The device employed in this work is shown in Fig。 4, which can be used for measuring the residual stresses quantitatively and for qualitative analysis。 It may take a variety of different forms, depending on the desired use。 The polariscope was manufactured in collaboration with the Palacký University Olomouc in the Czech  Republic。

The optical equipment described in this analysis to perform the photoelasticity experiments was based on two models of arrange- ment, basically:

a) plane polariscope

b) plane polariscope  with compensator

The two arrangements can be observed in Fig。 5, a) and b), respectively。 They were equipped with a light source L, the polariscope P, which polarized the light in the right vibration plane, the model or sample to be analyzed M, the compensator C used in the arrange- ment (b), which determined the retardation value δ, the analyzer A, and finally the screen or the person who performed the experi- ment T。

The compensator (model LWC-100) is calibrated using an interferometric filter transmitting a 10-nm band at the “standard” wavelength described in ASTM test methods F218 and D4093 (with a wavelength value of 570 nm for plastics) (Strainoptics™, Inc。, USA)。

The stress-optic Eq。 (4) establishes that double refraction is directly proportional to the difference in principal stresses, which is equal to the difference between the two indices of refraction exhibited by the stressed material。 By knowing the difference in the indices of

Fig。 4。 Optical equipment  — polariscope use  in the qualitative and  quantitative    analyses。

refraction and the stress-optical or Brewster's constant, it is possible to calculate the difference in the principal stresses。 The difference in the indices of refraction can be determined by piding the retardation value by the material thickness。 The stress equation is

where r is the reading at the point of interest by the compensator model LWC-100, c is the calibration factor supplied by Strainoptics, and N is the fringe order, which can be calculated as  follows:

δ  。 nm fringes 。

Nfringes ¼ ðrÞðcÞ ¼ λ

or : 6

pision pision

Our interest is mainly focused on the values of retardation (δ) or reading (r), for which maximal or minimal values of N are attained。 If δ/λ = N where N = 0,1,2…, the interaction between two waves is called constructive interference。 On the other hand, if it takes its minimum value, zero in this ideal case, for δ/λ = 1/2, 3/2…, this situation is called destructive interference and causes dark fringes。

Once the part has been analyzed by plane polariscope, Fig。 6, it is possible detect the areas where the flow shows a complex path after starting to fill the cavity of injection molding tool IMT through the sprue, area close to the gate where is located the highest

Fig。 5。 Optical equipment arrangement, a) plane polariscope, b) plane polariscope including compensator to obtain the values of δ。

Fig。 6。 Plastic cover lens analyzed under polarized light where streamlines are detected close to the gate。

concentration of isochromatic lines and residual stress。 This is called qualitative analysis and allows us to start with the investigation by the non-destructive  technique。

2。3。 Chemical attack technique

When products are in service, the ability to predict the lifetime failure rate is highly beneficial。 Plastic parts should be tested in real time and in environments similar to those to which the part will be exposed。 PCLs made of thermoplastic polycarbonate are known to be highly susceptible to environmental stress cracking when exposed to organic compound liquids。 This type of failure can be prevented by insuring that the level of mechanical stress on plastic parts is below the characteristic critical cracking stress for the par- ticular test medium described by test method DIN EN ISO 22088-2。 Destructive analysis was carried out by two test fluids which have different reaction thresholds and are presented in Table 1。 These test fluids were used to determine the mechanical stress level that develops in the PCL during the IMP, which, combined with stresses from the assembly operation such as mechanical attachment and stresses which the part encounters in service, becomes  critical。