On the fracture of reinforced thermoplastics

Jourmd of

Materials Processing Technology ELSEVIER

Journal of Materials Processing Technology 55 (1995) 229-233

On the fracture of reinforced thermoplastics V.V. Meshkov a'*, O.V. Kholodilov a, A.I. Sviridenok b "Metal Polymer Research Institute, Belarus Academy of Sciences, 246652 Gomel, Belarus, CIS bDepartment for Resources Saving, BAS, 230023 Grodno, Belarus. CIS Received I April 1994

Industrial summary

In the designing of engineering fiber-reinforced thermoplastics and of the processing technology to make such, their behaviour at failure should be taken into consideration. The processes of deformation and failure from compression were investigated for polycarbonate and polyarylate reinforced with polyaramide fiber, particularly, poly-m-phenyleneisophthalamide and poly-pamidobenzimidazole, the results obtained being described in this paper. The acoustic emission (AE) technique was used to reveal the essential dependences of the deformation and the fracture on the fiber length and elastic modulus of the composites. Chemical, plasma or biochemical pre-treatment of the fiber surface appeared to considerably influence the kinetics of damage accumulation in the material. The relationship between the interphase interaction of the components and the characteristics of the fracture surface is considered for fiber-reinforced thermoplastics. Morphological differences were established by the secondary electron emission technique using the scanning electron microscope and a computer system. The statistical characteristics of the fracture surfaces of composites differing in the fiber origin and in the degree of interaction between the components are described. Thestrengthpr•perties•fthe•ber-reinf•rcedtherm•p•asticsaredescribedf•rquasistatic(def•rmati•nrate 10 4s-ljanddynamic (deformation rate 10z s- 1) loading conditions.

1. Introduction

The leading industries such as mechanical engineering, the chemical industry and electrical engineering are in a great need of polymeric composite materials. Those materials should possess high strength, deformability, and impact strength; also, particular specific requirements often need to be met. Of promise seem to be polymeric materials prepared from engineering thermoplastics, particularly aromatic polyesters (polyacrylate, polycarbonate) with fibrous reinforcements. In recent years organic fiber has been introduced more widely. Despite voluminous information available on the composition, the production processes and the properties of such thermoplastics obtained under different service conditions, their development and application are restrained owing to scarce and contradictory information on their behaviour under static and dynamic loading conditions, regarding the mechanisms of damage accumulation and fracture. Valuable information on flaws accumulated and microcracks generated in the materials can be obtained from measurements of the energy parameters of the acoustic emission (AE) resulting from the collective *Corresponding author. 0924-0136/96/$15.00 c) 1996 Elsevier Science S.A. All rights reserved SSDI 0 9 2 4 - 0 1 3 6 ( 9 5 } 0 2 0 1 I - A

breaking of interatomic bonds and the energy produced by deformation. The magnitude of this energy depends on the mechanisms of plastic deformation or failure. Although the AE recording technique has been employed frequently to investigate processes of solids deformation and fracture, the mechanism of acoustic emission originating in polymeric engineering composites is understood inadequately. Experiments have been conducted mostly on carbon-and glass fiber-reinforced thermoplastics prepared with thermosetting resins. The elastic-deformation region, i.e. the low frequency AE range has been analysed; the results obtained being of qualitative character [1-3]. Information on AE resulting from the deformation of filled thermoplastics is not abundant and rather fragmentary. The purpose of this paper is to describe the peculiarities of the fracture processes detected in composites fabricated from aromatic polyesters and organic fiber. 2. Experimental work 2.1. Materials

Thermoplastic polymers used widely to prepare advanced engineering materials, viz. polyacrylate grade

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DV-105 (PA DV-105) and polycarbonate grade PC-2 (PC-2) were investigated. The filler was of aromatic polyamide fiber, e.g. poly-m-phenyleneisophthalamide (registered name Fenilon) and poly-p-amidobenzimidazole (registered name SVM). The fiber surface was treated biochemically [4], chemically [5], and by the low-frequency charge plasma technique. The specimens were prepared by the molding procedure. 2.2. Testing p r o c e d u r e

