Using a photoelastic coating technique, the authers conducted a series of experiments on wood specimen (Buna ・・・Fagus crenata BLUME) having a glue layer, to measure the strains on the surface and the glue layer under a tension, as shown in Fig. 1.
The following results were obtained :
(1) The distribution patterns of the isoclinics and the principal prajectory vary depending on the depth of the cutting slot as shown in Fig. 3. Especially, isoclinics of 20-30 degree are uniformly distributed on the glue layer between opposite slots in specimen of type B and C. Both of these types are suitable for the present experimental object.
(2) As shown in Fig. 4, the fringe orders of isochromatics originate from the extreme point of slots in wood. And as the tensile load increases, it increases to 2nd or 3rd order. The fringe orders of isocromatics expands along the glue line toward the center of both slots. Both fringe orders which appeared at both slots combine with each other, and then the combined fringe orders expand toward the width direction of wood specimen.
(3) The values of the shearing strain obtaind in this study are shown in Fig. 5. The shearing strain on the glue layer (LM) is uniformly distributed in case of lower stress (9kg/<cm>^2), and then is wholly increased, but is concentreated on the extreme point of slots as the stress increased. In the width direction of the specimen, the shearing strain is remarkably concentrated in and around the glue layer.

Taking into account the distribution of the shearing strain shown in Fig. 5, employment of the shape of specimen which is shown in Fig. 1 (type B) for measisuring the glue joint strength of wood block having a glue layer is considered appropriate.