31 Tough-Brittle Transition of Glass Fiber Composites by Impact Testing HIDEMITSU HOJO, YUKIO OKADA, and TOSHIO MAEDA Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan
Charpy impact tests were made to show the effects of temperature and specimen size on the impact strength and fracture mode of glass-polyester composites. The tough-brittle transition depended on temperature and was controlled by the weaker intrinsic strength of the materials. The law of similarity for size effects held for all materials when the fracture mode was unchanged, and a clear transition of fracture mode occurred for cloth laminates. This transition occurred at the critical size ratio depending on temperature. Consequently we could predict the impact behavior over a wide range of temperature, independent of specimen size, by using standard specimens.
Numerous studies have been made of the mechanical properties of fibrous composites; these include recently published papers on impact properties by Izod (1,2,3,4) and Charpy (5,6) and drop weight (7) tests. We reported the Charpy impact fracture behavior of various glass-polyester composites regarding the effects of temperature (8,9,10), specimen size (8), and fiber orientation (10). This paper describes the effects of the tough-brittle transition in the impact behavior of glasspolyester composites which occurs with a variation of temperature and specimen size. Materials and Test Procedure The materials tested were mainly unidirectional and bidirectional satin cloth laminate. Unsaturated polyester resin was used as a matrix. The tensile strength ratio of longitudinal fiber to transverse fiber was 374
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6.6 to 1.0 for the unidirectional laminate and almost unity for the bidirectional laminate; their glass contents (GC) were 62 and 60 wt % . ASTM E 23-66 was applied for the dimensions of standard specimens except that the mat reinforced specimen ( G C : 33 wt % ) used ASTM D 256-56. Specimens were taken from a molded plate along the longitudinal fiber direction, and V notches were cut edgewise, perpendicular to both fiber direction and molding surface. Occasionally a specimen notched on the molding surface (flatwise) was used. Tests were performed with an instrumented Charpy impact tester (JO) with a 30 kg-m or 150 kg-cm capacity. Test temperatures were varied from — 196°-250°C. Transition
of Fracture Mode Depending on
Temperature
Four typical impact fracture modes—tensile, compressive shear, compressive buckling, and interlaminar shear—were recognized from the broken specimens (9) and load-time curves (10). Among these four the tensile fracture had the lowest absorbed energy and behaved as a brittle material; compressive shear fracture had higher absorbed energy and behaved as a tough material. A mixed mode of fracture was often observed except for buckling failure at high temperature. The temperature dependence of the absorbed energy for standard specimens is shown in Figure 1. Impact behaviors and fracture modes of edge notched specimens depending on temperature were reported elsewhere (9, 10). In edge notched specimens of unidirectional roving reinforced material ( G C : 68 wt % ), cloth laminates, and mat reinforced material the fracture mode changed with temperature; all materials re500r-i
°-200
1
.
-100
.
.
.
0 Temperature (°C)
.
100
.
.
n
200
Figure 1. Effect of temperature on impact strength of glass fiber composites
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°-200
Figure 2.
-150
-100
-50
0
50
Temperature (°C)
100
150
200
Variation of static tensile and compressive strength of cloth laminates with temperature
vealed sharp peaks at high temperatures which seemed to correspond with the glass transition temperature in impact testing. After reaching the peak the specimens buckled by compressive load along the longitudinal direction. In unidirectional cloth specimens the fracture mode was essentially compressive; there was compressive shear at lower temperatures and buckling after the peak, but tensile fracture apparently became dominant at moderate temperatures. For bidirectional cloth specimens the fracture is almost controlled by tension but it shifts to compressive shear and then to buckling at higher temperatures. Local indentation at the point of impact may contribute to picking in absorbed energy with increasing temperatures. There was clear indentation for unidirectional roving reinforced specimens that seemed to increase with increasing temperatures. For cloth specimens a slight indentation was observed above 100 °C. In specimens notched flatwise the absorbed energy is constant at lower temperatures and decreases with increasing temperatures. Fracture is caused almost entirely by interlaminar shear, and the number of delaminated layers is reduced at higher temperatures; however in mat specimens tensile fracture is added to interlaminar shear, and there is no clear transition of fracture mode in these cases. Therefore the clear, tough-brittle transition exists, with reference to the temperature variation, when the notch was cut on the edge of
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cloth specimens. To evaluate these results static tensile and compressive strength data were obtained as a function of temperature (see Figure 2). This behavior in static strength coincides with that of dynamic strength obtained by the load-time curve (JO) and the transition of fracture mode in Figure 1. In unidirectional cloth laminate compressive strength is weaker than tensile strength for the temperature range tested, but at 100 °C the difference between both strengths becomes smallest; in this point the fracture area ratio of tension to compression is maximum (JO). For bidirectional cloth laminate, the weaker strength changes at 110°C, corresponding to the change of fracture mode from tension to compression. Consequently the fracture mode is controlled by the weaker intrinsic strength of the materials at the test temperature. Effect of Specimen
Size
To clarify the relation between absorbed energy and specimen size, tests were made with various specimen widths (b) and thicknesses under the notch—i.e., remaining depth (h). Law of Similarity. An example of size effects is shown in Figure 3. The absorbed energy increases linearly with an increase in width and continuously increases with an increase in remaining depth for unidirectional cloth notched flatwise. The same behavior occurred for other specimen sizes. Figure 4 shows a logarithmic plot of absorbed energy and the remaining depth for resin and various composites at room temperature. The law of similarity, Equation 1, held for all the materials between absorbed energy (E) and specimen size when the same fracture mode was maintained regardless of specimen size. =
E
Cb h m
(1)
n
where C, m, and n are the material constants; m is unity for most cases, and n varies from 1.0 to 2.0. C depends on the temperature and fracture mode, and m and n are constant and independent of temperature when specimens have the same fracture mode. The total absorbed energy (E) consists of E i (proportional to the sectional area bh) and E (proportional to the volume bh ). Therefore E is given in the form 2
2
E
= Ei
+
E
2
=
CM
+
C bh 2
2
«
Cbh
n
(2)
Tough-Brittle Transition in Cloth Laminates. For cloth specimens notched edgewise transition of fracture mode occurred with a variation of specimen size although the law of similarity held for each region having the same fracture mode. Figure 5 shows this transition for bidirectional cloth specimens at room temperature. With the increase of remain-
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Figqure 4. Absorbed energy vs. specimen size relationship for various glass fiber com posites at room temperature
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Transition
100r o h = 8 mm (const.) • b = 10 mm (const.)
o
e Tensile type^
cn Compressive type
8 .Q
