**4.2 Stress concentration and strength of specimens with various DHS configurations**

The experimental tensile strength results for the unnotched and notched longitudinal [0]4 laminated specimens with different DHS configurations are shown in **Table 3** and schematically in **Figure 4**. The present DHS configuration is given as (*D*/(*A*, *A*)/(*d*, *d*)) where *D* is the main hole diameter, *A* is the auxiliary hole diameter, and *d* is the distance between the centers of the main and the auxiliary holes.


#### **Table 1.**

*E-glass/epoxy laminate properties.*


**177**

(see **Table 3**).

**Figure 4.**

**Table 3.**

**4.3 Strain distributions using DIC**

*Strength Improvement and Stress Analysis of E-Glass Laminated Plates with Circular Notches…*

**Case Strength load (MPa) Improvement**

Case 1-a 5/(4.5,4.5)/(6,6) 527.04 353.54 413.17 16.87 Case 2-a 5/(4.5,4.5)/(7,7) 527.04 353.54 412.39 16.65 Case 3-a 5/(4,4)/(6,6) 527.04 353.54 386.34 09.27 Case 4-a 5/(2.5,2.5)/(6,6) 527.04 353.54 348.57 −1.41

*Experimental strength loads for [0]4 laminates with different DHS configurations.*

**Unnotched plate With** 

**SH**

**With DHS**

**%**

In this experimental investigation, the obtained strength values were compared

As discussed earlier, the DIC technique was used in order to get the strain fields developed in composite samples with different open hole configurations loaded in tension at a rate of 0.5 mm/min. It is shown that the technique provides quantitative information that can be used to identify the strain distribution. A speckle pattern was applied manually on the specimen surface, and the quality of the speckle pattern affects the strain distribution; so it needs to be carefully dropped. The correlation subset size was large enough to ensure that there was a sufficiently distinctive pattern contained in the area used for correlation. In order to calculate the SCF around the

to plate with a single hole, and the main hole diameter in this case is 5.0 mm. **Figure 4** shows the experimental stress-strain curves, and it can be clearly seen that the strength values were related not only to the main notch diameter but also the auxiliary notch diameters and their locations. The ultimate strength value varies greatly with the auxiliary hole sizes and locations, and the laminate strength increases as the auxiliary notch size increases and is located too near to the main one

*Typical stress-strain curves for (*D *= 5 mm) laminated plates with different DHS configurations.*

*DOI: http://dx.doi.org/10.5772/intechopen.87089*

#### **Table 2.**

*Strength and SCF values of unnotched and notched specimens with various notch diameters.*

*Strength Improvement and Stress Analysis of E-Glass Laminated Plates with Circular Notches… DOI: http://dx.doi.org/10.5772/intechopen.87089*


**Table 3.**

*Composite and Nanocomposite Materials - From Knowledge to Industrial Applications*

In this section, the stress concentration factors and the tensile strengths of laminated plates with different notch diameters are analyzed. The results will be given first for plates with a single hole, and then the same analysis will be done for

**4.1 Stress concentration and strength of specimens with a single hole (SH)**

**4.2 Stress concentration and strength of specimens with various DHS** 

**Material Elastic properties**

The experimental tensile strength results for the unnotched and notched longitudinal [0]4 laminated specimens with different DHS configurations are shown in **Table 3** and schematically in **Figure 4**. The present DHS configuration is given as (*D*/(*A*, *A*)/(*d*, *d*)) where *D* is the main hole diameter, *A* is the auxiliary hole diameter, and *d* is the distance between the centers of the main and the auxiliary holes.

E-glass/epoxy 22.54 10.94 3.54 0.30

**Material Ultimate strength Stress concentration values**

Unnotched 527.04 — — 2.5 427.57 3.756 4.114 5.0 353.54 3.937 4.025 7.5 314.93 4.194 4.799 10.0 274.86 4.601 5.072

*Strength and SCF values of unnotched and notched specimens with various notch diameters.*

**(MPa) FEM Experimental results**

*E***1 (GPa)** *E***2 (GPa)** *G***12 (GPa)** *v***<sup>12</sup>**

The performed experimental study aimed to determine the ultimate tensile strength of unnotched/notched samples as well as the stress-strain distributions in laminates weakened by various notch diameters in order to evaluate the stress state. The obtained results from the characterization tests are summarized in **Table 1**. **Table 2** presents the stress concentration and the strength (ultimate stress) obtained experimentally for specimens without notches (unnotched plates) and with a single hole. It can be clearly seen that the SCF and the degradation in strength values are related to the notch dimension. The ultimate strength values vary greatly with the notch size, and the laminate strength decreases as the hole size increases. It turned out that as the notch diameters increased from 2.5 to 10.0 mm, the strength values steadily decreased from 427.57 to 274.86 and the SCF increased from 4.114 to 5.072. The SCF findings presented in **Table 2** are the average of three values obtained for each sample. One can see that the numerical results are in good agreement compared to the experimental data and the difference between them is considered insignificant.

**4. Results and discussion**

**configurations**

*E-glass/epoxy laminate properties.*

plates with different DHS configurations.

**176**

**Table 2.**

**Table 1.**

*Experimental strength loads for [0]4 laminates with different DHS configurations.*

**Figure 4.**

*Typical stress-strain curves for (*D *= 5 mm) laminated plates with different DHS configurations.*

In this experimental investigation, the obtained strength values were compared to plate with a single hole, and the main hole diameter in this case is 5.0 mm.

**Figure 4** shows the experimental stress-strain curves, and it can be clearly seen that the strength values were related not only to the main notch diameter but also the auxiliary notch diameters and their locations. The ultimate strength value varies greatly with the auxiliary hole sizes and locations, and the laminate strength increases as the auxiliary notch size increases and is located too near to the main one (see **Table 3**).

#### **4.3 Strain distributions using DIC**

As discussed earlier, the DIC technique was used in order to get the strain fields developed in composite samples with different open hole configurations loaded in tension at a rate of 0.5 mm/min. It is shown that the technique provides quantitative information that can be used to identify the strain distribution. A speckle pattern was applied manually on the specimen surface, and the quality of the speckle pattern affects the strain distribution; so it needs to be carefully dropped. The correlation subset size was large enough to ensure that there was a sufficiently distinctive pattern contained in the area used for correlation. In order to calculate the SCF around the

**Figure 5.**

*Finite element and DIC engineering strain fields for [0]4 notched laminated plate with a 5/(4.5, 4.5)/(7, 7) DHS configuration.*

zone of discontinuities, the stress-strain relationship for planar composite structures was used. DIC technique measurements were carried out in all the samples.

The SCF results tabulated in **Table 2** are actually the average of three values obtained for each specimen. One can see that the experimental results are in good agreement with finite element outputs and the difference between them is insignificant. In addition, one can clearly see that the stress concentration near circular cutouts can be reduced using two auxiliary holes (see **Figure 5**). So, it can be concluded that the stress concentration reduction explains the tensile strength load improvement in specimens with DHS.
