Fatigue Strength of Laser Welded Joints of PP and PC Components

The paper describes research of fatigue properties of laser welded joints of PP and PC reinforced composites. Welded specimens were tested on dynamic testing machine to obtain data for fatigue strength and fatigue life evaluation. Materials used for testing were PP and PC plastics filled with various amounts of glass fibers. Nine combinations of PP specimens and 6 combinations of PC specimens were welded and loaded on testing machine by fluctuating stress with constant stress ratio. Presented results include fatigue strength of welded joints and also the S-N curves based on experimental data and theory are derived.

Fatigue behavior of reinforced composite welded joints is still object of research and the literature doesn't cover this field fully [6] [7]. There is much blank space for studying and that is also a motivation. Besides the fatigue strength research, the object of interest was to survey the influence of filling of glass fibers on the welded joint strength.
In this paper, which follows on the older studies [8], the laser welded process is used to bond glass reinforced polypropylene (PP) and polycarbonate (PC). Fatigue strength of welded joints is measured by the lap shear strength test method.
The composite materials, welding parameters and testing methods are described in the next paragraph. Next, the fatigue tests are evaluated, and results are discussed, and the S-N curves are derived. Finally, some conclusions are suggested.

Materials Welding and Methods
The inspected materials were multi-directional short glass fiber reinforced PP and PC. The volume ratio of the filler was 0%, 20% and 40% for PP plastics and 0%, 15% and 30% for PC plastics. The specimens for experimental tests were cut from the plate produced by injection molding.
Laser welding requires one material to be transparent so the light beam gets through to the non-transparent material working as an absorber of the light. Although jointing of two transparent composites is possible if special conditions are fulfilled, common method is adding of darkening absorbers additives, usually carbon black. These additives slightly change material properties. Here, the absorbing material is marked by letter A (absorber), the transparent material is marked by letter T (transparent). The term ahead of the letter T or A means the volume ratio of glass fiber in the composite (in percentage). In total 15 material combinations of 10 specimens for each combination were welded ( Table 1). The combination with PC GF30T couldn't be welded on the laser station due to unrealizable conditions.
The lap shear strength test was proposed as a tensile test of two welded overlapped pieces of material. Figure 1 shows the scheme of welded specimen. Dimensions of the welded bars were 120 × 20 × 2 mm, 1 mm continual wide weld was produced in the middle of 20 mm long overlap.
Specimens for cyclic tests were welded on laser station LM05/05P.W. Both parts were pressed to the base by a tool that consisted of four pneumatic cylinders Table 1. Combinations of materials for shear tests. The letter T means transparent material, letter A means absorbing material and number after GF the volume fraction of glass fibers (in percentage). ensuring adequate contact pressure between the welded bars. A longitudinal hole in the middle of the tool let the laser beam through. Laser station ready to welding process is in Figure 2.
Settings of welding parameters were quite demanding because the weld joints need to be thin as the preliminary tests have shown that larger welds have higher strength than the material itself. The presence of glass fibers also complicated the welding process because the laser beam was dispersed by the fibers. Welding parameters were unique for each combination of welded materials to achieve a good weld joint. Nevertheless the weld area differs for each specimen and had to be evaluated after every single shear test. Examples of welded specimens are in Figure 2.
All tests were performed on a hydraulic Instron Electropuls E3000 testing machine. Cyclic loading frequency was set to 20 Hz, which is near to the maximal frequency that the machine can control for this method. The mean and amplitude value of cyclic loading was chosen to have constant stress ratio R = σ min /σ max = 0.1. The attempt was focused on fatigue strength measure at the two million cycles. Since the real weld area was unknown before the test it was difficult to estimate proper values. Therefore it happened that some specimens have broken early (if the weld area was smaller than expected) and some specimens not even after five million cycles (if the weld area was larger than expected).

Experiments
The input data to the testing machine were mean and amplitude values of load-  an image processing from an image obtained by a microscope. If the weld joint didn't rupture during the test the specimen was broken on a static testing machine for subsequent weld area measurement.
The graphical representation of measured data in semi-logarithmical coordinates is in Figure 3. There are shown only six graphs due to saving the space.
The present stress corresponds to a shear stress.

Discussion
It   Figure 3. Weld joint stress at specimens rupture after N cycles (orange), stress of intact specimen after the stopping the test (green) and static strength of weld joints (blue) for selected sets of material combinations. Four upper graphs relate to PP reinforced composites and two lower graphs relate to PC reinforced composites. The amount of tests and their quality doesn't allow to construct S-N curves using the linear regression method. However the S-N curves can be approximately described by analytical equation where N is a number of cycles, Δσ is a range of loading stress and m is a tangent of the line in bi-logarithmical coordinates. Parameters m and C are usually empirically defined only for the most frequent materials, in other cases they need to be guessed.
Parameters of weld joints S-N curves were determined so that the curves went through the measured data and the evaluated fatigue strength in compliance with Table 2. In several cases the S-N curve is a constant function equal to fatigue strength value or it couldn't be even created due to lack of relevant data. Parameters are shown in Table 3.
Small slope of S-N curves isn't unexpected; this behavior is for composite materials typical. Large decrease of weld joint fatigue is quite unusual as the common drop of fatigue strength is about 10% [9]. Selected S-N curves graphs based on the Equation (1) and parameters stated in Table 3 are shown in Figure 4.
The S-N curve is displayed in interval 10 4 -10 7 cycles for PP composites and 10 5 -10 6 cycles for PC composites.

Conclusion
The  Table 3. Parameters of analytical S-N curves shown in Figure 4. The symbol * means that corresponding S-N curve is a constant function equal to the fatigue strength.  . S-N curves of PP (four upper graphs) and PC (two lower graphs) composite weld joints for cyclic shear loading with stress ratio R = 0.1. Orange points mean the moment of the specimens rupture, green points mean the moment when the test was ended and the weld area measured, green points mean static strength of the weld joint.
weld joints is higher than the fatigue strength of PC weld joints despite the higher strength of PC plastics. The S-N curves were derived for designing weld joints.
With regard to large variances of measured data and behavior of weld joints during the experiments, it is necessary to design the joints with major safety.