULTS AND CONCLUSIONS

The experimental analyses were conducted on composites plates made of epoxy vinyl ester matrix (Derakane 470-30-S) reinforced with carbon fibers with dimensions 150x100[mm.sup.2], 2.5mm thickness, 8 unidirectional laminae and symmetric orientation [[0/-45/+ 45/90].sub.s]. The characteristics of lamina's plate were: [E.sub.1]=54GPa, [E.sub.2]=[E.sub.3]=4.5GPa, [G.sub.12]=[G.sub.23]=1.65GPa, [[upsilon].sub.12]=0.3. The specimens used, were cut off from plates which were manually manufactured and the resin was impregnated by brushing-on action obtaining a fibers volumetric ratio of about 35%. The impact test was done by the use of a device designed for this particular study, having the energy capacity of 1-50J obtained by adjusting the height and/or the impactor weight. The specimen was simply supported at the edges against a metal plate (30mm thickness) with interior cutting-out of 125x75[mm.sup.2] by the intermediary of a wooden plate (6mm thickness) in order to avoid the specimen crushing at ends. The specimen was supplementary fixed in four points with screws, manually tighten, having rubber disposed on the tip. The projectile had a 16-mm diameter semispherical head made of alloyed steel with increased hardness, 1.9kg weight and the impact was targeted at the plate's center. Behind the projectile, an accelerometer was attached (screwed), in order to measure the projectile's acceleration, see fig.(1). The signal was recorded by using an acquisition plate NI USB 6251 BNC. Figures (3,4,5) illustrate the variation related to time of the projectile's displacement, contact force and the energy transferred to the plate.

[FIGURE 3 OMITTED]

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[FIGURE 5 OMITTED]

It is noticed the maximum contact force and maximum displacement are simultaneously reached that means the impact is quasi static.

In the future the authors intend to investigate the damage introduced by impact, the level of the contact force that causes the damage and its effect on the residual properties.

4. ACKNOWLEDGEMENTS

This research was done with financial support of MECT and ANCS, contract PN II--IDEI, ID_187, 110 / 1.10.2007.

5. REFERENCES

Dogaru, F. (2005). Research concerning the behavior of mechanical structures made of composites materials subjected to impact loading, doctoral thesis, Transilvania University of Brasov

Fuoss E., Straznicky, P., V. et al. (1998). Effects of stacking sequence on the impact resistance in composite laminates Part 1: parametric study, Composite Structures, Vol. 30(1998), pp. 67-77

Fuoss E., Straznicky, P., V. et al. (1998). Effects of stacking sequence on the impact resistance in composite laminates Part II: prediction method, Composite Structures, Vol. 30(1998), pp. 177-186

Lifshitz, J.M., Gov, F. et al. (1995). Instrumented Low-Velocity Impact of CFRP Beams, I. J. Impact. Vol. 16, No. 2, pp. 201-215

Nesttles, A., T. & Douglas, M., J. (2000). Comparison of Quasi-Static Indentation to Low-Velocity Impact, NASA/TP-2000-210481