It had been discovered earlier experimentally that the AE signals generated by the deformation of thermoplastics are very weak, therefore special requirements were imposed on the recording devices. The present authors have designed a testing system [6] comprising a unit to measure the load with a pre-set velocity, and a device to vary the AE parameters; the latter device includes a receiving emission transformer, a low-noise pre-amplifier, a high- frequency filter, and a main amplifier. The system includes several peripheral devices to widen the feasibility, e.g. a memory oscillograph for visualizing purposes, a tape-recorder, a spectral and an amplitude analyzer, a digit-printing device, a graph-plotter. The dependence of the AE parameters on the loading pattern was studied for polyester-based materials with a random distribution of organic fiber of size 50 mm length and 1-2 mm length. The specimens were in the form of cylinders: d = 10 mm; h = 15 mm; and the test conditions were: quasi-static loading mode; deformation rate 10-4s - 1 The unit surface area, determined as the ratio of the real area to the nominal area, is one of the most important informative characteristics for understanding fracture processes in composite materials. At present, numerous procedures have been developed to estimate the unit surface area [7]. Usually these methods are used to evaluate such parameters as porosity, capillarity, etc. To apply the above methods correctly for the purposes under consideration, they should meet special requirements, viz., be able to monitor the dimensional range of asperities found within the unit surface area, and be invariant with respect to faults such as pores, deep capillaries or cracks. To estimate the unit surface area, the phenomenon of secondary electron emission, i.e. lowenergy electrons generated when a primary beam strikes the surface, was used [7,8]. The method was applied using the scanning electron microscope JSM-50A and a computer.

3. Results and discussion

Consider some general relationships typical of the deformation of the present materials. Comparison of the deformation and failure results revealed that unfilled

polymers are not able to develop AE up to a load of ca. 80%o of the ultimate value, unlike the situation with fiber-reinforced polymers. On increasing the load further, the emission rate is negligible, except for a sharp increase observed directly before the specimens failed. The acoustic emission occurring in deformed fiber-reinforced thermoplastics as early as 6% relative deformation is due to the polymers in question possessing high flexibility; therefore, the deformations originated by the applied load are developing slowly to give weak acoustic signals that do not exceed the noise level. Only after the main structural elements responsible for the integrity of the materials have been broken is a significant increase in emission rate detected. Organic fibers incorporated in a polymeric matrix differ substantially from the binder in their elastic properties. Therefore, considerable stresses can be generated at the interface of the components if the composition is subject to compression. As a result, the adhesional contact between the fiber and the matrix becomes broken to initiate cracks, being the sources of acoustic emission. The greater the relative stiffness of the reinforcing fibers (the ratio of the elastic modulus of the fiber to that of the matrix), the faster these sources become active. Further, the sources resulting from the fracture of individual monofilaments that are in a state of complex stress, when micro-cracks can be initiated to make a marked contribution to the acoustic emission. The results of earlier investigation [9] and preliminary evaluation suggested that the fibers under consideration can be arranged according to their adhesional bond strength, viz.: biochemically modified fibers, chemically modified fibers, plasmatreated fibers and original fibers. Differences in the adhesional strength, e.g. of fibers treated biochemically and chemically, amounted to more than 50%. Further experiments were therefore conducted with compositions containing biochemically-treated fibers. The analysis of the acoustic emission resulting from the initiation of flaws and their development under the mechanical loading of reinforced plastics containing original and biochemically-modified fiber revealed the following. In composites containing Fenilon fiber treated biochemically, Table 1, the total number of acoustic

Table 1 Parameters of AE resulting from the deformationof compositions of polyarylate + Fenilonfiber Fenilon fiber (length 50 mm, elastic modulus 18.5 GPa) Original Treated biochemically

Composition Total emission, Pulseenergy, Amplitude, rel. units tel. units rel. units 250

7992

5.24

9772

111957

9.59

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V.V. Meshkov et al. / Journal of Materials Processing Technolo~, 55 (1995) 229 233

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Deformation (%) Fig. 1. The relationship between stress (1, 2) and AE rate (1', 2') and deformation for compositions with original fiber (1, 1') and biochemically-modified fiber (2, 2').

