# Flexural Behavior of Epoxy under Accelerated Hygrothermal Conditions

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Experimental Program

#### 2.1. Test Specimens

#### 2.2. Material Properties

#### 2.3. Specimen Preparation and Curing

#### 2.4. Accelerated Environmental Conditions

#### 2.5. Test Procedures

## 3. Test Results

#### 3.1. Control Specimens

#### 3.2. 100 °C Conditioned Specimens

#### 3.2.1. 40 Cycles

#### 3.2.2. 100 Cycles

#### 3.2.3. 250 Cycles

#### 3.2.4. 625 Cycles

#### 3.3. 180 °C Conditioned Specimens

## 4. Discussions

#### 4.1. Flexural Strength

#### 4.2. Deflection

#### 4.3. Apparent Stiffness

#### 4.4. Mode of Failure

## 5. Conclusions

- (1)
- Under standard laboratory conditions, the maximum flexural loads, the maximum deflections, and the apparent stiffness all increase with an increasing time, which is related to the number of cycles. All specimens show brittle flexural failure.
- (2)
- The epoxy was sensitive to the effect of temperature. For all specimens conditioned at 100 °C, the strength initially increased up to 250 cycles. After 250 cycles, a decrease in strength was found. The 180 °C conditioned specimens showed strength reductions ranging from 39% to 62% in comparison to the corresponding 100 °C specimens at the same cycle numbers. In the case of 100 °C specimens, the majority of specimens showed very large deflections compared to the control due to a change in the failure mode from brittle to no failure; a small reduction in deflection was observed up to 250 cycles. Beyond 250 cycles, the reductions accelerated. On the other hand, the 180 °C specimens all show brittle failure; the maximum deflections were reduced by about 12–46% from that of the control. For all specimens, the apparent stiffness quickly reduced by about 11–33% from the stiffness of the control specimens within the first 40 cycles, and then remained about the same throughout the entire testing period.
- (3)
- The level of relative humidity appears to have an appreciable effect on the maximum strength and deflection of the epoxy specimens under both 100 and 180 °C environments. The combination of 100% humidity and long treatment periods produced a strong negative effect. After 625 cycle at 100 °C, the maximum deflection of the 0% humidity specimens was two to four times higher than that of the 100% humidity specimens.
- (4)
- The effect of temperature cycling vs. constant temperature at 100 °C was found to be insignificant in this study. When the maximum temperature was increased to 180 °C, the constant 180 °C environment would produce damage that was not likely to be observed during the service life of the FRP materials. Hence, such constant high temperature conditioning should not be employed in accelerating testing. On the other hand, a temperature cycling regime between room temperature and 180 °C appears to be adequate.
- (5)
- It should be noted that the abovementioned findings are based on the resin used in this study, which is common on the US market for impregnating FRP sheets. The interpretation of the hygrothermal effects on other resins must be made with caution.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 1.**Environmental chamber (model: Tenny T10RC-1.5 SPL) (Tenny Engineering, 1090 SPRINGFIELD RD, Union, NJ 07083, United States).

**Figure 5.**Flexural load-mid span deflection curves after 40 cycles under the conditions of 100 °C, humidity (0% and 100%), cycle period (Cp = 0 and 2).

**Figure 6.**Flexural load-mid span deflection curves after 100 cycles under the conditions of 100 °C, humidity (0% and 100%), cycle period (Cp = 0 and 2).

**Figure 7.**Flexural load-mid span deflection curves after 625 cycles under the conditions of 100 °C, humidity (0% and 100%), cycle period (Cp = 0 and 2).

**Figure 8.**Flexural load-mid span deflection curves after 40 cycles under the conditions of 180 °C, 2-h cycle period, humidity (0% and 100%).

**Figure 9.**Maximum flexural loads vs. cycle numbers for the control specimens. Each cycle is equivalent to 2 h.

**Figure 10.**Max. flexural load vs. number of cycles under 100 °C but different humidity levels (0% and 100%) and different cycle periods (Cp = 0 and 2).

**Figure 11.**Max. flexural load vs. number of cycles at 100 °C and 180 °C (Cp = 2, 0% and 100% humidity).

**Figure 12.**Max. deflection vs. number of cycles for the control specimens. Each cycle is equivalent to 2 h.

**Figure 13.**Max. deflection vs. number of cycles under 100 °C but different humidity levels (0% and 100%) and different cycle periods (Cp = 0 and 2).

**Figure 14.**Max. deflection vs. number of cycles at 100 °C and 180 °C (Cp = 2, 0% and 100% humidity).

**Figure 15.**Apparent stiffness vs. number of cycles for the control specimens. Each cycle is equivalent to 2 h.

**Figure 16.**Apparent stiffness vs. number of cycles under 100 °C but different humidity levels and different cycle periods.

Curing Schedule 72 h Post Cure at 60 °C (140 °F) | ||
---|---|---|

Property | ASTM | Typical Test Value |

Glass Transition Temperature, Tg | D-4065 | 82 °C (180 °F) |

Tensile Strength ^{1} | D-638 | 72.4 MPa (10,500 psi) |

Tensile Modulus | D-638 | 3.18 GPa (461,000 psi) |

Elongation (%) | D-638 | 5.0 |

Flexural Strength | D-790 | 123.4 MPa (17,900 psi) |

Flexural Modulus | D-790 | 3.12 GPa (452,000 psi) |

^{1}Testing temperature 21 °C (70 °F).

Beam No. | Max. Deflection ^{+} (mm) | Max. Load (N) | Stiffness (N/mm) | Flexural Strength (MPa) | Failure Mode |
---|---|---|---|---|---|

EB0 | 11.4 | 1084.9 | 87.2 | 51.7 | Flexural |

EB01 | 11.6 | 1077.8 | 87.3 | 51.4 | Flexural |

^{+}mid-span deflection at the maximum load.

Mode of Failure | |||||||
---|---|---|---|---|---|---|---|

Number Of Cycles | S.L.C. ^{1} | 100 °C | 180 °C | ||||

0% RH | 100% RH | 0% RH | 100% RH | ||||

Cp = 2 | Cp = 0 | Cp = 0 | Cp = 2 | Cp = 2 | Cp = 2 | ||

0 | Flexural | - | - | - | - | - | - |

40 | Flexural | x | Flexural | x | x | Flexural | Flexural |

100 | Flexural | x | x | x | x | Flexural | Flexural |

250 | Flexural | x | x | x | x | Flexural | Flexural |

350 | Flexural | - | - | - | - | Flexural | Flexural |

625 | Flexural | Flexural | Flexural | Flexural | Flexural | - | - |

^{1}S.L.C. is Standard laboratory conditions; (x) means no failure.

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Mohamed Elarbi, A.; Wu, H.-C.
Flexural Behavior of Epoxy under Accelerated Hygrothermal Conditions. *Fibers* **2017**, *5*, 25.
https://doi.org/10.3390/fib5030025

**AMA Style**

Mohamed Elarbi A, Wu H-C.
Flexural Behavior of Epoxy under Accelerated Hygrothermal Conditions. *Fibers*. 2017; 5(3):25.
https://doi.org/10.3390/fib5030025

**Chicago/Turabian Style**

Mohamed Elarbi, Abulgasem, and Hwai-Chung Wu.
2017. "Flexural Behavior of Epoxy under Accelerated Hygrothermal Conditions" *Fibers* 5, no. 3: 25.
https://doi.org/10.3390/fib5030025