UDC E F F E C T OF W O R K I N G M E D I A ON T H E GLASS-REINFORCED PLASTICS

RESIDUAL

STRESS

DISTRIBUTION

678:539.31.539.319 IN

G. K. Shreiber, S. M. Perlin, L. I. Obishchenko, and L. A. Borisenko Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 8, No. 6, pp. 737-740, 1967 Methods of determining residual (internal) stresses in glass-reinforced plastics are discussed, and some resuits o f an investigation of the influence of petroleum c h e m i c a l s on these stresses are reported. It was established [1, 2] that in the case of glass-reinforced plastics, i . e . , heterogeneous materials whose constituents (glass and resin) have sharply contrasting properties, the fracture in most cases starts at the resin-glass interface. When glass-reinforced plastic parts are made, different thermal expansion coefficients of glass and resin and the c h e m i c a l l y - i n d u c e d shrinkage produce residual stresses which, superposed in service on the working loads, m a y lead to the a p pearance of breaking stresses. It is known that residual stresses of this kind reach their m a x i m u m levet at the glass-resin interface. Liquid working m e d i a have a substantial influence on polymers in a stressed state [3]. Studies of the t i m e dependence of the character of residual stresses in glass-reinforced plastics under working conditions (i. e . , in the presence of working media, under working loads and at various temperatures) are of considerable interest. This investigation was concerned with the effect of elevated temperatures and m e d i a most often encountered in the oil and gas industries (petroleum, gas and oil well waters) on residual stresses in the m a t e r i a l under consideration. The tests were carried out in the following media: distilled water [4]; high-sulfur (3.8~ S) petroleum from the Arlansk deposits; low-sulfur petroleum from the Belorussian deposits. The effect of these m e d i a was studied at 20, 50, and 80 ~ C, i . e . , at temperatures most characteristic of conditions in the oil and gas industries. The tests were carried out in specially constructed m u l t i - s t a t i o n thermostats. The temperature in specimen holders was m a i n t a i n e d constant with •176 with the aid of a water jacket; the m a x i m u m test duration was two months.

Fig. 1. A photomicrograph of a cross section of a glass-reinforced plastic

(x 800). The e x p e r i m e n t a l work was planned in accordance with the theory of planning regressive experiments which m a d e it possible to d e t e r m i n e the necessary number of tests and the optimum test duration [5]. Accordingly, eight test t i m e intervals were selected: 0, 43, 77, 140, 250, 450, 810, and 1440 hr. At present there is no generally accepted method of determining residual stresses in glass-reinforced plastics. A photomicrograph of a cross section of a unidirectionally oriented specimen of this m a t e r i a l (Fig. 1) shows that the glass fibers are situated at the corners of squares or triangles (rhombs) which form a biperiodic l a t t i c e with conjugate periods. The high packing density of glass fibers makes it difficult q u a l i t a t i v e l y and quantitatively to estimate residual stresses by direct measurements on glass-reinforced plastics. The determination of residual stresses was therefore carried out on model specimens by a polarized light method. There are several optical methods of determining residual stresses. For instance, the double refraction effect producked in an optically isotropic substrate when a thin film of a glass-reinforced plastic is formed on it was used in [6] to

537

d e t e r m i n e the overall magnitude of residual stresses. Other workers constructed models of the e l e m e n t a r y cells of gIassreinforced plastics and used these models to study the behavior o f constituents near the glass/plastic interface. And so, a m o d e l "resin in glass" was used in [7] to a n a l y z e the deformation of a certain isolated volume o f the binder without t a k ing into account its action on the glass e l e m e n t . A model "glass in resin" was used in [8] to study the interaction of the binder with glass without taking into account the influence o f neighbor reinforcing elements.

|

!

i !

I

U

I

~0

a

m

A

.0 D

I

F

I

6'1

Glass rod

~sin

u

i

Fig. 2. Residual stress distribution along the cross section of a glass-reinforced plastic specimen.

Fig. 3. Q u a l i t a t i v e representation o f stress distribution.