pulses increased sharply by 30-40 times, whilst their total energy increased 10-15 times. The fiber length also was found to influence the kinetics of the AE amplitude distribution if the composites underwent deformation. Composites reinforced with shorter fibers are characterized by high residual stresses generated by shrinkage of the binder, around the fibers. The frequency-amplitude analysis of AE spectra shows that those stresses produce microdefects in the polymeric matrix long before an external load is applied, shrinkage producing radial compressive stresses as well as axial tensile stresses in the polymeric matrix that lead to earlier micro-damage of the matrix. Further increase in the load results in substantial damage of the inter-phase interface. There are few individual fibers broken that initiate AE signals of high amplitude. Obviously, discrete fracture occurs in short-fibers (1 2 mm)-filled compositions before a catastrofically-growing crack occurs. Fracture and the AE associated with it do not develop uniformly; particularly, for the time dependence of the AE energy, there are observed increases and declines. Long-fibers (50 mm)-filled compositions start to fail by microplastic flow and micro-damages of the matrix. However, amplitude spectral analysis revealed the sources activated by reinforcing fibers fracture to contribute largely in the AE in the case of the above materials. Since these sources consume much energy, the total energy of AE generated by the deformation of long fiber-filled compositions was much greater than by short fiber filled compositions. Biochemical treatment of the fiber appeared to increased the ultimate compressive strength of the material, by 1.7-1.8 times with stresses of up to 300-310 MPa. Curve 2 (AE intensity) has a second distinguished peak for such material. This is indicative of higher energy at which the inter-phase layers fail, and also that fracture of multiple fibers and even blocks is the prevailing process.

The appearance of the fractured surfaces of the FR composition was found to be largely dependent (with other conditions being equal) upon inter-phase interaction. Consider the changes in the fractured surfaces of composites with original fibers and those with fibers modified biochemically using polyacrylate + Fenilon fiber compositions. After the chemical affinity between the matrix and the fiber has been improved at the expense of a greater number of ester groups synthesized on the fiber, the adhesional bond strength at the inter-phase inter-face is increased. Unlike compositions with untreated fiber, in the present case much of the load is supported by the inter-phase surface and the fibers themselves, with fracture occurring mostly within the matrix. However, as the biochemical modification of fibers continues, no increase in the composition strength due to the greater number of groups synthesized on the fibers was observed. IR-spectroscopy, X-ray analysis and the DTA technique revealed irreversible changes in the composition and structure of deeper layers, leading to sharp deterioration of the deformation and strength properties of the materials. In this case the appearance of the fractured surface depends mainly on the properties of the thermoplastic matrix. Similar phenomena are typical (with some peculiarities) of the fracture surface in PA + SVM fiber composites for which the ratio of the elastic modulus of the fiber to that of the matrix is much greater (by several times). It was established experimentally that inter-phase interaction of the components influences the statistical characteristics of the fracture surfaces of the compositions. The lower the strength properties of the fibers, particularly the elastic modulus, and their chemical activity, the greater the unit area of the fracture surface and the extrema density, Fig. 2. Variations in the composition and structure of the inter-phase surface due to biochemical modification of

7 ~20

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¢~0 i 30 ~0 80 Asperity spacing, ~tm

f~

5 ¢5 2S ~5 Asperity spacing, ~arn

Fig. 2. Showing: (a) unit area of fracture; and (b) peak density; versus asperity spacings for compositions containing: 1, original Fenilon fiber; 2, biochemically-modified Fenilon fiber.

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V.V. Meshkov et al. /Journal o f Materia& Processing Technology, 55 (1995) 229 233 t06

550 500

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,.n'

t'

400

F

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.

~03

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-

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~oo

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g

o

o 5

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2o spacing,

40 lam

go

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Fig. 3. Selective estimation of the fracture unit area in polyarylate + biochemically-modified Fenilon fiber compositions.

