求翻译：图4 所示为复合材料的拉伸断口SEM 照片。由图（左)可见，未加外场情况下，断口上显示有大面积的脆性断裂区，局部存在有韧窝和撕裂棱，断裂机制是脆性-塑性断裂，此时伸长率为7.2%。由图（中）可见，加低频单一磁场时，复合材料的脆性平坦区减少，韧窝数量增加，且其形貌变小、变浅，呈现了较为明显的韧窝型断裂特征，但是韧窝中颗粒团聚现象仍然较为严重，伸长率为8.1%。由图（右）可见，加组合电磁场时，复合材料拉伸断口形貌中可见，材料的脆性平坦区已基本消失，韧窝数量很大程度的增多，韧窝尺寸细小，形貌均匀，呈现了韧窝型断裂特征，此时的伸长率达到最高9.6%。是什么意思？

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Figure 4 shows the composite photo SEM stretch Panne. The figure below (left) is visible, not the outside, it has a large display on gaps in brittle fracture zones, and there is a partial tear edges annealed Wo and are brittle, broken mechanism, the broken - plastic elongation rate of 7.2 per cent.

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Figure 4 shows the composite tensile fracture surface SEM photo. By map (left) visible lack of outfield situations, fracture, appear on the large area of brittle fracture zone, local existence and toughness Wo and torn edges, brittle-plastic fracture mechanism is broken, elongation of 7.2% at this t