A m o d e l proposed by Abibov and Molodtsov [9], in which the reinforcing e l e m e n t centers are situated at the corners o f a square, was used in this investigation. This model makes it possible simultaneously to a n a l y z e both the intera c t i o n of the binder film with glass fibers and the deformation o f a certain isolated volume o f the binder in the c e l l interior. However, m e t a l wires were used as the reinforcing elements in the m o d e l studied in [9], so that processes taking p l a c e in this m o d e l were not sufficiently representative of those taking p l a c e in e l e m e n t a r y cells of glass-reinforced plastics. The reinforcing e l e m e n t s in models used in this investigation were therefore m a d e of a l u m i n o - b o r o s i l i c a t e glass: The construction o f t h e m o d e l was based on the m o d e l l i n g and similarity theory; the following s i m i l a r i t y criteria were used:

Ke = E"

1, K . . = ~*" T'

K r = - T-i c= K,=

n (n > 1),

v' 1, K ~ = - - - - - 1 . ,r

D' 0.004 - - -o.oo D" 4

,

where K E, Kct, K a , , K T, and K v are s i m i l a r i t y coefficients for, respectively, the Young modulus E o f the binding p o l y mer, the difference between linear t h e r m a l expansion coefficients c~*, the t e m p e r a t u r e conditions T, and the Poisson c o e f ficient u; Ke is the g e o m e t r i c a l s i m i l a r i t y coefficient equal to the ratio o f the reinforcing e l e m e n t diameters. These conditions give the following relations for converting the stresses in the model to the real conditions:

where o is stress, e r e l a t i v e strain, and Ka. denotes a proportionality coefficient which, in this case is equal to one. The indices ' and " r e l a t e to the natural and m o d e l conditions, respectively. Studies of the residual stresses were carried out on an a u t o m a t i c bution pattern and makes it possible to c a l c u l a t e the stress l e v e l . The which covers all the points of the specimens at 0 . 5 m m intervals; the corded on a special film. Data obtained in this way m a k e it possible at various points of the specimen cross section.

538

p o l a r i m e t e r IPL-451 which produces a stress distridesign of the instrument ensures a scanning c y c l e results of the measurements are a u t o m a t i c a l l y reto d e t e r m i n e the difference of the principal stresses

The separation of principal stresses was carried out by the well known in photoelastic studied method of the tangential stresses difference [10] which is based on a numerical solution of differential equations of equilibrium in Cartesian coordinates. The difference between principal stresses (ol - a2) and isocline parameters ~ was used in the calculations. The difference between principal stresses was determined by a compensation method, isoclinic parameters being determined from the points. Stress measurements were done at points of two supplementary sections through the diagonal of the e l e m e n t a r y cell. Stress distribution curves were constructed from data for 80 points. Figure 2 shows the residual stress distribution curves for a specimen in the initial condition (continuous curves) and after two months in distilled water at 20" C (broken curves). It will be seen that the m a x i m u m stresses are produced at the glass-resin interface; in the case of specimens in the initial condition they reach 1, 8 - 2 . 5 5 k g / m m 2. The residual stresses decrease rapidly with the distance from the glass-resin interface, which is also shown by qualitative results reproduced in Fig. 3. Exposing the m a t e r i a l studied to distilled water produces a tenfold reduction in the residual stress level. The reduction due to the influence of crude petroleum is smaller and takes p l a c e during the first 15 days of the test, no significant changes being observed during subsequent exposure to the action of this medium. REFERENCES 1. Yu. M. Malinskii, B. Yu. Trifel,

2. 3. 4. 5. 6. 7. 8.

B. A. L. S. P. I. F.

and V. A. Kargin, Vysokomolekulyarnye soedineniya, no. 5, 1964. Yu. Trifel and Yu. M. Malinskii, Vysokomolekulyarnye soedineniya, no. 6, 1964. N. Tynnyi and' A. I. Soshko, FKhMM [Soviet Materials Science], no. 1, 1967. N. Arsen'eva, Trudy MINKhiGP im. Gubkina, no. 46, 1964. M. Perlin, Plasticheskie massy, no. 8, 1966. I. Zubov, L. A. Sukhareva, and V. V. Poturoev, Kolloidnyi zhurnal, no. 4, 1964. O. Outwater and G. H. Dewey, Mod. Plast, 39, 154, 1961. I. McGarry, The 17th Annual Technical and Management Conference of Rein Plast Div of SPI, Chicago,

1958. 9. A. L. Abibov and G. A. Molodtsov, Mekhanika polimemv, no. 4, 1965. 10. M. Frokht, Photoelasticity [in Russian], OGIZ, Moscow, vol. 1, 1948. 21 January 1967

Moscow Plastic Research Institute

539