the fibers leads to an ambiguous dependence of the unit fracture area and the extrema densities on the treatment conditions. For example, on increasing the duration of treatment, the unit area of the fracture surface was observed to decrease in polyarylate + poly-m-phenyleneisothalamide fiber compositions, whereas in polycarbonate + polyamidobenzamidazole fiber composition the dependence on the duration of the treatment was nonmonotonous. For all compositions and regimes of biochemical treatment tested, the unit areas of the fracture surfaces and the extrema densities tend to monotonously decrease with greater asperity spacings. Selective analysis of the unit fracture area showed [8] the greatest increments in unit fracture area to take place within the asperity spacing range of up to 10 ~tm and from 80 ~tm onwards, Fig. 3, which is probably associated with the fracture of monofilaments and inter-phase surfaces, and hence matrix damage. This is supported by their correlation with the typical size of respective structural formations on the fracture surfaces. On reducing asperities, the dependence of the extrema density on asperity spacings grows monotonously; the greatest increments in extrema density were observed for spacings of below 10 lam. The parameters characterizing the shape of the asperity inclination angle distribution curves, particularly the asymmetry and the kurtosis, vary in a complicated manner. However, for compositions with improved strength properties the asperity inclination angle distribution curve tends to symmetry. The differences in fracture mode determine the levels of strength and the deformation properties of materials containing original fiber and biochemically-modified fiber loaded dynamically and quasi-statistically, Fig. 4. For example, under quasi-static loading (a deformation rate of 10-~s - 1) the strength of composites

I

t

I

2 Deformation (%)

I

3

4

Fig. 4. Compressive stress and deformation relationships for compositions made of polycarbonate + SVM fiber: 1 and 1', the original fiber was used; 2 and 2', the biochemically-modified fiber was used. Loading conditions: quasi-static (1 and 2); and dynamic (1' and 2').

containing biochemically-treated aramide fiber was on marginally greater than that of compositions with original fiber, whereas under dynamic loading (a deformation rate of 102 s - ' ) the difference was significantly greater. Probably, greater energy of bonding at the thermoplastic/biochemically-modified fiber inter-phase surface leads to this result.

4. Conclusions

The following conclusions can be made in view of the research findings: 1. The fracture process develops irregularly with time in fiber-reinforced thermoplastics, as was revealed from acoustic emission parameters. 2. Biochemical pre-treatment of the aramide fiber surface was observed to affect essentially the kinetics of damage accumulation in thermoplastic-based compositions. 3. The fracture surface morphology analysed for different asperity spacing ranges appeared different for compositions containing biochemically-treated fiber, distinguished in inter-phase interaction, the main differences being associated with inter-phase surface peculiarities (composition and structure): they show at levels below monofilament size (5-7 lam) and lead to differences in the deformation and the strength properties of fibre-reinforced thermoplastics under quasi-static and dynamic loading conditions.

References [1] A. Peterlin, Acoustic Emission of Polymer Stretching, Latest Experimental Techniques for Studyin9 Polymeric Structures, Mir, Moscow, 1982, 21 (in Russian).

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[2] I.V. Viktorov and V.S. Dobrynin, AE-analysis of deformation and fracture in organic fiber-filled plastics, Machine Design, 4(1987) 115 (in Russian). [3] I.S. Kurov, G.B. Muravin and A.V. Movshovich, Mechano-andacoustic emission technique for studying fracture development in carbon fiber reinforced plastics, Mech. Composite Materials. 5 (1984) 918 (in Russian). [4] A.I. Sviridenok, T.K. Sirotina and V.V. Meshkov, Biochemical treatment of polymeric materials, USSR AS Reports, 298 (3) (1988) 666 (in Russian). [5] A.I. Sviridenok, V.V. Meshkov and V.V. Lisovsky, Effect of organic fibre modification on properties of polymer composites, Proc. Int. Syrup. Fiber Reinforced Plastics~Composite Materials, Nanjing, China, 7A 0988) 1/s 6/s.

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[6] O.V. Kholodilov, T.F. Kalmykova, V.V. Meshkov and E.V. Vinogradova. Acoustic emission from deformation of fiber-reinforced egineering thermoplastics, Trans. BSSR AS, 2 (1989) 53 fin Russian). [7] A.Ya. Grigoriev, N.K. Myshkin, N.F. Semeniuk and O.V. Kholodilov, Estimation of unit surface area by the secondary electron emission technique, Friction and wear, 9 (5) (1988) 792 (in Russian). I-8] A.I. Sviridenok, A. Ya. Grigoriev, V.V. Meshkov and T.K. Sirotina, Relationship between interphase interaction and fracture surface characteristics for fiber-reinforced thermoplastics, Mech. Composite Materials, 3 (1989) 444 (in Russian). [9] A.I. Sviridenok, T.K. Sirotina and E.V. Pisanova, Effect of biochemical treatment on the adhesion of poly-p-amidobenzimidazole fibre, J. Adhesion Sci. Tech., 5 (1991) 229.