RESIDUAL STRESS

Assessment of Induced Residual Stress in Blast Protection Visors.

Project Driver:

Blast visors are an essential element of protection for deminers. The proddingprocedure is manually intensive as the deminers cautiously and tentativelyinvestigate the presence of a mine and then proceed to remove soil fromaround mine casing. In doing so, the deminer's face is in relatively closeproximity to the mine as per the following diagrams.

Blast protection visors provide protection for the face and head shouldaccidental detonation of the mine result. Eye injuries which are common in deminingaccidents can be greatly minimised with the correct use of visors.

As an important tool for the demining process, a blast protection visorwas developed by the University of WesternAustralia Demining Project for use in the field. The visor is manufacturedfrom clear 4.5mm thick polycarbonate sheet bent to shape under temperature.The processes of thermal bending and other manufacturing steps ultimatelyinduce residual stresses within the visor screens. Although the visorsare field tested to validate their performance against live mines, inducedstresses within the material would more than likely introduce weaknesseswhich reduce strength and performance.

A procedure was thus devised to relatively qualify the intensity of residualstresses that are induced during the manufacturing process. Thus wouldthen facilitate process refinement to reduce the stresses. A keyobjective in the development of the testing procedure was to keep it simple,inexpensive and easily maintainable to ensure that the procedure couldbe easily performed in third world locations where the visors are beingmanufactured.

The Thermal Bending Process

To form the visors to the required shape, flat sheets of polycarbonateare clamped within a special spring loaded forming fixture and heated abovethe material bending temperature within a special oven. Any physical contacton the sheets damage the material as it is being heated. The pressure thatis exerted by the clamps along the edges of the polycarbonate sheet aregenerally regions of highest induced stresses.

Testing procedures Developed:

The residual stress test is used to analyse the stresses in the visor.Residual stress in the visor is the result of uneven bending of the plasticduring the thermoforming process. This stress weakens the visor and reducesits performance. When polarised light is shone through the clear plastic,areas under stress bend the light and this shows up when viewed throughpolarising film as bands of bright colour. The closer the bands of colourare, the greater the stress.A testing rig is required to examine the visorsfor stress (fig. 1). This rig uses two sheets of polarising film arrangedwith the polarising at right angles to each other (this occurs when thesheets are lined up so that when one is in front of the other no lightis allowed through). When the visor is placed between the two sheets, thebands of light indicating stress can be seen.

A curvature test is used to determine how closely matched the curvatureon each visor is. At first it was thought that it would be best to usea dial gauge to measure various points on the visor and compare the readingsas an indicator of the radius of the curve at that point. A measuring rigwas built (fig. 2) using the dial gauge. However this was found to be hardto use, as the gauge was difficult to position accurately and demonstraterepeatable results with. The cross sections of two different visors weretraced and compared (fig. 3). This method is suitable because of the largedifference in the curvature of the visors. Tests with a dial gauge deviceare still required for accurate and quantitative testing though.

Sample Set

To verify the procedure a sample set of five visors were evaluated. The set comprised of two visors from the Hamid Ali Research centre in Pakistan and three visors from Security Devices in Zimbabwe. This approach provided an opportunity to assess how differences in the manufacturing processes result in induced residual stresses.

Test Results:

The residual stress tests show that the visors exhibit stress at the sideswhere they are clamped for the thermoforming process and at the top andbottom edges where the edge effect of the bending creates a curl in theplastic. The HARC produced visors were compared to the Security Devices´visors and it was found that the Security Devices´ visors exhibit stresscontours that were less regular than the HARC visors.

When comparing the stress between the visors, the Security Devices´visor (fig. 4) shows highly irregular and concentrated stress patternson the sides of the visor, particularly around the mounting hole. The HARCvisor in figure 5 shows a much more regular stress pattern where the stressis concentrated on the edge of the visor and at the bend in the plastic.The different stress patterns are a result of the different clamping techniquesduring the thermoforming.

Figure 6 shows the stress through the centre of the Security Devicesvisor. The colour is lighter than through the centre of the HARC visor(fig. 7) which is almost opaque. Both visors show no stress patterns, butthe lighter colour of the Security Devices´ visor indicates that the entirearea is under slight stress, which is not evident in the HARC visor.

Residual stresses occur on the top and bottom edges of the visors asa result of the plastic curling near the edges during the bending process.This is an unavoidable result of the bending. The Security Devices´ visorin figure 8 shows multiple bands of colour at the edge whereas the HARCvisor in figure 9 has just one thick band of soft colour. The SecurityDevices visor is under substantially more stress near the edge than theHARC visor.

