Frayed or just fluffier? All brushes get fluffier after a decent wash; it's the fixatives, glues, oils and grease fully coming out of the hairs. Think of a post-wash clean fluffy brush as the true shape, not the shape it comes in when you first take it out of the plastic sleeve.
If your brushes are getting actually frayed after only a couple of washes, there's something too severe in your cleansing routine. Generally fraying or severe hair loss comes from when the brushes are cleaned by grinding the wet soapy hairs into the palm of your hand or onto a towel to remove any dirt, or if you're twisting and pulling on the brush hairs when they're wet or if you're soaking the brushes to the extent that the glue in the ferrule is becoming waterlogged and soft or you're drying them in direct heat (hot sun, hair dryer, hot a/c).
Be more gentle and you'll be surprised at how long a decent brush can last.