flexible without cracking or chipping when stressed. It should work very well at mending the broken segments when they are butted up. Just razor blade the squeezed out excess adhesive flush, after an hour of setting. Once a week has been done, the broken sections should be ridged as if one piece, when the reinforcement has bonded.

Using fiberglass or Bondo will be too firm and ridged, and may chip or crack away after time, especially being driven at speeds.

That's my take on such an item,

As already stated, the first step is to Identify the substrate ( the material the part is made of).  

I have been working in or around auto collision shops all my life and from the photo, I still can't tell what the substrate is!  With that said, it is possible you have a prior repair, with body filler over top of the repair, etc...  ( or a complete improper previous repair).    I would clean the part, strip the paint off and then bevel the edge of the broken area. Since creating a bevel edge is part of the repair process, this action will not hurt anything. In addition, during removal of the paint you encounter body filler or any type of filler, I would remove all of that material as well. Creating a beveled edge will help you in identifying the substrate a little easier.  

Once you clean and bevel the edge a close exam will help to determine if you have evidence of reinforcement strands such as in SMC or fiberglass parts. If not you might have a plastic part ( need to Identify the type if plastic ( Olefin or non-olefin)) to select the products and repair method for such. Most plastic repairs or repairs of plastic parts ( not Fiberglass or SMC) might and in many cases need a backing for reinforcement during a repair. 

Look at 3M auto plastic repair materials

Other companies for professional repairs: ( Lord Fusor, Kent, SEM, etc.)  Most of the professional plastic repair products are a 2-part product already packaged in a a double tube. You may need to buy an applicator gun to use such products. 

Prior to the 90's, plastic repair was not main stream repair in the collision shops. Plastic welders and some plastic repair materials were available in the 80's but nobody knew that you had to identify the type of plastic ( substrate) first to select the correct repair materials ( olefin vs. non-olefin). In the late 90's new products and ID of the type of plastic allowed for a quality repair. Skip a step and most repairs failed. Ask me how I know this so well! 

 

The reason for the history lesson, many plastic repairs made prior to 2000 were improper and failed. Most folks used the wrong materials or tried to repair a structural part (spoiler) with modified or fiberglass stand reinforced body fillers (as mentioned- Tiger hair, gorilla hair, etc). Obviously, body filler(s) of any kind would not have any rigid or structural strength and the repaired part would fail. ( almost before you could bolt the part back on the car, never mind driving down the road at 55 mph.)

Once you determine the substrate, then the repair process type can be determined and the applicable materials obtained.  You can learn a great deal about auto part repair ( of plastics and of SMC/Fiberglass) off of the internet and the 3M website.