When they mount the visors to the helmets, HARC do the mounting boltsup extremely tightly to discourage the deminers from lifting the visorsup. As can be seen in figures 10 and 11 the bolt creates stress in theplastic. It is difficult to ascertain what effect this stress has on thestrength of the visor. As the stress is acting onto the plane of the plastic,is compressive with no shearing forces and is only on the sides of thevisor it should not greatly affect the structural integrity and functionof the visor.

After the bolt was removed it was found to leave residual stress inthe plastic. The change can be seen when comparing figure 5, before thebolt was placed to figure 12, after the bolt was removed. This is alsoevident when comparing figures 4 and 13 for the Security Devices´ visor.The residual stress is relatively minor and present only over a very smallarea, so it is unlikely to have much effect on the strength of the visor.

Bubbles can be observed in the plastic of the Security Devices´ visors.Excessive heating during thermoforming or the plastic not being dried sufficientlycauses these bubbles. The bubbles create small high stress areas in theplastic. While the bubbles can´t be seen in the photos the high stressareas they cause can be seen above and to the left from the mounting holein figure 13.

Interestingly the polycarbonate prodder shields were examined with thephoto-elastic technique and found to be under severe stress (fig. 14).This ties in with Test results showing that the plastic splits when subjectedto a blast. The split is caused by the plastic reforming to relieve thestress when it is able to. This stress is caused by the residual stressesfrom the thermoforming, bubbles in the plastic creating localised stresses,stress from cutting the holes and the twisting of the shield required toget the mounting holes to line up.

The prodder shield is poorly manufactured. There are many bubbles inthe plastic, there is a lot of stress along the bend lines and the holefor the blade to fit through is poorly manufactured with jagged edges anda lot of residual stress. The main problem is that the mounting holes don´tline up with the handle, so for the handle to be screwed in place the plasticmust be twisted. This action places the entire shield under a great dealof stress and is no doubt the main cause of the splitting that occurs whenthe prodder is caught in a blast.

The curvature of the HARC visors was measured by simply comparing cross-sectionsof their curves. It was found that the HARC visors have an asymmetricalcross-section, which appears to be due to the uneven heating of the plasticsheet during thermoforming. The curve of the visor will have a smallerradius on the left or right hand side, subject to which side was closerto the heat source in the oven. The difference of orientation between thevisors can affect the radius of the curve by as much as five millimetres.

Conclusion:

The HARC visors appear to have less residual stress than the Security Devicesvisors and consequently should withstand higher stresses without failure.The Security Devices visors show bubbles, due to the plastic not beingdried sufficiently or being overheated. Irregular stress patterns are presentwhich are caused by the clamping technique during thermoforming. Some minorchanges to the manufacturing process should eliminate these problems.

The curvature of the HARC visors differs to a maximum of five millimetres.While this is a substantial difference and shows that a product of a definiteand repeatable standard is not being produced, it should have no effecton the strength of the visor. Some changes to the design of the oven shouldbe able to correct the heat difference and enable visors of a standardcurvature with a symmetrical cross-section to be produced.

Given the large difference between the visor curvatures a more practicalmethod of determining that the curvature fits within limits may be a simpletemplate with an acceptable visor cross-section. If a visor matches thistemplate (i.e. fits through the right shape hole) then the visor´s shapeand curvature is correct. A better device utilizing the dial gauge mayneed to be constructed to provide more accurate curvature data for quantifiableand recordable curvature results.

The prodder shield is of poor construction and the manufacturing processneeds to be improved to remove the large amounts of stress present in thecurrent design. The current level of stress has been shown in field teststo have a large detrimental effect on the strength of the prodder and itsability to withstand a blast. The prodders are also not of a standard qualityof production, and differ greatly in the quality and fit of their components.It is recommended that the quality of the prodders be improved before anysale is made.

All the products tested have scope for improvements to their manufacturingand quality control. The prodder shield is in particular need of attentionas its quality has a proven impact on its performance. Field trials wouldbe the best way to determine the effect that the faults in the visors'have on their performance and to compare the effectiveness of the visors.

Formore information on any of these send us E-mail at demining@mech.uwa.edu.au
All graphics by Demining ResearchTeam. Aug 1998.
Last modified: 12:55.05 Tue22 Sept 1998 by Brian McLean