## 7 Testing Methods  
Both conventional and specialized methods are frequently used in tests on fiber-reinforced composites to assess their creep behavior and other mechanical characteristics. Understanding how fiber-reinforced composites react over time to different stressors, strains, and environmental circumstances is facilitated by these tests. The primary tests carried out in the study of creep behavior are presented below. Testing methods are related to the results that need to be obtained from testing. For this reason, there are several types of tests that are performed in the laboratory. A standard test machine is presented in Figure 9.  
The most common tests used to study these materials are creep tests, which assess the time-dependent deformation of fiber-reinforced composites under constant load (creep) over an extended period. A constant load (or constant stress) creep test is performed when the specimen is subjected to a constant load at a constant temperature. It is possible to measure the primary, secondary, and tertiary creep phases. Both thermoset and thermoplastic composites can be studied. Thus, the creep strain vs. time curves and the creep rate are obtained. The standard procedure normally used in experiments is the following:  
* Test specimens are loaded with a constant tensile or compressive load. * Deformation (strain) is measured at different time intervals to monitor how it changes over time. * Testing is carried out at various temperatures and environmental conditions. So, it is possible to study the composite's behavior under different operating conditions. With the help of these types of tests, linear or nonlinear creep can be studied, the creep rate can be determined (generally decreases over time), temperature sensitivity can be evaluated (typically exhibiting higher creep rates at elevated temperatures), and the increase in lifetime with an increase in fiber volume fraction can be studied. In addition to the standard creep study procedure, there are different variations in which these tests are performed or additional tests, which are related to the context for which the tests are performed:  
* Constant strain (or stress relaxation) tests are performed when the material is strained to a fixed level, and the decay in stress is measured over time. So, it is possible to study the viscoelastic behavior of polymer matrix composites (PMCs). The results obtained are stress vs. time and relaxation modulus. * A creep rupture test is very similar to that performed under constant load, but the focus is on time to failure under a constant stress and temperature. So, it is possible to obtain the durability/lifetime of a composite under long-term loads. It yields time to rupture vs. applied stress. * Accelerated creep testing represents a class of tests conducted at higher-than-service temperatures or stresses to accelerate failure. To interpret the results, the time-temperature superposition and time-stress superposition principles are used. * Dynamic mechanical analysis (DMA) is performed using oscillating stress to measure viscoelastic properties. This method is used mainly when short-term creep compliance and modulus measurements should be obtained. Figure 9: A standard test machine [138]. * A bending creep test (flexural creep) in a three-point or four-point setup is used to study the real-world loading in structures composed of beams or panels. * Thermal cycling tests are used for the study of composites under varying thermal conditions (expansion and contraction) to simulate real-world conditions experienced in aerospace, automotive, and other industries. In these tests, the specimens are exposed to high and low temperatures for a certain number of cycles, and thermal expansion, dimensional stability, and residual stresses are determined. Thermal expansion can lead to residual stresses, which may cause microcracking or delamination over time and matrix degradation, fiber-matrix debonding, or interlaminar shear failure.