Objectives:

Internal cracking and fracturing of fiber-reinforced composite can be evaluated by means of acoustic signals from the material. In this preliminary study three different fiber reinforced composite materials were tested using acoustic emission monitoring during loading event.

 

Methods: Three groups (n=6/group) of bars were fabricated; Group TV:  Targis-Vectris (Ivoclar, Liechtenstein), Group S: Stick (StickTech, Finland), Group ES: everStick. Continuous unidirectional glass fibers of TV were dimethacrylate monomer preimpregnated containing also inorganic particulate fillers between the fibers, ES fibers were dimethacrylatemonomer-PMMA preimpregnated and S fibers were polymer preimpregnated. S fibers were further-impregnated for 30 minutes with dimethacrylate monomer resin.  All specimens were photopolymerized with hand curing unit for 20 sec and after that for 15 min in light curing device. 

 

A step-by-step three-point loading process was the following: First loading cycle reached 50N and the load was remained for one minute. Then load was let down to zero Newton. Equal loading steps were proceeded to load 75, 100, 125, 150, 175, 200 and 225N, or until the test specimen fractured.

 

Two acoustic emission microphones were placed to both ends of each bar. Acoustic emission signals were measured during the loading cycle with MISTRAS 2001 using 4 MHz sample frequencies.

 

Results:

Group	Maximum Load (N)	Load (N) required for AE-activity initiation
TV	199.7 (SD 14.4)	133.3 (SD 40.8)
S	187.2 (SD 9.3)	120.8 (SD 36.8)
ES	196.6 (SD 22.8)	179.2 (SD 10.2)

 

ANOVA showed no difference between groups when maximum load were compered (p=0.412). The load values required for the initiation of AE-activity different similarity (p=0.0179).

 

Conclusion:

Two different modes of fractures could be seen from the AE-analysis. Loading rates under 150N gave low amplitude signals which were considered as a matrix micro cracking. High amplitude signals developed with loading rates above 150N, which were considered as fiber breaking.